Search

The link between biodiversity and grassland productivity remains a central puzzle for both theoretical and applied ecologists. Little is known about the response of functional diversity (FD, i.e. the extent of trait differences between organisms in a given community) to increased grassland productivity following fertilization. Species diversity is often used as a surrogate for FD but evidence shows that FD is often independent from species diversity. We conducted a three-year (2004–2006) experiment in a Tibetan alpine grassland to examine the effect of fertilization on FD and SD, and its relationship with productivity. We measured specific leaf area, plant height and seed size for 32 species in control, low- and high-fertilization treatments over three years, and quantified three components of FD: functional richness (FRic), functional evenness (FEve) and functional divergence using Rao's index (FDrao). While SD decreased rapidly, as expected, both FRic and FDrao increased significantly with fertilization. Neither species evenness nor FEve was affected by fertilization. Aboveground biomass was not significantly correlated with SD, but it was significantly positively correlated with both FRic and FDrao, with the strongest correlations found in 2006 for FRic and in 2005 for FDrao. The results showed that FD and SD have opposite responses to short-term fertilization. The increased FD suggests enhanced niche differentiation between species remaining after fertilization, which caused species loss. This study highlights the importance of examining FD in assessing fertilization-induced biodiversity loss and its impacts on ecosystem functioning in grasslands.

Summary 1. The goal of conservation and restoration activities is to maintain biological diversity and the ecosystem services that this diversity provides. These activities traditionally focus on the measures of species diversity that include only information on the presence and abundance of species. Yet how diversity influences ecosystem function depends on the traits and niches filled by species. 2. Biological diversity can be quantified in ways that account for functional and phenotypic differences. A number of such measures of functional diversity (FD) have been created, quantifying the distribution of traits in a community or the relative magnitude of species similarities and differences. We review FD measures and why they are intuitively useful for understanding ecological patterns and are important for management. 3. In order for FD to be meaningful and worth measuring, it must be correlated with ecosystem function, and it should provide information above and beyond what species richness or diversity can explain. We review these two propositions, examining whether the strength of the correlation between FD and species richness varies across differing environmental gradients and whether FD offers greater explanatory power of ecosystem function than species richness. 4. Previous research shows that the relationship between FD and richness is complex and context dependent. Different functional traits can show individual responses to different gradients, meaning that important changes in diversity can occur with minimal change in richness. Further, FD can explain variation in ecosystem function even when richness does not. 5. Synthesis and applications. FD measures those aspects of diversity that potentially affect community assembly and function. Given this explanatory power, FD should be incorporated into conservation and restoration decision-making, especially for those efforts attempting to reconstruct or preserve healthy, functioning ecosystems.

Fire is a globally distributed disturbance that impacts terrestrial ecosystems and has been proposed to be a global ''herbivore.'' Fire, like herbivory, is a top-down driver that converts organic materials into inorganic products, alters community structure, and acts as an evolutionary agent. Though grazing and fire may have some comparable effects in grasslands, they do not have similar impacts on species composition and community structure. However, the concept of fire as a global herbivore implies that fire and herbivory may have similar effects on plant functional traits. Using 22 years of data from a mesic, native tallgrass prairie with a long evolutionary history of fire and grazing, we tested if trait composition between grazed and burned grassland communities would converge, and if the degree of convergence depended on fire frequency. Additionally, we tested if eliminating fire from frequently burned grasslands would result in a state similar to unburned grasslands, and if adding fire into a previously unburned grassland would cause composition to become more similar to that of frequently burned grasslands. We found that grazing and burning once every four years showed the most convergence in traits, suggesting that these communities operate under similar deterministic assembly rules and that fire and herbivory are similar disturbances to grasslands at the trait-group level of organization. Three years after reversal of the fire treatment we found that fire reversal had different effects depending on treatment. The formerly unburned community that was then burned annually became more similar to the annually burned community in trait composition suggesting that function may be rapidly restored if fire is reintroduced. Conversely, after fire was removed from the annually burned community trait composition developed along a unique trajectory indicating hysteresis, or a time lag for structure and function to return following a change in this disturbance regime. We conclude that functional traits and species-based metrics should be considered when determining and evaluating goals for fire management in mesic grassland ecosystems.