Ecological processes depend on …

14 05 2014
© Cagan Sekercioglu

© Cagan Sekercioglu

I have been known to say (ok – I say it all the time) that ecologists should never equivocate when speaking to the public. Whether it’s in a media release, blog post, television presentation or newspaper article, just stick to ‘yes’ or ‘no’. In other words, don’t qualify your answer with some horrid statistical statement (i.e., in 95% of cases …) or say something like “… but it really depends on …”. People don’t understand uncertainty – to most people, ‘uncertainty’ means “I don’t know” or worse, “I made it all up”.

But that’s only in the movies.

In real ‘ecological’ life, things are vastly different. It’s never as straightforward as ‘yes’ or ‘no’, because ecology is complex. There are times that I forget this important aspect when testing a new hypothesis with what seem like unequivocal data, but then reality always hits.

Our latest paper is the epitome of this emergent complexity from what started out as a fairly simple question using some amazing data. What makes birds change their range1? We looked at this question from a slightly different angle than had been done before because we had access to climate data, life-history data and most importantly, actual range change data. It’s that latter titbit that is typically missing from studies aiming to understand what drives species toward a particular fate; whether it’s a species distribution model predicting the future habitat suitability of some species as a function of climate change, or the past dynamics of some species related to its life history pace, most often the combined dynamics are missing. Read the rest of this entry »





Too small to avoid catastrophic biodiversity meltdown

27 09 2013
Chiew Larn

Chiew Larn Reservoir is surrounded by Khlong Saeng Wildlife Sanctuary and Khao Sok National Park, which together make up part of the largest block of rainforest habitat in southern Thailand (> 3500 km2). Photo: Antony Lynam

One of the perennial and probably most controversial topics in conservation ecology is when is something “too small’. By ‘something’ I mean many things, including population abundance and patch size. We’ve certainly written about the former on many occasions (see here, here, here and here for our work on minimum viable population size), with the associated controversy it elicited.

Now I (sadly) report on the tragedy of the second issue – when is a habitat fragment too small to be of much good to biodiversity?

Published today in the journal Science, Luke Gibson (of No substitute for primary forest fame) and a group of us report disturbing results about the ecological meltdown that has occurred on islands created when the Chiew Larn Reservoir of southern Thailand was flooded nearly 30 years ago by a hydroelectric dam.

As is the case in many parts of the world (e.g., Three Gorges Dam, China), hydroelectric dams can cause major ecological problems merely by flooding vast areas. In the case of Charn Liew, co-author Tony Lynam of Wildlife Conservation Society passed along to me a bit of poignant and emotive history about the local struggle to prevent the disaster.

“As the waters behind the dam were rising in 1987, Seub Nakasathien, the Superintendent of the Khlong Saeng Wildlife Sanctuary, his staff and conservationist friends, mounted an operation to capture and release animals that were caught in the flood waters.

It turned out to be distressing experience for all involved as you can see from the clips here, with the rescuers having only nets and longtail boats, and many animals dying. Ultimately most of the larger mammals disappeared quickly from the islands, leaving just the smaller fauna.

Later Seub moved to Huai Kha Khaeng Wildlife Sanctuary and fought an unsuccessful battle with poachers and loggers, which ended in him taking his own life in despair in 1990. A sad story, and his friend, a famous folk singer called Aed Carabao, wrote a song about Seub, the music of which plays in the video. Read the rest of this entry »





Biogeography comes of age

22 08 2013

penguin biogeographyThis week has been all about biogeography for me. While I wouldn’t call myself a ‘biogeographer’, I certainly do apply a lot of the discipline’s techniques.

This week I’m attending the 2013 Association of Ecology’s (INTECOL) and British Ecological Society’s joint Congress of Ecology in London, and I have purposefully sought out more of the biogeographical talks than pretty much anything else because the speakers were engaging and the topics fascinating. As it happens, even my own presentation had a strong biogeographical flavour this year.

Although the species-area relationship (SAR) is only one small aspect of biogeography, I’ve been slightly amazed that after more than 50 years since MacArthur & Wilson’s famous book, our discipline is still obsessed with SAR.

I’ve blogged about SAR issues before – what makes it so engaging and controversial is that SAR is the principal tool to estimate overall extinction rates, even though it is perhaps one of the bluntest tools in the ecological toolbox. I suppose its popularity stems from its superficial simplicity – as the area of an (classically oceanic) island increases, so too does the total number of species it can hold. The controversies surrounding such as basic relationship centre on describing the rate of that species richness increase with area – in other words, just how nonlinear the SAR itself is.

