A niche based approach reveals how humans reshape the species richness-productivity pattern in bird assemblages
A well-documented ecological pattern is the increase in species richness with aboveground net primary productivity (NPP) at various spatial scales and extents.
The dominant, mutually non-exclusive, processes invoked to explain the increasing number of species with primary productivity include (i) an increase in total functional space filled by the community or (ii) an increase in species-specific niche specialization, with both allowing more species to coexist without increasing niche overlap. These hypotheses lead to the predictions that (i) community niche volume, as measured by functional richness, will increase linearly with NPP and species richness, or that (ii) species spacing within the functional volume, as measured by functional evenness, will increase linearly with these two factors, while the volume itself remains stable.
Currently realized NPP patterns are heavily reshaped by human activities, with varying and sometimes high human appropriation of net primary productivity (HANPP). This is expected to alter the realized link between NPP and species richness.
Using large-scale bird distribution data for the Eurasian zooregion in a 50*50 km grid, we assessed which of the above hypotheses was more likely in the absence of human activities and if these activities have altered the link between productivity and bird species richness.
The NPP in the Eurasian zooregion (occurring in the absence of human activities) ranged from almost null to 840 gC m-2 year-1whereas HANPP ranged from 0 to 87% of NPP.
As expected, the bird species richness increased linearly with the productivity in the absence of HANPP. However, the SR-NPP relationship was disrupted for higher HANPP levels, with a flatter and weaker response of SR to NPP.
At low HANPP, the functional richness increases linearly with species richness and NPP, providing support for the functional volume increase hypothesis. On the other hand functional evenness increased at higher NPP levels but not with species richness thus failing to provide support for the niche specialization hypothesis.
The saturating response curve of functional richness with increasing species richness and NPP at higher levels of HANPP suggest that the HANPP lead to shrinkage in functional space. These preliminary results suggest that the human use of ecosystems have a high impact on assembly rules through a redirection of energy flows out of ecosystems.