8.5 - 8.7 - Community Ecology

Community: A group of populations of different species living closely together and capable of interacting.

Community Structure

Community Structure: Determined by species composition and species diversity.

• Species Composition: The different species that make up the community.

• Species Diversity: The number of different species (species richness) and the relative abundance of each species (species evenness).

Measuring Species Diversity

Simpson’s Diversity Index: A measure of diversity that takes into account both the number of species present and the relative abundance of each species. As species richness and evenness increase, so does diversity.

Equation:

• n: The total number of organisms of a particular species.

• N: The total number of organisms of all species.

Interactions in Communities

Interspecific Interactions: Interactions between different species in a community can change its structure. Key types of interspecific interactions include:

1. Competition:

   • Occurs when different species compete for the same resource (e.g., food, space).

2. Predation:

   • Involves one species (predator) feeding on another species (prey).

Competitive Exclusion:

• Concept: When two populations use the exact same resources at the same time, the inferior competitor is locally eliminated.

• Definition: Competition is a relationship where individuals of different species compete for limited resources.

Niche Partitioning:

• Natural Selection: Drives competing species into different patterns of resource use or different niches (comfortable positions in life).

• Resource Partitioning: When species divide a niche to avoid competition for resources, leading to a realized niche. Species have evolved to share resources in the wild; otherwise, one species might die out.

Predation

Predation:

• Definition: A +/- relationship where one species (predator) kills and eats the other species (prey).

Trophic Cascades:

• Definition: Powerful indirect interactions that can control entire ecosystems.

• Mechanism: Occur when predators limit the density and/or behavior of their prey, thereby enhancing the survival of the next lower trophic level.

Keystone Species:

• Definition: Species that are not usually abundant, but other species in an ecosystem rely on them because of their role (niche). Removing them can have devastating effects on the ecosystem.

• Importance: Keystone species, producers, and essential abiotic and biotic factors contribute to maintaining the diversity of the ecosystem.

Biodiversity:

• Definition: The number of different living things present within a specific area.

• Importance: The more diverse an ecosystem is, the more resilient it is to change.

Disturbances:

• Definition: Events that change a community by removing organisms from it or altering resource availability.

• Examples: Fires, droughts, human activities, etc.

• Impact: Disturbances can influence species diversity and composition.

Invasive Species

Invasive Species:

• Definition: Organisms that become established outside of their native range/ecosystem, usually due to human activity.

• Example: A ship bringing produce from another country may have insects in the crates holding the produce.

• Impact: Cause harm to the environment by growing and reproducing quickly, often outcompeting native species.

Human Activity: The strongest disturbance to an ecosystem. Major threats to biodiversity include:

• Habitat Loss:

1. • Example: Deforestation leading to loss of habitats for countless species.

• Invasive Species:

1. • Example: Non-native species introduced into new environments, outcompeting local flora and fauna.

• Overharvesting:

1. • Definition: Organisms are harvested faster than their populations can rebound.

   • Examples: Harvesting of ivory in elephants (now banned), overfishing leading to depletion of fish stocks.

• Global Change:

• Definition: Alterations to climate, atmospheric chemistry, and ecological systems that reduce Earth's capacity to sustain life.

• Examples:

  • Air and water pollution

  • Acid rain

  • CO2 emissions

  • Ocean acidification

Specific Issues

1. Eutrophication:

   • Definition: Excess nitrogen runoff into water bodies leads to poor water quality and harmful algal blooms.

   • Impact: Suffocates fish and other aquatic life due to depleted oxygen levels.

2. Biological Magnification:

   • Definition: Increasing concentration of toxic substances in the tissues of organisms at successively higher levels in a food chain.

   • Example: Mercury buildup in fish that then gets transferred to humans who consume the fish.

3. Overharvesting:

   • Definition: Harvesting organisms at a rate faster than their populations can recover.

   • Examples: Overfishing, poaching of endangered species.

4. Global Change:

   • Impact: Alterations to climate and ecosystems that reduce Earth's capacity to sustain life.

   • Examples:

     • Air and Water Pollution: Contamination from industrial and agricultural activities.

     • Acid Rain: Result of air pollution, harming ecosystems.

     • CO2 Emissions: Contributing to global warming and climate change.

     • Ocean Acidification: Result of increased CO2, affecting marine life.