Community Ecology and Species Interactions

Interactions

• Competition (-/-): Two or more species compete for a resource that is in short supply.

• Exploitation (+/-): One species benefits by feeding upon another species, which is harmed. Includes predation, herbivory, and parasitism.

  • Predation: Predator kills and eats prey.

  • Herbivory: Herbivore eats part of a plant or alga.

  • Parasitism: Parasite derives nourishment from a host, which is harmed.

• Positive Interactions (+/+ or +/0): One species benefits, while the other benefits or is not harmed.

  • Mutualism (+/+): Both species benefit.

  • Commensalism (+/0): One species benefits, while the other is not affected.

Community Ecology

• An ecological community is a group of species living together at the same place and time.

• Major focus: Factors that structure communities by patterns of species composition & relative abundance.

• Communities are groups of interacting species occurring together in space and time.

• Community is often defined taxonomically.

  • Guild: group of species that use similar resources.

  • Functional group: species that function in similar ways but may or may not use similar resources.

Community Structure

• Patterns of species diversity and composition at the community or local scale.

• Influenced by:

  • Foundation species: Large or abundant species affecting community structure by providing habitat and food.

  • Interactions between species: Predation and other interactions.

  • Disturbances: Marine heat waves, storms, human activities, etc. that can remove organisms or alter resource availability.

Species Diversity

• Two components:

  • Species richness: Number of different species in the community.

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

• Two communities can have the same species richness but a different relative abundance

• Diversity can be compared between communities using a diversity index

• Shannon diversity index (H) is widely used by ecologists

  • H = -\sum{i=1}^{S} (pi \ln p_i)

Trophic Structure

• Feeding relationships between organisms in a community.

• Energy is transferred from autotrophs (primary producers) through herbivores (primary consumers) to carnivores (secondary and higher consumers).

• Decomposers are the final link in this chain, which is referred to as a food chain.

• The position an organism occupies in a food chain is called its trophic level.

• The energetic hypothesis suggests that length is limited by inefficient energy transfer

  • Only about 10% of the energy stored in organic matter at each trophic level is converted to organic matter at the next trophic level

Species Impact

• Certain species have a very large impact on community structure due to their abundance or pivotal role in community dynamics

• Keystone species have disproportionately large effects on the community

  • Larger than would be expected based on abundance or biomass

• Foundation species have strong effects due to their large size or high abundance

  • They often have community-wide effects because they provide habitat or food

  • They may be competitively dominant—superior in exploiting key resources such as space, water, nutrients, or light

• Ecosystem engineers cause physical changes in the environment that affect community structure

Bottom-Up and Top-Down Controls

• Organisms can be controlled by what they eat (“bottom-up” control)

• Organisms can be controlled by what eats them (“top-down” control)

Disturbance

• Abiotic disturbance: floods, landslides, fire, wind, wave action

• Biotic disturbance: treefalls, predation, herbivory, trampling, digging

• Event that changes a community by removing organisms from it or altering resource availability

• Creates opportunities for new individuals to become established and disrupts structure of a community and changes availability of resource

• Alters intensity & outcomes of species interactions and changes richness & composition of communities

• Intermediate disturbance hypothesis: Moderate levels of disturbance can foster greater species richness/diversity

  • High levels of disturbance exclude many slow-growing species

  • Low levels of disturbance allow dominant species to exclude less competitive species

Ecological Succession

• Pattern of colonization and species replacement that occurs in a community following a severe disturbance

  • Primary succession: Succession on landforms NOT previously influenced by species

  • Secondary succession: Succession on landforms where vegetation has been partially or completely removed

Biogeographic Factors

• Latitude and area are two key biogeographic factors that affect the species diversity of biological communities

• Species richness is especially great in the tropics and generally declines in a gradient toward the poles

• The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species