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Grazing by large herbivores has been shown to interact with primary productivity in determining plant community structure (Milchunas & Lauenroth 1993; Proulx & Mazumder 1998; Osem et al. 2002). In their conceptual model, Milchunas et al. (1988) predicted that modifications in plant community composition caused by grazing are more likely to occur, and will be larger, with increasing productivity, due to ‘divergent selection’ for grazing vs. competition for light. Competition for light will select for traits such as taller growth forms with larger leaves and faster growth (Keddy 1989; Gaudet & Keddy 1995; Grime 2001), but should make plants more vulnerable to grazing (Noy-Meir et al. 1989; McIntyre et al. 1995; Sammul et al. 2000; Díaz et al. 2001). On the other hand, adaptations to aridity such as shorter plants, small leaves, basal meristems and annual life cycle, should increase tolerance to, or avoidance of, grazing (‘convergent selection’) (Coughenour 1985; Milchunas et al. 1988). Therefore, moderate or small changes in the composition of plant communities in response to grazing are expected in semi-arid rangelands with lower productivity in which plant growth is usually limited by soil resources.
The conceptual model of Milchunas et al. (1988) was generally supported by an extensive meta-analysis of the effects of grazing on community structure over a global range of environments (i.e. large scale) (Milchunas & Lauenroth 1993). However, when comparing ecosystems from different geographical regions confounding effects may mask the interactive effects of productivity and grazing on community structure. This is due to other factors, such as vegetation composition and physiognomy, evolutionary history, grazing regime and dominant grazers. We propose that a small-scale approach, based on comparisons among neighbouring topographic sites differing in productivity, in which the plant community evolved under similar biotic and abiotic conditions, may allow a finer resolution of the interactive effects of productivity and grazing on plant community structure. We hypothesize that the trends observed in community structure due to grazing across ecosystems differing in primary productivity (i.e. large scale) also occur among neighbouring topographic sites differing in their productivity (i.e. small scale).
In their model Milchunas et al. (1988) proposed that plant growth rate and plant size are major traits involved in the mechanistic interactions between productivity and grazing, as they are related to both competitive ability and tolerance or avoidance of grazing and drought (Coughenour 1985; Grime 2001). This model and the supporting meta-analysis (Milchunas & Lauenroth 1993) were based mainly on grazing responses observed in perennial grasslands and shrublands, due to scarcity of information on annual grasslands, particularly those from semi-arid regions. However, the relative importance of growth rate and plant size traits along productivity gradients may differ between perennial and annual grasslands, as annual species lack temporal continuity in competitive interactions. Therefore, changes in vegetation structure of annual grasslands due to grazing may depend to a larger extent on seed-bank dynamics and seedling establishment than in perennial grasslands, particularly in semi-arid regions with lower plant density (Briske & Noy-Meir 1998).
Grassland species exhibiting similar responses to grazing have been traditionally categorized into grazing response groups, such as increasers, decreasers and invaders (Dyksterhuis 1949; Noy-Meir et al. 1989; McIntyre et al. 2003). Within these groups, species frequently share traits regarding life cycle, growth form, plant size, palatability and defence mechanisms that are presumably related to their tolerance to, or avoidance of, grazing (Noy-Meir et al. 1989; Díaz et al. 2001; McIntyre & Lavorel 2001; Vesk & Westoby 2001). The importance of these traits in understanding the dynamics of vegetation change and associated ecological processes in response to grazing is well established (Noy-Meir et al. 1989; Fernandez Alés et al. 1993; McIntyre et al. 1995; Lavorel et al. 1999). Plant size and height are traits frequently considered as robust predictors of species response to grazing (Noy-Meir et al. 1989; Lavorel et al. 1997; Hadar et al. 1999; Lavorel et al. 1999; Sternberg et al. 2000; Díaz et al. 2001; Dupre & Diekman 2001; McIntyre & Lavorel 2001). However, plant size may vary considerably along environmental gradients determining productivity, particularly in the case of annuals (Aronson et al. 1990). This variation should be taken into account when studying species response to grazing, as species ranking by size and their categorization in size groups may change along environmental gradients regardless of grazing impact. A related, cardinal question for species categorization into grazing response groups is whether their response to grazing, or to protection from grazing, is consistent (i.e. repeatable under different conditions), or dependent on grazing regime and habitat conditions (i.e. context dependent). Vesk & Westoby (2001) proposed that inconsistent responses might result from context characteristics, such as abiotic conditions, neighbour species, competitive relationships, grazing regime and the grazing herbivore. McIntyre et al. (2003) also reported that environmental variables, such as soil disturbances and water enrichment, influence species response to grazing. Lack of consistency in grazing response will complicate predictions about the effects of grazing on the behaviour of individual species, composition of response groups and assumptions about vegetation dynamics (Gitay & Noble 1997; Garnier et al. 2001).
In our research we hypothesized that: (a) the response of annual species to grazing (i.e. magnitude and direction of change in their abundance) is context dependent, and may vary with productivity level among years differing in rainfall and across neighbour topographic sites with different microclimatic and edaphic conditions; (b) species plant size and range of variation play a central role in determining the response of annual species to grazing along gradients of productivity; and (c) grazing impact on species composition of annual plant communities in semi-arid regions increases with increasing productivity and associated changes in plant size, as proposed by the Milchunas et al. (1988) model for larger scales.
We studied these hypotheses in an annual plant community, with and without protection from grazing, in neighbouring topographic sites representing a gradient of primary productivity. The study was conducted in a Mediterranean semi-arid rangeland in Israel. In this type of semi-arid rangeland primary productivity is limited by soil resources, mainly water and nitrogen (van Keulen & Seligman 1992), which are patchily distributed due to a large heterogeneity of annual rainfall and habitat conditions (Noy-Meir 1973; Ludwig 1986). This heterogeneity, together with the dynamic nature of annual plant populations regenerating every year from the seed-bank, causes a wide spatial and temporal variation in primary productivity, within the range of arid and subhumid rangelands (Osem et al. 2002). Such rangelands provides a good opportunity to study, at the small scale: (a) changes in species response to grazing due to variation of plant size as a function of productivity; and (b) the interactive effects of grazing and productivity on species composition of annual plant communities under similar climatic conditions, grazing regime and evolutionary history.