Disturbance and resilience

A wide range of natural phenomena, such as cyclones and other severe storms, hot weather, disease outbreaks, sediment movements, can remove resident plants and animals, creating “gaps” or patches.  These gaps provide opportunities for colonists to arrive, replenishing populations affected by disturbance, but they also allow opportunities for other species to arrive.  Interest in disturbance rose sharply among marine ecologists in the 1970’s, but despite initial progress, there exists no coherent theory of disturbance and its effects, notwithstanding its crucial role in maintaining populations.  Human activities also act as disturbances – the seabed is disturbed by fishing gear, nutrients and pollutants are released into the water, and new species are introduced and become pests. Human activities may be novel, with no natural counterpart, or they may represent changes in rates or intensities of existing natural disturbances.  Managing coastal environments sustainably depends on understanding how these human activities affect coastal ecosystems.

A primary aim of our research has been to understand the direct effects of human disturbances, particularly those that recur, and to compare them with natural disturbances.  In the intertidal zone of rocky shores, we have examined the effects of human recreational activities, particularly pedestrian traffic and the removal of animals from the shore for food or bait.  In the subtidal zone, we have examined the effects of pollutants, particularly heavy metals and nutrients, but also those of invasive species.  Many of these activities recur, either regularly, such as the influx of visitors to rocky shores during summer, or irregularly, such as pollutants that follow heavy rainfall events, and an understanding of disturbance must incorporate these complexities.

When dealing with human activities, there is a tendency to treat particular activities in isolation, especially when different agencies are responsible for management of threats.  The potential for synergistic effects has major implications for how we manage natural environments, and recent work has focused on testing for such synergies.

Our current research program on disturbance is aimed at developing a deeper understanding of resilience in temperate marine environments, with a focus on the factors that influence the capacity of a system to return to its original state or function following a disturbance, rather than flipping to alternative community states.

Outcomes

We have shown that recurrent disturbances vary strongly in the kinds of effects that they have on flora and fauna.  Some of the most significant findings are:

  • In the case of disturbance from pedestrian traffic, recurrent events may act as a series of independent disturbance and recovery cycles at one location, while at nearby locations, several disturbances may act synergistically to produce major change.  These results show new complexity in the responses to disturbance and represent one of the most rigorous, experimental studies of disturbance.  They have stimulated similar studies in other parts of the world, as well as providing valuable guidance to Australian natural resource managers
  • Our studies of the effects of pollutants have also produced novel results, which in this case have important implications for how we predict the effects of toxic chemicals.  The present, internationally standard approach to predicting effects relies on laboratory testing of a suite of test organisms under a set of standardized conditions, followed by extrapolation the natural environment.  We have shown that this approach is flawed, by developing a field experimental system that allows controlled release of pollutants and accurate measurements of actual exposure to animals.  We have used this system to show unexpected effects of pollutants.  In particular, brief pulses of toxic metals that occur at intervals of 4-8 weeks produce severe impacts on key species that are not predicted by current laboratory approaches.  In another novel finding, the removal of some species by pollutants frees others from competition and allows them to increase in abundance, with the result that they appear to be stimulated by pollutants.  This work will lead to field-based tools that can be used to predict effects of pollutants more accurately.
  • A third important outcome of this research is its highlighting of indirect effects, which occur when a species that is affected directly by an environmental process in turn affects other species. These effects are notoriously hard to detect or demonstrate.  We have shown a range of such indirect effects such as the shift in dominant species on rocky shores following the removal of habitat-forming seaweeds by intense trampling, the increase in weakly competitive species after pulses of pollutants.  For disturbances caused by harvesting of animals from the shore, a surprising lack of indirect effects, in sharp contrast to the dramatic changes that have been reported on rocky shores elsewhere in the world.