Community Structure Function A community has emergent properties

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Community Structure & Function A community has emergent properties beyond the population level –

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

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/( (pi 2)) pi =

Two measures of community diversity Simpson’s index D = 1/( (pi 2)) 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

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 Rank - Abundance Curves incorporate both richness and even-ness

Species Accumulation Curves also reveal richness and even-ness primary forest secondary forest plantation forest

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

Not all species within a community are equivalent KEYSTONE DOMINANT Species Ecological Effect RARE UMBRELLA INDICATOR Species abundance (or biomass)

Sea otter

Sea otter

Kelp

Kelp

Sea urchin

Sea urchin

OP OA OT Otter presence affects kelp density (especially understory kelps)

OP OA OT Otter presence affects kelp density (especially understory kelps)

Otter presence increases kelp density… by reducing urchin density.

Otter presence increases kelp density… by reducing urchin density.

OTTER RECOLONIZATION Typically leads to urchin declines - kelp recovery in some cases -

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)

Recruitment (of kelp) vs Offtake (of kelp by urchins)

Sea otter – urchin – kelp: trophic cascade

Sea otter – urchin – kelp: trophic cascade

Two Contrasting Views of Communities • Clements: ORGANl. SMAL concept of community structure: a

Two Contrasting Views of Communities • Clements: ORGANl. SMAL 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

Clements Gleason

Ecotones ARE easily recognizable breaks in community structure, but often arise due to adaptations

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

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