Ecological Communities and Biodiversity Notes

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Definition of Ecological Communities

  • Community: A collection of interacting populations of different species sharing the same environment.

  • Community Structure includes:

    • Species Diversity: Variety and abundance of species within a community.

    • Species Composition: Types of species present.

  • Importance: Provides a basis for hypotheses about community functions.

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Effects of Species Interactions

  • Direct Effects: Influence between species from direct interactions (like predator-prey).

  • Indirect Effects: Influence between species through intermediaries, complicating predictions in food webs.

Trophic Cascades

  • Definition: When a change in consumer abundance at one trophic level alters species composition at lower levels.

    • Example: Loss of apex predators (sharks) in coastal oceans leads to overpopulation of certain species, disrupting balance.

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Trophic Facilitation

  • Definition: Positive interactions between prey and other species that indirectly aid consumers.This can occur, for example, when certain plants provide habitat or resources that enhance prey availability, thereby benefiting both the prey and their predators.

Ecological Dynamics

  • Example: Removal of cats (invasive species) can lead to increased rat populations which further threaten native wildlife through predation.

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Impact of Invasive Species

  • Domesticated and feral cats pose threats to wildlife, leading to an indirect facilitation of native predator populations due to their role in controlling rat populations.

  • Statistical Impact:

    • Kill billions of birds and reptiles annually; removal can inadvertently harm native species due to ecological release.

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Key Species Types & Effects on Communities

  • Dominant Species: Most numerous, hugely influential on community dynamics.

  • Keystone Species: Disproportionate effect on community structure; critical for maintaining ecosystem balance.

  • Ecosystem Engineers: Species that physically modify environments to create habitats.

  • Foundational Species: Provide structure and habitat for others (e.g., trees, coral reefs).

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Examples of Species Roles

  • Examples of Keystone Species:

    • Sea otters, which control sea urchins, maintaining kelp forests.

    • Beavers create ponds, and hippos enrich aquatic ecosystems with nutrients.

  • Concepts: Not all species carry equivalent ecological weight; their role may vary with population dynamics.

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Biological Diversity

  1. Genetic Diversity: Variation among individuals within a species.

  2. Species Diversity: Total variety of different species in a region, characterized by species richness.

  3. Ecosystem Diversity: Variety of ecosystems present in larger landscapes.

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Factors Affecting Biodiversity

  • Habitat Heterogeneity: More microhabitats yield higher biodiversity.

  • Climatic Factors: Stable climates usually foster more biodiversity compared to extreme or fluctuating climates.

  • Tropical Regions: Highest biodiversity due to stable conditions, specialization opportunities, and longer growth seasons.

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Intermediate Disturbance Hypothesis

  • Moderate levels of disturbance promote higher species richness by allowing coexistence of various species types.

  • Types of Succession:

    • Primary Succession: Begins after total disruption of species (e.g., after glaciers).

    • Secondary Succession: Recolonization post-disturbance that only partially clears species.

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Pioneers and Climax Communities

  • Pioneer Species: Initial colonizers that set the stage for succession.

  • Climax Community: Stable, mature community that sustains until disrupted; creates gaps for species persistence.

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Historical Impacts on Biodiversity

  • Stability over time generally correlates with higher species richness, as seen in North America post-glaciation.

  • Younger environments like tundra have lower biodiversity compared with ancient tropical forests.

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Island Biogeography Theory

  • The number of species present depends on island size and distance from colonization sources.

  • Important Concepts:

    • Larger islands harbor more species due to higher colonization rates.

    • Distance impacts colonization more than extinction rates.

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Predicting Species Equilibrium

  • By understanding island characteristics, predictions on equilibrium states can be made.