Community approach to tropical forest fragmentation gets us closer to understanding mechanisms by revealing strong indirect effects

Habitat fragmentation is one of the most complex problems threatening biological diversity. Besides the loss of habitat area, fragmentation in general isolates habitat patches and degrades their quality, particularly on their edges. These multiple ecological stressors are difficult to distinguish from the additive, if not interactive effects of additional global change phenomena such as climate destabilization, diverse pollutants, direct human overexploitation of diverse populations, invasive species and altered biogeochemical cycles. Moreover, the stakes of understanding habitat fragmentation are particularly high in the tropics, where species diversity is maximal, 82% of earth’s threatened species reside (Birdlife International, 2000), habitat loss and fragmentation are rampant, and where many species may be disproportionately sensitive (e.g. Sodhi, Liow & Bazzaz, 2004; Stratford & Robinson, 2005). The challenge of fragmentation is that we barely understand it, particularly in the tropics, which precludes effective management. Conservation biologists have applied diverse tools to this challenge. An early tool was island biogeography, a community approach emphasizing the importance of patch area and isolation, and how extinction and re-colonization processes determine patch species richness. A variety of additional tools have emphasized population responses: for example, meta-population theory characterizes the population processes determining patch occupancy. Landscape ecology has helped us understand the importance to populations of habitat patch shape and configuration – especially connectivity – as well as effects of hostile habitat edges and the role of the intervening matrix of land-use. Technologies such as satellite and other remote-imaging methods, and more recently landscape genetics (for a review see Holderegger & Wagner, 2008) are contributing to our understanding of how habitat fragmentation erodes populations and communities (e.g. see Lees & Peres, 2006). The challenges posed by tropical forest fragmentation in particular, coupled with a bewildering array of analytical approaches typically requiring specialized skills, often result in biased, myopic (e.g. not recognizing relevant insights from other approaches) or provincial studies that are better at characterizing patterns than mechanisms. A few studies have refreshingly circumnavigated most of these pitfalls by attracting broad collaborations that could focus extraordinary talent and resources, an approach exemplified by the experimental fragmentation studies initiated by Tom Lovejoy in Brazil (Laurance et al., 2001); and more recently by John Terborgh and colleagues in Lago Guri, Venezuela. The paper by Feeley & Terborgh (2008), in this issue of Animal Conservation, extends the invaluable insights from Lago Guri by adopting an island biogeography (community level) approach, designed to assess mechanisms by which species richness, the main response variable, responds to fragment area. Feeley & Terborgh (2008) examine avian community relaxation, which began in 1986, when the reservoir created by the Raul Leoni Dam isolated hundreds of hilltop, landbridge islands from once continuous, semi-deciduous tropical dry forest. As Feeley and Terborgh point out, species richness declines on such landbridge islands, more and faster the smaller the island, but we do not understand why. Using their extensive knowledge of changes in plants (e.g. Terborgh et al., 2001, 2006), herbivores (Terborgh et al., 2001) and birds (e.g. Feeley & Terborgh, 2006; Feeley et al., 2007), this study used path analysis in a subset of 11 island fragments to partition response mechanisms into direct versus indirect effects of area, the latter in response to populations of two particular herbivores, howler monkeys Alouatta seniculus and leaf-cutter ants (Atta spp.). On small islands where howler monkeys became hyperabundant, bird species extirpations tended to be retarded, whereas hyper-abundant leaf-cutter ants tended to accelerate bird extirpations. Feeley & Terborgh (2008) argue convincingly that their study is important both because it provides a compelling alternative hypothesis – indirect ecological interactions – to explain the area effect of extirpations in general, and because it reveals the importance in fragmented landscapes of indirect ecological processes, which were up to seven