Port Phillip Bay is almost an inland sea, having only a narrow 3½ kilometre-wide entrance at the Heads. Despite its size, the Bay has very few islands. However, situated near the Heads, 5½ kilometres from the mainland, is a small and unique group of islands - Mud Islands. They are remarkable not only for their physical features, but also for their bird-dominated ecology. 

Mud Islands have long been of scientific and conservation interest. This is reflected in their listing on the Register of the National Estate and on the Ramsar list of wetlands, and more recently by their inclusion within the Port Phillip Heads Marine National Park. 

Mud Islands are the surface expression of the Great Sand, the largest shoal in Port Phillip Bay. They consist of three shrubby sand islands enclosing a shallow tidal lagoon, which is fringed by salt marsh. The island group is 1200 x 900 metres in size, has a total area of 86 hectares with a land area of 60 hectares, and reaches a height of 4 metres. Despite the name, the islands, including their outer beaches, are mainly composed of shelly sand. Resembling an atoll, Mud Islands form a unique feature in the southern Australian landscape. 

Formed by wind and wave action, Mud Islands are 'anchored' by outcrops of phosphate rock. This rare rock type forms below guano deposits as phosphate from guano (accumulations of bird droppings) leaches down and combines with shelly sand below to form hard calcium phosphate. Phosphate rock is resistant to marine erosion and keeps the entire system in place. Birds have thus played a fundamental role in the physical evolution of the system. 

The guano itself was mined in the 1860s and 70s, amidst much controversy at the time as to the possible effects on the islands. Archival records show the original extent of the guano in 1859, coinciding with the distribution of the phosphate rock. Chemical analysis of the rock gives phosphorus concentrations of between 4 and 8 per cent, well within the range for phosphorus in phosphate rock elsewhere. 

The Mud Islands system is remarkably physically dynamic, continually changing in size and configuration, due to storms, beach and dune sand movement, and growth of salt marsh. Nautical survey charts show that the system has been changing in size and configuration since at least 1836, with the phosphate rock anchor points providing basic stability. The inlets have been particularly dynamic, having opened, closed or shifted many times. 

Superimposed on this ever changing physical environment are the flora and fauna, and these also undergo enormous change. Birds dominate the ecology of Mud Islands. A total of 87 species are recorded and 15 of these have been recorded nesting. The entire area above high water mark is used for nesting, with some species forming extensive colonies. The major species are Silver Gull, Straw-necked Ibis, Australian White Ibis and White-faced Storm-Petrel. Other colony forming birds are Australian Pelican, Crested Tern and Caspian Tern. The central lagoon is visited by thousands on intercontinental waders in the warmer months. 

Colonial breeding birds interact with their vegetation habitat in several ways. These include manuring, trampling, perching, nest construction and introduction of plant species. Each species has a characteristic relationship with its vegetation habitat, which may be mutualistic, where the birds and vegetation both benefit, or antagonistic, where either the birds or the vegetation are adversely affected. 

The vegetation of Mud Islands is highly dynamic. The islands support the largest stands of Coast Saltbush Atriplex cinerea in Victoria, and the largest stands of Coast Hollyhock Malva sp. aff. australiana throughout its geographic range. These shrubs compete for space on the dunes but have contrasting ecology: Coast Saltbush is a primary coloniser of dunes favoured by marine influences, Coast Hollyhock a secondary coloniser of bird colonies favoured by avian influences. Two factors force continual change: (1) marine events such as storms, tides, erosion and deposition; (2) the intensive nesting activity of colony forming birds. These factors interact to produce conditions variably affecting the two competing shrubs. 

Comparison of six reliable flora surveys between 1906 and 1996 reveal many changes in the flora and vegetation over time. The flora of Mud Islands was relatively stable during most of the 20th century, apart from a gradual loss of native species due to erosion at North Cape. But after 1972 there was a dramatic increase in species numbers, particularly in introduced species but also in native species. By 1996 the flora had actually tripled in size. 

Silver Gulls underwent a population explosion in the 1970s and they are clearly implicated in the increase in the flora. Gulls cause vegetation change within their colony sites around the world, most notably by increasing the number of exotic species in the vegetation. The main factors involved appear to be soil disturbance, increased nutrients and the transport of seeds. 

Silver Gulls were recorded nesting in 1838 but they later abandoned the islands as a breeding site, probably due to human occupation of the islands by fishing families and guano miners in the second half of the 19th century. Over a century later, in 1959, they returned to breed when two pairs constructed nests, one in a beach-washed wooden fruit case. The subsequent population explosion was due to eviction of the gulls from their traditional breeding site at Port Melbourne due to industrial development and the increasing availability of food at urban rubbish tips. Mud Islands now support the equal largest breeding colony of Silver Gulls in the world, with an estimated 40-50 thousand pairs breeding by 1986 (the other large colony is Big Island off Port Kembla, New South Wales). These two major colonies dwarf all the other Silver Gull colonies. Gulls banded as chicks on Mud Islands have been recovered in many other parts of Australia including 1,439 kilometres away in Queensland. 

From 1990 onwards, Straw-necked and Australian White Ibis commenced nesting on Mud Islands, and in huge numbers. This led to further dramatic change in the vegetation as the previously dominant Coast Saltbush was replaced by Coast Hollyhock over large areas. Coast Saltbush is a brittle shrub that ibis readily use to construct large, elaborate stick nests. Coast Hollyhock is so fibrous that it cannot be used by ibis when alive, and it rapidly colonises bare areas created by ibis. Thanks to the ibis, the hollyhock population has risen to over 5000 mature plants, making it the largest known population of this rare species in the world. 

Bird and bat breeding colonies are the most nutrient-rich ecosystems in nature, with phosphorus and nitrogen concentrations many times the background levels. This has led to the evolution of plant species that specialize in these highly disturbed, extremely nutrient-rich but well separated habitats. These guano-loving or 'ornithocoprophilous' species are dispersed by birds. Coast Hollyhock is such a species, and it is superbly adapted to life in bird colonies. It thrives on guano, and is one of 18 known plant species in the world flora that are largely or entirely restricted to bird colonies. Its tough fibrous stems and branches make it indestructible by even the largest nesting birds. It can also tolerate birds tunnelling in its root system, unlike other species such as Coast Saltbush. 

Mud Islands displays all of the characteristics of bird colony ecosystems. However, the islands are unique in several ways. In particular, the proximity to large human populations has facilitated the growth of huge populations of commensal birds such as gulls and ibis. This has intensified the bird-dominated ecological processes to extreme levels. Although the system is indirectly affected by humans in this way, Mud Islands are a haven for many species of shorebird, and remain the spectacular natural wonder of Port Phillip Bay. 

Further reading 

Bird, E.C.F. (1973). Physiographic changes at Mud Islands, Port Phillip Bay. The Victorian Naturalist 90: 157–165. 

Menkhorst, P.W., Kerry, K.R. & Hall, E.F. (1988). Seabird islands 181: Mud Islands, Port Phillip Bay, Victoria. Corella 12: 72–77. 

Yugovic, J. 1998. Vegetation dynamics of a bird-dominated island ecosystem: Mud Islands, Port Phillip Bay, Australia. PhD thesis, Monash University.