Edge effects on trophic cascades in tropical rainforests

Abstract

The cascading effects of biodiversity loss on ecosystem functioning of forests have become more apparent. However, how edge effects shape these processes has yet to be established. We assessed how edge effects alter arthropod populations and the strength of any resultant trophic cascades on herbivory rate in tropical forests of Brazil. We established 7 paired forest edge and interior sites. Each site had a vertebrate-exclosure, procedural (exclosure framework with open walls), and control plot (total 42 plots). Forest patches were surrounded by pasture. Understory arthropods and leaf damage were sampled every 4 weeks for 11 months. We used path analysis to determine the strength of trophic cascades in the interior and edge sites. In forest interior exclosures, abundance of predaceous and herbivorous arthropods increased by 326% and 180%, respectively, compared with control plots, and there were significant cascading effects on herbivory. Edge-dwelling invertebrates responded weakly to exclusion and there was no evidence of trophic cascade. Our results suggest that the vertebrate community at forest edges controls invertebrate densities to a lesser extent than it does in the interior. Edge areas can support vertebrate communities with a smaller contingent of insectivores. This allows arthropods to flourish and indirectly accounts for higher levels of plant damage at these sites. Increased herbivory rates may have important consequences for floristic community composition and primary productivity, as well as cascading effects on nutrient cycling. By interspersing natural forest patches with agroforests, instead of pasture, abiotic edge effects can be softened and prevented from penetrating deep into the forest. This would ensure a greater proportion of forest remains habitable for sensitive species and could help retain ecosystem functions in edge zones.

Rethinking extinctions that arise from habitat loss

“…Species from higher trophic levels (the upper levels of the food chain), such as predators, require larger areas to maintain their populations compared with species from lower trophic levels, so the number of individuals supported by smaller habitat fragments might not suffice to maintain populations of top predators or consumers, and hence would produce shorter food chains and alter the ecosystem structure 15 . Differences in extinction rates between trophic levels can cause striking changes in ecosystem functioning at habitat edges 16 , jeopardizing the functioning and ecosystem-service provision as natural habitats diminish in size 11 .…”

“…Chase and colleagues' approach is good for providing a general overview of the extent of these effects, but to understand exactly how ecological processes are changing locally, a higher level of detail will be needed. This will require going beyond the studies of trophic chains 14,16 to assess more-complex food webs 15 , and to gather information on changes in species' functional responses and trait diversity in increasingly smaller habitats. Ultimately, this information will reveal which ecological processes are decaying, and what the consequences of such ecosystem decay are for the maintenance of fully functional biodiversity.…”

Weak effects of birds, bats, and ants on their arthropod prey on pioneering tropical forest gap vegetation

“…This may produce a negative correlation between herbivores and their APs (Bosc et al, 2018). Studies reporting both top-down effects of vertebrate insectivores on arthropod communities and their cascading effects on plant community composition in tropics are scarce (Harrison & Banks-Leite., 2020;Letourneau et al, 2004;Maas et al, 2016). Thus, specific way in which predators may be affecting their arthropod prey and vegetation in pioneer tropical forest gaps remains uncertain.…”

Synergistic effects of habitat configuration and land-use intensity shape the structure of bird assemblages in human-modified landscapes across three major neotropical biomes

“…Within native habitat remnants of once continuous expanses of primary habitat, species diversity is expected to be reduced by both smaller habitat areas, which constrain population sizes and limit habitat diversity, and isolation from other native habitats (Stouffer and Bierregaard Jr. 1995), which further affects species colonization rates (MacArthur and Wilson 1967). Moreover, edge effects tend to modify the vegetation structure, particularly at more irregularly-shaped fragments, often depressing the availability of trophic and structural resources (Laurance et al 1998), which negatively affects biodiversity (Laurance 2004;Harrison and Banks-Leite 2019). Within non-native matrix habitats, species diversity is further affected by their quality as suggested by the structural fragmentmatrix contrast (Driscoll et al 2013).…”