Connectivity among wetlands matters for vulnerable amphibian populations in wetlandscapes
Introduction
Wetlands are important ecosystems as they provide several functions and services (Calhoun et al., 2016; Yao et al., 2017; Creed et al., 2017) and constitute an important source of biodiversity (Costanza et al., 1997; Gibbs, 2000). In recent decades, many wetlands have been drained because of urban or agricultural expansion (Davidson, 2014; Dixon et al., 2016; Golden et al., 2017). Wetland loss impacts on biodiversity both directly, by removing habitat (Gibbs, 2000), and indirectly, by increasing the distance among remaining wetlands and resulting in functional isolation and increasing mortality of organisms migrating from one wetland to another (Baguette et al., 2013). Wetlands are not isolated features; on the contrary, they are dynamic, complex ecosystem with biotic and abiotic connections to other wetlands on the wetlandscape (Cohen et al., 2016; Thorslund et al., 2017). Understanding the ecological dynamics of wetlandscapes is important to sustaining biodiversity (Semlitsch and Bodie, 1998; Gibbs, 2000).
In particular, amphibians’ survival is influenced by both wetland habitat and wetland connections to other wetlands (Dudgeon et al., 2006). In fact, these wetland qualities determine the success of amphibians’ breeding (Mushet et al., 2012): wetland habitat is used by adults for mating and by offspring to complete their metamorphism from eggs. Factors such as availability of resources and dispersal capabilities influence the amphibian population in wetlands (Pechmann et al., 1989; Semlitsch, 1996). Availability of resources depends on wetland habitat properties (e.g., area, vegetation) and on the number of amphibians competing for available resources. Amphibian dispersal relies on wetland distribution within the surrounding terrestrial habitat. Every year, at the end of the summer, amphibians start their migration through the terrestrial habitat and the following spring they reach a new aquatic breeding habitat (Pittman et al., 2014). Alteration of wetland habitat and distribution within the landscape, such as wetland loss, negatively influences both breeding and dispersal success by decreasing wetland density and increasing travel distances for amphibians (Gibbs, 1993).
Management strategies have been implemented to protect biodiversity promoted by wetlands. Many of these management strategies focus on wetlands of special importance (Amezaga et al., 2002). Policy goals vary from “no net loss” to “net gain” (Accatino et al., 2018) to general statements about the need to address adverse impacts to these wetlands (e.g., Calhoun et al., 2016). Few of these management strategies focus on the physical, chemical, or biological connections among wetlands (e.g., Cohen et al., 2016). Although it is widely recognized that wetland connectivity is important for biodiversity (Semlitsch, 1996; Semlitsch and Bodie, 1998; Skelly et al., 1999; Marsh and Trenham, 2001; Cushman, 2006), concrete strategies in policies are still not well formulated. The lack in wetland policies of clear operational strategic schemes based on wetland connectivity is at least in part due to the lack or rarity of quantitative assessments of the role of wetland connectivity in sustaining wetland biodiversity. Important steps forward would be to determine if and how wetland connectivity plays a role in sustaining biodiversity in a wetlandscape (Fortuna et al., 2006; Albanese and Haukos, 2017), and to explore if management interventions on the wetland itself (i.e., wetland removal or restoration) are influenced by wetland connectivity.
The “sink-source” framework describes the distribution of species in the variety of interconnected habitat patches within a region (Pulliam, 1988; Watkinson and Sutherland, 1995). According to this framework, a productive patch serves as a source of individuals, which are dispersed to less productive patches called sinks (Pulliam, 1988; Dunning et al., 1992). Pulliam (1988) argued that in sink habitat patches reproduction is insufficient to balance local mortality, whereas in source habitat patches reproduction balances local mortality; the population in sinks is thus maintained by immigration from sources. Most studies classify sinks and sources only by demographic measures (i.e., birth and death rate) (Watkinson and Sutherland, 1995). The role of connectivity in the classification of sinks and sources has not been explored.
