Community Invasibility Spatial Heterogeneity, Spatial Scale, and Productivity

Kendi F. Davies, University of Colorado, Boulder

Introduction: Why Are Some Communities More Invasible than Others? Invasive species are one of the most significant threats to native species diversity, and identifying the factors that make places more or less invasible has been one of the most important issues in the study of invasions (Wilcove et al., 1998; Pimentel et al., 2000). From a theoretical perspective, the reasons some communities are more invasible than others is a question that continues to intrigue ecologists (Levine et al., 2004; Davies et al., 2005; Fridley et al., 2007; Stohlgren et al., 2008; Cadotte et al., 2009) because it underlies fundamental concepts in community ecology: species coexistence and assembly (Chesson, 2000; Tilman, 2004). Therefore, its explo- ration offers insights into why communities are structured as they are. Serpentine systems have provided significant insight into the reasons some communities are more invasible than others because the environment within these systems is often extreme. Spatial heterogeneity, spatial scale, and productivity have all proven to be critical elements in understanding the invasibility of communities. Serpentine systems have allowed researchers to examine these elements in greater depth. First, serpentine systems are very spatially heterogeneous in soil chemistry, texture, rockiness, and toxicity. Importantly, this spatial heterogeneity can exist at very small scales, making it easier to examine the effects of it on communities. Second, because spatial heterogeneity can be at both small scales and large scales,

Serpentine: The Evolution and Ecology of a Model System, edited by Susan Harrison and Nishanta Rajakaruna. Copyright  by The Regents of the University of California. All rights of reproduction in any form reserved.

237 238 serpentine as a model in ecology serpentine systems have allowed researchers to uncover the importance of processes operating at both small (local) and large (regional) scales and how these processes relate to spatial heterogeneity. Finally, serpentine systems can range from very harsh and unproductive to quite productive. Thus, they provide an un- usually wide range of productivity, uncovering processes that might not be possible to see in other systems. In what follows, I take each of these elements—spatial heterogeneity, scale, and productivity—and contrast the contributions of studies in serpentine systems compared with studies in systems that are not serpentine.

