Investigating the relative strengths of top-down and bottom-up forces on species’ distributions and abundances is a major theme in community ecology. While bottom-up effects on higher trophic levels are well documented, the means by which top-down forces are transmitted is less clear. In particular, recent studies have proposed that prey species’ behavior and distribution can be strongly influenced by ‘landscapes of fear’ and attraction to ‘enemy free space’.
Although most studies on these topics have worked with large mammals in large landscapes, here we explore the applicability of such top-down forces to forest floor communities in the Northeast US. Our focal species, the red-backed salamander (Plethodon cinereus), is the most abundant vertebrate in these ecosystems and is thought to differ in abundance among forest types, suggestive of bottom-up effects. However, as a key link in the forest floor food web, P. cinereus is also frequently preyed on by small mammals, like shrews, raising the possibility of significant top-down effects on its distribution and behavior. We used artificial cover objects (ACOs) arrayed between two forest types and indicators of small mammals (tunnels) under these ACOs to compare the relative strengths of bottom-up and top-down forces on the distribution of P. cinereus.
Results/Conclusions
We found significant evidence of bottom-up effects on the distribution of P. cinereus: on average, salamanders were 2.15 times more likely to be found under ACOs in Eastern Hemlock (Tsuga canadensis) dominated areas than in adjacent deciduous forest patches. However, evidence of top-down effects was also apparent: P. cinereus was 1.9 to 3.8 times more likely to be found under ACOs that were free of small mammal tunnels compared to ACOs with small mammal tunnels, a trend that was significant for several survey dates. These results indicate that the distribution of P. cinereus is strongly influenced by a combination of bottom-up and top-down forces, with localized top-down effects sometimes exceeding those of broader forest habitat context. An unexpected positive association was also detected between P. cinereus and its intra-guild competitor, Carabid ground beetles, suggesting that both species were converging in similar enemy free spaces, away from small mammals. This pattern suggests that the risk of predation might outweigh the lesser costs of spatial association with a competitor in this forest floor “landscape of fear”.