Capturing complexity in measures of biodiversity.

“My introduction to measuring biodiversity came during my PhD research at the University of Ulster, when I set out to compare ancient Irish oakwoods in County Derry, Northern Ireland, with commercial conifer plantations composed of non-native Sitka spruce. This was at a time when the Troubles overshadowed our lives and so I had the interesting challenge of explaining to the British Army soldiers patrolling the area where I was working that surveying quadrats and setting light traps for moths were innocent pursuits.

“As part of this research, I had to grapple with the methods needed to quantify biodiversity, and drew on this experience to write my first book, Ecological Diversity and its Measurement. This book and its successors became standard references for biodiversity measurement, and are still used all around the world.

“At the time, my question about the differences in the biodiversity of natural versus planted forest was regarded as rather an esoteric quest. Now, of course, the restoration and expansion of woodland in Scotland and elsewhere is a priority, but balancing demands for carbon sequestration with the protection of our native fauna and flora can be difficult. The need to accurately measure biodiversity has not gone away.

A smiling lady standing in front of the sea
Professor Anne Magurran

Universal themes

“I have been very fortunate during my subsequent career to be able to work with colleagues in many countries and on many systems including the flooded forests of the Amazon, the Northern Range in Trinidad and Tobago, and the Western Ghats in India. Although these ecosystems differ in multiple ways, there are common challenges. One universal theme is that biodiversity is complicated to quantify. Part of the reason for this 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.

“A second universal observation is that biodiversity is not a single entity, but rather a constellation of concepts. It includes the amount of diversity, such as the number of plant species found in a woodland, and the composition of that diversity, for instance the degree to which two woodlands differ in the types of species present. We can measure this diversity through the lens of the identity of the species present, or using their functional traits, or by taking account of their position on the evolutionary tree.

“It is increasingly clear that different metrics tell different stories about how biodiversity changes over space and time. In the same way that no single metric, such as tusk size, or weight, or gut microbiome, is the single best descriptor of an elephant, there is no one correct way to measure biodiversity. Instead we need to draw on a range of different measures to present a complete picture of biodiversity change.

Tackling the biodiversity crisis

“My most recent work focuses on temporal change in biodiversity. This has involved analyses of large databases, where the numbers and abundances of the species present at a locality have been recorded over several years. This research – which includes marine, terrestrial and freshwater ecosystems from tropical, temperate and polar regions – is showing that the reorganisation of ecological communities, through turnover in the types of species present, is occurring at rates that appear to have little precedent in historical times.

“Factors such as climate change and the presence of non-native invasive species can be linked to these changes. For example, the communities of fish found in the northern and southern portions of seas to the west of Scotland are now less distinctive than they were 30 years ago. This ‘biotic homogenisation’ is correlated with warming. An analogy we use to describe this is the ‘shopping mall’ effect – the phenomenon whereby the same brands increasingly dominate retail opportunities around the world.

“However, resorting to the generic term ‘biodiversity loss’ to describe these changes causes much of the nuance to be lost, and the assumption that species are invariably in decline (which isn’t necessarily the case) takes hold. Too often headlines convey the sense that it is all hopeless, and can give the impression that there is no point in trying to do anything to tackle the biodiversity crisis. But there are good news stories as well, and although the task is an exceptionally demanding one, there is still time to act in ways that will protect the natural ecosystems on which we all depend.

“Scotland is a great place to do this work because biodiversity, and its conservation, is a national priority. Yes, there are major challenges, but by thinking globally and acting locally we really do have an opportunity to help mitigate both the biodiversity and climate crises.”

Anne Magurran is Professor of Ecology and Evolution at the University of St Andrews. She has a longstanding interest in biological diversity. Find out more here.

This post is part of a series showcasing Scotland’s innovative, high-impact research supporting biodiversity conservation, in partnership with Scottish Government and NatureScot. Read the rest of the series here.

Further reading

Magurran, A.E. 1988. Ecological Diversity and its Measurement. Dordrecht: Springer.

McGill, B.J., et al. 2007. Species abundance distributions: moving beyond single prediction theories to integration within an ecological framework. Ecology Letters 10(10): 995—1015.

Magurran, A.E., et al. 2010. Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time. Trends in Ecology and Evolution 25(10): 574—582.

Magurran, A.E., et al. 2015. Rapid biotic homogenization of marine fish communities. Nature Communications 6: 8405.

Magurran, A.E., et al. 2018. Divergent biodiversity change within ecosystems. Proceedings of the National Academy of Sciences of the United States of America 115 (8) 1843—1847.

Kortz, A.R. & Magurran, A.E. 2021. Complex community responses underpin biodiversity change following invasion. Biological Invasions 23: 3063—3076.

Trinidade-Santos, I., Moyes, F. & Magurran, A.E. 2022. Global patterns in functional rarity of marine fish. Nature Communications 13: 877.