Hotspots of exotic free‐spawning sex: man‐made environment facilitates success of an invasive seastar

Abstract

Summary1. The introduction of exotic species to new areas poses a major threat to the environment. For those introduced species that establish and survive beyond the short term, the opportunities to manage the risk of continuing spread often rest with limiting reproductive output. The introduced northern Pacific seastar Asterias amurensis is an ecologically important pest that has established in the Derwent Estuary (Australia). Driven by oversupply of bivalve prey, it is persistent, abundant and fecund when associated with man‐made structures such as docks, marinas, jetties and piers, that is, ‘wharves’.2. As a free‐spawning invertebrate, fertilization by A. amurensis is a critical life‐history stage constrained by strong Allee effects. Eggs must be released in close proximity to sperm sources because the chances of achieving fertilization drastically reduce with increasing distances of spawner separation.3. Investigation of zygote production in the Derwent Estuary using a spatially explicit model of free‐spawning fertilization shows that A. amurensis at wharves, while representing <10% of the total population in the estuary and concentrated in <0·1% of the total area, may contribute >90% of total zygote production. Given the seastars’ long‐lived and highly dispersive larvae, we show that wharves not only represent important sites of invasion but also facilitate propagule pressure promoting secondary invasions.4. Synthesis and applications. In the absence of effective pest control solutions, focusing on reproductive hotspots has the potential to reduce further spread of established marine pests and to alleviate ongoing ecological impacts. In the case of the northern Pacific seastar, elimination of highly localized wharf populations annually prior to spawning can reduce overall zygote production by up to estimated ∼90%. The long‐term protection of key sources of larval production is a common goal for marine reserve design and fisheries management. However, the same concept but in reverse, whereby larval production is minimized at key sources, could be effective in the management of introduced pests in subtidal marine environments.