Influence of wetland networks on bat activity in mixed-use landscapes
Introduction
Wetlands are a valuable natural resource in need of increased protection or restoration (Zedler, 2003, Zedler and Kercher, 2005), especially in the face of current and future global change. Wetlands immobilize carbon and nutrients and are therefore important for carbon sequestration and water purification (Mitsch et al., 1995, Arrigoni et al., 2008, Knox et al., 2008); they store and slow down water and are therefore important for storm water retention and shoreline stabilization (Cohen and Brown, 2007, Hogan and Walbridge, 2007); and they produce large amounts of plant biomass and are therefore important in food web support (Bunn and Boon, 1993, Hargeby et al., 1994, Lindholm et al., 2007). Despite the recognized need to manage wetlands effectively (Zedler, 2003, Tiner, 2005, Zedler and Kercher, 2005), most wetlands are considered in isolation without consideration of the broader landscape connections (wetland ↔ wetland, land ↔ water) that many mobile species depend on to complete their life cycle (Roshier et al., 2002). Wetland management and restoration activities that do not consider the broader landscape may fall short of their target goals and waste limited funds.
Effective landscape and habitat conservation planning depends critically on the spatial arrangement and connections among habitats (Jordan et al., 2007). A single habitat patch often does not encompass the entire home range of individuals, which need to disperse to cope with fluctuating environments (Roshier et al., 2001). Graph networks have been increasingly and successfully used to describe habitat networks for conservation and restoration (Rothley and Rae, 2005, Estrada and Bodin, 2008, Lookingbill et al., 2008) and to assess connectivity among landscape elements for individual species (Bunn et al., 2000, Neel, 2008). These analyses depend critically on spatial scale (Keitt et al., 1997) where the number of nodes (i.e., patches) within a network, the area of the network, and the ‘length’ of the network (e.g., graph diameter; Fig. 1) are all strongly dependent on the distance at which a focal organism perceives landscape elements (Lima and Zollner, 1996, Pascual-Hortal and Saura, 2007). Thus, quantifying the characteristic spatial scale at which organisms are associated with landscape structure is a crucial first step in any assessment of habitat connectivity.
We studied the importance of wetland habitat connectivity and landscape context to bat activity in National Parks characterized by their mixed land-use setting in the eastern United States. In addition to being a group of species of conservation concern globally (Andelman and Willig, 2002) and locally (Johnson et al., 2008), we selected bats as our model organism as they are potentially affected by both urbanization and wetland habitat availability. One limitation on bat distributions in urban settings is the availability and distribution of roost sites (Rhodes et al., 2006, Neubaum et al., 2007). The availability and distribution of wetlands and riverine environments might also be an important determinant of bat foraging activity due to the abundance of insects in these areas (Ford et al., 2006). For example, higher activity levels have been observed for the little brown myotis (Myotis lucifugus) at lakes and rivers as compared to forests and other terrestrial habitats (Broders et al., 2006), and Johnson et al. (2008) found evidence suggesting that wetland area might be a potentially important control on foraging behavior in urbanized landscapes.
We hypothesized that not just the total area of wetlands but also the spatial distribution of wetlands is critical in allowing bats to effectively use landscapes of heterogeneous land cover. We predicted that bat activity would be higher for more connected wetlands than for unconnected wetlands, as has been shown for waterbirds and other water dependent species (Roshier et al., 2001). We further predicted that the importance of a connected wetland network would differ among species, which, based on expert opinion and evidence from the literature (Francl et al., 2004, Menzel et al., 2005, Broders et al., 2006, Johnson et al., 2008), appear to differ in their affinity for foraging around water, home range sizes, and roosting preferences. Because connectivity analyses are scale dependent, we assessed the characteristic spatial scale at which bat activity was positively associated with wetlands and used this value as the maximum connectance distance when constructing network models. A graph theoretic framework was chosen to construct network models of potential landscape connectivity for the different bat species and to illustrate the value of network analysis in guiding the coordinated management of two of the National Parks’ most valued natural resources – wetlands and bats.
Section snippets
Study area
The National Capital Region (NCR) of the US National Park Service, centered on the District of Columbia and including parts of three US states (Maryland, Virginia, and West Virginia), has seen rapid urbanization over the past several decades. Today the region is a highly fragmented patchwork of forest, agriculture, and developed land. The NCR National Parks, embedded within this matrix, are situated in a variety of landscape contexts from rural to urban. The parks were designed to protect both
Results
Bat activity at the 96 sample locations varied widely between parks (from an average of 0.6 detections per hour for the five focal bat species at Antietam National Battlefield Park to 5.6 detections per hour at Rock Creek Park) and between species (from 0.2 detections per hour for the little brown myotis to 5.6 detections for big brown bats) (Fig. 3). The average variation in activity among species within a park (st. dev. = 3.16) was similar to the variance in activity among parks (st. dev. =
Discussion
The primary objective of this study was to evaluate the efficacy of network analysis in guiding the restoration and conservation of two valued yet threatened natural resources – wetlands and bats. The network graphs produced information for the parks that was distinct from the measures of land cover (percent wetland, percent urban, etc.). For example, Monocacy and Antietam National Battlefields are both preserved in a mixture of open field and forest land covers, and have roughly the same total
Management implications
Wetlands offer many valuable and irreplaceable ecosystem services to human societies including the provision of important habitat for fauna and flora. Ironically, wetlands are some of the most threatened ecosystems of the world (Sala et al., 2000) as wetlands occur where land and water meet, the same areas that are traditionally preferred for human settlement. Protection and effective management of this valuable resource is therefore paramount, and many countries have laws in place that require
Acknowledgements
The authors gratefully acknowledge Joseph Ferrari for assistance with coding our network analysis software. Funding was provided through a cooperative agreement between the UMCES and the NPS I&M Program through the Chesapeake Watershed Cooperative Ecosystem Study Unit, Task Agreement J3992-07-0104. Shawn Carter and Emily Minor reviewed earlier versions of the manuscript.
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