“Fifteen forms of biodiversity trend in the Anthropocene”

CITATION:
Brian J. McGill, et al. 2015. Trends in Ecology & Evolution, Vol. 30, No. 2, pp. 104-113.
ON-LINE AVAILABILITY:
ABSTRACT:
Humans are transforming the biosphere in unprecedented ways, raising the important question of how these impacts are changing biodiversity. Here we argue that our understanding of biodiversity trends in the Anthropocene, and our ability to protect the natural world, is impeded bya failure to consider different types of biodiversity measured at different spatial scales. We propose that ecologists should recognize and assess 15 distinct categories of biodiversity trend. We summarize what is known about each of these 15 categories, identify major gaps in our current knowledge, and recommend the next steps required for better understanding of trends in biodiversity.

Let me start with the bottom line: the scientific discussion is in full bloom and it is pretty complicated. The concept of biodiversity has been touched upon several times in our blog, most recently by Kiza, Antonio, and Noah, but we have never taken the time to dig a little deeper. I will use a recent publication to start discussing this important concept further.

We need to start with a few definitions: what is biodiversity? And how is this concept related to some of the ideas we have been working with here on this blog?

First of all, biodiversity is a vey complex, multidimensional concept (see the Wikipedia article for a good overview). The concept tries to capture the diversity of life-forms on the planet. It has two main aspects, having to do with the spatial and temporal dimensions of biodiversity. Let’s briefly talk about the temporal aspect first: over time biodiversity (simply as number of species), has waxed and waned. Starting with very few species in deep evolutionary time, the number of species has typically increased, punctuated by periods of “explosive” growth in species numbers, and then mass extinctions. The Cambrian explosion around 450 Million years ago is a great example for a relatively quick increase in the number of life-forms. On the other hand, the Cretaceous–Paleogene extinction event about 66 Million years ago would be a great example for a mass extinction. This particular period of extinctions was formative for the biodiversity we currently see on the planet because it wiped out the dinosaurs and opened up relevant ecological niches to birds and mammals. The balance of these two processes, speciation and extinction, leads to the biodiversity observed at any given time. But neither should be thought of as events, but rather as slow processes and therefore a bit difficult to study. My own work on speciation focuses on trying to understand the role of abiotic factors in “sympatric speciation” in a fish model system. In contrast with traditional models of speciation, which involve geographic isolation over extended periods of time, in sympatric speciation new species can arise even when they are in the same geographical area, although the processes that allow this are not well understood.

We are currently in a phase of rapid extinction of species induced by human activities ranging from habitat destruction to climate change. Since biodiversity is tightly linked with ecosystem services, losing biodiversity is a very bad thing from many angles. From the scientific point of view, however, one key issue is that this particular mass extinction – although it is happening very rapidly on an evolutionary timescale – has come so recently that we are still trying to get out to study what is happening. Of course the crisis in federal funding and the widespread denial of science among our political leaders does not help.

This is the context in which I read a recent paper by Brian McGill (who was at OU for a seminar in April) and his team. They argue that, as the title says, there are fifteen forms of biodiversity in place under Anthropocene conditions, and illustrate the very complex nature of the spatial dimension of biodiversity and of the interaction between biodiversity’s spatial and temporal aspects. The paper captures the existing confusion among scientists concerning somewhat contradictory trends in the analysis of geographical data. First we need to distinguish between alpha and beta diversity (see also the Whittaker paper referenced below). Alpha diversity is the number of species in one community. As we remove species from that community, alpha diversity goes down. Interestingly, if we replace the lost species with invasive species (see my post on this topic), alpha diversity would remain the same, even though the community has changed substantially. Beta diversity has to do with a larger geographic scale and comparing multiple local communities. In their paper, McGill and colleagues combine geographical and temporal scales into one large table with all possible combinations. The table leads to a summary of what we know about biodiversity in the Anthropocene, but more importantly highlights where we need to add more knowledge. Interestingly, we know quite a bit about biodiversity trends at the local scale and at the global scale, but the geographic scales in between are poorly understood.

All the authors of this article are well respected biologists, and most of us would stop after laying out the facts, but McGill and his colleagues go a step farther: after defining questions for future research they charge the scientific community with developing policy actions! And the final action they suggest is to “communicate the complexity to the public” (p. 111). This last suggestion indicates that they think that the public has a stake in biodiversity–i.e. that biodiversity contributes to the habitability of the territory over which the public has political responsibility.


FURTHER READING:
Robert J. Whittaker, Katherine J. Willis and Richard Field. 2001. “Scale and species richness: towards a general, hierarchical theory of species diversity.”  Journal of Biogeography, Vol. 28, No. 4, pp. 453–470. In this paper Whittaker, Willis and Field present an overview of the theory of biodiversity, focussing on ways of understanding spatial distribution of species. They offer a more intuitive classification of types of diversity in terms of local, landscape and macro-scale.
Peter Kareiva, Michelle Marvier and Robert Lalasz. 2012. “Conservation in the Anthropocene: Beyond Solitude and Fragility.” The Breakthrough, Winter, 2012. This (controversial) article on the “New Conservation” gives an idea of what is at stake in discussions of biodiversity, since preserving biodiversity has traditionally been a goal of conservation policy. The authors deal directly with the issue, touched on by McGill, of conflicts between biodiversity and human interests.
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