Marine management affects the invasion success of a non‐native species in a temperate reef system in California, USA

Abstract

Despite promises that 'healthy' marine systems show increased resilience, the effects of ecosystem management strategies on invasion success in marine systems is still unclear. We show that resistance to the invasive alga, Sargassum horneri, in a temperate reef system occurs through alternate mechanisms in different ecosystem states. In an old marine protected area (MPA), invasion of S. horneri was suppressed, likely due to competitive pressure from native algae, resulting from protection of urchin predators. In a nearby fished urchin barren, invasion of S. horneri was also suppressed, due to herbivory by urchins whose predators are fished. Within newer MPAs with intermediate levels of interacting species, S. horneri was abundant. Here, neither competition from native algae nor herbivory was sufficient to prevent invasion. We confirm that invasion in marine systems is complex and show that multiple mechanisms in single systems must be considered when investigating biotic resistance hypotheses.

Non‐native species are a global issue for marine protected areas

“…NNS may be suppressed by direct competition and predation but in other cases have thrived when newer MPAs have less dominant native populations (Caselle et al . ) or when predation on native species has increased space resources for NNS (Coma et al . ).…”

“…These inconsistent responses are likely due in part to insufficient research for analyzing overarching trends but also to complex interactions with native species. NNS may be suppressed by direct competition and predation but in other cases have thrived when newer MPAs have less dominant native populations (Caselle et al 2018) or when predation on native species has increased space resources for NNS (Coma et al 2011). The complexity of shifting interactions between NNS and native species as population pressures change with protection makes it difficult to predict outcomes.…”

Predator-induced selection on urchin activity level depends on urchin body size

“…Across the wide geographic distribution of purple urchins along the west coast of North America (Ebert, 2010), urchins are eaten by a diversity of predatory species including the sunflower sea star ( Pycnopodia helianthoides ), otters ( Enhydra lutris ), sheephead ( Semicossyphus pulcher ), and California spiny lobsters ( Panulirus interruptus ). These predators can play key roles in regulating herbivory by urchins on giant kelp (Burt et al, 2018; Caselle, Davis, & Marks, 2018). However, the role of variation in urchin size structure and individual behavioral variation in their susceptibility to predation remains mysterious, even though such variation in mortality could explain variation in community susceptibility to urchin-driven state shifts, like between kelp forests vs. urchin barrens, a process often referred to as a “tipping point” (Pruitt et al, 2018; Selkoe et al, 2015).…”

Cobble community DNA as a tool to monitor patterns of biodiversity within kelp forest ecosystems

“…The ability to monitor multispecies assemblages and species-specific interactions on small and large spatial scales strengthens the ability to understand species trajectories that allow forecasting changes in species composition (Caselle, Davis, & Marks, 2018;Pinsky et al, 2018). While our ability to identify abrupt regime shifts in natural ecosystems has improved (Hughes et al, 2018), detection of potential early-warning signals is still very limited, particularly in structurally complex and productive communities (Melis, Ceccherelli, Piazzi, & Rustici, 2019).…”