Supply-side ecology, barnacle recruitment, and rocky intertidal
Transcription
Supply-side ecology, barnacle recruitment, and rocky intertidal
Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e Supply-side ecology, barnacle recruitment, and rocky intertidal community dynamics: Do settlement surface and limpet disturbance matter? Bruce A. Menge ⁎, Melissa M. Foley 1, Jacque Pamplin, Gayle Murphy, Camryn Pennington Department of Zoology, Oregon State University, Corvallis, OR 97331-2914, United States a r t i c l e Keywords: Artificial collectors Balanus glandula Bulldozing Chthamalus dalli Cyprids Limpets Oregon Recruitment Rocky intertidal Safety-walk plates Settlement i n f o a b s t r a c t The supply of new recruits can be a critical determinant of community structure, but estimating recruitment density can be challenging due to variation in larval supply, settlement, substratum, species interactions and physical stresses. We evaluated the effect of surface type and limpet “bulldozing” on settlement and recruitment of barnacles, using replicated field experiments sampled monthly at each of four sites along the central Oregon coast in 2001, 2003, and 2008. A particular goal was to evaluate the efficacy of Saf-T-Walk, a widely used textured surface, in quantifying actual patterns and levels of recruitment. In 2001 and 2003 experiments, surfaces were bare rock (ROCK) and Saf-T-Walk (STW). In 2008 experiments, to further evaluate the influence of surface rugosity, we added two additional artificial substratum types — smooth PVC plates (no texture) and travertine plates (TRAV, low texture). Limpet activity was manipulated using barriers of anti-fouling paint. Results indicated the overwhelming importance of surface texture. Recruitment density was highest in 2001, lower in 2003, and lowest in 2008. Recruitment on STW, the most textured surface, ranged up to 87 times greater than on ROCK, and the rank order of surfaces for recruitment density was STW N ROCK N TRAV N PVC which matches that for texture. Settlement differences were even greater, with up to 1180-fold higher settlement on STW than ROCK. Limpets usually had a negative effect on recruitment of Balanus glandula on ROCK, likely due to bulldozing and direct consumption, but had a positive effect on STW. These impacts were reduced or negated when recruitment density was higher (e.g., 2001) or lower than average (e.g., 2008). Limpets had no consistent effect on Chthamalus dalli. Abundances of settlers and recruits on the different surfaces were positively correlated (explaining up to 71% of the variance), and correlations between STW and other surfaces generally were stronger in the absence of limpets. Although compared to bare rock, recruitment to STW overestimates absolute rates of recruitment, use of STW settlement plates provides an effective and relatively efficient means of determining spatial and temporal patterns of recruitment at local to geographic scales. © 2010 Elsevier B.V. All rights reserved. 1. Introduction After decades of research focused at local scales, ecology has undergone a dramatic shift in recent years towards investigation of how patterns and dynamics of species interactions vary across multiple spatial and temporal scales (Levin, 1992; Brown, 1995; Holyoak et al., 2005). Investigations now routinely incorporate multiple study sites often over vast spatial scales into their design (Jenkins et al., 2000; Sagarin and Gaines, 2002; Menge et al., 2004; O'Riordan et al., 2004, Navarrete et al., 2005; Coleman et al., 2006). Such studies are challenging because of variation among coinvestigators, the biota, and physical conditions. For example, ⁎ Corresponding author. E-mail address: [email protected] (B.A. Menge). 1 Current address: Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064, United States. 0022-0981/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2010.04.032 incorporation of spatial scale requires standardization to ensure that differences observed are independent of method. In marine ecosystems, several recent large-scale studies exemplify this approach and its challenges (Jenkins et al., 2000; Menge et al., 2003; O'Riordan et al., 2004; Coleman et al., 2006). These studies are typically carried out by a consortium of institutions and a number of investigators at each institution. For example, the PISCO (Partnership for Interdisciplinary Studies of Coastal Oceans) consortium is focused on the linkages between coastal ocean environments and benthic communities, and how these vary through time and across space (Menge et al., 2004; Schoch et al., 2006). A key potential determinant of community structure and an important link between the coastal ocean and benthic habitats is the supply of new recruits (Connell, 1985; Gaines and Roughgarden, 1985; Connolly and Roughgarden, 1999). PISCO has focused much effort on the investigation of recruitment patterns of sessile invertebrates to rocky intertidal regions and fish to kelp bed habitats along the coasts adjacent to the California current (Amman, 2004; B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Broitman et al., 2008). In these large-scale investigations, an important tool has been the adoption of standardized units to quantify recruitment that are inexpensive, uniform in size and composition, and readily deployed, retrieved and processed. These devices include plastic mesh ovoids used to quantify mussel recruitment (Menge, 1992), and, our focus in this paper, settlement plates covered with a textured surface to quantify barnacle recruitment (Farrell et al., 1991). Barnacles are ubiquitous in marine habitats, and in rocky intertidal systems are usually dominant components of ecological communities (Lewis, 1964; Stephenson and Stephenson, 1972; Morton, 2004). As expressed in many influential studies, they are central to the dynamics of populations and communities around the world, serving as competitors, facilitators, and prey in a wide variety of settings (e.g., Connell, 1961; Dayton, 1971; Menge, 1976 and many others). Barnacles are particularly prominent in studies of recruitment because larvae, settlers and recruits are identifiable, abundant, and readily studied in the field (e.g., Hawkins and Hartnoll, 1982; Wethey, 1984; Connell, 1985; Caley et al., 1996). In field investigations, two approaches have been taken to quantify settlement and/or recruitment of barnacles. Some have investigated recruitment to natural rock substrata (e.g., Hawkins and Hartnoll, 1982; Raimondi, 1988; Menge, 1991; Jenkins et al., 2000, 2008; Bertness et al., 2006) or to human-fabricated rock surfaces (Noda, 2004), while others have used artificial settlement surfaces (e.g., Pomerat and Weiss, 1946; Sutherland, 1974; Menge et al., 1999; Menge, 2000a). With artificial surfaces or collectors, a variety of materials have been used including plexiglass plates with pits (Raimondi, 1990), unglazed ceramic plates (Broitman et al., 2005), and fiberglass, plexiglass, or PVC plates coated with a textured surface (Farrell et al., 1991; Pineda, 1994; Menge, 2000a) among many others. The multiplicity of surfaces used in studies of recruitment raises questions of comparability among studies using different methods and the extent to which artificial surfaces reflect patterns that occur on natural rock surface (Raimondi, 1990). Further, rock surface variation such as differences in rock type or texture may also cause variation in settlement on natural surfaces (Pomerat and Weiss, 1946; Barnes, 1956; Crisp, 1961; Raimondi, 1988). In addition, species interactions can influence estimates of recruitment (Dayton, 1971, Hawkins, 1983). For example, Dayton (1971) showed that barnacle abundances were reduced by limpet “bulldozing,” defined as dislodgement of barnacle cyprids and recent metamorphs by the leading edge of the limpet shell as it grazes over the rock. Thus, if bulldozing by limpets varies with their size, abundance, and species composition, patterns of variation in recruitment of barnacles may be influenced by limpet abundance. Here we address these issues using results from factorial experiments to evaluate the importance of substratum (natural rock or artificial settlement plates) and limpet bulldozing (present or absent) in determining barnacle recruitment density. Following earlier research (Connell, 1985; Raimondi, 1990), we define settlement as the number of larvae attaching and surviving 24 h and recruitment as the number of settlers surviving for up to one month. We addressed six hypotheses: 161 H5. Because of texturing that can provide refuges for new recruits, limpet bulldozing will have smaller effects on recruits on artificial surfaces than on natural rock. H6. recruitment of the coexisting acorn barnacle species Balanus glandula and Chthamalus dalli will be affected in similar ways by substratum and limpets. 