Out of all the species on earth, why is it that communities in nature assemble into the patterns of species coexistence and abundance that we observe, many of which repeat themselves over and over again across the landscape? It has long been a goal of community ecology to determine the rules that govern which species can coexist in a given community, and what determines the structure of that community as indicated by species abundances (Elton 1946, Preston 1948, Hutchinson 1959, Cody and Diamond 1975). Alhough many notable ecologists have attempted to find generalizable rules regarding community assembly (Diamond 1975, Weiher and Keddy 1995), community ecology’s holy grail remains elusive (Lavorel and Garnier 2002), and some would say that it does not truly exist (Lawton 1999, Simberloff 2004). Still, we do know that abiotic factors such as habitat, resource availability and disturbance, as well as biotic factors such as dispersal, competition and predation both contribute to patterns of community assembly observed in nature (Chesson 2000, Grime 2006).
In this paper I will give a brief overview of processes driving community assembly, as well as the historical context behind modern theory. I will then discuss the more recent emergence of trait-based approaches to examining community assembly, with a focus on plant assemblages. This approach is widely thought to be the best way forward for finding general laws in community ecology (Weiher and Keddy 1995, McGill et al. 2006). I will then demonstrate how recent studies using trait-based approaches can be applied specifically to investigate how grassland plant assemblages respond to grazing.
Overview of community assembly processes
The regional species pool is comprised of all potential species that could colonize a given site based on dispersal processes. Species richness is limited by colonization from the species pool, and successful immigrants tend to show trait-based species sorting along environmental gradients (Houseman and Gross 2011). Environmental constraints provide a filter on the regional species pool based on which species are able to survive under the given conditions in the absence of competitors (Diaz et al. 1998). This filter tends to cause convergence among species traits, in which species that are able to colonize have certain similarities that enable them to survive in the given environment (Chesson 2000, Messier et al. 2010).
Though environmental filters are important for determining species presence, they tend to be weaker than the biotic filters that determine abundance (Cingolani et al. 2007). However, this may depend on the productivity of the site. There tends to be more abiotic filtering at low productivity sites, since many species cannot establish even in the absence of competitors. In high productivity sites, biotic filtering is stronger as many species are only able to establish in the absence of competitors (Houseman and Gross 2011).
Biotic interactions are most important in determining species abundances rather than presence. For example, strong competitors will outcompete weak competitors for a given resource supply, and will be found in higher abundance (Mouillot et al. 2007). This idea relates to niche theory, in which species are thought to occupy distinct ecological niches in terms of function and resource utilization within an n-dimensional space of environmental conditions (Hutchinson 1959). Traditionally, niches are viewed in pairwise species interactions, where the fundamental niche, or set of environmental conditions that could be occupied by the species is restricted to the realized niche, or conditions in which the species actually does occur, due to competition with a second species with an overlapping niche (Cody 1991). In this way, species differentiate their resource use to minimize niche overlap.
This process can select for divergence among species traits, in which species that are more functionally dissimilar are better able to coexist (Macarthur and Levins 1967, Stubbs and Wilson 2004, Kraft et al. 2008). These niche differences represent tradeoffs among multiple traits, and tend to have a stabilizing effect on community structure since they cause species to limit the growth of their own populations more than the growth of their competitor’s populations (Adler et al. 2007, Levine and HilleRisLambers 2009). Because the abiotic and biotic filters function simultaneously with opposite effect, the results can sometimes appear to cancel each other out (Mouillot et al. 2007).
In theory, species should be able to perform optimally at the center of their realized niche (Macarthur and Levins 1967). However, in natural communities species abundance is not necessarily highest at this supposed performance optimum (McGill et al. 2006). Instead, plants often share preference for benign environmental conditions and high resource availability. In these cases, abundance is determined by growth strategy. The strategy employed represents a tradeoff between fast growth rates under favorable conditions and tolerance of unfavorable conditions. Among plant assemblages, this shared niche preference may actually be more common than preference for distinct niches (Wisheu 1998).
