The ecological niche of a species describes its specific role within an ecosystem, encompassing all the biotic and abiotic factors necessary for its survival and reproduction. Ecologists distinguish between the fundamental niche and the realized niche. The fundamental niche represents the full range of environmental conditions, such as temperature, humidity, and resource availability, in which a species could thrive in the absence of negative biological interactions. This potential space is determined primarily by the species’ physiological tolerance limits.
The realized niche, in contrast, is the actual, often smaller, set of habitats and conditions a species occupies in a real-world setting. This reduction occurs because species are constantly constrained by interactions with other organisms in the community. The realized niche is thus a subset of the fundamental niche, carved out by limiting biological factors such as competition, predation, and disease.
Constraints Imposed by Interspecific Competition
The presence of other species vying for the same limited resources is the most classically studied factor that shrinks a species’ fundamental niche. Interspecific competition forces an organism to retreat from areas where the environment is physiologically suitable but resources are heavily contested. The competitive exclusion principle states that two species cannot indefinitely occupy the exact same niche if resources are scarce. The resulting realized niche is the habitat or resource space that remains after superior competitors have claimed their portions.
A classic example involves the zonation of barnacles on intertidal rock faces. The small barnacle Chthamalus is physiologically capable of surviving across a broad range of the intertidal zone, defining its fundamental niche. However, in the lower, moister portions of its range, it is outcompeted for space by the larger, faster-growing barnacle Semibalanus. This interference competition restricts the realized niche of Chthamalus to the upper intertidal zone, where the superior competitor cannot survive the increased desiccation stress.
Competition can also be indirect, known as exploitative competition, where organisms consume shared resources faster than their rivals. For example, the protozoan Paramecium aurelia was shown to outcompete P. caudatum by utilizing the limited food source more efficiently. Competitive pressure often leads to resource partitioning, where species are forced to use different parts of a shared resource to coexist, such as feeding at different times or locations. This differentiation minimizes niche overlap, allowing multiple species to persist, but the realized niche of each remains narrower than its fundamental potential.
Limits Due to Predation and Herbivory
The threat of being consumed often prevents a species from occupying parts of its fundamental niche, even when all physical conditions are ideal. This top-down pressure from predators and herbivores can restrict a species’ realized distribution or behavioral patterns. An area with perfect abiotic conditions may become a lethal sink if predator density is too high, forcing the prey species to occupy less desirable locations.
This phenomenon is demonstrated by changes in the foraging behavior of bluegill fish in the presence of predators like the largemouth bass. Without predators, bluegills prefer to forage on benthos in the open water, which is their fundamental niche for feeding. When bass are present, smaller, more vulnerable bluegills abandon this energetically rich habitat and retreat to dense, less productive vegetation beds for refuge. Their realized niche becomes a less-efficient foraging space, minimizing the risk of consumption at the cost of resource quality.
In plant communities, herbivory similarly defines a species’ realized range. The California annual plant Collinsia sparsiflora illustrates how insect herbivores constrain its serpentine ecotypes to a narrow range of suitable habitats. Where soil conditions are marginal, the plant’s defenses may be lower, making it more susceptible to insect attack and preventing persistence. Plants may also gain refuge from large ungulates by growing directly under protective shrubs, limiting their physical distribution to areas of least consumption risk.
The Role of Pathogens and Disease
Non-consumptive antagonistic interactions, such as those involving pathogens and parasites, also place significant restrictions on a species’ distribution. An environment may satisfy all a host species’ resource and temperature requirements, but if that area favors a high prevalence of a debilitating disease, the host cannot successfully occupy it. The realized niche is limited to areas where the species can maintain sufficient fitness and reproductive success to sustain its population.
Abiotic factors, which normally define the fundamental niche, can indirectly limit the realized niche by optimizing conditions for the pathogen. For example, warmer temperatures accelerate the life cycle and transmission rate of vector-borne pathogens, such as those causing malaria or dengue fever. In these warmer regions, the host’s physiological tolerance is met, but the increased risk of severe infection makes the area unsuitable for long-term survival. This pushes the host’s realized niche toward cooler, less disease-prone areas.
The host-pathogen dynamic creates a constraint where the disease’s environmental requirements often dictate the host’s range boundary. In the flax rust system, the host plant (Linum lewisii) is restricted by geographical and environmental features. The flax rust fungus (Melampsora lini) is less constrained, meaning it can survive across the entire potential range of the host plant. The pathogen effectively tracks the host, preventing expansion into parts of the fundamental niche where the disease load is high enough to cause population collapse.