Addressing context dependence in ecology

A phrase that you are bound to hear many times at any ecology conference is “it depends”. We see context dependence – variation in the magnitude or sign of ecological relationships depending on the conditions under which they are observed (Fig. 1) – in just about every study and every system. Such variation, especially when unexplained, can lead to spurious or seemingly contradictory conclusions across studies, which can limit understanding and our ability to transfer findings across studies, space, and time. Because of the wide prevalence of observed context dependence and the critical need to tackle it, a group of us recently knocked heads (and read lots of fabulous papers!) about how it can be addressed. Our reading, thinking, talking, drawing and writing culminated in this open access paper in TREE

Figure 1: Context dependence may be invoked when the observed relationship between two variables varies in (a) magnitude (strength), (b) sign (direction), and (c) uncertainty, applied here to hypothetical examples from plant invasions.

In the paper, we identify two types of context dependence resulting from four sources (Fig. 2). Mechanistic context dependence occurs when a relationship, say between variables X and Y, fundamentally differs under different ecological and spatiotemporal conditions. Such relationships arise from (i) interaction effects of another variable, Z, which modifies the effect of X on Y, reflecting ecological processes. Apparent context dependence occurs when the relationship between variables X and Y does not differ but appears to due to: (ii) the presence of confounding factors that are either unaccounted for or are measured and accounted for in some studies but not others; (iii) problems of statistical inference where studies differ in sampling accuracy and precision, statistical power, or interpretation of statistical measures; and (iv) methodological differences among studies whereby studies observe and measure variables or relationships in different ways.

Figure 2: Four sources of variation in the relationship between independent variable X and dependent variable Y, with illustrative examples and actions that can reduce unexplained variation and the likelihood of apparent context dependence.

We illustrate our typology using examples from biological invasions, a field where context dependence is prominent and widely discussed, but we propose that the typology is applicable across all areas of ecology (and it may well extend to all natural and biological sciences…). We conclude the paper by outlining steps for addressing the different types and sources of context dependence, and provide a decision tree that outlines key actions likely to be helpful. We believe that by recognising the different ways in which context dependence can arise, we can better account for context dependence and reduce the prevalence of unexplained variation in ecology.

Full paper: Catford, J.A., Wilson, J.R.U., Pyšek, P., Hulme, P.E. & Duncan, R.P. (in press) Addressing context dependence in ecology. Trends in Ecology & Evolutionlink (open access)

Invasive Species Week: UK Invasions in a Changing Climate

Regan Early (University of Exeter) and I wrote this piece to mark Invasive Species Week for the British Ecological Society blog. In it, we discuss the possible synergies between climate change and species invasions, and some associated policy challenges.

 Vandan Patel

Whether it is parakeets streaking across the sky, grey squirrels pirouetting in your local park, or seed pods of Himalayan balsam going pop, alien species are familiar sights and sounds in the UK.

Last week’s release of the IPBES report and this week’s Invasive Species Week remind us of the serious threat posed by alien species that become invasive. Not only do they impact the economy – costing the UK some £1.8 billion each year, threaten human health and degrade cultural values, but they are the fifth biggest threat to biodiversity globally.

Together with the BES policy team, we recently responded to Parliament’s Invasive Species inquiry.

Among the questions the Inquiry posed was whether climate change would exacerbate invasions. Would the two interact to make a problem bigger than the sum of its parts?

Increasing number of invaders under climate change 

Somewhat surprisingly, the way that climate change will affect UK invasion is yet to be comprehensively assessed. Evidence suggests it’s unlikely that numbers of invasive species will increase simply because the UK climate will become suitable for species that otherwise couldn’t live here. Rather, more invaders may arrive in the UK because their populations grow in mainland Europe, and through human responses to climate change1.

If climate change makes invaders more abundant in continental Europe, the number of emigrants will increase, driving up immigration into the UK2. For example, numbers of moths migrating each year to the southern UK (but not establishing populations) has increased by 1.3 species/year, associated with warming temperatures in Spain and France3, but there is no direct evidence that climate change is the cause.

Greater use of biofuels, more intensive agriculture, and introduction of new plant species (or plant varieties) for gardens and agriculture may help us mitigate or adapt to climate change, but may inadvertently facilitate invasion4,5. New varieties of pasture plants that grow quickly and can cope with varying weather conditions are being developed; unfortunately, these are among the traits that can make species invasive6. Seaweeds are increasingly being used for biofuel production7 – many of them alien – and tests into seaweed farms are now underway across the UK. This developing aquaculture industry may pose a future invasion risk.

 Shane Stagner

Increasing impact of invaders under climate change 

Climate change will likely increase the impacts of invaders in the UK because many invaders are opportunistic generalists with wide environmental tolerances, good dispersal ability and rapid growth rates1. These characteristics mean that they’re well placed to take advantage of environmental change and of increases in disturbances like floods and storms8. Additionally, as climate change makes life tougher for natives, they will be less able to repel the advances of invaders9,10. For example, a decline in perennial native grasses with increasing temperatures has facilitated exotic annual grass invasion in California11. Under a new climate, currently successful management may become less effective, allowing invaders to proliferate and spread4,12.

The policy challenge of range-shifting species 

No matter how hard we try, a degree of climate change is inevitable, and this will drive shifts in species’ ranges and abundances13. Such range-shifting species are not being introduced directly by people, and so don’t fit into the traditional invasive species paradigm. In areas strongly affected by environmental change, species’ range shifts are likely essential for their survival, so could species native to mainland Europe that colonise the UK merit protection here? Species colonising from nearby locations are less likely to be invasive, and indeed no European native that has thus far colonised the UK is considered invasive14. On the other hand, anecdotal evidence raises concerns. St Piran’s hermit crab colonised Cornwall from Europe in 2016, and has reached extraordinarily high numbers on one beach, with no native hermit crabs to be found. Distinguishing desirable range shifts of climate-adapting “environmental refugees” from undesirable species invasions remains a key challenge.

To our knowledge, policy makers are not yet examining this issue and we predict that European native species colonising the UK will cause conflict in conservation goals. This seems like a key challenge that the invasion and conservation science communities – among others – need to resolve. Exciting times ahead!

 Shane Young

References

  1. Catford, J. A. & Jones, L. P. (2019) “Grassland invasion in a changing climate” in Grasslands and Climate Change (eds D.J. Gibson & J. Newman). Cambridge University Press, p. 149-171.
  2. Lockwood, J. L., Cassey, P. & Blackburn, T. M. (2009) The more you introduce the more you get: the role of colonization pressure and propagule pressure in invasion ecology. Diversity and Distributions 15, 904-910.
  3. Sparks, H. T., Dennis, L. H. R., Croxton, J. P. & Cade, M. (2007) Increased migration of Lepidoptera linked to climate change. European Journal of Entomology 104, 139-143.
  4. Bradley, B. A. et al. (2012) Global change, global trade, and the next wave of plant invasions. Frontiers in Ecology and the Environment 10, 20-28.
  5. Haeuser, E., Dawson, W. & van Kleunen, M. (2017) The effects of climate warming and disturbance on the colonization potential of ornamental alien plant species. Journal of Ecology.
  6. Driscoll, D. A. et al. (2014) New pasture plants intensify invasive species risk. Proceedings of the National Academy of Sciences  111, 16622–16627.
  7. Czyrnek-Delêtre, M. M., Rocca, S., Agostini, A., Giuntoli, J. & Murphy, J. D. (2017) Life cycle assessment of seaweed biomethane, generated from seaweed sourced from integrated multi-trophic aquaculture in temperate oceanic climates. Applied Energy196, 34-50.
  8. Diez, J. M. et al. (2012) Will extreme climatic events facilitate biological invasions? Frontiers in Ecology and the Environment 10, 249-257.
  9. Kraft, N. J. B. et al. (2015) Community assembly, coexistence and the environmental filtering metaphor. Functional Ecology 29, 592-599.
  10.  Catford, J. A., Downes, B. J., Gippel, C. J. & Vesk, P. A. (2011) Flow regulation reduces native plant cover and facilitates exotic invasion in riparian wetlands. Journal of Applied Ecology 48, 432-442.
  11.  Bansal, S. & Sheley, R. L. (2016) Annual grass invasion in sagebrush steppe: the relative importance of climate, soil properties and biotic interactions. Oecologia 181, 543-557.
  12.  Hellmann, J. J., Byers, J. E., Bierwagen, B. G. & Dukes, J. S. (2008) Five potential consequences of climate change for invasive species. Conservation Biology 22, 534-543.
  13.  Inderjit, Catford, J. A., Kalisz, S., Simberloff, D. & Wardle, D. A. (2017) A framework for understanding human-driven vegetation change. Oikos 126, 1687-1698.
  14.  Fridley, J. D. & Sax, D. F. (2014) The imbalance of nature: revisiting a Darwinian framework for invasion biology. Global Ecology and Biogeography 23, 1157-1166.

Species invasions and climate change: can grasslands cope?

Lizzie Jones and I contributed a chapter about grassland invasions to Grasslands and Climate Change, the latest volume of the British Ecological Society’s Ecological Reviews series. This is a blog post that Lizzie – a former MSc student of mine at Southampton – and I wrote for the Journal of Ecology blog.

Imagine a typical grassland ecosystem. You might see American prairies, rangelands of Australia, or African savannah. Either way, you’re probably thinking of wide-open spaces, dominated by resilient grass-like species. Yet, despite covering over 35% of the ice-free land surface, grasslands are an increasingly fragile ecosystem, experiencing some of the highest levels of exotic plant species invasion of all ecosystems. While there are strong links between levels of grassland invasion and human activity (as work by the Nutrient Network shows), climate change is also thought to be a key driver of such invasions.

© Getty Images

It is well established that there will be both winners and losers with climate change, where some species experience increases in range and population sizes, while other experience reductions. A key prediction nevertheless remains that exotic species invasion will increase with climate change, especially with rises in temperature and increases in extreme climatic events. Given that – like native species – individual exotic species can be helped or hindered by climate change, why does this remain a general prediction? It makes sense that some species will benefit from changes in climate regimes, and others will not, but why should some species experience an advantage simply because they are non-native?

In our chapter of Grasslands and Climate Change, we address these questions by concentrating on the effects of climate change on exotic plant invasion in global grasslands. We specifically ask whether climate change will favour exotic species, why that might be the case, and what sort of species (including their functional traits) will be favoured. In the chapter we used a systematic approach to review three key environmental changes that may give advantage to invasive species: changes in background climate conditions including temperature and rainfall; increased disturbance from extreme events such as storms and droughts; and human responses to climate change, either to mitigate its effects or to adapt to them.

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A warming and biodiversity grassland experiment at Cedar Creek Ecosystem Science Reserve: experiments like these provide insights into the impact of climate change, including species invasions. Photo by Jacob Miller.

Exotic species are well-adapted to capitalise upon change – their very invasion shows that that they are able to expand their distributions and deal with what might be unfamiliar ecological conditions. Increases in the frequency and magnitude of storm events, floods, fires and other disturbances will increase opportunities for invasion, and species that can reproduce and spread quickly will be particularly well placed. For example, some Bromus grasses can recover very rapidly when drought eases, which has allowed them to invade and convert woody scrubland areas in North America. The ability to seize opportunities and cope with a broad range of environmental conditions means that climate change will favour many exotic species – especially compared with native species, which may be less able to keep pace with changing conditions.

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Finally, humans have a huge impact on grassland invasion. In our efforts to mitigate, offset and adapt to our changing climate, we are unwittingly exacerbating the invasion of exotic species globally. A key culprit is the production of biofuel, such as Miscanthus species, now widely used in North America and Asia and predicted to spread with climate change.

So, to respond to the question “can grasslands cope with species invasions and climate change?” – native grassland species are certainly under threat not only by exotic species but by a multitude of human and climate-related issues. But, as this book shows, work towards adapting current conservation and management strategies is already underway to keep pace with our changing climate, not only in grasslands but in all other ecosystem types.

Lizzie P. Jones (Royal Holloway, University of London and Institute of Zoology, London, UK) and Jane A. Catford (King’s College, London, UK)


Grasslands and Climate Change is part of the Ecological Reviews series. BES members get 25% off all titles in the series when buying directly from Cambridge University Press. See also, David Gibson’s blog post: Grasslands and Climate Change.

How can exotic invasions drive native species extinct?

Catford et al 2018 cartoon

Read the full paper here: 

Catford, J. A., Bode, M. & Tilman, D. (2018) Introduced species that overcome life history tradeoffs can cause native extinctions. Nature Communications 9: 2131.

Weed or feed? New pasture plants intensify invasive species risk

Pasture plants gone rogue: Phalaris is highly invasive but new varieties are still being developed
Pasture plants gone rogue: Phalaris is highly invasive but new varieties are still being developed

To meet increasing demands for livestock production, agribusinesses around the world are breeding new varieties of pasture plants. Unfortunately, many of the plant characteristics promoted for use in pasture – higher growth rates, greater resistance to disease, higher tolerance of environmental extremes and higher reproduction – are shared by invasive species. Coupled with the fact that many pasture species are already highly invasive, this effectively means that agribusiness may be inadvertently breeding “super weeds”, which farmers then spread across the landscape.

And, just to make matters worse, this increased weed threat is going largely unchecked: even countries with leading biosecurity do not consider the weed risk posed by plant varieties that are developed within-country.

But all is not lost!

As described in a new PNAS paper led by Don Driscoll, there are various ways in which this problem can be fixed.

Read more about this issue in Nature, The Conversation and ESA’s Hot Topics or see more about it in Don’s video.

Invasive Gamba grass, planted for pasture, can increase bush fire intensity five fold
Invasive Gamba grass, planted for pasture, can increase bush fire intensity five fold

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.

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…

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: The intermediate disturbance hypothesis and plant invasions: implications for species richness and management

Jan 2012 – Some colleagues and I have recently written a paper that examines the relationship between the intermediate disturbance hypothesis (IDH) and alien plant invasions.  Published in Perspectives in Plant Ecology, Evolution and Systematics, the paper is structured around two questions: in accordance with IDH, 1) at what disturbance frequencies is alien plant colonisation most likely and why, and 2) where along the disturbance continuum (at which successional stage) are alien plants likely to reduce community diversity and why?  We use understanding of community and invasion ecology to answer these questions, drawing on empirical evidence from a variety of terrestrial ecosystems.  We conclude the paper by discussing implications and strategies for managing plant communities and how patterns of invasion might change in the future.

You can find a summary of the paper on our lab website.

If you’d like a copy, please shoot me an email.

Newsletter article: Causes, impacts and ways to manage exotic plant invasion along the River Murray

Dec 2011 – Condensing a rather large topic down to a single page, I discuss some of the processes that lead to high levels of invasion in riparian zones in this article. I also present  information on why we should be concerned about riparian plant invasion, as well as some of the management approaches that are available. Published in the Inland Rivers Network News, you can find the article on page 9 of the Summer 2012 edition.

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