The Influence of Ecology and Genetics on Behavioral Variation in Salamander Populations across the Eastern Continental Divide

Overwintering and breeding patterns of monarch butterflies (Danaus plexippus) in coastal plain habitats of the southeastern USA

Understanding variability in species' traits can inform our understanding of their ecology and aid in the development of management and conservation strategies. Monarch butterflies (Danaus plexippus) are native to the western hemisphere and are well-known for their long-distance migrations but have experienced significant population declines in recent decades. Here we use a 5-year capture-mark-recapture dataset to compare monarch distributions, mating activity, and larval host plant use between two coastal plain habitats in South Carolina, USA. We observed seasonally specific habitat use, with maritime habitats serving as overwintering areas while nearby inland swamps support significant breeding in spring, summer, and fall seasons due to an abundance of aquatic milkweed (Asclepias perennis). We also observed mating activity by fall migrating monarchs and their use of swallow-wort (Pattalias palustre) in the spring as an important larval host plant in maritime habitats. This phenology and habitat use of monarchs diverges from established paradigms and suggest that a distinct population segment of monarchs may exist, with significance for understanding the conservation status of monarch butterflies and associated habitats in eastern North America. Further research should explore how monarchs along the Atlantic coast of North America relate to other eastern monarch populations.

Interspecific interactions are conditional on temperature in an Appalachian stream salamander community

Differences in the rates of responses to climate change have the potential to disrupt well-established ecological interactions among species. In semi-aquatic communities, competitive asymmetry based on body size currently maintains competitive exclusion and coexistence via interference competition. Elevated temperatures are predicted to have the strongest negative effects on large species and aquatic species. Our objectives were to evaluate the interaction between the effects of elevated temperatures and competitor identity on growth and habitat selection behavior of semi-aquatic salamanders in stream mesocosms. We observed interference competition between small and large species. Elevated temperatures had a negative effect on the larger species and a neutral effect on the smaller species. At elevated temperatures, the strength of interference competition declined, and the smaller species co-occupied the same aquatic cover objects as the larger species more frequently. Disruptions in competitive interactions in this community may affect habitat use patterns and decrease selection for character displacement among species. Determining how biotic interactions change along abiotic gradients is necessary to predict the future long-term stability of current communities.

Context-dependent responses to light contribute to responses by Black-bellied Salamanders (Desmognathus quadramaculatus) to landscape disturbances

Behaviour often regulates population responses to environmental change, but linking behavioural responses to population patterns can be challenging because behavioural responses are often context-dependent, have an instinctive component, and yet may be modified by experience. Black-bellied Salamanders (Desmognathus quadramaculatus (Holbrook, 1840)) occupy forested streams where dense canopies create cool, dark environments. Because riparian deforestation negatively affects salamander-population connectivity yet some individuals choose to persist in these gaps, we sought to evaluate whether phototaxis could explain these patterns and whether phototactic behaviour would be influenced by experience (capture from forested or deforested areas) or context (refuge type and availability). Our results demonstrated that larval D. quadramaculatus exhibited negative phototaxis, but that larvae from forested streams exhibited stronger negative phototaxis than individuals from deforested streams. Larvae also selected habitat closer to light when refuge was available. Our results show that light alters habitat use by larval D. quadramaculatus, but the magnitude of that effect depends on refuge availability and experience with well-lit conditions associated with forest removal. As human activities reduce canopy cover and refuge availability, negative phototaxis may be one explanation for behavioural barriers to movement. Ultimately, the ability of salamanders to exhibit behavioural plasticity will determine their potential for local adaptation facilitating persistence in the face of environmental change.

