Local species richness may be declining after all

Recently two papers seemed to turn what we thought we knew about changes in biodiversity on their head. These papers by Vellend et al. and Dornelas et al. collated data from multiple sources and suggested that species richness at local scales is not currently declining. This was counter-intuitive because we all know that species are going extinct at unprecedented rates. However, it is possible that the introduction of non-native species and recovery of previously cultivated areas may offset extinctions leading to relatively little net change in local species richness.

This week a paper has been published that calls these findings into question. The paper by Andy Gonzalez and colleagues published in the journal Ecology, suggests that there are three major flaws with the analyses. These flaws mean that the answer to the question ‘Is local-scale species richness declining?’ currently remains unanswered and is unanswerable.

The papers of Vellend et al. and Dornelas et al. were meta-analyses of previously published papers. One issue with meta-analysis is that it is very prone to bias. Like any study if the samples (in this case ecological studies) are not representative of the population (in this case locations around the globe) then any results will be flawed. To test the representativeness of the datasets used by Vellend and Dornelas Gonzalez et al. examined how well they represented biodiversity and threats to biodiversity. This analysis (see below) showed that the papers were not representative of biodiversity or the threats faced by biodiversity (though curiously, the analysis of Dornelas et al. showed an overrepresentation of areas highly impacted by human impacts).

Gonzalez_1.png
Figure 1 – Spatial bias of the Vellend et al. (2013) and Dornelas et al. (2014) data syntheses. For more information see the paper by Gonzalez et al. (2016).

The paper also suggests that using short time series can underestimate losses. By analysing the effect of study duration and changes in species richness (see below) Gonzalez et al. claim that increases in study duration were correlated with a decline in species richness. This supports previous theory which suggests that there is often a time lag between disturbance events and species extinctions – termed ‘extinction debt.’ However, I’d be intrigued to see the results of removing the studies with the longest duration from this analysis since the authors admit that the analysis is sensitive to their inclusion. I’ve seen recent similar work that suggests the same kind of relationship might be seen for studies monitoring individual animal populations.

Gonzalez_2
Figure 2 – The effect of study duration on apparent changes in species richness.

Thirdly, Gonzalez et al. assert that including studies in which ecosystems were recovering from disturbance (e.g. regrowth on former agricultural fields) without taking into account historical losses that occurred during or after the disturbance biases estimates of change. The paper by Vellend et al. in particular combined studies of the immediate response of biodiversity to disturbances such as fire and grazing along with studies of recovery from the very same disturbances. Gonzalez et al. show that once studies of systems that were recovering are removed from Vellend et al’s analysis there is a negative trend in species richness changes.

The biases prevalent in the Vellend and Dornelas papers lead to Gonzalez et al. to suggest that the papers cannot conclude what the net changes in local species richness are at a global scale. However, they note that the results of Dornelas and Vellend are in sharp contrast to other syntheses of biodiversity changes which used reference undisturbed such as those by Newbold et al. and Murphy and Romanuk which reported average losses of species richness of 14 and 18% respectively.

In their conclusion Gonzalez et al. suggest that though meta-analysis is a powerful tool, it needs to be used with great care. Or to put it another way, with great power comes great responsibility. As someone who regularly uses meta-analysis to form generalisations about how nature works I completely agree with this statement. Traditionally scientists have used funnel plots (graphs with study sample size on the y-axis and effect size on the x-axis) to identify biases in their analyses. I’ve always been skeptical of this approach, especially in ecology where there is always a large amount of variation between sites. In the future syntheses would do well to follow the advice of Gonzalez et al. and really interrogate the data they are using to find any taxonomic, geographic, climatic or any other biases that might limit their ability to generalise. I know it’s something I’ll be taking more seriously in the future.

