Effects of a Nonnative Species of Poaceae on Aquatic Macrophyte Community Composition: a Comparison with a Native Species
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J. Aquat. Plant Manage. 53: 191–196 Effects of a nonnative species of Poaceae on aquatic macrophyte community composition: A comparison with a native species SARA REGINA DE AMORIM, CRISTIANE AKEMI UMETSU, DOUGLAS TOLEDO, AND ANTONIO FERNANDO MONTEIRO CAMARGO* ABSTRACT INTRODUCTION Invader-dominated systems of exotic species frequently The introduction of exotic species is one of the most damage native communities, mainly because there is a shift concerning consequences of human activities that have in the competition–facilitation balance, and competition ecological and economic implications (Pimentel et al. 2001). intensifies in communities structured by facilitation. We Because of the increase in biological invasions, the effects of tested whether areas dominated by the exotic species these species on natural ecosystems have become a focal African signalgrass [Urochloa arrecta (Hack. ex T. Dur. & area of ecological studies (Naeem et al. 2000, Battaglia et al. Schinz) O. Morrone & F. Zuloaga] can affect the assemblage 2009, Quinn et al. 2011). Some exotic, invasive species have structure of aquatic plants in tropical freshwater ecosys- several detrimental impacts on native biota, such as negative tems, compared with the native species, anchored water- effects on species richness, diversity, and community hyacinth [Eichhornia azurea (Sw.) Kunth]. We predicted that composition (Hedja et al. 2009), as well as the disassembly the dominance of African signalgrass (expressed as an and alteration of community organization (Sanders et al. increase in biomass) would reduce species richness, diver- 2003). sity, and functional diversity of the macrophyte assemblag- Once established, exotic species can reproduce consis- es, when compared with anchored waterhyacinth. Species tently and sustain dominant populations (Richardson et al. cover and the occurrence of associated species were assessed 2000). Systems dominated by exotic invader species suffer in quadrats (1 m2), located in sites dominated by exotic species loss primarily because there is a shift in the species (African signalgrass), native species (anchored competition–facilitation balance, and competition inten- waterhyacinth), and without dominance. The effects of sifies in communities structured by facilitation (Richard- dominance on species richness and diversity of aquatic son et al. 2012). The dominance of invasive species can lead macrophytes were assessed through generalized linear to the exclusion of less-competitive species (Gurevitch and model and composition with detrended correspondence Padilla 2004). However, it is possible to find indicators that analysis. African signalgrass negatively affected species exotic species can facilitate native species (Rodriguez 2006) richness and Shannon diversity, whereas anchored water- by providing a habitat, serving as functional substitutes for hyacinth showed no effects on species richness and Shannon extinct taxa, and supplying desirable ecosystem functions diversity. However, native species positively affected func- (Schalaepfer et al. 2011). Thus, we compared an exotic tional diversity. Our study showed that invasive species Poaceae African signalgrass [Urochloa arrecta (Hack. ex. T. reduced the presence of rooted-submerged species, whereas Dur. & Schinz) O. Morrone & Zuloaga], which is known to native species facilitated the occurrence of rooted-sub- affect subtropical communities (Reinert et al. 2007, Pott et merged and free-submerged species. Thus, African signal- al. 2011), with a native species anchored waterhyacinth grass was able to change the composition of the macrophyte [Eichhornia azurea (Sw.) Kunth]. These two species have the assemblage and can represent a threat to native communi- same emergent/floating life form, i.e., rooted plants with ties of tropical freshwater ecosystems. vegetative parts emerging above the water’s surface, as well Key words: dominance, Eichhornia azurea, exotic species, as long floating steams. Recently, African signalgrass was generalized linear models, tropical freshwater ecosystems, recorded in the Itanha´em River basin (located in the south Urochloa arrecta. littoral of the Sa˜ o Paulo State, Brazil); this species is native to Africa and has infested tropical and subtropical zones *First author: Programa de Pos-gradua ´ ca ¸ ˜ o em Biologia Vegetal, worldwide. It is a perennial, aquatic grass with long, Universidade Estadual Paulista, Rio Claro, Sa˜ o Paulo CEP 13506-900, floating branches that can form thick mats or tussocks Brasil. Second author: Programa de Pos-gradua ´ ca ¸ ˜ oemAquicultura, Universidade Estadual Paulista, Jaboticabal, Sa˜ o Paulo CEP 14884-900, with dense, usually