Wetland connectivity: understanding the dispersal of organisms that occur in Victoria’s wetlands draft

Waterbirds

Aerial counts of waterbirds in south-eastern Australia over three latitudes encompassing Victoria and the most southerly part of New South Wales show large annual variations in waterbird numbers, with total counts varying from under 20 000 to over 140 000 between 1983 and 2004 (Kingsford and Porter 2006). Habitat depletion has greatly reduced waterbird populations, and several species are classed as threatened (Appendix 2).

1. Habitat preferences

Some waterbirds that occur in Victoria are common and familiar birds that occupy a range of habitats (e.g. Pacific Black Duck, Masked Lapwing and White-faced Heron). Others have more specialised requirements and only occupy habitats with certain levels of aquatic vegetation cover and salinity. Some of the less common species tend to be associated with large and complex wetlands that provide a range of habitat resources.

2. Movement

In Australia, changes in resource availability as wetlands flood and dry strongly influence waterbird dispersal (Kingsford and Norman 2002, Chambers and Loyn 2005). A regular seasonal pattern of movement takes place in response to changes in habitat availability in some species. For example, in summer, many waterbird species in inland south-eastern Australia become concentrated in large swamps as smaller ones dry. As densities increase, a portion of the population disperses. Although dispersal occurs in all directions, most follow the Murray River and its tributaries to South Australia and Victoria and return inland in winter (Frith 1977).

The geographical arrangement of wetlands in the landscape and the dispersal capacity of waterbirds influence the dynamics of waterbird populations. For example, a study of waterbirds in 30 marshes in Iowa, USA, found that clustered wetlands contained more species than isolated wetlands, and the total wetland area within 5 km of each wetland explained the most variation (42%) in species richness among wetlands (Browne and Dinsmore 1986). The clustering of wetlands probably supported higher species richness for two reasons. Firstly, clustering of wetlands of various sizes produces a mosaic of diverse habitats both spatially and temporally as habitats fill and dry at different rates. For example, Pacific Black Duck and other species with limited salt tolerance probably only forage in saline wetlands when freshwater wetlands are accessible nearby (Kingsford and Norman 2002, Loyn et al. 2006). Secondly, the proximity of diverse habitat types allows a wider range of bird species, differing in habitat requirements and dispersal capacity, to utilise them.

On a larger spatial scale, Roshier et al. (2001) examined the availability of wetlands for waterbirds in the Lake Eyre Basin of central Australia, where there are thousands of temporary wetlands. Although the availability of wetland habitats in this arid region is highly variable at small spatial scales, at larger spatial scales habitat availability increases as wetlands fed by different rivers fill and dry at different rates creating a mosaic of wetlands that hold water at different times. This increases the availability of habitats through time and may provide core habitat for waterbirds capable of dispersing the long distances that separate individual wetlands. Highly nomadic waterbirds are able to use habitat patches separated by hundreds of kilometres and are therefore more resilient to habitat loss than more sedentary species.

Although waterbirds are capable of dispersing long distances, movement can be restricted by breeding or moulting. Some waterbirds return to the same breeding or moulting sites each year, producing a narrower pattern of movement than suggested by their mobility (Kingsford and Norman 2002). During breeding, the need to feed and protect flightless juveniles imposes a reliance on nearby wetlands for foraging (Bryan and Coulter 1987). For example, in North America the foraging flights of nesting adults of the Western Great Egret (Casmerodius albus), Snowy Egret (Egretta thula), Tricolored Heron (Egretta tricolor) and American White Ibis (Eudociumus albus) were all within 30 km of nesting sites (Smith 1995). Where foraging flights exceed 25-30 km in length breeding success has been found to decline in several ciconiiform wading birds (see Smith 1995). Breeding can also limit movement patterns if juveniles have more specific habitat requirements than adults. Nasal glands that secrete salts and help maintain salt regulation are not fully developed in juvenile ducks (Riggert 1977), so access to freshwater sources is required, restricting habitat utilisation (Halse 1987). These examples illustrate that it is during these critical life stages that the patterning of wetland resources in the landscape probably exerts its greatest influence on waterbird populations.

Although there is some information about the dispersal capacity of waterbirds over large scales, there is little information on the frequencies or patterns of waterbird movements between wetlands over small spatial and temporal scales. Knowledge of these finer-scale movements, particularly during critical life stages, is needed to identify the landscapes elements required to sustain waterbird populations. Haig et al. (1998) has suggested that critical information gaps exist in understanding seasonal movements between multiple sites, including:

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  how well waterbirds survey habitats before breeding sites are selected
  
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  whether early nest failure prompts movement to new sites to re-nest
  
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  utilisation of foraging areas away from nest sites
  
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  utilisation of brood rearing habitat away from the nest site
  
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  moult migration
  
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  movement of young adults to access age-specific resources
  
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  movement within migration and winter sites.
  

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  wetting and drying cycles are important in providing food resources for waterbirds
  
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  floods of sufficient size and duration are needed to trigger waterbird breeding
  
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  mosaics of wetlands that fill and dry over different cycles increase habitat availability over time
  
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  the influence of temporal and spatial patterns of wetland types in the landscape probably exert their greatest effects on waterbird populations during critical life stages (breeding and moulting), when dispersal capacity is more limited and resource requirements are more specialised
  
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  for waterbirds with more limited capacity to migrate large distances, cluster of wetlands separated by a small distance are likely to be beneficial
  
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  temporary habitats can provide stepping stones that facilitate movement among habitats but also have the potential to trap less mobile waterbirds when they dry