Anthropogenic disturbance facilitates a non-native species on offshore oil platforms.
Abstract
Anthropogenic practices that facilitate species introductions must be identified and modified to improve management and control the spread of non-native taxa in many environments. Maintenance practices that remove dense epifaunal invertebrates attached to offshore structures create a disturbance that may facilitate the establishment of non-native species. We evaluated the effect of disturbance on the abundance (percent cover) of a non-native bryozoan, Watersipora subatra, on an offshore oil platform in the Santa Barbara Channel, USA by removing the existing epifaunal community in experimental plots and comparing Watersipora cover in these plots to that in undisturbed control plots over a 15-month period. We explored the importance of larval supply and the epifaunal community in driving observed patterns, using measurements of Watersipora larval availability and colony recruitment and growth in the disturbed and control plots. We also examined the effect of disturbance on Watersipora establishment at the larger, platform-scale over c. 18 months with comparative surveys of cleaned and uncleaned portions of another oil platform. Both the experimental disturbance and the larger platform cleaning facilitated Watersipora establishment, with cover increasing from c. 5% to 20-60% at shallower (≤12 m) depths within 15 to 18 months following disturbance. Initial Watersipora recruitment to the disturbed plots occurred during a period of elevated larval availability, as indicated by recruitment onto settlement plates. However, 1 year after the experimental disturbance, sessile invertebrates occupied all available settlement space, and there was little recruitment of Watersipora into disturbed plots despite the availability of larvae. Synthesis and applications. Maintenance operations for offshore structures can include the manual removal of subtidal epibenthic invertebrates attached to the structure. Our study at offshore oil platforms found that this anthropogenic disturbance enhanced the establishment of the non-native invertebrate Watersipora subatra. The timing of disturbance relative to Watersipora's reproductive season was an important driver of this pattern. Scheduling maintenance practices to occur soon after the reproductive period of Watersipora could allow adequate time for native species to recruit and occupy the available bare space, thereby reducing the potential for establishment of this non-native species.