Answer:
The introduction of a keystone species can potentially be a threshold in an aquatic environment because keystone species play a crucial role in shaping the structure and function of their ecosystems. These species have a disproportionate impact on their environment, and their presence or absence can have a significant effect on the overall health and biodiversity of the ecosystem.
For example, in an aquatic environment, the introduction of a keystone predator, such as a large predatory fish, can have a cascading effect on the entire food web. This predator can help regulate the populations of its prey species, thereby preventing any one species from becoming too dominant and outcompeting others. By controlling the populations of its prey species, the keystone predator can help maintain a diverse and balanced ecosystem, which can be more resilient to environmental changes and less susceptible to invasions by non-native species.
However, if the keystone species is introduced to an ecosystem where it does not belong, it can have a very different effect. In this case, the keystone species may not have the same level of control over its prey species, and its presence could lead to an imbalance in the ecosystem. This imbalance could make the ecosystem more vulnerable to invasions by non-native species and less resilient to environmental changes.
In summary, the introduction of a keystone species can potentially be a threshold in an aquatic environment because keystone species play a crucial role in shaping the structure and function of their ecosystems. Their presence or absence can have a significant effect on the overall health and biodiversity of the ecosystem, and their introduction to an ecosystem where they do not belong could lead to an imbalance in the ecosystem, making it more vulnerable to invasions by non-native species and less resilient to environmental changes.