Conservation genetics of extremely isolated urban populations of the northern dusky salamander (Desmognathus fuscus) in New York City

Urbanization is a major cause of amphibian decline. Stream-dwelling plethodontid salamanders are particularly susceptible to urbanization due to declining water quality and hydrological changes, but few studies have examined these taxa in cities. The northern dusky salamander (Desmognathus fuscus) was once common in the New York City metropolitan area, but has substantially declined throughout the region in recent decades. We used five tetranucleotide microsatellite loci to examine population differentiation, genetic variation, and bottlenecks among five remnant urban populations of dusky salamanders in NYC. These genetic measures provide information on isolation, prevalence of inbreeding, long-term prospects for population persistence, and potential for evolutionary responses to future environmental change. All populations were genetically differentiated from each other, and the most isolated populations in Manhattan have maintained very little genetic variation (i.e. <20% heterozygosity). A majority of the populations also exhibited evidence of genetic bottlenecks. These findings contrast with published estimates of high genetic variation within and lack of structure between populations of other desmognathine salamanders sampled over similar or larger spatial scales. Declines in genetic variation likely resulted from population extirpations and the degradation of stream and terrestrial paths for dispersal in NYC. Loss of genetic variability in populations isolated by human development may be an underappreciated cause and/or consequence of the decline of this species in urbanized areas of the northeast USA.

Projected loss of a salamander diversity hotspot as a consequence of projected global climate change

BACKGROUND

Significant shifts in climate are considered a threat to plants and animals with significant physiological limitations and limited dispersal abilities. The southern Appalachian Mountains are a global hotspot for plethodontid salamander diversity. Plethodontids are lungless ectotherms, so their ecology is strongly governed by temperature and precipitation. Many plethodontid species in southern Appalachia exist in high elevation habitats that may be at or near their thermal maxima, and may also have limited dispersal abilities across warmer valley bottoms.

METHODOLOGY/PRINCIPAL FINDINGS

We used a maximum-entropy approach (program Maxent) to model the suitable climatic habitat of 41 plethodontid salamander species inhabiting the Appalachian Highlands region (33 individual species and eight species included within two species complexes). We evaluated the relative change in suitable climatic habitat for these species in the Appalachian Highlands from the current climate to the years 2020, 2050, and 2080, using both the HADCM3 and the CGCM3 models, each under low and high CO(2) scenarios, and using two-model thresholds levels (relative suitability thresholds for determining suitable/unsuitable range), for a total of 8 scenarios per species.

CONCLUSION/SIGNIFICANCE

While models differed slightly, every scenario projected significant declines in suitable habitat within the Appalachian Highlands as early as 2020. Species with more southern ranges and with smaller ranges had larger projected habitat loss. Despite significant differences in projected precipitation changes to the region, projections did not differ significantly between global circulation models. CO(2) emissions scenario and model threshold had small effects on projected habitat loss by 2020, but did not affect longer-term projections. Results of this study indicate that choice of model threshold and CO(2) emissions scenario affect short-term projected shifts in climatic distributions of species; however, these factors and choice of global circulation model have relatively small affects on what is significant projected loss of habitat for many salamander species that currently occupy the Appalachian Highlands.

Use of multiple dispersal pathways facilitates amphibian persistence in stream networks

Although populations of amphibians are declining worldwide, there is no evidence that salamanders occupying small streams are experiencing enigmatic declines, and populations of these species seem stable. Theory predicts that dispersal through multiple pathways can stabilize populations, preventing extinction in habitat networks. However, empirical data to support this prediction are absent for most species, especially those at risk of decline. Our mark-recapture study of stream salamanders reveals both a strong upstream bias in dispersal and a surprisingly high rate of overland dispersal to adjacent headwater streams. This evidence of route-dependent variation in dispersal rates suggests a spatial mechanism for population stability in headwater-stream salamanders. Our results link the movement behavior of stream salamanders to network topology, and they underscore the importance of identifying and protecting critical dispersal pathways when addressing region-wide population declines.

