Effects of Fragmentation on Connectivity: Implications for Pool Dependent Herpetofauna in the Northeastern United States

Transcription

1 Eric C. NRS 534 April 29, 2013 Effects of Fragmentation on Connectivity: Implications for Pool Dependent Herpetofauna in the Northeastern United States Vernal pools of the northeastern United States are small, shallow features that fill with water in the late fall, winter and spring and dry down in the summer, when evapotranspiration exceeds precipitation (Colburn 2004). They can be found across a variety of landscapes and come in many different shapes and sizes. Vernal pools can be geographically isolated, or they can found within larger wetland systems (Liebowitz and Brooks 2007). However, they cannot have a continuous surface connection to other permanent waters during the growing season (Colburn 2004). This is part of what makes them unique. The absence of a continuous surface water connection to permanent waters and highly dynamic hydroperiod/hydroregime, allows vernal pools to contain a unique assemblage of species, some of whom are specifically adapted to and rely upon these environments. These organisms are called obligate vernal pool species. Other species, which may use the pool out of convenience, but do not require the pool for any portion of their life cycle are referred to as facultative species. Fragmentation from timber harvesting, urbanization, agriculture, and other factors has exerted tremendous pressure on amphibian populations worldwide (Liebowitz and Brooks 2007). In fact, one inthree amphibian species are currently threatened or endangered (Cushman 2006). Pool dependent amphibians, such as the wood frog (Rana sylvatica) and ambystomatid salamanders, are particularly at risk, due to their ecology and the physical habitat within which they reside. This white paper briefly looks at the effects of fragmentation on pool dependent amphibians in the northeastern United States and attempts to articulate some of the causative factors behind these effects. The ecology of wood frogs and ambystomatid salamanders is complex and not well understood (Cushman 2006). What is critical to this discussion is that these organisms only spend a portion of their life cycle in vernal pools. Adults migrate to pools in the early spring, oviposit, and then disperse back to adjacent terrestrial habitat. Once metamorphosis occurs later in the spring, juveniles also move to terrestrial habitat. Page 1 of 9

2 These species show a high degree of philopatry (Gamble et al. 2007). During a long term (7 year) study Gamble et al. (2007) found amongst marbled salamanders (Ambystoma opacum) that 91% of first time breeders and 96.4% of experienced breeders returned to their pools. Also of importance is vagility and dispersal distance. Although species have varying degrees of vagility, dispersal distance is fixed, which limits the distance an individual will travel to find suitable habitat (Liebowitz and Brooks 2004). It has been demonstrated that the ecology of wood frogs and ambystomatids result in a high sensitivity to fragmentation, due to reduced connectivity between other pools and wetlands, terrestrial habitat and refugia. Gibbs (1998a) found that forest cover percentages below 30 percent resulted in the absence of adult wood frogs and spotted salamanders (Ambystoma maculatum), which indicates that pool dependent amphibians have a preference towards forested cover. This preference is probably tied to the avoidance of desiccation and predation (Houlahan and Findlay 2003). Roads have been implicated in creating barriers to migration (Gibbs 1998b), and degrading adjacent habitat through indirect factors, such as salt and nutrient accumulations (Forman and Deblinger 1999). In fact, road effects have been found to extend a significant distance (>300 feet) from the actual roadway (Forman and Deblinger 1999). Fragmentation can also result in isolation effects (Cushman 2006). In such cases, post metamorphic survival needs to be high in order to maintain populations in the pool, as the probability of emigration from potential source populations is low. The effects of fragmentation appear to operate at different spatial scales. Homan and Windmiller (2004) observed critical thresholds for spotted salamanders at 100 m (28%), 300 m (34) and 1000 m (51%) scales. Likewise, critical thresholds were observed at 30 m (88%) and 1000 m (44%) for the wood frog. Classic metapopulation theory can be used to describe the spatial dynamics of pool dependent amphibians and can yield insights into the causative factors behind the effects of fragmentation (Cushman 2006). Using this theory, each pool would represent a patch that contains a certain assemblage of species. Due to some stochastic event, there are periods within most vernal pools where some or all species may become temporarily lost. Recolonization occurs via migrants from nearby source pools. Thus, connectivity between pools is critical to the persistence of the population. Page 2 of 9

