Community Ecology Notes

Ch55 Community Ecology

Community Definition

• A community consists of species that occur at a particular locality.
• It's characterized by:
  • Species richness: The number of species present.
  • Primary productivity: The amount of energy produced.
• Darwin noted that the most diverse ecosystems tend to have the greatest productivity.

Ecological Niche Concept

• Niche: The total of all the ways an organism uses the resources of its environment.
• Fundamental niche: The entire niche a species is capable of using, based on physiological tolerance limits and resource needs.
• Realized niche: The actual niche in which the species can establish a stable population.
• Niche restriction: Can occur due to:
  • Predator absence or presence.
  • Absence of pollinators.

Competition and Niche

• Example: Barnacles (Chthamalus and Semibalanus)
  • Chthamalus fundamental niche is larger than its realized niche due to competition with Semibalanus.
  • Semibalanus realized niche is restricted by Chthamalus in the upper tidal zone.
  • When S. balanoides is removed, C. stellatus fundamental and realized niches are identical.

Competitive Exclusion

• A species that uses a limited resource most efficiently at some location will eventually eliminate others.
• Often results in resource partitioning.
• Example: Sympatric lizard species dividing resources (e.g., different perches).

Character Displacement

• Differences in morphology between sympatric species.
• May lead to speciation.
• Example: Finch beak depth on different islands. Graph of individuals in each size class (%) vs finch beak depth (mm) for G. fuliginosa and G. fortis shows allopatric and sympatric distributions in Los Hermanos Islets, Daphne Major Island, and and San Cristóbal and Floreana Islands.

Predator-Prey Interactions

• Predation: Consuming of one organism by another.
• Often triggers a coevolutionary 'race' driving adaptation in both predator and prey.
• Example: Snowshoe hare and lynx population cycles. Graph of number of pelts (in thousands) vs Year showing snowshoe hare and lynx populations from 1845 to 1935.

Plants as Prey (Herbivory)

• Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves.
  • Chemical defenses: Secondary compounds such as oils and chemicals to attract predators to eat the herbivores, poison milky sap, and others.
  • Nicotine is poisonous in high volumes.
• Herbivores coevolve to continue eating the plants.

Chemical Defenses in Animals

• Monarchs evolved to be able to eat poisonous glycosides in milkweed and incorporate them for protection from predation.
• Poison-dart frogs produce toxic alkaloids in the mucus that covers their brightly colored skin.
• Poisonous Animals are often brightly colored as a warning.

Defensive Coloration

• Camouflage or cryptic coloration help nonpoisonous animals blend with their surroundings.
• Animals using camouflage do not usually live in groups.

Mimicry

• Allows one species to capitalize on defensive strategies of another.
• Resemble distasteful species that exhibit warning coloration.
  • Batesian mimicry: Mimics look like distasteful species.
  • Müllerian mimicry: Several unrelated but poisonous species come to resemble one another.

Symbiosis

• Ongoing interactions between different organisms.
• Potential for coevolution.
• Three major types:
  • Mutualism: Both species benefit (e.g., pollinators).
  • Parasitism: One benefits, one harmed (can be plants).
  • Commensalism: One benefits, other unaffected.

Mutualism Examples

• Coevolution: Flowering plants and insects (pollination).
• Ants and acacias.
• Plants parasitic wasp.

Parasitism Details

• Parasitoids: Insects that lay eggs on living hosts.
• Ectoparasites: Feed on the exterior surface of an organism.
• Endoparasites: Live inside the host.
• Extreme specialization by the parasite as to which host it invades.
• Structure of the parasite may be simplified because of where it lives in its host.
• Many parasites have complex life cycles involving more than one host.

Example: Oxpeckers and Grazing Animals

• Oxpeckers and grazing animals: eat parasites off of grazers, but sometimes pick scabs and drink blood.

Keystone Species

• Effects on composition of communities much greater than one might expect based on their abundance.
• Example: Beavers.

Interactive Effects

• Barnacles can occupy entire tidal zone - crowding out other species.
• Starfish eat barnacles, allowing species diversity.
• Experiment: Removal of predatory sea stars (Pisaster ochraceus) from rocky intertidal shoreline.
  • Result: Population of the mussel Mytilus californianus exploded, occupying all available space and eliminating many other species from the community.

Indirect Effects

• Species interactions may involve many species/pathways
• Rodents and ants compete for seeds
• Rodents removed lead to an increase of ant colonies. Graph shows number of ant colonies vs sampling periods.

Change in Communities

• Communities are constantly changing as a result of:
  • Climatic changes.
  • Species invasions.
  • Disturbance events - like fire, floods, hurricanes.
• Nonequilibrium models that emphasize change rather than stability are used to study communities and ecosystems.

Succession

• Primary succession: Occurs on bare, lifeless substrate (rocks).
• Organisms gradually move into an area and change its nature.

Secondary Succession

• Occurs in areas where an existing community has been disturbed but organisms still remain.
• Examples: Field left uncultivated, forest after a fire.

Why Succession Happens

• Succession happens because early species alter the habitat for later species.
• Three dynamic concepts:
  • Establishment: Early species are r-selected, fast growing, and tolerant of harsh conditions.
  • Facilitation: Greater soil nutrition/more habitat diversity allows K-selected species to enter.
  • Inhibition: K-selected species begin to outcompete r-selected species - species diversity can go down.

Intermediate Disturbance Hypothesis

• Disturbance is common, rather than exceptional, in many communities.
• Communities with moderate frequency of disturbance will have higher levels of species richness.
• Patches of habitat will exist at different successional stages.
• Often required for healthy ecosystem.