Predation on Caecilians (Caecilia orientalis)

Herpetological Review, 2008, 39(2), 162–164.
© 2008 by Society for the Study of Amphibians and Reptiles
Predation on Caecilians (Caecilia orientalis) by
Barred Hawks (Leucopternis princeps) Depends
on Rainfall
HAROLD F. GREENEY
and
RUDY A. GELIS
Yanayacu Biological Station, Cosanga, Napo, Ecuador
and
W. CHRIS FUNK
Department of Biology, Colorado State University
Fort Collin, Colorado 80523-1878, USA
e-mail: [email protected]
Caecilians are limbless, subterranean or aquatic amphibians
found throughout much of the tropics (Duellman and Trueb 1994;
Himstedt 1996). Although amphibians are declining dramatically
(Stuart et al. 2004), the conservation status of caecilians is largely
unknown due to lack of information on their ecology and natural
history (Gower and Wilkinson 2005). A handful of studies have
documented caecilian life histories (e.g., Funk et al. 2004; Gans
1961; Kupfer et al. 2005; Malonza and Measey 2005; Parker 1936,
1958; Sanderson 1937; Sarasin and Sarasin 1887–1890; Taylor
1968; Wake 1980). Nevertheless, 114 out of 172 species (66%) of
caecilians remain too poorly known for an accurate status assess-
162
ment, and thus are listed as “Data Deficient” by the IUCN (2006).
Perhaps because of their elusive nature, there is an increasing interest in the biology of caecilians (Kupfer et al. 2006; Measey and
Herrel 2006).
Predator-prey interactions are widely recognized to have important effects on population dynamics (e.g., Krebs et al. 1995;
Lotka 1925; Volterra 1926), but in the case of caecilians, little is
even known about which taxa act as predators. Snakes are considered the main predators of caecilians (Duellman and Trueb 1994;
Kupfer et al. 2003), although some other predators such as turtles
(Zamprogno and Zamprogno 1998), spiders (Boistel and Pauwels
2002), and ants (Measey 2004) have been documented preying on
caecilians. Identifying predators of amphibians is important in the
context of amphibian declines, because predation may tip already
declining populations over the edge toward extinction (Corn 1993;
Parker et al. 2000). Here we show that a tropical hawk acts as an
important predator of the caecilian Caecilia orientalis and that
this unexpected ecological interaction depends strongly on weather.
Methods.—We filmed a Barred Hawk (Leucopternis princeps)
nest using a hidden camera from 15 February–8 May 2004 and 7–
28 January 2005 for a total of 599 h in the private reserve of
Cabañas San Isidro, next to Yanayacu Biological Station (00°35'S,
77°53'W; 1950 m elev.). During most days of filming, the nest
was filmed continuously during daylight hours (from morning to
evening) when the hawks were active. The Barred Hawk is a rare,
large hawk (total length = 52–61 cm) found from northern Peru to
Costa Rica (700–2200 m; Ridgely and Greenfield 2001). The 1700
ha reserve comprises a mosaic of primary and secondary growth
in humid, montane, evergreen forest about 3 km W of the town of
Cosanga in the Napo Province of northeastern Ecuador (for a more
complete site description, see Greeney et al. 2006).
Each year, the same Barred Hawk pair raised a single chick in
the same nest. The nest was located on a rocky ledge 5 m from a
rushing waterfall. The blind was installed 3.5 m above and 10 m
from the nest, on the opposite side of a stream. All videos were
transcribed at a later date. In addition to recording prey brought to
the nest, we recorded whether it rained during each hour-long time
interval. Since video quality was excellent, most taxa were clearly
identifiable, but seven unknown taxa were excluded from the analysis. Because caecilians surface primarily during heavy rains and
snakes are active at Yanayacu when it is clear and sunny, we hypothesized that Barred Hawks would bring more caecilians to the
nest, but fewer snakes, when it was raining. We tested this in 2004
using a Fisher’s exact test. This was the second Barred Hawk nest
ever documented (Muela and Valdez 2003) and the first closely
monitored to document feeding behavior. Details of the breeding
ecology of these Barred Hawks are being prepared separately for
an ornithological journal (R. A. Gelis and H. F. Greeney, unpubl.
ms.).
