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.

This appeals to a lot of people because it doesn’t necessarily require vast tracks of public or private land (makes the farmers happy, makes the urban sprawlers happy, keeps the greenies happy because it looks good for conservation, keeps the politicians happy because they don’t have to wade in and make unpopular decisions). Or at least, that’s the idea.

As it turns out, ‘connectivity’ per se between habitat patches is probably not quite as important as we once believed for population persistence. I’ve blogged before about several papers that have over-turned the ‘connectivity paradigm’ through experimental manipulation of microcosms (mini ecosystems), and coincidentally, I found another by Mike Bull & colleagues that just came out today in Austral Ecology showing that roadside vegetation corridors did little to explain variation in reptile abundance.

Why might this be the case? Many, many papers (including many of my own) now identify that population size and habitat quality are far more important for population persistence than connectivity per se. While we have also identified that isolated and small populations are more temporally variable (and hence, more prone to extinction), it seems like the evidence is mounting that just ensuring some degree of connectivity doesn’t really do what many think it should do in terms of reducing extinction risk.

So, what are the alternatives? Well, I still think connectivity needs to be endorsed to reduce overall fragmentation, but it must be done smartly and guided by the evidence-based principles of extinction biology. As mentioned above, the most important driver of extinction risk is population size, and as overall habitat area is increased, so too does population size. Thus, a thin thread of a corridor attaching fragments is insufficient – we should view these ‘corridors’ instead as patches themselves, with the aim to maximise their size (i.e., total area) instead of their connectivity enhancement per se. Think a big, fat chunk of restored habitat rather than a thin line of vegetation.

I’ve more or less indicated this to the Corridors Plan people in my submission, and I had the opportunity to meet with the committee a few weeks ago in Adelaide to reiterate my recommendations. Despite all the uncertainty about the true conservation effectiveness of wildlife corridors, we do know for certain that more area is better. Let’s at least follow this one principle.

CJA Bradshaw



9 responses

24 08 2015
What conservationists should recommend to philanthropists |

[…] We know that isolation, like small size, is bad. If they can buy up bushland adjacent to existing reserves or protected areas, they effectively get the biodiversity benefits of all that existing land as well. Better yet, if they can buy up land between separated reserves and somehow link them, then they’ve created their own biodiversity corridor. […]

13 09 2013
Conservation: So easy a child could do it |

[…] In other words, these are perhaps some of the easiest and most mundane solutions to the biodiversity crisis. While we do our best to protect what’s there, clearly it isn’t working. For example, over half of tropical protected areas are still losing their biodiversity, and Australia’s largest national park is experiencing a pathetic collapse of its vertebrates. Add all this to our governments’ charge to remove protections of our national reserve system, and you can see that we have to do so very much more. Biodiversity corridors are another potential disaster if not done very, very carefully. […]

26 04 2012
Michael Drielsma

We do need to keep banging on, even though it often seems to no avail. In conservation planning we are dealing with a complex space. It seems that often we are not well equipt to conceptualise that complexity, instead we instinctively reach for simple rules. As John Sterman has shown (e.g. Sterman, J. D. & Sweeney, L. B. 2002, ‘Cloudy skies: assessing public understanding of global warming ‘. System Dynamics Review, vol. 18, no. 2, pp. 207-240.) we humans struggle with complexity – but this needs to change especially in order to catalyse action on climate change, but also for biodiversity conservation. That’s why the role of scientists is increasingly to engage others in this wonderous complexity rather than to ‘simply’ hand down instructions.

6 05 2012

Agreed. Hence, this blog – my little way of making the complex just that bit more digestible.

26 04 2012
Michael Drielsma

I mostly agree with the blog. When its unpacked, I think that what is being said is that corridors for corridors sake is a mistake, that corridors are not a silver bullet and that the concept can be misused to promote destructive habitat loss and avoid the hard decisions needed to restore biodiversity in a meaningful way. All very good.

What isn’t in the blog is that under the banner of corridor planning or connectivity conservation, a lot of rigorous science takes place where connectivity is integrated with consideration of habitat quality and quantity, along with community engagement.

I guess a lot of us who understand the limitations of the concept of corridors have reliuctantly embraced it as a way to engage non-scientific people, while working very hard to ensure that detailed planning addressed population viability into the future.

