Key words :
Too Much of Too Many Good Things
3 May, 2007 12:04 pm
The idea of niche dimension is that greater diversity at lower levels in a food web allows greater diversity at higher levels: greater diversity of limiting nutrients (like nitrogen, phosphorus and potassium) leads to greater diversity of plants; greater plant diversity might lead to greater herbivore diversity, and so on. Plants in natural habitats are specialized for and compete for many different and relatively scarce nutrients and other environmental factors. Differences in how plants relate to these many factors is part of their niche. Niche dimension is basically how many niches there are ? more niches means more species can coexist.
We tested this niche dimension idea in a grassland near Santa Barbara, California (see W.S. Harpole & D. Tilman. 2007. Grassland Species Loss Due to Reduced Niche Dimension. Nature. 446:791-793). This grassland was made up of many species of grasses and wildflowers. These coexisting species compete for multiple soil resources: nitrogen, phosphorus, potassium and water are all simultaneously limiting to plant growth. We found that more and more species were lost as we added greater numbers of different nutrients to grasslands. For example, we saw the greatest loss of species only when we added nitrogen, phosphorus, potassium and water all at the same time. One exotic grass, Bromus diandrus, or “rip gut”, took over in the plots with high soil nutrients, and as a consequence created very dark light conditions below its canopy.
Similar patterns consistent with this niche dimension idea have been seen in lakes, and we expect that it should work in marine systems as well and may therefore be a general mechanism that determines species diversity. Inputs of nutrients to aquatic systems often leads to growth of algae and decreased light levels and loss of species diversity, similar to how one particular fast growing annual grass took over our experimental plots and shaded out other species. However, aquatic systems differ in some ways from terrestrial systems. Nutrient pollution to aquatic systems can lead to oxygen depletion once the algae die and decompose; oxygen depletion can drive away or kill fish. The dead zone in the Gulf of Mexico is the classic example of this. Another consequence of nutrient pollution for aquatic systems is that it can promote the growth of harmful and toxic algae.
Adding multiple nutrients to grasslands can have a very long-lasting effect. We looked at data from the world’s longest running ecological experiment, The Park Grass Experiment at Rothamsted, Harpenden, U.K, which was established in 1856. Of the plots we looked at, those getting applications of four different nutrients showed the greatest loss of species. Diversity has not recovered in these plots for over 150 years. Given the amount of nutrient pollution occurring world wide, the long-term consequences of environmental homogenation to biodiversity are of great concern.
Many natural systems are experiencing nutrient pollution as a result of human activities. One example is atmospheric nitrogen deposition resulting from fossil fuel combustion that produces nitrogen compounds that eventually come back to earth in rain or dust. High levels of nitrogen deposition can occur downwind of major urban centers. Phosphorus and other nutrients can also runoff from agricultural land and cause nutrient pollution of water systems. We may be losing species because nutrient pollution is effectively homogenizing habitats and making them fit for only a small number of species.
W. S. Harpole and D. Tilman, Nature 446, 791-793 (12 April 2007)
Key words :
Global conservation priorities based on human need
Tropical forests worth more standing
The Island Rule: Made to be Broken?
Marine reserves and climate change: study finds no-take reserves do not increase reef resilience
The Rockefeller Foundation and climate change