Food web structure shaped by habitat size and climate across a latitudinal gradient

Abstract

Habitat size and climate are known to affect the trophic structure and dynamics of communities, but their interactive effects are poorly understood. Organisms from different trophic levels vary in terms of metabolic requirements and heat dissipation. Indeed, larger species such as keystone predators require more stable climatic conditions than their prey. Likewise, habitat size disproportionally affects large-sized predators, which require larger home ranges and are thus restricted to larger habitats. Therefore, food web structure in patchy ecosystems is expected to be shaped by habitat size and climate variations. Here we investigate this prediction using natural aquatic microcosm (bromeliad phytotelmata) food webs composed of litter resources (mainly detritus), detritivores, mesopredators, and top predators (damselflies). We surveyed 240 bromeliads of varying sizes (water retention capacity) across 12 open restingas in SE Brazil spread across a wide range of tropical latitudes (-12.6° to -27.6°, ca. 2,000 km) and climates (Δ mean annual temperature = 5.3°C). We found a strong increase in predator-to-detritivore mass ratio with habitat size, which was representative of a typical inverted trophic pyramid in larger ecosystems. However, this relationship was contingent among the restingas; slopes of linear models were steeper in more stable and favorable climates, leading to inverted trophic pyramids (and top-down control) being more pronounced in environments with more favorable climatic conditions. By contrast, detritivore-resource and mesopredator-detritivore mass ratios were not affected by habitat size or climate variations across latitudes. Our results highlight that the combined effects of habitat size, climate and predator composition are pivotal to understanding the impacts of multiple environmental factors on food web structure and dynamics.

Effects of bromeliad flowering event on the community structuring of aquatic insect larvae associated with phytotelmata of Aechmea distichantha Lem. (Bromeliaceae)

“…Diptera larvae are very abundant in bromeliads (Araújo et al, 2007), pitcher plants (Baiser et al, 2011) andtree-holes (Blakely et al, 2012), and are considered the main predators in the phytotelmata of several bromeliad species, where they exert a great influence on microorganisms (Walker et al, 2010;Baiser et al, 2011). Although several studies show that zygoptera larvae are the main aquatic predators within phytotelmata (Petermann et al, 2015a;Romero et al, 2016), in the bromeliads used in our study no larval forms of these organisms were observed. Paradise (2000) suggest that alterations in pH due to the decomposition of organic matter by Scirtidae larvae (Coleoptera) within phytotelmata may lead to a reduction in the mortality and an increase in the abundance of Diptera larvae in this habitat.…”

Geographic shifts in the effects of habitat size on trophic structure and decomposition

“…The dominant predators in the bromeliad phytotelmata are often damselfly nymphs (Odonata: Zygoptera), which are large-bodied relative to their prey, voracious generalists, and have been demonstrated to be sensitive to habitat size in several sites (Petermann et al 2015, Romero et al 2016. Bromeliads contain a food web based on fallen leaf litter supporting macroinvertebrate prey species -composed of detritivores feeding directly on leaf litter, as well as filterers and grazers on algae and the microbial community -and predators.…”

Terrestrial vertebrate predators drive the structure and functioning of aquatic food webs

“…, Romero et al. ), a decrease in the biomass of this functional group might lead to changes in the shape of trophic pyramids (Fig. c, d).…”

“…, Romero et al. ). A larger biomass ratio of predators in relation to prey causes greater interaction strength in a food web (Berlow et al.…”

“…Top‐heavy pyramids also occur in large bromeliads, where a higher amount of biomass of aquatic apex predators (e.g., Odonata) accumulates, which can be maintained in the system by higher turnover biomass rates of lower trophic levels (Romero et al. ). Therefore, an increase in aquatic top predators in the system, caused by exclusion of terrestrial predators, may result in top‐heavy pyramids (Prediction 2, Fig.…”