An invasive seagrass drives its own success in two invaded seas by both negatively affecting native seagrasses and benefiting from those costs

The nature and strength of interactions between native and invasive
species can determine invasion success. Species interactions can drive,
prevent or facilitate invasion, making understanding the nature and
outcome of these interactions critical. We conducted mesocosm experiments
to test the outcome of interactions between Halophila stipulacea, a
seagrass that invaded the Mediterranean and Caribbean Seas, and native
seagrasses (Cymodocea nodosa and Syringodium filiforme, respectively) to
elucidate mechanisms explaining the successful invasions. Mesocosms
contained intact cores with species grown either mixed or alone. Overall,
in both locations, there was a pattern of the invasive growing faster with
the native than when alone, while also negatively affecting the native,
with similar patterns for shoot density, aboveground and belowground
biomass. In the Caribbean, H. stipulacea increased by 5.6 ± 1.0 SE shoots
in 6 weeks when grown with the native while, when alone, there was a net
loss of −0.8 ± 1.6 SE shoots. The opposite pattern occurred for S.
filiforme, although these differences were not significant. While the
pattern in the Mediterranean was the same as the Caribbean, with the
invasive grown with the native increasing shoots more than when it grew
alone, these differences for shoots were not significant. However, when
measured as aboveground biomass, H. stipulacea had negative effects on the
native C. nodosa. Our results suggest that a seagrass that invaded two
seas may drive its own success by both negatively affecting native
seagrasses and benefiting from that negative interaction. This is a novel
example of a native seagrass species facilitating the success of an
invasive at its own cost, providing one possible mechanism for the
widespread success of this invasive species.