Edge effects in tropical dry forests of Madagascar: Additivity or synergy?
The study of habitat fragmentation is complicated because multiple, potentially synergistic, ecological processes may be acting simultaneously. Further, edge effects themselves may be complex in that additivity from multiple edges may give rise to heterogeneous nearest‑edge gradients. We used two‑dimensional heat conduction as a proxy for additive edge effects and used an inverse modelling approach to apply it in two study landscapes in order to test whether: 1) the additive edge model explained edge effects better than a non‑additive one and 2) two key observations that have been attributed to synergy could be more simply explained by additivity; namely, increasingly shallow nearest‑edge relationships with decreasing fragment area and variation in slopes of species‑area relationships as a function of distances to fragment edges. We sampled vegetation structure in dry tropical forests in northwestern Madagascar at various distances from the edge in 12 fragments and 3 sites along continuous forest edge (n = 139 plots).
With increasing edge proximity, woody stem densities decreased, and as predicted by the model, smaller fragments had lower stem densities and shallower nearest-edge relationships than larger ones. It did not appear to be necessary to invoke processes other than edge additivity in explaining the fragmentation effects: area effects were no longer significant once additive edge effects were accounted for, and parameterization of the model based just on the edge of continuous forest was successful in predicting stem densities in fragments. For a published study of beetle communities in fragmented forests in New Zealand, the model was successful in predicting shifts in species-area relationships as a function of nearest edge distances for both matrix and forest specializing beetle species, although observed richness for the latter group in the smallest fragments was lower than predicted. Thus, we found that two key observations attributed to synergy could be explained by edge additivity. The model is particularly useful in that it can be applied at the landscape scale and can help to disentangle the complex sets of processes acting in fragmented landscapes.