99 Ecology, 81(1), 2000, pp. 99���109 q 2000 by the Ecological Society of America A TEST OF THE EFFECTS OF FUNCTIONAL GROUP RICHNESS AND COMPOSITION ON GRASSLAND INVASIBILITY AMY J. SYMSTAD1 Department of Ecology, Evolution and Behavior, 1987 Upper Buford Circle, University of Minnesota, St. Paul, Minnesota 55108 USA Abstract. Although many theoretical and observational studies suggest that diverse systems are more resistant to invasion by novel species than are less diverse systems, experimental data are uncommon. In this experiment, I manipulated the functional group richness and composition of a grassland community to test two related hypotheses: (1) Diversity and invasion resistance are positively related through diversity���s effects on the resources necessary for invading plants��� growth. (2) Plant communities resist invasion by species in functional groups already present in the community. To test these hypotheses, I removed plant functional groups (forbs, C3 graminoids, and C4 graminoids) from existing grassland vegetation to create communities that contained all possible combinations of one, two, or three functional groups. After three years of growth, I added seeds of 16 different native prairie species (legumes, nonleguminous forbs, C3 graminoids, and C4 graminoids) to a 1 3 1 m portion of each 4 3 8 m plot. Overall invasion success was negatively related to resident functional group richness, but there was only weak evidence that resident species repelled functionally similar invaders. A weak effect of functional group richness on some resources did not explain the significant diversity���invasibility relationship. Other factors, particularly the different responses of resident functional groups to the initial disturbance of the experimental manipulation, seem to have been more important to community in- vasibility. Key words: Cedar Creek Natural History Area, Minnesota, USA community assembly community composition disturbance diversity ecosystem properties grassland invasion plant functional groups. INTRODUCTION The long-distance dispersal of species via human activities is a major component of global change (Vi- tousek 1994) and can profoundly affect populations, communities, and ecosystems (Mooney and Drake 1986, Drake et al. 1989, D���Antonio and Vitousek 1992, Vitousek et al. 1997). As a result, considerable work has been done to understand the properties of species that determine their invasive potential (e.g., Newsome and Noble 1986, Rejma ��nek and Richardson 1996, Rei- chard and Hamilton 1997) and the properties of com- munities that determine their resistance to invasion (e.g., Elton 1958, Robinson and Dickerson 1984, Fox and Fox 1986, Robinson et al. 1995, Tilman 1997). Much of this work has been based on computer mod- eling (Case 1990, 1991), correlational analyses (e.g., MacDonald and Frame 1988, Planty-Tabacchi et al. 1996, Wiser et al. 1998), or microcosm studies (Rob- inson and Dickerson 1984, McGrady-Steed et al. 1997). Studies investigating community assembly rules (sensu Diamond 1975 e.g., Drake et al. 1993, Wilson and Roxburgh 1994, Law and Morton 1996) have also con- Manuscript received 30 January 1998 revised 4 January 1999 accepted 6 January 1999. 1 Present address: Illinois Natural History Survey, Savanna Field Station, P.O. Box 241, Savanna, Illinois 61074 USA. E-mail: amysym@internetni.com tributed to the debate about which, if any, traits char- acterize the relationships between invasible commu- nities and the species that invade them. Direct, exper- imental tests of hypotheses concerning such traits are necessary to resolve this debate. One commonly cited characteristic of invasible com- munities is that they have a low diversity of resident species (Elton 1958, Lodge 1993). Various causes of the relationships between diversity and invasibility have been suggested but not experimentally tested. El- ton (1958) suggested that greater community diversity caused greater invasion resistance and explained it through the concept of ������ecological resistance,������ par- ticularly for plant communities. According to this con- cept, competition for the resources required by all plants is greater in diverse plant communities compared to communities with fewer species. This more intense competition for resources tends to prevent newly in- troduced species from becoming established in the spe- cies-rich communities. The mechanisms for this hypothetical competitive resistance have only recently been explored in exper- iments (Naeem et al. 1994, 1995 Tilman et al. 1996, 1997a Hooper and Vitousek 1997, Hooper 1998) and theory (Tilman et al. 1997b, Loreau 1998) investigating the relationship between biodiversity and ecosystem functioning. Recent experiments have shown negative

