Increased frequency of catastrophic events may stimulate collapse in marine creatures, says study

Researchers have discovered that increased frequency and severity of catastrophic events could hinder marine population's ability to recover and, in turn, may stimulate collapse.

Mass mortality events can aggravate the risk of extinction for species that are prone to Allee effects (correlation between population size or density and the mean individual fitness (often measured as per capita population growth rate) of a population or species), particularly species harvested commercially. When species experience Allee effects, they exhibit diminished reproductive success at decreased population density. Managed fisheries frequently keep populations at low densities.

The research published in The American Naturalist focused on one marine species impacted by Allee effects-the green abalone or Haliotis fulgens (species of large sea snail) near Baja California Sur, Mexico.

Using the Isla Natividad fishery as an example, the authors examined whether spatial management strategies (primarily reserves) are more effective than non-spatial management strategies in alleviating the damage caused by catastrophic events.

In particular, the authors sought to ascertain whether designating marine protected areas (MPAs) in fisheries will help combat collapse when a population facing catastrophe is also susceptible to Allee effects.

While recent hypoxic (a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level) events led to significant abalone decreases at Isla Natividad, the "no-take reserves" in the region sustained a higher density of abalones and produced higher recruitment levels after the disruption in comparison to the areas where fishing occurs.

The authors proposed that the implementation of marine protected areas (MPAs) that incorporate "no-take reserves" offers a spatial management strategy that ensures available resources to restore depleted populations and encourage recruitment in low-density areas after a disaster.

When a fishery is devastated by a mass mortality event, there are also non-spatial strategies that can be employed. A fishery, in response, could temporarily close to give the population a recovery period. The authors referred to this strategy as a "dynamic post-catastrophe fishery closure," or the DC strategy. Another option is for the fishery to forgo closing down entirely. The authors referred to this strategy as the "no-closure (NC) management strategy."

Utilising a spatially explicit integral projection model (IPM), the authors ran simulations to determine how each of these three management strategies would influence recovery after a mass mortality event. The simulations measured the number of instances where a catch was able to return to a level above the threshold of collapse.

The authors also conducted sensitivity analyses to determine how other factors, such as dispersal distance, MPA size, and catastrophe severity, impacted population recovery.

Results indicate that the implementation of MPAs significantly aids in preventing population collapse.

"Our model predicts that a network of protected areas that reduce or possibly eliminate anthropogenic disturbances can minimise the risk of population collapse caused by large-scale extreme climatic events for species whose dynamics at low density are characterised by an Allee effect. Networks of protected areas can effectively increase resilience if their size and spatial layout are able to maintain a breeding population sufficient to rebuild the reproductive potential despite the presence of Allee effects," the authors wrote.