Competition

Ecology Chapter 13: Competition

13.1 Resources

Competition: A biological interaction that negatively affects the fitness of one or more individuals due to limited resources. This interaction is commonly referred to as a density-dependent mechanism, which means its effects intensify as population density increases.

Types of Competition:

  1. Exploitative (Resource or Scramble) Competition: This occurs when individuals compete indirectly by consuming or monopolizing shared resources, usually leading to decreased availability of those resources for others.

  2. Interference (Contest) Competition: Involves direct interactions such as aggressive acts between individuals, where one or more individuals actively prevent others from accessing resources.

Occurrences of Competition:

  • Intraspecific Competition: Competition that takes place within the same species, often leading to self-regulation of population densities.

  • Interspecific Competition: Competition that occurs between different species, which may drive evolutionary adaptations.

Competing for Resources:

Competing species contend for various resources essential for survival, including:

  • Space: All organisms require physical space for living and reproduction.

  • Food: Access to food is often a critical factor for survival.

  • Nutrients: Essential for growth and reproduction, particularly in plants.

  • Water: A vital resource for survival across species.

  • Habitat: Different species require different habitat types for optimal living.

  • Light: Particularly important for photosynthetic organisms.

  • Pollinators: Many plants compete for pollinator services to ensure reproduction.

Case Study: Barnacles

Barnacles serve as an example of competition for space in the intertidal zone, where several species may settle on the same rock surface. Understanding the nature of this competition is crucial as it poses questions about whether survival is due to competitive advantages or the resilience of the species' habitat.

Invasive Ants vs. Native Crabs

In the South Pacific, invasive ant species compete with native crabs for food resources. The ants use formic acid in aggressive behaviors to deter crabs from foraging, effectively reducing food availability for the native species and potentially disrupting the local ecosystem.

Types of Intraspecific Competition

  • Clumped Resources: This typically leads to interference competition, where individuals vie for limited resources, leading to direct aggressive interactions.

  • Spread Out Resources: This situation encourages exploitative competition, where the first individuals to access resources deplete them, leaving less for others. Notably, when resources are abundant, competition is often minimal, allowing populations to thrive.

Evidence of Intraspecific Competition

Yoda's Law (Self-Thinning Rule): Observes that as biomass increases within a population, density decreases; indicating that competition forces some individuals to succumb, thus allowing for better growth of remaining individuals. The mathematical representation is:

  • Equation: Log(w) = -3/2 (log(N)) + log(c)

  • where:w = mean plant weightc = growth constantN = plant density (number of plants/m2)This law is applicable to both plant populations and sessile animals, affecting overall community structure.

Evidence of Interspecific Competition

Detecting interspecific competition typically involves:

  • Identifying negatively correlated distributions of species.

  • Observing Character Displacement, where competing species diverge in traits to reduce niche overlap.

  • Removing dominant species from a community and monitoring the resulting changes to the ecosystem dynamics. Example observed with ants and crabs shows that resource availability can dramatically influence competitive interactions.

13.2 Competitive Exclusion

Competitive Exclusion Principle

A fundamental ecological rule stating that strong competitors tend to completely eliminate weaker competitors within shared niches. For instance, peacock butterflies may dominate the availability of nectar plants, leading to the exclusion of other butterfly species from those resources.

Coexistence vs. Competitive Exclusion

The outcomes of competition can be influenced significantly by niche characteristics. Conditions that favor coexistence include:

  • Environmental Variation: Fluctuating conditions over time can create opportunities for coexistence.

  • Niche Shift: Evolutionary changes may allow competitors to exploit different aspects of the environment.

  • Niche Partitioning: The process through which competing species share resources by utilizing different resources or areas within a habitat, thus reducing direct competition.

13.3 Coexistence

Environmental Variation

An example of this is seen in mosquitofish, which have varying competition levels based on salinity. In high salinity, mosquitofish demonstrate less aggressive competitive behavior, leading to reduced resource consumption.

Niche Shifts

Adaptive changes allow species to modify their niche dimensions, such as habitat, diet, or timing. For example, minks may alter their activity patterns to coexist with recovering otters, avoiding direct competition for similar resources.

Niche Partitioning

Occurs in shared habitats where species utilize resources at different depths. For instance, bromeliad plants can house different species of mosquito larvae that occupy varying depths of water pools, allowing them to coexist despite competing for similar aquatic resources.

Key Takeaways

  • Recognize the different types of competition: direct interference vs. scramble competition.

  • Understand Yoda's law and its implications for population dynamics in both plants and animals.

  • Identify key signs of competition, including negatively correlated distributions and character displacement.

  • Comprehend the outcomes of competition: the balance between competitive exclusion and coexistence, as well as the conditions that promote coexistence among species.