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. Continue reading →
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.
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.
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.
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).
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).
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.
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.
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.
There’s been a lot said about relationships between species diversity and ecosystem function over the last two decades. The general view of these relationships is that diverse ecosystems are more productive, use resources more efficiently and are more stable.
But, and it’s a big but, almost none of this work has been done in forests and even less in mega-diverse tropical forests. Because of this diversity productivity relationships can’t really be described as general. How do we know what is general across the globe if we have only concentrated on temperate grasslands?
This is something a new paper in Global Ecology and Biogeography by Kyle Cavanaugh and colleagues hopes to set straight. Their study drew on a dataset of carbon storage and tree biodiversity from 59 plots across the tropics produced by members of the Terrestrial Ecosystem Assessment Monitoring (TEAM) Network.
The great thing about this work is that all plots were surveyed using the same methods, meaning they should be reasonably comparable. All sites collected measures of aboveground carbon storage, genus diversity, functional diversity – by measuring wood density of trees and potential maximum diameter, and the mean value for wood density and maximum diameter for each plot. All of this was then analysed while trying to account for climatic differences between sites.
The general findings of the study were that both genus diversity and the mean potential maximum diversity of species appear to be positively related to aboveground carbon storage.
This enforces the view that diverse ecosystems are more productive and that large species may contribute a disproportionate amount of biomass – as I have written before. Very few studies have shown a relationship between diversity and biomass in tropical systems before, so this is exciting stuff.
And yet, I still have a few queries about some findings. The study failed to find any relationship between climate and carbon storage – a connection that is fairly well established. Also it uses stepwise model selection, which is beginning to become one of my (and others’) pet peeves . I am of the feeling that testing all possible models and then averaging amongst them based on the ones that have greatest support is the best way to do things, and this often comes up with very different findings to stepwise selection.
Previous similar work has suggested that carbon – biodiversity relationships are scale-dependant, with positive relationships in small plots and mixed results at larger plot sizes. Given the increasing number of tropical forest research networks I am sure this study will not be the last of its type. Once these get published we will have a better idea of how general these findings are. At the moment I am not completely convinced.
Anyway, there has recently been a bit of back-and-forth regarding the state of large, old trees at a global scale.
These trees are key in both forest and non-forest ecosystems. The definition of what is ‘old’ and ‘large’ is specific to each region but it is widely accepted that these trees tend to store lots of carbon and are valuable for many species because of their structural complexity.
This is all important stuff and they had me convinced. It makes sense. Large, long lived species are disproportionately vulnerable to threats because they take a long time to reach maturity and they are targeted simply because they are large – for animals see hunting of ungulates, for trees see selective logging.
However, a recent letter by Edward Faison has made me doubt the claims of Lindenmayer. Faison points out that there have been increases in the abundance of large trees in forests in Sweden, Spain, Hungary, Italy and Switzerland. In addition there have apparently been relatively few declines of large trees in North America.
Lindenmayer and colleagues have since rebutted this letter, saying that there is a difference between large old trees and simply large trees. They point out that there have been increases in Europe and North America but that these increases have been from a very low point since both regions have historically cleared large swathes of forest for agriculture. They also point out the loss of large trees as a result of logging in the tropics as well as in Australia, North America and Siberia.
And yet I am still not entirely convinced.
Don’t get me wrong, I believe that large old trees are probably in decline in the ecosystems they talk about, but is this a general trend all over the place?
I am also a little scared that all of the discourse on this so far has been in the form of reviews/essays that could easily cherry-pick some cases and then craft a nice narrative around them. It is easy to believe the stories we tell ourselves and this is where we as scientists should be most self critical. After all how many beautiful sounding theories have been seen to have nothing to do with how things work in the real world? The only way to confront such problems is with cold, unemotional statistical analysis.
One of the things that kept coming up on the course was what people considered a to be a trait.
Apparently the strict definition that the people running the course was something along the lines of “morpho-physio-phenological traits which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance” taken from Violle et al 2007.
