Beta-diversity – What is it good for?

A while ago I wrote a post asking whether everyone’s favourite measure of biodiversity, species richness, was useful. In it, I concluded that it is probably one of the bluntest, least informative measures of ecological communities we have and that we should try to use alternative metrics when possible. Recently, I started wondering about what other measures of biodiversity might be informative, and what they can be used for. And then a neat review of beta-diversity by James Jacob Socolar ( correction courtesy of James Gilroy on Twitter – thanks James!) and colleagues came out in Trends in Ecology and Evolution, so today I’ll focus on that, borrowing from some of their thoughts and hopefully adding some of my own along the way. In the future, at some point, I’ll write something about temporal changes in ecological communities at individual sites.

So, firstly what do I mean by beta-diversity? Beta-diversity broadly reflects the differences in community composition between sites.  Gamma diversity (regional diversity) is a product of both beta- and alpha-diversity (diversity at a single site). And there are lots* of different ways of measuring beta-diversity. The simplest metric for beta-diversity is termed ‘true beta-diversity’ and was defined by Whittaker in 1960 as:

\beta=\frac\gamma\alpha

This metric is perhaps the easiest to interpret, but it also needs a reliable estimate of gamma diversity, so may be difficult to use in practice. Using this method allows the relationship between alpha and gamma diversity to be investigated. Other measures can be based on dissimilarity matrices, identifying pairwise differences between sites. These metrics can then be used to look at drivers of these differences, such as the geographic distance between individual sites and environmental differences. However, dissimilarity matrix methods don’t allow the relationship between alpha and gamma diversity to be investigated. The above explanation probably explains the ubiquity of species richness as a metric in ecology – we can all (more-or-less) agree on what it means.

Changes in beta-diversity when humans alter natural landscapes can be unpredictable. When human disturbances are patchy, such as in the case of selective logging, beta diversity has been shown to be stable or increase due to an influx of generalist species in forest gaps.

berry_et_al
Differences changes in tree community dissimilarity with increasing distance between sites in unlogged and logged forest. Note that logged forests show a more rapid rate of change, suggesting that logging results in more variable ecological communities. Figure modified from Berry et al 2008.

In contrast, when human land-use change results in the conversion of natural ecosystems to a relatively homogeneous system in which only a small subset of species can survive, beta-diversity tends to decrease. Examples of such drivers include agricultural conversion and urbanisation. However, even high intensity farming can result in an increase in beta-diversity particularly if species populations decrease leading to greater dissimilarity purely as a result of random processes.  In summary, the response of beta-diversity to anthropogenic change appears to be relatively idiosyncratic.

All of this is well and good, but what use is beta-diversity to practical conservation? At first inspection, this is not clear. The general perception of species richness is that more species = better**. Does higher beta-diversity = better? Well, no, not necessarily. Given that the aims of conservation vary from place to place, it is not surprising that how beta-diversity can be used also varies.

The most obvious use of beta-diversity is in spatial planning of protected areas. In landscapes which show a high spatial turnover of species, managers might favour the use numerous distinct reserves to capture this variation. However, in a landscape in which beta-diversity results from differents in species richness a single protected area might be favoured. Also, if a natural ecosystem is particularly distinct from other candidate sites it may be considered a priority for protection.

High beta-diversity can also result from dispersal limitation in a landscape. For example, secondary forests in fragmented landscapes plants with seeds dispersed by wind may colonise sites more readily than those dispersed by animals that may not cross non-forest areas. So in cases where beta-diversity amongst patches of a similar habitat in a fragmented landscape is high, this may point to the need for restoration to increase connectivity. Successful restoration may result in a decrease of beta-diversity as dispersal between patches increases. For example, Renata Pardini’s work has shown that the small mammal communities of more highly connected fragments of Atlantic forest are more similar to other patches than unconnected fragments. However, as far as I know, there is relatively little evidence empirical that restoration has similar effects.

In the paper I mentioned earlier, Jacob Socolar and colleagues suggest that beta-diversity may also be useful in informing the land-sharing vs land-sparing debate (which i have previously written about here, here an here). They argue that the use of beta-diversity as part of this debate may show that heterogenous landscapes that include agri-environment schemes, management of natural systems and high intensity agriculture are better at maintaining alpha- beta and gamma-diversity. Thus, the incorporation of metrics other than population sizes of species, the classic approach for such comparisons, may produce different conclusions to current studies, which largely suggest land-sparing as a favoured approach. As always with conservation, this depends on what you think we should try to protect. Should we focus on particular species? Or should we look attempt to conserve the processes that maintain coarse-scale diversity?

