What factors can limit the size of a species population?

In ecology, factors that limit the size of a species population are broadly categorized into two types: density-dependent and density-independent factors.12

Density-Dependent Factors

These forces, often biological in nature, influence population size in response to the number of individuals per unit area. As population density increases, these factors intensify, helping to regulate the population within a specific range. Key examples include:31
* Competition: As individuals become more crowded, competition for essential resources—such as food, water, nesting sites, and sunlight—increases, which can limit growth.34
* Disease: Pathogens often spread more rapidly in dense populations, increasing mortality rates.13
* Predation: Predators may exert more pressure on a population as it grows, or conversely, their presence can limit the numbers of a highly successful competitor, thereby influencing community diversity.54

Density-Independent Factors

These factors affect population size regardless of how many individuals are present in a given area. They typically arise from physical or chemical phenomena rather than biological interactions. Examples include:613
* Weather and Climate: Extreme temperatures or shifts in climate can impact survival rates.61
* Natural Disasters: Events such as wildfires, flooding, and landslides can cause sudden population declines regardless of density.6
* Environmental Stress: Factors like pollution or the sudden unavailability of critical resources (e.g., oxygen) can limit a population.62

Ultimately, the growth of most populations is governed by a combination of these factors, which together determine the environment's carrying capacity—the upper limit of a population that a specific environment can sustain.142