The preliminary results are now in: urbanisation at the landward boundary of coastal salt marshes is associated with a two-fold reduction in the density of molluscs inhabiting adjacent coastal salt marsh. Our results found, however, that despite this reduction in overall molluscan abundance, urbanisation had no effect on the number of different mollusc species (i.e. richness) present in adjacent salt marshes.
The overall aim of this project was to examine effects of shoreline urbanisation on the unique molluscan fauna within endangered salt marsh of south-eastern Australia. My student, Geoff Clarke did a stellar job with this project, counting and identifying over 7,500 individual snails from six species, most of which are restricted to salt marsh or similar vegetation that borders estuaries and coastal embayments.
Geoff surveyed 9 ‘urbanised’ and 9 ‘forested’ (which he termed natural) reference sites across three embayments (see accompanying satellite images). Each site consisted of a 40 m x 10 m quadrat at the landward boundary between marsh and casuarina forest. Molluscs were sampled from within 15 50 cm x 50 cm subplots that were randomly distributed throughout each site.Geoff also recorded the number and abundance of resident plant species and litter cover.
Interestingly, salt marsh patches adjacent to urbanised and forested landscapes were similar in terms of number and abundance of plant species, litter cover, bare ground cover and extent of tidal inundation.
What, then, is the mechanism by which urbanisation drives a reduction in molluscan community productivity? We are exploring the following ideas in out next field season:
(1) Chemical pollutants in urban storm-water runoff limits growth of algal soil crusts upon which gastropods graze, thus indirectly limiting population sizes.
(2) Urbanisation inhibits the fecundity and recruitment of resident molluscs, perhaps through chemical disruption of embryo development and recruit establishment.
(3) Urbanisation increases rates of predation of resident gastropods (Phallomedusa and Ophicardelus) by fishes and birds.
It would be great to hear from like-minded benthic ecologists interested in these and other interactions and getting involved with future projects at University of Wollongong!
It is muddy work but immensely enjoyable!!! This project was cosupervised by Matt Rees, member of FishThinkers research hub (http://fishthinkers.wordpress.com/)