Presents-absents of woody species on termite mounds and the savanna matrix in Africa

Questions: Although the biogeography of plant communities has been well
studied, mechanisms controlling plant community diversity are still poorly
understood. Stochastic and deterministic processes are hotly debated as
being key in structuring communities across taxa. Here we ask, to what
extent are plant communities on termite mounds shaped by either stochastic
or deterministic processes within and between regions on the African
continent? Methods: We performed a meta-analysis on 23 studies that
satisfied our selection criteria out of 369 that we initially identified.
All these studies sampled woody plants that occurred on termite mounds and
the surrounding matrix. In order to disentangle the relative importance of
deterministic and stochastic processes in generating β-diversity between
mound-mound and matrix-matrix plots, within and between regions, we
applied null models. We also assessed the relationship between species
richness and environmental variables for each site. Results: Of the
environmental variables assessed at each site, only altitude had a
significant (negative) relationship with species richness. Within each
region comparison of mound-mound, matrix-matrix and matrix-mound species
composition did not significantly deviate from the null models of random
expectation. Between regions, the observed pattern was significantly
higher than expected for both mound-mound and matrix-matrix plots between
East and West Africa. Surprisingly, the comparison between southern Africa
and both East and West Africa did not significantly deviate from the null
expectation, regardless of increased spatial separation. We attributed the
within region pattern to homogenizing dispersal and reliance on the same
species pool (γ) for matrix and mound plots. Between regions pattern, was
influenced by both deterministic and stochastic processes. 
Conclusion: Our results suggest that the processes responsible for
vegetation composition between mound-mound, matrix-matrix and mound-matrix
are stochastic in nature due to lack of difference from the null
expectation. However, regional patterns are governed by both deterministic
and stochastic processes.