Competition in Ecology

Competition in Ecology

  • Types of Competition

    • Interspecific Competition

    • Competition between different species.

    • Intraspecific Competition

    • Competition among individuals of the same species, or conspecifics.

  • Nature of Competition

    • Competition is described as a lose-lose situation since the consumption of resources by one species leaves fewer resources available for others.

    • Competition is widespread, occurring among nearly all species within a trophic level. For instance, when a fox consumes a rabbit, that prey is no longer available to other predators.

    • It is crucial to note that competition is not always the primary driving force in ecological communities.

Flavors of Competition

  • Exploitation Competition

    • This occurs when two species compete for a limiting resource by using it up.

    • Example: Two predator species feeding on the same prey results in a situation where prey consumed by Species A is not available for Species B.

  • Interference Competition

    • This is characterized by two species competing by physically preventing each other from accessing a limiting resource.

    • A common example is competition for light among plants.

Understanding the Niche

  • Niche Definition

    • A niche is an n-dimensional space that represents all the physical and biotic conditions necessary for a species to exist.

    • Resources such as Resource A and Resource B are often included in this niche model.

    • When multiple species occupy the same niche, their niches will overlap, which results in competition.

  • Degree of Niche Overlap

    • The extent of the overlap between niches is contingent upon the degree of niche overlap; more overlap leads to increased competition.

Competition Dynamics Over Time

  • Long-term Outcomes of Competition

    • Over time, competition tends to reduce:

    • One species may go locally extinct.

    • One species may achieve total victory.

    • Species may partition resources, resulting in what is known as the realized niche.

  • Asymmetrical Nature of Competition

    • Competition is not uniform across different habitats or conditions.

    • A “winner” in one environment may not be the winner in another due to factors like foraging ability and predator avoidance.

    • Temporal variability also plays a role; a species may prevail one year and another species the next.

Competitive Exclusion Principle

  • Definition

    • The competitive exclusion principle states that no two species can coexist if their niches are exactly the same.

    • In instances where niches are identical, the competition intensity is so high that neither species can find a refuge from competition, leading to one species going extinct.

  • Difficulties in Demonstration

    • Although competitive exclusion is arduous to demonstrate empirically, it is almost impossible to falsify, owing to the complexity of niches being n-dimensional and multifaceted.

    • Species may seem indistinguishable in terms of niche occupation, yet they may have unseen niche separations not evaluated.

Case Study: Desert Rodents

  • Genera Example

    • The genera Chaetodipus and Dipodomys exemplify species that consume the same resources (seeds).

    • Initially viewed as an anti-competitive exclusion example with co-occurrence of up to 8 species, further investigation indicated that these species partition their resources.

    • Some forage early while others forage late.

    • Some forage in the open, while others prefer under bushes.

    • Adaptations vary; some species dig for seeds, others gather seeds from the surface.

Lotka-Volterra Competition Model

  • Model Introduction

    • The Lotka-Volterra competition equation models interspecific competition and introduces competition coefficients (α) which represent the impact of one species on another.

    • If α is large, the effect of competition is significant; if α is small, the effect is minimal.

  • Realized Population Growth Rate

    • The growth rate of species 1 (N1) is affected by both the degree of competition (α) and the abundance of species 2 (N2).

  • Impact of Competition Coefficient (α)

    • Small α: Minimal impact on species 1’s growth from species 2, regardless of species 2's abundance.

    • Large α: Significant impact on species 1's growth depending on abundance of species 2.

Projecting Competition Outcomes

  • Estimating Values

    • By estimating values like N, K (carrying capacity), and α, conclusions can be drawn about competitive outcomes:

    • Species 1 may win, species 2 may win, or both may coexist.

    • Example scenarios where K = 100 indicating variable competition outcomes based on α.

Competitive Equilibria

  • Unstable Equilibrium

    • In instances where species can either coexist or lead to one species going extinct, the outcome hinges on the starting abundances of each species.

  • Stable Equilibrium

    • In a stable scenario, two species will consistently coexist over time.

Competition Implications in Ecology

  • Ecological Importance

    • Understanding competition is vital in ecology and is regarded as a foundational concept, often referred to as the "holy grail" of ecological studies.

    • While the theoretical framework supporting competitive theory is robust, demonstrating actual competition with significant effects on population dynamics is complex.

    • Easy cases include species like canids (e.g., wolves, coyotes). In contrast, many instances show subtle competition observed through correlation analyses, leading to negative correlations in abundance between two competing species.

Apparent Competition

  • Definition

    • Apparent competition arises when two species appear to be competing, but the competition is not direct; it is mediated by a third factor, such as a predator limiting the growth of both species.

    • Example: If two prey populations have a negative correlation in abundance, removing one may crash the population of the remaining species due to increased predation.

Commensalism

  • Commensal Interaction Definition

    • A form of interaction where one species benefits while the other remains unaffected.

    • True commensal relationships are difficult to find, as the impact on the unaffected species can often be subtle or overlooked.

    • A frequently referenced example includes birds nesting in trees. The presumed benefit to birds does not significantly harm the tree, but the presence of birds may still influence the tree through foraging behavior.

  • Nesting Behavior

    • Birds as central place foragers often favor foraging in their host trees over neighboring trees, impacting insect populations and herbivory rates on the host tree, which can render a benefit.

  • Conclusion on Commensalism

    • Although many cases are considered for commensalism, further examination often reveals some degree of cost or benefit to the second species involved, challenging the strict definition of commensal interactions.