Transcript Describing communities

Community Structure & Function
A community has emergent properties
beyond the population level
– Number of species
– Relative abundance of each species
– Coadaptation of species within community
– Associated abiotic conditions
Community Structure and Diversity
• Species richness (S) = the number of species
in the community
• Species even-ness (or relative abundance) =
the proportion that individuals of each species
contribute to total individuals of all species
Two measures of community diversity
Simpson’s index
D = 1/((pi2))
pi = relative abundance of each species = ni / N
D
= 1 if only 1 species is present (no diversity)
Dmax = S if all species have equal numbers
(perfect even-ness)
Two measures of community diversity
Shannon (or Shannon-Weiner) index
H = -(pi)(lnpi)
pi = relative abundance of each species = ni / N
H
= 0 if only 1 species is present (no diversity)
Hmax = ln(S) if all species have equal numbers
(perfect even-ness)
Species Rank - Abundance Curves incorporate both
richness and even-ness
Species Accumulation Curves
also reveal richness and
even-ness
primary forest
secondary forest
plantation forest
Barlow J et al. PNAS 2007;104:18555-18560
Not all species within a community are equivalent
KEYSTONE
DOMINANT
Species Ecological
Effect
RARE
UMBRELLA
INDICATOR
Species abundance (or biomass)
Sea otter
Kelp
Sea urchin
OP
OA
OT
Otter presence affects kelp density (especially understory kelps)
Otter presence increases
kelp density…
by reducing urchin density.
OTTER RECOLONIZATION
Typically leads to urchin declines
- kelp recovery in some cases
- but not in others (hysteresis)
Recruitment (of kelp) vs Offtake (of kelp by urchins)
Sea otter – urchin – kelp: trophic cascade
Two Contrasting Views of Communities
• Clements: ORGANlSMAL concept of
community structure: a community is a set
of species that are co-adapted to biotic
interactions with one another
• Gleason: INDIVIDUAL concept of
community structure: community is a set of
species, but each species responds to
environmental gradients independently
Clements
Gleason
Ecotones ARE easily recognizable breaks in community structure,
but often arise due to adaptations to abiotic factors,
rather than biotic interactions among species
Some support for each of the two views