3 In order for the metapopulation model to work, source populations must be within the dispersal distance of the lost species. This presents a potential issue for wood frogs and ambystomatid salamanders, who have fixed dispersal distances. Where this is the case, a higher density of pools may be required to maintain connectivity with potential source patches and thus, improve the resilience of a particular population (Liebowitz and Brooks 2007). Densities of vernal pools in the northeastern United States range from 0 to 35 pools per mi 2 (Colburn 2004). There have been some authors who have questioned the validity of the metapopulation model as it relates to vernal pools (see Marsh and Trenham 2001). However, current scientific understanding still holds the model to be true and has been corroborated by several studies, including Ribeiro et al. (2011), who used network analysis to find that structural and functional connectivity is correlated to species richness of pool dependent amphibians in Spain. It was long thought that species with a high vagility and dispersal distance were better equipped to survive in a fragmented landscape. However, for amphibians at least, this does not appear to be the case. Gibbs (1998a) found that species with greater vagility were more susceptible to fragmentation than those with lesser vagility. Although such species have greater tendency to move about the landscape and may have the ability to travel greater distances to find suitable habitat, fragmentation creates inhospitable environments, which retard movement and/or increase the likelihood of mortality during migration (Gibbs 1998a). Over the long term, species with low vagility can also be affected by fragmentation if isolation effects occur (Cushman 2006). Should post metamorphic survival be low, the loss of the species within the pool will likely occur, as potential emigrants may not reach the pool. As a wetland scientist and environmental consultant, I spend a great deal of time studying vernal pools and helping individuals navigate through the various local, state and federal regulations which protect them. So, as you can imagine, this topic was of immense interest to me. Many of the current regulations within southern New England focus on the use of terrestrial buffer zones to protect core habitat and critical terrestrial habitat adjacent to the core habitat (see Semlitsch 1998). Some regulations (e.g., the Massachusetts General Permit) establish buffer zones based off of dispersal distance. Other regulations (e.g., the Massachusetts Wetlands Protection Act Regulations [310 C.M.R et seq.]) set buffer zones using arbitrary distances or outdated science. Current research clearly indicates that connectivity and scale play a critical role in the resilience of pool dependent Page 3 of 9

4 herpetofauna. As such, it is time to re evaluate our existing regulations and move towards a landscapebased approach to conservation. Page 4 of 9

5 Annotated Bibliography Colburn, E. A. (2004). Vernal Pools: Natural History and Conservation. Blacksburg, VA: The McDonald and Woodward Publishing Company. This book is a part of the seminal literature on vernal pools of the northeast region. I used Chapter 1 to inform my discussion relative to the physical attributes of a vernal pool. I think the most important information in this introductory chapter is related to hydrologic connectivity of vernal pools with other wetlands and water bodies. In my seven years as an environmental consultant, I ve found that many individuals are unaware that vernal pools do not have to be geographically isolated. It appears counterintuitive, but vernal pools can, in fact, have surface connections to other bodies of water. The key is that they do not have a continuous surface connection to a permanent water body (which would preclude the presence of sustainable populations of fish). A typical example is a pool located within a flood plain of a perennial stream. During high discharge, low frequency storm events, surface water may enter the flood plain and connect the pool with the stream temporarily. Once the water recedes, connectivity is lost. Cushman, S. A. (2006). Effects of habitat loss and fragmentation on amphibians: A review and prospectus. Biological Conservation. 128(2), This review article takes a look at the current state of knowledge related to the effects of habitat loss and fragmentation on amphibians. I selected this article because it provided a starting point for my research; determining seminal literature that has paved the way towards current scientific understanding. One of the important research needs identified in the article was related to the integration of molecular genetic methods and spatial modeling to help determine movement of pooldependent amphibians. I took a look at the literature to see what research had been done in this area since the article was written (seven years ago). While there was plenty of research conducted on higher order species (e.g., mammals), very few of the articles were related to herpetofaua, and none were related to pool dependent herpetofauna. This research is highly important to the future of such organisms, who rely on connectivity to increase a population s resilience against perturbation. Page 5 of 9

