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

Summary

An examination of the literature revealed that the dispersal characteristics of wetland biota are highly variable and will result in diverse temporal and spatial patterns of connectivity in the landscape Waterbirds are the most mobile group examined, capable of dispersing over large distances unconstrained by the nature of the landscape. However, the scale of movement varies among species and over time. Although some waterbirds move regularly over hundreds of kilometres, others do so only rarely. Even among the most mobile waterbirds, dispersal capacity becomes constrained during breeding and moulting. During these critical life stages the proximity of suitable foraging wetlands to breeding and moulting sites is likely to play an important role in the persistence of waterbird populations.

In contrast to waterbirds, amphibians have limited mobility, and the nature of the intervening landscape strongly influences movement patterns. Mobility ranges from species that are very sedentary and move less than 0.5 km to the most vagile that may move up to 3 km, although longer distances may be travelled under optimal conditions. Areas of the landscape that offer high resistance to amphibian movement include dry terrestrial areas, saline areas, urban areas and transport networks.

Fish differ in their movement abilities; large fish generally disperse farther than small fish. Water depth can constrain fish movement in different ways. Shallow water can limit movement in large fish that require deeper water to swim, whereas small fish may avoid deep water because the risk of predation is higher. Fish movement between rivers and floodplain wetlands can be influenced by hydroperiod, distance, elevation gradient and the presence of water control structures such as regulators, weirs, levees, and dams. Movement between river drainage systems can occur when flood waters provide temporary connections across the landscape, or when they produce plumes of freshwater in the sea that permit the temporary passage of freshwater species to other catchments.

Aquatic invertebrates are a large group with diverse dispersal abilities. They can be segregated into winged species that disperse by flight and wingless species that rely on vectors to disperse dormant life stages. Among the winged invertebrates, species can be further segregated into strong and weak flyers. Strong flyers may disperse hundreds of kilometres, some using high-altitude currents to increase the distance they disperse.

Wingless insects and plants rely on vectors to disperse dormant eggs, seeds or vegetative fragments. Wind, water and waterbirds are the most important natural dispersal vectors in aquatic systems. Each dispersal vector produces different spatial and temporal patterns of connectivity in the landscape. Wind-mediated dispersal is likely to be more effective in connecting wetlands when they are close together and aligned with the direction of the prevailing winds. Adaptations for dispersal by wind are common in plants seeds, but adaptations in invertebrate propagules, other than a small size, are lacking.

Water serves as a vector for the dispersal of plant and invertebrate propagules. In rivers, flow facilitates longitudinal connectivity by carrying propagules from upstream sites to downstream sites. Flooding facilitates lateral connectivity when floodwaters carry propagules between the river and floodplain. Floods are particularly important for dispersal as they flush propagules that have accumulated in vegetation into streams, and they can fragment and uproot plants which may then be dispersed in floodwaters to distant sites. Buoyant propagules are usually dispersed farther than those that sink. River regulation has altered the timing, frequency and duration of hydrological connections between rivers and floodplains, disrupting dispersal processes and driving changes in community assemblages.

Waterbirds disperse propagules that become attached to the feet, feathers or bill, or are consumed and survive passage through the gut. Waterbirds are an important dispersal vector because they can disperse propagules over long distances and to sites not connected by water-mediated or wind-mediated dispersal. Differences in foraging behaviour or movement patterns between species of waterbirds can influence the types of propagules dispersed and the spatial patterns of connectivity across the landscape.

Assessing wetland connectivity requires a multispecies approach to represent the diverse biota of wetland systems. For such an approach to succeed, maps of connectivity are needed for key groups that share similar modes of dispersal. An appraisal of landscape connectivity models found that only a few are capable of incorporating rules for landscape permeability and the direction and scale of movement to produce maps of connectivity for the organism(s) of interest. Circuit theory accommodates most of these requirements but is computationally demanding, and this currently limits its application at a state-wide scale. Alternative techniques are available in a GIS framework, which can be carried out with more modest computing requirements and processing times, and currently provide the best option for modelling connectivity of Victoria’s wetlands.

Integrating patterns of connectivity for different groups of aquatic taxa is desirable to guide strategic investment and planning, and to improve our understanding of how wetland systems interface with other ecosystems. Producing a map that represents patterns of connectivity for wetland biota could be achieved by combining models for individual groups. Although an integrated map of wetland connectivity will provide a useful tool for guiding policy, an understanding of the patterns of connectivity at a group or species level is needed to inform more targeted management interventions.