2. Methods 2.1. Study sites This study was carried out at five sites along the central Oregon coast (Fig. 1), with subsets of four sites used in each of 2001, 2003, and 2008. Replicate sites were located in regions of contrasting oceanographic conditions, which are associated with variation in the width of the continental shelf (Menge et al., 1997; Barth and Wheeler, 2005; Kosro, 2005). In all years the two northern sites were located in the narrow continental shelf region (Fogarty Creek [both years] and Boiler Bay [both years], 44°51′ N and 44°50′ N, respectively) while the southern sites were located in the wider continental shelf region (Yachats Beach [2003, 2008], Strawberry Hill [all years] and Tokatee Klootchman= Gull Haven [2001], southern sites; 44°17′N, 44°15′N, and 44°13′N, respectively). At the intertidal areas at Fogarty Creek and Boiler Bay (hereafter, FC and BB), abundance of macrophytes was relatively high, while abundances of filter-feeders, predators, and macroherbivores were relatively low (Menge et al., 1994, 1997; Allison, 2004). In contrast, at Yachats Beach, Strawberry Hill, and Tokatee Klootchman (hereafter YB, SH, and TK) the abundance of macrophytes was relatively low and abundances of filterfeeders, predators, and macroherbivores were relatively high (Menge H1. Recruitment of barnacles on natural rock surfaces will be positively correlated with recruitment on artificial surfaces. H2. With increasing texture on artificial surfaces, settlement and recruitment densities will increase. H3. Recruitment of barnacles will be lower and more spatially variable on natural rock surfaces than on artificial surfaces because natural rock has lower and more spatially variable surface texture. H4. Because of the negative effects of bulldozing, recruitment of barnacles will be lower in the presence of limpets than in their absence. Fig. 1. Study sites along the Oregon coast. Cape Blanco and the Columbia River are shown for reference. Site codes are FC = Fogarty Creek, BB = Boiler Bay, YB = Yachats Beach, SH = Strawberry Hill, and TK = Tokatee Klootchman. 162 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 et al., 1994, 1997, 2004). Differences in community structure were reflected in differences in key dynamic processes. Rates of predation, mussel recruitment, and grazing were all higher at southern sites than at northern sites (Menge et al. 1994, 2004; Freidenburg et al., 2007). Barnacle recruitment, however, did not vary consistently among the five sites (Barth et al., 2007). These community and process differences have persisted since at least the early 1980s to present (Menge et al., 2004, Menge, B. A., pers. obs.). 2.2. Study design To evaluate the simultaneous effects of limpet grazing and settlement surface on the density of barnacle settlement (attached cyprids) and recruitment (B. glandula and C. dalli), we established experiments at wave-exposed locations at each site. All plots were positioned at the lower edge of the mid zone, just below the mussel bed. In 2001, five replicates of five treatments were set up in waveexposed areas at FC, BB, SH, and TK (Table 1). In 2003, we substituted YB for TK to make wave action more consistent across all sites. Six treatments were assigned randomly within each of five replicate blocks (Table 1). In 2008, we included two additional settlement surfaces to the two used in the 2001 and 2003 designs to evaluate the influence of varying texture in addition to the effects of limpet grazing. Eight treatments (four substrata × two grazing) in five blocks were included in the 2008 experiment (Table 1). Although several barnacle species settled in these experiments (the above two plus Balanus crenatus, Balanus nubilis, and Pollicipes polymerus), recruitment of the latter three was sporadic and usually very low except for occasional annual pulses of the gooseneck barnacle Pollicipes. Thus we focused our analyses on B. glandula and Chthamalus. Cyprids of these species are identifiable, but our method of monitoring bare rock plots lacked sufficient resolution to separate these species so we combined all cyprid counts for analysis. Earlier sampling of daily plates indicated that cyprid metamorphosis occurs within hours of settlement. We tested this assumption, and also evaluated the rate at which metamorphosis to juvenile barnacles occurred by carrying out daily sampling for two four-day periods in July 2002 at the FC site. Three replicate settlement plates were placed in the mid exposed zone on 09 July 2002 and collected daily through 14 July. This process was repeated 22–26 July. Based on this study (see Results), we define cyprids attached to the rock as settlers and assume they reflect settlement over the past 24 h. Table 1 Design of experiments testing the effects of substratum and limpets on recruitment of barnacles. Year Surface Treatment code Treatment description 2001 ROCK − limpets + limpets − limpets − limpets + limpets − limpets + limpets + limpets − limpets + limpets + limpets − limpets + limpets − limpets + limpets − limpets + limpets − limpets + limpets − limpets + limpets Paint barrier Marked plot Painted plate Painted z-spar barrier Unpainted plate Paint barrier Marked plot Paint control Painted plate Unpainted plate Paint control (partial paint barrier) Painted z-spar ramp (YB only) Unpainted z-spar ramp (YB only) Paint barrier Marked plot Painted plate Unpainted plate Painted plate unpainted plate Painted plate Unpainted plate STW 2003 ROCK STW 2003 STW 2008 ROCK STW TRAV PVC 2.2.1. Substratum effect Many types of artificial surfaces have been tried (see Introduction), and over the years we have tested unglazed ceramic tiles, a building material called “Hardiflex,” plastic plates coated with latex paint with neoprene microspheres mixed in, Saf-T-Walk (see below), travertine and polyvinyl chloride plate. Although each offers relative homogeneity in surface texture, all have shortcomings. For example, tile undersides were smooth except for a square of grooves. Barnacles settled primarily along these grooves, leaving most of the surface unoccupied (Gaines, S. D., pers. comm.; Blanchette, C. A., pers. comm.). Hardiflex and neoprene microsphere surfaces also proved similarly inadequate; we often observed recruits on the rock nearby but almost none on Hardiflex plates. In this study, we used four different types of settlement surface. In the 2001 and 2003 experiments, to evaluate the effect of settlement surface, we used 10 × 10 cm bare rock plots and 10 × 10 cm polyvinyl chloride (PVC) plates covered with Saf-T-Walk® tape (3M Company, St. Paul, Minnesota, USA)(Farrell et al., 1991; Menge, 2000a; Shkedy and Roughgarden, 1997). Saf-T-Walk tape (hereafter STW) is a rubbery textured surface that, since barnacles prefer to settle into pits and rugosities (e.g., Barnes, 1956, Crisp, 1961), provides barnacle cyprids with nearly ideal settlement surfaces and ensures substratum uniformity (Farrell et al., 1991; see Fig. 2). The rock surface at all sites was basaltic, a considerably less rugose surface than STW. Bare rock (hereafter ROCK) plots were chosen to be representative of the texture of the rock at the site (Fig. 2), but we avoided surfaces with crevices, pits, holes, grooves, and similar heterogeneities to make these as comparable to the artificial collectors as possible. This treatment allowed us to determine if plates covered by STW reflected the patterns of recruitment that occur on natural substrata. We note that this design does not specifically test the effects of rock texture or substratum type since neither of these was varied systematically in the design. Because of the differences in degree of texturing on ROCK vs. STW, we expected to observe lower recruitment densities on rock, but predicted that ROCK densities would be a consistent fraction of the density on STW (Hypothesis H1). In 2008, to evaluate the effects of surface texture, we added two additional surfaces to the design. The smooth surface of PVC plates with no STW (hereafter PVC) provided artificial surfaces with no texture and travertine plates (hereafter TRAV) provided artificial surfaces with low to moderate texture (Fig. 2). Ranked by texture, the surface order is thus no texture (PVC), low texture (TRAV), moderate texture (ROCK) and high texture (STW)(Fig. 2). All artificial plates were soaked in seawater for at least a week to condition them before use in the field. 