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.
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.
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 & Evolution. link (open access)
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.
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!
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.
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 Distributions15, 904-910.
Sparks, H. T., Dennis, L. H. R., Croxton, J. P. & Cade, M. (2007) Increased migration of Lepidoptera linked to climate change. European Journal of Entomology104, 139-143.
Bradley, B. A. et al. (2012) Global change, global trade, and the next wave of plant invasions. Frontiers in Ecology and the Environment10, 20-28.
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.
Driscoll, D. A. et al. (2014) New pasture plants intensify invasive species risk. Proceedings of the National Academy of Sciences 111, 16622–16627.
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.
Diez, J. M. et al. (2012) Will extreme climatic events facilitate biological invasions? Frontiers in Ecology and the Environment10, 249-257.
Kraft, N. J. B. et al. (2015) Community assembly, coexistence and the environmental filtering metaphor. Functional Ecology29, 592-599.
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 Ecology48, 432-442.
Bansal, S. & Sheley, R. L. (2016) Annual grass invasion in sagebrush steppe: the relative importance of climate, soil properties and biotic interactions. Oecologia181, 543-557.
Hellmann, J. J., Byers, J. E., Bierwagen, B. G. & Dukes, J. S. (2008) Five potential consequences of climate change for invasive species. Conservation Biology22, 534-543.
Inderjit, Catford, J. A., Kalisz, S., Simberloff, D. & Wardle, D. A. (2017) A framework for understanding human-driven vegetation change. Oikos126, 1687-1698.
Fridley, J. D. & Sax, D. F. (2014) The imbalance of nature: revisiting a Darwinian framework for invasion biology. Global Ecology and Biogeography23, 1157-1166.
Founded in 1829 by King George IV and the Duke of Wellington (and featuring a duel, no less!), King’s is the fourth oldest university in the UK, and is one of the world’s top 25 universities.
KCL has one of the strongest Geography departments globally, with research and education that extends across physical, environmental and human geography. I will be in the Environmental Dynamics theme, connecting hydrological, geomorphological, atmospheric and ecological processes – right where I love to be!
Based at the Strand campus, with views over the Thames, it is hard to be more central – not just literally in terms of London itself, but figuratively too. As a global city, London is a hotspot of education, research, culture – and has incredible connections with the rest of the world.
If you’re interested in working or studying with me, or would like to visit, please do get in touch. You can reach me on jane.catford<at>kcl.ac.uk.
Not only was this exercise great fun, but I am aware that – as a nature English speaker – I enjoy the privilege of reading (the bulk of) international science and science communication in my first language. So, I thought that others – both inside and outside of the science, research and education worlds – would like the opportunity to do the same.
I’m unsure how widely these cartoons will travel – and how well they’ll reach their intended audience (i.e. speakers of these languages), but can’t hurt to try, eh?
In July, I will be shifting my office some 17,107 km to the University of Southampton. I’ll be starting a lectureship (equivalent of Assistant Professor) in Community Ecology in the Centre for Biological Sciences where I’ll be part of the Environmental Biosciences research group.
A member of the UK’s Russell Group, Southampton is one of the leading teaching and research universities in the UK and is currently ranked 81st in the World by the QS World Rankings 2015. Students come from across the UK and from over 135 other countries, making it an exciting place for researchers, teachers and students.
It is also in a great – and the sunniest! – part of the UK. An hour and twenty minutes from central London, it is on the south coast, nestled between New Forest and South Downs National Parks, which are less than 20 km away. Southampton has a local airport with flights throughout Europe and Heathrow is less than an hour away, making national and international travel easy.
I’m incredibly excited about establishing a lab and research program in Southampton. I’ll be actively trying to grow my lab, so please get in touch if you’re interested in working with me as a student, postdoc or collaborator. I’ve got ideas for potential projects but would love to hear others. You can reach me on: catfordj[at]unimelb.edu.au.
For those interested in postgraduate research, here are some starting points for information on courses, scholarships and life in Southampton:
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.
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.
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.
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.
While perusing my emails recently, I had a delightful surprise: an email notifying me that I have been awarded the 2015 Australian Society for Limnology Early Career Excellence Award.
The award is given to limnologists based on the contributions they have made in the first ten years of their professional life. I am thrilled, humbled and very honoured to receive such an acknowledgment from my freshwater colleagues.