Gonzalez et al. also point out that most ecological research is carried out in Europe and North America. If we want to monitor biodivesity we need to increase efforts in biodiverse tropical regions, as well as boreal forests, tundra and deserts. We need to identify where these gaps need filling most and then relevant organisations need to prioritise efforts to carry out monitoring. I am positive that this can be achieved, but it will cost a lot money, needs to be highlighted as a priority and will ned a lot of political good will. Even with this effort some of the gaps in biodiverse regions, such as the Democratic Republic of Congo, will be extremely difficult to fill due to ongoing armed conflict

My take-home message from this paper is that we need to be more careful about how we do synthesis. However, I also think that species richness isn’t the only metric that we should focus on when talking about biodiversity change. Studies have shown that measures of the traits of species present in a community are generally more useful for predicting changes in ecosystem function than just using species richness. Species richness is the iconic measure of biodiversity, but it probably isn’t the best. Ecologists should view species richness in the same way as doctors view a thermometer – it’s a useful tool but you still need to be able to monitor blood pressure, take biopsies and listen to a patient’s lungs before you diagnose them*.

 


 

*Thanks to Falko Bushke whose analogy I stole from a comment he made on my blog post here.

 

“Like walking through an open cemetery”

“I have been working in human-modified tropical forests for the past 14 years, but seeing these fires first hand was devastating,” wrote Erika responding to one of my questions “The smell of wet soil was gone and I could only smell smoke…even the usual cacophony of forest sounds disappeared…it was like walking through an open cemetery.”

Erika de Berengeur Cesar, an up and coming Brazilian forest researcher, works at Lancaster University. For last two months, she has been slogging away in the field collecting data for her team’s project on human-modified forests. But this year hasn’t gone to plan. This isn’t a case of bad planning though, as with so may projects – 7 of her 20 sites had burned in some of the most widespread fires in recent times. After seeing her tweeting about this, I thought “I need to write something about this. It feels important.” So we fired a few tweets and emails back and forth, with Erika fitting answering my questions between her days in the field. After I had waited impatiently for a couple of days, Erika messaged me:

“Sorry, trying not to work weekends…not going very well though…Today I just learned that 9 of my 20 plots have burned.” 2 more plots. Aside from the wider situation, this was the stuff of researcher’s nightmares.

B260 T5 - Before and after the fire.1
Erika de Berengeur Cesar in one of her logged plots before it burned (top) and the same plot after recent fires (bottom). Photo courtesy of Erika.

Fires in Brazil reached record levels in 2015, with more than a quarter of a million separate fires recorded. However, these fires are not generally ‘natural’ – “Fires in the region always have a human ignition source.” Erika told me “They are used in slash-and-burn agriculture, to clear pastures of weeds and also to burn downed timber in newly deforested areas.” This year’s strong El Niño has caused drier conditions than normal making it “easier for agricultural fires to escape the targeted area and sweep through the forests.” Indonesia is facing a similar problem, where forests have been burned to clear space for new oil plantations, in what the Guardian’s George Monbiot  has described as the ‘greatest environmental disaster of the 21st century – so far.’

When I queried why it matters that the forest is burning, Erika was clear what the major issue is – the loss of unique biodiversity. “Every year over 100 new species are found in Amazonian forests. To see all this going up in smoke is a crime against humanity. It is a tragedy.”

“How are these fires likely to affect biodiversity?” I asked.

“The Amazon has not co-evolved with periodic fires…This means that Amazonian forests are not used to these events and…do not cope very well with it. In terms of plant communities, there is a sharp increase in the abundance of pioneer species, while high-wood density climax species disappear….Fires negatively affect…rare bird species, and the habitat specialists, such as the ant-following insectivores and the terrestrial gleaners. Overall, burned forests are significantly less diverse than their unburned counterparts.”

Amazonian forests that have burned repeatedly may eventually come to resemble more open savannahs and contain  very different species to relatively undisturbed old-growth forest.

But it’s not just biodiversity that is affected by these fires, but humans as well. In Indonesia there were evacuations of children by the navy, although some of the children, according to reports, still died from breathing difficulties . In Brazil the fires have “affected many of the local people…who reported a number of respiratory problems, such as dry cough, difficulties in breathing, and sore throats,” according to Erika. “People had to spend days building fire breaks to protect their land, instead of directly working on their crops.” People working on these farms already have a tough life as it is, without having to worry if their source of income will go up in smoke.

