A new look at an old problem: Measuring the importance of dispersal in human-impacted systems

In this post Angela Strecker discusses the recent paper ‘A fresh approach reveals how dispersal shapes metacommunity structure in a human-altered landscape‘, by Barbara Downes, Jill Lancaster, Alena Glaister and William Bovill.

One of the largest challenges that ecologists face is understanding the importance of dispersal across patches in ecosystems.  In the process of dispersal, organisms move to different habitats, allowing them to potentially escape poor quality habitats that may have been degraded or where competition for resources is high (amongst other reasons).  This can maximize the survival of species in the system as a whole.  In some systems, dispersal can be measured directly, but in many systems, dispersal must be inferred using statistical approaches.  Understanding the importance of dispersal gives scientists critical information about whether species are responding to environmental changes directly or rather, that there are some limitations on dispersal that prevent species from accessing better habitat.  Ultimately, these factors influence species persistence, thus it is imperative to understand their independent effects.

A recent study by Barbara Downes and colleagues from the University of Melbourne takes a new analytical approach to inferring the importance of dispersal relative to changes in the environment (i.e. habitat degradation).  The authors use survey data from two streams in an agricultural region of southeast Australia that span substantial environmental gradients.  The survey collected data on the abundances of stream invertebrates, including those found in benthic habitats, as well as dispersing invertebrates found in the drift and in the air (i.e. terrestrial adult insects).  The study used combinatorics (i.e. the probability that clusters occur by chance) to determine if environmental factors are relatively more important than dispersal in shaping invertebrate communities by comparing the benthic vs. the actively dispersing invertebrates.

Fig 1
Photo credit: Jason Neuswanger, Troutnut.com.

The study found that their approach was far more sensitive than other traditional analytical approaches for detecting a dispersal signal, suggesting that previous studies have likely underestimated that strength of dispersal.  Additionally, traits were not useful in predicting which species were likely to be more influenced by dispersal or environmental conditions.

Fig 2
Photo credit: Oregon State University

From a management perspective, this study presents many interesting findings.  For instance, the lack of adequate dispersal has been identified as a challenge in achieving river restoration goals of increasing biodiversity1. The methods proposed in this study could greatly improve monitoring and analytical approaches used to assess restoration goals, as well as the placement of restoration projects.  Additionally, many agencies and organizations use stream invertebrates as indicators of environmental health and water quality, yet this study suggests that dispersal may have a larger influence on species’ distributions than the environment in some cases.  Importantly, this research provides a new approach for the ecologists’ toolbox, which may have a substantial influence on management and restoration actions in the years to come.


1  Palmer, M.A., K.L. Hondula, and B.J. Koch. 2014. Ecological restoration of streams and rivers: Shifting strategies and shifting goals. Annual Review of Ecology, Evolution, and Systematics. 45:247-69.

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