Even a cursory understanding of maths reveals the importance of estimating this curve correctly. As the area of an ‘island’ (habitat fragment) decreases due to human disturbance, estimating how many species end up going extinct as a result depends entirely on the shape of the SAR. Get the SAR wrong, and you can over- or under-estimate the extinction rate. This was the crux of the palaver over Fangliang He (not attending INTECOL) & Stephen Hubbell’s (attending INTECOL) paper in Nature in 2011.

The first real engagement of SAR happened with John Harte’s maximum entropy talk in the process macroecology session on Tuesday. What was notable to me was his adamant claim that the power-law form of SAR should never be used, despite its commonness in the literature. I took this with a grain of salt because I know all about how messy area-richness data can be, and why one needs to consider alternate models (see an example here). But then yesterday I listened to one of the greats of biogeography – Robert Whittaker – who said pretty much the complete opposite of Harte’s contention. Whittaker showed results from one of his papers last year that the power law was in fact the most commonly supported SAR among many datasets (granted, there was substantial variability in overall model performance). My conclusion remains firm – make sure you use multiple models for each individual dataset and try to infer the SAR from model-averaging. Read the rest of this entry »





To corridor, or not to corridor: size is the question

24 04 2012

I’ve just read a really interesting post by David Pannell from the University of Western Australia discussing the benefits (or lack thereof) of wildlife ‘corridors’. I’d like to elaborate on a few key issues, and introduce the most important aspect that really hasn’t been mentioned.

Some of you might be aware that the Australian Commonwealth Government has just released its Draft National Wildlife Corridors Plan for public comment, but many of you might not really know what a ‘corridor’ constitutes.

Wildlife or biodiversity ‘corridors’ have been around for a long time, at least in terms of proposals. The idea is fairly simple to conceive, but very difficult to implement in practice.

At least for as long as I’ve been in the conservation biology biz, ‘corridors’ have been proffered as one really good way to make broad-scale landscape restoration plausible and effective for (mainly) forest-dwelling species which have copped the worst of deforestation trends around Australia and the world. The idea is that because of intense habitat fragmentation, isolated patches of primary (or at least, reasonably intact secondary) forest can be linked by planting some sort of long corridor of similar habitat between them. Then, all the little creatures can merrily make their way back and forth between the patches, thus rescuing each other from extinction via migration. Read the rest of this entry »





Life, death and Linneaus

9 07 2011

Barry Brook (left) and Lian Pin Koh (right) attacking Fangliang He (centre). © CJA Bradshaw

I’m sitting in the Brisbane airport contemplating how best to describe the last week. If you’ve been following my tweets, you’ll know that I’ve been sequestered in a room with 8 other academics trying to figure out the best ways to estimate the severity of the Anthropocene extinction crisis. Seems like a pretty straight forward task. We know biodiversity in general isn’t doing so well thanks to the 7 billion Homo sapiens on the planet (hence, the Anthropo prefix) - the question though is: how bad?

I blogged back in March that a group of us were awarded a fully funded series of workshops to address that question by the Australian Centre for Ecological Synthesis and Analysis (a Terrestrial Ecosystem Research Network facility based at the University of Queensland), and so I am essentially updating you on the progress of the first workshop.

Before I summarise our achievements (and achieve, we did), I just want to describe the venue. Instead of our standard, boring, windowless room in some non-descript building on campus, ACEAS Director, Associate Professor Alison Specht, had the brilliant idea of putting us out away from it all on a beautiful nature-conservation estate on the north coast of New South Wales.

What a beautiful place – Linneaus Estate is a 111-ha property just a few kilometres north of Lennox Head (about 30 minutes by car south of Byron Bay) whose mission is to provide a sustainable living area (for a very lucky few) while protecting and restoring some pretty amazing coastal habitat along an otherwise well-developed bit of Australian coastline. And yes, it’s named after Carl Linnaeus. Read the rest of this entry »





How fast are we losing species anyway?

28 03 2011

© W. Laurance

I’ve indicated over the last few weeks on Twitter that a group of us were recently awarded funding from the Australian Centre for Ecological Synthesis and Analysis – ACEAS – (much like the US version of the same thing – NCEAS) to run a series of analytical workshops to estimate, with a little more precision and less bias than has been done previously, the extinction rates of today’s biota relative to deep-time extinctions.

