AP Biology Notes Outline Chapter 54: Community Ecology
CONCEPT 1: Communities
A community is an assemblage of populations of various species living close enough for potential interaction
CONCPET 2: Interspecific Interactions (Symbiosis)
Competition:
Interspecific competition – occurs when species compete for a particular limited resource. Strong competition can lead to the elimination of 1 of the 2 competing species = competitive exclusion because one species will use the resource(s) more efficiently and, thus, reproduce more rapidly than the other.
Competition affects an organism’s niche = its ecological role (how it “fits into” an ecosystem):
• fundamental niche = an organism’s potential niche
• realized niche = the niche an organism actually occupies due to competition for limited resources
The Niche:
A niche is the full range of physical and biological conditions in which an organism lives and the way in which the organism uses those conditions. An organism’s niche includes:
1. Its place in the food web
2. The range of temperatures that the organism needs to survive
3. The type of food the organism eats, how it obtains that food, and which other species use the organism as food
4. When and how the organism reproduces
Predation:
Predation is a relationship in which one species (the predator) kills and eats the other (the prey). Predators possess adaptations for securing prey:
1. acute senses
2. claws, teeth, fangs, stingers, poison
Prey have adaptations that help them avoid predators:
1. behavioral defenses (hiding, fleeing, self-defense, alarm calls)
2. cryptic coloration (camouflage)
3. aposematic coloration (bright warning coloration)
4. Batesian mimicry (harmless species mimics a harmful one)
5. Müllerian mimicry (two unpalatable species mimic each other)
Parasitism:
Parasitism is a symbiotic relationship in which one organism (the parasite) derives its nourishment from another organism (the host) which is harmed in the process:
} types of parasites:
} endoparasites – live within the body of their host
} ectoparasites – feed on the external surface of their host
} parasitoidism – insects lay eggs on or in living hosts and, then, the larvae feed on the host
CONCPET 3: Community Structure
Species diversity is the variety of different kinds of organisms that make up a community. It has two components:
• species richness = total # of different species in the community
• relative abundance = the proportion of the total # of individuals in the community that each species represents
Measuring species diversity is essential for understanding community structure & for conserving biodiversity.
CONCEPT 4: Trophic Structure
Trophic structures are feeding relationships between organisms in a community. Food chain = the transfer of food energy up the trophic levels. Food chains are usually only a few links long (≤5). There are two hypotheses why:
1. energetic hypothesis – suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain
2. dynamic stability hypothesis – proposes that long food chains are less stable than short chains because population fluctuations at lower trophic levels are magnified at higher levels
Most of the data available support the energetic hypothesis. Another factor that may limit food chain length is that animals in a food chain tend to be larger at successive trophic levels. Food webs – made up of linked food chains.
CONCEPT 5: Dominant, Keystone, & Foundation Species
Dominant Species are those species in a community that are most abundant or have the highest biomass and therefore exert a powerful control over the occurrence and distribution of other species (i.e. sugar maples are the dominant plant species in many North American forest communities).
Keystone Species are species that are not necessarilythe most abundant but exert strong control on community structure by their pivotal ecological roles/niches (i.e. sea star/mussels).
Foundation Species are organisms that influence community structure by causing physical changes in the environment that affect the structure of the community. They act as facilitators that have positive effects on the survival and reproduction of some of the other species in the community (i.e. lichen and moss break down rock into soil).
CONCEPT 6: Models of Community Organization
1. bottom-up model
postulates a unidirectional influence from lower to higher trophic levels (nutrient availability ª plant #s ª herbivore #s ª predator #s) thus, to change the community structure, you need to alter biomass at lower trophic levels
2. top-down model
postulates that it is mainly the predators that control community organization (predators limit herbivores which limit plants which limit nutrient levels) a change at the higher trophic levels moves down the trophic structure as a series of +/– effects (trophic cascade)
CONCEPT 7: Ecological Disturbances
In a process called ecological succession, a disturbed area may be colonized by a variety of species which are, in turn, replaced by other species until a climax community is reached. There are two types of ecological succession:
} primary succession – occurs in a virtually lifeless area, where there were originally no organisms and where soil has not yet formed
} secondary succession – occurs where an existing community has been cleared by some disturbance that leaves the soil intact (i.e. natural disasters; human disturbance; abandoned farmland, etc.)
Primary Succession
On land, succession that occurs on surfaces where no soil exists is called primary succession.
Example: on the surfaces formed as volcanic eruptions build new islands or cover land w/ lava and ash or on bare rock exposed when glaciers melt.
Stages:
- No soil, just ash and rock
- First species to populate is called the pioneer species (often lichen)
- As lichen grows, it can break up rock, and when they die, they add organic material to help form soil in which plants can grow
Pioneer species – first individuals to inhabit an area of primary succession – break down rock, erode surfaces, and turn it into soil (i.e. lichen and moss). Climax Community – the final community of any succession.
What causes an increase or decrease in biodiversity as a succession progresses?
• Some populations facilitate biodiversity/succession (by developing conditions more suitable for other species and/or developing conditions less suitable for their progeny.
• Some populations inhibit biodiversity/succession (by developing conditions less suitable for other species and/or developing conditions more suitable for their progeny.
• Increase in plant stratification (increased layering of plants; ex: canopy, understory.
• More niches/habitats formed (plants, animals, decomposers).
• Pioneer plant species – dominants (more shade-tolerant plants emerge).
• Increase in producer diversity brings about increase in consumer diversity.
• Shift from more opportunistic (r) to more equilibrium (K) species.
What types of factors cause successions?
What are the short and long term effects of successions?
CONCEPT 7: Biogeographical Factors & Species Richness/Diversity
A species-area curve shows that, with all other factors being equal, the larger the geographic area of a community, the greater the number of species. A likely explanation is that larger areas offer greater diversity of habitats and microhabitats than smaller ones: