About two months ago, our paper from one of my collaborative projects got published in PLOS ONE*. I know it’s rather late, but I’ve finally found time to write up a summary of the paper, with some additional background and details.
*Katayama, N, Amano, T., Naoe, S., Yamakita, T., Komatsu, I., Takagawa, S., Sato, N., Ueta, M. and Miyashita, T. (2014) Landscape heterogeneity – biodiversity relationship: effect of range size. PLOS ONE 9: e93359.
Being an ecologist, I’ve always found the word ‘heterogeneity’ to appear frequently in what I see and read (but realised after moving to the UK that this word is not that commonly used by non-ecologists!) Either way, the heterogeneity of habitats and landscapes is undoubtedly key to biodiversity. In fact, thousands of papers have reported that heterogeneity leads to increased species richness.
But how general is this relationship? A recent global meta-analysis has successfully shown that the overall mean effect of heterogeneity on species richness is positive across taxa and regions. This conclusion is reasonable, but we became interested in the other side of the same issue; when we look at individual studies, the relationship can be highly variable, ranging from very negative to very positive effects. So, what factors explain this context dependency?
We developed a hypothesis, which attributes the varying effect of heterogeneity to differences in species composition and historical processes that have formed it. That is, in historically-heterogeneous regions, species adapted to spatially- or temporally-variable environments are expected to be widespread, creating the overall pattern of heterogeneity-richness association. On the other hand, this means that in such regions species adapted to homogeneous environments may have already become extinct or be distributed only over a narrow range.
We tested this hypothesis using avifauna in Japan. Japan is a mountainous country with > 60% forest cover, but a mosaic of landscapes have long been maintained, over thousands of years, by its topographic heterogeneity and more recently, farming activities. So it’s an excellent study system to test this hypothesis.
Mosaic landscapes in Japanese farmland
The results showed a clear support for our hypothesis. Total bird richness was the highest at the intermediate level of forest cover (in Japan, this is a good measure of mosaic landscapes, called Satoyama), showing a positive heterogeneity-richness relationship (panel (a) below). This relationship was, however, mostly due to the same relationship observed in wide-ranging species (b), and narrow-ranging species actually showed the opposite pattern, i.e., they were the most numerous in open or forest habitats (c).
The same was true even when looking at patterns in the abundance of each species; many wide-ranging species (e.g., Japanese bush warbler and meadow bunting) showed the highest abundance in mosaic landscapes while most narrow-ranging species (like ruddy kingfisher and black-browed reed warbler) did not.
These results have given us two insights about the heterogeneity-richness relationship.
First, the effect of heterogeneity clearly depends on species composition. Even if an overall pattern shows a positive association, responses by wide-ranging species may be masking those by rare species favouring homogeneous habitats. We cannot assume that maintaining heterogeneity always benefits biodiversity. Instead, we should manage landscapes specifically for the target species or systems. I think this is similar to the conclusion regarding the effectiveness of Entry Level vs Higher Level Stewardship for farmland biodiversity conservation in the UK.
Second, as we hypothesised in this study, the context-dependent relationship between heterogeneity and species richness may be explained by how species composition in the region has been formed historically. In this project, we are keen to test the hypothesis in historically homogeneous regions and are also trying to develop a framework for understanding this relationship in a more general way.
Going back to Japan, recent anthropogenic impacts have been transforming landscapes dramatically, which may alter the historically-developed relationship between heterogeneity and species richness. For example, recent loss of Satoyama landscapes in Japan has caused a range contraction in grey-faced buzzards, a well-known ‘mosaic-habitat’ species. So in the near future, if this trend continues, we might start seeing the positive effect of heterogeneity even in rare, narrow-ranging species.