Soil fertility can vary greatly at any given latitude and can affect species diversity. For example, Stevens et al. (2004) found that in Great Britain, species adapted to infertile conditions were systematically reduced at high levels of nitrogen deposition. In earlier studies in which fertilizers were added to small plots, enrichment resulted in a loss of species diversity (Huston 1979). However, Tilman (1987) pointed out that lower diversity in enriched plots occurs because plant communities do not have time to adapt to changed conditions. When plant communities have evolutionary time to adapt to soil conditions, those on richer soils will have higher diversity than those on poor soils. A decrease in diversity following addition of fertilizer merely indicates that certain species are better able to take advantage of changing conditions than others. The former gain a competitive advantage and crowd out the latter. However, this is a short-term result. When we speak of high diversity on rich soils, we refer to an ecosystem where there has been time for species to immigrate or to adapt. Thus the number of plant species on highly weathered soils of the lowland rain forest of the upper Rio Negro in Venezuela (Clark and Liesner 1989) is lower than those found on younger, richer soils of Ecuador (Gentry 1988).
However, species richness in tropical forests may not always peak in the richest soils. Davis and Richards (1933-1934) found that in wet seasonal lowland Guyana, species richness peaked in mixed forest on moderately low-nutrient yellow sand Ultisols, and was somewhat lower on richer alluvial loams, and much lower on the acid, nutrient-poor Spodosols. Rich soils also will have lower biodiversity at high latitudes than rich soils at the equator, because of the latitude effect. Poor soils, such as those of the Galapagos Islands, often have a high incidence of endemism, but endemism is not to be confused with diversity. An ecosystem can be high in endemics but low in diversity, compared to other ecosystems at the same latitude.
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