Spatial Scale, Spatial Heterogeneity, and the Invasibility of Communities Elton (1958) first proposed that a high richness of native species armors sites against invasion by making resources less available to newly arriving species. This idea has been supported by many empirical studies that detected negative relationships between native and exotic diversity at small spatial scales—the scale of interaction between individuals (Elton, 1958; Turelli, 1981; Case, 1990; Tilman, 1997; Knops et al., 1999; Stachowicz et al., 1999; Levine, 2000; Naeem et al., 2000; Lyons and Schwartz, 2001; Brown and Peet, 2003; Levine et al., 2004). Thus, the idea that competitive exclusion (Grime, 1973; Tilman, 1999) is in place at small scales, leading to resistance to invasion, is well established and has been confirmed experimentally: a recent meta-analysis of biotic resistance to invasion, incorporating 65 experiments from 24 different studies, revealed that competition from resident species has strong and significant effects on both establishment and performance of invaders (Levine et al., 2004). In the late 1990s, Stohlgren et al. (1999) and Lonsdale (1999) showed that native and exotic diversity could be positively correlated when scales larger than that of local interaction were considered. Their results first raised the possibility that the processes determining invasibility and its relationship to diversity depend on spatial scale. Since then, many other studies have reported positive relationships between native and exotic diversity at large scales and furthermore that the most diverse regions are also often the most invaded, particularly for plant communities (Levine, 2000; Davies et al., 2005, 2007; Richardson et al., 2005; Stohlgren et al., 2005; Harrison et al., 2006). Since the studies of Stohlgren et al. (1999) and others, the potential for scale dependence of the native–exotic relationship has stimulated much research, and studies have reported both negative relationships at small scales and positive relationships at large scales within the same system (Levine, 2000; Brown and Peet, 2003; Davies et al., 2005; Knight and Reich, 2005). Why native and exotic diversity should be positively correlated at large scales so that diverse regions are also the most invaded regions has puzzled ecologists. Two community invasibility 239 hypotheses could explain the positive correlation of native and exotic diversity at large scales. First, the environmental favorability hypothesis suggests that sites or landscapes with favorable environmental conditions for native species also have favorable conditions for exotic species (Levine and D’Antonio, 1999; Stohlgren et al., 1999; Levine, 2000; Brown and Peet, 2003). In other words, factors such as soil fertility or propagule supply rates vary between sites and cause between-site variation in both native and exotic diversity. Shea and Chesson (2002) generalized this idea and showed how a positive relationship at large spatial scales can arise by combining data from a series of negative relationships at smaller scales, where differences in diversity at larger scales were caused by environmental differences in the mean conditions between sites. However, their model only accounts for patterns in the mean diversity (alpha diversity) rather than the cumulative diversity (gamma diversity) of communities (Davies et al., 2005). Second, the environmental heterogeneity hypothesis suggests that not only variation in mean conditions between sites (as in the environmental favorability hypothesis) but also heterogeneity of conditions within sites can contribute to the positive relationship of native and exotic diversity at large scales. What causes the relationship between native and exotic diversity to change from negative at small scales to positive at large scales? Davies et al. (2005) suggested, in line with evidence just discussed, that negative relationships tend to be detected at scales at which the environment and resources are relatively homoge- neous and classic niche partitioning (and competitive exclusion) dominate (Grime, 1973; Tilman, 1999). They suggested that the relationship between native and exotic diversity becomes positive at scales at which spatial heterogeneity in the environment is such that coexistence mechanisms that depend on heterogeneity be- come dominant, resulting in communities that could be considered unsaturated. A comprehensive review of the diversity-invasibility paradox (Fridley et al., 2007) came up with a similar framework built on the shift from biotic to abiotic drivers as scale increases. Eight processes that could generate either negative or positive relationships were identified but all could be fitted within Fridley and colleagues’ (2007) framework. Recently, Stohlgren et al. (2008) suggested that invasion, rather than diminish- ing the diversity of native species in a community, has the overall effect of increasing the diversity of communities by adding exotic species. They based their con- clusion on evidence that communities are unsaturated at large spatial scales, and, at least for plants, the lack of known species extinctions driven by competitive interactions. Consequently, we should generally find positive relationships between native and exotic diversity at large scales. Stohlgren and colleagues (2008) suggest that ultimately competition appears to play no role in structuring communities at large scales, even if it is important in structuring communities at small scales. With further investigation, however, this final point is proving to be incorrect (see 240 serpentine as a model in ecology also Harrison, 2008). Recent evidence from phylogenetic relatedness data suggests that competition still structures communities at large scales, in particular in determining which species can be present in a community, despite positive relationships between native and exotic diversity (Davies et al., submitted). I expand on the evidence from phylogenetic studies shortly.

Insights from Serpentine Systems In a nested data set for grassland plants in Californian serpentine grasslands in the University of California McLaughlin Reserve, Davies et al. (2005) detected negative relationships between native and exotic diversity at small scales and positive relationships at large scales. Furthermore, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial heterogeneity hypothesis but not the favorable conditions hypothesis. Thus, Davies et al. (2005) demonstrated that the observed relationship between native and exotic diversity flipped from negative to positive at scales at which spatial heterogeneity in the environment came into play (correlated with native and exotic diversity and beta diversity). These serpentine studies were able to show that spatial heterogeneity in species composition (beta diversity) and spatial environmental heterogeneity within metacommunities drove the positive relationship between native and exotic diversity at large scales (supporting the environmental heterogeneity hypothesis), rather than differences in mean (extrinsic) conditions between metacommunities (environmental favorability hypothesis). These observations are consistent with invasion and coexistence theories in heterogeneous environments. Habitat heterogeneity increases the number of both native and exotic species in metacommunities by allowing more species to invade while placing the resident native species at lower risk of extinction because of the presence of more niche opportunities for both natives and exotics in the presence of heterogeneity (Shea and Chesson, 2002; Pauchard and Shea, 2006). Recently, Davies and colleagues (submitted) further tested these hypotheses using the phylogenetic relatedness of natives and exotics in this same system. They tested two hypotheses. First, if competitive exclusion and classic niche partitioning dominates, successful invaders should have niches that are different from those of the natives present. Thus, native and exotic species should be more distantly related than expected from a random assemblage model. Second, given the role of coexistence mechanisms dependent on environmental heterogeneity, native and exotic species should not be more distantly related than expected, and may be more closely related than expected if the environment filters membership of communities at large scales. community invasibility 241

