Community Ecology
Community Ecology
Community: all the individuals in a given area.
Species Richness: the number of different species in the community.
Relative Abundance: proportion of individuals each species contributes to the total.
Species Importance
Factors that indicate a species' importance:
Density: Number of individuals of that species in the community.
Coverage: Size of each individual of the species.
Frequency: Widespread versus localized distribution.
Keystone Species: A species that has a disproportionately large effect on its community; removal of such a species affects others.
Example: Sea otters maintain kelp forest health by controlling sea urchin populations.
Tropic Structure of a Community
Illustrates the flow of energy among species.
Interactions between species:
Interactions in Ecosystems
Interspecific Competition
Competition between different species for the same resources.
Competitive Exclusion Principle: When two species compete for identical resources, the more reproductively advantageous species will dominate.
Example: Eastern fox squirrels outcompete native gray tree squirrels due to faster reproduction rates.
Ecological Niche
Total biotic and abiotic resources utilized by a species.
Example: Marbled murrelet requires cool coastal waters, small fish, and old-growth conifer forests for nesting.
Resource Partioning
Species narrow their niches utilizing a sliver of resources, reducing competition.
Fundamental Niche: The potential niche a species could occupy without competitors.
Predation & Herbivory
Predation: Organisms consuming non-plant organisms.
Herbivory: Organisms consuming plants.
Plants defend via:
Chemical defenses: bitterness, toxicity, irritation.
Structural defenses: physical barriers to herbivores.
Species Interactions
Facilitation
Presence of one species enables another to thrive.
Example: Abandoned rabbit burrows provide habitat for other species.
Symbiosis
Two or more species cohabitating.
Commensalism: One species benefits, the other is unaffected.
Example: Remora fish benefiting from sharks.
Mutualism: Both species benefit.
Example: Giraffes and red-billed oxpeckers.
Parasitism: One species benefits while harming the other.
Example: Tapeworms in animal guts.
Parasitoids: Organisms that consume their host but are not symbiotic.
Mimicry
Müllerian Mimicry
A form of mimicry where two or more species that are harmful, toxic, or unpalatable to predators evolve to look similar.
Example: the monarch butterfly and viceroy moth look the same and are unpleasant to eat
Batesian Mimicry
A form of mimicry where a harmless species (the mimic) evolves to resemble a harmful or unpalatable species (the model).
Example: Hoverfly and wasp
Disturbance and Community Recovery
Effects of Disturbance
Natural disturbances (e.g., fire, flood) can drastically alter communities short-term.
Ecological Succession: Process of community recovery post-disturbance.
Secondary Succession: Recovery in an area where some habitat remains (e.g., soil with seeds).
Primary Succession: Recovery from a disturbance that leaves no life (e.g., lava flow).
Intermediate Disturbance Hypothesis
High species diversity in a community is maintained with intermediate frequencies/severity of disturbance.
Fire Adapted Ecosystems
Most California ecosystems: adapted to burn every 30-150 years.
Plant Adaptations to Fire:
Thick tuberous roots can survive fires and resprout.
Some plants germinate after fire, stimulated by heat or smoke.
Fire Return Interval
Fire Returnal Interval: Classification based on fire frequency.
Yellow-Red: Frequent fires, Blue-Purple: Infrequent fires.
Both lead to biodiversity loss as invasive species can take over native plant habitats.
Invasive Species
Common invaders include:
Bullfrogs: Generalist feeders that can eat a wide variety of prey.
Common Carp: Disturb water sediment and consume large quantities of aquatic life.
Fox Squirrels: Compete with other squirrel species.
Invasive Plants: Such as wild mustard or ice plant destabilize ecosystems leading to landslides.