Human disturbance, mangrove invasion and the structure of endangered salt marsh communities

A project undertaken at The Institute of Conservation Biology and School of Biological Sciences, University of Wollongong, and supervised by Dr Todd Minchinton

Salt marshes are highly productive habitats that support the healthy functioning of estuarine and coastal ecosystems and provide food and shelter for a diverse suite of marine and terrestrial species of ecological and economic importance.  Despite their significance, salt marshes are one of the most heavily impacted coastal habitats and are currently listed as an Endangered Ecological Community in New South Wales.  Indeed, recent surveys in southeast Australia have discovered a recent decline in the area occupied by salt marsh and a concurrent “invasion” of marshes by native mangroves.  This landward encroahment of mangroves poses an immediate threat to the remaining salt marshes in estuaries of New South Wales, but the causes of mangrove invasion are unknown.  Several explanations have been put forth to account for the increased colonisation of salt marshes by mangroves, including natural processes, sea-level rise, and human influence due to increased nutrient loading caused by urban and agricultural development of the estuarine landscape.

This project was developed to redress our current ignorance about the causes of mangrove invasion and to determine the consequences of nutrient enrichment for plant species distributions in endangered salt marshes.  A mechanism that might explain how mangroves can successfully invade salt marshes is that, because salt marshes are nitrogen limited habitats, increased nutrient load has created conditions conducive to the colonization of mangroves, allowing them to gain competitive advantage over and displace the other species of plants in the marsh.  The main objective of this project is to use small-scale field experiments and large-scale surveys to determine to what extent nutrient load is altering plant species distributions across temperate salt marshes in southeast Australia by facilitating invasion of the mangroves and the competitive displacement of salt marsh plants.  This project is significant because it will experimentally test how increases in the terrestrial run-off of nutrients due to sewage and fertilizers might facilitate invasion of mangroves into salt marshes.  The research will identify management strategies to control the invasion of mangroves and conserve salt marshes in southeast Australian estuaries.