I can’t tell you how thrilled I was to receive the email notifying me that Journal of Ecology would like to honour me as the 2021 Eminent Ecologist. Journal of Ecology has always been one of my favourite journals. My very first PhD paper was published there, which gave me a huge boost in confidence as a young researcher. I will always be grateful for the extremely kind and productive editor and reviewer comments on that first paper, and have taken it on board throughout my career as a lesson learnt on the importance of thoughtful and supportive reviewing.
Many of my papers of which I am most proud have been published in Journal of Ecology over the decades since 1992. I thank the Editors for putting these together into a Virtual Issue and for the opportunity to reflect on them and acknowledge the many remarkable mentors, colleagues and students that I worked with on these various research projects. I have chosen 12 papers that fall into three areas: Plant strategies and phylogeny; Evolutionary ecology of seed size; and Plant invasion success.
PLANT STRATEGIES AND PHYLOGENY
When I began my PhD in 1990, ecologists were already talking about climate change, the potential impacts on ecological communities and how to model vegetation dynamics with climate change at the global scale. It has taken a long while for the rest of the non-science world to catch up but finally the understanding of the urgency required to understand and tackle climate change has arrived. One of the early challenges for dynamic global vegetation models in predicting vegetation change with climate change, was to find a way to classify plant species into sensible groups. That was the starting point for my PhD – to use objective approaches to grouping species based on their plant functional traits. My supervisor Mark Westoby (note to any aspiring PhD students reading this – choose your supervisor wisely. I certainly did) recommended that I consider working on the semi-arid woodland flora of western New South Wales, Australia. I found a well-located and comfortable field station (CSIRO Lake Mere) about 1000 km west of my Sydney home and immersed myself in a landscape that I knew nothing about. Armed with my newly-found bible (Cunningham’s Plants of Western New South Wales) I collected, pressed and identified hundreds of plant species and collated as much trait information on each that I could. My first selected paper (Leishman & Westoby 1992 – Virtual Issue paper 1) was a multivariate analysis of this trait dataset which showed that the 300 species clustered into five main groups corresponding largely with growth form. Interestingly the vegetative, life-history and phenological traits that underpinned these plant groups were poorly associated with seed biology traits.
When I started compiling this trait dataset I had no idea that I was right at the beginning of the rapidly expanding field of comparative plant ecology, which is now a major globalised research area. There were very few large site-based trait datasets at that time, with Phil Grime and colleagues’ book on the comparative ecology of the Sheffield flora being a notable exception (that book became my second bible!). This field has now grown from trait databases of several hundred species to global-scale databases of thousands of species (such as the TRY Plant Trait Database www.try-db.org), leading to major advances in our understanding of the evolution of plant traits in relation to climate (Zanne et al. 2014; Wright et al. 2017) and biogeography (Moles et al. 2009 – Virtual Issue paper 2), and in our understanding of the major dimensions of variation that underpin plant ecological strategies (Westoby et al. 2002; Wright et al. 2004; Diaz et al. 2016).
One of the spin-offs that came from this early comparative plant ecology research was an at times heated debate about “the phylogenetic correction”. Research to understand the major dimensions of trait variation that underpin plant strategies requires assessment of correlations among plant traits across many species. Some researchers argued that species could not be considered as independent in these types of analyses, requiring a ‘phylogenetic correction’ to make the correlations statistically valid. We wrote a Forum piece (Westoby et al. 1995a – Virtual Issue paper 3) which led to an extended exchange (including Westoby et al. 1995b, c – Virtual Issue papers 4,5), arguing that ecology and phylogeny are not mutually exclusive, and that consideration of one should not trump the other (for an extended discussion of this please see Mark Westoby’s earlier Eminent Ecology blog (‘How to interpret it when a trait is phylogenetically conservative’). It was an important contribution to make to comparative plant ecology, but as a young postdoc, I confess that I felt like I had a target on my back at international conferences and workshops!
