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Lecture 34: Diversity, Productivity, Stability
Reading: None.
Community Structure Species Richness, Productivity, and Stability It is an old idea that communities with high levels of biodiversity (=species richness) may benefit from increased total production (carbon fixation) and stability (resistance to disturbance). The following statements were made by Charles Darwin in The Origin of Species (1859). "...if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and dry herbage can be raised in the latter than in the former case" "...the truth of the principle that the greatest amount of life can be supported by the great diversification of life, is seen under many natural circumstances" (p 85). Elton (1958) building on these ideas suggested that low diversity in a community results in decreased ecological stability. For example, the frequency of pest outbreaks in simple communities (crop lands) is greater than that in more complex communities (grasslands or forests). Modern Evidence Ecotron experiments (Naeem, Thompson, Lawler, Lawton, and Woodfin, 1994) Ecotrons are small isolated microcosms consisting of annual plants, insects, and soil fauna. Productivity was highest for more diverse assemblages and least productive for species poor assemblages in the ecotrons. Natural Prairie experiments (Tilman, Wedin and Knops, 1996) (Experiment 1) Experimental plots (147) in Minnesota prairie planted with 1, 2, 4, 6, 8, 12, or 24 species of native prairie plants. Replicates were random draws from the 24 species pool to make the various species richness treatments. Diversity-Productivity Hypothesis: Predicts that increased richness (diversity) should result in both greater production and greater utilization of limiting nutrients to attain greater production, so there should be lower rates of nutrient leaching with increased diversity.
Results (after Tilman, Wedin and Knops, 1996, Fig. 1): Increased plant cover (production) with increased species richness Increased utilization of root zone nitrate with increased species richness Decreased leaching of root zone nitrate with increased species richness
Similar patterns in plant production and nitrate utilization were observed in native grasslands (after Tilman, Wedin and Knops, 1996, Fig. 2).
Conclusion: Increased species richness yields increased total productivity, nutrient utilization, and nutrient retention. This conclusion was criticized by some researchers because species richness is potentially confounded by the types of species present in a plot (functional composition), and the number of different types of species present (functional diversity). A separate Minnesota prairie ecosystem experiment was conducted (Tilman, Knops, Wedin, Reich, Ritchie, and Siemann, 1997) (Experiment 2) in which species diversity, functional diversity, and functional composition were evaluated for their contributions to plant production, plant nitrogen content, soil nitrogen. A total of 289 plots (169 m2) were sown and maintained with 0, 1, 2, 4, 8, 16, or 32 species of perennial plants that represented 0, 1, 2, 3, 4, or 5 functional groups. Functional groups (5 groups): Legumes (fix nitrogen), C3 grasses, C4 grasses, woody plants, and non-woody forbs (herbs)
Results (after Tilman, Knops, Wedin, Reich, Ritchie, and Siemann, 1997, Fig 1 and 2):
Increased plant production with increased species diversity, functional diversity, or functional composition when considered separately, but the effects are caused by functional diversity and composition Plant total nitrogen increased and soil nitrogen levels decreased with functional diversity and functional composition but not species diversity. However, productivity and plant and soil nitrogen levels changed with species diversity within each of the functional groups except woody plants.
Conclusion: Functional diversity and functional composition are more important than species diversity (alone) in their influence on ecosystem production and nutrient utilization.
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