stoloniferous, root systems. This Brasil.Thirdauthor:ProgramadeP´os-Graduaca ¸ ˜ oemEstat´ıstica e species, recently identified as tropical signalgrass [Urochloa Experimentaca ¸ ˜ oAgronˆomica, Escola Superior de Agronomia ‘‘Luiz de subquadripara (L.) T.Q. Nguyen] (Michelan et al. 2010a; see Queiroz,’’ Piracicaba, Sa˜ o Paulo CEP 13418-900, Brasil. Fourth author: Michelan et al. 2013) has high regeneration potential, and Professor, Departamento de Ecologia, Universidade Estadual Paulista, Rio Claro, Sa˜ o Paulo CEP 13506-900, Brasil. Corresponding author’s E- recent studies have shown that this species can compete mail: [email protected]. Received for publication May 15, 2014 and in with emergent species and has negative effects on species revised form December 16, 2014. richness and functional diversity (Michelan et al. 2010b). J. Aquat. Plant Manage. 53: 2015 191 Figure 1. A map of the area studied in the Itanha´em River basin, Brazil. The circles indicate the patches that were sampled. However, this last investigation did not compare its results hypothesized that the increase in biomass from the exotic with the effects of a similar native plant and, therefore, species would negatively affect the structure of the aquatic could not be generalized. macrophyte assemblages. We predicted that the domi- Anchored waterhyacinth is a native, rooted, aquatic nance of African signalgrass (expressed as an increase in macrophyte; its long, floating stems develop a few biomass) would reduce species richness, diversity, and centimeters below the water’s surface, forming dense functional diversity of the macrophyte assemblages, when stands and ensuring additional structural complexity in compared with anchored waterhyacinth. littoral regions (Agostinho et al. 2007). There is no record that this species is considered invasive, but it seems to MATERIALS AND METHODS increase structural complexity in aquatic ecosystems and provide habitats for both animal and plant assemblages (Boschilia et al. 2008, Padial et al. 2009, Cunha et al. 2012). Data sampling Both African signalgrass and anchored waterhyacinth are Our samples were made in the Itanha´em River basin, dominant species in almost all habitats of the Itanha´em located in the south littoral of the Sao Paulo State, Brazil River basin, and that is the reason why we chose these two ˜ emergent species for this study. Thus, our study can (Figure 1). Field samples were collected in the summer of provide an excellent opportunity to show how the 2012 in the littoral zone of the rivers in three visually dominance of exotic and native species, evaluated in a different patches of aquatic macrophytes: patches dominat- gradient of biomass, affects the community composition. ed by exotic species African signalgrass (UA), patches Many studies test the effects of exotic species on the dominated by the native anchored waterhyacinth (EA), attributes of communities and rarely compare the effects and patches without dominance by either exotic or native of other species on community structure (Hulme et al. species (WD). We defined dominant quadrats as those in 2013). Moreover, there are several investigations showing which the exotic and native species had more than 50% of the effects of exotic plants on terrestrial native plants the total biomass, by visual inspection, confirmed later in (Hedja et al. 2009, Souza et al. 2011), but there are few the laboratory, and quadrats without dominance had less investigations testing whether the same remains true for than 50% cover and 50% of total biomass. Samplings were 2 aquatic ecosystems (e.g., Madsen et al. 1991). We tested carried out in all patches using a 1 m quadrat (1 m 3 1 m), whether areas dominated by one exotic species of aquatic totaling 90 samples. Each river sampled received approxi- macrophytes can affect the structure of the assemblage of mately the equivalent number of quadrats (approximately aquatic plants, compared with one native species. We 23), subdivided into UA, EA, and WD. In each quadrat, we evaluated the effect of increased biomass from the exotic identified aquatic macrophytes species and measured the African signalgrass and the native anchored waterhyacinth frequency of occurrence (presence/absence) for all species. on community structure of aquatic macrophytes. We Species that could not be identified in the field were 192 J. Aquat. Plant Manage. 53: 2015 TABLE 1. A LIST OF AQUATIC MACROPHYTES SPECIES PRESENT IN PATCHES DOMINATED BY AFRICAN SIGNALGRASS (UA), ANCHORED WATERHYACINTH (EA), AND WITHOUT DOMINANCE (WD), WITH THEIR GROWTH FORMS AND CODES USED IN AN ORDINATION ANALYSIS. Family Scientific Name Common Name Growth Form Code UA EA WD Araceae Lemna minuta Kunth Minute duckweed Free-floating Lm x x x Araceae Pistia stratiotes L. Waterlettuce Free-floating Ps x x x Araliaceae Hydrocotyle bonariensis Comm. ex Lam. Coastal