Species Interactions

Introduction to Community Interactions

Species interactions are fundamental processes that shape the structure and diversity of biological communities. These interactions can benefit, harm, or have no effect on the species involved, influencing community composition and ecosystem function.

• Community: A group of populations of different species living close enough to interact.

• Interspecific interactions: Relationships between individuals of different species in a community. These can be positive (+), negative (−), or neutral (0) for each species involved.

Types of Species Interactions

There are several main types of interspecific interactions, each with distinct effects on the species involved:

• Competition (−/−): Both species are harmed as they compete for the same limited resources.

• Predation ( +/−): One species (predator) benefits by feeding on the other (prey), which is harmed.

• Herbivory ( +/−): An animal eats part of a plant or alga, benefiting the consumer and harming the plant.

• Parasitism ( +/−): The parasite benefits by feeding on or in a host organism, which is harmed.

• Mutualism ( +/+): Both species benefit from the interaction.

• Commensalism ( +/0): One species benefits while the other is neither helped nor harmed.

Community Structure and Diversity

Trophic Structure

The trophic structure of a community describes the feeding relationships among organisms, determining the flow of energy and nutrients. It is often represented by food chains and food webs.

• Producers: Organisms (usually plants or algae) that convert solar energy into chemical energy via photosynthesis.

• Consumers: Organisms that obtain energy by eating other organisms. These include primary consumers (herbivores), secondary consumers (carnivores), and higher-level consumers.

Diversity and Composition

Diversity refers to the variety of species in a community, while composition describes which species are present. Both are influenced by species interactions, disturbances, and environmental factors.

• Species richness: The number of different species in a community.

• Relative abundance: The proportion each species represents of all individuals in the community.

Disturbance and Community Dynamics

Disturbance Influence

Disturbances, such as fires, storms, or human activities, can alter community structure by changing species diversity and composition. Some disturbances can increase diversity by preventing competitive exclusion.

• Intermediate Disturbance Hypothesis: Moderate levels of disturbance can foster greater species diversity than low or high levels of disturbance.

Biogeographic Factors

Effects on Community Diversity

Biogeographic factors, such as the size of a habitat and its distance from other habitats, influence community diversity. Larger and less isolated areas tend to support more species.

• Island Biogeography Theory: Explains how species richness is affected by island size and distance from the mainland.

Pathogens and Community Structure

Local and Global Effects

Pathogens can alter community structure by affecting the abundance and diversity of species. Disease outbreaks may reduce populations of certain species, leading to changes in community composition.

• Example: The introduction of a new pathogen can cause declines in native species, allowing other species to become more dominant.