Herbivore and predator pressure in tidepools along an intertidal gradient: no consumption refuge for invasive species!

Non-indigenous species (NIS) can invade marine ecosystems worldwide not only because of higher growth rates and reproductive potential but also due to their ability to escape from native consumers either by defensive traits or by the colonization of spatial refuges. Spatial consumption refuges can be present in tidepools, especially in those from the high shore, with harsh environmental conditions. In order to test this hypothesis, we quantified consumer pressure on NIS in tidepools with different tidal elevations. We deployed tethering assays within 73 tidepools at 30° S (Chile) in 2021 using the following four bait types: two NIS (the green seaweed Codium fragile and the tunicate Ciona robusta) and two reference baits (blades of the kelp Lessonia and dried squid). The two reference baits as well as the two NIS were consumed across all tidepools. Thus, there was no tidal gradient in NIS consumption: higher tidepools did not offer a spatial refuge as native consumers can reach them and consume NIS. Possibly, on wave-exposed coasts, high shore tidepools are frequently splashed, mitigating harsh conditions, and, therefore, reducing the potential for spatial refuges from consumption. While both NIS were eaten by native consumers in tidepools, they are nevertheless successful invaders. Therefore, our results suggest that NIS can compensate for consumer losses in tidepools, which allows them to colonize tidepool systems even if these do not offer spatial refuges from consumption. Tidepools might represent a first invasion window for NIS to enter natural communities, potentially causing important effects on biodiversity of rocky intertidal habitats.