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(c) Percentage of Earth's net primary production

Net Primary Production
55.5 Net Primary Production of Terrestrial Ecosystems Variations in temperature and water availability over Earth's land surface influence the annual production of ecosystems.

latitude deserts, where plant growth is limited by lack of moisture, annual primary production is low. At still higher latitudes, even though moisture is generally available, annual primary production is also low because it is cold much of the year (Figure 55.5). Production in aquatic systems is limited by light, which decreases rapidly with depth; by nutrients, which sink and must be replaced by upwelling of water; and by temperature (see Chapter 58). Primary productivity strongly influences two other important features of ecological communities: species richness and food web structure.

Species richness is influenced by primary productivity

The number of species living in a community (its species richness) is correlated with gross primary productivity, but the relationship between these two factors is complex. Ecologists first observed that species richness often increases with productivity up to a point, but then decreases (Figure 55.6). The increase occurs because the number of individuals an area can support increases with productivity, and with larger population sizes, species extinction rates are lower. But why should species richness decrease when productivity is still higher?

One hypothesis proposed to explain this decrease postulates that interspecific competition becomes more intense when productivity is higher, resulting in competitive exclusion of some species. This hypothesis is supported by the re sults of a long-term experiment at the Rothamstead Experiment Station in England in which fertilizer has been added to some plots of land to increase productivity. Fertilized and unfertilized plots have been monitored continuously at Rothamstead since 1856. Over this time period, the number of plant species in unfertilized plots has remained roughly constant, whereas species richness has declined in the fertilized plots.

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Essentials of Human Physiology

Essentials of Human Physiology

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