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Lecture 35: Biodiversity and Biogeography
Reading: Economy of Nature, pp. 546-571.
Biological Diversity Worldwide Biodiversity
Importance of Diversity
Patterns in Species Richness Number of species decreases with latitude, tropical communities support more species than do temperate of polar communities. This applies to all habitats, terrestrial, aquatic, and marine for a wide variety of organisms. Some examples of species richness gradients are given below for tunicates (marine invertebrate chordates), marine bivalves, birds, ants, mammals, and trees.
Species richness patterns among marine bivalve mollusks (Ricklefs, 1996, p 548, Fig. 24.1).
Species richness gradients are observed among ant species (Ricklefs, 1996, p 548 Fig. 24.1).
Species richness patterns among breeding birds in the northern hemisphere are given below:
Mammal species (species/150 square miles) in North America (Ricklefs, 1996, p 550, Fig. 24.4).
Tree Families, Genera and Species (Ricklefs, 1996, p 569, Fig. 24.20).
Cause for species richness gradients is not clear, but these gradients follow latitudinal gradients in primary production (after Begon, Harper, and Townsend, 1990, p 653, Fig. 18.2).
Consumer species richness follows that of primary producers as seen in bird species (Ricklefs, 1996, p 549, Table 24.1). Plant productivity and the average number of species of birds in representative temperate zone habitats
Hypotheses on causes for species richness gradients are at different levels of analysis and are not mutually exclusive. The great species richness of the tropics could result from: 1. More resources or more resource diversity 2. More specialization (smaller niches) but similar resource abundances 3. More niche overlap 4. Communities more fully saturated (more niches) (Ricklefs, 1996, p 550, Fig. 24.3) 5. More time with stable conditions More niches are expected in communities that have a greater diversity of plant foliage heights. The relationship between species diversity and foliage height diversity is clearly shown for bird species (Ricklefs, 1996, p 550, Fig. 24.3)
Community fragmentation and isolation may increase with geological time. Tropical conditions are the oldest and largest regions on earth. Tropical region surface areas are greater than either temperate or polar (arctic and antarctic) regions. Tropical rain forests cover 7% of land surface but contain more than 50% of all terrestrial species.
Habitat destruction leads to species extinctions.
Tropical Soils Old, deeply weathered soils (oligotrophic soils) as in the Amazon basin
Distribution of mineral nutrients in the soil and living biomass of a temperate and a tropical forest ecosystem.
Not all tropical soils are the same. Eutrophic versus Oligotrophic Soils
(Ricklefs, p. 176, Table 8.2) Table 4. Standing crops and fluxes of dry biomass and calcium in a nutrient-rich and a nutrient-poor tropical rain forest
Soil carbon content (humus) critical for nutrient and water retention Temperate and oligotrophic tropical soils differ in humus content prior to disturbance and lose humus at different rates following cultivation. Tropical soils are generally at much greater risk as a consequence of temperate zone type cultivation.
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