Species richness is associated with<br>Productivity and stability<br>To test the hypothesis that specs-richs is more<br>Stable sspecies-poor communities, David Tilman and his colleagues at the University of Minnesota cleared 120 outdoor plots,<br>In whatthey ford grasses in mixtures ranging from 2 to 22<br>grass species. At the end of each growing season, they measured<br>Total plant cover (a measure of grass biomass, and so of net primary production) and the population densities of all the grasses<br>in each plot. Over a period of 11 years, what is an a serious<br>drought, the plots with more species were more more more (slof 57.16A), and their sproductivity wass es sys variable year<br>To year. These findings werei a tha a' tha a' tha a' tha a'<br>species richness promotes s productivity and keeps productivity<br>stable. Moreover, in the plots with the greater specis, soil<br>nitrogen was used more more efficiently (Figure 57.16B). However,<br>The population densities of individual species in the thins were<br>Not stable over the years (regardless of a plot's species richness)<br>Because of the specs specis synd ed better fod years<br>and wet years. In other words, higher species richness sans ad<br>The stability ity of the productivity in the plots, but not the stability of the<br>bztion sup.<br>Researchers sy to debate what-why-specisd d<br>is responsible for the stability or is simply correlated<br>With stability. This question is important be be many of the cause<br>alterations humans have made in the meth of the natural communities have rheed disseth srichness, and many of them<br>These human-altered communities-not audness-a-rifany unstable. ...
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