Community Level Ecology

Introduction to Community Ecology

Community ecology is the study of the interactions between species within a given area and the resulting patterns of species richness, diversity, and abundance. A biological community is defined as a group of populations of different species living in the same place at the same time.

Key Concepts in Community Ecology

Species Diversity:
- Species Richness: The number of different species in a community.
- Relative Abundance: The proportion each species represents of all individuals in the community.
- Shannon Diversity Index (H): A common metric used to quantify species diversity, taking into account both richness and relative abundance. The formula is: H = -\sum{i=1}^{S} (pi \ln p_i)
  - Where S is the total number of species in the community.
  - p_i is the proportion of individuals belonging to species i
Interspecific Interactions: Relationships between species in a community that can affect their survival and reproduction.
- Competition ($-/-$): Occurs when species compete for the same limited resources, leading to negative effects for both.
  - Competitive Exclusion Principle: States that two species competing for the exact same limited resources cannot coexist indefinitely; one will be driven to extinction.
  - Resource Partitioning: Differentiation of ecological niches, enabling similar species to coexist in a community.
- Predation (+/-): An interaction where one species (the predator) kills and eats the other (the prey).
  - Adaptations: Prey develop defenses (e.g., mimicry, camouflage, chemical defenses), while predators develop enhanced hunting strategies.
- Herbivory (+/-): An interaction where an herbivore eats parts of a plant or algae.
  - Plants develop defenses (e.g., thorns, toxins), and herbivores adapt to these defenses.
- Symbiosis: A close and long-term interaction between two different species.
  - Mutualism (+/+): Both species benefit from the interaction (e.g., flowering plants and pollinators).
  - Commensalism (+/0): One species benefits, while the other is neither harmed nor helped (e.g., cattle egrets and grazing animals).
  - Parasitism (+/-): One species (the parasite) lives on or in another species (the host) and harms it (e.g., ticks on deer).
Trophic Structure: The feeding relationships between organisms in a community.
- Food Chains: Transfer of food energy from its source in plants and other autotrophs through herbivores to carnivores and ultimately to decomposers.
- Food Webs: Branching food chains within an ecosystem, more complex and realistic representations of feeding relationships.
- Trophic Levels: Position an organism occupies in a food chain.
  - Producers: Autotrophs (e.g., plants, algae).
  - Primary Consumers: Herbivores.
  - Secondary Consumers: Carnivores that eat herbivores.
  - Tertiary Consumers: Carnivores that eat other carnivores.
  - Decomposers: Break down organic matter from all trophic levels.

Community Dynamics

Ecological Succession: The process of change in the species structure of an ecological community over time.
- Primary Succession: Occurs in an area essentially devoid of life, where soil has not yet formed (e.g., new volcanic rock, sand dunes).
- Secondary Succession: Occurs in areas where a disturbance has removed most, but not all, of the organisms and soil remains intact (e.g., after a forest fire or logging).
Disturbance: An event, such as a storm, fire, flood, drought, or human activity, that changes a community by removing organisms from it or altering resource availability.
- Intermediate Disturbance Hypothesis: Suggests that moderate levels of disturbance can foster greater species diversity than either high or low levels of disturbance.

Important Considerations

Foundation Species: Species that play a major role in structuring a community by creating or maintaining a habitat (e.g., kelp in kelp forests).
Keystone Species: Species whose impact on its community is larger than its biomass or abundance would suggest, often by influencing food webs or modifying the physical environment (e.g., sea otters).
Invasive Species: Non-native species that are introduced to a new environment and spread rapidly, often outcompeting native species and altering ecosystems.