So what will happen to these forests in the future? Given time and, vitally, protection they can recover but Erika thinks this is unlikely “These burned forests may never recover. After the fire, several large trees die, creating a number of gaps in the forest canopy, through which more light and wind can reach the forest floor, making it drier and, as a consequence, more vulnerable to further fire events.”

The research Erika and her team are carrying out will help to answer the question of how burned forests recover but it is obvious that degraded forests, such as these, need to be seen as a greater conservation priority. More than 50% of the globe’s forests are degraded in one way or another. We cannot afford to only protect primary forests anymore.

Edit: I got an email from Erika a bit ago after I asked her what the best solution would be. I thought I should include it here:

“Funnily enough there are already quite a few good policies in place. The problem is that none is followed. For example, every year there is a ‘burning calendar’ establishing when farmers can use fire to burn their pastures or their croplands. During the peak of the dry season, the use of fire is forbidden. In 2015, given the extreme drought, some states even extended the prohibitive period. So all quite reasonable and good, right? The problem is that no one follows this rules and there is no law enforcement in place. So people carry business as usual and the forests carry on burning. To put in practice the existing laws would be the best solution.”


If you want to read more about the situation in Brazil take a look at the excellent article Erika has written  for ‘The Conversation.’

There are also a pair of videos that Erika’s team have made documenting the fires that you can see here and here.

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.

 

Impacts of selective logging of tropical forests on tree damage, biomass and tree species richness

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A few days ago we put some of my thesis work on the impacts of tropical forest logging on the preprint server PeerJ. The work is currently in review elsewhere but I thought I should put up a blog post about our findings, even if they change a bit after the review. I am really pro the idea of making results available as soon as possible so that they can be read and cited. I have lost count of the number of times a piece of work I have seen presented at a conference and wanted to cite has taken 1-2 years to come out as a paper. In short I think preprints are the future, so feel free to read, comment on and critique ours over at Peerj (and you can even cite it if you like).


A fifth of tropical forests have been logged in the recent past. Though logging is an important source of timber and jobs it also faces questions about its long-term sustainability particularly in its impacts on biodiversity and carbon. However, as I have written before, the results of studies on the impact of logging are very variable making generalisation difficult. Previous meta-analyses of the impacts of logging have indicated that biomass losses can be as high as 66% or as low as 4%, while tree species richness may be reduced by as much as 53% or show increases of up to 27%. However, none of these meta-analyses of the impacts of logging on tree biodiversity or biomass have explored the potential reasons for these differences. A recent study showed that differences in the impact of logging on animal species richness was explained by the variation in intensity of logging at sites, measured as the volume of wood removed per hectare. Interestingly this study showed that while species richness generally declined for amphibians, mammals and invertebrates with increasing intensity, bird species richness actually increased slightly.

Another recent piece of work by Jake Bicknell and colleagues has shown that a method called reduced impact logging (RIL), a technique which aims to reduce logging damage by altering extraction methods, reduces the negative effects of conventional logging techniques on animal population sizes. This is an important result as RIL has long been championed as a potential solution to the problems of logging sustainability and Jake’s work is the first to really show that it has positive effects across a number of sites.

