The final entry in this series on ecological principals as they relate to aquariums focuses on choosing livestock to help promote a healthy environment and the idea that behavior of individuals within a community can change with increases or decreases in population sizes within that community.
How does the livestock you choose effect the ecosystem
Focusing on a specific biotope can sometimes be easier for hobbyists because the interactions between all the organisms that live in that habitat will have been studied to some extent and information can be found in scientific literature. You may also have better luck asking questions to the community of other keepers of your target biotope. However you choose to develop your aquarium, it is important to consider how the organisms will interact over time. The behavior of fishes can change as they age in ways that may harm your aquarium. For example, aggressive species will often become more aggressive as they get larger and many fish change their dietary preferences with maturity.
How does behavior change with an increase or decrease in population size.
To complicate matters more, many organisms interact differently at different population sizes. Aquarists have used this knowledge to there advantage with tumbling bio reactors. The biological filter can process up to 10 times the amount of ammonia and nitrite when new bacteria is growing. By tumbling sand or bio media, the physical shearing of the media sheds old growth bacteria and frees up space for new bacteria growth. Behavioral changes with population density are actually very common in aquatic environments. Ask any African cichlid keeper how to reduce aggression and they will tell you to add more cichlids. Keepers of more diverse aquariums like reefs or planted freshwater find that many organisms may be beneficial at relatively low numbers but become a detriment at higher population sizes.
There is an interesting case study that I have since lost the citation for. From my memory, researchers were attempting to restore lobsters to a habitat from which they were extirpated, or locally extinct. What they knew of the environment was that algae were eaten by nerite snails, whelks ate the nerite snails, and the lobsters ate the whelks. Restoring the lobsters to the habitat was supposed to control the whelks which would allow the nerites to recover and control the algae. The researchers introduced 1,000 lobsters. The next day, they found that all of the lobsters had been eaten by the whelks, the snails which had previously been their food source. However, the high population size of the whelks allowed them to overwhelm the lobsters- all 1,000 of them!
There are many animals and plants that are considered pests or nuisances in aquariums, that have specific and important roles for the ecosystem. What makes them a pest is sometimes their population size. As such aquarist should adopt a more nuanced view of these pests. For example: bristle worms and asterina starfish in reef tanks. They are beneficial at lower population sizes for their roles in recycling nutrients. It is only when conditions allow there overabundance that they start exhibiting detrimental behavior. As such, their increased population size can be a metric for measuring an imbalance in your aquarium that you cannot otherwise test for.
In short, animal behavior is fluid and can be affected by many factors. Carefully consider how you will maintain harmony in your aquarium over time and how you can use some unlikely or even unintentional organisms to your advantage by monitoring their growth.