Limiting Factor Definition
A limiting factor is a resource or environmental condition which limits the growth, distribution or abundance of an organism or population within an ecosystem. These can be either physical or biological factors which can be identified through a response of increased or decreased growth, abundance, or distribution of a population, when the factor is changed and when the other factors necessary to life are not.
Limiting factors are theorized under Liebig’s Law of the Minimum, which states that “growth is not controlled by the total amount of resources available, but by the scarcest resource”.
The limiting resource within an ecosystem determines the carrying capacity (indicated in ecology by the letter, “K”), which is the maximum number of individuals in a population that a habitat can support without environmental degradation.
In an ecosystem with unlimited resources, no predators and no disease, populations may experience exponential growth. The carrying capacity therefore acts as a moderator of population size; once limiting resources start to become depleted by increasing numbers of individuals, intraspecific competition occurs and the growth rate of the population begins to slow as individuals die or fail to reproduce. Eventually the growth rate levels off at a plateau – this plateau is the carrying capacity. Once the carrying capacity of an environment has been reached, individuals may begin to search for resources elsewhere, migrating away from the original population and creating new populations. If the populations become separated indefinitely, this can lead to speciation.
Although sunlight tends to be a factor which controls the temperature of a habitat, and thus affects photosynthesis in terms of photon energy, correct temperature is also important for catalyzing enzymes in photosynthetic reactions. Above the optimum temperature, enzymes are catalyzed at an increased rate, which can lead to denaturing of the enzymes. This is called a light independent reaction. Increased temperature also leads to desiccation of leaves, as it causes increased evapotranspiration and removes too much water from the plant. Conversely, if temperatures fall too low, frost may form on leaves, which damages the cell walls and cell contents.
The amount of precipitation in an environment is also important for plants. The absorption of water as a resource is vital for plant growth and other functions, so lack of rainfall can lead to wilting, scorching and damaged cells. Precipitation is also important because many plants are evolved to withstand different amounts of atmospheric humidity. As the thin, tough leaves of cacti make them specifically adapted to surviving in hot and arid conditions, too much rainfall can affect their ability to reproduce, which in turn restricts the population growth. Too much rainfall may also flood the soil, reducing the amount of oxygen available to the roots, causing root loss or leaving the plants susceptible to fungal damage.
The population density of predators and prey are limiting factors for each of these parties. If a population, for example deer, reaches high numbers because there has been an increase in a resource that is their limiting factor – such as increased tree growth after a warm summer – their predators, for example, wolves, will experience an increase in their food source. The wolf population, with a surplus of deer to eat, may then be able to reproduce more freely, increasing their own population. As the population of wolves increases, they will require the high number of deer, which was available as their limiting factor when the population grew. As the wolves heavily predate the deer populations, the resources will eventually become scarce, and the wolf population will no longer be sustainable. As wolves die, pressure on the deer population is reduced and the numbers will be able to increase again; thus the cycle continues.
Human Limiting Factors
The increase in human population is responsible for placing many limiting factors on species that did not historically exist. Density dependent limiting factors such as decreased availability of space due to deforestation is a global issue, causing decline and extinctions in many populations. Resources are also increasingly scarce due to hunting and leaching of nutrients from soil, which causes intraspecific and interspecific competition within and between populations. Removal of predators has also disturbed the balance of natural biotic, cycle of predators and prey; in some cases, prey animals have been able to thrive in the absence of predators, exceeding the carrying capacity of ecosystems and causing environmental damage. Predators have also been introduced as invasive species into ecosystems, putting pressure on prey populations and thus on the prey’s natural predators.
