Flow regulation and drought drive riparian plant invasion

Ecologists, like epidemiologists, are often confronted with the challenge of trying to determine causality by piecing together bits of information observed in nature. When the presence or absence of a species at a site is affected by the characteristics of the environment and community, the availability and dispersal success of propagules, stochastic events and the peculiarities of the species itself, it can be very difficult to isolate the likely mechanisms that lead to the occurrence – or lack thereof – of a particular species, especially when the influential factors are highly correlated.

The joys of being in control.  (Source: thecampaignworkshop.com)
The joys of being in control.
(Source: thecampaignworkshop.com)

Experiments are obviously made for getting around such problems; by controlling and isolating one factor at a time, the relative importance of different factors can be quantified. However, experiments are not always possible, desirable or ethical. Take plant invasions along rivers, for example: they occur at large spatial and temporal scales; many factors may drive the invasion process; introducing and augmenting the supply of invasive species is unpalatable and likely prohibited; plus, river environments are very hard to control and manipulate, as any manager will tell you. So, if we are limited to potentially confounded survey data, how can we more effectively identify the drivers of plant invasion so that we know which factors to target in weed management?

In a paper recently published in Diversity and Distributions, my colleagues and I contend that incorporating data about species characteristics into survey-based approaches provides an additional line of evidence that can be used to improve inferences drawn from patterns. We illustrate how using information about environmental gradients, species distributions and species characteristics can increase understanding of ecological phenomena – here, riparian plant invasion, which can help inform management responses.

Using this approach, we find that, of four hypotheses examined, hydrological modification (indicated by flood magnitude) most likely drives invasion in River Murray wetlands. Flow regulation may inhibit native species adapted to the historical hydrological regime, facilitating exotic species with different environmental ranges. A symptom of environmental change, invasion may have been exacerbated by drought, although it is unclear why.

By hitching a ride on walkers' shoes and boots, exotic plants may even invade places like this.  Columbia River Valley, Oregon. (Source: JAC).
By hitching a ride on walkers’ boots, exotic plants may even invade places like this.
Columbia River Valley, Oregon. (Source: JAC).

There was no indication that human-increased propagule pressure or colonisation ability facilitated invasion. Exotic cover was unrelated to proximity to towns, recent flood frequency and cattle grazing intensity. Additionally, similar proportions of exotic and native species were used in cultivation and, despite a higher proportion of exotics being known weeds, weed status was unrelated to exotic species occupancy. Overall, colonisation ability was unrelated to species’ origin or response to water depth and hydrological change. Although exotics had higher specific leaf area and shorter longevity (indicative of higher colonisation ability), they had heavier (not lighter) seeds and did not differ in height from natives.

Based on our findings, we conclude that (i) using environmental flows to reinstate mid-range floods and (ii) augmenting the propagule supply of native species with characteristics suitable for modified conditions may help limit invasion in these wetlands.

For more, have a look here or drop me a line and I’ll send you a copy. I’d be delighted to hear any thoughts, comments or queries that you may have.

Project kick-off: propagule pressure, functional traits, resource availability and plant invasion

As mentioned in a previous post, I was lucky enough to be awarded one of the inaugural ARC Discovery Early Career Researcher Awards (DECRA) late last year. I officially started my DECRA research in April, so I thought it was time that I introduce it – albeit rather briefly.

In essence, I am planning to investigate the susceptibility of native vegetation edges to alien plant invasion using quantitative and experimental approaches.  The project will contain both theoretical and applied elements and will primarily examine plant invasion through a community ecology lens (or is it community assembly through an invasion lens??!).

I’ll specifically be looking at the combined (and interactive) effects of species traits, resource availability and propagule pressure on invasion success using Bayesian meta-analysis, causal modelling and a field experiment.  As stated in my grant application, “disentangling effects of alien species’ seed supply, high resource availability (light, water, nutrients) and species’ traits on invasion will indicate their relative influence on plant invasion and community assembly.  As a result, new knowledge will be gained on the efficacy of invasive species prevention and control by indicating which invasion pathways to target, and under what conditions.”

The project will run for three years and I’ll be splitting my time between Australia and the US to achieve it. The plan is to work with CEED/NERP folk on the more quantitative aspects of the project while in Australia (principally with people like Brendan Wintle, Cindy Hauser, Mick McCarthy and Peter Vesk in the QAEcology group at Melbourne Uni, but also with Phil Gibbons and David Lindenmayer at the Australian National University; more on that later). I’ll conduct the experiment at Cedar Creek Ecosystem Science Reserve in Minnesota working with David Tilman.  I’m planning to spend two months at the University of Minnesota this year (July-August) and then 6 months for the following two years (roughly April-Sept/Oct).  As a lover of warm weather, an endless summer comes as an added bonus!

New paper: Quantifying levels of biological invasion: towards the objective classification of invaded and invasible ecosystems

Oct 2011 – I recently published a paper in Global Change Biology with Peter Vesk, Dave Richardson and Petr Pyšek that identifies the best way to quantify the level of invasion by non-native animals and plants.

For a summary of the paper, check out this post on our lab website,

or to download a free copy of the paper, go here.

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