2.2.2. Limpet effect To determine the effects of limpets on barnacle recruitment, we established treatments with and without limpets (+limpet and −limpet, respectively). To exclude limpets from ROCK and STW plots, we surrounded one ROCK plot and one STW plate (2001 only) in each replicate with a 3 cm wide band of marine epoxy, z-spar™ (Kop-Coat Inc., Los Angeles, California), which was then painted with a copperbased anti-fouling paint (hereafter termed z-spar + paint barriers). Copper paint barriers have been shown to impede limpets and chitons, in most cases with no artifacts due to the presence of the paint (Cubit, 1984; Paine, 1992; Menge, 2000b; Freidenburg et al., 2007; Guerry, 2008; Guerry et al., 2009; see Benedetti-Cecchi and Cinelli, 1997 for critique). Since past experiments indicated that the z-spar + paint barrier had to be repaired often, we also tested the effectiveness of copper paint in deterring grazers when coated on the sides and bottom of the artificial plates (all years). Because differences between the painted plate and plates with z-spar+ paint barriers were minimal (see Results) we deployed only painted plates in the 2003 and 2008 studies. 2.2.3. Control experiment The z-spar + paint barriers were relatively flat, so plate edges protruded above the surrounding substratum by about 5 mm. This abrupt edge could possibly create differences in flow over plates vs. B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 163 Fig. 2. Close-up photographs of the artificial surfaces used in the experiments, and of the range of rugosity observed in the rock settlement plots. The holes in the plates are 8 mm in diameter. The area in the rock photos is scaled similarly to the area shown on the artificial plates. PVC = polyvinyl chloride plates, TRAV = travertine plates and STW = safety-walk plates. The differences in surface texture and relative homogeneity of the artificial plates is evident in the photos, as is the heterogeneity in natural rock surfaces. ROCK plots, or even deter limpets from +limpet treatments, and could also have affected barnacle recruitment. In 2003, we tested this potential artifact at YB by deploying additional treatments that included z-spar “ramps” around the plates to minimize eddies over the plate edges. The two treatments were ramps with (−limpet) and without (+limpet) paint. All treatments are summarized in Table 1. 2.3. Monitoring procedures The experiment was monitored monthly from June to September in 2001 and 2003, and biweekly in July and August in 2008. Plates were replaced at all sites during the same tide series, usually on consecutive days. Limpets on plates (bottom and top) were counted in the field. Because small limpets are difficult to identify to species in the field, we identified them to genus (Lottia) only. The species most likely to be included in this group are Lottia digitalis, L. pelta, L. strigatella, and an unidentified Lottia species. After collection, barnacle settlers (cyprids) and recruits (juveniles) on plates were counted and identified to species in the laboratory under dissecting microscopes. ROCK plots were also monitored during the same tide series. All limpets in ROCK plots were counted and in −limpet plots, removed. Barnacle recruits to the ROCK plots were counted and identified to species in the field using a field microscope (2001, 2003) or hand lens (2008). Field microscopes have greater acuity than hand lenses but sampling the entire surface was impractical with microscopes so we sampled five haphazard fields of view (0.1 cm in diameter) in each plot. Subsequent analysis suggested that subsampling introduced much variability, especially at lower densities. Thus, in 2008 we switched to hand lenses and counted the entire plot. Recruit densities were standardized to an area of 100 cm2 for analysis. In the field, cyprids could not be resolved to species with microscopes, so in 2001 and 2003 we lumped all taxa into a single count of “cyprids.” Hand lenses have lower acuity than microscopes, so in 2008 field counts of cyprids were unreliable and cyprid counts were only available from STW plates, which unlike other surfaces, were switched out and processed in the laboratory. ROCK plots were scrubbed clean using a stainless steel wire brush after counts and rechecked to make sure all individuals were removed. On all surfaces filamentous algal blooms occurred at times, especially in the absence of grazers. To help counting accuracy, we used bleach to remove the algae, either in the field (ROCK and TRAV) or in the lab (PVC). Counts in areas where recruits were visible prior to bleaching indicated that only algae was removed, not settlers or recruits, allowing unobstructed counts of settlers and recruits prior to their removal. Prior to removal, percent cover of algae was recorded in all plots to allow testing for potential interactions between ephemeral algae and recruits. 2.4. Data analysis All analyses were done using JMP v. 8 (SAS, 2008). Establishment of experimental replicates takes 1–2 days, so it would be impractical to redo experimental setup each month to vary the location of each. Hence, ROCK plot locations were fixed in space as was the location of the plates, although the latter were replaced monthly. Because plot and replicate locations were fixed, we used repeated measures analysis of variance to 164 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Fig. 3. Overall responses within each year of limpets, barnacles and algae in + limpet and − limpet treatments on STW and ROCK surfaces, arranged by cape from north (FC, BB) to south (TK/YB, SH). TK is actually south of SH, but is grouped with YB, the site used in 2003 and 2008. Note that scales for limpets are arithmetic, for barnacles are ln, and for algae are percent cover. test the effects of limpet bulldozing, site, region, and substratum on barnacle recruitment. Linear contrasts were used to test differences among specific combinations of treatments. We used ordinary least squares regression to test the magnitude and direction of the relationship among settlement, recruitment, algal cover, and limpet density. In most cases, raw data were not normally distributed but normality was generally achieved using log-transformations. 3. Results 3.1. Effectiveness of treatments: limpet exclusion Overall, as has been demonstrated before (Cubit, 1984; Paine, 1992; Menge, 2000b; Freidenburg et al., 2007), copper-based paint barriers were effective in excluding limpets in all years (Fig. 3A–H, Table S1a). Average densities of limpets in +limpet vs. − limpet treatments were 8.6 ± 0.9 (mean ± 1SE) vs. 1.0 ± 0.2 in 2001 (n = 120 vs. 120), 13.0 ± 1.2 vs. 1.0 ± 0.3 in 2003 (n = 311 vs. 158), and 4.6 ± 0.6 vs. 0.3 ± 0.07 in 2008 (n = 302 vs. 296), or differences of ∼ 9- to ∼ 15-fold. The limpet exclusion treatment accounted for the majority of the variance (Table S1a). However, the limpet response also varied through time by year, site and substratum type (Fig. 3A–H, S1, S2, Table S1a, S2). In 2001, these effects were expressed in the withinsubjects effects of the month × site × surface interaction and the month × surface × limpet interaction (Table S2, p = 0.0004 and p = 0.03, respectively). In 2003, the four-way within-subjects effect of month × site × surface × limpet indicated a highly context-dependent effect of all factors in combination (Table S2, p = 0.01). In 2008, variable limpet response appeared in both month × site × limpets and month × surface × limpet interactions (Table S2, p b 0.0001). Despite these statistical interactions, limpet densities were higher in + limpet than in − limpet ROCK treatments in all years (Figs. S1, S2). The same general pattern held on STW (Figs. S1, S2), although in 2001 and 2008 the differences occurred at only two of the four sites (BB and SH in 2001, FC and YB in 2008; Figs. S1, S2). In 2003, limpet densities did not differ between