6 Forman, R. T., & Deblinger, R. D. (1999). The Ecological Road Effect Zone of a Massachusetts (U.S.A.) Suburban Highway. Conservation Biology. 14(1), Roads are well known barriers to species with high vagility. However, I selected this article because it looks at the potential effects of roads beyond their traditional role as a migratory barrier. What is fascinating is that the authors found that, for all ecological factors the measured, the road effect zone extends over 300 feet from the road. In addition, they found that some factors extended as far as a half mile. This has huge implications for amphibians, whose habitats can be degraded by salt, invasive species, etc., and may be eventually lost. Areas that have a high density of roads could result in isolation effects that prevent re colonization of pools that have been degraded by other road effects. Gamble, L. R., McGarigal, K., & Compton, B. W. (2007). Fidelity and dispersal in the pond breeding amphibian Ambystoma opacum: Implications for spatio temporal population dynamics and conservation. Biological Conservation. 139, The metapopulation paradigm has been used to describe the spatial dynamics of vernal pools. During my research, however, I discovered a few articles that suggest this might not be the case (see Marsh and Trenham 2001, below). I selected this article because it looks to confirm the accuracy of the metapopulation paradigm in modeling the spatial dynamics of pool dependent amphibians. They found a high degree of philopatry in the marbled salamander, although some individuals did migrate to other pools. While this suggests that metapopulation theory still can describe population dynamics, it s unclear to what extent migration buffers populations from local extinction. The one concern with this paper was that it did not get into the relative importance of each pool, which would be critical in determining whether migration is occurring at random or from important pools (e.g., higher quality habitat that serves as a source pool for other pools which may be more susceptible to extinction). Gibbs, J. P. (1998a). Distribution of woodland amphibians along a forest fragmentation gradient. Landscape Ecology. 13, This article looks at how fragmentation may affect the distribution of herpetofauna. It was one of the first studies to determine that species richness was correlated with percent canopy cover, indicating that herpetofauna either actively avoid areas with decreased canopy cover or these areas result in greater mortality (perhaps both). Although the causative factors for this phenomenon could not be addressed, it set the stage for further research. What I found most interesting was that the study found that species with greater vagility appear to be affected by fragmentation to a greater extent than those Page 6 of 9

7 with less vagility. This is rather counter intuitive, but upon reviewing additional literature on the subject, this makes sense, as those species with high vagility are more prone to mortality in fragmented environments from roads, desiccation and predation. Gibbs, J. P. (1998b). Amphibian movements in response to forest edges, roads, and streambeds in southern New England. The Journal of Wildlife Management. 62(2), This study takes the study of amphibian response to fragmentation a step further. In Gibbs (1998a), the author looked at whether herpetofauna were affected by fragmentation. In this study, Gibbs looks at how amphibians respond to different landscape components. The goal was to identify potential conduits and filters that could aid in development of conservation strategies that preserve connectivity between pools, which is critical for increasing resilience of populations of pool dependent species. The reason I selected this article was that it identified which landscape components hindered movement and which ones facilitated movement. Interestingly, the study found that edge permeability decreased with increasing vagility. This builds off of Gibbs (1998a), which found negative correlations between increased vagility and decreased canopy cover. Homan, R. N., Windmiller, B. S., & Reed, J. M. (2004). Critical thresholds associated with habitat loss for two vernal pool breeding amphibians. Ecological Applications. 14(5), This article looks at threshold levels of habitat loss across multiple spatial scales for the wood frog (Rana sylvatica) and spotted salamander (Ambystoma maculatum); two species that are considered obligate vernal pool species in the northeast region. I selected this article because it not only demonstrates that there are certain thresholds that affect species (see Gibbs 1998a), but it also shows that the effects of fragmentation operate at different scales. This has implications for conservation, as it becomes more important to understand what scale is most appropriate to a target species when developing conservation strategies. Houlahan, J. E., & Findlay, C. S. (2003). The effects of adjacent land use on wetland amphibian species richness and community composition. Canadian Journal of Fisheries and Aquatic Sciences. 60, This article provides an excellent analysis of some of the potential effects of land use on herpetofauna. While the study was conducted in Canada and was not restricted to pool breeding species, many of the Page 7 of 9

