Data from: Opposite effects of nutrient enrichment and an invasive snail on the growth of invasive and native macrophytes

Many ecosystems are now co-invaded by alien plant and herbivore species.
The evolutionary naivety of native plants to alien herbivores can make the
plants more susceptible to detrimental effects of herbivory than
co-occurring invasive plants, in accordance with the apparent competition
hypothesis. Moreover, the invasional meltdown hypothesis predicts that in
multiply invaded ecosystems, invasive species can facilitate each other’s
impacts on native communities. Although there is growing empirical support
for these hypotheses, facilitative interactions between invasive plants
and herbivores remain underexplored in aquatic ecosystems. Many freshwater
ecosystems are co-invaded by aquatic macrophytes and mollusks and
simultaneously experience nutrient enrichment. However, the interactive
effects of these ecological processes on native macrophyte communities
remain an underexplored area. To test these effects, we performed a
freshwater mesocosm experiment in which we grew a synthetic native
community of three macrophyte species under two levels of invasion by an
alien macrophyte Myriophyllum aquaticum (invasion vs. no-invasion) and
fully crossed with two levels of nutrient enrichment (enrichment vs.
no-enrichment) and herbivory by an invasive snail Pomacea canaliculata
(herbivory vs. no-herbivory). In line with the invasional meltdown and
apparent competition hypotheses, we found that the proportional
above-ground biomass yield of the invasive macrophyte, relative to that of
the native macrophyte community, was significantly greater in the presence
of the invasive herbivore. Evidence of a reciprocal facilitative effect of
the invasive macrophyte on the invasive herbivore is provided by the
results showing that the herbivore produced greater egg biomass in the
presence than in the absence of M. aquaticum. However, nutrient enrichment
reduced the mean proportional above-ground biomass yield of the invasive
macrophyte. Our results suggested that herbivory by invader P.
canaliculata may enhance invasiveness of M. aquaticum. However, nutrient
enrichment of habitats that already harbor M. aquaticum may slow down
invasive spread of the macrophyte. Broadly, our study underscores the
significance of considering several factors and their interactions when
assessing the impact of invasive species, especially considering that many
habitats experience co-invasion by plants and herbivores and
simultaneously undergo various other disturbances, including nutrient
enrichment.