100 Ecology, Vol. 81, No. 1 AMY J. SYMSTAD relationships between plant species or functional group richness and resource availability (Naeem et al. 1994, 1995 Tilman et al. 1996, 1997a Hooper and Vitousek 1997, Hooper 1998). One hypothesized mechanism for these relationships is that diverse communities have a greater variety of methods for capturing resources than do simple communities (Naeem et al. 1994, Tilman et al. 1997b). Another possible mechanism for the neg- ative correlation between diversity and resource avail- ability is that the probability of having the most highly competitive species for a given resource increases as community diversity increases (Tilman et al. 1997b). In this case, the composition of a community is im- portant because of the influence of individual species on resources (Tilman 1982, Wedin and Tilman 1990, Naeem et al. 1996, Symstad et al. 1998). Extending either of these hypotheses into invasion theory suggests that greater resident diversity confers resistance to in- vasion due to lower resource availability for an invad- ing species. Resource availability within a given eco- system, however, is affected by factors other than plant diversity. Disturbance in particular can temporarily un- hinge the relationship between resident plant species and ecosystem properties, thus potentially weakening the relationship between community diversity and in- vasibility. Associated with the diversity���invasion hypothesis is another commonly cited characteristic of invasible communities they lack species that are ecologically similar to the invader (Mooney and Drake 1989, Lodge 1993). This notion of niche limitation is a central tenet of community assembly theory (e.g., Wilson 1995 and references therein) and is related to Elton���s ������ecological resistance,������ in that if two species��� ecological charac- teristics are too similar, one may competitively exclude the other (Case 1990, 1991, Pacala and Tilman 1994). Even if the invader is the superior competitor, the res- ident community may ������repel������ the invader because of the priority effect that established residents have over invaders (Case 1990, 1991). Because functional defi- nitions of plants (sensu Vitousek and Hooper 1993) are often based on similarities in the manner in which spe- cies use and compete for resources, the invasion re- sistance of a plant community may depend on the func- tional group composition of the community and the functional type of the potential invader. In this paper, I report on an experimental test of the effects of two aspects of plant functional group diver- sity, the number of functional groups (richness) and the types of functional groups (composition), on the invasibility of a grassland. I also examine the possi- bility of ������ecological resistance������ as the mechanism through which diversity affects invasion resistance. Specifically, I asked three questions: (1) Are commu- nities with more functional groups more resistant to invasion? (2) If functionally diverse communities are more resistant to invasion than simple communities, does variation in ecosystem functioning account for this relationship? (3) Do species more readily invade communities that lack species similar to them? I ad- dressed these questions by experimentally manipulat- ing plant functional group richness and composition in a series of plots for three years, after which I added a mixture of species to them as seed and measured their success for the following two growing seasons. For brevity, the species added as seed will henceforth be referred to as ������invaders.������ METHODS Field site This experiment was conducted at Cedar Creek Nat- ural History Area, which lies on a glacial outwash sand plain in east-central Minnesota. The soils are nutrient poor and nitrogen limited (Tilman 1987). The experi- mental plots were in an old field last cultivated in 1934 and now dominated by the prairie species Schizachyr- ium scoparium (25% of plant cover), Ambrosia psilos- tachya (15%), Poa pratensis (12%), Helianthus pau- ciflorus (7%), Solidago nemoralis (7%), and Artemisia ludoviciana (5%). (Nomenclature follows Gleason and Cronquist [1991].) Although the research site was an old field, previous work has shown that the vegetation at this site is fairly stable and nonsuccessional (Tilman 1987). Experimental design The three functional groups defined for this exper- iment, C3 graminoids (including grasses and sedges), C4 graminoids (including grasses and sedges), and forbs, comprised .99.9% of the biomass in the ex- perimental field. Legumes were not distinguished as a separate group because they were uncommon (,1% of plant cover). I classified the species into functional groups based on their phenology and morphology, as- suming that these characteristics are also related to tem- poral and spatial patterns in nutrient use. C3 grami- noids, mainly Poa pratensis, Panicum oligosanthes, and Elytrigia repens, grow primarily during the cool part of the growing season (spring thaw to mid-June, and September to snow cover), set seed by early sum- mer, and tend to be shallow rooted. C4 graminoids, mainly Schizachyrium scoparium and Sorghastrum nu- tans, are warm-season plants, generally growing from June through August and setting seed in August and September. Although all forbs in this experiment have the C3 photosynthetic pathway, they tend to differ from the graminoids in their growth form, rooting depth, and allocation to seed. During the summer of 1993, nine treatments were applied in a completely randomized fashion to 4 3 8 m plots. The treatments consisted of all possible com- binations of zero, one, or two plant functional groups removed at a time, plus two extra control treatments in which 25% and 55% of the biomass was removed. These extra controls were included in order to separate