A sixth-level habitat cascade increases biodiversity in an intertidal estuary

Abstract

Many studies have documented habitat cascades where two co-occurring habitat-forming species control biodiversity. However, more than two habitat-formers could theoretically co-occur. We here documented a sixth-level habitat cascade from the Avon-Heathcote Estuary, New Zealand, by correlating counts of attached inhabitants to the size and accumulated biomass of their biogenic hosts. These data revealed predictable sequences of habitat-formation (=attachment space). First, the bivalve Austrovenus provided habitat for green seaweeds (Ulva) that provided habitat for trochid snails in a typical estuarine habitat cascade. However, the trochids also provided habitat for the nonnative bryozoan Conopeum that provided habitat for the red seaweed Gigartina that provided habitat for more trochids, thereby resetting the sequence of the habitat cascade, theoretically in perpetuity. Austrovenus is here the basal habitat-former that controls this “long” cascade. The strength of facilitation increased with seaweed frond size, accumulated seaweed biomass, accumulated shell biomass but less with shell size. We also found that Ulva attached to all habitat-formers, trochids attached to Ulva and Gigartina, and Conopeum and Gigartina predominately attached to trochids. These “affinities” for different habitat-forming species probably reflect species-specific traits of juveniles and adults. Finally, manipulative experiments confirmed that the amount of seaweed and trochids was important and consistent regulators of the habitat cascade in different estuarine environments. We also interpreted this cascade as a habitat-formation network that describes the likelihood of an inhabitant being found attached to a specific habitat-former. We conclude that the strength of the cascade increased with the amount of higher-order habitat-formers, with differences in form and function between higher and lower-order habitat-formers, and with the affinity of inhabitants for higher-order habitat-formers. We suggest that long habitat cascades are common where species traits allow for physical attachment to other species, such as in marine benthic systems and old forest.