25.3: Environmental Limits to Population Growth
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Learning Objectives
- Compare and contrast between exponential and logistic growth patterns.
- Give examples of exponential and logistic growth in natural populations.
- Describe the role of carrying capacity in population growth.
- Describe the influences of intraspecific competition in population size.
Although life histories describe the way many characteristics of a population (such as their age structure) change over time in a general way, population ecologists make use of a variety of methods to model population dynamics mathematically. These more precise models can then be used to accurately describe changes occurring in a population and better predict future changes. Certain models that have been accepted for decades are now being modified or even abandoned due to their lack of predictive ability, and scholars strive to create effective new models.
Exponential Growth
Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, and then population growth decreases as resources become depleted. This accelerating pattern of increasing population size is called exponential growth .
Though bacteria are often the noted examples of species that grow exponentially, we do see this in many photosynthetic species as well. For example, algae and cyanobacteria will often grow exponentially for a period of time with warmer temperatures, both in freshwater (ex: lakes) or salt water (ex: oceans). Exponential growth occurs when the population growth rate —the number of organisms added in each reproductive generation—is accelerating; that is, it is increasing at a greater and greater rate. For example, a population of 1000 can increase by 1000 in one hour, but then increase by 2000 the second hour, and to 4000 the third hour, and 8000 by the fourth hour. The number of organisms increases faster at every reproduction event. After 1 day and 24 of these cycles, a population could have increased from 1000 to more than 16 billion. When the population size is plotted over time, a J-shaped growth curve is produced (Figure \(\PageIndex{1}\), left graph).
We often see examples of exponential growth for a period of time in nature. For example, after recovery, a plant species may exponentially grow for a period of time while establishing its previous niche. Non-native, invasive species , can often also grow exponentially, as they may not have the same environmental pressures (predators, parasites, competitors) in the introduced area and can increase dramatically once established.
Logistic Growth
In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off (Figure \(\PageIndex{1}\), right graph). The pattern formed in this type of growth is called logistic growth , or S-curve. This population size, which represents the maximum population size that a particular environment can sustain, is called the carrying capacity .
There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.
Role of Intraspecific Competition
The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources.
In the real world, phenotypic variation among individuals within a population means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition (intra- = “within”; -specific = “species”). Intraspecific competition for resources may not affect populations that are well below their carrying capacity—resources are plentiful and all individuals can obtain what they need. However, as population size increases, this competition intensifies. In addition, the accumulation of waste products can reduce an environment’s carrying capacity.