Dispersal and species coexistence – updated

Collaborators: Jill LancasterCR7 9 (Project leader) & Barbara Downes

Funding: Australian Research Council (Discovery) Grant: 2012-2015

A central question in ecology is to understand what drives species coexistence in landscapes with patchy habitat. Key hypotheses predict that species’ dispersal abilities are either “traded off” against other ecological traits in ways allowing species coexistence or are important in allowing species to “track” environmental change. Field experiments to test these hypotheses are lacking due to logistic barriers. SettingUpSITE8BungleBooriWe are field-testing these hypotheses using species living in streams flowing through agricultural landscapes in central Victoria  that have been filled with sand from erosion, using novel experimental designs that allow tests at landscape scales. The channel widens considerably downstream and the sand swallows much of the resources of leaves and bark that drop into these streams.  TrapSITE6 2We manipulated the amount of leaves and bark by adding simple traps to experimental sites, while leaving others to act as controls. We are testing whether the additions of resources changes the density and diversity of species present and whether species found only in intact, upstream sections will disperse downstream when resources are available to colonise.

 Update – 2016

Our experiment has been an unqualified success in demonstrating that firstly detrital resources are in short supply and secondly that animals in these rivers disperse frequently and will colonise sites where resources have been increased.

Control site on Hughes Ck

Control site on Hughes Ck

Our control sites continue to have broad expanses of sandy bed with little detritus, even during summers when most bark and leaves drop into streams. Our samples show that they have low densities and diversities of invertebrates.

Experimental site on Hughes Ck

Experimental site on Hughes Ck

In contrast, experimental sites have over 10x the amount of bark and leaves present. They have also become complex hydrologically because the traps have created “islands” of sand and the river has become much more braided, especially during the summer when water levels are low.

Detritus trap with plant growing out of it

Detritus trap with grass growing out of it

Additionally many macrophytes and grasses have been caught by the traps and are now growing out of them.

Experimental sites have in some cases twice as many species and over twice the densities of invertebrates as seen at controls.  Samples taken after two years show that the increases in species diversity and densities has been maintained. Our data on animals in the drift (using the water current to disperse downstream) and on the terrestrially based adults of some insects trapped along the stream side show that many of the species now present at experimental sites have almost certainly travelled from upstream locations.

What this experiment shows is that firstly dispersal is key to understanding species diversity  and community structure in this stream. Although downstream sites have harsh environments (water temperatures routinely exceed 30 °C in summers), species living in cooler upstream locales are dispersing to downstream locations and are capable of living there if adequate resources are supplied. These outcomes back findings from a new method we devised to assess the role that dispersal plays in structuring communities, which had suggested also that animals are routinely dispersing along Hughes Ck (Downes et al. 2017).

Second, our experiment shows how we can restore the biological diversity of streams, like Hughes Creek, that have been inundated with sand and lost most of the physical structure (like tree branches) that retains detritus. Putting in simple traps improves retentive capacity, meaning that a lot of detritus is retained on the stream bed rather than being swept downstream or buried. As a result, resources of living space, food and oviposition habitat are increased markedly, all allowing animals dispersing along this creek to survive. Our experiment shows that the Field of Dreams hypothesis for ecological restoration (which outlines the dependency of restoration projects on dispersal) is supported for this stream.

The first paper describing this experiment has been published (Lancaster & Downes 2017) and there will be more to come.

What’s next?

This work is being expanded through the award of an ARC Linkage grant (see Current Projects), in which we will test whether improving retentive capacity will increase the diversity of stream fauna in an array of other creeks.