Mechanisms of Invasion Resistance of Aquatic Plant Communities

Abstract

Invasive plant species are among the major threats to freshwater biodiversity. Few experimental studies have investigated whether native plant diversity can provide biotic resistance to invaders in freshwater ecosystems. At small spatial scales, invasion resistance may increase with plant species richness due to a better use of available resources, leaving less available for a potential invader (Complementarity effect) and/or the greater probability to have a highly competitive (or productive) native species in the community (Selection effect). In submerged aquatic plant communities, we tested the following hypotheses: (1) invader establishment success is greatest in the absence of a native plant community; (2) lower in plant communities with greater native species richness, due to complementary and/or selection effects; and (3) invader establishment success would be lowest in rooted plant communities, based on the limiting similarity theory as the invader is a rooted submerged species. In a greenhouse experiment, we established mesocosms planted with 0 (bare sediment), 1, 2, and 4 submerged plant species native to NW Europe and subjected these to the South African invader Lagarosiphon major (Ridl.) Moss. We used two rooted (Myriophyllum spicatum L., Potamogeton perfoliatus L.) and two non-rooted native species (Ceratophyllum demersum L., Utricularia vulgaris L.) representing two distinct functional groups considering their nutrient acquisition strategy which follows from their growth form, with, respectively, the sediment and water column as their main nutrient source. We found that the presence of native vegetation overall decreased the establishment success of an alien aquatic plant species. The strength of this observed biotic resistance increased with increasing species richness of the native community. Mainly due to a selection effect, the native biomass of mixed communities overyielded, and this further lowered the establishment success of the invader in our experiment. The strongest biotic resistance was caused by the two native plant species that were of the same functional group, i.e., functionally most similar to the invader. These results support the prediction of Elton's biotic resistance hypothesis in aquatic ecosystems and indicate that both species richness and functional group identity can play an important role in decreasing establishment success of alien plant species.

Keywords: biotic resistance; diversity-resistance hypothesis; functional group identity; limiting similarity; niche partitioning; sampling effect; species diversity; species richness.

FIGURE 1

Effect of native plant species richness on the invader Lagarosiphon major (A) root formation and, given their presence, on (B) log-transformed root biomass production. Data points were jittered in the graph (A) so the binomial (presence/absence) of roots at many of the treatments could be seen. Significance level at p < 0.05.

FIGURE 2

Effects of native plant species richness on the invader Lagarosiphon major (A) shoot biomass, (B) log-transformed root:shoot ratio given their root formation, and (C) relative growth rate (RGR). Significance level at p < 0.05.

FIGURE 3

Effect of native plant species richness on total native community biomass production (g DW) per mesocosm.

FIGURE 4

Partitioning of the diversity effect on native community biomass of the highest species richness level (4 species) into a complementarity effect and a selection effect by applying the additive partitioning diversity effect method proposed by Loreau and Hector (2001).

FIGURE 5

Median, SE (boxes), and minimum and maximum (whiskers) values of (given the presence of roots), (A) log-transformed root biomass, (B) shoot biomass, (C) log-transformed root:shoot ratio, and (D) relative growth rate (RGR) of the invader Lagarosiphon major. Different lowercase letters indicate statistically significant differences between functional group treatments (Tukey’s post hoc test). Significance levels were determined after Bonferroni’s correction (p < 0.008).