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Introduction to Species Interactions

  • Central part of ecology focusing on the interactions between species.

  • Types of Species Interactions:

    • Mutualism: Both species benefit.

    • Commensalism: One benefits, the other is unaffected.

    • Amensalism: One species is harmed, the other is unaffected.

    • Parasitism: One benefits, the other is harmed (includes diseases).

Competition in Species Interactions

Understanding Competition

  • Definition: Active demand for a resource in short supply.

  • Resources of competition can include food, water, nesting sites, etc.

  • When resources are abundant, competition does not occur.

Resource Utilization Curves

  • Graphically illustrates how species utilize resources:

    • Y-Axis: Resource utilization or fitness.

    • X-Axis: Gradient of resource size (e.g., seed size).

    • Bell Curve: Indicates efficiency in resource utilization; species utilize resources preferentially within a certain range.

  • Overlapping Curves: Indicates potential for competition as both species utilize the same resource size.

Mechanisms of Competition

  • Exploitative Competition: One species is more efficient in utilizing a resource. Competition occurs indirectly through the resource itself.

    • Example: One bird species may be better at cracking seeds, leading to more resources consumed than the inferior competitor.

  • Interference Competition: Direct behavioral interactions where one species excludes another from the resource through aggression.

    • Example: A larger bird may physically chase away a smaller bird from its nesting site, leading to competitive exclusion.

Case Study: Canids in North America

  • Historical changes in coyote and wolf populations due to interference competition.

    • Gray Wolves: Large canids that once dominated regions.

    • Coyotes: Increased in number following wolf population declines due to human intervention (trapping, hunting).

    • Red Foxes: Also impacted by coyote behavior as they nest in similar environments.

Documented Effects in Yellowstone National Park

  • Reintroduction of wolves led to:

    • Decrease in coyote population (by 50% within a few years) and increase in red fox population.

    • Evidence of competition dynamics, including coyote territory changes and prey availability effects due to wolf predation.

Niche Dynamics in Competition

Niche Shifting

  • Species may change their resource utilization to adapt to the pressures of competition.

    • Niche Partitioning: Both species may adjust their niche to decrease overlap, allowing coexistence.

    • This can include changes in food preferences (e.g., different seed sizes).

Evolutionary Implications

  • Persistent competition may lead to evolutionary adaptations over generations (e.g., changes in gut morphology, bill size, behavior).

    • Behavioral shifts could precede genetic changes, establishing niche differences based on competition history.

Assessing Competition in Ecology

Methods of Identifying Competition

  • Niche Partitioning Analysis: Inferring competition by measuring resource utilization curves and their overlaps.

  • Behavioral and Genetic Basis Testing: Assessing whether observed changes in resource use are due to competition or pre-existing adaptations (ghost of competition past).

    • Example of Bobcats and Mountain Lions in Texas showing resource partitioning based on size preferences for prey.

Evidence of Niche Differentiation

  • Sharp-shinned hawks, Cooper's hawks, and Goshawks study demonstrating prey size specialization aligned with body size differences.

    • Suggests alternative evolutionary paths influenced by historical competition.

Conclusion

  • Next Steps: Further exploration into predation and interactions.

  • Reminder that the PowerPoint will be posted soon for review.