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Because many animals nest only in particular habitats, they may need to disperse into neighbouring habitats to breed and to find food. Accordingly, plants that depend on these animals for pollination or seed dispersal are also affected by the ability of pollen or seed vectors to move through the landscape (Tewksbury et al. 2002; Kreyer et al. 2004). As animals exploit multiple habitats to meet their various requirements, landscape-level approaches may provide insight into identifying how changes of physical environments at different spatial scales influence both plant and animal populations (Wiens 1997; Haynes & Cronin 2004).
The resources needed by many insect pollinators, including mates, nectar and pollen, nesting materials and larval food plants, may occur in different habitats in the landscape. For example, different pollinator guilds (bumblebees, honeybees and large wild bees) prefer patches of different qualities in the mosaic of habitats in an agricultural landscape (Hirsch et al. 2003). Strong fliers, such as bumblebees and honeybees, often forage great distances from their hives in order to exploit the most rewarding floral patches (Heinrich 1979; Visscher & Seeley 1982; Beekman & Ratnieks 2000). By contrast, weak-flying pollinators, such as solitary wild bees and some butterflies, may have limited ranges and forage for resources only near the nest or roosting site (Sutcliffe & Thomas 1996; Gathmann & Tscharntke 2002).
Heterogeneous landscapes and the spatial arrangement of habitats therein may also influence dispersal abilities and movement patterns of foraging insects that disperse into adjacent habitats. For example, different prairie edge types have been shown to influence the emigration rates of two butterfly species, a habitat generalist and a habitat specialist (Ries & Debinski 2001). Thus, the inherent nature of mosaic habitats often creates ecological barriers or movement corridors for many insect pollinators, and, as a result, impedes or facilitates dispersal ability to meet these species-specific requirements (Sutcliffe & Thomas 1996; Haddad 1999).
The ability of insect pollinators to move between habitats to fulfil their resource requirements may depend in part on their size, strength and flight ability. Studies of foraging ranges of insect pollinators in various habitats suggest that maximum flight distance varies greatly among species (Osborne et al. 1999; Beekman & Ratnieks 2000; Steffan-Dewenter & Kuhn 2003). For example, honeybees fly between 1 and 6 km away from the hive for food, depending on the floral richness of the habitat (Visscher & Seeley 1982; Waddington et al. 1994), while bumblebees fly up to 2.2 km for food (Kreyer et al. 2004). Although flight distance estimates for small-bodied, weak-flying pollinators are rare, the maximum foraging distances for several species of solitary bees range from 150 to 600 m, and foraging distance is positively correlated with length of the body (Gathmann & Tscharntke 2002). Therefore, pollinators that have long-distance flight capabilities, particularly large-bodied bees and large butterflies, may play a vital role in the reproduction of plants in the isolated areas of heterogeneous landscapes (Janzen 1971; Bawa 1990).
Plants that require pollinators to transfer pollen in isolated or fragmented habitats may be reproductively disadvantaged because pollen vectors may be in low density or have restricted flight capabilities (Ågren 1996; Steffan-Dewenter & Tscharntke 1999). In Asclepias tuberosa (Apocynaceae), annual differences in insect visitor abundances, together with differences in pollinator visitation rates, have been used to explain pollinator effectiveness (Fishbein & Venable 1996).
The Everglades landscape is a natural mosaic of mostly pristine habitats that are structured by the natural flow of water through the ecosystem (Davis & Ogden 1994; Lodge 1994). Tree islands, higher elevation outcrops bearing temperate and tropical shrubs and trees, pepper the landscape in various habitats and plant communities. This habitat type is particularly abundant in marl prairie, a marsh habitat characterized by a thin layer of calcitic soil (marl) covering the limestone substrate, which typically is inundated with water (hydroperiod) for 3–7 months. Marl prairie typically experiences the shortest hydroperiods of all marsh types in the Everglades (Olmsted et al. 1980; Lodge 1994).
We investigated the effects of the spatial arrangement and size of tree islands on the distribution, diversity and abundance of insect pollinators in both the tree island and the marl prairie communities. We also examined the effects of these patterns on the pollination success of Asclepias lanceolata, a wetland species growing at varying distances from tree islands of varying sizes. We selected A. lanceolata as our study species because it grows in marl prairie habitat but not in tree islands and we were able easily to quantify both removal of pollinia from flowers (Wyatt & Broyles 1994) and presence/absence of pollinia attached to appendages of foraging insects.