But it’s the interactions between these things that should guide my decisions
I cannot change; the courage to change the things I can; and wisdom to know the difference.” It’s a great piece of advice for any conservation manager struggling to deal with multiple threats with inadequate resources. However, in an age when our most important (and much loved) ecosystems are under growing pressure, we’d like to suggest that true wisdom lies not only in knowing the difference between the things we can and can’t change, but also understanding how these things interact. Indeed, understanding the nature of these interactions should be central to the decisions we ultimately make.
Seagrass cans and can’ts
Managers tasked with protecting the health of ecosystems have to deal with multiple threats. Some of those threats have more to do with processes happening in and around the ecosystem they are attempting to protect. Some of them, however, operate at a much bigger scale; climate change, for example, is a threat operating at a global scale.
A good example of this is the plight of seagrass article. Seagrasses are under pressure from nutrients and sediments running off the land, and conversion as bays and harbours are dredged for commercial developments. These are big challenges but they involve stressors that have local causes. To some degree they can be remediated by local action.
But seagrasses will also be impacted by climate change. Increasing water temperature and rising sea levels will both impact on seagrass ecosystems but these stressors cannot be remediated by local action (not to any significant degree, anyway).
So managers at a local scale mostly cannot directly mitigate the effects of climate change but they can improve water quality with the aim of stopping declines in the health of seagrass ecosystems. The questions then become: should we improve water quality in places where the impacts of climate change are likely to be large or small? And, how much do we need to improve water quality to compensate for the impacts of climate change? To answer these questions, our new study shows that a manager must consider how the impacts of global and local stressors can interact to affect ecosystems (Brown et al. 2013).
Stressor on stressor
Stressors to ecosystems can interact additively, antagonistically or synergistically. An additive interaction means that the impact of two stressors in combination is simply the sum of their effects if they were acting in isolation. For instance, if warming increases the rate of seagrass decline by 8% and poor water quality by 4%, then we would expect that together they would increase the rate of decline by 12%. Additive interactions are the typical assumption when ecologists have not been able to measure the type of interaction.
“Management that remediates local stressors in places vulnerable to climate change will have the greatest benefit if there are synergistic interactions between stressors.”
Stressors can also have synergistic impacts to ecosystems, which increase the rate of decline. Returning to our seagrass example, the two stressors in combination may increase the rate of decline to be greater than 12%, say 15%. Synergistic interactions occur when one stressor increases the susceptibility of the ecosystem to additional stressors. Synergisms have been a cause for concern because they imply the typical assumption of an additive interaction will underestimate the true rate of decline in an ecosystem. Consequently, many studies have focussed on identifying synergisms.
This focus on synergisms has come at the expense of documenting antagonisms. Antagonisms imply a slower rate of decline for an ecosystem faced by multiple stressors, so in the past they have been of less concern for ecological studies. Antagonisms can take several forms, although there are two particularly important types: dominance antagonisms and mitigative antagonisms.
Returning to the seagrass example, the two stressors together may only cause seagrass to decline at a rate of 10%. This can occur if the first stressor, poor water quality, removes plants that are sensitive to stress. The first stressor has dominated. Then additional stressors have little additional impact, because only stress tolerant plants remain.
Antagonisms may also be mitigative if the actions of one stressor mitigates the impact of the other. Mitigative antagonisms are likely to be rare, however one example comes from certain coral species. Poor water quality and warming both kill corals, however combined together, poor water quality can reduce the effects of heat stress to corals, by shading corals from light stress.
Decisions around interactions
How important is it that we know the nature of stressor interaction? We examined two case-studies for managing a local stressor when there is an interaction with a global stressor. One case-study was for seagrass and another was for a coral reef fish community. In both case-studies we found consistent results.
Antagonistic interactions between local and global stressors may be more challenging to manage than synergistic or additive interactions. If antagonistic interactions occur then improving a local stressor, such as poor water quality, may have little benefit for an ecosystem impacted by global climate change. In rare cases, improving the local stressor may even worsen the impacts of climate change.
Conversely, while synergisms imply greater rates of decline, they also mean that management that mitigates a local stressor can be of greater benefit to the ecosystem. For instance, improving water quality will reduce its direct effect to seagrass, but will also mean seagrass is more tolerant of global warming impacts (as discussed on pages 14,15).
How can information about interactions inform the management of vulnerable ecosystems?
Deciding how to spend a budget to improve an ecosystem must consider the cost of different actions, but also the effectiveness of those actions. The effectiveness is determined, in part, by the type of interaction. If there are multiple local stressors impacting an ecosystem, management should try to remediate those stressors that interact synergistically with climate change. This will ensure the greatest benefits are obtained from management.
Choices around synergism and antagonism
If the choice is between places, then management that remediates local stressors in places vulnerable to climate change will have the greatest benefit if there are synergistic interactions between stressors. Alternatively, if there is an antagonistic interaction between stressors, management should seek to remediate local stressors in refuges from climate change impacts.
Unfortunately, we know little about what types of interactions occur in nature. Recent review studies indicate however, that antagonisms are likely to be just as common as synergisms. This suggests a cautious approach – management should try to remediate local stressors in refuges from climate change. That way, they are assured of a benefit to the ecosystem, regardless of the interaction type.
Our study also suggests that future ecological studies are needed to identify the types of interactions between stressors that occur in nature. In particular, we need more studies that consider interactions between local, manageable stressors and global unmanageable stressors.
So, sure it’s wise to know the difference between what you can do and what you can’t. However, before making your final decision, make sure you know how all these things interact. It could be that some of the things you can do, may not be sensible choices after all.
More info: Chris Brown firstname.lastname@example.org
Brown CJ, MI Saunders, HP Possingham & AJ Richardson (2013). Managing for Interactions between Local and Global Stressors of Ecosystems. PLoS ONE 8(6): e65765. doi:10.1371/journal. pone.0065765 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0065765