No systematic local biodiversity loss: Where, when, and relative to what baseline?

Background/Question/Methods

Biodiversity is undisputedly declining at a global scale in response to several anthropogenic drivers, including land use changes, nutrient enrichment, invasive species, and climate change. There is, however, currently some debate as to whether and how biodiversity is changing at local scales. Some meta-analyses have found that many human impacts tend to decrease local biodiversity, whereas other meta-analyses have found no systematic loss of local biodiversity. These two groups of studies have used different types of data, and have defined species loss with respect to two different baselines: either to less impacted places or to earlier points in time. Here I critically consider the strengths and limitations of both types of studies, and offer new interpretations of these seemingly conflicting results. I also explore the implications of such changes in biodiversity for ecosystem functioning.

Results/Conclusions

Previous meta-analyses together provide considerable evidence that many anthropogenic impacts substantially decrease local biodiversity, and that reducing such impacts often leads to substantial recovery of local biodiversity. Many of the observed local plant species gains occur during post-disturbance succession or recovery. Thus, these are species gains above disturbed, not intact, levels of biodiversity. For example, one meta-analysis reported a significant increase in local terrestrial plant diversity worldwide; however, this increasing trend was driven by a single study in which plant diversity recovered after deforestation. Existing evidence suggests that there has been systematic loss of local biodiversity across most of Earth's land surface during recent centuries and millennia, relative to levels of local biodiversity present prior to major human impacts, such as cropping, over-grazing, or deforestation. Trends in local biodiversity cannot be extrapolated or understood without acknowledgement of where and when observed trends occurred, and the state of the community used as a baseline for determining the magnitude and direction of change.