Consumer depletion alters seagrass resistance to an invasive macroalga.
Few field studies have investigated how changes at one trophic level can affect the invasibility of other trophic levels. We examined the hypothesis that the spread of an introduced alga in disturbed seagrass beds with degraded canopies depends on the depletion of large consumers. We mimicked the degradation of seagrass canopies by clipping shoot density and reducing leaf length, simulating natural and anthropogenic stressors such as fish overgrazing and water quality. Caulerpa racemosa was transplanted into each plot and large consumers were excluded from half of them using cages. Potential cage artifacts were assessed by measuring irradiance, scouring by leaf movement, water flow, and sedimentation. Algal invasion of the seagrass bed differed based on the size of consumers. The alga had higher cover and size under the cages, where the seagrass was characterized by reduced shoot density and canopy height. Furthermore, canopy height had a significant effect depending on canopy density. The alteration of seagrass canopies increased the spread of C. racemosa only when large consumers were absent. Our results suggest that protecting declining habitats and/or restoring fish populations will limit the expansion of C. racemosa. Because MPAs also enhance the abundance and size of fish consuming seagrass they can indirectly promote algal invasion. The effects of MPAs on invasive species are context dependent and require balancing opposing forces, such as the conservation of seagrass canopy structure and the protection of fish grazing the seagrass.
Department of Earth and Environmental Sciences, University of Pavia, Via S. Epifanio14, I-27100 Pavia, Italy.
Sarah
S
Caronni
Department of Earth and Environmental Sciences, University of Pavia, Via S. Epifanio14, I-27100 Pavia, Italy.
Chiara
C
Calabretti
Department of Science for Nature and Environmental Resources, University of Sassari, Via Piandanna 4, I-07100 Sassari, Italy.
Maria Anna
MA
Delaria
Marine Protected Area Tavolara Punta Coda Cavallo, Via Dante 1, I-07026 Olbia (OT), Italy.
Giuseppe
G
Bernardi
Marine Protected Area Tavolara Punta Coda Cavallo, Via Dante 1, I-07026 Olbia (OT), Italy.
Augusto
A
Navone
Department of Earth and Environmental Sciences, University of Pavia, Via S. Epifanio14, I-27100 Pavia, Italy.
Anna
A
Occhipinti-Ambrogi
Marine Protected Area Tavolara Punta Coda Cavallo, Via Dante 1, I-07026 Olbia (OT), Italy.
Pieraugusto
P
Panzalis
Department of Science for Nature and Environmental Resources, University of Sassari, Via Piandanna 4, I-07100 Sassari, Italy.
Giulia
G
Ceccherelli
D000818:Animals; D046068:Caulerpa; D017753:Ecosystem; D004777:Environment; D005399:Fishes; D058865:Introduced Species; D007558:Italy; D012624:Seaweed