(4-10) Competition and Niche Space

Introduction

• Ecologists consider competition a pervasive and important driving force in natural selection.
• Competition allows individuals with certain traits to be better competitors.
• Out-competing others leads to greater fitness, resulting in populations of better competitors.
• Previous lectures covered:
  • What goes on within individuals.
  • How genes/traits are passed down.
  • How genetic diversity changes.
  • How populations grow.
  • How new species are formed.
• Upcoming lectures will cover:
  • Costs of interactions within species.
  • How different species interact.
• Interactions are analyzed in terms of costs and benefits, similar to an economist's balance sheet.
• Interactions are defined by considering those who benefit (+), sustain a cost (-), or have no effect (0).

Competition

General Definition

• Competition is when individuals interact and both sustain some cost (-/- interaction).
• Many confuse the definition, thinking it’s a (+/-) interaction, but this refers to the ultimate payoff.
• The process of competition is costly to both; without the other, there is no cost.
• Competition is for resources such as:
  • Food.
  • Nesting sites.
  • Sunlight.
  • Mates.
  • Water.
• Two classifications of competition:
  • Intraspecific (within species).
  • Interspecific (between species).
• Intraspecific competition can have potentially higher costs due to complete overlap in requirements between individuals.

Intraspecific Competition (Competition within individuals within a species)

• Slowing population growth at high densities produces a sigmoidal pattern around K (carrying capacity).
• Intraspecific competition for limited resources causes population growth to slow and fluctuate.
• Intraspecific competition is the major cause of density-dependent growth.
• Increased competition decreases resource availability for reproduction, making the population follow K.
• The General Adaptation Syndrome (GAS) is a physiological response where short-term survival gains in high-density situations are paired with long-term costs.
  • Short-term benefit of "fight or flight" carries costs of decreased immunity and reproductive potential.
• Example: Spongy Moth explosion (Lymantria dispar).
  • Summer of 2012: significant increase in spongy moth in NE U.S.
  • Larvae pupated; adults mated, and females laid egg masses on trees.
  • May/June 2013: caterpillars hatched and began feeding.
  • Forests were defoliated; trees took on a winter appearance.
  • Many trees were completely defoliated before caterpillars could complete larval development.
  • Result: massive die-off of animals; few completed metamorphosis.
  • Illustrates how competition among members of one species for a finite resource (food) caused a sharp drop in population size.
  • The effect was density-dependent.
• Superior competitors will have greater fitness than lesser competitors.
• Fine-tuning of a species' trait can cause specialization; the species can only use this resource.
• Optimization comes with the cost of not being able to effectively compete/use other resources that require different behaviors to obtain.

Interspecific Competition and Niche Space

• Niche: Summarizes the environmental factors that influence the growth, survival, and reproduction of a species.
  • A species’ niche consists of all the factors necessary for its existence – approximately when, where, and how a species makes its living.
• The term "niche" has been around for almost 100 years.
• The importance of niche space was fully realized within interspecific competition.
• G.E. Hutchinson (1957) defined the niche as an n-dimensional hypervolume, where n equals the number of environmental factors important to the survival and reproduction of the species.
• Niche is multidimensional, with multiple, complicated interconnections with its environment.
• Hutchinson’s hypervolume, which specifically defines the accumulated factors permitting a species to survive and reproduce, is the fundamental niche of a species.
• Fundamental niche defines the physical conditions under which a species might live, in the absence of interactions with other species (abiotic environment).
• Interactions, such as competition, may restrict the environments in which a species may live: the realized niche.
• The realized niche is the actual niche of a species that is restricted through its interactions (competition, predation, disease) with other species.
• Example of niche-overlap: Coyotes and Wolves

Interspecific Competition

• Interspecific Competition: Competition between individuals of different species for the same resources.
• Interspecific competition can lead to an Evolutionary Arms Race.
  • Adaptations of one species are overcome by the selection for traits within another species, which is countered by the evolution of new traits in the first to overcome adaptations by the second.
• G.F. Gause's study of interspecific competition in 1934 led to the Competitive Exclusion Principle.
  • The Competitive Exclusion Principle states that two species with identical niches cannot coexist in that niche indefinitely.
  • The costs of competition are great enough that if 2 species have identical niches, the better competing species will exclude the lesser competitor from the niche.
  • The species that is out-competed will likely go extinct from the area.
  • If the niches do not completely overlap, the lesser competitor MAY alter its niche space to better fit that which does not overlap with the other species (realized niche).
  • Sometimes both competitors will shift their niche space.
• Resource partitioning describes the differentiation of niches, both in space and time that enables similar species to coexist in a community.
  • Example: MacArthur’s warblers.
• Resource segregation: one difference between species and it reflects results of past competition. By understanding where species are different, we can see where competition was the greatest.
• Example: Lack’s passerines: Among passerines (small perching birds), most segregation takes place by habitat preference, followed by body size, feeding habit, geography, and wintering range.

Mechanisms of Competition

• Competition mechanisms fall into two categories:
  • Scramble or exploitative competition: individuals target the resource, and there may be no direct interaction between the two.
  • Interference competition: individuals target the competitor, and not necessarily the resource.
• Examples:
  • Dung beetles
  • Barnacles
  • Hyenas vs. scavengers
  • Salvia sp. vs. other plants