Did climatic stability allow for the evolution of toxic plants, voracious specialist herbivores and fierce predators in the tropics?

Background/Question/Methods

Previous literature reviews show how trophic cascades are, on average, stronger in the tropics.  A possible reason might be that tropical species are, on average, more specialized with more co-evolved interspecific interactions.  This study aims to tie the strength of trophic interactions to trophic niche specialization and study their relationships at a global scale by collecting and georeferencing effect sizes from studies on plant defenses, specialist versus generalist herbivory and predation, conducted during the past fifty years. Thus in this study we explore the correlation between temperature and precipitation versus the strength of herbivory, plant defenses and predation.  Then, we ask if the relative strength of specialist versus generalist herbivores changes in temperate versus tropical climatic conditions.  Finally, we test the hypothesis that the stronger effects of: 1. specialist herbivory on plant biomass, 2. plant defenses on herbivore biomass and 3. predation on herbivore biomass, are correlated with lower magnitude of climatic oscillations over thousands to millions of years. We achieve this by quantifying the amplitude of climate change from the last glacial maximum (LGM) to the present and correlating it to the locations at which the strongest interactions were recorded in the literature.

Results/Conclusions

Analysis of 500 effect sizes from trophic cascade studies distributed across the globe showed that specialist herbivores had stronger negative effects in tropical biomes compared to their milder effects specialist herbivores had at temperate latitudes. Whereas generalist herbivores had similar plant damage effects across latitudes.  Stronger negative effects in herbivore biomass in response to both predation and plant defenses suggest there are higher selection pressures for herbivore specialization at tropical latitudes.  The results suggest that warm and stable climatic conditions in the tropics allowed for the coevolution of plant defenses, specialist herbivory, and close associations between predators and plants, but that climatic upheavals and geographic range shifts towards higher latitudes has selected against strong coevolutionary associations.