Community Ecology and Species Interactions

Biological Hierarchy and Community Definition

• Biosphere interacts globally to form ecosystems, which combine biotic communities with the abiotic environment.

• Communities consist of different species living in the same area and interacting.

• Evolution occurs in populations; natural selection acts on individuals/organisms.

• Community defined as an assemblage of all species in an area living close enough to interact.

Describing Communities

• Community Composition: The identity of all species present.

• Species Richness: Total count of species, regardless of their individual abundance.

• Species Evenness: The relative commonness or rarity of species compared to one another.

• Species Diversity: A weighted combination of richness and evenness.

Quantitative Diversity: The Shannon Index

• Used to calculate species diversity based on proportions.

• Variables:   * $p_i = \text{proportion of individuals in the ith species}$   * $s = \text{number of species in the community}$

• Example calculation provided result: $H = 1.57$

Types of Interspecific Interactions

• Commensalism ($+/0$): One species benefits, the other is unaffected (e.g., antbirds following ants to catch stirred-up insects).

• Competition ($-/-$): Individuals use the same limiting resources, lowering fitness for both.   * Intraspecific: Between members of the same species.   * Interspecific: Between different species.

• Consumption ($+/-$): One organism eats or absorbs nutrients from another.

• Mutualism ($+/+$): Interaction benefits both species.

Competition and Niche Theory

• Gause's Competitive Exclusion Principle: Complete competitors cannot coexist.

• Niche Types:   * Fundamental Niche: Resources used and conditions tolerated in the absence of interactions.   * Realized Niche: Resources used and conditions tolerated in the presence of species interactions.

• Asymmetric Competition: One species suffers a significantly greater fitness decline than the other; can lead to the extinction of the weaker competitor if niches overlap completely.

• Symmetric Competition: Both species experience a similar decrease in fitness.

Case Studies in Competition

• Joe Connell and Barnacles: Experimental removal of Semibalanus showed that Chthamalus is restricted to the upper intertidal zone because it is outcompeted in the lower zone, not because it cannot survive the physical conditions.

• Paramecium Experiments: Studies by Gause using Paramecium aurelia and Paramecium caudatum demonstrated competitive exclusion.

Mechanisms of Coexistence and Evolution

• Fitness Trade-Offs: Organisms good at competing for one resource may be less proficient at enduring drought, disease, or predation.

• Niche Differentiation (Resource Partitioning): Evolutionary change in resource use to reduce competition and niche overlap.

• Robert H. MacArthur: Documented resource partitioning in five species of warblers (Cape May, Blackburnian, Black-throated green, Bay-breasted, and Yellow-rumped) coexisting in the same tree species by feeding in different zones.

• Character Displacement: Evolutionary change where phenotypes of competing species become more distinct over time to reduce competition.