Habitat provided by native species facilitates higher abundances of an invader in its introduced compared to native range

The journey of hull-fouling mobile invaders: basibionts and boldness mediate dislodgement risk during transit

Vessel hull-fouling is responsible for most bioinvasion events in the marine environment, yet it lacks regulation in most countries. Although experts advocate a preventative approach, research efforts on pre-arrival processes are limited. The performance of mobile epifauna during vessel transport was evaluated via laboratory simulations, using the well-known invasive Japanese skeleton shrimp (Caprella mutica), and its native congener C. laeviuscula as case study. The invader did not possess any advantage in terms of inherent resistance to drag. Instead, its performance was conditioned by the complexity of secondary substrate. Dislodgement risk was significantly reduced when sessile fouling basibionts were added, which provided refugia and boosted the probability of C. mutica remaining attached from 7 to 65% in flow exposure trials. Interestingly, the invader exhibited significantly higher exploratory tendency and motility than its native congener at zero-flow conditions. Implications in terms of en-route survivorship, invasion success and macrofouling management are discussed.

Sublethal effects of a rapidly spreading native alga on a key herbivore

Multiple anthropogenic stressors are causing a global decline in foundation species, including macrophytes, often resulting in the expansion of functionally different, more stressor-tolerant macrophytes. Previously subdominant species may experience further positive demographic feedback if they are exposed to weaker plant-herbivore interactions, possibly via decreased palatability or being structurally different from the species they are replacing. However, the consequences of the spread of opportunistic macrophytes for the local distribution and life history of herbivores are unknown.The green alga, Caulerpa filiformis, previously a subdominant macrophyte on low intertidal-shallow subtidal rock shores, is becoming locally more abundant and has spread into warmer waters across the coast of New South Wales, Australia.In this study, we measured (a) the distribution and abundance of a key consumer, the sea urchin Heliocidaris erythrogramma, across a seascape at sites where C. filiformis has become dominant, (b) performed behavioral field experiments to test the role of habitat selection in determining the local distribution of H. erythrogramma, and (c) consumer experiments to test differential palatability between previously dominant higher quality species like Ecklonia radiata and Sargassum sp. and C. filiformis and the physiological consequences of consuming it.At all sites, urchin densities were positively correlated with distance away from C. filiformis beds, and they actively moved away from beds. Feeding experiments showed that, while urchins consumed C. filiformis, sometimes in equal amounts to higher quality algae, there were strong sublethal consequences associated with C. filiformis consumption, mainly on reproductive potential (gonad size). Specifically, the gonad size of urchins that fed on C. filiformis was equivalent to that in starved urchins. There was also a tendency for urchin mortality to be greater when fed C. filiformis.Overall, strong negative effects on herbivore life-history traits and potentially their survivorship may establish further positive feedback on C. filiformis abundance that contributes to its spread and may mediate shifts from top-down to bottom-up control at locations where C. filiformis has become dominant.