COMMUNITY

BIODIV

ERSITY

&

DEVELOPM

ENT

CHAPTER 5

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

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 diversityAdditional plants provide more opportunities for ‘microhabitats’ and create additional niches

SPECIES RICHNESS EXPLANATIONS…

2. Geographic isolationInversely 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 historyOlder, 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 soil2.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 partsearly stages like infancy, climax community like adulthood

Individualist model – each species has its own particular abiotic requirements and there is no cooperation