In general, species diversity is lower in ecosystems that are stressed than in those in which conditions are optimal for life. Thus ecosystems in dry regions have lower diversity than those where rainfall equals potential evaporation. For example, in the dry forests on the southwestern coast of Puerto Rico, diversity of trees is lower than in the rain forest of the eastern end of the island (Murphy et al. 1995). However, diversity can be low where excess rain causes soils to be frequently saturated. Nutrient imbalance in soils can also cause stress: ecosystems on nutrient-poor soils have fewer species than those on richer soils. Ecosystems on soils with high levels of potentially toxic elements such as selenium are also relatively low in species. Salt marsh ecosystems have low diversity because of the stress of salt and tides. Low temperatures and large daily fluctuations in temperature at high elevations cause stress, resulting in lower diversity at higher elevation. Highest diversity is almost always found in ecosystems that are not stressed, that is, they are well watered, have deep rich soils, and optimum temperatures for photosynthesis.
Some ecologists may argue that there is no such thing as a natural ecosystem that is stressed because there are always evolutionary adaptations to conditions such as infertile soils, low rainfall, and extreme temperatures, and thus the plants that live under these conditions cannot be classified as stressed. The problem is with the definition of stress. There are environmental conditions where plant growth is best. These optimal conditions consist of water availability that exceeds evapotranspiration but not to the extent where the soils are waterlogged. Nutrient elements are present in sufficient amounts in the soil. Temperatures are optimal, that is, somewhere between 22 and 30 °C, depending on the species (Aber and Melillo 1991). Above the optimal temperature, respiration exceeds photosynthesis and growth decreases. Below the optimal temperature, metabolic rates are low. Thus both high and low temperatures can be considered stressful. For the purposes of discussion, stress can be defined as any condition deviating from the optimum for plant growth. When we take this definition of stress, we can say that diversity generally decreases along a gradient of increasing stress, that is, where climate becomes very hot or very cold, rainfall is low or very high, and where soils are low in certain essential elements, but high in others, species diversity will decrease.
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