So what’s the issue? The Earth’s impressive diversity of life has experienced at least five mass extinction events over geological time. Species’ extinctions have kept pace with evolution, with more than 99 % of all species that have ever existed now gone (Bradshaw & Brook 2009). Despite general consensus that biodiversity has entered the sixth mass extinction event because of human-driven degradation of the planet, estimated extinction rates remain highly imprecise (from 100s to 10000s times background rates). This arises partly because the total number of species is unknown for many groups, and most extinctions go unnoticed.

So how are we going to improve on our highly imprecise estimates? One way is to look at the species-area relationship (SAR), which to estimate extinction requires one to extrapolate back to the origin in taxon- and region-specific SARs (e.g., with a time series of deforestation, one can estimate how many species would have been lost if we know how species diversity changes in relation to habitat area). Read the rest of this entry »





Webinar: Modelling water and life

27 08 2010

Another quick one today just to show the webinar of my recent 10-minute ‘Four in 40′ talk sponsored by The Environment Institute and the Department for Water. This seminar series was entitled ‘Modelling as a Tool for Decision Support’ held at the Auditorium, Royal Institution Australia (RiAus).

“Four in 40″ is a collaboration between The University of Adelaide and the Department for Water, where 4 speakers each speak for 10 minutes on their research and its implications for policy. The purpose is to build understanding of how best to work with each other, build new business for both organisations and raise awareness of activity being undertaken in water/natural resource management policy and research.

CJA Bradshaw





Faraway fettered fish fluctuate frequently

27 06 2010

Hello! I am Little Fish

Swimming in the Sea.

I have lots of fishy friends.

Come along with me.

(apologies to Lucy Cousins and Walker Books)

I have to thank my 3-year old daughter and one of her favourite books for that intro. Now to the serious stuff.

I am very proud to announce a new Report in Ecology we’ve just had published online early about a new way of looking at the stability of coral reef fish populations. Driven by one of the hottest young up-and-coming researchers in coral reef ecology, Dr. Camille Mellin (employed through the CERF Marine Biodiversity Hub and co-supervised by me at the University of Adelaide and Julian Caley and Mark Meekan of the Australian Institute of Marine Science), this paper adds a new tool in the design of marine protected areas.

Entitled Reef size and isolation determine the temporal stability of coral reef fish populations, the paper applies a well-known, but little-used mathematical relationship between the logarithms of population abundance and its variance (spatial or temporal) – Taylor’s power law.

Taylor’s power law is pretty straightforward itself – as you raise the abundance of a population by 1 unit on the logarithmic scale, you can expect its associated variance (think variance over time in a fluctuating population to make it easier) to rise by 2 logarithmic units (thus, the slope = 2). Why does this happen? Because a log-log (power) relationship between a vector and its square (remember: variance = standard deviation2) will give a multiplier of 2 (i.e., if xy2, then log10x ~ 2log10y).

Well, thanks for the maths lesson, but what’s the application? It turns out that deviations from the mathematical expectation of a power-law slope = 2 reveal some very interesting ecological dynamics. Famously, Kilpatrick & Ives published a Letter in Nature in 2003 (Species interactions can explain Taylor’s power law for ecological time series) trying to explain why so many real populations have Taylor’s power law slopes < 2. As it turns out, the amount of competition occurring between species reduces the expected fluctuations for a given population size because of a kind of suppression by predators and competitors. Cool.

But that application was more a community-based examination and still largely theoretical. We decided to turn the power law a little on its ear and apply it to a different question – conservation biogeography. Read the rest of this entry »





How to make an effective marine protected area

22 09 2009

Here’s a nice little review from the increasingly impressive Frontiers in Ecology and the Environment which seems to be showcasing a lot of good conservation research lately.

© USGS

© USGS

As we know, the world’s oceans are under huge threat, with predictions of 70 % loss of coral reefs by 2050, decline in kelp forests, loss of seagrasses, over-fishing, pollution and a rapidly warming and acidifying physical environment. Given all these stressors, it is absolutely imperative we spend a good deal of time thinking about the right way to impose restrictions on damage to marine areas – the simplest way to do this is via marine protected areas (MPA).

