Habitat Cascade: An overlooked process maintaining biodiversity in New Zealand estuaries

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1. Abstract

Past research into the drivers and maintenance of biodiversity has overlooked the

importance of ‘habitat cascade’ (HC). In HC, primary habitat-forming species facilitate

secondary habitat-forming species, thereby indirectly facilitating those species

(hereafter inhabitants) that depend on the secondary habitat-former. Examining an

ecosystem through the lens of HC may reveal facilitative processes that allow species

persistence and high biodiversity. In this study, we used surveys and experiments in the

Avon-Heathcote Estuary, Christchurch, to document how HC can affect distribution and

abundance of species often overlooked in inventories and biodiversity assessments.

First, we demonstrated the existence of a long cascade incorporating up to six habitat

formers. Cockles provided hard substrate for seaweeds, which provided habitat for

trochid snails, which in turn facilitated encrusting and sessile animals, more seaweeds,

and more snails. Second, we identified underlying patterns of this process. The strength

of the HC was strongly density-dependent; increasing the amount of secondary habitat

formers positively influenced the abundance of inhabitants. Increasing the amount of

sampled habitat formers also allowed us to document the presence of ‘ubiquitous’

species that nevertheless rarely are considered parts of estuarine biota. In addition, we

found that inhabitants had varying affinities for different habitat formers, as both

generalist and specialist species were documented. The diversity of habitat structure, in

combination with varying habitat-affinities, allows the persistence of many species in the

estuary. We conclude that HC has been an overlooked process but can be a valuable

tool for examining biodiversity in many ecosystems.

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