Latitude, altitude, productivity, spatial heterogeneity, size, age, and disturbance regime.
Latitudinal Gradients
For most groups of terrestrial plants and animals, diversity decreases with increasing latitude. Diversity is highest in the tropics and lowest at the poles. There are many factors correlated with this gradient that may affect species diversity, such as average temperature and precipitation, the variability in temperature and precipitation, annual net primary productivity, and geological history.
Altitudinal Gradients
Species diversity generally decreases with increasing elevation. Many physical conditions, such as air temperature, rainfall, wind patterns, and the variability of those conditions, change along altitudinal gradients. For example, air temperatures, in general, decrease 6oC with a 1000 m increase in elevation (Holdridge 1967).
Productivity
Primary productivity, the solar energy that is captured by plants and converted to carbon compounds, has been correlated with biodiversity in many instances. Precipitation, length of growing season, potential evapotranspiration, air temperature, and solar radiation, have all been demonstrated to be strongly correlated with species richness for a variety of taxa (Huston 1994).
Diversity and the Size of the Sample Area
Large areas generally have more species than small areas. Within a homogeneous area, samples of increasingly larger area will randomly sample an increasing proportion of the total population, and are thus likely to detect increasingly rarer species as the size of the sample increases.
Diversity and the Spatial Heterogeneity of the Sample Area
Spatial heterogeneity is, in general, positively correlated with biodiversity (Huston 1994). There are many causes of environmental heterogeneity, including those acting on large scales (geology, climate, topography, and major disturbance) and small scales (vertical complexity created by plants, and minor disturbance).
Diversity and the Age of the Sample Area
Age, the time over which living organisms have been continuously present in an area, is correlated with biodiversity over temporal scales ranging from days to millions of years. The number of organisms potentially present in an area increases with time as more and more species arrive. Biologically produced changes in heterogeneity. The balance between speciation rates and extinction rates.
Diversity and Disturbances
Disturbance is correlated with biodiversity on many different scales, with some positive correlations and some negative correlations (Huston 1994). Infrequent, massive disturbances such as glaciation, can lower diversity by killing or driving off organisms and altering the landscape. On the other hand, frequent and less severe disturbances have often been shown to increase biodiversity.
Diversity and Biological Interactions
Biological interactions, such as competition, predation, mutualism, parasitism, and disease, also influence species diversity, but the importance of these interactions varies greatly. In stable and productive environments, the effect of biological interactions on species diversity tends to be stronger than in unstable or unproductive environments (Szaro 1996).
Temporal Variation in Species Diversity
Diversity is constantly changing at many temporal scales. Fluctuations in species diversity are a natural phenomenon in all ecosystems. They occur in response to seasonal cycles, long-term climatic variations, and ecological processes such as succession. In some groups of organisms, diversity measured at one time of year is very different from that measured at different time of year (Huston 1994).
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