Ecology/Resource Competition

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Chapter 10. Resource Competition

Population Growth

A population is defined as a group of interbreeding organisms inhabiting the same region at the same time. In demography, the study of changes in population structures (e.g., birth rates, death rates), the age structure of a population is often considered. Age structure is defined by how many individuals fall within a particular age class. Typically, demography-forming life tables, in which there are several different types, are constructed. Each life table is more advantageous for a specific population structure than other life tables might be.

A time-specific life table, which is a snap-shot of a long-lived species, is one type of life table used. It usually starts out as a survivorship curve and is usually normalized to 1,000 individuals. Time-specific life tables assume that there are equal birth rates among all age classes.

An age-specific life table is another type of life table which works best for short-lived species like insects (butterflies and flies for example). Age-specific life tables follow a cohort. A cohort is a group of individuals sharing an attribute. Age-specific life tables require a little more sampling than the time-specific life tables.

Studying population structure is also associated with studying life expectancy. Life expectancy is the number of additional classes expected. Life expectancy may be used for both time-specific life tables and age-specific life tables. In other words, life expectancy may be used on long-lived or short-lived species. Studying populations also deals with studying the type of organisms that are present in the environment (either K-selected or r-selected organisms). K-selected organisms are those organisms that are adapted to survive and compete in a stable environment with limited resources. K-selected organisms are better competitors when it comes to limited resources as compared to r-selected organisms. An r-selected organism is one that is adapted to reproduce quickly to take advantage of a highly disturbed environment. R-selected organisms are more likely to be generalists.

As previously mentioned, both life tables have their advantages of when and how to be used for a particular population. Both can be used to calculate fecundity. Fecundity is the average number of offspring per breeding individual. They can also be used to see which species are out-competing other species for resources (the species that outcompetes other organisms tend to live longer). Overall, calculating the growth of a population (fecundity) is used by deterministic growth models. Deterministic growth models are usually an exponential curve shape.

Competition Types

Competition is a natural occurrence between organisms occupying the same space at the same time. Thus, competition can occur between organisms of the same species or between organisms of a different species. Hence, two main types of competition exist. Interspecific competition refers to two different species vying for the same resource, and intraspecific competition refers to individuals of the same species competing for the same resource. The term resource can describe water, food, shelter, territory, light, or any means to maintain life and reproduce. Intraspecific competition is usually a major determiner of population density, and interspecific competition can result in the extinction of a local species.

Different theories have been demonstrated by the effect of two species interacting in an environment, especially if the two species are in competition for the same niche. In 1934 G.F. Gauze studied protists and determined that only one species can occupy a single niche. The Competition Exclusion Principle signifies that if two species occupy exactly the same niche, the species that uses the shared resources less efficiently will eventually go extinct where is coexists with the more efficient competitor. In order for competing species to coexist, there must be some difference(s) in their niches.

Competition can take a variety of forms. Exploitation competition is when a species reduces resources, negatively affecting another species using that same resource. This is also called scramble or exploitative competition. It occurs when a number of organisms (of the same or of different species) utilize common resources that are in short supply. Interference competition is when one species physically excludes another species from using a particular resource. Overgrowth competition occurs when an organism physically grows on top of another (usually sessile) organism and, in turn, limits the underlying organism's ability to capture a resource like food or light.

Density-Dependence is a major form of intraspecific competition that regulates population size in an environment. This occurs when an increase in population density slows down or halts population growth or when a decrease in population density stops further decrease in population size. This type of self regulation can be explained by an increase in prey, which will result in an increase in predators, controlling the population. The same decrease in prey will lead to a decrease in predators.

Modeling Interspecific Competition

In the 1920s, Vito Volterra and Alfred Lotka independently developed realistic models of interspecific competition between two species (1932). They took into account that real populations do not live in isolation but share habitats with a variety of other species. These coexisting species interact by predation, parasitism, mutualism or competition. Lotka and Volterra developed a model that investigated the conditions that would allow species who compete for resources to coexist indefinitely.

As mentioned above, the competitive exclusion principle generally says that if there is excessive niche overlap, one species will have to exclude the other to survive. This principle was formed from the Lotka-Volterra Interspecific competition model. The Lotka-Volterra model of two-species competition is as follows:

       Population 1:  N_1,t+1= N_1,t+R₁N_1,t(K₁-N_1,t-a₁₂N_2,t)/K₁

       Population 2:  N_2,t+1= N_2,t+R₂N_2,t(K₂-N_2,t-a₂₁N_1,t)/K₂

The key component to this model is the competition coefficient, a₁₂, which expresses the effect of one new member of population 2 on the growth rate of population 1, and the opposite for a₂₁, which expresses the effect of one member of population 1 on the growth rate of population 2.

