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.

 

Land-sparing/sharing in tropical logged forests

The dichotomy between land-sharing and land-sparing has been used a lot in studies on the impacts of agriculture on biodiversity to compare between relatively intense, highly productive agriculture that spares natural ecosystems from conversion and extensive, wildlife friendly agriculture with lower yields. The comparison between these two extreme ends of the land-use spectrum could potentially be applied to a whole host of problems relating to how we use land, such as urban planning, electricity production and timber production. While making changes to a manuscript I have been pondering the last of these problems a bit, in the context of tropical selective logging.

Our recent preprint, as well as in 2 other papers in the last year (here and here), showed how the impact of logging biodiversity and carbon storage vary over a gradient of logging intensity. Where large volumes of wood are extracted species richness of trees and animals are negatively impacted, animal populations are reduced as is carbon storage in tree biomass. This gradient of logging extraction represents potential different intensities at which tropical forests could be logged, extensive and low intensity, or high intensity and spatially concentrated. Though it is a topical subject (indeed there has been an NCEAS working group set up to deal with it and who have a flashy website here) there has been little empirical study of land-sparing/sharing in the context of tropical forests, with the only study published so far suggesting that land-sparing presents a better option for birds, dung beetles and ants in Borneo. Typically the gradient of timber extraction is calculated as the volume of trees felled per hectare. However, there are a number of problems that make this metric far from ideal.

Firstly, it tends to be calculated at very large scales, often covering an entire forest concession of hundreds of hectares. To get a better idea of the impact of logging across a gradient the scale of the measurement needs to be reduced so that variation between plots can be examined. Also, though the volume of trees felled obviously tells us quite a lot about the gradient of disturbance, it doesn’t actually tell us what we want to know – the yield. Just as crop yields are what is most important for a farmer the yield of timber from a logging concession is the primary concern of logging companies. Importantly the volume of trees harvested is not always very good at measuring this, since some logging operations are more efficient than others. For example, some trees that are felled but never actually make it to the sawmill. Such wastage is more likely in unplanned logging when lack of co-ordination can result in logs being left behind after being cut, and as a result the yields per hectare can be lower than measures of logging intensity might otherwise suggest.

Though there is currently some discussion of whether sharing or sparing are likely to result in better outcomes in tropical logged forests, the truth is that we currently don’t know much since we lack the sufficient evidence. In order to get this much needed evidence we need to make sure that when studies are designed to answer the land sparing/sharing question in tropical forests they use timber yield, not logging intensity as their gradient and species density as their response variable. Doing this will require closer collaboration with logging companies in order to get detailed information. Some people have cited the fact that logging appears to have relatively little effect on species richness at low intensities, however as I have discussed these relatively modest reductions in species richness may mask large changes in what species are present. As such species richness has no place in the debate about the configuration of landscapes in the context of tropical logging.

In addition to the populations of priority conservation species any future assessment of land-sharing/sparing must recognise that recovery times for carbon and timber tree populations are likely to be longer when logging intensities are high. Given this it seems likely that in order to reach as many goals as possible logging intensity should be high enough to reduce the area impacted but low enough to allow recovery within cutting periods – often around 30 years. Finding this balance will be difficult in the current data vacuum.

Species richness – what is it good for?

Species richness is the iconic measure of biodiversity. It is simple to interpret* and it is one of the most commonly measured metrics in ecology. From the early beginnings of ecology Darwin, Wallace and von Humbolt noted the striking differences in the number of species found in different places and ecologists are still fascinated by it . However, over the last few months I have begun to question how useful it is for applied research.
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Knowledge gaps for urban land sharing & sparing

Photo courtesy of villes on flickr.
Photo courtesy of villes on flickr.

If you are reading this there is a good chance that you live in a city.

You’re not alone. About half of all humans now live in cities.

These cities will continue to grow for the next century and the rise of the megacity with more than 10 million souls will continue apace.

Because of this we need to think seriously about how we plan our cities so they can fulfil our needs as well as possible. They should be easy to get around, they should be a pleasant place to live and they should be as nature friendly as we can make them.

When you talk about cities and nature people often give you odd looks. “But surely all the nature is out there, in fields,” they say. They have a point. But when it comes to direct experience of nature most of us do that in cities.

Regular readers will know I have form in this area and this week a really interesting paper came out in the journal of Applied Ecology looking at the potential for using the land-sharing/land-sparing idea for urban planning.

I have to be honest that the initial thought when I saw this paper was “someone’s robbed the idea from my blog post!” After I calmed down and actually read the paper I realised that the authors had thought about it all a hell of a lot more than I had. I could hardly accuse them of stealing my ideas – after all there are a finite number of subjects out there, much like the material for jokes. With enough monkeys and enough typewriters and all that…

Anyway, the paper points out the similarities between the design of landscapes for agricultural production and urban areas, summarised below.

Sparing vs sharing urbanPanels (a) and (c) are the extreme ends of the land sparing continuum for agriculture and urban planning. Panels (b) and (d) are the land sharing ends of this spectrum.

