This #psejclub post is by Michael Van Nuland and Rachel Wooliver. If you’d like to write a guest post, please get in touch! Get involved with the discussion of this post using the comments section below, or on Twitter or Facebook using #psejclub.
One of the most fundamental questions in ecology is to better understand the distribution and structure of biodiversity in nature. Plant-soil feedback (PSF) studies have brought lots to the table in this regard over the last decade by expanding what we know of plant community dynamics and patterns of succession. However, our understanding of these scenarios is rarely the complete picture (is anything?). Incorporating evolutionary ideas into plant-soil linkage studies offers an important and fruitful research direction because the current situation of species and communities can be as much a reflection of their evolutionary history as their contemporary interactions. In other words, species in the ecological theatre may play out on the evolutionary stage (Post & Palkovacs 2009), but everyone has a story.
The novelty of the recent Anacker et al. (2014) study is in using species evolutionary relatedness to understand plant-soil feedbacks in the context of whole communities. This work used a combination of previous studies (notably Klironomos et al. 2002, 2003), mixed with some phylogenetic inference (more on that below), to evaluate similarity in PSFs among ~60 old-field plant species. One of the most interesting approaches in the study comes from comparing phylogenetic signals in PSF to that of plant abundance in the field (i.e., bringing things back down to those basic questions in ecology).
University of Guelph Arboretum, home of the long-term mycorrhizal research station used in the Klironomos 2002, 2003 studies (photo: Gardens of Canada)
The major take-home message is that evolutionary history can leave plant species sharing similar interactions with their soil microbes, which turns out to be a key predictor of plant abundance in these old-field communities. This was particularly clear for the net whole-soil and conspecific AMF feedback responses, but not so much for the soil treatment with the generally abundant AMF Glomus etunicatum. Given the diversity and complexity of soil communities, it makes sense that one particular member might not have a clear or dominant influence on the direction of PSFs across an entire community of plant species. However, in terms of predicting abundance in the field, all soil biotic treatments (net whole-soil, conspecific AMF, and G. etunicatum) were retained as critical factors in the best models, indicating that the whole suite of belowground interactions likely has important consequences for structuring plant abundance.
That the authors have examined PSFs in a phylogenetic framework is what makes this study exciting for moving these ideas forward. Their results provide a mechanism for explaining plant abundance in the field: as a plant, you and your relatives interact the same way you do with soil communities. In human terms, an analogy would be that you and your siblings, cousins, parents, and Uncle Eddy have similar careers, shop at similar stores, and eat at the same restaurants (i.e., construct and interact with a similar environment), all of which shapes how big your family is in relation to everyone else in town.
This begs the question of why you are so similar to your family. Is it simply because you are related? To an extent, it might be expected that you are more similar to your family given the nature of evolution (i.e., inheritance and descent with modification). Or is it because you are somehow constrained (e.g., due to lack of variation or strong trade-offs that lead to the extinction of anyone who is different) to keep what you’ve inherited? Constraints are the essence of phylogenetic conservatism, and there are several evolutionary processes that can lead to this pattern (Crisp & Cook 2012). Yet, in beginning to answer this question, it is important to clarify what is meant by the magical term “phylogenetic conservatism”.
Some evolutionary and ecological causes that can strengthen (or weaken) a pattern of phylogenetic niche conservatism (from Crisp & Cook 2012)
The most thorough explanation that we’ve come across has been Losos (2008), a perspectives piece motivated by the widespread misinterpretation of (and lack of clear distinction between) the terms phylogenetic signal and phylogenetic conservatism. Essentially, phylogenetic signal is a pattern “in which ecological similarity between species is related to phylogenetic relatedness”. Given the phylogenetic methods used in this particular study, a ‘significant’ Blomberg’s K value would indicate phylogenetic signal. Phylogenetic conservatism, however, is a pattern in which the “similarity of closely related species is significantly greater than would be expected based on phylogenetic relatedness” (Losos’s emphasis). In terms of K values, only those values that are greater than one indicate phylogenetic conservatism. All reported K values in the Anacker et al. study are significant, but less than one, which is indicative of phylogenetic signal but not phylogenetic conservatism. In other words, you and your family may all be doctors, but little Johnny is not constrained to this and instead pursues a related career in dentistry. The point here is that just because plants are similar doesn’t mean that they are constrained to be similar – it is simply because they have inherited similar traits from their recent common ancestors.
Because the paper could not offer more definitive conclusions on phylogenetic niche conservatism, we wish the authors had speculated or explored more about how conservation of traits might be related to feedback responses among groups. We know that plant traits determine PSF by shaping interactions with soil communities and controlling nutrient processes, and we thought this topic deserved more attention given the implications of this work. For example, are there traits that are more important for predicting feedback responses when incorporating species’ evolutionary histories (i.e., is there a phylogenetically conserved suite of traits that underlie the observed phylogenetic effects on PSF)? Is it possible for PSFs to promote the conservation of certain traits, and if so how might you test that? Are the traits always organism-level (e.g., r- or k-selected growth strategies), or is it possible to use more specific shoot/root traits with phylogenetic inference to predict PSFs and their consequences well? Is this even a reasonable approach to take, or should we be interpreting phylogenetic signal differently when thinking about PSF given the cyclical nature of the relationship?
We also appreciated the section on potential limitations of the study, because their conclusions did encompass multiple independent experiments that were not quite replicated in the same manner or for the same amounts of time. This is especially important because we know PSF studies can be finicky based on experimental designs (Brinkman et al. 2010, van der Voorde et al. 2012) – one obvious example being their drastically different results between net whole-soil and conspecific AMF feedback patterns. However, one concern they did not address is the potential over interpretation of species-level effects when the comprising studies largely did not account for within-species variation. There are exceptionally few studies (let alone PSF experiments) that span individual to phylogenetic levels, but this is still worth considering given that the majority of experiments finding interspecific differences in PSF typically do not appropriately considered intraspecific variation.
The future of community and ecosystems ecology is with an evolutionary perspective, and overall we recommend this paper since it provides an important step in that direction. We wonder how widespread these trends of phylogenetic signal on feedback responses are across more than just old-field communities, and if we can begin to identify broader patterns of important traits or selective pressures that are related to niche conservatism in PSFs. There’s a whole other barrel of fun with their native/exotic comparison, which we won’t go into explicitly but would love to hear your thoughts on as well.
Anacker BL et al. (2014) Phylogenetic conservatism in plant-soil feedback and its implications for plant abundance. Ecology Letters, doi: 10.1111/ele.12378.
Brinkman EP et al. (2010) Plant-soil feedback: experimental approaches, statistical analyses and ecological interpretations. Journal of Ecology, doi: 10.1111/j.1365-2745.2010.01695.x.
Crisp MD & Cook, LG (2012) Phylogenetic niche conservatism: what are the underlying evolutionary and ecological causes?. New Phytologist, 196, 681-694.
Klironomos JN (2002) Feedback with soil biota contributes to plant rarity and invasiveness in communities. Nature, 417, 67-70.
Klironomos JN (2003) Variation in plant response to native and exotic arbuscular mycorrhizal fungi. Ecology, 84, 2292-2301.
Losos, JB (2008) Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology letters, 11, 995-1003.
Post DM & Palkovacs, EP (2009) Eco-evolutionary feedbacks in community and ecosystem ecology: interactions between the ecological theatre and the evolutionary play. Philosophical Transactions of the Royal Society B, 364, 1629-1640.
van de Voorde TF et al. (2012) Soil inoculation method determines the strength of plant–soil interactions. Soil Biology and Biochemistry, 55, 1-6.