Davies et al. (submitted) found strong support for the first hypothesis, provid- ing further evidence that competitive exclusion dominates at small scales. How- ever, the second hypothesis was rejected: native and exotic species were more distantly related at large scales than expected. However, importantly, natives and exotics were significantly more distantly related at small scales than they were at large scales. These results suggest that as we transition from small to large scales, the effect of biotic resistance exclusion is relaxed but still present. In other words, although communities were saturated at small scales, they were not saturated at larger scales: more species could enter the community, thus increasing species richness. However, species did not invade indiscriminately at large scales; which species successfully invaded was determined in part by competitive exclusion. At large scales, communities were still resistant to invasion by particular invaders. Exotic species that were closely related to the species already established in the community were excluded. The extreme and small scale of heterogeneity in abiotic conditions was critical for the foregoing insights. In particular variability in soil depth, soil nitrogen and aspect were important. Typical of serpentine systems, the underlying bedrock and the rockiness of the soil itself was very heterogeneous in space and contrasted strongly with the more “normal” soils that intersperse the serpentine soils. Simi- larly, the microtopography was very heterogeneous in space so that many micro- aspects were represented within a small area. Because of the mixture of serpentine and nonserpentine soils, the range of soil nitrogen was large. These extremes in heterogeneity likely made the effects of heterogeneity easier to detect. Similarly, the small scale of heterogeneity likely made it possible to contrast the effects of small and large spatial scales over a relatively small (and therefore intensely stud- ied) area.

Productivity/Harshness, Scale, and the Invasibility of Communities Site productivity or harshness has also been used to try to understand why communities differ in their invasibility. Harsh conditions can alter the invasibility of communities (Cleland et al., 2004; Beisner et al., 2006; Davies et al., 2007; Perel- man et al., 2007). I define harsh conditions to mean those that result from physi- ological or resource stress, resulting in low population densities (Chesson and Huntly, 1997). Although harshness alone cannot lead to lack of saturation (Ches- son and Huntly, 1997), it could reduce saturation by interacting with other processes. Then the invasibility of communities may vary with productivity, even at the small spatial scales where competitive exclusion is expected to be the dominant mechanism. These ideas can essentially be traced back toE lton’s (1958) hypothesis 242 serpentine as a model in ecology that increased diversity should lead to increased resource capture by the community but leave fewer resources available to potential invaders (e.g., Tilman, 1982). Three hypotheses explain how productivity interacts with community invasibility: the environmental heterogeneity, environmental favorability, and the facilitation hypothesis. The environmental heterogeneity hypothesis proposes that harsh sites are more internally heterogeneous at small scales than productive sites are, so that in harsh sites, local scale plots with greater heterogeneity have more niches for both natives and exotics, leading to coexistence and a positive relationship between native and exotic diversity (Davies et al., 2005; Melbourne et al., 2007). In comparison, at productive sites, homogeneity of resources would lead to competitive exclusion of exotics by natives and a negative relationship. First, harsh sites might simply have greater environmental heterogeneity than productive sites at small scales. Sources of heterogeneity within sites could include rockiness, soil depth, and soil composition. A second possibility, suggested by Tilman (1988, 1987), is that competition occurs predominantly above ground at productive sites, where species compete for light, a single limiting resource, and shifts below ground at unproductive sites, where species compete for below-ground resources that tend to be more heterogeneous (e.g., Wilson and Tilman, 1991, 1993, 1995). Both possibilities lead to positive relationships between native and exotic diversity in harsh sites, even at very small spatial scales—the scale of interaction between individuals. The environmental favorability hypothesis covers coexistence mechanisms that depend on mean conditions rather than heterogeneity in conditions (as already discussed). This hypothesis suggests that native and exotic diversity are positively correlated at large spatial scales because sites with favorable conditions for natives also have favorable conditions for exotics. Although this hypothesis was developed to explain large-scale positive relationships, it can also be applied at small spatial scales if communities are unsaturated so that there are weaker effects of resident species on the ability of new species to invade. This results in positive relationships between native and exotic diversity even at small spatial scales. In the context of community invasibility and its relationship to productivity, most research examines the environmental favorability hypothesis, and researchers have found evidence to support it. An analysis of four long-term observational data sets for herb communities from long-term ecological research sites found that invaders were less likely to establish in high-productivity sites. The probability of invasion was reduced both within and following years with high productivity, except at a desert grassland site where high productivity was associated with increased invasion (Cleland et al., 2004). In harsh years, community saturation was reduced, changing the dynamic from one of competitive exclusion of invaders to coexistence. Finally, in Flooding Pampas of Argentina, sites that were harsher (more saline, shallower soils) were less invaded, but native and exotic diversity community invasibility 243 were still positively correlated across sites at large spatial scales (Perelman et al., 2007). Thus, results are somewhat mixed. These studies focus on how diversity and invasibility are related at large scales, but to truly understand the effect of productivity, we need to study its effects at small scales, the scale of local species interaction. This has usually been done only in the context of an experimental manipula- tion that looks at a single invader. For example, in Jamaican rock pools, high productivity (through experimentally boosted resources) increased the success rate of establishment of an invasive crustacean (Beisner et al., 2006). Local-scale, community-level analyses are needed. The third hypothesis concerns facilitation, which can be more likely to occur at harsh, unproductive sites (Bertness and Callaway, 1994; Callaway, 1998; Choler et al., 2001; Elmendorf and Moore, 2007). Theory connecting facilitation to community invasibility is undeveloped, so it is unclear how facilitation would be expected to alter the diversity–invasibility relationship in a metacommunity context (but for invader facilitating invader. see Simberloff, 1999, 2006).H owever, facilita- tive interactions may be more common in general and more common than competitive interactions under harsh conditions resulting in different patterns of invasibility of communities at harsh and unproductive sites. This area warrants further investigation.

