Wetland connectivity: understanding the dispersal of organisms that occur in Victoria’s wetlands draft
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A conceptual model of dispersalThe processes that determine the dispersal of aquatic taxa across the landscape are represented schematically in Figure 1 (developed from Morris et al. 2009). The number, size and spatial arrangement of habitat patches in the landscape, the availability and behaviour of dispersal vectors, and the features of the landscape that impede or facilitate movement combine with species dispersal traits to produce unique patterns of connectivity in landscapes. Population size — Population size can affect the strength of dispersal because more individuals are likely to disperse when the population is large, thus increasing the number of individuals that reach more distant sites. Realised landscape permeability — For mobile species such as fish, frogs, waterbirds and winged insects, the distances individuals are able to move and their responses to the landscape determine the pattern and scale of dispersal (Figure 1). For sessile taxa such as plants and many invertebrates, patterns of dispersal result from interactions between propagule traits and the behaviour and availability of dispersal vectors (Figure 1). Dispersal vectors: wind, water and waterbirds — Wind, water and waterbirds are considered the most important natural dispersal vectors in wetland systems (Amezaga et al. 2002). Propagules adapted for wind dispersal are very small and light, or have special adaptations that help them to stay in the air. Wind-mediated dispersal will connect wetlands that are close together and aligned with the direction of the prevailing wind. Water connects habitats longitudinally, laterally and vertically. In rivers, flow facilitates longitudinal connectivity by carrying propagules from upstream sites to downstream sites. Flooding facilitates laterally connectivity when floodwaters carry propagules between the river and floodplain. Vertical connectivity occurs when organisms move between the riverbed and the hyporheic (subsurface) zone. Floods are particularly important for dispersal because they flush propagules that have accumulated among vegetation into streams, and they fragment and uproot plants which are then dispersed in floodwaters to distant sites. Buoyant propagules are usually dispersed farther than those that sink. Propagules are dispersed by waterbirds when they become attached to the feet, feathers or bill, or when they are consumed but survive the passage through the gut. Waterbirds can disperse propagules to sites not connected by water and are important in dispersing propagules between catchments. In this way waterbirds play a critical role in connecting aquatic habitats. Realised dispersal — Realised dispersals are dispersals that result in successful establishment or the exchange of genetic material (e.g. pollen transfer). Upon arrival at a site, the success of colonisation will depend on the habitat requirements of the species. If the conditions are unsuitable the dispersers will perish, although plants and invertebrates with dormant propagules may remain viable in the soil seed bank. The potential for plant and invertebrate propagules to establish later if conditions become suitable will depend on how long the propagules remain viable and how variable the local conditions are. 
Figure 1. Conceptual model of dispersal processes for different groups of aquatic organisms. White boxes indicate species traits that will influence dispersal and establishment (developed from Morris et al. 2009). Positive feed-back mechanisms — The establishment of new populations results in an increase in local species richness, with benefits that extend to the regional population (metapopulation). Increased population size increases the strength of dispersal regionally, and this in turn increases the likelihood of further colonisation and reduces the probability of extinction, creating a positive feedback effect. Temporal variability — Wetland connectivity is a highly dynamic process. Wetting and drying cycles determine the frequency with which water connects habitats, and wetlands can alternate from high connectivity during floods to extreme fragmentation during droughts. Australian systems in particular are characterised by such extreme hydrologic variability (Arthington and Pusey 2003). Hydrology not only affects the structural elements that define the connectivity of the landscape — it also shapes the composition, abundance and fecundity of wetland taxa, and thus the type and abundance of organisms that disperse. Many methods have been developed to capture the movement of organisms among habitats, but most provide only a snapshot of dispersal. The following sections discuss five methods that differ greatly in cost, effort, level of accuracy and temporal and spatial resolution.
Mark–recapture studies are a traditional method of assessing the movement of large animals. Individuals are marked in some way (e.g. tags, leg bands, clipping or notching, paint or dye marks) and subsequently identified at other locations over time. This has been the key method for tracking migratory birds, but recovery rates of marked individuals can be low. For example, only about 18% of birds banded by Frith (1959) were recovered 15 months later, and of 5060 juveniles American Wood Frogs (Rana sylvatica) marked only 9% were recaptured (Berven and Grudzien 1990). Some organisms such as invertebrates may be too small to mark or may be easily damaged by this method, although dragonflies have been successfully tracked by marking wings. As individuals move farther away from the site where they were marked, the potential dispersal area may become too great to search efficiently, and low recapture rates limit the detection of long-distance dispersal using this approach (Whitlock and McCauley 1999). |