Biogeographical comparison of the emergent macrophyte, Sagittaria platyphylla in its native and introduced ranges

Highlights

• Sagittaria platyphylla, native to southern USA is highly invasive in Australia and South Africa.

• We conducted a biogeographical survey to identify factors that might explain its invasion success.

• Achene production of introduced range plants was 40% greater than for native range plants.

• We hypothesise that enemy release may induce reproductive plasticity in S. platyphylla.

Abstract

Understanding why some plant species become invasive is important to predict and prevent future weed threats and identify appropriate management strategies. Many hypotheses have been proposed to explain why plants become invasive, yet few studies have quantitatively compared plant and population parameters between native and introduced range populations to gain an objective perspective on the causes of plant invasion. The present study uses a biogeographical field survey to compare morphological and reproductive traits and abundance between the native range (USA) and two introduced ranges (Australia and South Africa) of Sagittaria platyphylla (Engelm.) J.G. Sm (Alismataceae), a highly invasive freshwater macrophyte. Introduced and native populations differed in sexual reproductive output with the number of achenes per fruiting head and individual achene weight found to be 40% and 50% greater in introduced populations respectively. However, no other morphological traits were found to be consistently different between the native and both introduced ranges, especially after taking into account differences in environmental conditions between the three ranges. Although populations in introduced regions were larger and occupied greater percentage cover, no differences in plant density were evident. Our results suggest that, apart from sexual reproduction, many of the trait patterns observed in S. platyphylla are influenced by environmental and habitat conditions within the native and invaded ranges. We conclude that the enemy release hypothesis best explains the results observed for sexual reproduction. In particular, we hypothesise that a release from natural enemies, specifically a pre-dispersal seed predator, may induce reproductive plasticity in S. platyphylla.

Introduction

For many decades, invasion ecologists have sought to determine the key attributes that make some plant species successful invaders (Elton, 1958, Baker et al., 1965, van Kleunen et al., 2014) and why some communities are more susceptible to invasion than others (Crawley, 1987, Kolar and Lodge, 2001). Understanding why certain plants become invasive is important for predicting and preventing future weed threats (Groves et al., 2001) and for identifying appropriate management strategies (Blumenthal, 2006; Herrera et al., 2011).

The contribution of traits to the success of invasive species must be context dependent (Pyšek and Richardson, 2007, van Kleunen et al., 2014). Nevertheless, certain traits appear to be common amongst many invasive plant species. These traits include high phenotypic plasticity, rapid growth to sexual maturity, sexual and asexual reproductive strategies, high tolerance to environmental heterogeneity, dispersal capabilities and an ability to outcompete other species (Baker et al., 1965, Sakai et al., 2001). Such traits may be inherent within the species making the species pre-adapted to being invasive (Baker et al., 1965). Alternatively, rapid evolutionary change within the species may predispose the species to being a successful invader following introduction into a new range (Müller-Schärer et al., 2004).

Along with intrinsic factors, extrinsic factors such as climate, disturbance regimes, resource availability and competitive abilities of the resident species, may also facilitate invasion in the new range (Crawley, 1987, Shea and Chesson, 2002).

Many hypotheses have been proposed to explain why some plant species become invasive (Keane and Crawley, 2002, Shea and Chesson, 2002, Blumenthal, 2005, Catford et al., 2009). A myriad of approaches have been used to test their validity, such as observational and experimental, biogeographical and community comparative studies (see review by van Kleunen et al., 2010).

Prior to testing any hypotheses, empirical comparisons of plant performance between the introduced and native range should be conducted to determine whether there are explicit differences that can be attributed to any of the invasion hypotheses, such as enemy release, evolution of invasiveness, empty niche or novel weapons (Hierro et al., 2005). Surprisingly, few studies have quantitatively compared plant and population parameters between native and introduced ranges to gain an objective perspective on the causes of plant invasion (but see Hinz et al., 2012, Williams et al., 2010), and even fewer have considered more than one invasive range.

Sagittaria platyphylla (Engelm.) J.G.Sm (delta arrowhead, or colloquially referred to as “sagittaria” in Australia) (Alismataceae), is an emergent freshwater perennial macrophyte with phyllodial submersed or emersed lanceolate petiolate leaves up to 150 cm tall (Supplementary material Fig. S1) (Haynes and Hellquist, 2000). It is indigenous to south eastern United States of America (USA) but has become highly invasive in some introduced areas, such as Australia and the Republic of South Africa (South Africa) (Supplementary material Fig. S2), where it rapidly dominates shallow fresh water environments such as wetlands, streams, riverbanks, ditches and irrigation channels (Adair et al., 2012). Plants have a prolonged 5–6 month flowering season (summer-fall) and produce multiple fruiting heads arranged in whorls of three on erect, racemose inflorescences (Haynes and Hellquist, 2000). An average of 850 achenes per fruiting head and 6900 achenes per inflorescence has been recorded on S. platyphylla in Australia (Flower, 2004). The light, buoyant achenes are dispersed along waterways collecting in still pools and river berms (Broadhurst and Chong, 2011). When water levels recede, exposed achenes germinate readily on the saturated mud (Kwong unpublished data). Vegetative reproduction via stolons is prolific throughout spring and summer. Tubers are produced at the base of roots in autumn and allow populations to rapidly regenerate in spring (Adair et al., 2012).

For the present study, we undertook a four-year biogeographical field survey to compare morphological and reproductive traits and abundance of S. platyphylla between the native range (USA) and two introduced ranges (Australia and South Africa). Research questions examined include: (1) Are S. platyphylla populations in Australia and South Africa more abundant in terms of population size and plant density?; (2) Are individual plants larger in the introduced ranges?; (3) Is sexual reproductive performance greater in introduced ranges compared to the native range?; and (4) Are observed effects maintained after accounting for differences in native and introduced environments? The findings are used to hypothesise possible mechanisms of invasion success for S. platyphylla in its introduced ranges.