25.4: Population Dynamics and Regulation
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Learning Objectives
- Compare and contrast density-dependent growth regulation and density-independent growth regulation.
- Compare and contrast K -selected and r -selected species using examples of their life history traits.
The logistic model of population growth, while valid in many natural populations and a useful model, is a simplification of real-world population dynamics. Implicit in the model is that the carrying capacity of the environment does not change, which is not the case. The carrying capacity varies annually: for example, some summers are hot and dry whereas others are cold and wet. In many areas, the carrying capacity during the winter is much lower than it is during the summer. Also, natural events such as earthquakes, volcanoes, and fires can alter an environment and hence its carrying capacity. Additionally, populations do not usually exist in isolation. They engage in interspecific competition : that is, they share the environment with other species, competing with them for the same resources. These factors are also important to understanding how a specific population will grow.
Nature regulates population growth in a variety of ways. These are grouped into density-dependent factors, in which the density of the population at a given time affects growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Note that in the former, the effect of the factor on the population depends on the density of the population at onset. Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation.
Density-dependent Regulation
Most density-dependent factors are biological in nature (biotic), and include predation, inter- and intraspecific competition, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population’s rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. High population density can also increase the transmission of parasites and prevalence of disease.
Density-independent Regulation
Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. A frosty night that emerging in early spring does not kill a higher or lower percentage of seedlings based on the density of the seedling population. Its chances of survival for the seedling are the same whether the population density is high or low.
In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than a sparse population.
Life Histories of K -selected and r -selected Species
While reproductive strategies play a key role in life histories, they do not account for important factors like limited resources and competition. The regulation of population growth by these factors can be used to introduce a classical concept in population biology, that of K -selected versus r -selected species.
K -selected species are species selected by stable, predictable environments. Populations of K -selected species tend to exist close to their carrying capacity (hence the term K -selected) where intraspecific competition is high. These species have few, large offspring, have more parental care, and have a longer period of maturation development period. In plants, scientists think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are determining factors in the time to the next reproductive event. Oak trees are great examples of K -selected species (Figure \(\PageIndex{1}\)). Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as acorns. As many as 50,000 acorns can be produced by an individual tree, but the germination rate is low as many of these rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number of acorns, and these years may be on a two- or three-year cycle depending on the species of oak ( r -selection).
As oak trees grow to a large size and for many years before they begin to produce acorns, they devote a large percentage of their energy budget to growth and maintenance. The tree’s height and size allow it to dominate other plants in the competition for sunlight, the oak’s primary energy resource. Furthermore, when it does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly established ( K -selection).
In contrast, r -selected species have a large number of small offspring (hence their r designation (Table \(\PageIndex{1}\)). This strategy is often employed in unpredictable or changing environments. Many weedy plants are considered r-selected. Dandelion are great examples of r-selected species, producing many small seeds on one flower head that are seed dispersed long distance (Figure \(\PageIndex{3}\)). Many seeds are produced simultaneously to ensure that at least some of them reach a hospitable environment. Seeds that land in inhospitable environments have little chance for survival since their seeds are low in energy content. Note that survival is not necessarily a function of energy stored in the seed itself.
| Characteristics of K -selected species | Characteristics of r -selected species |
|---|---|
| Mature late | Mature early |
| Greater longevity | Lower longevity |
| Increased parental care | Decreased parental care |
| Increased competition | Decreased competition |
| Fewer offspring | More offspring |
| Larger offspring | Smaller offspring |
- Contributors
Curated and authored by Kammy Algiers from the following sources: