How plant traits determine where native and alien species occur along rivers

Ever wondered why there are so many plant species – and so much weed invasion – along rivers?

At first glance, one might attribute this to the lush conditions of riparian ecosystems: lots of nutrients + loads of water = ideal conditions for plants. This coupled with the fact that riparian systems are on the interface of aquatic and terrestrial ecosystems, so are home to a rich mix of wet- and dry-loving species? Makes sense.

 I have much respect for this tree!
Balancing on a rock can’t be easy. I have much respect for this Mekong River tree!
(Source: JAC)

However, when you really think about the conditions of riparian ecosystems – physical disturbance from floods, inundation and desiccation, ice scour in cold places – then they perhaps don’t seem quite so hospitable. Like rocky intertidal platforms, presumably the species that live in riparian zones need to have some pretty specialised adaptations to be able to cope.

In a recently published Tansley Review in New Phytologist, Roland Jansson and I focus on the key structuring forces of riparian zones, plant ecophysiological traits and mechanisms of species coexistence to resolve the apparent conundrum between the high floristic diversity of riparian ecosystems and their challenging environmental conditions.

Would you drop your offspring in there?
(Source: JAC)

We describe 35 traits that enable plants to cope with riparian conditions. These include traits for tolerating or avoiding anoxia and enabling underwater photosynthesis, traits that confer resistance and resilience to hydraulic disturbance, and attributes that facilitate dispersal, such as floating propagules. This diversity of life-history strategies illustrates that there are many ways of sustaining life in riparian zones, which helps to explain high riparian biodiversity.

Using community assembly theory, we examine how adaptations to inundation, disturbance and dispersal shape plant community composition along key environmental gradients, and how human actions have modified communities. Dispersal-related processes seem to explain many patterns, highlighting the influence of regional processes on local species assemblages.

Treating alien plant invasions like an (uncontrolled) experiment in community assembly, we use an Australian and a global dataset to examine possible causes of high degrees of riparian invasion. We found that high proportions of alien species in the regional species pools have invaded riparian zones, despite not being riparian specialists, and that riparian invaders disperse in more ways, including by water and humans, than species invading other ecosystems.

You can find the paper details here and a free pdf copy here.

Please get in touch if you have any comments or questions – I’d love to hear them.

How do riparian plants even begin to survive a nordic winter?  Read our paper to find out!
How do riparian plants even begin to survive a nordic winter?!
Read our paper to find out!
(Source: Johanna Engström)

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:
The joys of being in control.

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.

Managing the Murray’s water to combat invasive species

An invasive exotic species, Sagittaria platyphylla, dominating a wetland in Cobrawonga, Victoria (2008)

About this time last year, I wrote an article for H2O Thinking, a water management magazine published by eWater (until recently the eWater CRC). While the turnaround time is nothing to envy, the piece found its place on the web earlier this week.

In the article, I focus on two questions that anyone* who has spent any time along a river will surely have asked:

Why are river banks, floodplains and floodplain wetlands so susceptible to alien species invasion?

And what can we do about it?

Well, I’m not going to give the game away, but lets just say that the words “flow” and “regulation” do make an appearance. Click here for more scintillating reading (?!).

A very nice wetland near Cobram, Victoria (2007)

*OK, anyone like me would surely have asked…

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