The science of MPA network design has matured over the last 10-20 years such that there is a decent body of literature now on what we need to do (now the policy makers just have to listen – some  progress there too, but see also here). McLeod and colleagues in the latest issue of Frontiers in Ecology and the Environment have published a review outlining the best, at least for coral reefs, set of recommendations for MPA network design given available information (paper title: Designing marine protected area networks to address the impacts of climate change). Definitely one for the Potential list.

Here’s what they recommend:

Size

  • bigger is always better
  • minimum diameter of an MPA should be 10-20 km to ensure exchange of propagules among protected benthic populations

Shape

  • simple shapes best (squares, rectangles)
  • avoid convoluted shapes to minimise edge effects

Representation

  • protect at least 20-30 % of each habitat

Replication

  • protect at least 3 examples of each marine habitat

Spread

  • select MPA in a variety of temperature regimes to avoid risk of all protected reefs succumbing to future climate changes

Critical Areas

  • protect nursery areas, spawning aggregations, and areas of high species diversity
  • protect areas demonstrating natural resilience or rapid recovery from previous disturbances

Connectivity

  • measure connectivity between MPA to ensure replenishment
  • space maximum distance of 15-20 km apart
  • include whole ecological units
  • buffer core areas
  • protect adjacent areas such as outlying reefs, seagrass beds, mangroves

Ecosystem Function

  • maintain key functional groups of species (e.g., herbivorous fishes)

Ecosystem Management

  • embed MPA in broader management frameworks addressing other threats
  • address and rectify sources of pollution
  • monitor changes

Of course, this is just a quick-and-dirty list as presented here – I highly recommend reading the review for specifics.

CJA Bradshaw

ResearchBlogging.orgMcLeod, E., Salm, R., Green, A., & Almany, J. (2009). Designing marine protected area networks to address the impacts of climate change Frontiers in Ecology and the Environment, 7 (7), 362-370 DOI: 10.1890/070211





Save the biggest (and closest) ones

12 11 2008

© somapsychedelica

© somapsychedelica

A paper we recently wrote and published in Biological Conservation entitled Using biogeographical patterns of endemic land snails to improve conservation planning for limestone karsts lead by my colleague Reuben Clements of WWF has recently been highlighted at Mongabay.com. Our main result was that following the basic tenets of the theory of island biogeography, the largest, least-isolated limestone karsts in South East Asia (biologically rich limestone outcrops formed millions of years ago by the deposition of calcareous marine organisms) have the greatest proportion of endemic land snails (a surrogate taxon for uniqueness among other species). I’ll let Rhett at Mongabay.com do the rest (see original post):

Researchers have devised a scientific methodology for prioritizing conservation of limestone karsts, biologically-rich outcroppings found in Southeast Asia and other parts of the world. The findings are significant because karsts – formed millions of years ago by sea life – are increasingly threatened by mining and other development.

Using data from 43 karsts across Peninsular Malaysia and Sabah, authors led by Reuben Clements of WWF-Malaysia reported that larger karsts support greater numbers of endemic snails – a proxy for biological uniqueness among other species – making them a priority for protection.

“Larger areas tend to have greater habitat diversity, which enables them so support a higher number of unique species.” said Clements, species conservation manager for WWF-Malaysia.

With a variety of habitats including sinkholes, caves, cliffs, and underground rivers, and separated from other outcroppings by lowland areas, karsts support high levels of endemism among insects, snails, fish, plants, bats and other small mammals. Animals that inhabit karsts provide humans with important services including pest control, pollination, and a sustainable source of income (swiftlet nests used for bird nest soup, a Chinese delicacy, are found in karst caves). But karsts are increasingly under threat, especially from mining for cement and marble. An earlier study by Clements showed that limestone quarrying is increasing in Southeast Asia by 5.7 percent a year – the highest rate in the world – to fuel the region’s construction boom. The biodiversity of karsts – especially among animals that move to surrounding areas to feed – is also at risk from destruction of adjacent ecosystems, often by loggers or for agriculture.

Clements says the new study, which is published in the November issue of the journal Biological Conservation, will help set conservation priorities for karsts.

“The protection of karsts has been mainly ad hoc and they are usually spared from quarrying by virtue of being situated within state and national parks, or if they possess some form of aesthetic or cultural value,” he said. “Taking Peninsular Malaysia for example, our results suggest that we should set aside larger karsts on both sides of the Titiwangsa mountain range for protection if we want to maximize the conservation of endemic species. Protecting karsts on one side of the mountain chain is not enough.”

