Ecological interactions, especially those that are beneficial (i.e. mutualism) or detrimental (i.e. parasitism), play important roles during the establishment and spread of alien species. This chapter explores the role of these

Biotic invasions can result in the displacement of native species. This can alter the availability of native prey and the choices made by native predators. We investigated prey selection by 2 native South African predators, the west coast rock lobster Jasus lalandii and the starfish Marthasterias africana in response to the invasive mussels, Mytilus galloprovincialis and Semimytilus algosus, and native mussels, Aulacomya atra and Choromytilus meridionalis. As the diets of lobsters and starfish are broad and have been suggested to reflect prey availability, we hypothesized that they would consume the most abundant prey, regardless of its native or alien status. Laboratory studies presented predators with varying proportions of native and invasive mussels that represented pre-and post-invasion scenarios. Mussel parameters (shell strength, adductor muscle size, and energy content) that may be of importance in selection by predators were compared among species. Both predators exhibited preference towards the native mussel C. meridionalis, even when it was the least abundant prey. The selection of native species occurred despite mussel parameters suggesting that invasive species would be easier to consume. These findings highlight the potential for facilitation of prey invasions, especially when predators avoid alien prey and select for native comparators that may offer resistance to the invasion through inter-specific competition. It is presently unclear how often such a lack of predator-driven biotic resistance acts in combination with indirect facilitation, but interrogating the behaviors that drive such outcomes will advance our understanding of successful invasions.

Two invasive mussel species are known from South Africa, Mytilus galloprovincialis and Semimytilus algosus. Most of the existing research on these invaders has focused on the intertidal zone, with little attention paid to subtidal habitats. This study addresses this knowledge gap by quantifying the relative abundance and size of native and alien mussels from the high-shore down to the subtidal zone, while accounting for the effects of wave exposure. This was achieved through extensive surveys along the west coast of South Africa and the Cape Peninsula. At all shore zones, mussel abundance varied among species and wave exposures. In intertidal habitats, invasive species were recorded in greatest abundances at wave-exposed sites. Specifically, M. galloprovincialis was dominant in the high-shore, but this pattern changed down the shore. In the mid-shore, the invaders were equally dominant over native mussels, while in the low-shore S. algosus became the most abundant. Notably, the native Choromytilus meridionalis was absent intertidally. In the subtidal M. galloprovincialis was rarely present, while S. algosus maintained a strong presence. The maximum size of native Aulacomya atra and invasive S. algosus in the subtidal was roughly double that recorded in the intertidal zone. Importantly, these results highlight that observations made from intertidal studies of mussel invasions cannot be used to infer subtidal patterns.

The presence of multiple predators can lead to variation in predator behavior and ultimately altered risk for shared prey. This concept has seldom been accounted for in studies that consider predator-driven biotic resistance from native marine predators against invasive prey. This study compared the prey selection of whelks and rock lobsters when co-occurring and when foraging in isolation. When in isolation, both predators preferred the native mussel Choromytilus meridionalis, regardless of the abundance of alternative prey. However, when cooccurring, predation risk for all prey species, including the invasive mussel Semimytilus algosus, increased. This was largely driven by greater variation in prey selection by rock lobsters in the presence of whelks. This indicates that predatory efforts from co-occurring predators can result in stronger predation pressure on invasive prey than would be recognized if predators were assessed in isolation.

Predator-driven biotic resistance is known to be more effective in marine systems than in terrestrial and freshwater environments. However, there is little consensus about when such resistance can be expected to succeed or fail. Here, we reviewed case studies that investigated interactions between native marine predators and alien prey, with the aim of establishing which characteristics are important in determining the outcome of such interactions. Four potential biotic resistance outcome scenarios were identified, with these scenarios progressing from a state of no resistance to successful resistance, i.e. when an alien species is successfully excluded from the native community. Characteristics of native predators and alien prey that likely affect the outcome of biotic resistance were identified, and their presence and absence were noted for each case study. The outcome of each native predator-alien prey interaction was assigned to one of the four biotic resistance outcome scenarios, based on the conclusion of the original study. Multivariate statistics were used to examine potential differences in the suites of characteristics typifying each outcome scenario. These characteristics were found to differ significantly among scenarios, with failure of predator-driven biotic resistance occurring in cases where the alien prey typically had high fecundity, high recruitment and substantial dispersal potential. Conversely, successful biotic resistance was related to the characteristics of native predators including high abundance, strong predation pressure on alien prey, coupled with high feeding rates. This research emphasises the need to integrate information from both trophic groups to strengthen predictions about the outcomes of novel predator-prey interactions.