Juan Miguel Artigas Azas,
Cichlid Room Companion

The exotics effect

By , 2012. image
Last updated on 30-May-2012

A recent paper dealing with the effect of introduced species in a given habitat called my attention. The paper in question (Lehtonen, McCrary & Meyer, 2012) reports on the observations of the behavior of some native species after a predatory exotic has been introduced into their habitat. In normal conditions, the number of native species is balanced and their responses toward sympatric predatory species are predictable. Predators each have a specialized hunting technique, and as soon as it is placed into action, a protective response is triggered in potential prey which can dissuade its effectiveness. The environment is then balanced.

The researchers found that in Lake Apoyo, Nicaragua, the Eleotrid Gobiomorus dormitor (big-mouth sleepers), natively present in other crater lakes like Xiloa but absent in Apoyo, was introduced. They then noticed that breeding pairs of the native and critically endangered cichlid Amphilophus zaliosus, when protecting their fry, would allow the exotic to approach at an unusual distance. This gave the big-mouth sleeper goby an advantageous position to prey on their fry, something they would not allow with other native predators. In fact, in the ecologically similar Lake Xiloa, the similar cichlid species A. sagittae, which has co-evolved with the predator, would not allow them that close to their fry.

These results suggest that exotic species have a tremendous advantage over natives, which is in agreement with the common observation that when initially introduced in a habitat, exotic species have tremendous reproductive success and a subsequent population boom, and it is only with time that their numbers reduce. At that point, they likely caused large damage to the environment, as their biomass of course displaced that of other native organisms.

Already Charles Darwin in the “Voyage of the Beagle” reports that in 1832 he observed the relationship between plants and insects of the same families in distant countries, though with different species, and how that relationship is broken when the man is the agent in introducing a new species into a country. Near Rio De Janeiro, Brazil, he witnessed that “the leaves of the cabbages and lettuces (recently introduced), which in England (introduced a long time ago) afford food to such a multitude of slugs and caterpillars, in the gardens near Rio are untouched“. On several occasions, Darwin notes the danger posed by exotic introductions to native fauna and flora, as he witnessed.

Over the years since Darwin we have countless examples of the previous, like the disastrous effects on native fauna of introductions by early European settlers into Australia. We also know of the adverse effect to the environment of officially supported introductions, like the Nile Perch in the native cichlid fauna of Lake Victoria, as a popular example. I won’t go any further with this. It is clear the damage that a balanced environment suffers with the introduction of exotic species. With time the environment may balance again (too much time for humans to witness perhaps), but in the meantime, we are deprived of the magnificence and beauty and of all the knowledge we can extract from balanced environments. Nowadays, introductions are likely one by one the leading global threats to biodiversity.

It is for this reason that I believe that aquarium specialized clubs should pose significantly more effort to educate young and/or new aquarists about the great dangers to native creatures that the introduction of exotics bring. The value of keeping long-term populations of natural fish would also bring more appreciation for them, much more so in my opinion than the rapid-breed-and-get-rid current programs, as I have mentioned in past editorials.

We as aquarists must adopt a significantly more responsible position on this matter than we currently have. We owe this to nature!! We should not turn our faces in another direction on this, a new philosophy of giving at least as much as you get, and missing no opportunity to stress the values of conservation and respect for the environment should be prevalent. Long-term commitment and participation or support of conservation programs and dissemination of this knowledge should be in the mission statements of every responsible aquarium club, backed up by effective action, with sufficient resources devoted to this end significantly increased. Aquarium clubs should be more than sources of cheap fish and social gathering places. It is in the interest of all of us, and of nature.

We don’t have much time left!

See you next month!

References (1):


Artigas Azas, Juan Miguel. (May 30, 2012). "The exotics effect". Cichlid Room Companion. Retrieved on Sep 23, 2023, from: