AP Bio
Ch. 53 Notes
Community Ecology
A biological community: populations of various species living
close enough to have potential interaction
Community interactions:
Interspecific interactions – key relationships in the life of an
organism are its interactions with other species in the
community.
Competition: interspecific competition occurs when species
compete for a particular resource that is in short supply. Ex.
Weeds competing with garden vegetable plants for the soil
nutrients and water. Strong competition can lead to the of
elimination of one of the two competing species, this is called
Competitive exclusion.
Ecological niche: the sum total of a species’ use of the biotic
and abiotic resources in its environment. Two species cannot
coexist in a community if the niches are identical. Similar
species can coexist in a community if there is one or two
significant differences in their niches. This differentiation of
niches is called resource partitioning.
Character displacement: the tendency for characteristics to be
more divergent in sympatric (geographically overlapping)
populations two species than in allopatric (geographically
separate) populations of the same two species.
Predation: this is a +/- interaction in which the predator kills
and eats the prey. Eating and avoiding being eaten are
prerequisites to reproductive success, the adaptations of
predators and prey tend to be refined through natural
selection.
Predator adaptations include: claws, teeth, fangs, stingers,
toxins, to help capture and subdue organisms.
Prey adaptations include: hiding, fleeing, and self defense
(behavioral). Cryptic coloration (camouflage), mechanical or
chemical defenses (morphological, physiological defense
adaptations). Aposematic coloration - animals that display
bright warning colors Organisms that mimmick the appearance
of poisonous species.
Herbivory: this is a +/- interaction in which herbivores eat parts
of a plant or algae. Most herbivores are small invertebrates,
grasshoppers, and beetles. In the ocean, snails, sea urchins,
some tropical fish. Herbivores have specialized adaptations
including the ability to distinguish toxic and non-toxic plants,
and which plants are more nutritious. Plant adaptations
include, chemical toxins, thorns, spines.
Parasitism: is a +/- interaction in which one organism (parasite)
derives nourishment from its host which is harmed in the
process. Parasites that live within a body of a host are called
endoparasites. Parasites that live on the external surface of a
host are called ectoparasites.
Symbiosis: the interaction in which two organisms of different
species live together in direct contact, including parasitism(+/-),
mutualism (+/+) benefits both species, and commensalism (+/0)
benefits one of the species but neither harms or helps the
other species.
Disease: pathogens, typically bacteria, viruses, or protists.
Most are microscopic and lethal.
Species Diversity: the variety of different kinds of organisms
that make up a community. 2 components of species diversity:
1st, species richness – total number of different species in the
community. 2nd, relative abundance of different species, the
proportion each species represents of the total individuals in
the community.
Trophic Structure: feeding relationships between organisms
(food chain). It’s the transfer of food energy up the the trophic
levels from its source (photosynthetic organisms – primary
producers) through herbivores (primary consumers) to
carnivores (secondary and tertiary consumers) and eventually
to decomposers.
Food Webs: diagrams of who eats whom. Usually, only a few
links long – rarely no longer than 7 links from producers to top
level consumers. Two main hypotheses for the short food
chains: 1st : Energetic hypothesis which states that the food
chain is limited by the inefficiency of energy transfer along the
chain (only 10% of the energy in organic matter at each trophic
level is converted to organic matter at the next trophic level.
2nd: the Dynamic Stability Hypothesis which states that long
food chains are less stable than short chains. Population
fluctuations at the lower trophic levels are magnified at the
upper levels.
Species impact environment:
Dominant species: the most abundant in a community and
have the highest biomass (total mass of all individuals in a
population). Ex. Sugar maple trees in eastern North America.
Invasive species: species (generally introduced by humans, that
take hold outside their native range) can attain in environments
that are lacking in their native predators and pathogens
Keystone species:
Exert strong control (not by their abundance) but by their
pivotal ecological roles or niches. Ex. Sea star
Foundation species: (ecosystem engineers) causing physical
changes in the environment that affect the structure of the
community. Ex. Beaver
Bottom-Up, Top Down Controls:
Bottom-Up postulates a unidirectional influence from lower to
higher trophic levels. Ex. The presence of absence of mineral
nutrients (N) controls plants (V) numbers, which controls
herbivores (H) numbers, which controls predator (P) numbers.
N -> V -> H -> P
Top-Down postulates that the influence moves in the opposite
direction: mainly the predation controls community
organization. N <- V <- H <- P Also called the trophic cascade
model
Disturbance:
An event such as a storm, fire, flood, drought, overgrazing or
human activity, that change a community, removes organisms
from it, and alters resource availability.
The intermediate disturbance hypothesis, states that a
moderate levels of disturbance can create conditions that
foster greater species diversity than low or high levels of
disturbance.
Human disturbance:
Humans have the greatest impact on biological communities
worldwide. It is often severe and usually reduces species
diversity in communities.
Ecological Succession:
A disturbed area is colonized by a variety of species, which are
gradually replaced by other species, which are in turn replaced
by still other species.
Primary succession: when the process begins in a virtually
lifeless area where soil has not yet formed. Ex. after a volcano
eruption, glacial retreat
Secondary Succession:
Occurs when an existing community has been cleared by some
disturbance and the soil is left intact. Ex. Yellowstone fires of
1988