Classics: the Allee effect

22 12 2008

220px-Vortex_in_draining_bottle_of_waterAs humanity plunders its only home and continues destroying the very life that sustains our ‘success’, certain concepts in ecology, evolution and conservation biology are being examined in greater detail in an attempt to apply them to restoring at least some elements of our ravaged biodiversity.

One of these concepts has been largely overlooked in the last 30 years, but is making a conceptual comeback as the processes of extinction become better quantified. The so-called Allee effect can be broadly defined as a “…positive relationship between any component of individual fitness and either numbers or density of conspecifics” (Stephens et al. 1999, Oikos 87:185-190) and is attributed to Warder Clyde Allee, an American ecologist from the early half of the 20th century, although he himself did not coin the term. Odum referred to it as “Allee’s principle”, and over time, the concept morphed into what we now generally call ‘Allee effects’.

Nonetheless, I’m using Allee’s original 1931 book Animal Aggregations: A Study in General Sociology (University of Chicago Press) as the Classics citation here. In his book, Allee discussed the evidence for the effects of crowding on demographic and life history traits of populations, which he subsequently redefined as “inverse density dependence” (Allee 1941, American Naturalist 75:473-487).

What does all this have to do with conservation biology? Well, broadly speaking, when populations become small, many different processes may operate to make an individual’s average ‘fitness’ (measured in many ways, such as survival probability, reproductive rate, growth rate, et cetera) decline. The many and varied types of Allee effects can work together to drive populations even faster toward extinction than expected by chance alone because of self-reinforcing feedbacks (see also previous post on the small population paradigm). Thus, ignorance of potential Allee effects can bias everything from minimum viable population size estimates, restoration attempts and predictions of extinction risk.

A recent paper in the journal Trends in Ecology and Evolution by Berec and colleagues entitled Multiple Allee effects and population management gives a more specific breakdown of Allee effects in a series of definitions I reproduce here for your convenience:

Allee threshold: critical population size or density below which the per capita population growth rate becomes negative.

Anthropogenic Allee effect: mechanism relying on human activity, by which exploitation rates increase with decreasing population size or density: values associated with rarity of the exploited species exceed the costs of exploitation at small population sizes or low densities (see related post).

Component Allee effect: positive relationship between any measurable component of individual fitness and population size or density.

Demographic Allee effect: positive relationship between total individual fitness, usually quantified by the per capita population growth rate, and population size or density.

Dormant Allee effect: component Allee effect that either does not result in a demographic Allee effect or results in a weak Allee effect and which, if interacting with a strong Allee effect, causes the overall Allee threshold to be higher than the Allee threshold of the strong Allee effect alone.

Double dormancy: two component Allee effects, neither of which singly result in a demographic Allee effect, or result only in a weak Allee effect, which jointly produce an Allee threshold (i.e. the double Allee effect becomes strong).

Genetic Allee effect: genetic-level mechanism resulting in a positive relationship between any measurable fitness component and population size or density.

Human-induced Allee effect: any component Allee effect induced by a human activity.

Multiple Allee effects: any situation in which two or more component Allee effects work simultaneously in the same population.

Nonadditive Allee effects: multiple Allee effects that give rise to a demographic Allee effect with an Allee threshold greater or smaller than the algebraic sum of Allee thresholds owing to single Allee effects.

Predation-driven Allee effect: a general term for any component Allee effect in survival caused by one or multiple predators whereby the per capita predation-driven mortality rate of prey increases as prey numbers or density decline.

Strong Allee effect: demographic Allee effect with an Allee threshold.

Subadditive Allee effects: multiple Allee effects that give rise to a demographic Allee effect with an Allee threshold smaller than the algebraic sum of Allee thresholds owing to single Allee effects.

Superadditive Allee effects: multiple Allee effects that give rise to a demographic Allee effect with an Allee threshold greater than the algebraic sum of Allee thresholds owing to single Allee effects.

Weak Allee effect: demographic Allee effect without an Allee threshold.

For even more detail, I suggest you obtain the 2008 book by Courchamp and colleagues entitled Allee Effects in Ecology and Conservation (Oxford University Press).

CJA Bradshaw

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(Many thanks to Salvador Herrando-Pérez for his insight on terminology)

Foiling the frog-killing fungus

15 12 2008

© F. Brem

© F. Brem

Something I picked up the other day that is an interesting application of ecology and engineering – extreme interventions like this may become more and more necessary, especially for particularly vulnerable taxa like amphibians. This one from New Scientist:

A fungal disease is decimating amphibian populations around the world, and so far the only way to save a species at risk is to remove individuals from the wild. Is it time to try taking out the disease as well?

So far the majority of amphibian conservation efforts have focused on identifying species at high risk of extinction, and establishing captive breeding programmes in biosecure units where they will be protected from Batrachochytrium dendrobatidis Bd, the chytrid fungus that is responsible for the devastating scourge.

“The immediate response has been the right one: to get species that are at risk into captivity,” says Trent Garner of the Institute of Zoology in London. However, he adds, “we’re potentially missing out on some very important species” because, inevitably, some are prioritised over others.

Now an alternative strategy is emerging, which many had previously thought impossible: to reduce the amount of Bd in the wild, and perhaps even to enable amphibians to survive alongside it.

In lab experiments, Garner and colleagues have shown that it is possible to cure tadpoles infected with Bd by bathing them in the antifungal drug itraconazole for 5 minutes a day for seven days. “Even using extremely low doses, we showed that you can eliminate Bd from tadpoles,” says Garner, who presented his results at a meeting on amphibian decline at the Zoological Society of London last week.