Diametrically opposed to niche theory is Hubbell’s unified neutral theory (Hubbell 2001). In recent years, neutral theory has gained attention with the idea that neutral processes alone can explain many patterns of community assembly (Bell 2001, Volkov et al. 2003). This theory operates under the assumption that all species are functionally equivalent, and processes such as dispersal, historical contingence, and other sources of demographic stochasticity are responsible for community patterns rather than niche differences. However, while this assumption may be true in some areas (Hubbell 2005), it has been shown in many studies that while neutral processes may play some role, they are not the only drivers of community assembly (Fargione et al. 2003, Harpole and Tilman 2006, Mouillot et al. 2007, Levine and HilleRisLambers 2009).
With this evidence there is a push to move away from viewing niche and neutral processes as a mutually exclusive dichotomy, and toward the more useful endeavor of quantifying the relative importance of these two mechanisms in a given system (Adler et al. 2007, Schamp et al. 2008, Stokes and Archer 2010). Niche processes are defined as those that stabilize interactions between species with large differences in fitness, whereas neutral processes are defined as weak stabilizing mechanisms acting on species of similar fitness. The relative contributions of fitness differences and stabilizing mechanisms can be determined by examining the relationship between per capita growth rates and relative abundance in a community (Adler et al. 2007).
In the past, community assembly has been largely focused on which combinations of species are able to coexist. Jared Diamond’s classic work (Diamond 1975) categorized species on islands of New Guinea into different types of incidence functions based on the probability of the species occurring on an island containing a given number of species. He observed that only certain combinations of species occurred together on the same island, and he categorized pairs that never coexisted as “forbidden combinations”. However, these rules were almost entirely pattern-based and lacked any mechanistic basis.
Diamond’s work received criticism for not taking into account whether the patterns he observed were significantly different from what one would expect by a random distribution of species (Connor and Simberloff 1979). In response to this criticism, the use of null models, simulated from random sorting of species, have become widely used. These null models are useful tools to verify that communities assemble in a non-random fashion. In this way they have helped provide support for the existence of niche processes, and have helped to illuminate mechanisms causing divergence and convergence among species traits (Weiher and Keddy 1995, Grime 2006).
In order to find rules that could be more applicable across systems, ecologists have tried to move away from simply looking at pair-wise species interactions. Instead, species are often classified into functional groups based on responses to environmental factors and effect on ecosystem functioning. For example, grassland plant species could be categorized as C3 graminoids, C4 graminoids, shrubs, forbs, or legumes. Functional groupings led Fox and Brown to come up with new assembly rules, which specify that communities containing equal numbers of species from each functional group will be favored, and therefore more commonly found in nature (Fox and Brown 1993).
Grouping by function is still not a perfect solution for finding general rules, as there is a great deal of ambiguity surrounding the appropriate way to aggregate species into functional groups (Petchey and Gaston 2006). For example, plant species could be classified by photosynthetic pathway, reproductive strategy, or by growth habit. Transferring continuous trait data into categorical data also presents a difficulty, since it requires a degree of subjectivity in making what is sometimes an arbitrary decision. Furthermore, there seems to be no consensus on the best way to measure a community’s functional diversity, as represented by differences in function among the species involved (Mouillot et al. 2005, Ricotta 2005). This is why there has been much focus lately on using a trait-based approach for investigating community assembly (Keddy 1992, McGill et al. 2006, Shipley et al. 2006).
Many ecologists are now asserting that assembly rules will be generalizable only if based on traits, and that species-based assembly rules can only be site-specific and they rely on unsure taxonomy (Weiher and Keddy 1995). Trait-based approaches to community assembly utilize quantifiable measurements of traits that are linked to performance rather than grouping species into named categories. This approach provides a more mechanistic look into assembly processes.
Functional traits can pertain to physiology, morphology, or life-history characteristics, but it is important that they strongly affect the performance of the organism. In the case of plant assemblages, these traits could be related to resource acquisition (i.e. rooting depth, tissue stoichiometry), photosynthetic rates (i.e. CO2 intake per leaf dry mass), or reproductive strategy (i.e. seed production, tillering rates), among other things. In this way, traits can act as a common currency among species, allowing comparisons across communities in which the taxa are not necessarily shared (McGill et al. 2006, Shipley et al. 2006). These species traits are then related to a given performance currency, such as growth rate (r2) or productivity. Because it is assumed that trait differences alter species performance in communities, this approach is inherently in opposition to neutral theory.
