6.5.2. Resource competition

r- and k- species

Depending on their reproductive strategies, species can be characterized as r or k species. Here r is the instantaneous rate of population increase while k is the carrying capacity. The r-species possess characteristics of high biotic potential, rapid development, early reproduction, single period of reproduction per individual, short life cycle, and small body size. On the other hand k-speciespossess characteristics of low biotic potential, slow development, delayed reproduction, multiple periods of reproduction per individual, long life cycle and larger body size. Populations of r-species usually remain below the carrying capacity and are regulated by the density-independent (affect populations regardless of population density) factors; while populations of k-species are usually maintained near the carrying capacity and regulated by density dependent (have a different effect when populations are high than when they are low) factors. In disrupted habitats r-species are more common while k-species are common in stable habitats. Many of our agricultural pests are r-species. Most organisms however actually have attributes that fit both r and k species.

r Unstable environment, density independent k Stable environment, density dependent interactions
Organism size Small Large
Energy used to make each individual Low High
# Offspring produced Many Few
Timing of maduration Early Late (with much parental care)
Life expectancy Short Long
Lifetime reproductive events One Moret han one
Survivorship curve Type III Type I or II

Survivorship curve for the Tyrannosaurus rex(600x664)(c)Survivorship Curves

Survivorship Curves, it is a graph showing the number of proportion of individuals surviving at each age for a given species or group. There are three types of curves. Type 1, Type 2 and Type 3.

Type 1, in this type there are more deaths at older ages. Humans are great example for this type.

Type 2, these survivorship curves graph as a straight line on semi-logarithmic graph paper. The individuals in populations that display a type 2 curve are those that both do not age and are born as fully fit as adults. Individuals are lost in these populations mostly to accidents and predation.

Type 3, type 3 survivorship species have a very large rate of mortality when young, but should they survive their youth, they put significant energy into continued survival since the longer they survive, the more progeny they will produce. In this type there are more deaths at young ages. Sea turtles and trees are great example of this type.

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
  • 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 contributor to population density and interspecific competition can result in the extinction of a local species.
Competition can be described in various ways concerning species interaction with each other:
  • Exploitation competition is when resources used by one species is reduced and negatively affects 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 organism and in turn, limits the underlying organism ability to capture a resource like food or light. This type usually refers to sessile organisms.

Density-Dependence is a major form of competition that regulates population in an environment. This is a proportional relationship between slowing down or halting the population increase in population density or stopping a decrease in population with a decrease in density. This type of self regulation can be explained by an increase in prey which will bring an increase in predators to control the population. The same decrease in prey will lead to a decrease in predators. Scramble competition is an example of density dependence overcompensating on survivorship in intraspecific competition. All competing individuals are affected so unfavorably that all individuals cease to exist.

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