Biological diversity is raising, for scale’s sake!

Estimating the current trend in direction of biological diversity (or ‘biodiversity’) is not as easy as it looks.
Like the assessment of CO2 concentration in the atmosphere, biological diversity status is key in understanding the amplitude of anthropogenic impact on natural systems and to inform policy.
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However, the assessment of these two metrics of human impact differ in that one is more easy to assess than the other. On one hand, the measurement of atmospheric CO2 is unresponsive to the spatial scale at which it is carried out. As a result, we can measure CO2 concentration in a random cubic micrometer of air, or we can sample the whole atmosphere volume and yet we will still obtain the same value (by the way, we are near 400 ppm; ). On the contrary, biological diversity (whatever the facet that you measure) estimates are highly dependent on the scale at which they are carried out. This, apparently, unimportant difference explains why nobody challenges the increasing trend in CO2 concentration in the atmosphere, whereas there are mixed indications on the current direction of trends in biological diversity.
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There is an increasingly well-documented global trend in biodiversity loss for marine and terrestrial domains and across the majority of taxa [1]. On the other hand there is mixed evidence for biological diversity decrease at a local scale [2]. However, even though the lack of evidence for biological diversity loss at local scale can be seen as a positive fact for natural systems health, it is wiser to delve deeper into the matter before drawing comfortable conclusions.
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The intensification of trade and transport causes an increasing rate of invasions of exotic taxa which, in turn, leads to biotic homogenisation at a local scale. Biotic homogenisation, defined as “the process of gradual replacement of native biotas by non indigenous and locally expanding nonnative species” [3], may potentially lead to a global loss (extinction) of species without coping with any loss at the local scale. A subtle effect of this process is that in some regions characterised by low diversity due to isolation (i.e. islands) the species diversity may actually increase as a result of invasions of non-native species! Given the local increase in biodiversity, should conservationists and nature lovers sleep soundly at night? Unluckily for them no! Indeed, biotic homogenisation, which supports biological diversity at local scale, implies a substitution of less competitive species by more competitive ones (in a determined natural system with certain environmental characteristics). Well, it is extremely unlikely that the “invader” carries the same functional features of the native species, with potentially catastrophic consequences for ecosystem functionality and services. As a result, studying the local trend in biodiversity, we may potentially observe a reassuring gain underpinning a biological catastrophe. A quick and intuitive example can be drawn considering the conversion of Amazon rainforest into cattle pasture. Countless times the Amazon conversion has been addressed as a huge threat for biodiversity conservation and Earth system balance. However, when scientists measured the response of soil microbial diversity to this conversion, surprisingly they found an increase after the conversion [4]. In other words, patches of rain-forest converted to cattle pastures showed a higher soil microbial diversity due to the conversion. This outcome puzzled the scientists for a while, but, a prompt further analysis of the data showed that these microbial communities exhibited high inter-community similarity, where specialized forest bacteria had been substituted by more generalist bacteria. Therefore, the increase in local (alfa) diversity together with a decrease in (beta) diversity between communities brought about an overall net loss in microbial diversity (gamma; have a look at the image below where I show this trend using rarefaction curves).
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Rarefaction curves of an hypothetical high beta diverse site (red) and low beta diverse site (blue).
Rarefaction curves of an hypothetical high beta diverse site (red) and low beta diverse site (blue).

The take home message is that “the biodiversity trends are as bad as expected” [5], the illusion of good news is just a problem of scale!

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1. Butchart, S. H. M. et al. Global Biodiversity: Indicators of Recent Declines. Science 328, 1164–1168 (2010).
2. Sax, D. F. & Gaines, S. D. Species diversity: from global decreases to local increases. Trends in Ecology & Evolution 18, 561–566 (2003).
3. Olden, J. D. & Poff, N. L. Ecological processes driving biotic homogenization: testing a mechanistic model using fish faunas. Ecology 85, 1867–1875 (2004).
4. Rodrigues, J. L. M. et al. Conversion of the Amazon rainforest to agriculture results in biotic homogenization of soil bacterial communities. Proc. Natl. Acad. Sci. U.S.A. 110, 988–993 (2013).
5.Teyssèdre, A. & Robert, A. Biodiversity trends are as bad as expected. Biodivers Conserv 24, 705–706 (2014).

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