Direct and indirect effects of native plants and herbivores on biotic resistance to alien aquatic plant invasions

1- Biotic resistance to alien plant invasions is mainly determined by
ecological interactions in two layers of the food web: competition with
native plant species, and herbivory by native herbivores. While the direct
effect of native plants on alien plant performance via competition has
been well documented across ecosystems, less is known about the direct and
indirect effects of herbivores in providing biotic resistance. Our main
aims were to determine whether temperate native aquatic plants and
herbivores can provide biotic resistance to plant invasions, understand
the underlying mechanisms and search for potential interactive effects of
competition and herbivory on invader performance (i.e. growth). 2- We
mimicked natural temperate mesotrophic and eutrophic freshwater lakes in
mesoscosms, by growing three native submerged plant species in
monocultures (Ceratophyllum demersum, Myriophyllum spicatum and
Potamogeton perfoliatus) at three competition levels (no, low and high)
without and with the native aquatic generalist snail Lymnaea stagnalis. We
subsequently simulated an early stage of establishment of the South
American highly invasive alien plant species Egeria densa. 3- We found
that competition by native plant biomass significantly reduced invader
performance but depended on native species identity. Herbivory had no
direct negative effect on invader performance as the snails fed mainly on
the available filamentous algae, which are commonly found in meso- and
eutrophic systems, instead of on the plants. However, the consumption of
filamentous algae by herbivores indirectly had positive effects on the
invader total biomass, thus facilitating the invasion by E. densa.
Nonetheless, these indirect effects worked through different pathways
depending on the native plant identity. 4. Synthesis. We found evidence
for biotic resistance through competition by native plant species.
However, we show that herbivores can indirectly facilitate South American
plant E. densa invasion promoting its growth through selective feeding on
filamentous algae, but this effect depends on the native plant species
involved. Our experiment illustrates the important role of indirect
interactions to understand the potential of biotic resistance in natural
ecosystems.