One-Third for the Birds

For this post I was looking for something a little different. Not so much a weird topic, but more like a topic that I haven't really posted on before. I came across an interesting article about predator-prey relationships as they relate to population sizes. It is one of those basic dynamics that is taught in almost every biology and/or ecology class there is, and rightfully so, because when you get down into it it is actually pretty interesting, no to mention really important.

Most people think they know predator-prey relationships. One thing eats another thing. The organism doing the eating is the predator and the one that is eaten the prey. That is perfectly true. Now think about it in terms of evolution. The predator and the prey evolve together, what is usually referred to as the "evolutionary arms race." If the predator does not catch any food then it does not survive; those that evolve better ways to catch food survive (speed, stealth, smell, sight, etc.). If the prey gets caught by the predator then it (obviously) does not survive, and so it evolves better ways to evade predators (camouflage, speed, poison, etc.). It is important to point out that both the predator and they prey are both adapting, a type of co-evolution. Next, think about the system in terms of population. The interaction of predators and prey greatly affect their population sizes, often on several levels within the food chain/web (or tropic pathway/cascade). When predators eat prey they: (1) decrease the population size of the prey, (2) survive, thereby not decreasing their own population, and (3) are healthy, and alive, enough to breed additional predators, increasing their population. So when there are lots of predators you see a big dip in the size of the prey population. You will can make the same types of conclusions when you take the point of view of the prey. When prey is eaten by the predator: (1) they are removed from the population, decreasing population size, (2) are no longer healthy and alive to reproduce, (3) release their food source (if there is one) to increase their population, and (4) by decreasing their population there is that much less food for their predators. These interactions can result in very predictable, natural boom and bust cycles within these populations and is more the focus of today's topic. If you want pictures and graphs then check out the classic lynx-hare relationship and the Lotka-Volterra Model.

A new paper published in Science takes a look at the role of top predators in marine ecosystems. These upper tropic level (UTL) species include seabirds, marine mammals, and large predatory fish. All groups that have been depleted due to human activities. Fisheries impacts cause direct mortality in the targeted species and indirect, often more subtle, mortality lower down in the food chain/web. Fisheries that specifically target lower tropic level (LTL) species (small fish, squid, crustaceans, etc.) threaten those higher tropic level species by directly removing their food sources. However, it can be very challenging to assess the impacts of fishing on food webs, an ecosystem level approach. Think about it: all those predator-prey interactions in the ocean. That's tough. Often what you see is studies, even large ones, taking on a big chunk of the system, modeling how it works based on gathered data, and drawing conclusions or feeding it into an even bigger model. This study in Science specifically looks at seabirds.

Figure 1: A map of the distribution of
seabird and prey species. (click for
larger view)

Seabirds are a good system because they are conspicuous members of the marine ecosystem globally, have been studied and measured for decades, can reflect change at several scales, long-term breeding datasets from several species in several environments exist, and data from systems where prey availability has been measured suggests that seabirds can be used as indicators for forage fish population fluctuations. This study looked at the response between seabird breeding success and forage fish abundance across various species and ecosystems. The modellers compiled 438 data points spanning 15 to 47 colony-years per breeding site, in a total of seven marine ecosystems spanning 19 time series, 14 seabird species and their prey. They crunched a whole lot of numbers, using statistical methods that I'm not going to mention here, to quantify the fluctuations in food abundance and breeding success.

They found something that you usually don't see when you crunch that much data - all their species showed the same response. The results showed that the number of fledglings per breeding pair started to decline and was more variable when their forage fish food source dropped below one third of its maximum observed amount. The prey becomes scarce and as a result the hunting becomes more inefficient. The birds do not have enough food to successfully raise that many young.

Sure, there are the normal high and low cycles that I mentioned at the beginning, and those are natural. However, adding high rates of fishing from humans into it and, as with many other systems, we exacerbate the problem or collapse the system completely. The fish populations never recover from low seabird numbers because predation and habitat destruction by humans still puts a large amount of pressure on the fish, causing chronic food scarcity for the birds. And, as this study shows, this scarcity can have long-term affects on breeding success, can reduce survival in adult birds, and may affect the trajectory of their populations. The thresholds revealed by this study should inform management objectives in balancing predator-prey interactions, a "keep one-third for the birds" approach. If done well, these management decisions can sustain healthy UTL predator populations, maintain LTL fish populations, and could be applied to other marine ecosystems.

Read the paper here:
Philippe M. Cury, et al. (2011) Global seabird response to forage fish depletion -- One-third for the birds. Science: 334(6063), 1703-1706. (DOI: 10.1126/science.1212928)

And a write-up in ScienceNOW called "A Surprising Threshold for Seabird Survival"

Learn more about predator-prey interactions at
University of Michigan's Introduction to Global Change Curriculum's "Trophic Links: Predation and Parasitism"
New England Complex Systems Institute (NECSI)'s Predator-Prey Relationships page
University of Wisconsin-Madison's SSCC page on Predator-Prey Models (Warning: Contains potentially scary math)
A neat demo from Wolfram of a Predator-Prey Model of foxes and rabbits

Also, revisit my post called "To Eat or Not to Eat, That is the Fishy Question" to learn more about how to purchase seafood from green stores or restaurants and guides on choosing the correct seafood.

(image from readysetwhoa.wordpress.com)