What Causes Population Fluctuations in Nature?

A biological population is a group of individuals of the same species living within a specific area. The number of individuals in a population is not static; it naturally rises and falls over time. This ongoing variation is known as population fluctuation, reflecting the relationship between a species and its environment. These changes can be gradual or sudden and are a characteristic of all life on Earth.

Factors Driving Population Changes

The forces causing population changes fall into two categories. The first is density-dependent factors, which have a greater influence as a population becomes more crowded. As density increases, individuals must compete for finite resources like food, water, and nesting sites. This competition can lower reproductive rates and survivability, slowing population growth.

A denser population also facilitates the spread of diseases and parasites, as pathogens move more easily between hosts in crowded conditions. Similarly, a high concentration of prey can attract more predators, increasing the death rate. The accumulation of waste products in a dense area can also degrade the environment, impacting health and reproduction.

The second category is density-independent factors, which affect populations regardless of their size. These factors are often physical or chemical, arising from sudden environmental events. A wildfire, flood, or volcanic eruption can eliminate individuals indiscriminately. Abrupt weather changes, like a hard freeze or prolonged drought, can also cause significant mortality.

Patterns of Fluctuation

Environmental pressures result in several patterns of population change. Some populations experience cyclic fluctuations: predictable, regular increases and decreases in numbers over time. These cycles are often driven by interactions between species, such as a predator and its prey. The classic example is the 10-year cycle of the snowshoe hare and Canada lynx. As hare populations grow, they provide more food for the lynx, whose numbers rise, which in turn causes the hare population to crash, leading to a decline in lynx from starvation.

Another pattern is irruptive fluctuation, a sudden increase in population size followed by a steep decline. These events are not as predictable as cycles. Locust swarms are a well-known example; under certain environmental conditions, their numbers can skyrocket and decimate vegetation before the population plummets. Algal blooms in aquatic environments follow a similar pattern.

Many populations exhibit irregular fluctuations with no predictable pattern. These changes result from a complex combination of both density-dependent and density-independent factors. A population limited by resource competition for several years might be suddenly impacted by a severe storm, making its trajectory unpredictable.

The Role of Carrying Capacity

Underlying these fluctuations is the principle of carrying capacity, the maximum population size an environment can sustainably support. Carrying capacity is not a fixed number and is determined by the availability of resources like food, water, and space.

As a population approaches its environment’s carrying capacity, the density-dependent factors mentioned earlier become more pronounced. These pressures cause the population’s growth rate to slow. If the population overshoots the carrying capacity, the death rate will exceed the birth rate, causing the population to decline until it falls back below that limit.

Human Influence on Natural Cycles

Human activities significantly alter population fluctuations. The destruction and fragmentation of habitats reduce the area and resources available to a species. This lowers the environment’s carrying capacity, supporting smaller populations that are more vulnerable to random events.

Climate change is another disruptive factor. Shifting temperature and precipitation patterns can alter the timing of seasons, affecting food availability. This can create a mismatch between a species’ reproductive cycles and its food supply. For some insects, warmer conditions may allow them to expand their range or produce more generations per year, leading to outbreaks.

The introduction of invasive species by humans can cause sharp declines in native populations. Non-native predators, competitors, or pathogens can impact species that have not evolved defenses against them. These new arrivals outcompete native organisms for resources, disrupting population cycles and leading to shifts in the ecological community.