CH 56 COMMUNITY ECOLOGY
Community: many populations that live in the same place at the same time
56.1 Differing Views of Communities
- Organismic model: (Clements) view of community with predictable and integrated associations of species separated by sharp
- Individualistic model (Gleason): community as an assemblage of species coexisting primarily because of similarities in their
physiological requirements and tolerances.
o Distinctly structured ecological communities usually don’t exist, species distribute independently along an
environmental gradient – no sharp boundaries.
o Clement’s model abandoned for this one.
- Principle of species individuality: each species is distributed according to its physiological needs and population dynamics, most
communities integrate and competition doesn’t create distinct vegetational zones.
- There are 4 hypotheses explaining distribution patterns of plants and animals on the gradient:
1. Competition causes sharp boundaries between groups of species; certain species group together.
2. Competing species exclude one another along sharp boundaries but don’t become organized into groups of species.
3. Competition doesn’t cause sharp boundaries but adaptation of species to similar physical variables result in groups of
species with similar distributions
4. Competition doesn’t produce sharp boundaries; different species group together due to physical variables.
- Fourth hypothesis is correct look at mountain side when hiking.
56.4 Species Richness and Community Stability
Diversity-Stability Hypothesis states that species-rich communities are more stable
- Elton: disturbance in a species-rich community would be cushioned by large numbers of interacting species.
o Ex. Outbreaks of pests found in species poor system like cultivated land.
- Counterarguments: disturbed or cultivated land may suffer from pests not because it’s simple but because individual
species, including exotic pest species and native species of natural enemies often have no evolutionary history with one
- Tilman measured biomass of everyspecies in each grassland plot and found how much this biomass varied from year to
- Variation was lower in plots with greater species richness.
- Diverse plots were more likely to contain disturbance resistance species .
o Ex. Drought harms competitively dominant species, unharmed drought-resistant species increased in mass
and replaced them.
56.5 Succession: Community Change
- Succession: gradual and continuous change in species composition and community structure over time. o Primary: succession on a newly exposed site not previously occupied by soil and vegetation – hundreds of
years required for plants to build up soil.
o Secondary: succession on a site already supported life but undergone a disturbance – fire, tornado, hurricane,
- Climax community: distinct end point of succession.
- Sere: phase of succession; primary seral stage is initial sere
- Facilitation: each colonizing species made environment different and maybe better for some species which invaded and
outcompeted the earlier residents.
- When the dominant species had colonized, the community was said to be at climax.
Facilitation Assumes Each Invading Species creates a more favourable Habitat for succeeding species
- Ex. Retreat of Alaskan glaciers
- As glaciers retreat, they leave moraines, deposits of stones, pulverized rock, and debris taht serve as soil.
1. Bare soil has low nitrogen content and scant organic matter, it’s colonized by black crust of cyanobacteria, lichens,
moss...(nitrogen fixers) but soil depth and litterfall (dead plant material) are still minimal
2. At about 60 years, alders form dense, close thickets and have4 nitrogen-fixing bacteria living in roots. Spruce
begins to invade and overtop alders.
- In aquatic environments, some species enhance the quality of settling and establishment sites for another species.
o Ex. Cnidarians enhanced the attachments of tunicates and mussels.
Inhibition implies that early Colonists prevent later arrivals from replacing them
- Inhibition: space is important, the plant that gets there first determines subsequent community structure.
o Ex. Setaria faberi removed E. Annus increased in biomass.
o S. Faberi released a toxic compound when decomposing inhibits E. Annus.
Tolerance suggests that early colonists neither facilitate nor inhibit later colonists
- Tolerance: any sp