Impacts of dugong foraging on seagrass habitats:observational and experimental evidence for cultivation grazing

Abstract

ABSTRACT. In Moreton Bay, Australia, dugongs (Dugong dugon) often graze in large herds at the same location for weeks to months. Such grazing reduced seagrass shoot density by 65 to 95 %, aboveground biomass by 73 to 96% and belowground biomass by 31 to 71 % at 3 sites ranging in size from 2 to 75 ha. Following even the most intense and sustained grazing, the space between surviving tufts of seagrass remains small (<l m') and recovery is usually rapid (months). In this regard, intensive grazing differs from disturbances caused by storms, sedimentation or disease. However, recovery of seagrass meadows can be suppressed by low levels of sustained grazing pressure. The species composition of seagrass meadows can be altered by intensive grazing, which favours rapidly growing, early pioneer species, such as Halophila ovalis, at the expense of slower growing but dominant species such as Zostera capricorni. In Moreton Bay, H. ovaljs is the most nutritious (high nitrogen, low fibre) and the most preferred seagrass grazed by dugongs. 2. capricorni is the least preferred specles. By preventing the expansion of Z. capricorni and increasing the abundance of H. ovalis, this grazing system, termed cultivation grazing, can improve the quality of the dugong's diet.

What was natural in the coastal oceans?

“…Moreover, the much better documented and more recent demise of the dugong (Dugong dugong) in Australia suggests populations of these enormous relatives of the manatee of about 1 million along the Australian coast only a century ago (73). Dugongs plow through seagrass beds in Australia, reducing shoot density and biomass of by up to 90% (74). We will likely never know the equivalent ecological consequences of manatee grazing in pristine seagrass environments.…”

A study into the export of saline water from Hervey Bay, Australia

“…The region is also home to a large number of dugongs and sea turtles that were listed as vulnerable to extinction by the International Council for the Conservation of Nature and Natural Resources. Severe weather events and extreme high river-runoff led to a major sea grass loss and impacted adversely upon the dugong population in the past (Preen 1995, Preen and Marsh 1995 highlighting the regions vulnerability to extreme physical climatic events. Sea grass recovery was monitored for several years (Campbell and McKenzie, 2004).…”

Rocky Reefs Provide Foraging Habitat For Dugongs In The Darwin Region Of Northern Australia

“…Aerial surveys (Marsh and Saalfeld, 1989;Preen et al 1997), land-based and boat-based observations (Anderson 1982(Anderson 1994Aragones 1994) and satellite tracking data (Marsh and Rathbun 1990) indicate that dugong distribution is closely associated with seagrass beds. Algae is known to occur in food samples from dugongs (Heinsohn and Birch 1972;Marsh et al 1982;Erftemeijer 1994;Preen 1995) but its presence in the diet has been described as incidental (Marsh et al 1982) or linked to large-scale losses of seagrass (Spain and Heinsohn 1973;Marsh et al 1982;Preen and Marsh 1995). This note contains opportunistic observations of long-term feeding by D. dugon on algal covered rocky reefs in the Darwin region of the Northern Territory.…”

“…Foraging studies throughout their range indicate that their diet is dominated by seagrass (Marsh et al 1982;Preen 1995;Preen and Marsh 1995;Anderson 1998;Das and Dey 1999). Aerial surveys (Marsh and Saalfeld, 1989;Preen et al 1997), land-based and boat-based observations (Anderson 1982(Anderson 1994Aragones 1994) and satellite tracking data (Marsh and Rathbun 1990) indicate that dugong distribution is closely associated with seagrass beds.…”