Reduced patch size and kelp density trigger declines in the biodiversity associated with kelp holdfasts

Kelp forests are some of the most productive and important ecosystems on our planet, yet their persistence is being challenged by global changes. Unlike other ecosystems under threat that depend on ecosystem engineering species, kelp forests are often dependent on the population dynamics of a single or very few ecosystem engineering species. This extreme lack of functional redundancy makes kelp forest ecosystems sensitive to kelp population contractions, with potentially severe consequences for biodiversity and human populations. In a large-scale, controlled field experiment, we simulated 2 different trajectories for population contractions in Australasia’s most widespread ecosystem engineering kelp, Ecklonia radiata . Specifically, we simulated the fragmentation and thinning of kelp forests by manipulating the density of kelp transplanted onto experimental reefs. Then, we quantified differences in biodiversity among kelp-holdfast assemblages on our experimental reefs. We show that the assemblages of individual holdfasts in our densest, largest kelp forests supported 1.32 times more taxa than holdfasts in our sparsest, smallest kelp forests. Sparser, smaller kelp forests supported fewer species because of species loss, particularly the loss of colonial, filter-feeding organisms. Moreover, although larger, more sparsely populated reefs may have the same number of kelp as smaller, more densely populated reefs, differences in the spatial arrangement of kelp on these reefs explained more of the variation in taxa prevalence than the number of kelp alone. Overall, our results highlight the widespread declines in biodiversity and simplification of temperate kelp forest ecosystems due to population contractions in a dominant ecosystem-engineering kelp.