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, coastal salt marsh is one of the most heavily impacted coastal habitats and is currently listed as an Endangered Ecological Community in New South Wales.  

Recent surveys in southeast Australia have discovered a decline in the area occupied by salt marsh and a concurrent “invasion” of marshes by the native mangrove Avicennia marina (Photo 1).  This landward encroachment 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.  The main objective of this project was to use field experiments and 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.

Field surveys determined that propagule dispersal and seedling abundance of the mangrove Avicennia marina varied along the environmental stress gradient of the salt marsh, with greater invasion in less stressful areas lower in the marsh (and closer to the mangrove forest) and fewer mangroves in more stressful areas higher in the marsh.  These patterns were likely due to extremely localized dispersal of mangrove propagules into the marsh and the increasingly stressful conditions in the salt marsh from low to high tidal elevations.

In small-scale field experiments we fertilised small patches of coastal salt marsh and examined the invasion dynamics of the mangrove Avicennia marina.  Enhancing nutrient load not only had a positive influence on salt marsh plant production (Photo 2), but also facilitated mangrove invasion (Photos 2 and 3).  Nutrients did not influence the establishment of mangroves, but promoted their growth and reproduction by alleviating physiological stress experienced by mangrove seedlings under harsh salt marsh conditions.  Interestingly, salt marsh plants facilitated the establishment of mangrove propagules by buffering the hypersaline soil conditions, although this varied with marsh plant species and position along the intertidal environmental stress gradient of the marsh.

Therefore, over time in marshes subjected to nutrient enrichment the pattern of mangrove invasion within marshes will likely be diffusion from the landward edge of the mangrove forest to the terrestrial border of the salt marsh.  Our study also revealed that mangrove establishment in salt marshes was greatest in years of heavy rainfall, such as occurs during La Niña conditions in this region.  We therefore speculate that mangrove invasion proceeds sporadically over time and is facilitated when there is a coincidence of nutrient and freshwater runoff promoting relatively benign conditions conducive to the establishment and early growth of mangroves in the typically harsh abiotic environment of coastal salt marshes.

Outcomes of this research can be used by environmental managers to develop strategies to control mangrove invasion into Endangered Coastal Salt Marsh.  For example, it appears that, in general, factors that promote mangrove propagule dispersal and reduce harsh abiotic stresses of salt marshes (nutrients, salt marsh plants, fresh water input) facilitate mangrove invasion.  Managers can identify and determine how local salt marshes (and the adjacent landscape) have been modified to increase these factors that promote mangrove invasion success and target how these might vary in space (i.e. with future landscape developments) and time (e.g. ENSO, climate change).



Photo 1. Invasion of salt marsh by the mangrove Avicennia marina.

Photo 2.. Experimental fertilisation of salt marsh Sarcocornia quinqueflora (bright green patches) promotes mangrove invasion.

Photo 3. Seedlings of Avicennia marina growing in fertilised patches of salt marsh.

Photo 4. Increased nutrient load facilitates mangrove invasion and reproduction.