“With our findings, we hope that governments would reconsider issuing mining concessions for larger karsts as they tend to be more biologically important,” Clements said.





Conservation Scholars: Daniel Simberloff

30 10 2008

This series on ConservationBytes.com takes a page out of our book Tropical Conservation Biology (Sodhi, Brook & Bradshaw) – therein we produced a series of ‘Spotlights’ describing the contributions of great thinkers to conservation science. Each highlight of a Conservation Scholar includes a small biography, a list of major scientific publications and a Q & A on the person’s particular area of expertise.

Our fifth Conservation Scholar is Daniel Simberloff

Biography

As a child in rural Pennsylvania, I was fascinated by nature, collecting insects from age five and keeping insects, turtles, and fish as pets. This idyll crashed to a halt at age 11 when we moved to an industrial suburb of New York City. I attended Harvard College, where I became excited by maths and majored in it. My junior year, while enjoying a non-majors biology course and realising that I wasn’t so enthusiastic about a maths career, I consulted a biology department advisor about postgraduate work. Frank Carpenter, an insect palaeontologist, astounded me by saying I could go to graduate school in biology with a little coursework in my final year. He also directed me to Ed Wilson as a potential graduate advisor. Ed introduced me to ecology and argued that maths is crucial to the maturation of ecology. He taught me an enormous amount of biology and encouraged me in a great doctoral dissertation project, testing the theory of island biogeography that he had recently propounded with Robert MacArthur. We collaborated in fumigating small Florida mangrove islands and studying their recolonisation by insects. Bill Bossert taught me about computers well before everyone knew about them. Beginning with my doctorate, my main interest has been how different species fit (or do not fit) together in communities, and this interest led to research in conservation issues, most notably on refuge design and impacts and management of introduced species, as well as more academic aspects of ecology, like the role of inter-specific competition. As a faculty member at Florida State University and now the University of Tennessee, I have learnt an enormous amount from excellent colleagues, postgraduate students, and post-doctoral fellows. I also quickly interacted with policy makers and non-governmental organisations on conservation issues, first at the local and state levels, then nationally. My most important advice to prospective conservation biology students is to interact with challenging people doing interesting research and to engage in local conservation issues.

Major Publications

Questions and Answers

1. What are the defining characteristics of an alien organism that make it ‘invasive’?

An invasive introduced species is one that spreads into more or less natural ecosystems and thrives there, affecting native species.

2. Is there merit in the idea that tropical ecosystems are more resistant to invasion due to their species richness and complexity?

I don’t believe either premise, that tropical ecosystems are particularly resistant to invasion or that complex ecosystems with many species are particularly resistant to invasion. There are many invasions into tropical ecosystems, including species-rich ones, and extensive research fails to support Elton’s hypothesis that biological invasions are greatly facilitated by low native species richness. Any ecosystem is invasible by the right invader.

3. How common are ‘invasional meltdowns’, and what are the best tropical examples?

It is too early to know just how common invasional meltdowns are, but every year more cases are suggested, and some are buttressed by careful study. An important tropical example is the near destruction of the forest ecosystems of Christmas Island (Indian Ocean) by the introduced yellow crazy ant, whose population explosion was facilitated by later invasions of scale insects that produced a honeydew fed on by the ants. The ants devastated populations of the keystone species, the famed red land crab, and their impact on the forest was exacerbated by a plant pathogen (a sooty mould). See O’Dowd D. J., Green P. T. & Lake P. S. (2003). Invasional ‘meltdown’ on an oceanic island. Ecology Letters 6, 812-817.

4. Biological, chemical, mechanical, genetic or ecosystem control: any to recommend?

The best defence against invasive introduced species is to avoid introducing them in the first place, by having more stringent regulations on movement of goods and by better inspections of luggage and cargo. The second line of defence is an effective early warning/rapid response system, which requires both ongoing monitoring and the institutional and legal mechanisms to act quickly when an invasion is discovered. Many introduced species have been eradicated before they spread too far, and many more could have been eradicated if a good early warning/rapid response system was in place. Once a species is established, biological, chemical, and mechanical control, genetic intervention, and ecosystem management all have roles to play in particular cases in maintaining invader populations at low levels. However, ecosystem management and genetic intervention, though widely discussed, have so far rarely been used to deal with introduced species, particularly in natural environments. By contrast, there are many successful uses of biological, chemical, and mechanical control. It is nonetheless important to recognize that the great majority of biological control projects do not control the target pest, that species introduced for biological control can and sometimes do attack non-target native species, and that once a biological control agent is well-established, it is difficult if not impossible to eradicate it, even if it turns out to be problematic. The latter feature contrasts with mechanical and chemical control, which can simply be terminated if they are not working as planned. For this reason, I feel it is important to consider all possible means of dealing with introduced pests, and to take account of the irreversibility and historically low success rate of biological control.