the two + limpet treatments (marked plot without paint and paint control) except for SH (Fig. S1). Limpet density in +limpet treatments averaged over all times within each year was greater in ROCK plots than on STW plates at FC and SH but not different between these surfaces at BB, TK and YB. Limpet density in +limpet treatments differed by year at BB (2001 = 2003 N 2008 on both STW and ROCK), TK/YB (01b 03N 08 on STW, 01 b 03 = 08 on ROCK) and SH (01 = 03 N 08 on STW) but otherwise was similar among years (Fig. 3A–H, Table S1a, linear contrasts in 4-way ANOVA, p b 0.05). Overall densities across sites were greater on ROCK than STW in 2001 (11.1 ± 1.2 vs. 6.2 ± 1.2, n = 60 each), but in 2003 limpet densities on ROCK were similar to those on STW (11.4 ± 0.9 vs. 14.7 ± 2.2; n = 159, 152). In 2008, densities in + limpet treatments varied with surface (ROCK: 7.3 ±1.2, n = 149; STW: 1.3 ± 0.25, n = 157; TRAV 0.8 ±0.19; n = 138; and PVC 0.4 ± 0.13, n =154). The variation among the artificial surfaces (STW= TRAV N PVC; linear contrasts, p b 0.05) indicates that surface texture influenced grazer abundance. Among years, the most striking differences were the near absence of limpets at BB in 2008, a site that had high densities in 2001 and 2003 and the large among-year differences at the southern sites (TK, YB and SH). Similar annual variation has been observed previously (Freidenburg et al., 2007). At the southern sites in particular, the limpets involved in these large among-year differences are mostly small individuals (b5 mm), which are likely new recruits, suggesting among-year differences in recruitment can be substantial. The B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 165 Fig. 4. Settlement of cyprids to STW coated PVC plates and to bare rock plots (ROCK) in the presence (+L, open symbols) and absence (− L, closed symbols) of limpets at four sites in 2001 and 2003. Sites were Fogarty Creek (FC), Boiler Bay (BB), Tokatee Klootchman (TK) and Strawberry Hill in 2001, and the same sites except for Yachats Beach (YB) rather than TK in 2003. Differences among sites by surface type (in left panels for each year; SW = safety walk, BR = bare rock) were calculated using linear contrasts. increasing densities observed through the summer (Figs. S1, S2) indicate that limpet recruitment pulses tend to occur from July to September. 3.2. Potential artifacts 3.2.1. Paint barrier vs. paint control As noted in Methods, we tested artifactual effects of paint in 2003 by adding a paint control treatment (partial paint barrier on bare rock or partially painted underside of STW plates). Testing the overall effect of paint (partial barrier vs. unpainted control) revealed no effect on recruitment of Chthamalus (1-way ANOVA; p = 0.27), Balanus (p = 0.10), cyprids (p = 0.54), total recruit density (p = 0.23), and limpets (p = 0.14). Algal cover, however was greater in paint controls (9.6 ± 1.5%) than in unpainted plots (4.1 ± 1.5%; p = 0.001), suggesting a weak effect of the paint on limpet access to the plot during submergence. 3.2.2. Test of z-spar ramp vs. no ramp In 2003 at YB we also tested the effect of the presence or absence of a z-spar ramp. Abundances of Chthamalus, Balanus, cyprids, and cover of algae did not differ in the presence or absence of the ramp (p = 0.30, 0.75, 0.96 and 0.87, respectively). Limpets tended to be more abundant on plates without a z-spar ramp (p = 0.052) but this was opposite of the expected effect. The absence of a z-spar ramp evidently had little effect on our tests of the effects of limpets and substratum on recruitment of barnacles. 3.3. Settlement responses 3.3.1. Overall effects Surface had by far the strongest effect on settlement of cyprids (Table S3, Fig. 4). Settlement was always much higher on STW than on ROCK (Fig. 4; Table S3). The effect of surface varied among sites in both 2001 and 2003 (Table S3). In 2001, settlement was somewhat higher at the southern sites, TK and SH, while in 2003, among-site variation was more idiosyncratic with the site of lowest settlement on ROCK, BB, differing from the southern sites, and the site of next lowest settlement, FC, differing only from the site of highest settlement, SH (Fig. 4). As noted above, in 2008, settlers were not counted in the field. Limpet effects on cyprids were relatively weak. In 2001, limpet effects on cyprids varied through time (Table S3, within-subjects, month × limpet interaction) but overall (Table S3, between-subjects) limpets had no influence either as a main effect or through interactions. In 2003, the limpet effect was stronger, but varied with surface (Table S3, between-subjects, surface × limpet interaction), with positive effects on STW and negative effects on ROCK (Fig. 4). The lack of an effect in 2001 may have been due to the limpets being swamped by the much higher numbers of cyprids and to the less effective exclusion of limpets in 2001 compared to 2003 (Table 3). 3.4. Recruitment responses 3.4.1. Overall effects For both species, recruitment varied among years, sites, surfaces and between limpet treatments (Figs. 3I–Y and 5). Surface had the largest effects on recruitment of both barnacle species which was always higher on STW than on ROCK (Figs. 3I–Y and 5; Tables S1b, c). Further, recruitment was progressively lower on increasingly smooth surfaces, with a rank order of densities for both barnacle species of STW N ROCK N TRAV N PVC (Fig. 5). In addition to surface type, site and limpets also had strong effects, but these varied among years and by species, surface and site (Figs. 3I–Y and 5; Tables S1b, S4–S9). Among years for Balanus, recruitment on STW was higher in 2001 and 2003 than in 2008 except at TK/YB (Fig. 3I–L; linear contrasts, p b 0.05) while on ROCK no interannual differences were observed (Fig. 3M–P, linear contrasts, p N 0.05). For Chthamalus, recruitment on both STW and ROCK was higher in 2001 and 2003 than in 2008, again excepting TK/YB (Fig. 3Q–Y, linear contrasts, p b 0.05). For Balanus, recruitment varied among years by site and the surface effect varied by year and presence of limpets (Table S1B; two-way interactions). Recruitment on STW was higher at the northern sites (FC and BB) than at the southern sites (TK and SH) in 2001, did not differ in 2003 and was reversed in 2008 (Figs. 3 and 5; linear contrasts). Recruitment on ROCK varied idiosyncratically among sites but revealed no regional trend across years (Figs. 3 and 5). Rank orders of Balanus recruitment were similar on the other two surfaces (TRAV and PVC), and recruitment densities were lowest at FC (Fig. 5). Limpet effects on Balanus recruitment averaged across sites varied by year and surface (Fig. 3). On ROCK, negative effects were observed in 2001 and 2003 but limpets had no effect in 2008. On STW, limpets had no effect in any year (Fig. 3). For Chthamalus, average responses across months varied among years, both by site and by surface but limpets had no impact (Fig. 3 and 5, Table S1C). On STW, recruitment was strikingly lower in 2008 166 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Fig. 5. Recruitment of Balanus glandula and Chthamalus dalli on STW, ROCK, TRAV and PVC in + limpet and − limpet treatments at four sites in 2008. Rank order of recruitment density for each surface type across sites is shown in the left panels. than in 2001 and 2003 while on ROCK, recruitment was lower in 2001 and 2008 than in 2003 (Fig. 3, linear contrasts, p b 0.01 or less). By site, recruitment of Chthamalus was generally similar among sites except in 2008 when it was higher at YB than the other sites (Fig. 3, linear contrasts, p = 0.03 or less). In 2008, no differences occurred among sites on TRAV or PVC, which had very low recruitment (Fig. 5, linear contrasts, p N 0.05). 