Trait-based approaches are most useful when studied across environmental gradients, as they help to determine which traits correspond most strongly to given environmental factors, and therefore can provide more general insight to community assembly under varying abiotic conditions (McGill et al. 2006). However, while many traits may contribute to performance, it is unrealistic to measure all of them. It is necessary for researchers to select the most important contributors to performance based on the gradient of interest. This can also be accomplished by reducing a suite of traits that represent a given tradeoff into a single axis of variation. A prime example of this would be the leaf economics spectrum, in which several different leaf traits correspond to a tradeoff between conservative growth strategies that limit tissue loss, and extravagant growth strategies that maximize resource acquisition (Wright et al. 2004).
One key assumption of trait-based approaches is that trait variation within a species is less than trait variation among species. While this assumption is generally safe to make, intraspecific differences can account for a significant amount of variation between communities as well (Jung et al. 2010). Intraspecific varitation can drive both convergence due to habitat filtering and divergence through niche differentiation. Including intraspecific trait variation in models can help detect and illuminate these processes (Jung et al. 2010).
As emphasized by McGill et al. in their 2006 paper, trait-based approaches to community assembly allow researchers to ask a modified suite of research questions compared with traditional species-based approaches. Instead of looking at species diversity, we can look at variation in traits between and within communities; instead of asking what environments species occur in, we can look at what traits are important in determining the range of environmental conditions in which a species can survive; instead of looking at pair-wise competition between species, we can look at traits that confer competitive dominance (McGill et al. 2006). These questions provide a framework with which to tackle the unenviable task of finding general assembly rules for communities.
How do grassland plant communities respond to grazing?
Plants employ two main strategies to cope with grazing: defense (avoidance) and regrowth (tolerance), and the preferred strategy may be related to site resource environment, grazing intensity, or history of grazing (Vandermeijden et al. 1988). Avoidance strategies, which serve to make plants less palatable, are conservative growth strategies that often indicate that tissue replacement is expensive (Herms and Mattson 1992). This strategy is characterized by traits including higher stem to leaf ratios, short stature, physical defenses such as spines or thorns, thick and/or waxy leaves, and presence of recalcitrant or toxic compounds in tissues. Tolerance strategies involve rapid regrowth from defoliation and are characterized by allocation to resource acquisition traits. These include higher nitrogen (N) content in leaf tissues, higher specific leaf area (SLA), and more allocation to root growth (Caldwell et al. 1981).
Many recent studies have used trait-based approaches to investigate how grassland plant assemblages respond to grazing by assessing these community traits corresponding to grazing avoidance and tolerance strategies (Diaz et al. 2007). Trait based approaches in measuring plant response to grazing have been found to be more informative than simply looking at species responses (Pakeman 2004). This is particularly relevant because communities that favor avoidance strategies tend to have low palatability and forage quality, whereas tolerance strategies often result in higher forage quality. However, it is not well understood under what conditions each strategy is favored, and how grazing can cause negative or positive feedbacks in terms of forage quality. Here I will synthesize results from recent studies that have investigated plant traits associated with these two strategies in response to grazing. I will specifically look at how these responses vary across gradients of resource environment, grazing intensity, and evolutionary history of grazing. I will also discuss how herbivore selectivity corresponds to these traits. The traits I have selected to focus on are: height, SLA, tissue N content, leaf toughness, and root:shoot ratio.