I agree roughly with this definition.
But there were a few naysayers in our group. Some of them argued that distribution size was a trait or that habitat that a species preferred was a trait. Personally I think these two things are the result of a trait-environment interaction, and are not themselves traits. However,even papers in the Holy Grail of publications, Nature, can get this wrong so I can understand the confusion.
For example, take a regional species pool that is made up of species that vary in traits which impact their fitness. These traits can determine whether a species is present at a particular site.
If you add up all the areas that a species is present in you have an idea of a species range and also the habitats they occupy. Therefore it is fairly obvious that these two things are not traits but are rather the products of traits.
This is not to say they aren’t useful in some way. Range size (or more accurately area of occupancy or extent of occurrence) is fundamental to one of conservation biology’s flagship projects – the Red List, and knowing the habitats a species uses can be useful in lots of ways.
It would be helpful if we could all agree what we were talking about when it come to traits, as very few of us seem to have thought about it. When people review a paper that uses the word ‘trait’ they should make sure that the term has some meaning and isn’t just used as a sexier synonym for ‘characteristic’ or ‘feature.’
Ecology is fraught with problems of definition (as I have discussed here and here) and personally I think it is one of the things that holds back our science. If our aim is to form meaningful generalisations about how the natural world works, we can’t do it until we agree what the hell we’re talking about in the first place.
Even if you think climate change is green hype there are reasons to worry about logging. The time given to logged foreststo recover is often not enough to allow timber species to recover properly. If this is widespread, it would put the long term sustainability of logging practices in the tropics in doubt.
Recovery of biodiversity, carbon, timber stocks and a whole host of other things are vital to work out how long forests should be left to recover between each logging period. Despite this there is actually relatively little data on recovery following logging, and this is particularly lacking from Africa.
Predictably logging forests sharply reduced their biomass and timber stock, with biomass reduced by about 30% and timber stock by 50%. More interestingly biomass then increased back to levels similar to undisturbed forests in by 2011, while timber stock did not.
This is alarming because this forest was logged much less intensively than those on other continents, but still did not recover its timber stocks. We should be worried by this. It means that even at the relatively low intensities of logging that happen in Central Africa, it might not be sustainable.
The authors’ argue that in order to make logging more sustainable the diameter at which trees are cut should be increased, whilst encouraging thinning to promote regrowth. I agree. However, we also need innovative solutions that go beyond those proposed already.
Reduced Impact Logging, which aims to reduce the amount of damage done to non-timber trees may help carbon stocks, but is less likely to aid timber stock recovery. A possible solution for tropical logging may be a combination of reduced logging techniques with planting of timber species which have been grown off site, in a similar way to that done by restoration projects. This could be lead by more researchers engaging with logging companies and encouraging projects to benefit sustainability of biodiversity and timber.
At the moment the problem of logging is similar to that of fisheries: forests provide a resource that is difficult limit access to and is difficult to track once exploited. As well as improving logging practices it is vital that we improve governance, but if I knew how to do that I wouldn’t be sat here writing a blog for free.
We do need to make progress on this issue though, we can’t stick everything in protected areas – that smacks of green colonialism and people need to get resources in one way or another. Idealists may accuse those who work with logging companies of dining with the devil. Fair enough. But we’re not going to solve these problems by ignoring them.
Tropical forests are getting ever more fragmented, human population in the tropics is increasing and guns are now widely available.
All of this has led to an explosion in the number of people hunting for food in the tropics.
This hunting can cause local extinction of bird and mammal species, with large bodied species being particularly at risk. This can lead to loss of species that eat fruit and therefore act as dispersers of plant seeds.
This dispersal is important since it means that species are dispersed widely around forest, rather that just being concentrated in small areas around their parent plant. However, those species that eat seeds, causing them to be damaged and therefore unable to germinate, may also be lost as a result of hunting. The balance between the losses of these two types of species will determine their effects on plant reproduction.
