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Communities
Biol/Env S 204
Spring 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 own
biotic limits that don’t necessarily coincide
completely with those of the members of its
immediate community.
E.g., specialists vs. generalists
Communities
Specialists can have very narrow abiotic
requirements or very restricted biotic
requirements (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’s
of years).
Or, large scale changes such as continental drift
in 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 species
are the essence of communities.
Communities—Interactions
4 broad categories
• Predation
• Competition
• Commensalism
• Mutualism
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.)
Cuban
anole
Eastern
(green)
anole
Communities—Interactions
Commensalism (+0): one species
benefits, the other is unaffected by
the interaction; rare.
Possible example: “hitchhikers” such
as barnacles on whales.
Communities—Interactions
Mutualism (++): both species benefit;
plants and pollinators, plant roots and
mycorrhizal fungi; symbiosis (e.g.,
lichens).
Communities—Interactions
Interactions structure communities.
Examples from Vandermeer
and Perfecto, 1995, Breakfast
of 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—Interactions
Seed 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 reproduction
in the tropical rainforest.
Here are 4 possible solutions that
allow reproduction and also help
physically 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