5. Do large-scale, co-ordinated efforts like the United Nations Global Invasive Species Program (GISP) offer the best means of addressing the problems of alien species?

Large international efforts, such as GISP, are important tools in dealing with introduced species. They can particularly aid in slowing down the rate at which non-native species arrive and in publicizing the problem and educating people about how to deal with particular invasions (as GISP has done). However, the most important response to introduced species will always be the efforts that each nation will mount against invaders that breach its borders, in terms of early warning/rapid response, attempted eradication, and effective maintenance management.

CJA Bradshaw

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(with thanks to Navjot Sodhi, Barry Brook, Ward Cooper, Wiley-Blackwell and Daniel Simberloff for permission to reproduce the text – buy your copy of Tropical Conservation Biology here)





Classics: Fragmentation

3 10 2008
Synergies among threatening processes relative to habitat loss and fragmentation. a) A large population within unmodified, contiguous habitat occupies all available niches so that long-term abundance fluctuates near full carrying capacity (K). b) When habitat is reduced (e.g. 50 % area loss), total abundance declines accordingly. c) However, this simple habitat-abundance relationship is complicated by the spatial configuration of habitat loss. In this example, all remaining fragmented subpopulations might fall below their minimum viable population (MVP) sizes even though total abundance is the same proportion of K as in panel B. As such, limited connectivity between subpopulations implies much greater extinction risk than that predicted for the same habitat loss in less fragmented landscapes. Further synergies (positive feedbacks among threatening processes; black arrows) might accompany high fragmentation, such as enhanced penetration of predators, invasive species or wildfire, micro-habitat edge effects, and reduced resistance to drought with climate change.

Figure 2 from Brook et al. (2008): Synergies among threatening processes relative to habitat loss and fragmentation. a) A large population within unmodified, contiguous habitat occupies all available niches so that long-term abundance fluctuates near full carrying capacity (K). b) When habitat is reduced (e.g. 50 % area loss), total abundance declines accordingly. c) However, this simple habitat-abundance relationship is complicated by the spatial configuration of habitat loss. In this example, all remaining fragmented subpopulations might fall below their minimum viable population (MVP) sizes even though total abundance is the same proportion of K as in panel B. As such, limited connectivity between subpopulations implies much greater extinction risk than that predicted for the same habitat loss in less fragmented landscapes. Further synergies (positive feedbacks among threatening processes; black arrows) might accompany high fragmentation, such as enhanced penetration of predators, invasive species or wildfire, micro-habitat edge effects, and reduced resistance to drought with climate change.

This is, perhaps, one of the most important concepts that the field of conservation biology has identified as a major driver of extinction. It may appear on the surface a rather simple notion that the more ‘habitat’ you remove, the fewer species (and individuals) there will be (see MacArthur & Wilson’s Classic contribution: The Theory of Island Biogeography), but it took us decades (yes, embarrassingly – decades) to work out that fragmentation is bad (very, very bad).

Habitat fragmentation occurs when a large expanse of a particular, broadly defined habitat ‘type’ is reduced to smaller patches that are isolated by surrounding, but different habitats. The surrounding habitat is typically defined a ‘matrix’, and in the case of forest fragmentation, generally means ‘degraded’ habitat (fewer native species, urban/rural/agricultural development, etc.).

Fragmentation is bad for many reasons: it (1) reduces patch area, (2) increases isolation among populations associated with fragments, and (3) creates ‘edges’ where unmodified habitat abuts matrix habitat. Each of these has dire implications for species, for we now know that (1) the smaller an area, the fewer individuals and species in can contain, (2) the more isolated a population, the less chance immigrants will ‘rescue’ it from catastrophes, and (3) edges allow the invasion of alien species, make the microclimate intolerable, increase access to bad humans and lead to cascading ecological events (e.g., fire penetration). Make no mistake, the more fragmented an environment, the worse will be the extinction rates of species therein.