In general, it appears that losses of animal species, particularly larger species, as a result hunting tends to lead to an increase in the abundance of plant species which don’t require animals for dispersion. However, the results of these studies can sometimes be unclear due to lack of replication and because they have tended to be over a relatively short period of time.
A new study in Ecology Letters aims to tidy up our view of how hunting affects plant species. This work studied the dynamics of Lambir forest in Malaysian Borneo, which looks like this:
Though it looks nice, this forest has been hunted for over 15 years and this has caused the local extinction of seed dispersing species like the white crested hornbill, which looks like this:
as well as the red giant flying squirrel, which looks like this:
However, seed predators such as the sambar deer have also become locally extinct
This situations mirrors that of other study sites and makes it hard to determine how hunting will affect plant biodiversity.
For their study Rhett Harrison and colleagues investigated the changes in diversity and distribution of plant species in Lambir by monitoring nearly 500,000 (!) individual trees. They found that the density of seedlings tended to increase – suggesting a reduction in the amount of seed predation going on as well as a reduction in dispersal by animals. They also found that tree richness was reduced, though this reduction was relatively modest.
Most interestingly the study also suggests that plant species that need animals to disperse their seed tended to become relatively more clustered than species which didn’t rely on animals.
All of these results suggest that hunting can have marked effects on tropical forest plant biodiversity – in the long run leading to a potential decline in some animal dispersed species.
Large areas of South East Asia, West Africa and the Atlantic forest in Brazil are facing similar pressures from hunting, so this phenomenon may be quite widespread. Though it is obviously less of a threat to biodiversity when compared to deforestation and other more dramatic degradation the subtle effects of hunting may occur both inside and outside protected areas going relatively unnoticed. To tackle this problem effectively we need to know the motivation for this hunting. Only then can we start to deal with what to do to stop it.
What you may not know about is the Reducing Emissions from Deforestation and Degradation (REDD+) initiative (those of you who do know, feel free to skip ahead to the next paragraph). In short REDD+ is a policy championed by some that aims to reduce the potential emissions from forests, largely by paying communities that live in and around them to manage them sustainably. Of all the tools we could use to help reduce deforestation this policy has been the one to generate most hype over the last few years. I’ve lost count of the number of talks, debates and papers I’ve seen discussing it since I became a PhD student.
Though it seems like a generally good idea (and yes I know there are lots of caveats to this), there have been fears about its potential effects on biodiversity. There is the potential that only forests which have high carbon density would be prioritised, thereby missing out large areas with unique biodiversity. This seems stupid for a policy which targets forests, especially when these forests are home to so many unique species. Equally however, just concentrating on biodiversity might not catch areas with high carbon density.
It is here where a new(ish) paper by Chris Thomas in Ecology Letters comes in. The paper acknowledges the problems of focussing solely on one goal, and explores how you could balance the two most effectively.
To do this they used maps of carbon, along with maps of species ranges in both the Americas and UK. This is, in my opinion, a bit unrealistic since they perceive a world in which protection of carbon everywhere is considered of equal value – at the moment REDD+ is very much targeted at forests in developing countries. Nonetheless, this paper provides a few pointers on how to target this policy. Using the carbon and biodiversity maps they used the program Zonation to come up with areas considered priorities for carbon storage and biodiversity by selecting the 30% of cells with the greatest value.
Doing this for carbon solely realised peoples fears about poor protection for biodiversity. In the Americas protecting the 30% of land with highest carbon could protect nearly half the carbon stocks of the continents, but only 34% of bird biodiversity. Similarly in the UK prioritising carbon solely could protect 59% of carbon stocks, but only 25% of biodiversity.
The picture was similar when they targeted biodiversity only. In the Americas this would lead to protection of 71% of biodiversity, but only 30% of carbon. In the UK this would protect 93% of biodiversity but only 25% of carbon.
Both of these results clearly present a problem, targeting one goal does not automatically mean that you do particularly well with the other even if areas of high carbon tend to be in areas of high biodiversity.
