COMMUNITY BIODIVERSITY & DEVELOPMENT CHAPTER 52

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

SPECIES RICHNESS EXPLANATIONS 1.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

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

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

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

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

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: 1.Primary succession – in an area not previously occupied; no soil 2.Secondary succession – in an area where a previous community existed but experienced some type of massive disturbance; soil is still present

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

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