Ecology: Competition and Coexistence

Species Interactions and Competition

• Species interact with various ecological outcomes that can be positive, negative, or neutral for the organisms involved
• Negative impacts typically involve energy expenditure or injury
• Symbiosis occurs when most or all of one organism's life cycle happens on or in another organism

Modes of Competition

• Interference competition involves direct interactions between individuals, such as defending territory or producing chemical toxins
• Resource limitation is another cause of Interference competition
• Intraspecific competition occurs among members of the same species
• Interspecific competition occurs between individuals of two different species, reducing the fitness of both

Intraspecific Competition Among Plants

• Plant biomass tends to be higher in low-density populations
• Competition for resources like nitrogen intensifies at higher densities
• Intraspecific competition often leads to mortality, a process known as self-thinning

Intraspecific Competition Among Planthoppers

• Denno and Roderick attributed competition among Homoptera to their aggregating habits, rapid population growth, and mobile food supply
• Intraspecific competition in planthopper populations often results from limited resources

Competitive Exclusion and Niches

• Gause's competitive exclusion principle states that two species with identical niches cannot coexist indefinitely
• One species will be a better competitor, leading to the exclusion of the other
• Coexisting species tend to have different niches
• Niches are often determined by only a few key factors

The Feeding Niches of Darwin's Finches

• Grant used beak size to represent the various feeding niches of finches
• Beak depths can estimate the size of seeds eaten
• Individuals with deeper beaks can feed on harder seeds
• Droughts highlight the importance of beak size
• During a 1977 drought, the availability of seeds decreased
• Finches first consumed small, soft seeds, leaving only hard seeds behind
• Small-beaked birds faced the highest mortality

Mathematical and Laboratory Models

• Mathematical and laboratory models are generally simpler representations of nature
• Metz summarized the use of models for the beetle Tribolium
• Models are abstractions and simplifications, not facsimiles of nature
• They are man-made constructs that are useful gaining insights into natural phenomena

Modeling Interspecific Competition

• dN₁ / dt1= r₁N₁ ((K₁ - N₁ - α₁₂N₂) / K₁) This represents the effect of interspecific competition on population growth of each species
• dN₂ / dt = r₂N₂ ((K₂ - N₂ - α₂₁N₁) / K₂) This represents the effect of interspecific competition on population growth of each species
• α₁₂: Represents the effect/impact of an individual of species 2 on the rate of population growth of species 1
• α₂₁: Represents the effect of an individual of species 1 on the rate of population growth of species 2

Lotka-Volterra Model

• The Lotka-Volterra model generally predicts the coexistence of two species when interspecific competition of both species is weaker than intraspecific competition
• Otherwise, one species will exclude the other

Isoclines of Zero Population Growth

• N₁ = K₁ – α₁₂N₂ and N₂ = K₂ – α₂₁N₁. Predicts when population growth for two species will stop
• These equations are referred to as isoclines of zero population growth
• Above the isocline, the population will decrease
• Below the isocline, the population will increase
• Coexistence of two species is possible only when isoclines cross

Experiments with Paramecia

• Gause demonstrated resource limitation using P. caudatum and P. aurelia with Bacillus pyocyaneus.
• When grown alone, carrying capacity is determined by intraspecific competition
• When grown together, P. caudatum quickly declined
• Competition increased with reduced resource supply

Experiments with Flour Beetles

• Tribolium beetles infest on stored grain products
• Park studied interspecific competition between T. confusum and T. castaneum
• Interspecific competition restricts the realized niches of both species to fewer environmental conditions

Competition and Niches

• Competition can limit species to their realized niches
• Strong and pervasive competitive interactions can produce an evolutionary response in the competitor population, resulting in changes to the fundamental niche

Niches and Competition Among Plants

• Tansley suggested that interspecific competition restricts the realized niche species: bedstraw (Galium spp.)
• G. saxatile grows of saxatile on acidic soils
• G. sylvestre grows on Gymestre limestone soils
• Experiments show each will grow on both soil types in the absence of competition
• When grown together, G. sylvestre quickly dominates on limestone soild
• When grown together, G. saxatile slowly dominates acidic soils

Niche Overlap and Competition Between Barnacles

• Connell observed the barnacles' interspecific competition
• Balanus is limited to the middle and lower intertidal zone by physical factors; this explains Balanus's lower limit, but not Chthamalus'
• Balanus also plays a role in determining the lower limit of Chthamalus within the intertidal zone
• Balanus excludes Chthamalus from the middle intertidal zone through intraspecific competition

Competition and Niches of Small Rodents

• Brown studied Chihuahuan Desert rodents' competition
• If granivorous rodents compete for food:
• Populations of small rodent would increase with removal of larger rodents
• Insectivorous rodents would show little or no response
• His experiments' results supported this hypotheses

Character Displacement

• Interspecific competition reduces competitors' fitness
• Individuals better able to limit or reduce competition have higher fitness
• Interspecific competition can lead to character displacement
• Species that co-occur differ more in areas where they overlap than in areas where their distributions do not overlap

Character Displacement in Darwin's Finches

• Medium (G. fortis) and small (G. fuliginosa) ground finches exhibit character displacement
• They are allopatric on Daphne Major and Los Hermanos:
• They have similar beak sizes
• They are sympatric on Santa Cruz:
• They have dissimilar beak sizes
• Allopatric G. fortis beaks are smaller than sympatric G. fortis beaks
• Allopatric G. fuliginosa beaks are larger than sympatric ones
• On Santa Cruz, the species have different feeding Niches

Character Displacement Criteria Parts I and II

• Taper and Case's criteria for character displacement Part I:
• Morphological differences between sympatric species are statistically greater than differences between allopatric populations
• Differences between sympatric and allopatric populations have a genetic basis
• Differences between sympatric and allopatric populations evolved in place, not derived from founder groups already differing in the character T- aper and Case's criteria for character displacement Part II:
• Variation in the character must have a known effect on resource use
• Competition must be demonstrated for the resource; competition must be directly correlated with character similarity
• Differences in character can't be explained by differences in resources available to each of the population

Test your knowledge of ecological competition, including Gause's principle, Lotka-Volterra models, and intraspecific competition. Questions cover natural selection, mathematical modeling, and the impact of environmental factors on species survival and coexistence. Explore the dynamics of species interactions and population regulation.