Community trophic structure typical of seasonal tropical climates

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Climate determines the arquitectures of trophic structures and human activities simplify them.

This is the main conclusion of a study published today in Nature Communications, by Manuel Mendoza and Miguel B. Araújo.

Using the geographic distributions and feeding preferences of all terrestrial large mammals, the authors found that trophic communities could be characterised by six main groups. The distribution of the groups is clearly related to climate, offering new grounds for accurately forecasting responses of animal communities to climate change.

Climate has an important role shaping the distributions of species, but we know little how it shapes the interactions of species at the community level. The study focuses on one of the most important biotic interactions: trophic interactions.

With help of water and nutrients, plants transform solar energy into biomass, such as leaves, fruit, and seeds. The biomass is then consumed by other organisms, such as folivores, frugívoros, and granivores. The starting hypothesis was that the quantity of energy available in every region, which is determined by the climate, limits the types of food webs that can exists therein. The results were surprisingly clear” explains Miguel B. Araújo.

We analysed the geographical distributions and feeding preferences of all terrestrial mammals with more than 3 kg and found that they are grouped into 6 clear trophic communities. These communities are linked to the climate in which they occur“, explain Manuel Mendoza. “There is an apparent determinism in the structuring of food webs that seems to self-organise in order to best exploit the available resources. Specifically, we found six basic types of trophic organisation that we termed boreal, temperate, semiarid, seasonal tropical, humid tropical and depauperate“, he notes.

The results also reveal that human activities directly affect the trophic structures of animal communities, leading to a reduction in the numbers of coexisting species that expected from trophic-climate relationships. “The depauperate structures are very interesting. We find them in oceanic islands, deserts, and polar regions, but also in regions highly affected by human activities. On islands, it is possible that trophic simplification comes about because the difficulties of dispersal of species from some guilds, that is, trophic profiles. It is also possible, they depauperate community are the outcome of past extinctions caused by human colonisation of the islands.  In fact, that is what we find in some boreal and temperate communities of Western Europe and Northeastern America that display clear patterns of trophic simplification, or tropical communities that are heading towards semiarid state“, explains Araújo. “These results will help project the effects of climate change as well as other human-induced changes in land use on biodiversity, since trophic structures are the way in which biodiversity organises itself to efficiently explore the resources available within ecosystems” concludes MiguelAraújo.

Figure. Six types of mammal trophic structures around the world: boreal, temperate, semi arid, seasonal tropical, and humid tropical environments. For convenience, trophic structures are represented as bipartite networks depicting energy flows between trophic guilds and their main resources. Resources: grass gr, leaves lv, fruits fr, flowers fw, forbes fb, seeds sd, invertebrates inv, fish fs, small vertebrates sv, mammals (1-10 kg) sm, 10-100 kg mm, (>100 kg) lm. Trophic guilds: selective herbivores SH, plant material feeders PF, small carnivores SC, frugivores FR, folivores FL, mixed feeders (being both grazers and browsers), MF, omnivores OM, grazers Gz, piscivores PS, invertebrate feeders IF, large carnivores LC. The size of the nodes for the guilds (white) is proportional to the number of species from the guild in the food web. Arrows represent the fluxes of energy from resources to guilds. The magnitude of the fluxes is represented by the thickness of the arrows and increases with the sum of the estimated percentage of that resource in the diet of all the species in that guild. The size of the nodes for resources (black) is proportional to their total contribution to the web. This contribution is equivalent to the sum of the estimated percentage of that resource in the diet of all species. Figure drawn by Pedro Salgado.