This model was designed to establish conditions in which two species could coexist indefinitely. Although this is pertinent information, the reality of whether two species can coexist depends on more than their competitive interactions with each other. It also depends on their interactions with their abiotic environment and other species not included in the model.

Kin Competition

Kin competition has played a major role in the evolution of social behavior. In turn, kin selection has altered specific behaviors affecting kin competition. Therefore, social behavior and kin structure of a population tend to co-evolve to maximize inclusive fitness. Inclusive fitness, an idea presented by W. D. Hamilton in 1964, states that an organism can increase its genetic success by promoting the reproduction and survival of genetic kin. In other words, social behaviors can enhance the fitness of individuals with closely related genes. Such an action increases the organism's indirect fitness. The forming of family groups is an evolved strategy with the purpose of maximizing inclusive fitness.

Kin competition is most commonly avoided by the evolutionary response of dispersion. In other words, mothers have evolved mechanisms in which they have their offspring in spatially dicrete patches. Also, in some cases, mothers can reduce competition related conflicts by biasing the sex-ratio of her offspring. Again, both the above are evolved strategies that minimize the competition between or among kin. An example of reduced competition can be seen in pollinating fig wasps. Mothers produce female-biased sex ratios to reduce local mate competition between her sons ^[1].

Competition Infocus

When thinking about competition, one might imagine two white-tail bucks sparring for the affection of a doe or wolves defending a kill. However, competition comes in many forms.

One great example of interference competition is shown in a study conducted by Rozen D. and his colleagues. In this study, they monitored the life of the burying beetle, Nicrophorus vespilloides. This particular beetle uses small mammal carcasses as a food source for growing and feeding their young. Rozen presented the beetles with mouse carcasses that were either freshly thawed or had been at room temperature for seven days. Rozen found that female Nicrophorus preferred to raise their young on freshly thawed carcasses. Rozen also discovered that offspring raised on carcasses at room temperature were on average 10% smaller in size ^[2].

*Nicrophorus vespilloides* known commonly as the burying beetle.

This smaller size is due to the presence of microbe competitors. Microbes have a huge advantage. They can rapidly reproduce, and they are found everywhere, including in the gut of the deceased animal. Microbes have the ability to produce secretions that cause a carcass to rot and become toxic to other organisms ^[3]. Female Nicrophorus have found two ways to fight microbes for resources, avoiding older carcasses and applying an antibiotic. The process of applying an antibiotic to suppress the growth of a competitor is known as antibiosis ^[4]. The battle of antibiosis between Nicrophorus and microbes is a fascinating example of competition.

Competition can and does occur between wild and domesticated animals. In African savannas, wild herbivore decline can be partially attributed to competition for forage with livestock. Such competition is intensified in periods of drought. Results of a 2008 study indicate that pastoralist decisions may play an important role on the interaction between wild herbivores and livestock through spatial partitioning ^[5].

Plants compete for things such as sunlight and nutrients. Gopel and Goal (1993) claimed that floating macrophyte (aquatic plant) species with similar growth rates compete the most for resources. Studies such as this have led to predictions that early stages of succession will be dominated by species with higher growth rates ^[6].

References

^ Lotka, A.J. 1932. The growth of mixed populations: two species competing for a common food supply. Journal of the Washington Academy of Sciences 22: 461-469.
^ Nelson, Ronald M., Jaco M. Greeff 2009. Evolution of the scale manner of brother competition in pollinating fig wasps. Animal Behaviour Vol.77, Iss.3, 693-700.
^ Rozen, D.E., D. J. P. Engelmoer, P. T. Smiseth 2008. Antimicrobial strategies in burying beetles breeding on carrion. PNAS vol. 105, no. 46. http://www.pnas.org/cgi/doi/10.1073/pnas.0805403105
^ Kaspari, Michael, Bradley Stevenson 2008. Evolutionary ecology, antibiosis, and all that rot. PNAS vol. 105, no. 49.
^ Sitters, Judith, Ignas M. A. Heitkonig, Milena Holmgren, Gordon S. O. Ojwang 2009. Herded cattle and wild grazers partition water but share forage resources during dry years in East African savannas. Biological Conservation Vol. 142, Iss. 4, 738-750.
^ Tipping, Philip W., Laurie Baurer, Melissa R. Martin, Ted D. Center 2009. Competition between Salvinia minima and Spirodela polyrhiza mediated by nutrient levels and herbivory. Aquatic Botany Vol. 90, Iss. 3, 231-234.

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