Thesis working title:

How does complexity promote biodiversity?

Research Summary:

Biogenic habitats - habitats directly created by organisms - are often physically complex structures which support diverse assemblages of other organisms.   Coral reefs, for example, support a huge variety of animals, many of which would not survive in the absence of the coral.   This study investigates the relationship between physically complex habitat created by bryozoans and the diversity of macroinvertebrates that live there.  

            Bryozoans are a phylum of filter-feeding, colonial animals.   Many marine bryozoans create carbonate structures to house themselves as they grow, in a similar way as do corals.   'Frame-building' bryozoans form colonies >=5 cm in 3D, but can be larger - 15-30 cm or more.   On the New Zealand continental shelf, large areas of the seafloor are dominated by them, forming a biogenic habitat known as bryozoan thickets or epifaunal reefs.   Like coral reefs, the bryozoan thickets can be great places for other animals to settle and grow from, or to hide in or feed upon.   Previous research showed over 200 species of invertebrate use this habitat, on Otago shelf alone!   The bryozoans are considered to be ecosystem engineers because they alter the water movement over the seafloor, slowing the boundary layer and creating eddies, which changes the availability of food for other animals and causes suspended sediment to be deposited.  

            My study will examine patterns in the occurrence of frame-building bryozoans and macroinvertebrates at the regional scale (i.e. New Zealand wide) using existing data, and investigate local (an area of continental shelf) and within colony (<10 centimetre) patterns using mimics deployed on, and samples collected from Otago shelf (80 m water depth), in order to answer the following questions:  

•  At which scales is there a relationship between bryozoan-generated habitat and the species richness of associated fauna?

•  Which aspects of the habitat are important to other organisms?   (e.g. bryozoan height, area covered, sediment type around colonies).  

•  How do bryozoans affect the taxonomic and ecological diversity of sample area?   (and conversely, what happens to the macroinvertebrate assemblage if the bryozoans are not there?)

•  What threats does this habitat-type face and has there been a loss of bryozoan-generated habitat in New Zealand?

The photographs show some aspects of fieldwork that was carried out on Otago shelf (September 2007).   The large van Veen-style grab operated well in the gravelly sediments.   Samples were washed over a nest of sieves (5mm, 1mm and 0.5mm), after epifauna was removed and bagged separately.   All samples were fixed and stored for further study in the months ahead.   The bryozoan pictured (approx 20 cm long) is the endemic Cinctipora elegans, plus a bright-orange sponge growing along one side of the bryozoan colony.