Sponge presence increases the diversity and abundance of fish and invertebrates in a subtropical seagrass bed

Ecosystem engineers (e.g., seagrasses) can have profound effects on
biodiversity and community structure. Sponges possess many traits (e.g.,
create complex structure) that suggest they may act as ecosystem
engineers, but we know little about their relationships with the animal
communities that inhabit seagrass beds. This study explored the effects of
the marine sponge, Ircinia felix, on fish and invertebrate taxa richness,
abundance, turnover, and community composition in a seagrass bed through a
1-year field experiment performed in The Bahamas. We recorded the fish and
invertebrate communities present in 5 × 5 m plots with the addition of
either a live sponge, a polypropylene sponge replica (structure), or no
additional structure (control). Both taxonomic groups responded similarly
to the addition of a live sponge. Taxa richness and abundance increased at
least 4-fold over a year in the presence of I. felix but decreased 2-fold
or more in the other plot types. Live sponge presence increased the number
of new taxa gained by at least 24% and reduced the number of taxa lost
compared to other plot types. Finally, we found that the fish community
shifted in a more consistent direction in the presence of a live sponge
than in the other plots. Our study shows that sponges can act as ecosystem
engineers in subtropical seagrass systems; however, additional research is
needed to determine the full extent and implications of their ecological
effects.