Reciprocal competitive effects of congeneric invaders, Trapa natans L. and Trapa bispinosa Roxb. var. iinumai Nakano, in established freshwater plant cultures
Introduction
Plant diversity is an important feature of healthy ecosystems, providing both function and numerous beneficial services (Isbell et al., 2011). Species interactions within diverse plant communities affect resource usage and productivity (Hooper, 1998; Cardinale et al., 2002). In hydrophyte communities, invasive plant introductions often lead to dominance of undesirable species and reduced overall species diversity through negative competitive interactions between invaders and other community members (Smart et al., 1994; Capers et al., 2007). The consequences of invasion may include damaged ecological services—water quality and habitat—or municipal services, including hydroelectric power generation, irrigation, and recreation. Understanding resource usage and competitive ability of a species, as it relates to invasiveness, is important in predicting interactions between invaders and recipient communities (McCreary, 1991; Hooper, 1998; Van et al., 1999; Capers et al., 2007).
One such invader within the United States is water chestnut (Myrtales: Lythraceae: Trapa natans L., European water chestnut). It was introduced to the northeastern United States in the late 1800′s and has been problematic for water resource managers since the 1920s by impeding navigation and degrading aquatic habitat by displacing beneficial natives (Gwathmey, 1945; Carter and Rybicki, 1994; Les and Mehrhoff, 1999). Native to Eurasia and Africa (Crow and Hellquist, 2000; Hummel and Kiviat, 2004), T. natans is an annual, floating-leaved plant identified by floating rosettes composed of triangular leaves, fruits bearing four spines, and flowers with white petals (Crow and Hellquist, 2000; Pemberton, 2002; Hummel and Kiviat, 2004). Trapa natans is not listed as a Federal noxious weed in the USA; however, it is established in nine states (Pfingsten et al., 2021). An additional nineteen states have listed this species as noxious and/or prohibited (EDDMapS, 2021).
Trapa natans was once thought to be the only species in the genus Trapa in the northeastern USA, but an inter-specific cryptic introduction (Morais and Reichard, 2018) of water chestnut (misidentified as its congener, T. natans) was recently identified in Virginia (Chorak et al., 2019; Dodd et al., 2019; Pfingsten and Rybicki, 2021). This species was first observed in 2014 within the freshwater reaches of the Potomac River near Alexandria, VA and is identified by fruits bearing two spines and pink flower petals. It has since been determined that populations of this cryptic morphotype (morphological variation) are established in numerous water bodies within the Potomac River watershed (one of several major tributaries to Chesapeake Bay); with one record reaching as far back as the mid-1990′s (Pfingsten and Rybicki, 2021). Genetic and morphological studies have determined this species to be most similar to specimens collected in Taiwan, taxonomically identified as Trapa bispinosa Roxb. var. iinumai Nakano (Huang, 1993; Chorak et al., 2019; Dodd et al., 2019). The biology and ecology of T. bispinosa var. iinumai outside of its native range is currently unknown. Given the importance of water resources in the Chesapeake Bay watershed, history of negative impacts by T. natans in the USA, and observations that T. bispinosa var. iinumai occupies similar shallow, stagnant, or slow-moving stream habitats of lakes, marshes, and wetlands (Mirani et al., 2012), it is especially important to understand potential problems associated with the recent introduction.
To abate expansion and manage current populations of T. bispinosa var. iinumai in its naturalized range of the northeastern United States, study of the genetics and biology (Chorak et al., 2019; Dodd et al., 2019), and ecology (Dodd and Schad, in press) of the species is needed to update management options for water resource managers. Whereas the effects of T. natans on water quality and native submersed aquatic vegetation (SAV) have been elucidated (Hummel and Kiviat, 2004), interspecific interactions between SAV and the cryptic introduction of the variety of T. bispinosa var. iinumai that is currently found in the US has not. To address this research need, we conducted a simple paired interaction study to evaluate and compare reciprocal competitive strengths of annual species T. bispinosa var. iinumai and T. natans when grown in the presence of important perennial SAV species. In this study, we tested the hypothesis that T. natans and T. bispinosa var. iinumai would outcompete two common SAV species found in the Chesapeake Bay, Vallisneria americana Michx. (American eelgrass) and Hydrilla verticillata (L. f.) Royle (water thyme), grown in mixture under controlled greenhouse conditions. A perennial, native V. americana is considered highly valuable for aquatic habitat (Moore et al., 2010). Non-native monoecious H. verticillata, a perennial, was introduced between 1985 and 1988 (Carter and Rybicki, 1994) to the upper tidal Potomac River. Hydrilla verticillata is considered highly invasive, although recent reports indicate interannual variation in coverage (Rybicki and Landwehr, 2007). Two biotypes of each H. verticillata and V. americana are recognized in the USA. H. verticillata biotypes are either dioecious (pistillate) or monoecious, forming overwintering subterranean turions (herein referred to as tubers) (True-Meadows et al., 2016). The southern (or broad-leaved) biotype of V. americana is evergreen with fibrous roots, and the northern biotype (or narrow-leaved) is a herbaceous perennial that forms winter buds (herein referred to as tubers) (McFarland and Shafer, 2008). Monoecious H. verticillata and northern V. americana were used in this study. We predicted that Trapa spp. growth (as determined by biomass, density, shoot:root ratio, and relative growth rate) and competitive strength (relative interaction intensity) would be greater than either V. americana or H. verticillata.