3.4.2. Temporal changes Analyses incorporating temporal variation within year modifies some of the above conclusions, likely due to seasonal progressions in recruitment intensity, limpet abundance and algal cover. Changes through time for both Balanus and Chthamalus were highly context dependent, varying with all factors tested (Figs. 6–8; Tables S4–S9; fourway interactions in 2001 for both species, in 2003 for Chthamalus, all Fig. 6. Recruitment of C. dalli and B. glandula to STW in + limpet and − limpet treatments at four sites by month in 2001 and 2003. Overall differences (between subjects analysis) in effects of limpets through time were analyzed using linear contrasts. ns = not significantly different at a significance level of p = 0.05. B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 three-way interactions in 2003 for Balanus, and two of three three-way interactions for both species in 2008). In 2001, temporal patterns varied by month, site and species as well as surface. Recruitment of both barnacles tended to increase over the summer at the northern sites (FC and BB) on STW but on ROCK recruitment peaks of the two species were inversely related, with Balanus peaking in August and Chthamalus tending to peak in July and September (Figs. 6A, B, E, F and 7A, B, E, F). Temporal trends differed at the southern sites (TK and SH) and again, patterns varied with species and surface (Figs. 6C, D, G. H and 8C, D, G, H). In contrast to weak overall limpet effects reported above, limpet impacts on ROCK tended to be positive for Chthamalus and negative for Balanus, but effects on STW were weak compared to ROCK (Figs. 6A–H and 8A–H). Contrarily, the only significant effect seen on plates was a positive effect of limpets on Balanus, the one instance showing the reverse of the usual negative effect on this species (Fig. 6E). In 2003, similar patterns were observed. Recruitment of the two species varied through time similarly on STW, with peaks later in summer, while patterns on ROCK were less similar (Figs. 6I–P and 7I–P). North–south differences were less pronounced, except for Balanus, which recruited more heavily at southern than at northern sites (Figs. 6M–P and 7M–P). The effects of limpet bulldozing were again contrasting, with negative effects on Balanus and positive effects on Chthamalus on bare rock and similar, but non-significant trends on STW (Figs. 6I–P and 7I–P). In 2008, patterns of recruitment of Balanus were similar across all surfaces, but Chthamalus was more variable in temporal recruitment pattern by surface and site and tended to recruit proportionally more in July (sample periods 1 and 2) than Balanus (Fig. 8). Although Balanus always recruited more densely than Chthamalus in 2001 and 2003 (roughly a ten-fold difference), in 2008 this difference was vastly 167 greater, ranging from a ∼20-fold to ∼900-fold difference (Fig. 8). As indicated by the necessity of plotting the data using different ranges in 2008 to be able to see the results clearly, among-site differences were also greater in 2008 than in 2001 and 2003 (Figs. 6–8). Recruitment was highest at YB and generally higher at the southern sites (YB and SH) than at the northern sites (FC and BB). Strikingly, the site rank of recruitment densities was generally consistent across surfaces, especially for Balanus, with YBN SH N BB= FC. For the northern sites, Balanus recruitment was similar on STW (Fig. 8E, F), higher at FC on ROCK (Fig. 8M, N), and higher at BB on TRAV and PVC (Fig. 8U, V, DD, EE). Overall, recruitment rate was predicted by texture, with rank order STWN ROCKN TRAV N PVC. Finally, limpet effects on recruitment appeared less consistent in 2008, but were generally positive for both species on STW and weak otherwise on the other surfaces. 3.5. Algal response in experiments In all years, algal abundance varied with all factors (Figs. 3, 9, 10, Table S1D, S10; interactions among month, site, surface, and limpets). In 2001, limpet grazing was the stronger effect but all factors but month contributed substantially to algal variation. In 2001 at the northern sites (BB and FC), algal cover was higher on ROCK and lower on STW. Southern sites (SH and TK) showed the opposite trend. In 2003, limpet effects were clearly the strongest influence and among site or region differences were not observed; cover on ROCK did not differ by site and cover on STW was variable in each region. In 2008, each factor had strong effects (Table S10), and regional differences again seemed important with higher algal abundance at the northern sites (Figs. 3 and 10). Fig. 7. Recruitment of C. dalli and B. glandula to ROCK in + limpet and − limpet treatments at four sites by month in 2001 and 2003. See Fig. 6 caption for further details. 168 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Fig. 8. Recruitment of C. dalli and B. glandula to STW, ROCK, TRAV and PVC in + limpet and − limpet treatments at four sites by sample period (1 = early July, 2 = late July, 3 = early August, 4 = late August) in 2008. Note that because of large differences in range of settlement, scales are different by species and site. 3.6. Recruitment correlations Although settlers and recruits were more abundant on STW than on ROCK, across all sites abundances on these two surfaces generally were correlated (Table S11). In 2001/2003, with the exception of Balanus in +limpet plots (p = 0.49), abundances of Chthamalus, Balanus, and settlers (cyprids) on ROCK increased with increasing numbers on adjacent STW (Table S11, p = 0.0004 or less, R2 from 0.16 to 0.68). These relationships were strongest for cyprids (R2 = 0.68 in +limpet plots, 0.51 in −limpet plots) and lowest for Chthamalus (R2 = 0.19 and 0.16) (Table S11). In 2008, similar relationships were observed for Balanus (pb 0.0001, R2 =0.24 in +limpet plots, 0.35 in − limpet plots) but not for Chthamalus (p=0.83, 0.74), likely due to the very low recruitment of this species in this year (Table S11). For Balanus, strong positive correlations were observed between recruitment on all surfaces tested (STW, ROCK, TRAV and PVC), explaining between 17% and 71% of the variance (Table S11). Limpet grazing appears to have affected the strength of these correlations, but differently in 2001/2003 vs. 2008. In 2001/2003, more variance was explained in +limpet than in −limpet plots for Chthamalus and cyprids, while in 2008, in all seven comparisons, less variance was explained in +limpet than in −limpet plots (Table S11). B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 169 Fig. 9. Algal cover in +limpet and −limpet treatments on STW and ROCK at four sites by month in 2001 and 2003. Significance levels for differences between limpet treatments are shown for each panel. In 2003, data are shown for both +L treatments (marked plot and paint control; see code in panel N), so upper significance level in each panel is for the −L to +L (marked plot) comparison and lower significance level is for the −L to +L (PC) comparison. ns= no difference. 3.7. Potential interactions The relationships among the cover of algae, the density of limpets, and the abundance of recruits varied between STW and ROCK (Table S12). Algal cover was inversely related to limpet density on ROCK in all three years, explaining 14%–47% of the variance but on STW this relationship was weak to nonexistent, explaining 0% to 7% of the variance (Table S12). Limpet-recruit regressions varied by year and surface but the two species responded similarly (Table S12). Both Chthamalus and Balanus recruit density was positively related to limpet density on STW and ROCK, but this relationship was relatively strong only on STW in 2003 and 2008 with a weak effect on ROCK in 2003. Recruit-algal regressions also varied by taxon, year and surface (Table S12). Chthamalus abundance increased with algal cover on STW in 2001, but was negatively affected on ROCK in 2003 and 2008 and on STW in 2008. Balanus abundance also increased with algal cover on STW in 2001 and on both surfaces in 2003, but was negatively affected on STW in 2008. Barnacles were neutral to algal cover in other cases (Table S12). 3.8. Daily samples The two bouts of daily sampling of recruitment indicated that inputs of larvae were much higher in early July than in later July, especially during the first three days of this period (Fig. 11, Table 3). They also showed that proportionately more Chthamalus had metamorphosed after 1 day (0.71 ± 0.04) than Balanus (0.20 ± 0.04) (Table 3; life stage × species interaction), suggesting that the former metamorphoses more quickly than the latter. The daily patterns suggest that a settlement episode was underway the first few days of the first period in July 2002, and that low but relatively steady numbers were settling beyond that time, perhaps lasting through the second episode (Fig. 11). 