Transcript Community Biodiversity and Development

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 ‘superorganism’ 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