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

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

Natural abundances of alliatorweed and water hyacinth were positively correlated.

<p>Correlations between water hyacinth and alligatorweed abundances in the field survey (square meters per linear meter of shoreline). The dashed line is the unity line (equal abundances) and the solid line is the RMA regression line (y = 0.77+1.32 ×, p = 0.0009).</p

Dependence of establishment and patch growth on the presence of another species.

<p>A) Establishment of hyacinth (floating) and alligatorweed (rooted) in bare shoreline plots (floating and rooted removed) versus plots in which the other functional group was not removed (but the response species was removed). B) Abundance of hyacinth and alligatorweed in plots in which no plants were removed versus plots in which the response species was growing in plots in which the other functional group was removed. Adjusted per plot means (+1 SE) from ANCOVA with starting abundance of response species as a covariate. Letters indicate means that were significantly different in adjusted means contrast tests (four independent sets of contrast tests). Initial abundances of water hyacinth and alligatorweed are shown as dashed lines in B.</p

Effects of treatments on plant cover.

<p>The dependence of area of different categories of cover on aquatic plant removal treatment in repeated measures ANCOVAs with initial abundance of the response group as a covariate. Univariate tests of hypotheses for within subject effects (repeated factors) are shown in the last four rows. Significant results are shown in bold.</p