Poplar-box woodland at the CSIRO Lake Mere research station near Louth in semi-arid Australia
EVOLUTIONARY ECOLOGY OF SEED SIZE
My PhD kicked off a ten year research endeavour in comparative seed and seedling ecology and my fascination with the remarkable diversity and beauty of seeds. While much was known about the ecology of individual species (John Harper’s Population Biology of Plants was a must-read for aspiring young plant ecologists), there had been few attempts to bring together trait data on lots of species to understand plant regeneration strategies. Seeds are a key component of a plant’s life history as they are critical for dispersal, colonisation and seedling establishment. In Leishman et al. 1995 (Virtual Issue paper 6) we tested whether the correlation patterns of seed size with other traits in the semi-arid woodland flora (Leishman & Westoby 1994 in The American Naturalist) were consistent across multiple floras. We brought together all the published multi-species datasets we could find that included seed size and other trait data – three Australian datasets from the Westoby lab (semi-arid woodlands of western NSW, arid woodlands of Central Australia, woodlands and forests of the Sydney region), Susan Mazer’s Indiana Dunes (Mazer 1989) and Phil Grime and colleagues’ Sheffield flora (Grime et al. 1988). This was before electronic datasets were readily available, and so the Indiana Dunes and Sheffield datasets were painstakingly transcribed from the original publications (I know, it’s hard to imagine for anyone born after 1980!). The remarkable finding from this work was that seed size ranged over at least five orders of magnitude in each flora, and that most seed size variation was associated with growth form, plant height and dispersal mode, rather than with differences among floras. It dawned on us that understanding diversity in plant strategies was more about processes within vegetation communities than environmental differences between different communities.
Much of the theory underpinning co-existence of seed size strategies focused on the trade-off between seed size and seed number for a given investment, which had been demonstrated on a per unit canopy basis (for example Henery & Westoby 2001). In research led by Angela Moles (Moles et al 2004 – Virtual Issue paper 7), we added plant lifespan to our analysis of seed size relationships with seed number and plant size, and showed that for longer-lived plants, seed size was not related to seed number when total seed production over a plant’s lifespan was considered. To understand the evolutionary ecology of seed size, we needed to include plant size, longevity, dispersal and seedling survival.
An aspect of plant ecology research that I really enjoy is designing and conducting experiments. I find it very satisfying to take a research question through from field observations and comparative analyses to testing hypotheses with manipulative experiments to determine causation. The field and glasshouse experiments described in Leishman & Westoby 1994 (Virtual Issue paper number 8) and in sister journal Functional Ecology (Leishman & Westoby 1994) were among the first manipulative experiments on relatively large numbers of species (18-23) to test hypotheses on the role of seed size in determining establishment success. Undertaking field experiments in relatively remote locations can be a challenge, and I was very grateful for the help from my lab colleagues (who learnt new fencing skills) and patient friends and family (who learnt how unpleasant working in temperatures of 35-40 C can be). These experiments showed clearly that seedlings from larger seeds are better able to tolerate stress such as shade and low water availability. In 2000 I published, with colleagues Ian Wright, Angela Moles and Mark Westoby, a chapter titled ‘‘The Evolutionary Ecology of Seed Size’ in the second edition of Michael Fenner’s book Seeds: The Ecology and Regeneration of Plant Communities. The chapter brought together the various threads of evidence from comparative analyses and experimental work on seed size as a key plant trait. I am very proud of that chapter which remains highly cited and is still widely regarded as the authoritative work on seed size.
Although I had started my research career with an Honours project looking at the relationship between nutrient enrichment and exotic plant abundance in urban Sydney, I didn’t return to the study of invasive plants until I was awarded on Australian Research Council Research Fellowship, starting in 2000. That earlier Honours research, plus follow-up research with Honours student Janet Lake who I supervised (Lake & Leishman 2004), demonstrated that exotic plant invasion was strongly correlated with nutrient enrichment on the low fertility soils of the Sydney region. Nutrient enrichment in these bushland areas typically occurs where there are areas of nutrient-rich stormwater run-on. To test the relative roles of physical disturbance, additional water and additional nutrients, we conducted a series of glasshouse and field experiments on 28 species encompassing both natives and exotics. I dragged my young children around weedy sites to collect seeds, organised for the collection of tonnes of soil from the nearby National Park (where fortunately roadworks at the time had created a large soil pile), and trucked in water to the Ecology Reserve bushland field site at the back of Macquarie University. These experiments (described in Virtual Issue paper 9) showed unequivocally that nutrient addition was the driver of exotic plant success in this low fertility vegetation community, and that physical disturbance or additional water were not important factors. This evidence has been translated directly to changes in the way stormwater is managed and how bushland restoration projects are done.
I wanted to understand why exotic species were able to take advantage of high soil nutrient conditions, while native species were not and were even adversely affected (Leishman & Thomson 2004). Virtual Issue paper 10 describes a field study comparing leaf trait relationships of native and exotic species from five habitats in eastern Australia, including riparian, woodland and rainforest vegetation communities. We showed that mean values of leaf traits differed between exotics and natives, with exotics generally having higher values of traits (specific leaf area, assimilation and dark respiration rates, foliar N) – these would confer capacity for fast growth. However we found that where exotics and natives co-occurred in disturbed (usually nutrient enriched) sites, they had similar trait values. The trait differences between exotics and natives was found when the natives were from undisturbed sites and the exotics from disturbed sites within the same vegetation type. Importantly, we found no difference in the carbon-capture strategies of natives and exotics – instead invasion success of a species is determined by the combination of resource availability of the environment and the species’ traits.