Insights from Serpentine Systems Davies et al. (2007) examined California statewide scale data set for serpentine herbs, composed of 109 sites. They investigated the native–exotic diversity relationship at three scales (range: 1 m2–4000 km2) in a system with a wide range in the productivity of sites—communities were established in conditions that ranged from very harsh to relatively productive. Native and exotic diversity were positively correlated at all spatial scales, even the smallest ones, at which individuals were expected to interact directly (1 m2). Positive relationships at large scales are expected and common in empirical studies; however, detecting a positive relationship at the small scales of interaction between individuals is rare. In contrast, results from the statewide data set suggest that sites were not saturated, even at small scales (1 m2). Davies et al. (2007) predicted that along with spatial scale, site productivity likely affected the invasibility of communities and thus the relationship between native and exotic diversity, especially at small scales, where competitive exclusion potentially varied with site productivity. They found that, at small-scale (1 2m ), high-productivity sites, native and exotic diversity tended to be negatively correlated, whereas at unproductive sites, native and exotic diversity tended to be positively correlated, resulting in a significant relationship between the slope of the native exotic relationship and productivity. Also, because the majority of sites were less productive, the average relationship at small spatial scales for all sites considered together was positive. Furthermore, when productive and unproductive sites 244 serpentine as a model in ecology were considered separately, productive sites had a common positive relationship between native and exotic diversity, whereas unproductive sites had a common negative relationship. Like most other studies concerning community invasibility and productivity, the environmental favorability hypothesis, likely explains Davies et al.’s (2007) finding.R ecall that the environmental favorability hypothesis incorporates coexistence mechanisms that depend on mean conditions rather than heterogeneity in conditions. Davies and colleagues hypothesized that in their serpentine system, the majority of sites, except for the small subset with the highest herb cover, are probably unsaturated so that there are weaker effects of resident species on the ability of new species to invade. The positive effects of shared responses to the environment (i.e., shared tolerances) more easily overwhelm the negative effects of competition, resulting in a small-scale positive relationship between native and exotic diversity at harsh sites. For example, the availability of critical nutrients may vary between 1 m2 quadrats so that quadrats with better conditions harbor both more natives and exotics. In contrast, at productive sites, competitive exclusion results in negative relationships between native and exotic diversity. The environmental heterogeneity hypothesis did not explain the observed relationships between native and exotic diversity because if heterogeneity was behind the positive relationships detected, the presence of greater heterogeneity at unproductive sites should have resulted in greater diversity of both native and exotic species at harsh sites, whereas the opposite was the case. The least productive sites also had the fewest species, making heterogeneity-dependent coexistence an un- likely driver of the change in slope between productive and less productive sites at small spatial scales. Finally, Davies et al. (2007) could not discount the facilitation hypothesis, but neither did they have evidence to support it. The presence of facili- tative (rather than competitive) local interactions at harsh compared to benign sites could have resulted in the relationships that were observed. Facilitation is more likely to occur at harsh, unproductive sites (Bertness and Callaway, 1994), but because theory connecting facilitation to community invasibility is undeveloped, it is not clear how facilitation would alter community invasibility in a metacommunity context. Critically, these findings illustrate that the mechanisms that determine the invasibility of communities, in this case measured as the change in slope of the relationship between native and exotic diversity, do not depend on spatial scale per se but can change whenever or wherever environmental conditions change to pro- mote species coexistence rather than competitive exclusion. In the 2007 serpentine study, this occurred within a single small spatial scale when the environment shifted from being locally productive (competitive exclusion) to unproductive (coexistence). It is very likely that the extreme range of productivity values at sites in this serpentine study system was critical to this discovery. community invasibility 245

