Friday :: May 25, 2007

one in six

by Christina Hulbe

The first comprehensive assessment of European mammal populations was released earlier this week. The bottom line: 15% of Euorpean mammal species are currently threatened with extinction (22% for marine mammals) and only 8% of species are increasing in numbers. The main threats are habitat degradation and loss, pollution, and over harvesting. The European Mammal Assesment website has more.

Plants build the foundations of terrestrial ecosystems, turning inorganic materials and sunlight into the organic energy sources that fuel other organisms. Yet while plants set the basic ecosystem structure, mammals do the "landscaping." Mammals modify vegetation structure and nutrient pathways, and in so doing, can modify species composition throughout an ecosystem.

a structure example
The eastern Serengeti is tropical grassland, a vast range of short grasses and small flowering plants (herbaceous dicots). Structure and composition of the grassland is maintained by nearly continuous grazing by migratory wildebeest and zebra herds. When rinderpest nearly wiped out the eastern Serengeti wildebeest in the first half of the 20th century, the grassland changed. Without heavy grazing, grasses are able to grow taller, a change that forces out the dicots, as well as birds and insects adapted to the short grass plain. New species move in to take their place, of course, and the ecosystem is changed. In effect, wildebeest grazing manages the landscape, favoring some organisms over others. Grazing is a primary structual control in both tropical and temperate grasslands.

a nutrient cycling example, also from the Serengeti
Non-migratory gazelles, topi, and hartebeest tend to congregate in locations where soils are rich in sodium (on which animals depend but plants do not). Large ungulate population density means rapid grazing and accompanying fecal deposition. Nutrients in the feces return rapidly to the soil, where they enhance the growth of forage plants. By congregating at a site that meets certain needs (sodium), the ungulates improve (fertilize) the soil and in turn improve foraging at that site. This creates a positive feedback that keeps ungulate population density high (scientific paper on this topic).

Grazing isn't the only thing that matters, of course. Landscape restructuring is pronounced when mammals are food limited. Where predators regulate population size, the effects of grazing on the landscape are reduced. So what governs the more complete relationship? In brief: body size (lions have a tough time catching and eating elephants), migration patterns (predators do not follow herds year-round), and diversity of both predators and prey in an ecosystem. The complete suite of interactions go all the way up from nutrients available in the soil to the top predator and all the way back down from predator to soil.

A "natural experiment" of sorts demonstrated the importance of these interactions when Venezuela's Caroni Valley was flooded in 1986. Islands created as the valley filled with water became fragments of the former landscape, on which different proportions of the orignal mammal cohort, some with and some without predators. The fates of the two groups of islands were very different. A news story from 2001 is at John Terborgh's Duke Universty research group's website here and a recent update is here. The 2001 article in the journal Science is here and the update is here. Fascinating stuff.

So to think critically about the broad meaning of mammal extinction driven by habitat degredation, we really need to embrace the ecology of entire ecosystems, not just the species of interest to us. For example, if we eradicate the top predators, that change will propagate all the way down to the soil.

further reading
A few years ago, the journal Nature published a special insight section on biodiversity. The Nature page for that section contains a bunch of broken links but I found another source for them here. A paper of particular interest to the present discussion, is Consequences of changing biodiversity (pdf) by Chapin and others.

Stuart Pimm, venerable ecologist, has written some great popular articles (and a book) over the years. A good place to start reading about contemporary extinction is his article The dodo went extinct (and other ecological myths) (here). Also check his research group's website.

Christina Hulbe :: 7:44 AM :: Comments (3) :: Digg It!