The new diversity: management gains through insights into the functional diversity of communities
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1. Biodiversity is being lost in our rapidly changing world. Most studies that examine biodiversity measure species richness, but loss of functional diversity (FD) – defined as the trait variation or dispersion in an assemblage – is equally important. FD is useful for management actions focused on ecosystem services or functions, especially when ecosystem function is ambiguously defined, or there are multiple functions of interest. Because FD is often a multivariate measure of species differences, it has additional relevance given that we seldom have a complete understanding of how traits translate to function.
2. This Special Profile includes six papers that examine how management activities or policy could benefit from consideration of the functional contributions of species. When focused on species richness, management activities may unintentionally reduce FD, with detrimental consequences for ecosystem services or stability. For example, two of the studies included in this Special Profile show that grassland management (e.g. mowing or grazing) can result in loss of FD, which may result in an unintended loss of ecosystem services.
3. A third paper reviews how species extinction and environmental change can negatively affect FD and, potentially, ecosystem function. At the individual species level, invasions offer insight into the importance of species traits. The fourth paper in this Special Profile describes how an invader is successful because of unique trait values, underlining the importance of examining FD when considering management options. In another paper, eutrophication is shown to drastically affect lichen functional groups (some decreasing and some increasing) with a minor effect on overall richness. The final paper demonstrates that the ability of aquatic plants to maintain biomass production in changing environments is determined by their complementary contributions to productivity. These complementary contributions to ecosystem function –especially given that they are more important in fluctuating environments – need to be included in evaluations of diversity.
4. Synthesis and applications. As FD is increasingly assessed in applied studies, it will call into question how we measure diversity and evaluate management success. It is clear that species richness cannot be the only measure considered. Given that environmental change and management activities can reduce FD as well as the number of species, management goals and criteria for success need to include FD.
With the exception of neutral theory (Hubbell 2001), the ability to predict and explain patterns of species richness rests on species having differential responses to environmental gradients and occupying different niches (Chesson 2000; Tilman 2004). Yet, the majority of community and ecosystem analyses and conservation policies focus on patterns of species richness, without considering how different or similar those species are to one another (McGill et al. 2006). Besides conserving biodiversity for its own sake, we protect and manage ecosystems because we wish to protect the services and functions that they provide. The amount and consistency of these services are a product of the differential contributions of individual species (Norberg et al. 2001), the complementary use of resources by ecologically different species (Tilman et al. 2001; Hector et al. 2002) and facilitative interactions among often very different species (Verdu et al. 2009). Understanding and measuring species differences becomes especially pertinent when trying to manage multiple ecosystem services, including biomass production, nutrient cycling, reduced pest outbreaks, pollination, water filtration, lower disease prevalence, etc.
Studies that compare the ability of species richness with the ability of other species diversity measures to explain patterns of ecosystem function show that accounting for species differences greatly increases explanatory power (Petchey, Hector & Gaston 2004; Fornara & Tilman 2008; Cadotte et al. 2009; Flynn et al. 2011). It is well known that species differences are important for ecology and conservation (e.g. Vane-Wright, Humphries & Williams 1991), yet metrics that quantify trait dispersion have only recently been created and evaluated (Walker, Kinzig & Langridge 1999; Petchey & Gaston 2002b; Petchey, Hector & Gaston 2004; Mouillot et al. 2005; Villeger, Mason & Mouillot 2008). Collectively, such measures are called ‘functional diversity’ (FD); they are assemblage-level measures of diversity, like richness or Shannon diversity. FD metrics fall into two categories: (i) counts of traits or functional groups or guilds or (ii) sum multivariate distances of individual species from group means –but more elaborate measures are appearing. The creation of metrics has drawn the attention of researchers. The number of papers containing the phrase ‘function diversity’ has been increasing exponentially, especially in the last 10 years (Fig. 1 in Cadotte, Carscadden & Mirotchnick 2011), and now appears in papers with an applied focus.
This Special Profile on FD reflects the increasing prevalence of papers that apply FD measures or reasoning to applied problems. The application of FD to evaluate management strategies can be especially illuminating. The management of particular ecosystem functions can have unintended cascading consequences. For example, management activities that focus on maximizing carbon sequestration usually select woody species with rapid growth. Selecting such species reduces overall ecosystem FD, and the loss of FD can alter ecosystems in other ways, such as declines in soil biodiversity (Dickie et al. 2011). The six papers included in this Special Profile apply FD measures or reasoning to applied ecological phenomena and collectively show that management needs to be cognizant of how practices may affect or be affected by species differences.