Evaluating existing movement hypotheses in linear systems using larval stream salamanders

Because of their linear nature, streams provide a restrictive framework to understand the movement ecology of many animals. Stream movements have been characterized under two competing hypotheses. The colonization hypothesis dictates that small individuals experience passive drift, but concurrent, upstream movement by larger individuals replaces the loss of small individuals. Alternatively, the production hypothesis suggests that downstream movements are a consequence of limited resource availability. Previous research suggests that large larvae should move upstream and vice versa for small larvae, which should therefore be found downstream more often. We conducted a mark–recapture study of larval red salamanders ( Pseudotriton ruber (Sonnini de Manoncourt and Latreille, 1801)) to assess the validity of these hypotheses. We found that no larvae exhibited downstream movement (skew = 0.361, p = 0.019; biased upstream), and large larvae were the only size cohort to exhibit directional movement upstream (skew = 0.901, p = 0.035). Contrary to predictions under the colonization hypothesis, small larvae were found upstream more frequently than large larvae (N = 871, H = 16.29, df = 2, p < 0.001). Our results suggest that larval movements are related to abiotic stream conditions, and we conclude that neither hypothesis fully explains stream movement. In the absence of drift, new movement hypotheses are necessary to describe persistent upstream movement in streams. These hypotheses should consider individual causes of movement and the direction of movements that will improve the fitness of the organism.

In-stream and overland dispersal across a river network influences gene flow in a freshwater insect, Calopteryx splendens

Gene flow in riverine species is constrained by the dendritic (branching) structure of the river network. Spatial genetic structure (SGS) of freshwater insects is particularly influenced by catchment characteristics and land use in the surroundings of the river. Gene flow also depends on the life cycle of organisms. Aquatic larvae mainly drift downstream whereas flying adults can disperse actively overland and along watercourses. In-stream movements can generate isolation by distance (IBD) at a local scale and differentiation between subcatchments. However, these patterns can be disrupted by overland dispersal. We studied SGS across the Loire River in the damselfly Calopteryx splendens which is able to disperse along and between watercourses. Our sampling design allowed us to test for overland dispersal effects on genetic differentiation between watercourses. Amplified fragment length polymorphism markers revealed high genetic differentiation at the catchment scale but the genetic structure did not reflect the geographical structure of sampling sites. We observed IBD patterns when considering the distance following the watercourse but also the Euclidean distance, i.e. the shortest distance, between pairs of sites. Altogether, our results support the hypothesis of overland dispersal between watercourses. From a conservation perspective, attention should be paid to the actual pathways of gene flow across complex landscapes such as river networks.

Amphibians used in research and teaching

Amphibians have long been utilized in scientific research and in education. Historically, investigators have accumulated a wealth of information on the natural history and biology of amphibians, and this body of information is continually expanding as researchers describe new species and study the behaviors of these animals. Amphibians evolved as models for a variety of developmental and physiological processes, largely due to their unique ability to undergo metamorphosis. Scientists have used amphibian embryos to evaluate the effects of toxins, mutagens, and teratogens. Likewise, the animals are invaluable in research due to the ability of some species to regenerate limbs. Certain species of amphibians have short generation times and genetic constructs that make them desirable for transgenic and knockout technology, and there is a current national focus on developing these species for genetic and genomic research. This group of vertebrates is also critically important in the investigation of the inter-relationship of humans and the environment based on their sensitivity to climatic and habitat changes and environmental contamination.

The importance of comparative phylogeography in diagnosing introduced species: a lesson from the seal salamander, Desmognathus monticola

BACKGROUND

In most regions of the world human influences on the distribution of flora and fauna predate complete biotic surveys. In some cases this challenges our ability to discriminate native from introduced species. This distinction is particularly critical for isolated populations, because relicts of native species may need to be conserved, whereas introduced species may require immediate eradication. Recently an isolated population of seal salamanders, Desmognathus monticola, was discovered on the Ozark Plateau, approximately 700 km west of its broad continuous distribution in the Appalachian Mountains of eastern North America. Using Nested Clade Analysis (NCA) we test whether the Ozark isolate results from population fragmentation (a natural relict) or long distance dispersal (a human-mediated introduction).

RESULTS

Despite its broad distribution in the Appalachian Mountains, the primary haplotype diversity of D. monticola is restricted to less than 2.5% of the distribution in the extreme southern Appalachians, where genetic diversity is high for other co-distributed species. By intensively sampling this genetically diverse region we located haplotypes identical to the Ozark isolate. Nested Clade Analysis supports the hypothesis that the Ozark population was introduced, but it was necessary to include haplotypes that are less than or equal to 0.733% divergent from the Ozark population in order to arrive at this conclusion. These critical haplotypes only occur in < 1.2% of the native distribution and NCA excluding them suggest that the Ozark population is a natural relict.

