Slower recovery forewarns tipping points in salt marsh ecosystems

Mathematical models have shown that there may be warning signs. An international team of researchers led by the Dutch NIOZ now shows that when the recovery of salt marshes slows down, a tipping point of this ecosystem is imminent. So when seemingly healthy salt marshes have trouble overcoming minor disturbances, this is a bad omen: the important ecological and coastal protection features of these areas may be under serious threat.

Salt marshes vulnerable to sea level rise

Sea level rise increases the pressure on salt marshes, which can result in the loss of important ecosystem services, such as the wave damping effect, storage of CO₂, and resting and breeding grounds for fish and birds. "To some extent, salt marsh plants adapt to and mediate the negative effects of sea level rise themselves.", says Jim van Belzen, the leader of the research. "However, when the sea level changes too quickly, the resilience of the salt marsh can become too low and even a minor disturbance like a storm, can be enough to push the ecosystem beyond the tipping point. The tidal marsh collapses and disappears, leaving a bare mudflat behind. " Van Belzen continues: "Our findings allow us to use the rate at which salt marshes recover from small perturbations as an important indicator to foresee such tipping points. This is a major step in keeping an eye on these important coastal ecosystems and taking precautions if necessary. "

Self-reinforcing feedbacks

Tipping points occur in various complex systems, such as ecosystems, the climate and the financial sector, and tend to arise when a process reinforces itself, to stabilize the system for instance. "Salt marsh vegetation reduces currents and waves, and accumulates sand and clay as a result," says Jim van Belzen. "In this way, a salt marsh plant improves its own condition: it becomes less inundated by salty sea water and therefore it grows better. This, in turn, results in more sediment capture." In other complex systems, such as the climate or society, we see similar self-reinforcing feedbacks that can result in tipping points. "Such processes are increasingly recognized in various complex systems as an important characteristic that improves stability." Van Belzen says, "However, on the downside: when the pressure on a system becomes too large, it can tip abruptly. Such a tipping point results in significant loss of important functions and recovery is often very difficult.”

Indicators of resilience

Yet, these self-reinforcing feedbacks also make it more difficult to see how resilient such a complex system is. The usual indicators for measuring the "health" of ecosystems, such as the total amount or number of plant species, are no longer reliable. However, mathematical models predict that the speed at which a complex system recovers from small perturbations is a good alternative warning signal that indicates when the pressure on a complex system increases and a tipping point is near. Indeed, such signals have been observed under controlled conditions in the laboratory. Yet, the direct observation of this phenomenon in a real ecosystem had not been done before. The researchers observed that the slowing down can be measured both in time series of aerial photographs, and field experiments at different salt marshes in the Netherlands and the United States.

Text: NIOZ Netherlands Institute for Sea Research
Photos: Jim van Belzen, NIOZ Netherlands Institute for Sea Research