Facilitation and Competition among Invasive Plants: A Field Experiment with Alligatorweed and Water Hyacinth

Emily J. Wundrow,Christopher A. Gabler,Evan Siemann,Katherine C. Horn,Juli Carrillo

doi:10.1371/journal.pone.0048444

Abstract

Ecosystems that are heavily invaded by an exotic species often contain abundant populations of other invasive species. This may reflect shared responses to a common factor, but may also reflect positive interactions among these exotic species. Armand Bayou (Pasadena, TX) is one such ecosystem where multiple species of invasive aquatic plants are common. We used this system to investigate whether presence of one exotic species made subsequent invasions by other exotic species more likely, less likely, or if it had no effect. We performed an experiment in which we selectively removed exotic rooted and/or floating aquatic plant species and tracked subsequent colonization and growth of native and invasive species. This allowed us to quantify how presence or absence of one plant functional group influenced the likelihood of successful invasion by members of the other functional group. We found that presence of alligatorweed (rooted plant) decreased establishment of new water hyacinth (free-floating plant) patches but increased growth of hyacinth in established patches, with an overall net positive effect on success of water hyacinth. Water hyacinth presence had no effect on establishment of alligatorweed but decreased growth of existing alligatorweed patches, with an overall net negative effect on success of alligatorweed. Moreover, observational data showed positive correlations between hyacinth and alligatorweed with hyacinth, on average, more abundant. The negative effect of hyacinth on alligatorweed growth implies competition, not strong mutual facilitation (invasional meltdown), is occurring in this system. Removal of hyacinth may increase alligatorweed invasion through release from competition. However, removal of alligatorweed may have more complex effects on hyacinth patch dynamics because there were strong opposing effects on establishment versus growth. The mix of positive and negative interactions between floating and rooted aquatic plants may influence local population dynamics of each group and thus overall invasion pressure in this watershed.

Highlights

Positive interactions among species are recognized as central drivers in structuring communities [1,2,3,4] and facilitation is increasingly recognized as a determinant of the invasive success of exotic species [5,6,7]
Establishment of water hyacinth was lower in plots with alligatorweed (Table 1, Fig. 1A, 2A) but the growth of water hyacinth patches was greater in plots where alligatorweed was present than in plots where alligatorweed had been removed (Table 1, Fig. 1B, 2B)
Our results demonstrate that distinct positive and negative interactions are occurring between exotic aquatic plant functional groups in terms of both growth and establishment

Summary

Introduction

Positive interactions among species are recognized as central drivers in structuring communities [1,2,3,4] and facilitation is increasingly recognized as a determinant of the invasive success of exotic species [5,6,7]. It has been observed that many ecosystems that are heavily invaded by an exotic species often contain several common exotic species [20,23], and particular combinations of exotic species may co-occur frequently, which suggests that their distributions are not independent [20,23,24] This could reflect independent responses to a single set of conditions, such as salinity or nutrient levels [25], or common pathways of introduction (such as from ballast water [26]), or it could be driven by facilitation or mutualistic interactions between exotic species, where the invasion of one species may make the subsequent invasion of one or more other exotic species more likely. This process of facilitation has been termed ‘‘invasional meltdown’’ in extreme cases because it has the potential to lead to an exotic species dominated ecosystem if such positive feedbacks between the initial invader and subsequent introduced species are sufficiently common or strong [20,27]

Methods

Results

Conclusion