What’s particularly sad about all this is that fragmentation was actually seen as a potentially GOOD thing by conservation biologists for many long years. The so-called SLOSS (Single Large or Several Small) debate pervaded the early days of conservation literature. The debate was basically the argument that several small reserves would provide more types of habitat juxtapositions and more different species complexes, making overall diversity (species richness) higher, than one large reserve. It was an interesting, if not deluded, intellectual debate because both sides presented some rather clever theoretical and empirical arguments. Part of the attraction of the ‘Several Small’ idea was that it was generally easier to find series of small habitat fragments to preserve than one giant no-go area.

However, we now know that the ‘Several Small’ idea is completely inferior because of the myriad synergistic effects of fragmentation. It actually took Bruce Wilcox and Dennis Murphy until 1985 to bring this to everyone’s attention in their classic paper The effects of fragmentation on extinction to show how silly the SLOSS debate really was. It wasn’t, however, until the mid- to late 1990s that people finally started to accept the idea that fragmentation really was one of the biggest conservation evils. Subsequent work (that I’ll showcase soon on ConservationBytes.com) finally put the nail in the SLOSS debate coffin, and indeed, we haven’t heard a whisper of it for over a decade.

For more general information, I invite you to read the third chapter in our book Tropical Conservation Biology entitled Broken homes: tropical biotas in fragmented landscapes, and our recent paper in Trends in Ecology and Evolution entitled Synergies among extinction drivers under global change.

CJA Bradshaw

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Classics: The Living Dead

30 08 2008

‘Classics’ is a category of posts highlighting research that has made a real difference to biodiversity conservation. All posts in this category will be permanently displayed on the Classics page of ConservationBytes.com

© M. Baysan
© M. Baysan

Tilman, D., May, R.M., Lehman, C.L., Nowak, M.A. (1994) Habitat destruction and the extinction debt. Nature 371, 65-66

In my opinion, this is truly a conservation classic because it shatters optimistic notions that extinction is something only rarely the consequence of human activities (see relevant post here). The concept of ‘extinction debt‘ is pretty simple – as habitats become increasingly fragmented, long-lived species that are reproductively isolated from conspecifics may take generations to die off (e.g., large trees in forest fragments). This gives rise to a higher number of species than would be otherwise expected for the size of the fragment, and the false impression that many species can persist in habitat patches that are too small to sustain minimum viable populations.

These ‘living dead‘ or ‘zombie‘ species are therefore committed to extinction regardless of whether habitat loss is arrested or reversed. Only by assisted dispersal and/or reproduction can such species survive (an extremely rare event).

Why has this been important? Well, neglecting the extinction debt is one reason why some people have over-estimated the value of fragmented and secondary forests in guarding species against extinction (see relevant example here for the tropics and Brook et al. 2006). It basically means that biological communities are much less resilient to fragmentation than would otherwise be expected given data on species presence collected shortly after the main habitat degradation or destruction event. To appreciate fully the extent of expected extinctions may take generations (e.g., hundreds of years) to come to light, giving us yet another tool in the quest to minimise habitat loss and fragmentation.

CJA Bradshaw

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Classics: Island Biogeography

19 08 2008

‘Classics’ is a category of posts highlighting research that has made a real difference to biodiversity conservation. All posts in this category will be permanently displayed on the Classics page of ConservationBytes.com

© Princeton University Press

© Princeton University Press

MacArthur, R.H. & Wilson, E.O. (1967). The Theory of Island Biogeography. Princeton University Press, Princeton, NJ

Although this classic book was written before the discipline of conservation biology really kicked off, it has to be one of the more influential in terms of reserve design and the estimation of extinction rates. The original theory was proposed as a determinant of total species richness on islands as a function of island size. Put (almost too) simply the bigger the island, the more species it contains. This ultimately lead to the branch of biogeography/conservation biology that applied ‘species-area’ relationships to habitat fragments to extrapolate total species number and more importantly (in the context of the extinction crisis), estimate rates of species loss. The species-area literature is a hot-bed of critique and polemic, yet no one can deny that this seminal paper really kicked off the idea that reduced and fragmented areas are bad for biodiversity. We wouldn’t have nature reserves today if it wasn’t for this simple, yet brilliant piece of work.

CJA Bradshaw

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