Special Feature: Is phylogenetic and functional trait diversity a driver or a consequence of grassland community assembly?

The latest issue of Journal of Ecology includes a Special Feature guest edited by Nicholas Barber, David Gibson and Holly Jones. Below, Holly discusses the inspiration behind the Special Feature & summarises each of the research articles.

When scientists think of healthy ecosystems, they often think of ecosystems that have lots of different species.  Diverse ecosystems are more resilient, because if ecosystems are disturbed and some species are lost, then there are many back-up species that can fill in those roles (niches).

Recently, ecologists have been using other measures beyond counting species to determine ecosystem health.  Specifically, we’ve been looking more closely at two measures: 

1. Phylogenetic diversity – or how many evolutionary histories are retained in an ecosystem.  This can be important because if many different evolutionary histories are represented in an ecosystem, then following disturbance, it is more likely that one of the evolutionary histories will be able to deal with and thrive in whatever type of new environment now exists. 
For example, if we have plants that are adapted to tolerate and thrive in drought and climate change causes more droughts, that ecosystem has a good chance of withstanding the adverse effects of climate change.  But even drought-adapted plants might not be able to tolerate severe drought, so if we have also have plants that evolved to avoid drought (maybe through deep-rooting, changing their leaf area or altering how often they open their stomates to suck in CO2 and expire water), then an ecosystem is even more likely to be resilient to climate change.  If an ecosystem has neither of those evolutionary histories or adaptations available, it might not survive climate change.

2. Functional diversity – how many different functional traits are retained in an ecosystem.  Functional traits are traits of species that contribute in various ways to ecosystem function. Some plant examples are plant height (contributes to biomass), leaf area (contributes to drought tolerance), rooting depth (contributes to water cycling), and leaf nitrogen content (contributes to decomposition).

Experimental restored tallgrass prairie plots used in Khalil et al. in southern Illinois, USA. (Photo: David Gibson)

Our Special Feature was put together to try to understand if those measures either drive changes in grassland plant communities – i.e. if we alter phylogenetic diversity and functional diversity of a restored grassland, will the resulting plant community and its functioning be different than if we didn’t mess with them at all? Or if they instead are passengers of change – they just change as a result of changes in the environment.  This is important to know because most restoration practitioners put seed mixes together by bean counting species.  But if we find out that seed mixes with higher phylogenetic and/or functional diversity are better able to tolerate disturbances, then that has important practical implications. 

This Special Feature was spawned from an Ignite session at Ecological Society of America’s 2018 conference; it highlights eight field studies and a review. These articles cover questions about how phylogenetic and functional diversity measures can be used to understand grassland community assembly following disturbance or during restoration. 

A common theme of this Special Feature was that scientists used standardized measures of species, functional, and phylogenetic measures – but often analyzed or manipulated them in novel ways.  Carmona et al. created an experiment that allowed them to ask whether trait effects on competition are due to simple trait dissimilarity, or to a trait-based hierarchy of species relative to a competitively dominant phenotype.  Khalil et al. measured how much internal filtering drives competitive interactions by comparing traditional intraspecific trait variation measures across spatial scales (e.g. population vs community intraspecific trait variation). Galland et al. used a relatively novel approach – they manipulated both functional and phylogenetic diversity in a fully factorial experiment in order to understand how trait/evolutionary uniqueness or redundancy impacted colonization. Brandt et al. investigated simultaneous impacts of disturbance and resource availability with a unique experimental approach – removal of aboveground biomass and N-additions. Yang et al. investigated the effect of elevated nitrogen deposition and increased precipitation on taxonomic and phylogentic diversity in a 9 year field experiment.  Barber et al. measured phylogenetic and species diversity in 120 tallgrass prairie restorations to quantify the consequences of a variety of management and climatic variables in an uncontrolled field setting.  Griffin-Nolan et al. experimentally manipulated water availability to investigate the impacts of drought over four years, while Miller et al. used a 19-year time series to investigate the impacts of real droughts.  Lastly, Cadotte et al. amassed results from 79 studies that compared functional and phylogenetic diversity in a review of studies worldwide. 

A diverse plant community in a semi-natural calcareous grassland in northern Bavaria, Germany (Photo: Nick Barber)

Overall, the studies complied in the Special Feature suggest that both functional diversity and phylogenetic diversity can act as drivers or be passengers of plant community changes.  They also found that both measures give us different information about ecosystems, meaning they can both be important to account for when restoring ecosystems.  One striking thing about the Special Feature was that the studies spanned geographies (Mongolia, China grasslands, North American grasslands, Czech grasslands) and manipulated phylogenetic diversity and functional diversity or asked how they responded to environmental changes in unique ways.  Studies include a variety of scales from replicated, controlled mesocosms to large-scale observational studies of grasslands. Therefore, the studies in the Special Feature can help scientists in the field find methodologies to ask these questions in their own systems.    

Holly Jones (Northern Illinois University, USA)

Read this special feature online: Is phylogenetic and functional trait diversity a driver or a consequence of grassland community assembly?

Read the full editorial article online: Is phylogenetic and functional trait diversity a driver or a consequence of grassland community assembly?” by Jones, H. P., Barber, N. A. & Gibson, D. J.

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