Results.—To our surprise, a caecilian species (Caecilia
orientalis) was the main prey item brought to the nest by two Barred
Hawk parents to feed a single chick brooded each year (Fig. 1;
videos available upon request). Caecilia orientalis is a large caecilian (total length = 31–62.5 cm) found in the Andes of Ecuador
and Colombia and is the only caecilian known from this site (Funk
et al. 2004; IUCN 2006). Prey items delivered to nestlings included
50 caecilians (48.1% of diet), 36 snakes (34.6%; Atractus
occipitoalbus and two unidentified colubrid species), five giant
Herpetological Review 39(2), 2008
FIG. 1. (A) Barred Hawk (Leucopternis princeps) with caecilian
(Caecilia orientalis) in talons (left). The white chick is seen in background
(upper right). A color version of this photo is available upon request. (B)
Proportion of Barred Hawk diet in 2004 and 2005 composed of different
taxa. (C) Barred Hawks bring significantly more caecilians than snakes
to the nest when it is raining (P < 0.00001, N = 58).
earthworms (4.8%), three young birds (2.9%), three small mammals (2.9%), and seven unknown animals (6.7%) (Fig. 1B).
Caecilia orientalis vouchers from this site are available in the
Museo de Zoología at the Pontificia Universidad Católica del Ecuador (QCAZ 21417–21419).
As predicted, we found that Barred Hawks brought significantly
more caecilians than snakes to the nest during hour-long time intervals when it was raining (Fisher’s exact test, P < 0.00001, N =
58; Fig. 1C).
Discussion.—It is surprising that an aerial predator, the Barred
Hawk, was able to find subterranean prey such as caecilians. A
previous report states that birds may occasionally prey on caecilians (Wake 1983), but this report did not provide specific bird
species names or details of this predator-prey interaction. During
several years of research at Yanayacu, caecilians were rarely encountered on the surface in the day even when it was raining (Funk
et al. 2004). Thus it is unknown how Barred Hawks are able to
consistently find these elusive amphibians. These results suggest
that Caecilia orientalis may actually be fairly common, as has
been found for some other caecilian species (Measey 2004), yet
C. orientalis is reported as “uncommon in Ecuador” by the Global
Amphibian Assessment (IUCN 2006). Because of the rarity and
huge ranges of Barred Hawks, we were only able to find and intensively monitor one pair over two years. It will likely take several years of intensive searching to find additional Barred Hawk
nests. Nonetheless, the dominance of caecilians in this pair’s diet
and consistent use of these amphibians over two years suggests
that caecilian predation by Barred Hawks will likely be widespread
at sites with abundant caecilian populations.
Rainfall has increased at some sites in Ecuador over the last 40
years (Haylock et al. 2006), although rainfall trends at Yanayacu
are unknown. If predation on caecilians is typical for Barred Hawks,
changes in rainfall could alter this predator-prey interaction and
potentially impact populations of both species. Climate change
has already been implicated in amphibian declines in Ecuador and
elsewhere in the Neotropics (Blaustein and Dobson 2006; Pounds
et al. 2006). Predicting the ecological impacts of climate change,
however, will require a better understanding of trophic interactions and the influence of weather on these interactions as documented here.
Determining the effect of predators on caecilian populations will
also require a much better understanding of caecilian population
dynamics. Studying the population ecology of these fossorial amphibians has proven difficult in the past due to low detectability, a
paucity of methods for individually marking caecilians, and the
rareness of some caecilians species. However, at Yanayacu Biological Station and some other sites (Bustamante 2005; Measey
2004; Péfaur et al. 1987), caecilians can be abundant and thus
potentially amenable to study. New methods have also recently
been developed for marking caecilians for capture-recapture estimation of vital rates and demographic parameters (Gower et al.
2006; Measey et al. 2001, 2003). Use of these methods in combination with population modeling (Biek et al. 2002) and molecular
genetic markers (Beebee 2005; Funk et al. 2005) should help illuminate the demography, ecology, and conservation status of these
fascinating animals.
Acknowledgments.—We thank Carmen Bustamante for permission to
conduct research on the San Isidro Reserve. We also thank J. Beatty, L.
Coloma, D. Gower, J. Matthews, J. Measey, and A. Sheldon for comments on the manuscript. Funding was provided by Matt Kaplan, John V.
and Ruth Ann Moore, a Pamela and Alexander F. Skutch Award, and a
Declining Amphibian Populations Task Force Seed Grant. This study was
conducted in compliance with Ecuadorian laws. This is publication number 59 of the Yanayacu Natural History Research Group.
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