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Understanding Biodiversity: Species Richness and Diversity Gradients

This chapter explores biodiversity by examining species richness, defined as the total number of species in a specific area. It highlights the importance of heterogeneity in ecosystems, with greater diversity occurring in regions with more complex habitats. The chapter also addresses factors influencing diversity gradients, such as climate, topography, and competition. It emphasizes the significance of evolutionary history and environmental stability on species diversity, especially in tropical regions. The interplay between habitat diversity and competition is discussed to illustrate how ecological factors shape biodiversity.

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Understanding Biodiversity: Species Richness and Diversity Gradients

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  1. Biodiversity – Chapter 22

  2. Biodiversity • Species richness – the total number of species in an area • Simplest measure of biodiversity • Heterogeneity - higher when there are more species and they are equally abundant

  3. Determining Species Richness • Species count depends on sample size • Relatively few species are very common • Estimated 5 – 30 million species exist on Earth: Only about 1.4 million are described • ~10% of all life • Taxonomists – people that describe and categorize species

  4. Diversity Gradients • Diversity increases towards the equator

  5. Factors That Might Cause Diversity Gradients • Eight Factors:

  6. History Factor • Evolution = speciation • Tropics  warmer and more humid, so they are more likely evolve and diversify faster • Tropical biotas are mature; temperate and polar are immature • All communities diversify over time

  7. Spatial Heterogeneity • The more heterogeneous and complex the physical habitat, the more complex the animal and plant community  the greater the diversity • Topographical relief important for species diversity • More habitats = more species • Highest diversity of US mammals occur in mountainous regions

  8. Habitat Diversity Between Habitat (Beta) Diversity Within Habitat (Alpha) Diversity

  9. Competition • In tropics: • Animals and plants are more restricted in their habitat requirements  increases between habitat (beta) diversity • Animals may also have a more restricted diet in each habitat, increasing within-habitat (alpha) diversity • Competition is keener in tropics, niches are smaller • Tropical species are more highly evolved and possess finer adaptations than do temperate species

  10. Niche Patterns # species determined by niche breadth # species determined by niche overlap

  11. Predation • Predators keep prey numbers so low, competition is reduced • Leads to an increase in types of prey, which leads to an increase in types of predators • Removal of a starfish from a tidal shore  decrease in prey diversity

  12. Climate and Climatic Variability • More stable the climatic parameters and the more favorable the climate  the more species • Similar to the history theory • Species richness limited by the available energy • Equator to the poles  decrease in diversity • See figure 22.18

  13. Productivity • The greater the productivity the greater the diversity: everything else being equal • Data does not support this theory • Can be supported when put in the context of length of growing season (stability hypothesis)

  14. Disturbance • If natural communities exist at equilibrium and the world is spatially uniform, then competitive exclusion rules and there will only be a few dominant species. • Moderate disturbance increases species diversity Intermediate Disturbance Hypothesis

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