Communities

Biol/Env S 204Spring 2008

Communities

Community: all of the organisms in a given area (habitat) and their interactions.

Communities

Community: An association of interacting populations of different species; often described by the dominant feature (e.g., tall grass prairie).

Communities

• Organisms that can exist in a given place are determined by the environment (abiotic factors) and their adaptations/interactions

Communities

• Organisms that can exist in a given place are determined by the environment (abiotic factors) and their adaptations/interactions

• Just because an organism can exist in a place doesn’t mean that it will be found there

Communities

• Organisms that can exist in a given place are determined by the environment (abiotic factors) and their adaptations/interactions

• Just because an organism can exist in a place doesn’t mean that it will be found there

• Species of a given community share similar abiotic requirements, but interactions also play a role

Communities

Abiotic factors: temperature, light, substrate, elevation, precipitation…

Boundaries can be sharp or gradual,depending on the gradient. Gentle gradients produce transitional zones.

Communities

Also, each species in a community has its ownbiotic limits that don’t necessarily coincide completely with those of the members of itsimmediate community.

E.g., specialists vs. generalists

Communities

Specialists can have very narrow abiotic requirements or very restricted bioticrequirements (e.g., one or few food items).

Examples: snail kite and apple snails limpkin and apple snails

Communities

Generalists can exist in a broader range of abiotic conditions or have a variety of food sources.

Examples: crows humans raccoons

Communities

History matters in community assemblage.

E.g., frequency of fire, flooding, other kinds of disturbance in ecological time (10’s to 1000’sof years).

Or, large scale changes such as continental driftin evolutionary time (10,000s to millions of years).

Communities

Chance also matters in community assemblage.

Species A + Species C + Species D

Species B + Species C + Species E

Communities—Interactions

Interactions between and among speciesare the essence of communities.

Communities—Interactions

•Predation•Competition

•Commensalism

•Mutualism

4 broad categories

Communities—Interactions

Predation (+-): predator/prey; parasitism (living on or in hosts, usually not killing outright but causing disease); herbivory.

Examples: spider catching a dragonfly fungus growing on a tree trunk lemur eating bamboo

Communities—Interactions

Competition (--): two or more species rely on similar limiting resources (food,nesting sites, nutrients, etc.)

Cubananole

Eastern(green)anole

Communities—Interactions

Commensalism (+0): one species benefits, the other is unaffected by the interaction; rare.

Possible example: “hitchhikers” suchas barnacles on whales.

Communities—Interactions

Mutualism (++): both species benefit;plants and pollinators, plant roots andmycorrhizal fungi; symbiosis (e.g.,lichens).

Communities—Interactions

Interactions structure communities.Examples from Vandermeerand Perfecto, 1995, Breakfastof Biodiversity (and others):

Herbivory

Seed dispersal

Pollination

Communities—Interactions

Herbivory

-leaves, stems and seeds are most common targets-exists in all ecosystems, both terrestrial and aquatic-plants can’t run away so they have evolved defenses (mainly mechanical, chemical and mutualistic)-tends to encourage a more dispersed distribution

Communities—InteractionsSeed dispersal

-seeds are energy-rich, attractive to herbivores-seed disperser can be a seed consumer-predator satiation: make a lot every so often and some will survive (common in SE Asian rainforests)-pulp as a reward for dispersing fruits or seeds-majority may drop near parents, but those further away tend to survive better, leading to a scattered distribution

Communities—Interactions

Pollination in rainforests:

-many species,but most are rare

-animal (not wind) pollination

Communities—Interactions

Rarity is a problem for reproductionin the tropical rainforest.

Here are 4 possible solutions thatallow reproduction and also helpphysically structure the community.

Communities—Interactions

Pollination: 1. Clumping of individuals

-individuals of a given species occur close together-greater likelihood of pollination and outcrossing-but also increases the risk of herbivory

Communities—Interactions

Pollination: 2. Selfing (or self-pollination)

-a lone individual can be self-fertilized or self-pollinate-therefore can be widely spaced-reduces risk of herbivory greatly-but over the long term this strategy leads to inbreeding and genetic problems

Communities—Interactions

Pollination: 3. Synchronous mass flowering

-all individuals of a species flower at the same time-once a year there is a bright display so long-distance pollinators have no trouble finding the trees-reduces risk of herbivory-but the pollinator must have a year-round food source-therefore there is a minimum number of tree species that must exist there

Communities—Interactions

Pollination: 4. Trap-lining

-birds or insects create a mental map for locations of particular food sources-pollinators know where the flowering individuals of a species are-each plant only produces a few flowers a day with a nectar reward-allows some distance between individuals but depends on pollinator

Communities—Keystone Species

• Have a disproportionate effect on the persistence of all other species in the community

• Can be predators, herbivores, pathogens, parasites, mutualists, earth-movers, system processors

• Keystone interactions often complex, subtle

Communities—Keystone Species

Sea otter is probably the best known example(Wilson, Ch. 9).

-lives in kelp beds from Alaska to S California-predator, one of main prey species is the sea urchin-hunting caused sea otter numbers to drop greatly-allowed population explosion of sea urchins which overgrazed the kelp beds-many other associated species also disappeared-reintroduction and protection of sea otters allowed the community to get back into balance

Communities—Keystone Species

Another example is figs in tropical forests.

-in a tropical forest in Peru, fruit-eating vertebrates depend on a variety of resources including nuts, fruits and nectar-many of these plant species flower and fruit synchronously-sometimes these resources are scarce, especially if forest patch is relatively small-suggestion is that figs, which flower continuously, may be keystone species in many parts of tropics

Communities—Keystone Species

• No general theory for identifying likely keystone species or for predicting their indirect effects

• Not all keystone species are equally vulnerable to local extinction

• Detection of keystones may be critical for the preservation of biodiversity