Ecology: Competition and Resources

• Competition occurs when one organism is harmed by another's actions.
• Competition shapes distribution, abundance, and evolution of organisms.
• Organisms compete for renewable resources like water, light, and nutrients.
• Organisms compete for nonrenewable resources like space and minerals in soil.

Types of Competition

• Interspecific competition occurs between different species.
• Intraspecific competition occurs among individuals of the same species.

Liebig's Law of the Minimum

• Growth is controlled by the scarcest, or most limiting, resource, not the total amount of resources available.
• Population growth is constrained by the depletion of critical resources.
• The availability of resources can be coupled to population growth.
• Species compete most strongly for the limiting resource.

Gause's Competitive Exclusion

• Two species competing for the exact same limiting resource cannot coexist indefinitely.
• One species will outcompete the other, leading to the extinction of the weaker competitor.
• When grown alone, Paramecium aurelia and Paramecium caudatum exhibit logistic growth.
• Logistic growth includes rapid growth, slowing as it approaches carrying capacity.
• When grown together, Paramecium aurelia outcompetes and excludes Paramecium caudatum because they compete for the same food.
• Paramecium caudatum's population declines and eventually goes extinct.

Interference Competition

• Organisms directly prevent others from accessing a resource.
• This can involve physical prevention, such as fighting, territoriality, or releasing toxins.
• Ants blocking rival ants from food is an example.

Exploitation Competition

• Involves indirect competition without direct aggression.
• Organisms consume shared resources faster or more efficiently, depleting them before others can.
• Two plants absorbing water from the soil without direct contact is an example.

Apparent Competition

• Two species share a common predator and compete for survival.
• An increase in one prey population leads to a rise in predator numbers, harming the other prey population.

Intraspecific Competition Among Plants

• Competition for resources is more intense at higher population densities.
• Plants compete for light, water, nutrients, and space.
• High mortality occurs among competing plants, leading to self-thinning where weaker plants die.
• Growth rate and weight are lower as each plant gets fewer resources.

Logistic Growth Model

• A single species growing alone can be modeled using the Logistic Growth Model: dN/dt = rN(K-N)/K
• in the equation: dN/dt is the rate of population growth, r is the intrinsic growth rate, N is the population size, and K is the carrying capacity.
• When N is small, growth is fast.
• As N approaches K, growth slows and eventually stops.

Lotka-Volterra Competition Model

• Describes two species trying to survive simultaneously while sharing resources.
• Requires accounting for each species' population size and the harm caused by the other species, modifying the basic logistic model.
• Species 1's Change in population is given by dN1/dt = r1N1(K1 - N1 - αN2)/K1
• α represents the effect of species 2 on the population growth of species 1.
• Species 2's change in population is given by dN2/dt = r2N2(K2 - N2 - βN1)/K2
• β represents the effect of species 1 on the population growth of species 2.

Species Growth Isoclines

• Setting dN/dt to 0 helps find the population sizes of species 1 and 2 where one species will have a zero growth rate.

Ecological Niche

• An organism's role within its ecological community including range of abiotic and biotic conditions it can tolerate, interactions with its predators, competitors, and others, including physical, environmental (abiotic), and ecological conditions.
• More than just a species habitat.
• A habitat is the physical setting where an organism lives and is determined by its niche.

Geographic Range

• The area where a species can be found during its lifetime.

Hutchinson Niche

• It explains the diversity of organisms in a specific habitat.
• A species' ecological niche is described as an n-dimensional hypervolume.
• 'n' represents the number of environmental factors important to survival and reproduction of a species.
• Diversity of a habitat is dependent on the environmental factors influencing each species.

Ecological Niche Modeling

• Well-known niches can be predicted where they occur and might occur in the future.
• This modeling is important for locating rare species, predicting range expansion for invasive species, and forecasting species movement due to climate changes.

Joseph Connell Experiment

• Chthamalus and Semibalanus barnacles live in the same habitat but have different distributions due to environmental and biological factors - specifically interspecific competition.

• Chthamalus is small, lives in the upper intertidal zones close to the high tide mark, is a weak competitor, and survives well without water (desiccation).

• Semibalanus is large, lives in the middle and lower intertidal zones down on the rocks, is a strong competitor, and needs water more often, not surviving desiccation.

• In lower zones, Semibalanus outcompetes Chthamalus because it grows faster and is bigger.

• In higher zones, Chthamalus outcompetes Semibalanus because Semibalanus dries out more often.

Fundamental vs. Realized Niche

• Fundamental Niche: the entire range of environmental conditions and resources a species could potentially occupy if there were no limiting factors like competition or predation.
• Realized Niche: the actual portion of the fundamental niche that a species actually occupies due to interactions with other species.
• Chthamalus' fundamental niche is the upper and middle intertidal zones, but its realized niche is only in upper zones because of competition with Semibalanus.

Niche Separation vs. Breadth

• Niche Separation/Partitioning: Difference in optimal use of a resource.
• Species divide available resources to minimize direct competition, illustrated by eating at different times, in different parts of trees, or different food sizes.
• Temporal Partitioning: Species may partition if they coexist.
• Common spiny mice and golden spiny mice are normally nocturnal.
• When they coexist, the golden spiny mouse becomes active during the day (diurnal).
• Niche Breadth: Diversity of resources used or environments tolerated by an individual.
• Broad niche breadth means a bird may eat many different insects.
• Narrow niche breadth means a bird may eat only one specific type of insect.

Character Displacement

• The evolution of phenotype differences between species due to competition.
• When niches overlap is high and competition is high, the species need the same resources.
• Natural selection favors individuals that use resources differently and therefore avoid direct competition.

Darwin's Finches

• An example of character displacement involving beak size and shape in finch species on the Galápagos Islands, which evolved due to competition for food.
• Medium beak finches are favored without competition because they can eat small and large seeds.
• Natural selection favors specialization with two alternate species evolved for the island niche.
• One species evolves a smaller beak to efficiently eat small seeds, and another evolves a larger beak to crack big seeds. This reduces overlap in resource use and minimizes competition.