Synthesis Three unique abiotic features of serpentine systems make them superior models for studying community invasibility. First, the environment can be heterogeneous at very small spatial scales, making it easier to measure the effects of spatial heterogeneity on community processes. It can be easier to set up strategies that sample large amounts of heterogeneity. It can also be easier to sample at multiple spatial scales that incorporate significant spatial heterogeneity in the environment (even small scales). For example, at the McLaughlin Reserve grid, variables like soil depth and soil nutrient concentrations, which turned out to be critical to community structure and invasibility, can vary significantly within 1–2 m. Second, not only can the environment be variable at very small spatial scales, the range of values encountered within any given environmental variable may be more extreme than in other systems, making it easier to detect the effects of spatial heterogeneity on community invasibility. For example, the range of soil nitrogen values between serpentine and interspersed nonserpentine soils can be large. Sim- ilarly, microtopography may be very heterogeneous in space so that many “micro” aspects are represented within a small area—the range of aspects present at small scales is larger than in other systems. Third, following from the last point, productivity can vary widely within serpentine systems from sites that are barren and rocky to sites that are productive. Again, this provides researchers with better opportunities for determining the importance of site productivity on community structure and community invasibility. A study system with a smaller range of productivity values may not have allowed researchers to detect the trends described herein.

Literature Cited Beisner, B. E., Hovius, J., Hayward, A., Kolasa, J., and Romanuk, T. N. (2006) Environmental productivity and biodiversity effects on invertebrate community invasibility. Biological Invasions, 8, 655–64. Bertness, M. D., and Callaway, R. (1994) Positive interactions in communities. Trends in Ecology and Evolution, 9, 191–93. Brown, R. L., and Peet, R. K. (2003) Diversity and invasibility of southern Appalachian plant communities. Ecology, 84, 32–39. Cadotte, M. W., Hamilton, M. A., and Murray, B. R. (2009) Phylogenetic relatedness and plant invader success across two spatial scales. Diversity and Distributions, 15, 481–88. Callaway, R. M. (1998) Competition and facilitation on elevation gradients in subalpine forests of the northern Rocky Mountains, USA. Oikos, 82, 561–73. Case, T. J. (1990) Invasion resistance arises in strongly interacting species-rich model competition communities. Proceedings of the National Academy of Sciences, USA, 87, 9610–14. 246 serpentine as a model in ecology

