Community Ecology

A biological community is defined as a group of populations of various species that live close enough to interact.

Community interactions can:
- Help species (+)
- Harm species (-)
- Have no effect (0)
These relationships are classified as interspecific interactions and can include:
- Competition
- Predation
- Herbivory
- Symbiosis (includes parasitism, mutualism, and commensalism)
- Facilitation
The effects of interspecific interactions can be summarized as follows:
- Positive (+)
- Negative (-)
- No effect (0)

Interspecific competition occurs when species compete for limited resources.

Competitive Exclusion

Competitive Exclusion occurs when strong competition leads to the local elimination of a competing species.
The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place.
This principle was demonstrated in Gause’s study from 1934.

Resource Partitioning

Resource partitioning refers to the differentiation of ecological niches, allowing similar species to coexist.
Example species:
- Cape May warbler
- Blackburnian warbler
- Bay-breasted warbler
- Yellow-rumped warbler
- Black-throated green warbler

A species’ fundamental niche is the niche potentially occupied by that species.
A species’ realized niche is the niche actually occupied by that species.
Connell studied these concepts using two barnacle species.

Predation (a +/− interaction) involves one species (the predator) killing and eating another (the prey).
Predators have certain feeding adaptations such as claws, fangs, and poisons.

Prey exhibit various defensive adaptations, which include:
- Behavioral defenses: hiding, fleeing, forming herds or schools, self-defense, and alarm calls.
- Morphological and physiological defenses: such as mechanical (spines, shells) and chemical defenses (poisons).
- Aposematic coloration: is bright warning coloration found in species with effective chemical defenses.
- Cryptic coloration: (camouflage) helps prey avoid detection.

Prey species may mimic other species for protection:
- Batesian Mimicry: a palatable species mimics an unpalatable model, capitalizing on the learned avoidance of predators.
- Müllerian Mimicry: two or more unpalatable species resemble each other, reinforcing the avoidance behavior in predators.

Herbivory (a +/− interaction) occurs when an herbivore consumes parts of a plant or algae.
This interaction has led to the evolution of plant defenses, both mechanical and chemical.

Symbiosis refers to a relationship where two or more species live in direct, intimate contact.

Types of Symbiotic Relationships

Parasitism (a +/− interaction): The parasite derives nourishment from the host, harming it in the process.
- Endoparasites: live within the body of the host.
- Ectoparasites: live on the external surface of the host.
Mutualism (a +/+ interaction): Both species benefit from the interaction.
- Can be Obligate (species are completely dependent) or Facultative (species can survive independently).
Commensalism (a +/0 interaction): One species benefits while the other remains unaffected. Difficult to document effectively in nature due to the interdependence of species.
Facilitation (a +/+ or 0/+ interaction): One species positively affects another without direct contact. Example: The black rush plant enhances soil conditions for other plants.

Communities can be characterized by:
- Structure: Types and numbers of species present.
- Dynamics: How communities change over time.
Understanding community structure and dynamics aids community ecologists in ecosystem management.

Species diversity refers to the variety of organisms in a community, which consists of:
- Species richness: Total number of different species in the community.
- Relative abundance: How common a species is relative to others.
Two communities may share the same species richness but differ in relative abundance.

Factors influencing species richness and diversity include:
- Latitude
- Altitude

Island biogeography seeks to explain the high species richness found on isolated islands. Factors include:
- Island size (larger islands support more species).
- Distance from the mainland (closer islands support more species).
Developed by MacArthur and Wilson in the 1960s.

Foundation species are the base of a community and have the greatest influence on its structure, modifying the environment to create habitats (e.g., coral reefs, kelp forests).
Dominant species are the most abundant and often outcompete others for resources (e.g., African grassland herbivores).
Keystone species exert significant control over community structure by their ecological roles but are not always the most abundant.
Ecosystem engineers are organisms that physically change the environment, affecting community structure (e.g., beaver dams).

Invasive species are nonnative organisms introduced to a new area, potentially threatening the ecosystem balance.

A food chain illustrates the path of energy and matter transfer between organisms.
A food web is a network of food chains showing how organisms feed within a community, showing energy and nutrient flow.

Intermediate disturbance hypothesis: Moderate disturbance levels can enhance species diversity more than high or low disturbance levels.
- High disturbance levels may eliminate slow-growing species.
- Low disturbance levels may allow dominant species to outcompete less competitive species.

Ecological succession is the sequence of community changes following a disturbance.
- Primary succession: occurs in areas where no soil exists.
- Secondary succession: takes place in areas where soil remains post-disturbance (e.g., abandoned farmland returning to its original state).

In addition to mutualism, facilitation can also improve conditions for other species, which can persist as a result.
- Interaction types include mutualism, facilitation, commensalism, parasitism, and antagonism.