The final analysis they did was to see how well the two goals could be achieved. By foregoing a loss of 10% of the maximum carbon in the America it was possible to maintain 91% of the maximal biodiversity value. The picture was similar in the UK where foregoing 10% of the maximum carbon meant it was possible to protect around 90% of biodiversity.
Given the previous concerns about targeting protection of carbon this paper seems remarkably positive. There seems no reason why, with careful and systematic planning, we can’t design policies to protect the two.
My criticisms of the paper still hold though, and it seems like a bit of an oversight that someone hasn’t done a similar analysis for countries which will realistically be part of REDD+ in the near future.
Having said that, we need to more analyses like this for more places – Africa and Asia, as well as Europe spring to mind, but we also need to get on and map and model ecosystem services other than carbon storage. This way we can get a better balance between managing the things we need to live and the biodiversity that underpins in. There is more to the benefits we get from ecosystems than just carbon
The Convention on Biological Diversity (CBD) aims to restore 15% of degraded ecosystems by 2020.
This is very ambitious. Even by the CBD’s standards.
But before we get to how we’re going to raise the money to do this, where we’re going to find the manpower to do all this work and what land is a priority we need to work out what we mean by degraded.
A lot of us struggle to define what degraded really means. Don’t worry though, you’re in good company – the CBD don’t know what it means either.
When I searched in google it came up with this:
1. Treat or regard (someone) with contempt or disrespect.
2. Lower the character of quality of
Not very useful right?
Looking at the literature a bit further you see that there have been constant attempts to define degraded forests in particular. The latest of these is a forest which ‘delivers a reduced supply of goods and services from the given site and maintains only limited biological diversity.’ This seems like a reasonable starting point.
However, it is not particularly useful in practice.
The main problems are:
What do you use as a reference? In some regions it might be relatively easy to find primary forest but other areas don’t really have undisturbed forest any more.
How much biodiversity/ecosystem service supply do you need to lose in order for the change to count as degradation?
Where does forest become non-forest? Is there a sensible threshold?
How can we avoid savannah being classed as degraded forest?
What ecosystem services are we talking about here? Trade-offs are inherent in any management of ecosystem services so even relatively small changes will reduce supply of one good or another.
That’s all I could come up with at the moment. I’m sure there are more.
All of these problems, and their lack of clarity in the CBD, completely scupper this 2020 goal. Forest biomes should those be for which it is easiest to define degradation, but this hasn’t been done.
Even though I have ranted about it here I realise it is not an easy thing to do. I am not going to solve this with a blog post, which is why I’m going to pursue the topic further in my personal research.
I have a few thoughts on how to push things forward as a starter. We need to be pragmatic and we can’t have woolly definitions in important international agreements if it stops us from balancing the needs of humans with conserving biodiversity.
For what its worth I think we need to:
Determine reference states for all broad biomes. Only then can we really start to measure degradation.
Work out thresholds below which ecosystems should be classed as degraded. This will obviously have to be ecosystem specific. It could include things like magnitude of changes in carbon pools, time required to recover from disturbance or some measure of species community similarity to reference states. Species richness should not be used as a biodiversity metric because many disturbed ecosystems have higher richness that neighbouring pristine systems.
We must develop a means of classifying ecosystem types for use in international agreements. Though this is a difficult task as there are many transition ecosystems, we still need to do it.
We need to recognise that ‘reduced loss in good and services’ means nothing. If you restore arable farmland to forest you would lose food production. Is this forest then degraded farmland? Obviously not. We must define what ecosystem services we are talking about for each biome and then use these as potential indicators of degradation.
We need to develop indicators of degradation since we will not be able to measure everything we would like everywhere. Canopy cover and tree height have been suggested for forests, but have rarely been tested.
This list is not exhaustive by any means, but I think its a good start.
I am constantly amazed by the ability of those who come up with CBD goals to forget about how we will actually measure progress towards them. I really think this needs to change in the future. For the moment we should try to develop indicators for the 2020 goals. Without them we will have little idea whether we’ve achieved them and what we might need to change in the future.
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