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
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.

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.

Logging intensity drives species richness loss

An area bigger than the entire Indian landmass is now used for timber production in the tropics. This logging is largely selective and leads to degradation with loss of specialist species and ecosystem services like carbon storage. However, many have also argued that these forests should be considered a priority for protection since they are at danger of conversion to other land-uses such as agriculture. In addition the impact of logging on tropical forest biodiversity appears to be less negative than other human impacts.

However, simply saying that logging is a less damaging option when compared to other way in which humans exploit forests misses a lot of what is going on. Logging operations differ massively from place to place in terms of the volume of trees cut for timber, the area affected by logging, the distance between logged and unlogged areas, I could go on… All of these differences have the potential to influence the the effect of logging on biodiversity.

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Forest regeneration provides cheap carbon and biodiversity benefits

First of all, hello again and apologies for my sporadic posting on here recently. I have now successfully defended my viva and have a few corrections to make but hopefully should be able to post on here a bit more regularly from now on.

One paper I read that really impressed me while on my hiatus from the blog was by my old commuting buddy James Gilroy and colleagues. This paper attempted to identify the potential biodiversity and carbon benefits of forest recovering in the Tropical Andes in Colombia, an area full of species found nowhere else many of which are under threat from agricultural conversion. The paper also attempted to look at the cost effectiveness of carbon payments for landowners who converted farmland to forest when compared to different land-use options like cattle farming.

Gilroy et al - Fig 1
Recovery of secondary forest carbon stock compared to that of pasture and primary forest (Taken from Gilroy et al. 2014)

I was actually quite surprised by what Gilroy and his team found. Their results suggested that carbon storage in recovering forests was fairly similar to that in mature forests in the area after around 30 years, much less than the 100 years or so that I estimated these stocks would take to recover in a previous study.

Gilroy et al - Fig 4

Gilroy et al - Fig 3
Relationships between carbon stocks and similarity of dung beetle and bird communities to primary forest communities (Taken from Gilroy et al. 2014)


More surprising still was that bird and dung beetle communities in the regenerating forests were fairly similar to those of mature forests, suggesting that they have high conservation value. Again previous studies have generally estimated that animal species that are forest specialists may take a long time to colonise secondary forests, and plants probably take even longer. The fast recovery times may be attributable to the relative closeness of recovering forest to intact forests in the study area, allowing immigration of  forest animals and increased likelihood of transportation of seeds from long lived tree species.

Gilroy et al - Fig 2
Relationship between the additional cost of undertaking forest regeneration and the price paid for carbon per tonne. The solid horizontal line shows where costs are equal to zero. This graph indicates that there are potentially net economic benefits for people undertaking forest regeneration projects when the carbon price is greater than $4 per tonne. (Taken from Gilroy et al. 2014)

More important than these findings though was the discovery that if forest regeneration schemes were implemented in the area, they could be more profitable to land-owners than current land-uses like cattle farming. This was true for all pastures in the area when carbon trading prices were greater than $4 per tonne of CO2 and given that the median price of carbon in 2013 was around $7.80 per tonne, paying for the carbon benefits of regeneration in these locations works out cheaply. This is the part that I thought was really neat, because all too often restoration schemes fail to account for the costs and benefits associated with such projects.

Given that the study area has fairly representative socioeconomic conditions to those found in the wider Colombian Andes, the results suggest that regeneration of cloud forest may provide a great opportunity for REDD+ carbon based conservation, which can deliver multiple environmental benefits at minimal cost. Though REDD+ has its critics it has the potential to transform forest conservation so we need to work hard to make sure it is done in the right way.

What traits drive response of birds to tropical land-use change?

Toucan for blog
Could fruit eating species such as the black-mandibled Toucan be disproportionately affected by land-use change?
Could fruit eating species like the black-mandibled Toucan be disproportionately affected by land-use change? (Photo credit to Ettore Bacocchi on flickr)

Everyone pretty much knows about the crisis of biodiversity loss facing the tropics.

In case you missed it tropical forests are being rapidly cleared, which human population increases and along with consumption. All this has lead to large losses of biodiversity in the tropics.

So far, so boring.

However, up until recently we didn’t have much of an idea how the characteristics of species in the tropics influenced their response to land-use change.

‘Why would we want to know that?’ – I hear you ask. Well if you’ve seen my blog before you will know that traits are  a good way of linking biodiversity change to changes in ecosystem function and services. This is the first step to working out the consequences of the massive changes in biodiversity we have seen over the last century. Simply put – we need to know this stuff.

Given what I think, it was great to find out at the recent BES 2012 annual meeting in Birmingham about a paper looking at how bird species with different traits respond to land-use change in the tropics.

Tim Newbold, a postdoc at the World Conservation Monitoring Centre in Cambridge, and colleagues compiled an impressive dataset of  >4500 records of >1300 bird species from 23 studies of  land-use change in the tropics. They then used data on habitat preferences, migratory status, diet, generation length and body size to determine how differences in these traits related to birds’ response to land-use change.

They found that long-lived, non-migratory, primarily frugiverous or insectivorous forest specialists were likely to be less abundant and less likely to occur in intensively used habitats.

Probabilities of presence and abundance relative to primary forest based on dietary preferences of tropical bird species
Probabilities of presence and abundance relative to primary forest based on dietary preferences of tropical bird species

Of these characteristics diet preference is perhaps the most easy to link to changes in ecosystem function and services.

The loss of insect eating species may impact the control of pest species with potentially negative consequences for tropical agriculture. However, this assumption depends heavily on pest species abundances not reducing in line with bird declines. It is also entirely possible that if pest species also reduce in abundance forest loss will lead to little change in crop damage.

The reduction in fruit eating bird species may have consequences for forest regeneration and maintainance of plant diversity. Many secondary forests that are isolated from primary forest have been shown to lack large seeded tree species. Any reduction in the abundance of fruit eating birds suggests another barrier preventing the recovery of plant species communities in secondary forests.

I really liked this paper. It shows the value of large datasets for making generalisations and the results are potentially important for investigating change in ecosystem function and services in tropical forest ecosystems. The good news is it looks like there is a lot more of this type of work on the way with the PREDICTS project aiming to do take a similar approach to many questions related to land-use change. I’m excited to see what they come up with next, provided they don’t scoop me in the process…