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Accounting for long-term change in ecosystem management
It’s no surprise to many wildlife managers that invasive species might affect the stability of an ecosystem. For example, invasive species could potentially edge out native ones.
An easy response would be to increase harvest of the invasives while leaving the native species alone to protect the balance of the ecosystem. Or managers might supplement the native species through translocation and reintroduction to bolster their numbers.
But the problem with both of these strategies is they take a short-term view on the ecosystem in question, which fails to account for the fact that systems like these are often in a state of flux rather than finished products.
“A system can look pretty stable over time, but internal dynamics in the ecosystem like competition among species and natural predator-prey dynamics can lead to sudden system shifts,” said Tessa Francis, the lead ecosystem ecologist at the Puget Sound Institute at the University of Washington Takoma.
When dealing with problems like these, most researchers often focus on outside stressors like climate change impacting the ecosystem.
But it isn’t always external forces that cause these problems, she said.
According to new research led by Francis, published recently in Nature Ecology and Evolution, mathematical models that look at ecosystems in a constant flux might assist managers in deciding the best course of action to avoid problems like extinction or declining water quality in lakes.
Francis and her colleagues used models to illustrate ecosystems not as final products, but as constantly changing systems. “The idea that ecosystems are not always stable has been around for a long time,” she said. “What we observe now may not be what we will see in the future.”
The authors use models to examine how features of ecosystem dynamics, like species competition, or the interaction between fisheries management and habitat in lakes, impact the stability of an ecosystem. In situations where the system is in a long transient and not a stable state, the models explore how alternative management strategies affect outcomes and ecosystem objectives.
When applied to a hypothetical invasive species scenario, for example, a model shows that supplementing native species or exclusively harvesting invasives might not fix the problem in the long-term, since it’s not possible to completely remove the invasives. Assuming the ecosystem is in a state of flux might help managers better tinker with the system to give the natives a chance. This example may have implications for the current invasion of European green crabs (Carcinus maenas) along the U.S. West Coast, as the nonnative crabs compete with native crab species.
“In this situation, the success of your management actions depends on whether the system is in a long transient,” Francis said, or if the system is approaching a stable state, for example where the native species is at carrying capacity and the invasive species is effectively undetectable.
In this case, accounting for a long transient reveals that harvesting both the native and the nonnative species would be the best strategy to eventually reach a balance where the native species can thrive. It would also help managers save valuable time and money working on impossible situations. “Basing your management intervention on misidentified system dynamics could cause you to invest more money and resources to control invasive species moving forward,” she said.
Another example modeled by the group focused on lakes and pollution. In some lakes, there is a stable state where a lake becomes eutrophic due to excessive phosphorous content. A lake that remains green with a thick cover of algal blooms for a long time doesn’t provide much habitat for species, and isn’t aesthetically pleasing for visitors. “Eutrophication continues to be one of the biggest threats to lakes worldwide,” Francis said.
One management strategy for combating eutrophication is to reduce phosphorous runoff from adjacent watersheds, for example by decreasing fertilizers used in agricultural systems. But because many farmers rely on fertilizer for food production, it may be more feasible to make smaller adjustments to fertilizer limits. “The aim may be to target a pollutant level at which a lake is less likely to become eutrophic,” she said.
However, a lake in a state of long-term change can maintain high phosphorus levels even while fertilizer runoff drops, due to internal recycling of phosphorus from sediments. In such a case, small adjustments to fertilizer use may not do much and farmers might not see a return for their sacrifice. In that case, investing in social and political capital to create bigger changes or finding novel alternative interventions would be a better option, she said.
Francis cautions that these theoretical models are not intended for use in setting specific management policies. Instead, she said, the value of the mathematical models is in revealing potentially unintended consequences of management actions in the absence of full knowledge about ecosystem dynamics, “which is usually the case,” she said. The models can be used to identify the parameters associated with important ecological mechanisms and dynamics in terms of management and social objectives, like species conservation, and then evaluate possible futures under ranges of those parameters and alternative management approaches. This is something the fisheries field is already doing. “In wildlife circles, this could be a powerful message,” she said.
Francis also said the paper can cause people to conceptually shift how they think about ecosystems. Managers should ask what they should be monitoring in an ecosystem to determine whether it’s going through long-term change.