Biodiversity loss is not the issue
- Publication Date
- Professor Anne Magurran
When Mary Sommerville wrote about growing up in Burntisland in Fife in the late 1700s, she recalled looking out across the Firth of Forth to see ‘whales spouting in every direction’ and the sea rippled by the shoals of herring.
The Firth today has much reduced fish stocks, and whales are an unusual site. It seems clear that its biodiversity has declined. Indeed, the term ‘biodiversity loss’ is often used to describe the current state of nature in Scotland, and further afield. But what is biodiversity, and is the situation one of unvarying biodiversity loss? More importantly, how should we draw on contemporary biodiversity science to help protect the ecosystems on which we all depend?
Every ecological assemblage consists of a few common and many rare species. A consequence of this unevenness in species abundances is that biodiversity metrics don’t scale neatly with sampling effort. For example, the number of species recorded during a bird count will increase with the time spent observing, or as larger areas are searched, but on a flattening curve.
Statistics are needed to make fair comparisons between places when sampling effort differs. However, even when this has been done, one perhaps surprising outcome is that analyses of temporal trends of species richness around the world – species richness being the number of species present in a locality – show no systematic declines.
In recent decades, then, there is no evidence of universal biodiversity loss as measured in this way. What’s more, in some places species richness is actually increasing. For example, the seas to the west of Scotland support more species than they did 30 years ago. In part this may be due to warming of the oceans.
But biodiversity is more than the numbers of species and their relative abundances. Natural ecosystems are co-evolved complexes that support species interactions which in turn enable resilience. These systems are not necessarily species rich. A Scottish peat bog, for instance, may not have a high diversity of plants, but the species that are there are play an essential role in supporting function, including carbon sequestration, as well as providing habitat for iconic birds.
Although it is rarely headline news, we are finding that the species composition of many assemblages around the world is changing at rates unprecedented during historical times. The causes are manifold – climate change, invasive species, agriculture, habitat degradation, urbanisation, etc. In some cases, this can mean that the composition of localities that were previously quite distinct are now much more similar. Such biotic homogenisation is sometimes dubbed the ‘shopping mall effect’, due to the parallels with similar brands increasingly dominating retail outlets around the world.
Nature based solutions have been advanced as a remedy for the climate crisis. It is however essential that these work with nature, rather than against it. Artificially increasing species richness above the ‘natural’ level for an ecosystem may impair its function, while planting exotic trees as a quick fix for carbon sequestration potentially disrupts the web of species interactions associated with the species that ‘belong’ to a place.
The way forward is to allow nature to tell us what the appropriate composition and diversity of the ecosystems around us should be, and to recognise that biodiversity is a multifaceted concept that cannot be reduced to a single number.
Professor Anne Magurran CBE FRSE is Professor of Ecology and Evolution at the University of St Andrews, and a Fellow of the Royal Society of Edinburgh.
This article originally appeared in ReSourcE Spring 2023.
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