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PeerJ
2018 Jan 01;6:e5332. doi: 10.7717/peerj.5332.
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Herbivore biocontrol and manual removal successfully reduce invasive macroalgae on coral reefs.
Neilson BJ
,
Wall CB
,
Mancini FT
,
Gewecke CA
.
Abstract
Invasive macroalgae pose a serious threat to coral reef biodiversity by monopolizing reef habitats, competing with native species, and directly overgrowing, and smothering reef corals. Several invasive macroalgae (Eucheuma clade E, Kappaphycus clade A and B, Gracilaria salicornia, and Acanthophora spicifera) are established within Kāne''ohe Bay (O''ahu, Hawai''i, USA), and reducing invasive macroalgae cover is a coral reef conservation and management priority. Invasive macroalgae control techniques, however, are limited and few successful large-scale applications exist. Therefore, a two-tiered invasive macroalgae control approach was designed, where first, divers manually remove invasive macroalgae (Eucheuma and Kappaphycus) aided by an underwater vacuum system ("The Super Sucker"). Second, hatchery-raised juvenile sea urchins (Tripneustes gratilla), were outplanted to graze and control invasive macroalgae regrowth. To test the effectiveness of this approach in a natural reef ecosystem, four discrete patch reefs with high invasive macroalgae cover (15-26%) were selected, and macroalgae removal plus urchin biocontrol (treatment reefs, n = 2), or no treatment (control reefs, n = 2), was applied at the patch reef-scale. In applying the invasive macroalgae treatment, the control effort manually removed ∼19,000 kg of invasive macroalgae and ∼99,000 juvenile sea urchins were outplanted across to two patch reefs, totaling ∼24,000 m2 of reef area. Changes in benthic cover were monitored over 2 years (five sampling periods) before-and-after the treatment was applied. Over the study period, removal and biocontrol reduced invasive macroalgae cover by 85% at treatment reefs. Our results show manual removal in combination with hatchery raised urchin biocontrol to be an effective management approach in controlling invasive macroalgae at reef-wide spatial scales and temporal scales of months to years.
Figure 2. Invasive macroalgae species found on study reefs in Kāne‘ohe Bay.(A) Eucheuma clade E, (B) Kappaphycus clade B, (C) G. salicornia, (D) A. spicifera (photo credit: Brian Neilson).
Figure 3. Invasive macroalgae control techniques applied in the field.(A) using the Super Sucker to manually remove Eucheuma clade E, (B) outplanting juvenile T. gratilla, (C) outplanted adult T. gratilla surrounded by G. salicornia and A. spicifera, (D) adult T. gratilla surrounded by Eucheuma clade E, (E) before and immediately (F) after manual removal of Eucheuma clade E revealing crustose coralline algae (CCA) and (G) before and (H) after removal of Eucheuma clade E revealing live and dead coral (photo credit: (A–B) DLNR/DAR, (C–H) Brian Neilson).
Figure 4. Mean percent cover of benthic cover types.(A) combined invasive macroalgae (Eucheuma clade E/Kappaphycus Clade B/Acanthophora spicifera/Gracilaria salicornia), (B) native macroalgae, (C) crustose coralline algae (CCA), (D) corals, and (E) SBT (sand/bare/turf). Values are mean ± SE; n = 24 (control) and n = 26–27 (treatment) for each sampling time. The first time point in each figure (Winter 2011) represents the “before” time period of the study and all subsequent time points represent the “after” period. Symbols (*) represent a significant difference (p ≤ 0.05) between the control and treatment.
Figure 5. Percent cover for invasive macroalgae species through time.(A) control reefs and (B) treatment reefs. Values are mean ± SE; n = 24 (control reefs) and n = 26–27 (treatment reefs) for each sampling time.
Figure 6. Mean percent cover for benthic community members at control and treatment reefs before applying treatments (Winter 2011) and two years after treatment application (Winter 2013).Values are mean ± SE; n = 24 (control) and n = 26–27 (treatment) for each sampling time.
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