A new study provides evidence that at over 100 long-term monitoring sites across the world, the number of species is not changing...
This surprising finding appears to contradict our notions of a "sixth mass extinction", and a "global biodiversity crisis". However, the study points out that all is not as it seems, and we should think much more carefully about how we measure and monitor biodiversity and how it is changing.
Biodiversity is usually measured using the number of species present (know as species richness). However, this study demonstrates that this measure does not allow us to detect changes in species composition. And while their global dataset does not show any change in number of species found in their plots (the left hand figure), there is a significant change in composition. This could be the result of two processes:
- A turnover rate created as species migrate poleward in response to climate change - this shift in species ranges has been observed in many groups in including insects, birds and plants
- A phenomenon known as "biotic homogenisation". This is a process where species invasions (mainly intentional or accidental introductions by humans) and extinctions (often as a result of these introductions) increase the genetic, taxonomic or functional similarity of different areas - i.e. ecosystems become more and more alike.
The second process is arguably more concerning (from a biodiversity perspective), as it suggests that global biodiversity is being eroded and the great variety of life is being reduced. In the figure on the right, the authors use a index of similarity to show that over time the plots in their database have become increasingly alike, suggesting that biotic homogenisation is occurring and is having a major effect on global biodiversity.
The global network of research plots have the same number of species as they ever did, but the composition of species is gradually becoming more and more alike - an alarming process of biodiversity globalisation.
Contrary to our expectations, we did not detect systematic loss of α diversity. However, community composition changed systematically through time, in excess of predictions from null models. Monitoring and understanding change in species composition should be a conservation priority.