So while two recent meta-analyses have indicated that logging intensity and method may have a profound influence on forest biodiversity no similar work has been done for trees, despite the fact that programmes that focus on conservation of carbon, such as REDD+, need this evidence to implement policy. So to fill this knowledge gap we performed a meta-analysis to determine what factors relating to logging intensity and method may cause variation in the impacts of logging on residual tree damage, aboveground biomass and tree species richness. In total we collected data from 62 studies across the tropics giving us 38, 43 and 9 data points for investigation of damage, biomass change and species richness change respectively. Prop_damaged_vol Promisingly we found that RIL seemed to reduce residual tree damage compared to conventional logging, with greatest differences to conventional logging found at low intensities. However, at higher intensities residual damage became more similar to that of conventional logging as suggested by previous work from Indonesia. Prop_volume2However, the same wasn’t true of biomass. Though there was an apparent statistical difference in the slopes for RIL and conventionally logged forests the relatively low overlap in the intensities at which conventional and RIL are carried out means that slope estimation is not fantastic. As such it is not entirely clear whether, at the stand scale, RIL reduces biomass losses because of lower intensities or differences in practice as the two are confounded. SR_volumeUnfortunately there wasn’t enough information on species richness from forests logged using RIL to allow a comparison. However, the results were still interesting. There was a general decline in species richness with increasing logging intensity, but the plot hints that richness might increase at low intensities. Whether or not this is as a result of intermediate disturbance hypothesis type relationships is not a fight I want to get into, but this work does confirm that tree species richness is relatively insensitive to logging even at high intensities.

So our work suggests that the evidence for the positive effects of RIL is mixed, once we account for differences in logging intensity.  I am well aware that this piece of work might annoy a few people who think I have something against RIL. Those I have spoken to at conferences where I have presented often think that I am saying that RIL doesn’t work. I’m not. It just isn’t entirely clear what it’s effects are. Frankly it would be remarkable if RIL and conventional logging had similar impacts at the stand scale given the differences in the two practices. What I think we lack is enough evidence to say what is going on.

So what would be a constructive way to determine the differences in impact of RIL and conventional logging? One thing we think would improve evidence is the quantification of logging intensity at the plot scale. Currently studies often report logging intensities for the entire landscape where plots are located, meaning that the variation between plots is not accounted for. There is likely to be a big difference amongst plots and so the impacts are likely to differ as well. As far as I can tell only a few studies have done this. One good example is the work of Lucas Mazzei and colleagues who showed that plots that had been more intensively logged showed a slower recovery in biomass. Metrics such as the basal area of trees removed per hectare might be useful and relatively easy to collect at the plot scale.

The results of our study and those of Zuzana Burivalova suggest that logging intensity drives carbon and species loss while Jake Becknell’s work suggest that RIL is less damaging for animal populations. As such, current evidence suggests that RIL at relatively low intensities is likely to be the best way to reduce carbon and biodiversity loss in tropical logged forests. However, given the massive area of tropical forest already designated for logging reductions in local intensity, and thus yield, may encourage expansion into previously unlogged areas. Recent work indicates that high intensity logging over a smaller area (‘land sparing’) may have better outcomes for tropical forest species than low-intensity extensive timber extraction (‘land sharing’) in Borneo, though there is a need for similar studies in other areas of the tropics. Although reductions in logging intensity may reduce impact, the high demand for timber requires novel solutions that do not drastically reduce current yields but reduce impacts on forest ecosystems. Methods such as silvicultural thinning techniques to remove pioneer species may aid recovery of floral community composition, carbon and timber stocks but further work is needed to assess their effectiveness. Although RIL may also provide a solution, further evidence is required to verify this for carbon storage in the form of above-ground biomass. Analyses that take into account plot level variation in logging intensities using collaborative networks such as The Tropical managed Forests Observatory offer a potential solution to this.


If you enjoyed this be sure to check out the preprint on peerJ or the posts ‘Logging intensity drives species richness loss’ and ‘How bad is logging for tropical biodiversity?’

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.

Looking to the past for insights into tropical forest resilience

A few weeks back Lydia Cole and colleagues published a really cool paper exploring recovery rates of tropical forests. Seeing as it’s something I’ve covered a here before in relation to my work on secondary forests recovering after agricultural clearance and recovery from selective logging, I invited Lydia to write a guest post giving a different perspective to a topic I have discussed here before. Thanks to Lydia for stepping up to the plate and I hope you find her post as interesting as I did.


Anyone reading this blog probably doesn’t need reminding of how important tropical forests are!  Birds, bees, berries and a whole load of other plants, animals and services that we probably underestimate our reliance on.  Despite the many arguments in favour of keeping tropical forests standing, vast areas continue to be deforested at rapid rates resulting in changes like that shown below (Fig 1), under pressures of expanding human population, rising consumption and the agricultural footprint to match (Geist & Lambin, 2002).

