Abstract
AbstractIn seasonal wet Neotropical forests, many studies have suggested that species-rich terrestrial frog assemblages are regulated bottom-up by the abundance of leaf litter. However, terrestrial frogs are prey to a diverse community of predators, and no studies have tested for top-down effects of predators on this or other anuran assemblages. Here, we used an extensive field dataset to model the relative contribution of food resources, microhabitat resources and predators towards the occupancy and detection of two frog species (Craugastor bransfordiiandOophaga pumilio) at La Selva, Costa Rica. Frog occupancy was most strongly influenced by predatory spiders and secondarily influenced by the abundance of leaf litter. Predators exerted stronger effects on frogs than food resources, and frogs avoided predators more as leaf litter decreased. Detection probability was elevated when predators were present. We found support for bottom-up effects of leaf litter on the terrestrial frog assemblage, but top-down effects by predators exerted stronger effects on frog occupancy and detection. Because predator avoidance varied along a resource gradient, predator and resource effects appear to be dependent, supporting interactions between top-down and bottom-up mechanisms. Climate-driven decreases in leaf litter may drive decreased availability of frog refugia and increased interactions between frogs and predators.
Highlights
A fundamental goal of ecology is to understand mechanisms that regulate the dynamics of populations and communities
Arthropod abundance and seasonality received zero support as effects on frog occupancy and detection (Table 4 and 5). Site occupancy of both C. bransfordii and O. pumilio in plot cells was more strongly predicted by the presence of spiders than other resources
Both frogs exhibited decreased occupancy when spiders were present, suggesting that frogs selected habitat to reduce co-occurrence with spider predators. Occupancy of both frogs and spiders was strongly influenced by leaf-litter abundance at sites, and we observed an effect in which predator avoidance by frogs decreased as litter abundance increased
Summary
A fundamental goal of ecology is to understand mechanisms that regulate the dynamics of populations and communities. In wet Neotropical forest ecosystems, a diverse assemblage of small terrestrial frogs occupies the leaf-litter layer on the forest floor, and a large body of literature has suggested that the ‘leaf-litter frog assemblage’ is regulated bottom-up (sensu Hunter & Price, 1992) by the abundance of leaf litter. Several studies have documented a positive relationship between standing leaf litter and abundance of terrestrial frogs and anoles in mainland ecosystems (Scott 1976, Lieberman 1986, Guyer 1988, Fauth et al 1989, Heinen 1992, Whitfield et al 2007), and Whitfield et al (2014) provided the first experimental demonstration of standing leaf litter as a density-limiting factor. A common thread among studies is the identification of leaf litter as a fundamental resource regulating the abundance of the leaf-litter frog assemblage, in ways consistent with bottom-up regulation (sensu Hunter and Price 1992)
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