That looks rare – I’ll kill that one

12 12 2008

Here’s an interesting (and disturbing) one from Conservation Letters by Gault and colleagues entitled Consumers’ taste for rarity drives sturgeons to extinction.

I like caviar, I have to admit. I enjoy the salty fishy-ness and the contrast it makes with the appropriate selection of wine (bubbly or otherwise). I guess a lot of other people like it too, to the extent that worldwide sturgeon population’s have been  hammered (all 27 species are listed in CITES Appendix I or II, and 15 species are still heavily exploited). Indeed, in the Caspian Sea from where 90 % of caviar comes, sturgeon populations have declined by 90 % since the late 1980s. Admittedly, I haven’t had sturgeon caviar very often, and I doubt I’ll ever eat it again.

Using a set of simple ‘preference’ experiments on epicurean (French) human subjects, Gault and colleagues found that when told that a particular type of caviar was rarer than the others (when in reality, they two choices were identical), these refined gourmets generally tended to claim that the rarer one tasted better.

This means that humans have a tendency to place exaggerated value on harvested species when they think they’re rare (in most instances, rarity is itself the result of over-exploitation by humans). This so-called ‘anthropogenic Allee effect‘ (see Courchamp et al. 2006) basically means that at least for the wildlife-based luxury market, there’s little chance that calls for reduced harvest will be heard because people continually adjust their willingness to pay more. This turns into a spiralling extinction vortex for the species concerned.

What to do? Ban all trade of caviar? This might do it, but with the reluctance to reduce highly profitable industries like this (see previous post on tuna over-exploitation here), there’s a strong incentive even for the harvesters to drive themselves out of a job. Consumer education (and a good dose of guilt) might help too, but I have my doubts.

CJA Bradshaw

© S. Crownover courtesy of Caviar Emptor

© S. Crownover courtesy of Caviar Emptor

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Our new Environment Institute: tackling environmental crises

9 12 2008

© T. Hampel

© T. Hampel

It’s official, the University of Adelaide has put in some major investment to get its environmental research specialists together to turns things into high gear. I’m privileged to be a part of the Institute, and I hopefully will be blogging about many of the exciting, topical and revolutionary research coming out this new ‘think tank’ (also, a ‘do tank’) over the coming years.

This report from AdelaideNow:

THE University of Adelaide will bring together experts in water management, climate change, economics, marine research, energy technology and ancient DNA to tackle Australia’s most pressing environmental challenges.

The new Environment Institute will be headed by water policy expert Mike Young who said Australia faced diabolical policy problems in relation to climate change and water resources.

“While climate change is the issue of greatest national importance, it is arguable that water is the issue of most interest to South Australia,” Professor Young said.

“The River Murray, our greatest ecological icon, is under terminal stress and we need to find alternative water sources.

“We should expect the adverse effects of climate change to first be expressed in water.”

Professor Young said research was needed to help reduce Australia’s carbon footprint, to restore and improve native habitats and restructure agricultural systems.

“Many of these issues have been dealt with in isolation in the past but this is no longer an option,” he said.

“All are linked and must be dealt with in a holistic and co-ordinated way.”

Also involved in the institute will be the university’s climate change expert Barry Brook and conservationist David Paton.

University vice-chancellor James McWha said all of the institute’s researchers had an outstanding track record and were internationally recognised in their fields.

“Collectively, they have been growing their research at a phenomenal rate over the past five years and they will play a critical role in building the state’s reputation as a global leader in environmental research,” Professor McWha said.

Some biodiversity with your coffee, Sir?

7 12 2008

Bird on coffee cherriesI really like my coffee. I’m sure there are a few billion humans who claim likewise, but I think I could safely categorise myself as a coffee snob. I cannot even contemplate placing powdery crystals into a cup of hot water and calling it ‘coffee’, let alone imbibing the toxic concoction. I spend way too much money on very slow-roasted, dark, oily beans that have to be ground to the exact espresso consistency to use in my Bialetti cafettiera, and I’ll search high and low for the best coffee produced in any city in which I live or to where I travel (N.B. Still haven’t found what I call a ‘great’ coffee in the CBD of Adelaide – suggestions welcome). I really, really like good coffee.

What the hell does all this meandering preamble have to do with biodiversity conservation? I’m happy you asked. With environmentally conscious consumers now demanding some sort of ‘green’ certification for many products (e.g., no palm oil, carbon-neutral, fair trade, etc.), coffee has also been targeted as a good product to certify for harvest and production of lower environmental impact than has been done traditionally. Well, how do you measure ‘green-ness’ in a product? For coffee, there are some good ways.

A recent paper (and candidate for the Potential list) by Aaron Gove and colleagues published in Conservation Letters entitled Ethiopian coffee cultivation – implications for bird conservation and environmental certification demonstrates how the cultivation of this NATIVE Ethiopian plant (Coffea arabica) can enhance or restore the biological value of lowland agricultural areas. This species of ‘highland coffee’ is harvested from forests (where it evolved and now grows naturally) and from more intensive farmland. Interestingly, this species needs some shade to grow, so trees must generally be planted in the agricultural areas to allow this. Result? Gove and colleagues found that birds who otherwise wouldn’t be seen dead in the agricultural areas were attracted there by the maintenance and proliferation of the shade trees, thus reducing regional extinction risk for fragmented populations dependent on forest remnants. The flip side was that coffee cultivation in forest remnants reduced bird diversity because of the obvious trade-off between some native trees and intensive agricultural crops.

So, the next time you’re thinking of buying certified coffee, think of this – the cultivation of INDIGENOUS (did I say that loudly enough?) coffee species requiring shade promotes the proliferation of native forest trees to reduce the extinction risk of threatened birds. The number of boxes to tick on my coffee-snobbery list has just grown by two.

CJA Bradshaw

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