4. Discussion Our results suggest that, as expected, recruitment densities were higher on STW than on ROCK. Densities on STW collectors were always higher and often much higher, ranging by year, site and taxon from 2.2 to 87 times higher, than on bare rock surfaces. As suggested by the 2008 experiments, which incorporated two additional types of artificial collector with successively smoother surfaces, this difference seems most likely due to differences in texture. In contrast, limpet effects on settlement and recruitment, although statistically significant, were much weaker. Our interpretation of these results is that settlement surface was the most important factor in determining settlement and recruitment density in our study. We suggest that artificial collectors are best viewed as providing estimates of relative recruitment or settlement as opposed to estimates of the actual number of new individuals entering populations at rocky intertidal sites. Below, we examine our results in some detail and further discuss 170 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 Fig. 10. Algal cover in + limpet and − limpet treatments on STW, ROCK, TRAV and PVC at four sites by month in 2008. Average rank of algal cover by site is shown in the third column of panels. Significance levels for differences between limpet treatments are as in caption to Fig. 9. their likely significance for studies of recruitment dynamics of rocky intertidal communities. 4.1. Effects of surface type In using tools such as artificial settling surfaces in ecological research, it is important to know what the surfaces are actually quantifying. As predicted by hypothesis H3 (see Introduction), this experiment revealed that settlement and recruitment were both substantially greater on STW surfaces than on bare ROCK surfaces. For example, average abundances of Chthamalus and Balanus on STW ranged from ∼ 2.2 to 87 times and from ∼ 1.45 to 86 times than on ROCK, respectively (Table 2). These differences were sensitive to interannual variation in recruitment. In 2008, recruitment of both taxa on STW was lower than in earlier years (overall, STW, Balanus: 2.3 to 4.2-fold difference, Chthamalus: 22.3- to 29.3-fold difference). Between-year differences were much less on ROCK and in fact, Balanus recruitment on ROCK was actually 2–3 times higher in 2008 than in 2001 and 2003. Settlement (cyprids) differed between STW and ROCK even more, ranging up to 1179 times greater on STW than Fig. 11. Daily settlement (cyprids) and recruitment (metamorphs) over two blocks of time for Balanus and Chthamalus at Fogarty Creek. B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 171 Table 2 Average (+ 1SE) abundances of recruits, settlers, limpets and percent algal cover on STW plates vs. ROCK plots in + limpet and − limpet treatments, averaged across the four sites in 2001, 2003, and 2008. 2003 data combined limpet (+ limpets − paint) and paint controls (+ limpets + paint) abundances. Coefficients of variation (CV) are based on ln-transformed data. STW N ROCK indicates the magnitude of the difference between the density of the taxon on STW plates vs. ROCK plots. − limpet + limpet Year Taxon Mean abundance STW Mean abundance ROCK CV STW N ROCK Mean abundance STW Mean abundance ROCK CV STW N ROCK 2001 C. dalli 7.7 ± 1.7 (60) 32.3 ± 9.0 (60) 7.2 ± 1.1 (60) 20.3 ± 5.3 (55) 15.96 ± 1.61 (160) 15.26 ± 2.14 (160) 11.4 ± 0.88 (159) 0.94 ± 0.63 (159) 2.7 ± 0.7 SW = 42.7 RO = 105.2 SW = 22.0 RO = 85.6 86.1 4.8 ± 1.4 (60) 88.0 ± 11.3 (60) 5.7 ± 1.1 (60) 28.5 ± 6.0 (55) 9.2 ± 1.8 (80) 143.8 ± 24.7 (80) 1.7 ± 0.5 (80) 74.9 ± 3.5 (80) 3.3 ± 0.8 SW = 31.8 RO = 154.2 SW = 20.3 RO = 41.5 86.1 C. dalli 662.7 ± 150.3 (59) 1120.2 ± 211.6 (59) 6.2 ± 1.18 (60) 6.88 ± 2.4 (59) 419.1 ± 42.2 (152) 1323 ± 134.4 (152) 14.72 ± 2.21 (152) 12.9 ± 1.9 (158) 18.1 ± 4.8 B. glandula (80) 634.0 ± 127.1 (80) 119.5 ± 43.2 (80) 2.6 ± 0.5 (80) 37.4 ± 4.5 (80) (80) 13.5 ± 2.1 (80) 4.5 ± 2.2 (75) B. glandula Limpets Algal cover 2003 C. dalli B. glandula Limpets Algal cover 2008 Limpets Algal cover SW = 76.2 RO=115.3 6.7 413.1 ± 66.4 (57) 3542.5 ± 1843.3 (57) 0.3 ± 0.1 (60) 19.6 ± 4.7 (57) 401.8 ± 60.0 (78) 1675.2 ± 220.6 (78) 0.4 ± 0.1 (78) 32.0 ± 4.1 (80) 4.8 ± 1.2 SW = 49.5 RO = 53.2 5.3 (80) 500.7 ± 104.6 (80) 227.0 ± 55.3 (80) 0.35 ± 0.14 (80) 59.2 ± 4.9 (80) (80) 0.6 ± 0.2 (80) 43.4 ± 5.0 (74) 34.7 0.86 0.34 SW = 23.1 RO = 66.1 SW = 20.0 RO = 78.2 26.3 86.7 1.3 13.7 on ROCK (Table 2). H1 also accurately predicted that variability of settlement and recruitment was greater on ROCK than on STW (Table 2). With the single exception of Balanus in − limpet plots in 2008, in every STW-ROCK comparison coefficients of variation (CV) were higher on ROCK than on plates, with the highest values for cyprids (Table 2). The basis for this difference is most likely the difference in texture. STW®, a plastic rubbery substance designed as an anti-skid surface for boat decks, swimming pools and other slippery areas, has a uniformly and highly textured surface with pits ranging in size up to 0.3 mm (e.g., Fig. 2). ROCK surfaces at our sites, in contrast, are far less textured varying from smooth to moderately rough and heterogeneous. Consistent with this interpretation, in the 2008 experiment, recruitment on the additional artificial surfaces, TRAV (with low surface texture; Fig. 2) and PVC (negligible texture; Fig. 2) was successively lower than on ROCK. Cyprids prefer pitted or textured surfaces over smoother surfaces (Barnes, 1956; Crisp, 1961; Connell, 1985; Sanford and Menge, 2001; Leslie, 2005), and range in average length from ∼ 0.087 to 0.107 mm (Menge, B. A. unpubl. data). The difference in settlement and recruitment can be easily understood in terms of not only rugosity but the size and density of the pits on STW vs. ROCK, TRAV and PVC. If settlement and recruitment densities are so different between rock surfaces and the artificial collectors, why use collectors at all? Or why not use collectors that do a better job of reflecting actual rock surface texture? As noted earlier, previous workers have used a variety of surfaces, from natural rock to various types of artificial surfaces (see Introduction). When we initiated our long-term studies of recruitment in the late 1980s and early 1990s, we conducted a number of preliminary investigations to evaluate the different options available. Our primary research goal was to gain insight into relative differences in recruitment rates in space and time. In this context, we rejected sampling on natural rock surfaces because field observations indicated that along the Oregon coast, surface texture and composition could vary dramatically at all conceivable spatial scales, from mm 0.19 8.3 40.3 0.05 0.69 SW = 25.7 RO = 108.2 SW=20.3 RO = 55.7 43.7 11.5 0.24 0.43 SW = 114.6 RO = 128.7 1.45 SW = 60.3 RO = 54.0 2.21 0.58 1.36 to thousands of km. As has been repeatedly demonstrated, surface texture is a major cue for cyprid settlement (e.g., Barnes, 1956; Crisp, 1961 and many others), and thus one goal was to reduce among-plot variance in recruitment invariably introduced by the naturally heterogeneous surfaces of intertidal rocks (e.g., Fig. 2). Another characteristic of intertidal rocks is variation in composition and hardness (e.g., rocks can consist of mudstone, sandstone, basaltic, granite, etc.), both of which qualitatively have been observed to be related to variation in density of recruits and adults of barnacles (Menge, B. A., pers. obs.). For example, barnacle recruitment is extremely low on sandstone and mudstone, and as a probable consequence adult densities are also low, even when densities are high on adjacent harder surfaces. Sandstone and mudstone erode at a high rate (Menge, B. A., pers. obs.), so barnacles settling on these surfaces face a very short lifespan (days to weeks). The low adult densities on such surfaces is not due to a lack of supply; STW plates attached to these substrata accumulate recruits at rates comparable to those attached to harder surfaces (Menge, B. A., pers. obs.). These considerations thus led to the conclusion that artificial collectors that controlled for texture, hardness and composition were the best choice for our purposes. We have observed that under conditions of exceptionally dense settlement, the smooth surfaces usually shunned by cyprids can be settled on after the rugosities in the substratum, whether natural or artificial, are filled (authors' pers. obs.). These massive settlement events on smooth surfaces rarely result in similarly high densities of juvenile barnacles, however, suggesting