The effect that organisms have on their abiotic and biotic environments is a central research topic in ecology. Over the past decade, one focus of this research has been an examination of the relationship between the number of species or functional groups in an ecosystem and the properties or the functioning of that ecosystem, or ‘biodiversity–ecosystem functioning’ research. This research was prompted by the massive current and predicted future loss in biodiversity, and the concern that this loss of species will have negative effects on ecosystem functions such as productivity and carbon storage (Hooper et al. 2005). Two meta-analyses (Balvanera et al. 2006; Cardinale et al. 2006) have shown that most studies support the hypothesis that decreases in species richness cause a decrease in ecosystem functioning. Early biodiversity–ecosystem functioning experiments often showed a significant effect of composition in addition to effects of species richness, promoting the idea that the type of species in a community may have as much impact as the number of species. The mass ratio hypothesis (Grime 1998) predicts that the influence of a species or group of species on ecosystem functioning is proportional to their input to primary production, i.e. ecosystem functioning is determined by the traits of the dominant plants. There have been few direct tests of this hypothesis, and experimental tests have both supported (Vile, Shipley & Garnier 2006; Mokany, Ash & Roxburgh 2008) and rejected (Spehn et al. 2002; Wardle, Lagerstrom & Nilsson 2008; Peltzer et al. 2009) this hypothesis.
Few experiments have examined the effects of diversity and composition on ecosystem properties in more than one environment, despite the knowledge that the processes that transform ecosystems, such as nitrogen deposition, may also result in loss or changes in the types of species present in a community (Hooper et al. 2005). It is imperative to simultaneously examine multiple environments, because changes in conditions may alter communities in ways that are difficult to anticipate (Doak et al. 2008). In those few experiments where biodiversity effects were examined in different environments or contexts, the nature of the relationship between diversity (Reich et al. 2001a, 2004; Fridley 2002; Dijkstra et al. 2007) or composition (Craine et al. 2003) and ecosystem properties often differed.
The majority of studies examining the impacts of diversity and composition on ecosystem functioning have been conducted in artificially created communities using random assemblages of species. These types of experiments are essential for determining a causal relationship between the number of species or functional groups and ecosystem properties, but may be less useful in determining the role of these groups in a natural community (Huston 1997; Loreau et al. 2001). More recently, removal experiments in natural communities have been promoted for biodiversity–ecosystem functioning studies (Diaz et al. 2003) because they use communities that have been formed through natural assembly processes, contain species at their natural abundance and also allow compensatory growth by the remaining species (Diaz et al. 2003). The role of a particular group of species in an intact community can be determined by observing how a community functions with a full complement of species compared with a community with that group of species removed. This method allows us to determine the direct influence of a group of species on ecosystem properties through its presence and abundance, and also its indirect effects on ecosystem properties through interactions with other members of the community.
The nature of the relationship between biodiversity and ecosystem functioning depends on the ecosystem property that is measured (Balvanera et al. 2006). Most studies have focused on the effects on primary productivity (Hooper et al. 2005; Balvanera et al. 2006). Although this is an essential component of a wide range of ecosystem properties, a broader range of properties must be examined to establish the generality of these results. Diversity and composition of the plant community have been reported to influence numerous other ecosystem properties including soil nutrient availability (Hooper & Vitousek 1998), invasibility (Emery & Gross 2006), soil C accumulation (Fornara & Tilman 2008) and soil moisture (Hooper & Vitousek 1998). Therefore, we chose to examine the impact of different plant functional groups on a fairly wide range of ecosystem properties.
In this study, we report results from a functional group removal experiment in which single functional groups (graminoids, legumes and non-leguminous forbs (hereafter called forbs)) were experimentally removed from a series of plots in a grassland in northern Canada. By comparing these plots from which species were removed to plots containing the entire suite of species, we examined the role of identity of the removed functional group in determining a suite of ecosystem properties in an intact community. Secondly, we tested the hypothesis that the dominant functional group, the forbs, would have the largest effect on ecosystem properties, as predicted by the mass ratio hypothesis (Grime 1998). The mass ratio hypothesis has been used to describe the effects of both species (Vile, Shipley & Garnier 2006; Mokany, Ash & Roxburgh 2008) and functional groups (Wardle, Lagerstrom & Nilsson 2008; Peltzer et al. 2009) based on their proportional abundance in a community. We examined impacts of functional group removal on the remaining members of the plant community through changes in biomass, and also on the potentially limiting soil nutrients, light and soil moisture. Thirdly, we examined whether the influence of a functional group in determining ecosystem function was dependent on environmental context, by using different fertilization and mycorrhizal environments. These environments were chosen because both are relevant to future environmental change. Global warming is expected to cause an increase in soil nutrient levels, especially in northern latitudes, because higher temperatures increase mineralization rates of both nitrogen and phosphorus (Chapin et al. 1995; Shaver et al. 2000). Additionally, the presence of mycorrhizal fungi may change a plant’s response to changes in nutrient status. Mycorrhizae are affected by soil nitrogen levels, both in terms of their functioning and the type of relationship with plants they exhibit on the parasitic–mutualistic continuum (Johnson 1993). With these three questions, we investigate the importance of functional group identity in determining ecosystem functioning to better predict the effects of their loss.