A limiting factor is any environmental condition or resource shortage that restricts the size, growth, or distribution of a population within an ecosystem. These factors are the fundamental checks on population increase, preventing a single species from consuming all available resources and destabilizing its environment. The interaction of these various constraints maintains the natural balance of an ecosystem over time.
Constraints Based on Population Size
Some constraints on population growth intensify as the number of individuals living in a specific area increases. These density-dependent factors create a negative feedback loop where a larger, denser population experiences a stronger limiting effect. The per capita growth rate naturally slows down as the population becomes more concentrated.
One of the most direct density-dependent factors is intraspecific competition, where members of the same species vie for finite resources like food, water, or suitable nesting sites. As a population grows, the available resource pool must be divided among more individuals, leading to stress, reduced reproductive success, and increased mortality rates. For example, a high density of deer in a forest can lead to over-browsing of vegetation, diminishing the food supply for all individuals and stressing the entire herd.
Predation pressure can also be a powerful density-dependent factor. When a prey population becomes dense, it offers a more concentrated and easily exploitable food source for predators, which may then increase their hunting efficiency or experience a population boom themselves. This dynamic is famously seen in the cyclical relationship between the snowshoe hare and the Canadian lynx, where the rise in the hare population is followed by a rise in the lynx population, which subsequently causes the hare numbers to crash.
The spread of infectious disease and parasites is further exacerbated by high population density. Pathogens and parasites transmit more easily and rapidly when individuals are in close and frequent contact, leading to higher rates of infection and death in crowded populations compared to sparse ones. This increased vulnerability to illness, coupled with the stress from resource competition, can trigger a significant population decline.
Constraints Unrelated to Population Size
Other limiting factors affect a population regardless of how many individuals are present per unit area. These density-independent constraints are typically non-living, or abiotic, environmental forces that impact the population uniformly. These factors often cause sudden, catastrophic reductions in population size.
Severe and unpredictable changes in climate and weather patterns represent a major density-independent constraint. Events such as extreme droughts, prolonged cold snaps, or intense hurricanes can destroy habitat, eliminate food sources, or kill organisms directly, with the same severity for every individual. For instance, a flash flood will sweep away animals in its path regardless of the local density of their species.
Natural disasters, including wildfires, volcanic eruptions, and earthquakes, similarly impact populations irrespective of their size. A large-scale forest fire will destroy the habitat and kill any animals trapped within its boundaries, affecting a sparse population just as dramatically as a dense one. These events act as powerful, non-selective mortality agents.
Broad anthropogenic factors can also function as density-independent limits across an entire geographic range. Widespread chemical pollution, such as an oil spill or the uniform application of certain pesticides, can degrade the environment and increase mortality rates without regard for how many organisms are present. Similarly, large-scale habitat destruction that uniformly clears an area affects all species in that location equally.
The Ecological Result: Defining Carrying Capacity
The combined pressure exerted by both density-dependent and density-independent limiting factors determines a fundamental ecological limit known as the Carrying Capacity, or K. This value represents the maximum population size of a specific species that a given environment can sustain indefinitely without undergoing permanent environmental degradation. K is an ecological ceiling, not a fixed number, as it can fluctuate with environmental changes.
The concept of carrying capacity helps explain why populations in nature rarely follow a pattern of exponential growth, which is characterized by a rapidly accelerating, J-shaped curve. Instead, most populations follow a logistic growth pattern, which is represented by an S-shaped curve. During logistic growth, the population initially grows quickly, but as it approaches K, the limiting factors begin to slow the growth rate until it eventually levels off.
When a population size is far below K, resources are abundant, and the birth rate exceeds the death rate, allowing for rapid growth. However, as the population nears K, the density-dependent factors—like increased competition for food or higher disease transmission—cause the birth rate to decline and the death rate to rise. At the level of K, the birth and death rates are essentially equal, resulting in zero net population growth.
Exceeding the carrying capacity can have severe consequences for both the population and the environment. If the population temporarily overshoots K, it will deplete resources faster than they can regenerate, leading to habitat degradation. This resource scarcity can trigger a sudden and sharp die-off, often called a population crash, which restores the population to or below the sustainable level. The carrying capacity itself is dynamic and can shift downward if an environmental change, such as a prolonged drought or pollution, permanently reduces the resource base.