The paper then discusses what we know about urban design in the context of the land sharing/land sparing debate. The answer is (spoiler alert!) not much.

To fix this the authors suggest 4 key areas we need more work on:

  • Understand how biodiversity reacts to urban intensification, particularly at the lower end of the scale.
  • Investigate how the shape and arrangement of fragments of habitat influences extinction risks.
  • Understand which urban ecosystems and designs are best for conserving populations and ecosystem services.
  • Undertake whole city analyses to compare between different city layouts and determine their ecological impact.

We live in an ever more crowded world, where people are shifting towards cities so we need to think about this stuff.  Go and read the paper.

 

Land sparing vs land sharing for town planning

Would you rather live here?

Planning regulation in the UK is set to be relaxed, allowing more homes to be built for the millions who can’t afford them at the moment. Planning minister Nick Boles recently evoked the right of every Brit to own a house with a small garden in his plans to increase the amount of land available for development from 9% of England to 12%.

It doesn’t sound like much when you put it like that. But that 3 percentage point difference represents nearly 4,000 square kilometre, an area around the size of Suffolk. Or French Polynesia if you’re feeling adventurous.

This is a big chunk of an already human dominated landscape. The UK  has one of the highest population densities in Europe and more than 60% of the landscape is given over to farming and urban areas.

Pressure groups like the Campaign for Rural for Rural England have objected to the recent changes in planning regulation but have been characterised as selfish, small-minded NIMBYs.

However, there has been little mention of nature or biodiversity as part of this debate. If we want to debate this it is important we ask how we should  plan our settlements to cause least harm to nature.

Similarity to debates on agriculture

In many ways this is similar to debates on land sparing versus land sharing in agriculture or to some extent the debates on strategies for energy production. We should be looking to divide our landscape up in a way that benefits society and balance this with the need to protect biodiversity.

I am constantly amazed that this issue is barely discussed with regard to town planning.

Should we live in settlements that cover large areas, that are spread out and have gardens? Should we live in settlements that are dense, most people living in  flats and fewer private gardens but with more public spaces? Or should we do something in between?

Would you rather live here?
Would you rather live here?

I tend to think that more intensive housing would probably be a quite good thing, the UK’s cities are already sprawling (though not like the US) and messy. It would probably bring biodiversity benefits as cities would have a smaller geographic, and carbon, footprint. However, there is very little primary research out there to base my opinions on, so at the moment I’m largely speculating.

For example the city I visit most outside of the UK is Bilbao which has a population of around 400,000, with a population density nearly twice that of London. This means you can more or less walk to the countryside from the city since it doesn’t spill out all over the place. This should benefit species that are specialists that might be sensitive to alteration of their environment by humans and would therefore suffer as a result of large suburban areas.

Or here?
Or here?

Most people in the UK don’t share my views. The majority of people probably want a house with a garden in a suburb somewhere. They also associate flats with poverty or old age, neither of which are exactly positive selling points.

None of these solutions will suddenly make the UK into a paradise for nature. Britain still has a population density twice that of its nearest neighbour France and is only topped by the Netherlands and Belgium in Europe.

We should, though, be thinking more strategically about how we plan our settlements. But before we get ahead of ourselves we need to carry out more work looking at how the density of settlements can affect biodiversity.

How I learned to stop worrying and love nuclear power.

nuclear

Biodiversity conservation by and large boils down to decisions about what we do and where we do it. Land use change is the major driver of biodiversity loss globally, mostly as a result of agricultural expansion. The discussion surrounding how to best divide up the landscape to best benefit biodiversity whilst meeting target of food production will run and run, but the land-sharing/land-sparing argument is equally applicable to other subjects.

One of these is power production. We all know that we are meant to be reducing emissions in the face of climate change. Some governments are doing this, while many others don’t really care. Meanwhile, with increasing global population it seems unlikely that energy demand will be reduced, at least in the short term.

To anyone reading this blog the consequences of not changing energy policies will probably be fairly clear. Increased severity of climate change will push species ever further polewards and disrupt the synchrony between species. Climate change could also disrupt agricultural production, which would be disastrous given that global human population is set to increase for at least the next 50 years. Given all this, it should be a no-brainer that we switch to less carbon intensive means of producing energy.

Most environmentalists would generally say that we should replace dirty coal and gas fired power stations with clean renewable energy generation. I agree, but only partially.

Renewables bring their own set of problems. Firstly, can they really meet our current energy demands? Probably not. It would need extremely quick uptake of green tech to do this. Even Germany, the EU country with the greatest percentage of its energy produced by renewables, can only manage 25%. Relatively speaking, this is great. However, they still need to do more.

Also there is the issue of space. How will we use our land to produce energy? Most renewables would take up relatively large chunks of land compared to the standard power station. Firstly, biofuels (which are a terrible idea for all sorts of reasons, but that’s the subject for another blog post) would need massive areas to be grown. They would also compete with agriculture for land, spelling problems for food security. Damming of rivers, like that recently announced for the Mekong, completely screw up the movement of aquatic species thus interfering with migration and breeding cycles.  All renewable energy production methods will require more space than carbon intensive methods (for a more detailed analysis see David MacKay’s excellent TED talk below). From a UK perspective it appears the best bet in terms of energy produced per square mile would, surprisingly, be solar.