high mortality and thus that such settlement is a poor choice and an act of “desperation” on the part of the cyprids. Under more typical conditions, the vast majority of settlement and recruitment occurs on rugose substratum. This raises yet another issue; are STW plates “magnets” for cyprids? That is, do the plates deplete the larval supply sufficiently that settlement densities on nearby rock are lower than those that would occur in the absence of the plate? We have no real way to address this issue without further research. However, given the 172 B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 turbulence of intertidal habitats and our observation that cyprids will settle on rock within cm of the collector plates, it seems unlikely that this possible behavior is the primary cause of the higher densities on STW plates and lower densities on rock. Shanks (2009) recently criticized the use of STW plates, citing results of his research at a site on the southern Oregon that suggested that STW plates heated rapidly in the sun, killing settled cyprids within hours. In his studies, he found much higher recruitment on ceramic floor tiles than on STW plates. His studies were carried out in 2007 from late May to the end of July. Our results are inconsistent with Shanks' (2009) results. For example, we find much lower recruitment on artificial tiles (both TRAV and PVC) than on STW. One difference that could account for our different results is that Shanks' studies were carried out in May through July while ours were carried out from June/July through August/September. The significance of this difference is that on the Oregon coast, timing of low tides gradually falls earlier in the day in each successive tide series through the summer. Thus, exposure of settlers and recruits to warmer mid-day temperatures gradually lessens as the summer progresses, presumably meaning lower exposure to thermal stress. This could explain why numbers of recruits on STW earlier in summer were generally lower in our experiments than they were later in summer (e.g., Figs. 6–8). However, numbers of recruits on other surfaces (TRAV, PVC and ROCK) were usually also lower earlier in summer (Figs. 6–8). If mortality explains the lower numbers on STW, and STW (and PVC) plates get hotter than ROCK or TRAV, then it seems likely that consistently higher numbers would be seen on the latter than on the former surfaces. Further, we have also measured plate vs. rock temperatures (B. Menge et al., unpublished data), and like Shanks found that STW plates often reached temperatures in the 40 °C range. It is possible that the differences between Shanks' (2009) results and ours may be influenced by other differences in spatial or temporal coverage. Shanks' study was done in one summer at one site while ours spanned three separate years at four sites per summer. Further, examination of our long-term data set spanning six sites along the central and southern Oregon coast (including Cape Arago, which is a few km south of Shanks' Bastendorff Beach site) and 20 years (1989– 2008) suggests other time/space differences that could be important (B. Menge et al., unpublished data). Average recruitment of Balanus was relatively high from March through November, with some sites having peaks of recruitment in August or September and others having relatively uniform recruitment through this 9-month period. Importantly, variance among successive months could be very large, especially at Cape Arago. Thus, it is possible that a repeat of Shanks' (2009) study would have a different outcome. Without further investigation, it is impossible to determine the causes for the different results obtained by Shanks (2009) and the present paper. What other alternatives to STW might we consider? Our sense is that the best form of standard collector would be one that mimicked Table 3 Three way analysis of variance of variation in numbers of settlers by period (early vs. late July 2002), life history stage (cyprid vs. metamorph) and species (Chthamalus vs. Balanus). Analysis was on ln-transformed data. R2 = 0.48. Boldface values are significant at p = 0.05. Analysis of variance Source SS df MS F p Period Life stage Species Period ⁎ life stage Period ⁎ species Life stage ⁎ species Period ⁎ life stage ⁎ species Error 40.70672 5.51549 5.91069 3.94577 1.08498 58.15874 5.70853 138.29542 1 1 1 1 1 1 1 100 40.70672 5.51549 5.91069 3.94577 1.08498 58.15874 5.70853 1.38295 29.43461 3.98819 4.27396 2.85314 0.78453 42.05399 4.12778 b 0.0001 0.049 0.04 0.09 0.38 b 0.0001 0.045 natural rock surface texture of moderate rugosity. After our initial trials, we explored the possibility of having unglazed ceramic tiles with appropriate surface texture manufactured for us, but high cost made this option unfeasible. We felt such tiles should be used only once since the porous nature of the tile could lead to accumulation of biofilm residues that might not be removable and might influence settlement in later uses (see Bourget, 1988; Keough and Raimondi, 1995; Thompson et al., 1998). (In contrast, STW plates are reusable; the STW can be removed and a new layer applied to the non-porous PVC plastic plate.) Similar restrictions applied to other surfaces considered such as travertine, slate, granite, and other types of natural rock. Nonetheless, given the extensive use of STW plates around the globe, the apparent success (contra Shanks, 2009) of these collectors in providing relative measures of recruitment in space and time, and the lengthy time series that have been accumulated in PISCO and other programs (Broitman et al., 2008; Navarrete et al., 2008), we are hesitant to recommend switching to new surfaces at this point. If the goals are to estimate absolute rates of input of recruits or the study is short-term, more local-scale, and not aimed at making comparisons with other investigations, however, then using natural rock surfaces or molds of natural rock surfaces would be a good choice. 4.2. Effects of limpets The results of the limpet manipulations suggest that their effects are more complicated than the simple, uniformly negative effects predicted by H4 (see Table 2). Overall, limpet activity had strong effects on settlement and recruitment of Balanus (2-way ANOVA with limpets and surface as factors, p = 0.008) but no effects on Chthamalus (2-way ANOVA, p = 0.25). Further, the effect on Balanus varied among years and surface type. Balanus recruitment densities were lower in +limpet than in −limpet plots in 2001 and 2003 (suggesting a negative effect of limpets), but in 2008 recruit density differences suggested a positive effect on STW and a negative effect on ROCK (Tables 2, S1B; surface × limpet interaction). Our results suggest that limpet effects were contingent on density of settlers or recruits. For example, on ROCK limpets negatively affected settlement of cyprids strongly in 2003 (p = 0.005) and weakly in 2001 (p = 0.049; linear contrasts in 3-way ANOVA), possibly due to swamping by much higher cyprid densities in 2001 (Table 2). On STW, limpets had no effect on cyprids in 2001, again, possibly due to swamping, but in 2003 had a positive effect (Table 3). As predicted by H5, this positive effect may be a consequence of the influence of rugosity on limpet crawling and grazing. Limpets, which range in length from about 2 to 15 mm, with most in the 5– 10 mm range, are too large to fit in the pits and thus skim over the pits on STW as they crawl. While the radula (tongue-like feeding apparatus) of a 5 to 10 mm limpet is likely to be ∼0.12 to 0.25 mm in width (Kitting, 1980; assuming proportional scaling using a limpet 24.2 mm with a radula 0.6 mm wide), and thus could fit into the larger pits, it seems unlikely to be able to affect most cyprids that settle into pits. In contrast, due to growth and change in morphology, metamorphs extend further out of the pits in STW and may be more vulnerable to bulldozing than are cyprids, at least during the first several days after metamorphosis. A direct effect of limpets on microalgae may also influence survival of cyprids by removing a potential barrier to settlement. Finally, a weaker effect of bulldozing on cyprids than on metamorphs may also reflect a shorter period of vulnerability of cyprids (≤24 h) than metamorphs (several days) coupled with spatial and temporal variability in limpet grazing. Thus, the positive