Edge effects on avian nest predation: the quest for a conceptual framework

Looks can be deceiving. The large and expanding body of literature on avian nest predation may suggest that mechanisms underlying variation in predation rates become gradually well understood. Yet, closer examination of the results yields a much less optimistic view, especially with respect to predation rates on natural – rather than artificial – nests of tropical forest species. Data on natural nest predation in tropical ecosystems indeed remain particularly scant, mainly because natural nests are often very difficult to detect and to monitor in complex vegetations, turning robust statistical analysis into a real challenge. While adequate sample sizes are available in a number of cases, little to virtually nothing is known about the identity of the other main actor of the biotic interaction, the predator. Since the latter is key to understanding how and why avian nest predation rates may vary in space and time, there is a growing consensus that future studies should at least reveal the predators’ identity, and if possible, also shed light on their feeding behaviour and population dynamics. Second generation predation studies should hence involve a small army of field assistants to assure adequate sample sizes, and sophisticated camera traps installed at each nest to monitor predator behaviour. But at what cost can this be done? Worldwide, only a handful of such studies would be feasible, more so in larger and more accessible areas and dealing with relatively easyto-study species that build conspicuous nests. Such studies may, indeed, turn into detective stories that describe ‘all details of the crime scene’ (Lahti, 2009). However, remote regions that harbour low densities of shy forest birds would most probably still drop out. Alternatively, networks of small-scale projects – such as the Taita forest project in south-east Kenya – may facilitate meta-analysis of largerscale patterns in nest predation, provided that raw data are made available to fellow researchers. In support of this approach, we here supplement the underlying raw data of our study (Spanhove et al., 2009a) in Appendix S1 and S2. As is the case with any ecological process, rates of nest predation are subject to strong variation. For example, the strength and direction of edge and fragmentation effects have been shown to vary within and among geographical regions, landscapes, nest types and species. As suggested by Robinson (2009), the quest for a conceptual framework on nest predation rates may therefore be a waste of time and effort, particularly so if the predator part of the equation remains largely ignored. Yet, despite this variability, a large number of nest predation studies show that habitat edge matters, be it in the form of increased (classic edge effect e.g. Gates & Gysel, 1978) or decreased (inverse edge effect e.g. Spanhove et al., 2009a; Spanhove, Lehouck & Lens 2009b) predation rates near habitat edges. Biological effects of habitat fragmentation on the dynamics and viability of bird populations can be expected to differ under classic or inverse edge effects. Furthermore, as discussed in Spanhove et al. (2009a) and supported in the commentaries (Lahti, 2009; Robinson, 2009; Sodhi, 2009), biotic and abiotic processes operating near habitat edges may affect habitat generalist and specialist (i.e. forest interior) species in largely different ways. To illustrate this point, we overlaid the hypothetical ranges of a forest generalist and forest interior species with a ‘high predation zone’ under classic and inverse edge effects (Fig. 1). Although quantitative population responses likely depend on fragment size, fragment shape and the level of habitat specialization of both prey (bird) and predator species, consistent qualitative responses can be expected under a range of environmental conditions. For instance, the broader niches of habitat generalists compared to specialists may allow the former to nest outside