Models are useful to test the “sink-source” framework for exploring organism dispersal through wetlandscapes, especially when empirical data are lacking or extremely difficult and costly to collect (Pittman et al., 2014). Patch-based models (e.g., Skelly and Meir, 1997; Trenham, 1998) focus on population dynamics within patches, which are important to describe fundamental ecological processes such as breeding (Marsh and Trenham, 2001), interspecific competition, and predation (Wilbur, 1997; Beebee et al., 1996). Patch-based models were successfully applied to wetlandscapes (Marsh and Trenham, 2001; Wilbur, 1997). However, an exclusively patch-based approach does not consider the role of wetland isolation and the mobility of individuals to other wetlands (Cushman, 2006). In contrast, network-based models focus on connectivity within a network’s node and they can be applied to wetlandscapes too (e.g., Albanese and Haukos, 2017). They make it possible to quantify changes to the connectivity of wetlands and identify wetlands critical to the maintenance of the whole system connectivity. Network-based models are useful tools for combining both the within-wetland population dynamics and the dispersal of individuals among wetlands (Estrada and Bodin, 2008). Network-based models can be used to identify keystone patches that are integral to the persistence of populations (Urban and Keitt, 2001; Keitt, 2003) and to quantify the robustness of populations to wetland loss (Bunn et al., 2000; Hanski, 2001; Jordán et al., 2003).
In this paper, we addressed the role of wetland connectivity in determining the role of different wetlands to sustain amphibian populations. We focused on amphibian species characterized by a bi-phasic life-cycle, migrating into different wetlands during the course of their life. We built a model of amphibian population dynamics in a wetland network and we formulated scenarios to address two research questions: how does the connectivity of a wetland influence the abundance of the local population in the wetland itself? And, how does a management intervention on a single wetland (e.g., wetland removal or wetland restoration) influence the total landscape population by changing connectivity within the wetlandscape?
Section snippets
Methods
We focused on amphibian species with life history traits characterized by a terrestrial and an aquatic phase, but the approach could be adapted to amphibian species with other life history traits. In summer, amphibians congregate in wetlands for mating. At the end of the summer, amphibians leave wetlands and migrate through the terrestrial habitat searching food and refuges for overwintering until the next spring, when they disperse again, looking for aquatic breeding habitat (Pittman et al.,
Single-network scenario
The simulation on the single-network scenario showed that the total population abundance, after a transient phase, reached a steady state. The local population in each wetland behaved in the same way. This shows that the length of the time horizon (60 years) was sufficient to represent the population dynamics of the wetland network. Hereafter, we present the results referred to the steady state. For the wetland network generated (Fig. 2a), we calculated the frequency distribution of the Indegree
Discussion
Wetlandscapes sustain sink-source population dynamics with individual wetlands playing different roles (sources or sinks) according to their Indegree. We identified source wetlands (where local birth rate is greater than mortality) and sink wetlands (where local mortality is greater than the birth rate, and the population is maintained by continued immigration from source wetlands nearby). Among sink wetlands, we identified two types: those in which the population goes extinct in the absence of
Conclusion
It is important to recognize the role of individual wetlands as defined by their connectivity within wetlandscapes. The adoption of a single-object perspective for wetland management is incomplete given the flux of energy, materials, and organisms that occur among wetlands and in light of our results. Wetlands can have different roles (sink, source, pseudo-sinks) in sustaining an amphibian population within wetlandscapes. Wetland Indegree is a key property for 1) quantifying wetland
Acknowledgement
We wish to acknowledge Politecnico di Milano that supported Marta and Patrizia’s stay at Western University, London, Ontario,Canada with the scholarship “Thesis Abroad”. We acknowledge the Natural Sciences and Engineering Research Council of Canada Canadian Network of Aquatic Ecosystem Services (CNAES) that funded the reseach activity with the grant no. 417353-2011. We also wish to acknowledge the Alberta Living Laboratory Project funding awarded to I.F. Creed.
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Cited by (0)
- 1
Department of Electronics, Information, and Bioengineering, Politecnico di Milano,Via Ponzio 34/5,20133, Milan, Italy (Current affiliation).
- 2
INRA, UR1115 Plantes et Systèmes de culture Horticoles (PSH), Domaine St Paul, Site Agroparc, 84914 Avignon Cedex 09 Avignon, France (Current affiliation).