When I went into a local pet store the other day to buy some flea medicine for my cat, I had the chance to walk by their tropical fish section. There was a chart on the wall that laid out which fish you could keep together. Some fish will eat smaller fish; some will relentlessly attack and harass others, so you can’t keep them together. This was familiar territory to me as a former pet store sales clerk. In fact, I learned a lot of the basic ecological ideas that I’m interested in now from fish keeping.
Fish keepers are aware that their pets require specific conditions. For instance, alkaline water is a necessity for cichlids from the African rift lakes. Some fish require hiding spaces. Some will only eat live food. Some can live off the algae on the side of the tank. Some live in warm water, some in cold, some in fresh, some in salt, some in brackish, and some will live just about anywhere. This set of requirements is what an ecologist calls a niche.
The niche concept is one of the fundamental ecological concepts and has been revised twice by important ecologists over the years. Originally, it was taken to mean the set of environmental conditions associated with the animal’s range, but Charles Elton revised it to mean the organism’s place in the community. “Algae eater,” a common name for any fish used to keep the tank free of algae, like “plecostomus,” is an example of this use of the word.
Another use of the niche concept describes the conditions required by the organism to survive, inverting the original concept by placing the requirements inside the organism, not in the environment.
I have seen geographically accurate tanks, particularly during my years as a volunteer at the Steinhart Aquarium in San Francisco, but the fish don’t care if they are grouped geographically. African fish can live with Asian fish or with Mesoamerican fish. They are fine as long as their niche requirements are met.
Since fish can thrive wherever their niche requirements are satisfied, they can be introduced in waters far from their native ranges. Species from other parts of the world, like bass and shad, have been introduced to the San Joaquin River, for instance. On the other hand, waters as far away as South America have proved satisfactory to one of our native fish, the Chinook salmon, the focus of a local restoration effort.
The niche is a fundamental idea in ecology, but it’s also fundamentally difficult. It’s problematic to determine the number of factors that go into a species’ niche; in fact the list could be extended indefinitely. This makes it difficult to measure a niche so we can test our ideas about it.
We believe that no two species can coexist if they have the exact same niche, for example, but if it seems like a pair of species violate this assumption we could always appeal to another unmeasured aspect of the species’ niches.
This ambiguity makes the niche an easy concept to learn but a hard one to master.