Is it likely then that exotic species will be more successful in areas that have similar climates to their native range? This assumes that species are constrained by climate which is one of the criteria used in weed risk assessment. In Virtual Issue paper 11, colleagues Rachael Gallagher, Linda Beaumont, Lesley Hughes and I compared the realized climate niches of native and exotic ranges for 26 species that have successfully invaded Australia, and showed that many species were able to occupy quite different climate niches when introduced outside their native range. Perhaps we shouldn’t be surprised by this – species’ distributions are constrained by many factors (e.g. interactions with other species, dispersal constraints) and we know from our botanic gardens and thousands of natural global horticulture experiments, that many species are capable of thriving in quite different conditions to those experienced in their natural distribution. This shows the importance of working with the ornamental horticultural industry to minimise the risk of plants ‘jumping the garden fence’.
In 2002 I attended the Australasian Weeds Conference in Perth, Western Australia and met Dave Richardson, one of the most highly published and cited plant invasion biologists. Over the next few years we met at various conferences and workshops, and applied for funding to do trans-continental comparisons of exotic species in their native and introduced ranges so that we could better understand the processes contributing to a plant’s successful transition from introduction to naturalisation and spread. This was a dream project, with travel between Australia, New Zealand and South Africa to sample plants in native and multiple invasive range sites (South Africa and Australia have generously gifted many of their species to each other over the last century for forestry, soil stabilisation and horticulture, often with disastrous outcomes for biodiversity). In the last paper in this Virtual Issue (Leishman et al. 2014) we presented data for 13 invasive plants species from 256 populations across the three countries and showed that plants in their new environment generally had reduced herbivore damage, allowing them to shift their resources to a faster growth strategy (driven by higher specific leaf area).
It’s been fun to look back on the papers in this virtual issue – I hope I have managed to get the right balance in providing insights into the back stories and context of the papers and the ecological research they present, without being too self-indulgent. And I admit that it still feels kind of strange to be considered sufficiently senior to be considered eminent! So where to now? The landscapes and species that were inspirational for me to study biology and become an ecologist are under increasing threat from human activities. Now much of my research focus is on climate change adaptation, plant conservation and urban greening, working closely with environment agencies to translate research outcomes to tools, resources and policy advice. But I will always be thrilled when those ‘manuscript accepted’ emails arrive from Journal of Ecology!
Michelle was interviewed by Senior Editor Jane Catford to discuss her inspirations and pivotal moments in her career and to reflect on advice that she would offer early career researchers today:
Cunningham, Geoffrey M., and J. H. Leigh. (1992) Plants of western new South Wales. CSIRO publishing.
Díaz, S., Kattge, J., Cornelissen, J. H., Wright, I. J., Lavorel, S., Dray, S., … & Gorné, L. D. (2016). The global spectrum of plant form and function. Nature, 529(7585), 167-171.
Grime, JP., Hodgson, JG & Hunt, R. (1988) Comparative Plant Ecology: a functional approach to common British species. Springer.
Harper, J. L. (1977). Population biology of plants. Population biology of plants.
Henery, M. L., & Westoby, M. (2001). Seed mass and seed nutrient content as predictors of seed output variation between species. Oikos, 92(3), 479-490.
Lake, J. C., & Leishman, M. R. (2004). Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores. Biological conservation, 117(2), 215-226.
Thomson, V. P., & Leishman, M. R. (2004). Survival of native plants of Hawkesbury Sandstone communities with additional nutrients: effect of plant age and habitat. Australian Journal of Botany, 52(2), 141-147.
Leishman, M. R., & Westoby, M. (1994). The role of large seed size in shaded conditions: experimental evidence. Functional Ecology, 205-214.
Leishman, M. R., & Westoby, M. (1994). Hypotheses on seed size: tests using the semiarid flora of western New South Wales, Australia. The American Naturalist, 143(5), 890-906.
Leishman, M. R., Wright, I. J., Moles, A. T., & Westoby, M. (2000). The evolutionary ecology of seed size. Seeds: the ecology of regeneration in plant communities, 2, 31-57.
Mazer, S. J. (1989). Ecological, taxonomic, and life history correlates of seed mass among indiana dune angiosperms: ecological archives M059-001. Ecological monographs, 59(2), 153-175.