What affects species distribution?
Species are often not spread evenly through an ecosystem. Their distribution describes where a species is found and their abundance describes how many individuals occupy that space. These patterns form in response to the availability of resources and environmental conditions.
Limiting Factors
• A limiting factor is an abiotic or biotic factor that restricts the number of individuals in a population.
• Density-dependent factors influence population size more as density increases. Effect: Higher population → stronger impact on growth and survival.
• Density-independent factors affect population regardless of density. Effect: Can cause sudden population declines irrespective of population size.
Limiting Factors: Biotic
Food Availability
•Food availability directly influences survival and reproduction.
•Areas with abundant food support higher numbers because individuals have the energy to grow, reproduce and avoid starvation.
•When food becomes scarce populations shrink or move elsewhere.
•For instance, herbivore populations tend to concentrate in regions where fresh grass is plentiful after seasonal rainfall.
Competition
•Competition arises when organisms seek the same resource.
•Intraspecific competition occurs within a species and can limit population growth when too many individuals need the same food or territory.
•Interspecific competition happens between species and can push one species out of an area.
Predation
•Predators reduce prey numbers through feeding but also by influencing where prey choose to live. Prey often avoid areas with many predators even if resources are present. Predation therefore shapes distribution as well as abundance. An example is the effect of dingoes, which can keep kangaroo numbers lower or force them into areas with fewer predators.
•The predator-prey relationship is a balance between the two populations.
•As the prey population increases, the predator population increases.
•As the prey population decreases, then so does the predator population.
Disease
•Disease can reduce population size by increasing mortality or weakening individuals.
•Disease spread is often density dependent, affecting large populations more severely than small ones.
•It can also limit a species’ distribution. For example, the fungal disease chytridiomycosis has caused severe declines in frogs, restricting populations to cooler upland streams where the fungus grows less effectively.
Limiting Factors: Abiotic
Water and Nutrients
•Water and nutrients determine plant productivity, which in turn shapes animal populations.
•High nutrient levels lead to increased plant growth, followed by increases in herbivores and the predators that feed on them.
•After significant rainfall wildflowers often bloom across arid regions, supporting insects and the birds that feed on them.
•Conversely, an excess of nutrients can cause algal blooms that deplete oxygen, harm aquatic life, and degrade water quality. This process, known as eutrophication, leads to low-oxygen “dead zones” that can kill fish and other organisms, while some blooms also release toxins that are harmful to organisms.
Space and Shelter
•Space affects access to nesting sites, feeding grounds and safe areas.
•Shelter protects organisms from predators and extreme conditions.
•Populations are often limited by the number of safe places to breed.
•For example, possums are limited by the availability of hollow trees, which restricts how many breeding pairs a habitat can support.
Environmental Conditions
•Temperature, pH, salinity and light intensity determine where species can survive.
•Each species has a tolerance range. If conditions move outside that range individuals may die or migrate.
•For example, corals bleach when temperatures exceed their tolerance.
•This reduces coral cover and changes the distribution of species that depend on the reefs for food and shelter.
•Conversely, heat-tolerant organisms can expand their range.
Extreme Climatic Events
•Natural disasters act as major disturbances. They can remove vegetation, alter soil structure and change access to food and shelter.
•These events often cause immediate drops in abundance because individuals are killed or displaced.
•Distribution patterns may also shift as surviving populations move to safer or more suitable areas.