effect of limpets on STW may result from the removal of microalgae from the ridges of the pits and a reduction in the ability of the limpets to bulldoze cyprids in pits. Conversely, the negative effect on ROCK seems likely due to the greater vulnerability of cyprids because of the much smoother texture of the rock surface and thus relative lack of settlement micro-refuges. Further investigation of the B.A. Menge et al. / Journal of Experimental Marine Biology and Ecology 392 (2010) 160–175 interaction between grazing activity, substratum and life history stage of barnacles seems warranted. Effects of limpets similar to those on cyprids were also observed with recruits, but counter to H6, which predicts similar effects of substratum and bulldozing on both species, the taxonomic resolution reveals yet another difference in how species responded to limpet bulldozing. With the exception of STW in 2008, when grazing had a positive effect (Fig. 3, 3-way ANOVA on site, surface, and limpets, surface× limpets interaction, p b 0.0001), limpets had either no or weakly positive effects on Chthamalus and negative effects on Balanus (Figs. 3 and 5; Tables S4–S9). These effects varied in space (among sites) and time, likely as a consequence of spatio-temporal variation in recruitment density, abundance of limpets and algal cover (Figs. 3, 6–10). The mechanism underlying the relative resistance of Chthamalus to bulldozing compared to Balanus is unclear. Chthamalus recruits are similar in size to those of Balanus (0.932 ± 0.01 vs. 0.95 ± 0.05 mm, means of means of 9 measurements per species across 8 sample dates in 2002), suggesting resistance is not based on size. However Chthamalus do appear to metamorphose faster than Balanus. Two sets of daily samples taken in July 2002 indicate that after one day, the majority of Chthamalus were metamorphs while the majority of Balanus were still cyprids (Fig. 11). Thus a possible mechanism underlying resistance is that Chthamalus can quickly metamorphose and thus attach more firmly faster than can Balanus. We conclude that the effects of limpet bulldozing on settlement and recruitment are context-dependent, varying with the density of settlers and recruits (as indicated by the differences between 2001 and 2003 for cyprids and 2001, 2003 and 2008 for metamorphs), surface rugosity, and differences in the abilities of settlers or recruits to resist limpet disturbance. Hence, the prediction of H5, that barnacle recruitment will be similarly affected by surface and limpet grazing is not supported. Bulldozing reduced the density of Balanus but had little effect on the density of Chthamalus (excepting a positive effect in 2008 on STW), most likely due to more rapid metamorphosis and thus firmer attachment of the latter. 4.3. Relationship between densities on different surfaces Hypothesis H1 predicts that even though magnitudes of settlement to different surfaces may differ, settlement and recruitment on STW will be correlated to that on ROCK. Our analyses support this prediction. For both settlers (cyprids) and recruits (abundances of Chthamalus and Balanus), numbers on STW were usually positively related to numbers on ROCK, the exceptions being Balanus in + limpet plots in 2001/2003 and Chthamalus in 2008 (Table S11). These positive relationships explained between 16% and 68% of the variance. In 2008, correlations among the four different surfaces were also relatively strong (explaining 17% to 71% of the variance; Table S11). These relationships seem to be relatively insensitive to counting method or location. In 2001 and 2003, counts on ROCK were made in five fields of view of known area using field microscopes and then adjusting numbers to 100 cm2. In 2008, we used hand lenses to count recruits in the whole area of ROCK, TRAV and PVC plates, and counted recruits on STW in the lab using both a hand lens and a dissecting microscope (the method also used in 2001/2003). Despite this variation in method, the amount of variance explained between 2001/2003 and 2008 on STW vs. ROCK tended to be similar for Balanus (again excepting the 2001/2003 +limpet plots)(Table S11). Further, STW counts using hand lenses and lab microscopes were strongly correlated (62% to 86% of the variance explained for Balanus; estimates for Chthamalus were compromised by very low recruitment of this species in 2008). On the other hand, hand lenses did not provide the resolution needed to detect and identify cyprids, which is why these were not counted in 2008. 173 4.4. Spatial variation Most analyses revealed strong differences among sites in recruitment, limpet densities, and algal cover (e.g., Fig. 3). The most consistent spatial trend, however, was the regional difference between the two northern sites on Cape Foulweather and the three southern sites on Cape Perpetua (see also Menge et al., 1997, 2004; Leslie et al., 2005; Barth et al., 2007; Freidenburg et al., 2007). Patterns at FC and BB were consistent for barnacle recruitment, algal abundance and limpet density. Similarly, patterns at SH tended to be the same as those at TK and YB despite the switch from TK to YB between 2001 and 2003, and, overall, different from FC and BB, at least in the three years covered by this study. For barnacles, however, longer-term data suggest that the regional differences observed during these experiments is not consistent. Examination of longer term (1989–2008) data indicated that in some years one cape had higher recruitment, in others, the other cape had higher recruitment and in still other years, the capes did not differ (e.g., Leslie et al., 2005; Barth et al., 2007; Broitman et al., 2008). 5. Conclusions We conclude that surface texture has the greatest effect on settlement and recruitment of these two barnacles, an observation that is consistent with classic studies in the United Kingdom and elsewhere (e.g., Barnes, 1956; Crisp, 1961; Chabot and Bourget, 1988). In addition, limpet grazing activity can influence settlement and recruitment, but this effect is variable through time, with the density of settlement and recruitment, with the specific barnacle taxa involved, and with surface rugosity. Important variation can also occur in space, both at the local site scale, and at the regional (cape) scale. A primary goal in this study was to evaluate the extent to which settlement on artificial surfaces provided relatively comparable estimates across space and time that were independent of variation in surface texture, and reflected actual recruitment rates to rock surfaces in the intertidal. Our conclusions are that recruitment on STW plates does provide a method for allowing comparisons of potential recruitment across multiple spatial and temporal scales, but that, not surprisingly, recruitment densities on STW substantially overestimate actual recruitment to rock. The tradeoff between two of the goals in a recruitment sampling program remains: STW plates provide the ability to sample many sites across large spatial scales over long time periods but overestimate recruitment densities, while bare rock counts provide more direct estimates of actual recruit densities but, due to much longer “handling times” in the field, are more limiting in spatio-temporal coverage. It is possible that with a correction factor, recruitment on STW potentially could provide an approximation of actual recruitment to bare rock surfaces and thereby allow for both precision in estimates and comparisons at multiple spatial and temporal scales. However, the fractions of recruits on ROCK compared to STW varied by species and year (e.g., for 2001, 2003, and 2008, respectively: Chthamalus, 1.1, 2.9, and 11.0% and Balanus, 2.3, 5.4, and 26.8%), suggesting the need to maintain parallel sampling schemes of both types of surface, at least at a few locations. Acknowledgements We thank the legions of undergraduate interns for their assistance in fabricating collectors, and deploying, replacing, and processing the many samples analyzed here. A. Iles, J. Tyburczy, S. Dudas, F. Chan, and G. Rilov and anonymous reviewers provided constructive criticism on the manuscript. The research was funded by an endowment from the Wayne and Gladys Valley Foundation, and grants from the A. W. Mellon Foundation, the David and Lucile Packard Foundation, and the Gordon and Betty Moore Foundation. 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