A common observation in ecology is that the outcome of interactions between species depends on the environment (Sharitz & McCormick 1973; Grace 1989; Keddy et al. 2002). Whereas many studies have explored how spatial heterogeneity alters species interactions, fewer authors have also considered temporal variation (Likens 1989). At any site, environmental conditions vary over time and species co-occurring in one habitat should therefore experience fluctuations in competitive interactions. For example, long-term data on community composition often show dramatic fluctuations in the abundance of individual species (Likens 1989; Dodd et al. 1995), with peaks in abundance sometimes related to peaks in climatic variables such as temperature or precipitation (Silvertown et al. 1994; Fitter et al. 1995; Herben et al. 1995; Dunnett et al. 1998). This suggests a strong link between climatic variation and community composition, but the mechanisms underlying this link cannot be assessed using observational data.
Environmental variation is widely thought to promote coexistence of species in competitive situations (Hutchinson 1961; Huston 1979; Fowler 1990). In order for variation in environmental conditions and interspecific interactions to promote coexistence among species, the response of an individual species to competition cannot be independent of its response to environmental variation (Chesson & Huntly 1989, 1997; Chesson 1994; see Muko & Iwasa 2000 for a different result in spatially heterogeneous environments). Coexistence is promoted if there is a non-zero interaction (negative covariation) between species responses to abiotic and biotic factors. This typically arises from differential sensitivity of juveniles and mature individuals to environmental variation; hence the whole phenomenon has been termed the ‘storage effect’ (Chesson & Warner 1981).
There are surprisingly few data sets available that are appropriate for testing either the assumptions or the predictions of these theoretical findings. Long-term data on community composition change (e.g. Likens 1989; Silvertown et al. 1994; Dodd et al. 1995; Dunnett et al. 1998) cannot be used to infer whether absence of particular climatic factors leads to lowered species richness or altered species composition; correlations with climate are always plagued with statistical problems. These data cannot be used to demonstrate the role of climate in changing species’ competitive abilities; rather, density of a species has to be manipulated experimentally over several years.
Two kinds of manipulative experiment can be used to assess how climate alters species competitive abilities through time. First, experiments could be designed to test predictions of the theory, i.e. to test the effect of temporal environmental variation on community composition by establishing treatments with constant and varying environments and comparing species composition and richness over these treatments. Such experiments have been performed in the context of global warming where whole (micro)ecosystems or communities are subjected to climate manipulation (e.g. Hillier et al. 1994; Chapin & Shaver 1996; Jonasson et al. 1999; Sternberg et al. 1999; Weltzin et al. 2000; De Valpine & Harte 2001; Graglia et al. 2001). Most of these experiments, however, are designed to assess the effect of the change of averages of specific climatic variables; this may predict change of species composition under different climate scenarios, but does not assess the role of climatic variation itself for species coexistence. Recent analyses indicate that climatic variation may also be subject to change (Easterling et al. 2000). Because these experiments generally manipulate whole communities, they cannot separate direct effects on the individual species from the indirect effects through altered interspecific interactions (Werkman et al. 1996). On the other hand, by treating the community as a whole, the role of species richness itself in the response to climate manipulation can be assessed (discussions in Grime 1999; Lepšet al. 2001).
In the second kind of experiment, the effect of temporal variation on species composition can be deduced by testing assumptions of the theory (see Hairston et al. 1996). In order to test the assumption that environmental variation leads to species coexistence, it is necessary to quantify the interaction between sensitivity to competition and sensitivity to temporal variation in the environment. The design of the experiment should therefore allow for testing this interaction. Few previous authors have, however, specifically addressed temporal variation in competition (Goldberg & Barton 1992). Although it is possible to argue that the space-for-time substitution (Pickett 1989) may be used in this context, justification for this argument depends on the range of environmental conditions encountered in space and time. If these vary in quantity or quality, the space-for-time substitution cannot be used, particularly if the response of a species to the environment is nonlinear.
Dunnett & Grime (1999) have identified an interaction between environmental conditions (spring heating) and competitive interactions between grassland species, but used artificial microcosms that were not similar to field conditions. This represents a rare test of non-additivity of the effect of year-to-year variation and competition, and suggests that competitive interactions can amplify climatic (between-season) effects on the performance of individual species (see also Cáceres 1997; Cerda et al. 1997; Lima et al. 1999).
We performed a long-term field experiment designed to test for changes in the competitive effects of a dominant species over several years in a mountain grassland. We used density manipulation treatments (removal of the dominant species) to detect changes in competitive response on the community-wide basis (Bender et al. 1984; Aarssen & Epp 1990; Laska & Wootton 1999). Although such experiments cannot be interpreted at the level of individual species’ response to density manipulation of the dominant (Wootton 1993; Laska & Wootton 1999), they provide a good measure of the community-wide response to this manipulation. Multidimensional tests can then be used to identify which species show opposite responses in competition as environmental conditions change.
More specifically, we tested the interaction between removal of the dominant grass species (Festuca rubra) and year-to-year variation in the effects of removal treatments on shoot frequency and above-ground biomass of all species in the community. We used a fully factorial design with two factors: removal of the dominant and year of the removal. Removal treatments were repeated over 3 years (removal beginning in 1994, 1995, 1996). The interaction between the two main factors (removal and year) was of primary interest; significance of this interaction was taken as a demonstration of year-to-year variation in competitive response of the community.