Chesson, P. (2000) Mechanisms of maintenance of species diversity. Annual Review of Ecol- ogy and Systematics, 31, 343–66. Chesson, P., and Huntly, N. (1997) The roles of harsh and fluctuating conditions in the dynamics of ecological communities. American Naturalist, 150, 519–53. Choler, P., Michalet, R., and Callaway, R. M. (2001) Facilitation and competition on gradients in alpine plant communities. Ecology, 82, 3295–308. Cleland, E. E., Smith, M. D., Andelman, S. J., Bowles, C., Carney, K. M., Horner-Devine, M. C., Drake, J. M., Emery, S. M., Gramling, J. M., and Vandermast, D. B. (2004) Invasion in space and time: Non-native species richness and relative abundance respond to inter- annual variation in productivity and diversity. Ecology Letters, 7, 947–7. Davies, K. F., Cavender-Bares, J., and Deacon, N. (Submitted) Native communities determine the identity of exotic invaders even at scales at which communities are unsaturated. Davies, K. F., Chesson, P., Harrison, S., Inouye, B. D., Melbourne, B. A., and Rice, K. J. (2005) Spatial heterogeneity explains the scale dependence of the native-exotic diversity relationship. Ecology, 86, 1602–10. Davies, K. F., Harrison, S., Safford,H . D., and Viers, J. H. (2007) Productivity alters the scale dependence of the diversity-invasibility relationship. Ecology, 1940–47. Elmendorf, S. C., and Moore, K. A. (2007) Plant competition varies with community composition in an edaphically complex landscape. Ecology, 88, 2640–50. Elton, C. S. (1958) The Ecology of Invasions. Methuen, London. Fridley, J. D., Stachowicz, J. J., Naeem, S., Sax, D. F., Seabloom, E. W., Smith, M. D., Stohl- gren, T. J., Tilman, D., and Von Holle, B. (2007) The invasion paradox: Reconciling pattern and process in species invasion. Ecology, 88, 3–17. Grime, J. P. (1973) Control of species density in herbaceous vegetation. Journal of Environ- mental Management, 1, 151–67. Harrison, S. (2008) Commentary on Stohlgren et al. (2008): The myth of plant species saturation. Ecology Letters, 11, 322–24. Harrison, S., Grace, J. B., Davies, K. F., Safford, H. D., and Viers, J. H. (2006) Invasion in a diversity hotspot: Exotic cover and native richness in the Californian serpentine flora. Ecology, 87, 695–703. Knight, K. S., and Reich, P. B. (2005) Opposite relationships between invasibility and native species richness at patch versus landscape scales. Oikos, 109, 81–88. Knops, J. M. H., Tilman, D., Haddad, N. M., Naeem, S., Mitchell, C. E., Haarstad, J., Ritchie, M. E., Howe, K. M., Reich, P. B., Siemann, E., and Groth, J. (1999) Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances, and diversity. Ecology Letters, 2, 286–93. Levine, J. M. (2000) Species diversity and biological invasions: Relating local process to community pattern. Science, 288, 852–54. Levine, J. M., Adler, P. B., and Yelenik, S. G. (2004) A meta-analysis of biotic resistance to exotic plant invasions. Ecology Letters, 7, 975–89. Levine, J. M., and D’Antonio, C. M. (1999) Elton revisited: A review of evidence linking diversity and invasibility. Oikos, 87, 15–26. Lonsdale, W. M. (1999) Global patterns of plant invasions and the concept of invasibility. Ecology, 80, 1522–36. community invasibility 247