Borneo-forest
Fig 1 – Forest disturbance like logging can lead to forests such as this one in Borneo being converted from intact (left) to heavily degraded (right).

Disturbance and recovery in tropical forests Despite this widespread clearance as a result of  recent international forest conservation initiatives and rising rural-to-urban migration (Mather, 1992), some degraded tropical forests are being given a chance to recover.  But how long does it take them to recover?  Much recent research has attempted to answer this question (e.g. the great work of Chazdon et al., 2007) but little has monitored change over time scales of >50 years. Since many tropical trees have lifespans much longer than this previous studies have only captured a snap-shot of the ecological process of recovery.  In our study, we attempted to answer the question again; this time by looking into the past to gather data over longer time scales that could offer a more complete picture of forest recovery post disturbance.

The palaeoecological approach

Palaeoecology, otherwise known as long-term ecology, uses fossils to decipher how plants and animals interacted with their environment in the past.  Fossil pollen grains come in all shapes and sizes, and their morphological characteristics can be used to identify the plant family, genus or even the species to which they belong.  When a collection of these grains are identified and counted from a layer of sediment, we can reconstruct what the vegetation was like at that point in time when those grains were deposited. In our project, we were interested in studies that documented disturbance-induced changes in fossil pollen from forested communities across the Tropics, over the last 20,000 years.  Types of disturbances ranged from climatic drying events and landslides, to shifting cultivation and human-induced biomass burning.  We found 71 studies published on tropical forest palaeoecology that satisfied our selection criteria (e.g. within 23oN/S of the equator, possessing a sufficient chronology), documenting 283 disturbance and associated recovery events.  The rate at which recovery was occurring across the different forests and disturbance events was the key variable of interest and was calculated as the percentage increase in forest pollen abundance per year relative to the pre-disturbance level.

How far and how fast have tropical forests recovered in the past?

Our results demonstrate that in the past the majority of forests regrew to less than 100% of pre-disturbance levels, prior to declining again or reaching a new baseline; the median recovery was to 95.5%.  They also recovered at a variety of speeds, ranging from rates that would lead to 95.5% regrowth in less than 10 years to those taking nearly 7,000 years; the average was 503 years.  This is significantly longer than the periods adopted by logging companies between extraction cycles!

What affects the rate of recovery?

Three of the different factors we investigated for their potential effect on the forest recovery rate seemed to be of particular importance: geographical location, disturbance type and frequency of disturbance events. Of the four key tropical regions, Central American forests recovered the fastest and those in Asia the slowest (Figs. 2 & 3).  This is concerning, given that forests in Southeast Asia are currently experiencing some of the greatest rates of deforestation of all tropical regions, primarily due to the economic profitability of oil palm agriculture (check out mongabay for details).

Tropical forest recovery
Fig. 2  Map of tropical forest distribution, the location of studies and relative recovery rates across regions.

The most common form of disturbance, and one from which forest regrowth happened relatively slowly, was anthropogenic impact, i.e. via logging, burning and/or for agriculture (Fig. 3).  The slowest rates of recovery occurred after climatic disturbances and the fastest after large infrequent events, e.g. landslides, hurricanes and natural fire.  This latter result is somewhat intuitive given that these perturbations are a natural part of all ecosystems, leading to the evolution of a dynamic response in the native plant communities.  

Figure 3
Fig. 3  Composite figure showing how the recovery rate varies with different variables.

Insights into resilience

When we looked at the standardised rate of disturbance events (SRD), i.e. the number of disturbance events per 1,000 years, we found that the greater the frequency events occurred in the past, the more quickly the forest responded to each subsequent disturbance.  This runs counter to contemporary theories on resilience that describe slowing rates and diminishing ability to recover with each subsequent perturbation (e.g. Veraart et al., 2012).  Our results suggest that over ecologically meaningful timescales, i.e. over the life-span of entire forest communities rather than single trees, increased exposure results in adaptation to that disturbance over time, leading to a greater ability to recover quickly from the perturbation.

