Print PageIn most parts of the world, forests ecosystems are now a mosaic of patches that are more or less large, old, and distant from each other. All these three factors are known to affect species richness and species composition at local scales. However, it is not clear whether or not they also affect the functional diversity (FD) of these communities. Here, we used structural equation modeling (SEM) to determine whether these factors have a direct effect on FD or only an indirect effect via their effects on species richness. To do this, we collected presence/absence data for all herb species found in 243 forest patches located in northern France whose age, area, and isolation were calculated based on historical and geographical data. Using six plant traits, we calculated three different functional diversity measures that we used to estimate the “true functional diversity” (a latent variable in our SEM).
Results/Conclusions
Our results showed that species richness is directly affected by age, area, and isolation with high species richness found in old, big, and less isolated patches. Species richness is the primary driver of herb FD in these patches, with a strong, positive and direct relationship between species richness and FD. Interestingly, we also observed that older forest fragments favor functionally similar species but this direct negative effect did not compensate for the increase in FD due to the higher species richness found in older forests. This may reflect the accumulation of species through time. Isolation also directly reduced FD suggesting that more isolated patches favor species with similar traits. Together these results demonstrate that preserving species richness and FD at local scales will require landscapes in which fragments are not too small or isolated from each other. To enhance FD at a regional scale will further require maintaining forest fragments of different ages in the landscape.