8 species described in the report overlap with those found here in the northeast. The study found that species richness and abundance was positively correlated with natural habitat (e.g., forest cover and wetlands) and negatively correlated with anthropogenic land uses (e.g., roads) and the by products of anthropogenic land uses (e.g., excess nutrients). These findings were generally consistent with much of the literature I reviewed. One interesting observation noted during the study was that the large scale of potential population drivers ( m). This once again points to the importance of developing landscape level conservation strategies for amphibian populations. Leibowitz, S. G., & Brooks, R. T. (2007). Hydrology and landscape connectivity of vernal pools. In Calhoun, A. J. K., & demaynadier, P. G. (Eds.), Science and conservation of vernal pools in northeastern North America (pp ). Boca Raton, Florida: CRC. Along with Colburn (2004), this book is another important part of the literature on northeastern vernal pools. I chose Chapter 3 of this book because it provided a coarse level overview of the effects of fragmentation and habitat loss on vernal pools, as well as population dynamics (including metapopulation theory). As with much of the literature I reviewed, this chapter underscored the importance of a landscape level approaches to conservation. Marsh, D. M., & Trenham, P. C. (2001). Metapopulation dynamics and amphibian conservation. Conservation Biology. 15(1), This article challenges the application of metapopulation theory to pool dependent species, indicating that the model potentially oversimplifies reality and more information is needed in order to confirm/deny its validity. I selected this article because I was interested in better understanding the potential issues with metapopulation theory, and confirm that it was still an accepted part of current scientific knowledge. It does appear that ponds as patches theory might be an oversimplification, as there are many unknowns relative to the ecology of pool dependent species (e.g., amount of migration, alternative biotic and abiotic factors influencing turnover, influence of deterministic extinction). However, the majority of the literature I reviewed that was published since this article (e.g., Liebowitz and Brooks [2007], Ribeiro et al. [2011]) points to metapopulation theory as the best means of describing the population dynamics of vernal pools. Page 8 of 9

9 Ribeiro, R., Carrentero, M. A., Sillero, N., Alarcos, G., Ortiz Santaliestra, M., Lizana, M., & Llorente, G. A. (2011). The pond network: can structural connectivity reflect on (amphibian) biodiversity patterns? Landscape Ecology. 26, This article uses graph theory to look at the relationship between connectivity (both structural and functional) and species richness. I selected this article because of its importance in linking structural and functional connectivity. These two approaches are often studied independently of one another, but in reality they are often linked. To effectively determine whether connectivity is vital for biodiversity in the pool environment, both must approaches be integrated. The study revealed that for certain distances, connectivity was correlated with species richness. It was also discovered that ponds at the center of the network were most important to connectivity. Semlitsch, R. D. (1998). Biological delineation of terrestrial buffer zones for pond breeding salamanders. Conservation Biology. 12(5), This article was one of the first to advocate for protection of terrestrial areas used by certain pooldependent species. These terrestrial buffer zones are based on the dispersal abilities of the fauna which require use of the pool. As noted in the conclusion of the white paper, many of the current vernal pool and wetland regulations within New England use terrestrial buffer zones as their conservation strategy of choice. These buffer zones appear to be reasonably effective at protecting most wetlands and watercourses. However, current scientific understanding of pool dependent herpetofauna shows that connectivity between other pools and wetlands is more critical to sustaining populations than simply protection of adjacent terrestrial habitat. Page 9 of 9