However, I think the best idea would be to mix renewables and nuclear. This would allow for greater energy production for the area used than a wholesale switch to renewables and would still reduce carbon emissions. If this policy was implemented harm to biodiversity would be reduced and there would be less threat to food crops from the expansion of biofuels. To me this seems blindingly obvious. There are, thankfully, other conservationists and green groups that have seen the light but in general the green movement seems to be opposed to nuclear.

I find this extremely frustrating. Particularly the backlash following the tsunami in Japan last year. Both Japan and Germany are decommissioning their power plants and other European countries are not planning to replace their ageing reactors. We need to make hard decisions, often choosing between the least bad options. We can’t live in a magical land where renewables can provide the energy we need ad infinitum. Those of us that realise this should pressurise groups like Greenpeace and the various green parties throughout Europe into rethinking their policies. Without this rethink, they could be doing more environmental harm than good.

Land sharing vs land sparing – meeting agricultural and biodiversity goals

poppy field

Earlier this month I went to the 2012 European Congress of Conservation Biology where the best session by far was one on land sharing vs land sparing (see a summary of the session by Joern Fischer here). This session was inspired by the Science paper by Ben Phalan and resulting back-and-forth.

The main idea behind the paper was that with increasing global population we need to increase food production. We can either do this by increasing the area of agriculture or intensifying production in the agricultural land we currently use. To limit the impact of both these options on biodiversity we could use wildlife friendly farming, termed land sharing. Another option is to spare natural ecosystems from conversion in the case of intensification, termed land sparing.

Thus land sharing aims to integrate goals for food production and biodiversity protection on the same land, while land sparing aims to separate intensive farming from protected ecosystems at the larger scale (A caricature of this continuum is provided below) .

Conceptual diagram of the land sparing-land sharing continuum, taken from the Fischer et al paper found here

Phalan et al took these options and tested their potential effects on bird and tree populations in Ghana and Northern India by looking at landscapes which represented these differing strategies. Using this data they plotted yield against species density to define how populations may change with increased yield. These changes allowed them to classify species as ‘losers’ or ‘winners’ following agricultural conversion as well as defining the land use options most likely to be beneficial for maintenance of their populations.

Proportion of bird and tree species classified as ‘Winners’ which would benefit under land sparing (Pink), or land sharing (Light blue), and ‘Losers’ which would benefit from land sparing (Red) and land sharing (dark blue). Grey represents species with more complex yield-abundance relationships. Taken from Phalan et al

On the whole they found that land sparing was the best option for most species, particularly for those species with small global ranges.This is important since it is largely these species that are considered to be conservation priorities.

However, some people have interpreted this as meaning that the authors advocate land sparing in all situations. Even if they do, it is obvious that land sparing might not be the best strategy in all situations. Different landscapes have different histories of land use, which will have inevitably had an effect on biodiversity and consequently what we see as priorities for protection.

For example, much of Western Europe has been cultivated for centuries if not millennia and has little forest cover. As a result the biodiversity we value here consists largely of  generalist species which thrive in low intensity farmland and require some form of agricultural practice for persistence. Meadows are a great example of a cultural landscape that is highly valued in Europe but requires disturbance, such as grazing, to exist. In situations like these it is entirely possible that land sharing may help to boost the populations of priority species.

Land sparing – great for pristine forest, rubbish for cultural landscapes? (Photo credit goes to Jenny Downing on flickr)

In addition the taxonomic group which you aim to protect will determine the scale at which management should be undertaken. What constitutes land sparing for an invertebrate will not be the same as that for a bird. Phalan et al’s paper arguably chose taxonomic groups which would be likely to benefit from large scale land sparing, it will be interesting to see how research into other taxa differs in their findings.

Ecosystem services will also be affected by these different land use strategies. Land sharing may favour services which rely on fragments of semi-natural habitat in order to be distributed throughout the landscape, such as pollination. Meanwhile services which are supplied far away from ecosystems which generate them, such as carbon storage and water purification, will be favoured by land sparing.

Though food production is an obvious priority, lots of conservation essentially adds up to how best to use particular parcels of land to meet multiple goals. Land sparing vs land sharing could be applied to urban planning and energy production to name just two. Hopefully, if my PhD doesn’t get in the way, I will explore this issues further in the coming weeks.

The land sparing vs land sharing debate is obviously set to run and run. However, it seems likely that with more research we may be able to form some generalisations. In areas where there are many species which depend on forests, or any other ecosystem incompatible with agriculture, land sparing may be best while land sharing may work best in areas with a long history of extensive farming and little forest cover.

Much work is needed to determine the consequences of these options for ecosystem services and also the social implications. For example, could promoting land sparing further add to our lack of connection with nature? What exactly is the relationship between provision of particular ecosystem services and these different options? I don’t have the answer to these questions. However, given that agriculture is the biggest single threat to biodiversity, but is something none of us can live without, I hope we will at least have a few more answers in the near future.