For me, the key point that the paper makes is that even though two recent high-profie studies recently suggested local-scale alpha-diversity is relatively constant***, global scale gamma-diversity is declining. This suggests that rare species are getting rarer and common species are increasing in abundance. If we can work out how and why beta-diversity responds to land-use changes we can better understand how to conserve gamma-diversity. However, before we do that we need to develop methods to upscale from alpha to gamma diversity and determine how different disturbances alter beta-diversity. Novel approaches offer the potential to solve this problem, but substantial testing is needed to determine how useful they are.


*Patricia Koleff identified 24 metrics for use with presence-absence data and my  old CEH office mate Louise Barwell tested 29 different beta-diversity metrics that incorporated abundance data. Give both of these papers a read, they’re well worth your time.

**I don’t agree with this perception, I’m just extrapolating based on things I have heard from a few people. Deeply unscientific, I know.

***I saw Andrew Gonzalez present some work on the problems of these two studies at the 2015 British Ecological Society annual meeting and hope to post something when the paper comes out. I can’t say much, but it was fascinating stuff.

 

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Tropical deforestation causes dramatic biotic homogenisation

Although species richness is most ecologists go-to metric to ‘take the temperature’ of an ecosystem, it is not always the most useful. Even when species richness doesn’t change much over time many species may be being added to or lost from a community. Changes in human land use can cause loss of a particular taxonomic or functional groups, which can have important implications for ecosystem processes such as pollination or seed dispersal. This non-random loss of species as a result of human impacts can result in biotic homogenisation – where the communities in different location become more similar to each other. Biotic homogenisation has been seen all over the world in response to drivers like urbanisation, agricultural land-use change, and eutrophication.

However, up until recently, there had been little work on how biotic homogenisation impacted multiple taxonomic groups across landscapes. Work has also been almost entirely carried out at a single spatial scale. Given that taxonomic groups are likely to differ in their response to disturbances and that landscape scale processes may play a critical role in species persistence. Fortunately last week a paper was published by Ricardo (aka Bob) Solar and colleagues in Ecology Letters that attempted to fill these knowledge gaps.

Specifically the paper attempted to determine how much of the change in community composition as a result of changes in tropical forest land-use change were attributable to replacement of species (termed turnover) and loss of species (termed nestedness). Bob and his colleagues did this for birds, dung beetles, plants, orchid bees and ants at 335 sites (!) in 36 different landscapes in 2 regions of Brazil. The sites used were either primary forest experiencing varying degrees of human disturbance, secondary forests, cattle pasture or arable farmland.

In short the paper shows that:

  • Species richness decreases as land-use intensity increases
  • Differences in community composition between deforested sites were much lower than for forested areas
  • Species turnover caused the majority of changes in community composition, but loss of species became more important as the intensity of disturbance increased
Bob_Solar_Fig5
The importance of loss of species (nestedness) in biotic homogenisation increased as intensity of disturbance increased at both (a) local and (b) landscape scales. Taken from Solar et al. 2015.

For me, the most interesting message of the paper the changes in community composition were largely attributable to replacement of species. This suggests that as species are lost following disturbance, colonisation of generalist species initially causes relatively little change in species richness. However, as land-use intensity increases the contribution of species loss to alteration in community composition became more important suggesting that communities in these locations tend to be made up of generalist species that are tolerant to human disturbances.

Conversion of forest to agricultural use led to much greater biotic homogenisation than degradation.
Conversion of forest to agricultural use led to much greater biotic homogenisation than degradation. Photo courtesy of Bob Solar.

Interestingly, the paper also shows that provided that forest cover is maintained there was relatively little biotic homogenisation. So while it is obvious from previous work that the maintenance of undisturbed forests is vital to conserve tropical forest biodiversity, it is also obvious that degraded forest can play an important role in conservation.  This is especially true where few undisturbed forests still exist or degraded forest is widespread such as in SE Asia and Central America.

This work effectively shows that taxonomic homogenisation is occurring at multiple scales as a result of human land-use change. The next step is to see what types of species are being lost/retained. This means looking at the interaction between species traits and the land-use gradient (see more on that here). Previous work has suggested that body size and feeding preferences may play an important role in determining whether bird species can persist in degraded forests. Looking at this will allow us to gain a greater understanding of how biodiversity change may alter ecosystem processes and ultimately the ecosystem services on which we all depend.