What Is Character Displacement in Ecology?
In the natural world, when two similar species encounter each other, they often undergo a remarkable evolutionary transformation called character displacement. This process involves two similar species, typically facing competition for the same limited resources, evolving to become more distinct in their traits. The changes allow them to minimize overlap and reduce the intensity of their competition, ultimately enabling them to coexist in the same geographic area.
The Competitive Spark
The primary force driving character displacement is interspecific competition, which occurs when individuals of different species vie for the same limited resources. These resources can include food, habitat, or even mates. When two species have similar ecological roles, or “niches,” there is an overlap in their resource needs. This overlap creates a selective pressure, as individuals that are more effective at acquiring resources without directly competing with the other species tend to survive and reproduce more successfully. The intensity of this competition is particularly high when resources are scarce or when population densities are high. Under these conditions, natural selection favors individuals within each species that exhibit traits allowing them to utilize different aspects of the environment or different parts of the resource spectrum. This evolutionary response helps reduce the direct competition that would otherwise lead to one species outcompeting and potentially excluding the other.
Evolving Differences
Character displacement leads to observable changes in species traits. These changes are evolutionary shifts that occur over many generations, becoming genetically embedded within the population, not merely behavioral adjustments within an individual’s lifetime. The diverging traits can be morphological, affecting physical structures such as body size or the shape of feeding apparatuses, like a bird’s beak or a mammal’s teeth specializing in different food items. Beyond physical form, character displacement can also lead to behavioral changes, including shifts in foraging times or alterations in mating calls to avoid inadvertently attracting individuals from the competing species. Physiological adaptations can also occur, where species evolve different metabolic rates or digestive capabilities to process distinct types of food or tolerate varying environmental conditions. These diverse adaptations collectively reduce the overlap in resource use, enabling species to carve out distinct ecological roles.
Documented Cases
Character displacement has been observed in various natural systems, providing compelling evidence for this evolutionary process.
Darwin’s Finches
A classic example involves Darwin’s finches on the Galápagos Islands. The medium ground finch (Geospiza fortis) and the large ground finch (G. magnirostris) show differences in beak size that are more pronounced when they live together on the same island compared to when they live separately. On Daphne Major, after G. magnirostris arrived, G. fortis evolved smaller beaks, allowing them to specialize in smaller seeds and reduce competition with the larger-beaked species that consumed larger, tougher seeds.
Anolis Lizards
Another well-studied case involves Anolis lizards in the Caribbean. When the brown anole (Anolis sagrei) invades islands inhabited by the green anole (Anolis carolinensis), the resident green anoles evolve to perch higher in trees and develop larger toe pads. This shift in perching height and associated morphological change allows Anolis carolinensis to avoid direct competition with the brown anole, which typically occupies lower perches.
Threespine Stickleback Fish
Freshwater threespine stickleback fish also provide a notable example. In some lakes, two species of sticklebacks, a “limnetic” form and a “benthic” form, coexist. The limnetic sticklebacks have many long gill rakers, adapted for feeding on zooplankton in open water, while the benthic sticklebacks have fewer, shorter gill rakers, suited for consuming invertebrates from the lake bottom. These distinct gill raker morphologies, which are genetically based, are driven by competition for different food sources within the same lake environment.
Broader Implications
Character displacement has broad implications for understanding the patterns of life on Earth. It contributes to biodiversity by enabling more species to coexist within a given habitat. By promoting niche differentiation, this process allows for a more efficient use of available resources within an ecosystem. Furthermore, character displacement can play a role in the formation of new species, a process known as speciation. The evolutionary divergence in traits between competing populations can eventually lead to reproductive isolation, where individuals from these populations no longer interbreed. This demonstrates how competition can act as a powerful selective force, shaping the diversity of life and influencing the structure of ecological communities over evolutionary timescales.