Community Biodiversity & development

1. Community Biodiversity & development Chapter 52

2. Species Richness vs. species diversity • Species richness = the total number of species in a community • high species richness – coral reefs, rain forests • low species richness – mountain tops, isolated islands • Species diversity = measures the relative importance of each species based on abundance, productivity or size • more diversity = more stability in the face of changes

3. Species richness explanations • Structural complexity • Often determined by the types of plants growing in an area • More types of plants (forest) = more diversity • Fewer plants (grassland) = less diversity • Additional plants provide more opportunities for ‘microhabitats’ and create additional niches

4. Species richness explanations… • 2. Geographic isolation • Inversely proportional: More isolation  less diversity • Distance effect – difficult for species in other communities to colonize • Some species may become locally extinct due to random environmental factors

5. Species richness explanations… • 3. Environmental stress • Also inversely proportional: more stress  less diversity • Only species that can tolerate extreme conditions can survive in highly stressful communities • Species richness-energy hypothesis: different latitudes effect species richness because of variations in solar energy

6. Species richness explanations… • 4. Position • Ecotone– the margins between two different communities • Diversity is greatest here when compared to the interior of each community – the edge effect • Contains a good number of habitats from each community

7. Species richness explanations… • 5. Geological history • Older, more stable areas tend to have more diversity  more time for evolution as well as immigration • This is known as the time hypothesis

8. Ecological succession • The stages of a community change over time • Occurs as one group of organisms is replaced by another • Generally look at the differences in plants, but these influence the animals • Two basic types: • Primary succession – in an area not previously occupied; no soil • Secondary succession – in an area where a previous community existed but experienced some type of massive disturbance; soil is still present

9. Why succession? • Climax community - early idea that succession always led to a ‘final’ community type, typical for that particular climate; this idea is currently out of favor • Intermediate disturbance hypothesis – all communities experience disturbances; periodic disturbances cause the community to revert to ‘earlier’ groupings of organisms

10. Community Models • Organismic model – a community is like a ‘super-organism’ and resembles an individual body with cooperation between the parts • early stages like infancy, climax community like adulthood • Individualist model – each species has its own particular abiotic requirements and there is no cooperation