Structure of bromeliad food webs shaped by habitat size and climate across a latitudinal gradient

Background/Question/Methods

Habitat loss and climate change are among the main drivers of biodiversity loss. Larger species, as keystone predators, are among the most threatened because they require large home ranges and thus are restricted to larger habitats. Therefore, it is expected that in patchy ecosystems predator-to-prey mass ratio is positively related with habitat size. Here we investigated to what extent this relationship is shaped by environmental conditions and species composition using natural microcosm (bromeliad) food webs composed by detritivores, intermediate predators and keystone predators (damselflies). We surveyed 240 bromeliads of varying sizes (bromeliad capacity, ml) across 12 Brazilian open restingas spread along a wide range of tropical latitudes (-12.6 to -27.6, ca. 2.000 km) and climates (Δ mean annual temperature = 5.3^oC). 

Results/Conclusions

Generalized linear mixed models for the whole regional species pool detected a strong increase of the damselfly-to-detritivore mass ratio with habitat size, thus representing a typical inverted trophic pyramid. In addition, this relationship differed dramatically among restingas; the slope of this relationship from each restinga increased along the latitude. A linear model including climatic components of latitude (19 bioclimatic measures from WordClim), restinga integrity and habitat size (average and CV per restinga) revealed that the slope decreased with increasing isothermality (i.e., mean diurnal range of temperature/mean annual range of temperature *100). In contrast, slopes of logistic models of presence-absence of damselflies with habitat size increased with isothermality. Detritivore-resource and intermediate predator-detritivore mass ratios were not affected by habitat size, presence of damselfly or overall predator biomass, neither varied among restingas. Our results add to a new consensus that composition of predators, habitat size and climatic conditions, especially components of temperature, may dramatically shape food web structure, and that the main changes begin ‘from the top’.

Financial support: FAPESP, FAEPEX, CNPq, CAPES