CONCLUSION

Our analyses suggest that the isolated population of D. monticola from the Ozarks is not native to the region and may need to be extirpated rather than conserved, particularly because of its potential negative impacts on endemic Ozark stream salamander communities. Diagnosing a species as introduced may require locating nearly identical haplotypes in the known native distribution, which may be a major undertaking. Our study demonstrates the importance of considering comparative phylogeographic information for locating critical haplotypes when distinguishing native from introduced species.

Biological Control Through Intraguild Predation: Case Studies in Pest Control, Invasive Species and Range Expansion

Intraguild predation (IGP), the interaction between species that eat each other and compete for shared resources, is ubiquitous in nature. We document its occurrence across a wide range of taxonomic groups and ecosystems with particular reference to non-indigenous species and agricultural pests. The consequences of IGP are complex and difficult to interpret. The purpose of this paper is to provide a modelling framework for the analysis of IGP in a spatial context. We start by considering a spatially homogeneous system and find the conditions for predator and prey to exclude each other, to coexist and for alternative stable states. Management alternatives for the control of invasive or pest species through IGP are presented for the spatially homogeneous system. We extend the model to include movement of predator and prey. In this spatial context, it is possible to switch between alternative stable steady states through local perturbations that give rise to travelling waves of extinction or control. The direction of the travelling wave depends on the details of the nonlinear intraguild interactions, but can be calculated explicitly. This spatial phenomenon suggests means by which invasions succeed or fail, and yields new methods for spatial biological control. Freshwater case studies are used to illustrate the outcomes.

Living in the branches: population dynamics and ecological processes in dendritic networks

Spatial structure regulates and modifies processes at several levels of ecological organization (e.g. individual/genetic, population and community) and is thus a key component of complex systems, where knowledge at a small scale can be insufficient for understanding system behaviour at a larger scale. Recent syntheses outline potential applications of network theory to ecological systems, but do not address the implications of physical structure for network dynamics. There is a specific need to examine how dendritic habitat structure, such as that found in stream, hedgerow and cave networks, influences ecological processes. Although dendritic networks are one type of ecological network, they are distinguished by two fundamental characteristics: (1) both the branches and the nodes serve as habitat, and (2) the specific spatial arrangement and hierarchical organization of these elements interacts with a species' movement behaviour to alter patterns of population distribution and abundance, and community interactions. Here, we summarize existing theory relating to ecological dynamics in dendritic networks, review empirical studies examining the population- and community-level consequences of these networks, and suggest future research integrating spatial pattern and processes in dendritic systems.

Linking direct and indirect data on dispersal: isolation by slope in a headwater stream salamander

There is growing recognition of the need to incorporate information on movement behavior in landscape-scale studies of dispersal. One way to do this is by using indirect indices of dispersal (e.g., genetic differentiation) to test predictions derived from direct data on movement behavior. Mark-recapture studies documented upstream-biased movement in the salamander Gyrinophilus porphyriticus (Plethodontidae). Based on this information, we hypothesized that gene flow in G. porphyriticus is affected by the slope of the stream. Specifically, because the energy required for upstream dispersal is positively related to slope, we predicted gene flow to be negatively related to change in elevation between sampling sites. Using amplified DNA fragment length polymorphisms among tissue samples from paired sites in nine streams in the Hubbard Brook Watershed, New Hampshire, USA, we found that genetic distances between downstream and upstream sites were positively related to change in elevation over standardized 1-km distances. This pattern of isolation by slope elucidates controls on population connectivity in stream networks and underscores the potential for specific behaviors to affect the genetic structure of species at the landscape scale. More broadly, our results show the value of combining direct data on movement behavior and indirect indices to assess patterns and consequences of dispersal in spatially complex ecosystems.