My final point is the observation that many people working in this space tend to align into opposing camps based on dogmatic ‘corridors are good’ or ‘corridors are bad’ judgements. Its time to get past this and look at each proposal on its merits.

26 04 2012

Well, this is the point, isn’t it? David Pannell rightly identified that there is a surprising lack of detail in the Plan on “…where connectivity is integrated with consideration of habitat quality and quantity…”. I understand that the necessity for political expediency often leads to vague and/or motherhood statements; hence, scientists like myself caution against the dangers of implementing corridors for corridors’ sake.

Of course corridors are good, if they’re planned well. We just have to keep banging on about the ‘well’ part.

26 04 2012
Tom Barrett

Hi Corey.. I think we need to seperate the concept of the ‘connectivity conservation’ movement from the idea of ‘corridors’. This is expanded on in the following paper:;jsessionid=289A6C8F499CD604DEFF68C9B4FFDB21.d02t01?userIsAuthenticated=false&deniedAccessCustomisedMessage=

In regard to the debate about the ‘science’ in the systematic conservation planning literature – the following is from my answer to a similar question raised in response to our article in ‘The Conservation’:

I believe there are two questions to answer here:

1. Does structural connectivity facilitate movement and improve the resilience of plant and animal populations and increase their chances of persisting into the future?
and the second is..

2. Does structural connectivity facilitate gene flow between populations or sub-populations of vertebrates (such as marsupials)? This is referred to as ‘functional connectivity’.

There is broad agreement that the answer to question one is ‘yes’. Habitat loss and fragmentation is identified as a key threatening process by most scientists and governments around the world. The following reviews of the literature support this:

A CSIRO Review of Australian studies found that “..studies with specific evidence of movement between patches also generally found that the presence of structural connectivity increased the rates and/or likelihood of such movement.”.

An recent analysis of 78 experiments from 35 studies “found a highly significant result that corridors increase movement between habitat patches by approximately 50% compared to patches that are not connected with corridors.”

As you correctly point out – the second question is the one causing the most debate and there is agreement that a lot more research is needed to allow scientists to make practical recommendations to land managers on how to achieve this (gene flow). It is certain this it will take more than just improving structural connectivity, as the following paper points out, there are many reasons behind the decline and loss of biodiversity (birds in this case) in Australia:

The CSIRO review of current corridor initiatives in Australia commissioned by the Australian Government makes the valid point “Australian science is an early leader in the study of landscape-scale fragmentation and the need for various forms of connectivity conservation but ecological understanding of patterns and processes at the scale of the Great Eastern Ranges or Gondwana Link is poorly understood”

So there is still a lot of work to be done to better understand the ecological processes occurring at larger scales. Studies have tended to focus on the spatial arrangement of habitat and ignore what is going on in the matrix around that habitat. A recent review found that studies “that use data on matrix composition report greater effects on abundance and occupancy of fragmented populations than studies that define connectivity without regard to the surrounding matrix (i.e. ‘binary’ studies that describe only characteristics of patch habitat)”

I believe this supports the need for the ‘connectivity conservation’ movement or a Landscape Ecology approach to the problem that considers all land uses and land management activities in landscape – of which ‘corridors’ are just one part.

26 04 2012

All good points, Tom. As I said, I don’t think anyone can argue that improving connectivity via a reduction in overall fragmentation will not augment persistence probability. It will: but over what time scales and by how much?

The concern I have is that even a large increase in ‘movement’ doesn’t necessarily rescue isolated populations fast enough to offset the losses due to past perturbations (extinction lags). Therefore, a vague ‘build a corridor and all will be right’ attitude could see a huge effort and cost wasted.

I reiterate that the most important determinant of extinction risk for any taxon you care to examine in detail is habitat area – you augment this component and you get your biggest bang for the buck. If you can connect fragments at the same time, then it’s a bonus that will likely add a bit more resilience to the fragments in question.

24 04 2012
Dr. Cagan Sekercioglu

Great point Corey. That’s what I’ve done with my proposal to create Turkey’s first wildlife corridor, which more than doubles the area of the national park it is connecting to the Caucasus forests. After 4 years of pushing, the government finally signed on it. Congrats on your corridor plan.

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