Many studies have shown that the resource environment and climate can mediate the plant response to grazing. This may be because environmental filtering from the regional species pool selects for a limited set of species that are more suited to certain grazing responses (De Bello et al. 2005). These responses have been particularly linked to annual precipitation and site fertility. Plants in arid environments have been found to exhibit more avoidance traits, such as short height and tougher leaves, whereas in more mesic sites have been found to exhibit more tolerance traits such as higher nitrogen (N) content in leaf tissues, indicating faster growth (Adler et al. 2004, Zheng et al. 2011). Similar results were found on soil fertility gradients (Rusch et al. 2009), indicating that low-resource environments and grazing avoidance may induce convergent traits. However, this response has been shown to differ over larger spatial and temporal scales (Sandel et al. 2010).
Plant strategy may also be contingent on grazing intensity. Studies have found that higher stocking rates are correlated with decreases in plant height and palatability (Pakeman 2004), with more allocation to defenses, and decreased allocation to reproduction and vegetative growth (De Miguel et al. 2010), as well as increases in root: shoot ratio (Evju et al. 2009). These studies indicate that at higher intensities of grazing, avoidance strategies may be most favored.
However, results from another study which held different levels of grazing intensities constant for a period of 15 years show the opposite to be the case (Cruz et al. 2010). In this study, higher grazing intensity was associated with high SLA, indicating that plants allocating to resource-acquisition and fast growth rates were favored. This discrepancy may be explained by taking into account intraspecific trait variation. In one such study, as grazing intensity increased, palatable species showed less allocation to resource acquisition, while unpalatable species increased in abundance as well as resource acquisition traits (Chen et al. 2005). This could lead to an increase in community-level acquisition traits (associated with tolerance) while the dominant strategy is still avoidance. Again, the dominant community response may depend on environmental filters on the regional species pool, and may interact with climatic factors (Zheng et al. 2011).
Regions that have a long evolutionary history of grazing may contain species that have had time to evolve better defenses (Diaz et al. 2007). This was found to be the case in a study comparing Patagonian grasslands to the sagebrush steppe of the western United States (Adler et al. 2004). They found that plant communities in Patagonia, which is known to have a longer history of grazing, tended to have lower forage quality (indicating defenses), while the sagebrush steppe communities tended to have taller stature plants. Decreased plant height seems to be a universal response to grazing, irrespective of climate variation (Diaz et al. 2007), which may be an indication of increased allocation to roots (Evju et al. 2009). Although belowground responses have not been well-studied (May et al. 2009). This study indicates that plant community traits such as height could be used as a proxy for evolutionary history of grazing.
These comparisons are based on the assumption that avoidance strategies are actually effective. Although it is commonly thought that reduced height is a grazing avoidance strategy (Coley et al. 1985), trait-based studies are increasingly finding that short stature may not be an indication of grazing avoidance. Diaz et al. (2001) found that species that respond positively to grazing tend to be shorter compared to species that respond negatively to grazing (Diaz et al. 2001). A study examining herbivore selectivity found that grazers actually seemed to preferentially eat shorter plants, as well as those with tougher leaves (Cingolani et al. 2005).
While SLA, which is an indicator of growth rate of the plant, was not correlated with herbivore selectivity at a species level, on a community level sheep tended to select for higher SLA communities (Cingolani et al. 2005). Also, plants with higher SLA (faster growing) tend to respond positively when grazed, compared with those with low SLA (Diaz et al. 2001, Cingolani et al. 2005). These studies found that grazing tends to reduce height, increase SLA, and increase plants preferred by sheep, indicating a positive feedback between grazing and forage quality, as indicated by both SLA and sheep preference. This is consistent with other studies indicating that sheep prefer plants with higher nutrition quality, but that selectivity was not a strong driver of the community response to grazing (Evju et al. 2009).
Although trait-based approaches to questions of community assembly may not be the answer to finding general laws in ecology, they provide an approachable framework for illuminating contingencies that hold true across disparate systems. In this paper I have demonstrated how trait-based approaches have been used to address the question of what conditions cause grazing to induce tolerance versus avoidance strategies in plant assemblages. This is a complex question that has not been usefully addressed in species-based studies. Results from the studies I have discussed may be generalizable across systems, but they are still highly contingent on site resource environment, history of grazing, and grazing intensity. Other factors such as herbivore selectivity, seasonality, scale, and regional species pool may also prove to be important contingencies that should be investigated further.
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