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The Latitude Effect

Rosenzweig (1995) attributes the high scatter in Fig. 2.1 in part to the small sampling areas (0.1 ha). However, other factors such as soils and local climate may be more important for explaining the high scatter of diversity at any given latitude. This raises the question, what is the factor that determines the latitude effect? The number of daylight hours during the year varies very little from equator to poles, and, consequently, the total annual amount of solar energy impinging on the earth's surface also varies very little. However, there is a great difference in the amount of light reaching the Earth's surface per year, when temperatures are above freezing. There is a strong decrease in the amount of light when temperatures are above freezing, along a line from equator to poles. Consequently, there is much less light available for photosynthesis at high latitudes than at low latitudes. There are also lower average yearly temperatures. However, average daytime temperatures during the growing (frost-free) season vary very little along a latitudinal gradient (at locations at the same elevation), so this cannot be the factor that causes differences in diversity. Light available during the growing season is the factor that constitutes the latitude effect (Jordan and Murphy 1978).

Because of the large variations in soils, rainfall, and temperature as affected by elevation that can exist within single latitudes, for an examination of the latitude effect on species diversity, the comparison must be restricted to ecosystems with similar soils, similar rainfall, and at similar elevations. The gradient of increasing diversity with decreasing latitude can only be found when comparing similar types of forest ecosystems such as lowland moist forests on good soils. To show unequivocally that species diversity changes as a function of latitude, all other variables must be held constant (Box 2.1).

Latitudinal gradients of species diversity

If latitude (yearly light available for photosynthesis) influences diversity, and if effects of soils, rainfall, and temperature as affected by altitude are eliminated, there should be a linear increase in the number of species per unit area as one approaches the tropics. It is quite difficult to find a region where these other factors are constant along a latitudinal gradient. However, Rosenzweig (1995), using data from Specht (1988), was able to show a latitudinal gradient in diversity of plants in sandy coastal habitats in Australia (Fig. 2.3), a single uniform biotope.

Likewise, Lewis (1991) found a gradient of increasing floristic richness with decreasing latitude in the subtropical forests of the eastern Chaco region in Argentina. In these forests, at an intermediate scale, different forest types are arranged according to environmental gradients correlated with topographic elevation. At a fine scale, many micro-sites can be discerned with different microenvironments colonized differentially by species. It is the greater number of species in the micro-sites at lower latitudes that results in the diversity gradient.

Fig. 2.3. Latitudinal gradient of number of plant species under similar conditions of climate and soils in Australia. (Adapted from Rosenzweig 1995, with permission from Cambridge University Press)

Fig. 2.3. Latitudinal gradient of number of plant species under similar conditions of climate and soils in Australia. (Adapted from Rosenzweig 1995, with permission from Cambridge University Press)

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Solar Panel Basics

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