Section snippets
Methods
Competition was investigated through an additive reciprocal design by growing Trapa spp. plants from seed in monoculture or in biculture (pairwise) with either V. americana or H. verticillata. To simulate established monotypical stands of H. verticillata and V. americana, plants were planted in spring and grown in containers for a period of one growing season and allowed to enter dormancy prior to biculture competition experiment. Treatments were replicated (n = 5) and included four
Experimental conditions
Water quality for pH (7.9 ± 1.1), alkalinity (52.5 ± 7.0 mM/L), conductivity (256 ± 30.5 μs m-2 s-1) and water temperature (20.4 ± 2.7 °C) were within ranges conducive for optimal growth and similar among planting combinations (Table 1, Table 2). Shading may have been a factor in light intensity for bicultures (Table 3). Both Trapa spp. experienced the least amount of shading with the highest significant light penetration, with monocultures of H. verticillata experiencing more light penetration
Discussion
Loss of biodiversity in aquatic ecosystems due invasions of exotic plant species can be attributed to numerous mechanisms including, but not limited to, competition between plant species in a community (Fleming and Dibble 2015). The spread and negative impacts of invasive plant T. natans to recipient aquatic plant communities in the US has been documented (Methé et al., 1993, Hummel and Kiviat, 2004; Caraco and Cole, 2002). Considering the problems associated with its congener, the newly
Authors’ statement
Lynde Dodd: Conceptualization, Methodology, Data curation, Orignial draft preparation, Funding Acquisition, Methodology, Formal Analysis, Visualization, Writing-Reviewing, Supervision, Project administration
Nathan Harms: Methodology, Formal Analysis, Writing-Reviewing
Aaron Schad: Methodology, Formal Analysis, Writing-Reviewing
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The US Army Engineer and Research Development Center in collaboration with the US Geological Survey supported this research with assistance from US Army Corps of Engineers North Atlantic Division: Baltimore District, New England District, and New York District. This work was funded by the US Army Corps of Engineers Aquatic Plant Control Research Program, under the work unit, “Investigations into water chestnut: genetics and ecology”. The authors would like to specifically thank Dr. Nancy
References (46)
- et al.
The effects of grazers and light penetration on the survival of transplants of Vallisneria americana Michx. in the tidal Potomac River, Maryland
Aquat. Bot.
(1985) Competition as a mechanism of submersed macrophyte community structure
Aquat. Bot.
(1991)- et al.
Competition between two floating-leaved plants
Aquat. Bot.
(2021) - et al.
Cryptic invasions: a review
Sci. Total Environ.
(2018) - et al.
Effects of sediment and salinity on the growth and competitive abilities of three submersed macrophytes
Aquat. Bot.
(2016) - et al.
Competition between Hydrilla verticillata and Vallisneria americana as influenced by soil fertility
Aquat. Bot.
(1999) - et al.
Measuring plant integrations: a new comparative index
Ecology
(2004) - et al.
Diversity increases indirect interactions, attenuates the intensity of competition, and promotes coexistence
Amer. Nat.
(2015) - et al.
Effects of salinity and light on biomass and growth of Vallisneria americana from Lower St. Johns River, FL, USA
Wetlands Ecol Manage
(2010) - et al.
Aquatic plant community invasibility and scale-dependent patterns in native and invasive species richness
Ecology
(2007)