Q.In an aquarium two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause's Principle, one of the species is to be eliminated in due course of time, but both are surviving well in the aquarium. Give possible reasons.
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Start your 14-day free trial to unlock the full solution →Both herbivorous fish species can coexist despite Gause's Principle if they occupy slightly different ecological niches—feeding at different depths, times, or on different phytoplankton types—thereby avoiding direct competition for identical resources.
Gause's Competitive Exclusion Principle states that two species competing for exactly the same limiting resource cannot stably coexist; the species with even a slight competitive advantage will eventually eliminate the other. The principle emerged from laboratory experiments with Paramecium species, where complete niche overlap led to one species driving out the other. Yet nature—and your aquarium—often tells a more nuanced story.
The survival of both herbivorous fish species in the aquarium suggests they are not, in fact, occupying identical ecological niches. Though both feed on phytoplanktons, several mechanisms allow them to partition resources and reduce direct competition:
Resource partitioning by feeding behavior: One species might graze primarily on phytoplankton near the water surface, while the other feeds in mid-water or near the substrate. Even subtle differences in preferred feeding depth create distinct micro-habitats within the same aquarium.
Temporal separation: The two species may feed at different times—one predominantly during daylight hours, the other more active at dawn, dusk, or night. This temporal niche differentiation means they exploit the same food source but not simultaneously, reducing competitive pressure.
Differential food preference: Phytoplankton is not a single entity but a diverse community of algae, diatoms, and cyanobacteria. One fish species might preferentially consume larger colonial forms, while the other specializes in smaller unicellular types. This dietary specialization means their realized niches overlap only partially.
Behavioral differences: One species might be more aggressive in feeding, quickly consuming abundant patches, while the other adopts a gleaning strategy, efficiently harvesting scattered or less accessible phytoplankton. Such behavioral complementarity reduces direct interference competition. …
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