What does this all mean for tropical forests?

From looking back into the past, it seems that tropical forests can take a long time to recover from disturbances, and that different regions may require different management regimes to encourage more complete reforestation after natural or anthropogenic events, such as fire.  Central American and African forests may bounce back from impacts more quickly than the other regions, with disturbances such as tropical hurricanes and climatic fluctuations being a more common component of these ecosystems than in the other tropical regions.  However, all of the forests we looked at demonstrated a greater vulnerability to anthropogenic impacts and climatic changes than large infrequent disturbances: the two major forms of disturbance occurring today and at levels that far exceed those experienced over the past 20,000 years – reasons for caution.

Sustainable management

Identifying and understanding the different ecological requirements of forests across the different geographical regions, and of the forest-types within those regions, is vital for developing more sustainable landscape management plans.  With increasing international concern over deforestation rates, the associated loss of biodiversity and elevated carbon dioxide emissions, the conservation and restoration of tropical forests is becoming more politically and economically feasible.  Indonesia, for example, has introduced ‘ecosystem restoration concessions’ in the last decade, providing a legal means for forest protection from the further expansion of industrial agriculture.  And the potential of Reducing Emissions from Deforestation and Forest Degradation (now REDD+) to save the World’s forests continues to generate international debate. Of importance to all of these programmes and initiatives, is the suggestion from our study that forests take time to recover, and if we give them that time, they will persist, and continue to provide their faunal inhabitants, including us, the greatest collection of biological riches on Earth.

Being positive about conservation

Over Christmas I was out walking in the Pennines with family and friends.

I got to talking to one of our old family friends (in that I have known her for a long time, not that she is old – I’d never be that rude) about what I was doing for my PhD.

I can’t quite remember the conversation but it went something like this:

Her: “The natural world is screwed. What kind of world do you think we’ll leave for our children and grand children if thinks keep on like this?”

Me: “I agree. But not everything is terrible. People think that deforestation in the Amazon is unsolvable, but recently deforestation has been going down.” (Note: this was true at the time, it’s just shown a ~30% increase)

Her: “Really? I didn’t know anything about that.”

Me: “Yeah, and the Brazilian government can now monitor deforestation monthly using its own satellites and potentially work out who is deforesting what.”

Her: “Wow. I didn’t know that either. Why don’t conservation people talk about these things more often?”

She had me there.

Why don’t we?

I think it’s fairly easy to understand why: tropical forests are being cleared rapidly, pollinator populations are in decline, as are carnivore populations and apparently populations of long-lived trees, not to mention the crisis in fisheries, the lack of a solid deal on carbon emissions… the list could go on.

However, buried amongst all that conservation has made some practical contributions to help save species and unique ecosystems from obliteration. We need to talk about these more often. If people think that everything is beyond hope – what is the point in doing anything?

Species like the golden tamarin have been brought back from the brink of extinction - so why don't we talk about them? Photo credit: 1000 wishes on Flickr
Species like the golden tamarin have been brought back from the brink of extinction – so why don’t we talk about them? Photo credit: 1000 wishes on Flickr

This is not a new idea. The late, great Navjot Sodhi and colleagues wrote a paper a few years ago identifying conservation successes at small, medium and large scales and others have been banging on about it for even longer. Shamelessly I am going to steal this idea.

So starting from now I will have an occasional series of posts called ‘Positive Conservation’, Or #positiveconservation for those of you on Twitter.

I ran through a series of names for this series #ponservation probably being the least appropriate, though #poncervation would  be a great name for hipster types doing conservation whilst rocking their oversized glasses (subnote – I don’t hate hipsters, I think I might be one).

I’ll keep this series as positive as possible and will write about individual case studies of conservation success, what the problems were and how people found solutions to them. Like upworthy for conservation but less cheesy, hopefully. If there are any examples that you particularly like and I don’t write about – send them my way. Let’s see how this goes.