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.