Managing functional diversity
Biodiversity has been the mantra uniting environmental and conservation policies. Writ large, biodiversity encapsulates all forms of biological variation, from intraspecific variation to ecosystem services, but the vast majority of ecological or applied studies that examine biodiversity only count the numbers of species. Biodiversity influences ecosystem function and services through the functional traits of the species present. Anthropogenic change often reduces biodiversity, but the way in which other aspects of biodiversity are affected is under-appreciated. Shifts to agricultural landscapes greatly reduce FD (Lin et al. 2011), and management strategies aimed at restoring species richness may not adequately protect FD. Rather special policies need to be in place to ensure the recovery of FD.
In the first paper in this Special Profile, Woodcock, McDonald & Pywell (2011) examine the ability of managed recreated grasslands to maintain FD (for both individual and multivariate traits). They show that hay cutting and grazing resulted in a convergence of FD. This analysis is important in demonstrating that natural grasslands often do not show this same convergence in FD as recreated grasslands. Woodcock, McDonald & Pywell go on to question policies that value recreated grassland on a par with natural grassland because recreated grassland is not as functionally robust to exploitative management. The authors express concern that recreated grassland will not be able to provide the full array of ecosystem services in future.
Cadotte, Carscadden & Mirotchnick (2011) review experiments that manipulate diversity and measure ecosystem function. In studies that measure functional traits, FD is the best predictor of ecosystem function, and so studies like those of Woodcock, McDonald & Pywell are correct to be concerned about the consequences of FD loss. Cadotte, Carscadden & Mirotchnick also review how FD changes across environmental gradients and compare this to changes in species richness. Gradients that place a strong filter on certain functional traits result in a high loss of FD, even when there are only small changes in species richness.
Richness and FD can respond very differently to management activities. Bernhardt-Römermann et al. (2011) examined how richness and FD responded to fertilization and mowing gradients in grasslands. They found that FD best explained patterns of biomass production in plots that were fertilized and frequently mowed. Conversely, richness best explained patterns in plots were mowed infrequently.
Functional diversity in a changing world
While the first three papers in this Special Profile reveal that richness and FD may not provide the same explanatory power or may respond differently to management activities, three further papers provide insight into how different aspects of our changing world are best understood by FD. Brym et al. (2011) examined the success of an invasive exotic shrub, Elaeagnus umbellata, in Michigan, USA. They show that E. umbellata occupies unique trait space, across six different traits, relative to the native community. This analysis lends support to the idea that this invader occupies unique niche space.
Nutrient deposition and eutrophication also cause changes in diversity. Pinho et al. (2011) examined lichen diversity responses to eutrophication caused by atmospheric ammonia in western Portugal. Overall richness showed a slight decline over the nitrogen gradient. This pattern may not prompt managers to take action. However, when Pinho et al. examined the responses of individual functional groups, defined by a priori estimation of responses to increased nitrogen, they found that two functional groups declined drastically, while a nitrogen-loving group greatly increased, resulting in an overall loss of FD.
Finally, species extinctions and diversity changes have cascading effects, and these need to be understood. Steudel et al. (2011) examined how much biomass was produced in assemblages of aquatic plants of differing richness. In a stable environment, richness did not explain variation in biomass. However, when there was a change in the environment (drought, increased salinity or shade), richness was important, and the speciose assemblages were more productive than low diversity ones. While they did not measure traits, the authors statistically separated the effects of species dominance causing biomass increases from complementary species combinations that result in greater biomass. They found that the benefit from diversity is through complementary combinations –presumably because of differing niches or functional contributions.
Where to from here?
As FD is increasingly evaluated in applied studies, it will call into question how we measure diversity and evaluate management success. It is clear that species richness cannot be the only measure considered. Non-random extinction can mean that FD is lost much more quickly than species richness (Petchey & Gaston 2002a). Establishing reserves based on species richness may under-protect FD (Devictor et al. 2010). Management goals and criteria for success need to include FD. It is easy to see how FD can be incorporated into traditional management activities. For example, restoration projects that strive to maximize ecosystem function as the primary benchmark (Thorpe & Stanley 2011) should consider FD explicitly. FD is an especially important tool for management when ecosystem function is ambiguously defined or where there are multiple functions of interest. In both these cases, management based on maximizing FD can ensure that ecosystems have the greatest array of species’ ecologies, thereby increasing the probability that multiple functions will also be maximized. In the vast majority of cases, there is an incomplete understanding of how traits translate to function, and by maximizing FD, managers would be protecting maximal trait diversity – underpinning effective biodiversity conservation.
Gillian Kerby deserves special thanks for all her help with this editorial and organizing the manuscripts included in this special profile.