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)
October has been an exciting month as we welcome Josh Brian and María Ángeles Pérez-Navarro to our lab group – and Junru Shen to London after she’s been doing her PhD remotely for a year!
María Ángeles has a PhD in Terrestrial Ecology (CREAF-UAB, 2020), and her research is mainly focused on understanding and predicting species distribution changes with global change. During her previous research at CREAF as a post-doctoral researcher, she investigated the impact of canopy-recruit interactions on climatic debt and analysed the differences in performance of planted and natural tree forest species. During her PhD, she analysed the relationship between species niche and population and communities responses to extreme climatic events, trying to disentangle some of the impacts of climatic variability driven by climate change on plant ecosystems.
Josh has recently completed his PhD at the University of Cambridge, studying the community ecology of freshwater mussel parasites and the implications for the conservation of their hosts. Prior to that, he worked on the diversity and ecology of the symbionts of coral reefs in Timor-Leste. He is interested generally in community ecology, symbiosis and invasion biology.
María Ángeles and Josh will both be working on our AlienImpacts project. We’re delighted to have them in the group and are really excited about the fun science that is now in store!
I absolutely love the journal (and the British Ecological Society more generally), so am delighted to have the opportunity to contribute to the journal, our science and our wonderful community of plant ecologists in this way.
Journal of Ecology is very excited to announce that Jane Catford will be joining our Senior Editor team! Jane has been an Associate Editor with our journal since 2016 and we’re thrilled to see her take on this new role.
Here we interview Jane about her current research, favourite plant species, ideal fictional lab partner & dream superpower!
In some very un-2020-like news (?!), I found out in December that my grant proposal to the European Research Council was successful. The words “ecstatic”, “ridiculously excited”, “over the moon” don’t begin to cover it. Safe to say, I didn’t sleep for several days after hearing the news.
My project, Predicting impacts of alien plant invasions on community diversity (AlienImpacts), aims to develop an approach for accurately predicting impacts of alien plants on floristic diversity, and to identify the circumstances under which negative impacts will occur, using temperate grasslands as a model system. The project combines experimental, observational, theoretical and quantitative approaches, and will enable us to undertake some incredibly exciting and important research.
Funded through the ERC Consolidator Grant scheme, the project will run for five years, officially kicking off in April 2021, and will support a team of researchers – including four postdocs (the ads of which will come out soon, so please watch this space!).
I’m incredibly grateful to the ERC for supporting this important work, to friends, colleagues and 2019 reviewers* who helped me hone the proposal, and to collaborators who will be involved in the research.
Project summary: The Anthropocene, the current geological epoch, is characterised by human-induced ecological changes, which have prompted a global biodiversity crisis. Human-introduced alien plants could help to offset native species loss, augmenting diversity and maintaining the services and capital that humans derive from nature. However, alien species that become invasive are themselves a key threat to biodiversity. Alien species thus presents a huge challenge for biodiversity conservation in the Anthropocene: should their arrival and establishment be inhibited or disregarded as they can potentially both exacerbate and ameliorate biodiversity loss? Coupling empirical and theoretical approaches, AlienImpacts will directly address this challenge by developing an approach for accurately predicting impacts of alien plant invasions on plant community diversity and identifying the circumstances under which negative impacts will occur. Using temperate grasslands as a model system, AlienImpacts will use innovative field experiments and global observations to systematically quantify – for the first time – how often, for how long, to what extent, under what conditions and in what ways alien plants can impact plant community diversity. AlienImpacts will develop mechanistic niche models, validated with empirical data from grasslands in North America, Europe and Australia, that will enable realistic scenarios of invasion biodiversity impacts to be forecast, now and in the future. Developing empirically accurate mechanistic models that predict invasions and their biodiversity impact is a highly ambitious goal. Its achievement will mark a step-change in ecological theory and understanding, will inform environmental policy and management, and address a critical research challenge of the Anthropocene: how to conserve the biodiversity of plants – the dominant life form on earth – under global environmental change.
[*2020 was my second attempt after falling short in 2019. Reviewer and panel comments from 2019 were incredibly helpful, and enabled me to strengthen my 2020 proposal –> so I owe them many thanks for their excellent feedback – thank you!]
Abiotic environmental change, local species extinctions and colonization of new species often co‐occur. Using a unique grassland experiment that isolates abiotic effects of warming from indirect biotic effects, we find that the biomass, richness and traits of plant colonists is more strongly affected by biotic resistance from residents than 6 years of 3°C‐above‐ambient temperatures. If these results were extended to invasive species management, preserving community diversity should help limit plant invasion, even under climate warming.
Full paper: Catford, J. A., J. M. Dwyer, E. Palma, J. M. Cowles, and D. Tilman. 2020. Community diversity outweighs effect of warming on plant colonization. Global Change Biology.linkaccepted author version
Perched in the middle of the southern part of the African continent is the Okavango Delta – an inspiring mosaic of wet and dry, with an abundance of wildlife, breathtaking landscapes, and grasses that would blow your socks off (and probably get attached to them given half a chance).
Last month, I was one of 24 lucky souls to spend about 10 days submersed in the Delta – a UNESCO World Heritage Site and Ramsar wetland that is formed when the Okavango River, flowing from Angola and Namibia, reaches a tectonic trough in Botswana where is it spills over the land to form a 15,000 sq km delta.
We had staff and students from all three universities plus the University of Botswana, so it was a wonderful melting pot of experience, expertise, backgrounds and interests – all set against the backdrop of this amazing system and river basin.
It was an unusually a dry year, but that didn’t detract from the place. The many, many highlights included:
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.
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.
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.
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.
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)
It’s been an exciting time recently – a few PhDs starting, a few others finishing (with two in and two out in the space of a week!). Love it!
Angela Bartlett, Stefanie Kaupa and Harry Shepherd recently crossed the starting line.
Angela is investigating the impact of rates of introduction, and introduction bias, on the richness and composition of alien plant and vertebrate species assemblages. She is supervised my Tim Blackburn (UCL) and me.
Stefanie is researching the impacts of agricultural land abandonment and associated plant invasions on hydrology in the mountain environments of Nepal and Colombia. She is supervised by Mark Mulligan (KCL) and me.
Harry is examining the potential for plant-soil interaction to enhance ecosystem restoration. He is supervised by Bjorn Robroek (Southampton) and me.
Bush House: home of King’s Geography (and my new office)
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.
No excuses not to visit if you’re ever in London
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.