Lyons, K. G., and Schwartz, M. W. (2001) Rare species loss alters ecosystem function— invasion resistance. Ecology Letters, 4, 358–65. Melbourne, B. A., Cornell, H. V., Davies, K. F., Dugaw, C. J., Elmendorf, S., Freestone, A. L., Hall, R. J., Harrison, S., Hastings, A., Holland, M., Holyoak, M., Lambrinos, J., Moore, K., and Yokomizo, H. (2007) Invasion in a heterogeneous world: Resistance, coexistence or hostile takeover? Ecology Letters, 10, 77–94. Naeem, S., Knops, J. M. H., Tilman, D., Howe, K. M., Kennedy, T., and Gale, S. (2000) Plant diversity increases resistance to invasion in the absence of covarying extrinsic factors. Oikos, 91, 97–108. Pauchard, A., and Shea, K. (2006) Integrating the study of non-native plant invasions across spatial scales. Biological Invasions, 8, 399–413. Perelman, S. B., Chaneton, E. J., Batista, W. B., Burkart, S. E., and Leon, R. J. C. (2007) Habi- tat stress, species pool size and biotic resistance influence exotic plant richness in the Flooding Pampa grasslands. Journal of Ecology, 95, 662–73. Pimentel, D., Lach, L., Zuniga, R., and Morrison, D. (2000) Environmental and economic costs of nonindigenous species in the United States. Bioscience, 50, 53–65. Richardson, D. M., Rouget, M., Ralston, S. J., Cowling, R. M., Van Rensburg, B. J., and Thu- iller, W. (2005) Species richness of alien plants in South Africa: Environmental correlates and the relationship with indigenous plant species richness. Ecoscience, 12, 391–402. Shea, K., and Chesson, P. (2002) Community ecology theory as a framework for biological invasions. Trends in Ecology and Evolution, 17, 170–76. Simberloff, D. (1999) Positive interactions of nonindigenous species: Invasional meltdown? Biological Invasions, 1, 21–32. Simberloff, D. (2006) Invasional meltdown 6 years later: Important phenomenon, unfortu- nate metaphor, or both? Ecology Letters, 9, 912–19. Stachowicz, J. J., Whitlatch, R. B., and Osman, R. W. (1999) Species diversity and invasion resistance in a marine ecosystem. Science, 286, 1577–79. Stohlgren, T. J., Barnett, D., Flather, C., Kartesz, J., and Peterjohn, B. (2005) Plant species invasions along the latitudinal gradient in the United States. Ecology, 86, 2298–309. Stohlgren, T. J., Barnett, D. T., Jarnevich, C. S., Flather, C., and Kartesz, C. (2008) The myth of plant species saturation. Ecology Letters, 11, 315–26. Stohlgren, T. J., Binkley, D., Chong, G. W., Kalkhan, M. A., Schell, L. D., Bull, K. A., Otsuki, Y., Newman, G., Bashkin, M., and Son, Y. (1999) Exotic plant species invade hot spots of native plant diversity. Ecological Monographs, 69, 25–46. Tilman, D. (1982) Resource Competition and Community Structure. Princeton University Press, Princeton, NJ. Tilman, D. (1987) On the meaning of competition and the mechanisms of competitive su- periority. Functional Ecology, 1, 304–15. Tilman, D. (1988) Plant Strategies and the Dynamics and Structure of Plant Communities. Princeton University Press, Princeton, NJ. Tilman, D. (1997) Community invasibility, recruitment limitation, and grassland biodiversity. Ecology, 78, 81–92. Tilman, D. (1999) The ecological consequences of changes in biodiversity: A search for general principles. Ecology, 80, 1455–74. 248 serpentine as a model in ecology

Tilman, D. (2004) Niche tradeoffs, neutrality, and community structure: A stochastic theory of resource competition, invasion, and community assembly. Proceedings of the Na- tional Academy of Sciences, USA, 101, 10854–61. Turelli, M. (1981) Niche overlap and invasion of competitors in random environments. 1. Models without demographic stochasticity. Theoretical Population Biology, 20, 1–56. Wilcove, D. S., Rothstein, D., Dubow, J., Phillips, A., and Losos, E. (1998) Quantifying threats to imperiled species in the United States. Bioscience, 48, 607–15. Wilson, S. D., and Tilman, D. (1991) Components of plant competition along an experimental gradient of nitrogen availability. Ecology, 72, 1050–65. Wilson, S. D., and Tilman, D. (1993) Plant competition and resource availability in response to disturbance and fertilization. Ecology, 74, 599–611. Wilson, S. D., and Tilman, D. (1995) Competitive responses of 8 old-field plant-species in 4 environments. Ecology, 76, 1169–80.