The Journal of Ecology editors are delighted to honour David Wardle in our continuing Eminent Ecologist series. David has put together a special Virtual Issue of some of his excellent contributions to the journal and has written a series of blog posts reflecting on his work. Read Part 1, Part 2 and Part 3 on the Journal of Ecology blog.

Firstly, I would very much like to thank the Editors for putting together a Virtual Issue of some of my publications in Journal of Ecology, and for the opportunity to reflect on these papers and to tell some of the stories behind them. It is quite some honour.

These papers each stem from exciting collaborative projects involving amazing and smart colleagues, postdocs and PhD students that I have been privileged to work with over the past quarter century. Many of these papers also emerge from fieldwork in locations that are somewhat isolated and fun to work in, and we have had several unforgettable adventures working in these locations over many years.

The 12 papers that I have chosen collectively represent three aspects of research that I have been involved with to varying extents for the past couple of decades or more.

PLANT SPECIES EFFECTS ON ECOSYSTEM FUNCTIONING

One of the biggest developments in ecosystem ecology over the past 25 years has involved the recognition that ecosystem processes are strongly impacted by the types of organisms that occur, or in addressing the question of ‘what do species do in ecosystems’ (Lawton 1993). Throughout the 1990s the Unit of Comparative Plant Ecology at the University of Sheffield was conducting pioneering studies to show how differences in attributes between plant species impacted on key processes such as plant litter decomposition (Cornelissen, 1996) and herbivory (Grime et al. 1996). My first selected paper (Wardle et al. 1998, also my first Journal of Ecology paper) was inspired by that type of work. At that time, I was working mostly on New Zealand grasslands, and we performed a comparative glasshouse study of 20 coexisting pasture weed species for which we measured a number of functional traits and community and ecosystem response variables. As such, my colleagues and I were able to demonstrate that across these species, certain combinations of traits were able to predict plant litter decomposability, competitive effects against phytometers, palatability to invertebrate herbivores, and effects on the biomass of soil microbes that carries out decomposition processes.

Coexisting but contrasting plant species in New Zealand grassland. Photo: D. Wardle

This sort of understanding is now of course common knowledge and the field of how species traits affect ecosystems has moved on a lot since then. One topic that has grown enormously during this decade, building on studies such as Albert et al. (2010), is based on the recognition that quite a bit of the community-level trait variability that we observe in nature is due not just to variability among species but also within species (Siefert et al. 2015). My second chosen paper (Jackson et al. 2013), driven by my then-PhD student Ben Jackson, sought to determine whether within-species variability of 16 co-occurring rainforest plant species in the Westland region of New Zealand might be important at the ecosystem level. The results from that work highlighted that while within-species variability was an important component community-level trait measures, there was also significant decoupling between within-species traits and plant litter decomposability. This led to the somewhat surprising outcome that within-species trait variability may be unimportant in explaining community-level decomposition patterns at least in our study system. Whether this outcome is also true in other systems or is just specific to Westland rain forests is, however, an open question.

Coexisting but contrasting plant species in New Zealand rainforest. Photo: D. Wardle

Sticking with the within-species variability theme, we have also been interested in how trait variation within species relates to community-level processes. My third chosen paper (Kumordzi et al. 2015) involved my then-PhD student Bright Kumordzi collecting an enormous amount of data from a system of 30 lake islands that we work with in northern Sweden and that collectively represent a long term succession (see next section). Specifically, he measured specific leaf area for ten individuals of each species on each island, which enabled quantification of within species variation of this trait both across and within islands. Following a visit by Bright to the University of South Bohemia to work with Francesco de Bello and Jan Lepš on the analysis of this data using rather sophisticated procedures that they had developed (and with further input from my then-postdoc Grégoire Freschet), we were able address questions about the contribution of within-trait variability to niche packing and species co-occurrence on these islands. Specifically, we were able to provide evidence suggesting that trait space distribution of plant individuals and therefore local species coexistence was linked in our system to plant successional age and species richness.

Coexisting epiphytic lichen species with contrasting ecological effects. Photo: P. Kardol

Nearly all work to date on how plant traits affect ecosystems has been on vascular plants, yet many ecosystems also have abundant ferns, mosses and lichens. As such, lichens dominate ground cover and help drive ecosystem functioning in many ecosystems worldwide (Asplund & Wardle, 2017). In 2000, Johan Asplund (a true lichen enthusiast and expert) obtained a postdoctoral award from the Swedish Research Council to work with me on the comparative ecology of lichens, and much of what I now know about lichens is through him. My fourth chosen paper (Asplund & Wardle, 2013) was aimed at determining the drivers of thallus palatability and litter decomposability across 28 species of lichens from a forest in south-east Norway. This work highlighted that the carbon-based secondary compounds produced by lichens was an important driver of both processes – conveniently these compounds can be easily removed from lichen thalli (while leaving the thalli otherwise intact) by acetone rinsing. It also highlighted important differences in both palatability and decomposability between lichen species from contrasting functional groups, and (in contrast to what we know for vascular plants) showed that decomposability and palatability are uncorrelated across species because they are driven by different functional traits.

David Wardle, Nanyang Technological University, Singapore 

You can read David’s 12 selected papers on the journal website: Eminent Ecologist 2018 Virtual Issue 

Read Part 1, Part 2 and Part 3 on the Journal of Ecology blog.