Using isolation‐by‐distance‐based approaches to assess the barrier effect of linear landscape elements on badger (Meles meles) dispersal

As the European badger (Meles meles) can be of conservation or management concern, it is important to have a good understanding of the species’ dispersal ability. In particular, knowledge of landscape elements that affect dispersal can contribute to devising effective management strategies. However, the standard approach of using Bayesian clustering methods to correlate genetic discontinuities with landscape elements cannot easily be applied to this problem, as badger populations are often characterized by a strong confounding isolation‐by‐distance (IBD) pattern. We therefore developed a two‐step method that compares the location of pairs of related badgers relative to a putative barrier and utilizes the expected spatial genetic structure characterized by IBD as a null model to test for the presence of a barrier. If a linear feature disrupts dispersal, the IBD pattern characterising pairs of individuals located on different sides of a putative barrier should differ significantly from the pattern obtained with pairs of individuals located on the same side. We used our new approach to assess the impact of rivers and roads of different sizes on badger dispersal in western England. We show that a large, wide river represented a barrier to badger dispersal and found evidence that a motorway may also restrict badger movement. Conversely, we did not find any evidence for small rivers and roads interfering with badger movement. One of the advantages of our approach is that potentially it can detect features that disrupt gene flow locally, without necessarily creating distinct identifiable genetic units.     

Do landscape barriers affect functional connectivity of populations of an endangered damselfly?

1. Landscape genetic approaches were used to assess functional connectivity of populations of the endangered damselfly Coenagrion mercuriale in a fragmented agricultural landscape in Switzerland. Spatial genetic clustering methods combined with interpolation by kriging and landscape genetic corridor analysis were applied to identify landscape elements that enhance or hinder dispersal and gene flow. 2. Spatial genetic clustering analysis divided the sampled populations into a northern and a southern genetic group. The boundary between the two groups coincided with a hill ridge intersecting the study area. Landscape corridor analysis identified five landscape elements that significantly affected gene flow. Elevation change, Euclidian distance, patches of forest and flowing waterbodies acted as barriers, whereas open agricultural land enhanced gene flow between populations of C. mercuriale. 3. This study showed that movement of C. mercuriale was not restricted to its preferred habitat (i.e. streams). Populations linked via continuous open agricultural land were functionally well connected if they were not more than about 1.5–2 km apart. In contrast, substantial elevation change and larger forest patches separated populations. These findings may serve as a basis to define conservation units and should be considered when planning connectivity measures, such as determining the locations of stepping stones, or the restoration of streams.     

基于最小费用模型的景观连接度评价

作为物种扩散的重要影响因子,景观连接度反映了景观促进或阻碍生物体在资源斑块间运动的程度,它也是区域土地可持续利用和生物保护领域的主要指标.最小费用模型起源于图论,其结合了景观中的详细地理信息和生物体的行为特征,通过费用距离分析可直观形象地描绘出物种在异质景观中的连接度,且可在GIS程序包中实现简便运算和适度的数据需求量,使其在大尺度景观连接度评价中受到广泛关注.本文结合当前的景观连接度研究,详细阐述了最小费用模型评价景观连接度的意义、原理及运算过程,并探讨了该模型应用中存在的问题,以期为进一步开展相关研究及生物多样性保护提供依据.     

Habitat selection,interspecific interactions and landscape composition

Summary I argue here that, from the perspective of any individual, most landscapes are composed of only three basic types of habitats. These are: (1) source habitat in which reproduction exceeds mortality and the expected per capita growth rate is greater than one; (2) sink habitat, in which limited, reproduction is possible but will not on average, compensate for mortality and the per capita rate of growth is between zero and one; and (3) unusable habitat, which comprises the matrix of all habitats that are never exploited by the species in question, and in which patches of source and sink habitats are embedded. Unlike earlier source-sink models, this model explicitly considers the effects that substituting one type of habitat for another has on the equilibrium size of a population and the interactions between species which can use both source and sink habitats. The model demonstrates that the equilibrium size of a species' population can sometimes be increased by substituting unusable habitat for sink habitat. Thus, even though the average patch quality in the landscape may be decreased, the overall quality of the landscape can increase. For two species with distinct habitat preferences, interactions between species can vary qualitatively as well as quantitatively as a function of the relative abundances of each of the habitat types. The model also shows that the interactions between species are particularly sensitive to the relative costs of moving between patches and sampling patches to determine their quality. Recent fragmentation of natural landscapes may increase the cost of searching for usable (source or sink) patches. Under some conditions, the interspecific interactions may be substantially more negative (competitive) than the interactions that evolved in the original natural landscape, further reducing population sizes and increasing the likelihood of competitive exclusion in fragmented modern landscapes.     

Scales and costs of habitat selection in heterogeneous landscapes

Summary Two scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.     

Population genetic structure and dispersal across a fragmented landscape in cerulean warblers (<Emphasis Type="Italic">Dendroica cerulea</Emphasis>)

Cerulean warblers (Dendroica cerulea) have experienced significant declines across their breeding range and presently exist in disjunct populations, largely because of extensive loss and fragmentation of their breeding and wintering habitat. Despite this overall decline, a recent north-eastern expansion of the breeding range has been proposed, and some researchers have suggested that the eastern Ontario population may be acting as a source population maintaining sink populations elsewhere. However, little is known about either the geographic distribution of genetic variation or dispersal in these birds. We assayed variation in five microsatellite loci and a 366 base-pair fragment of the mitochondrial control region among 154 cerulean warblers from five populations throughout the breeding range. No evidence of population genetic structure was found. Assignment tests suggested that six individuals were either inter-population migrants or descendants of recent migrants. The lack of population genetic structure is probably due to a combination of historical association and contemporary dispersal. Population decline does not appear to have reduced genetic variation yet. Overall results suggest that cerulean warblers from Ontario, Illinois, Arkansas and Tennessee should be considered a single genetic management unit for conservation.     

Commensal ecology, urban landscapes, and their influence on the genetic characteristics of city-dwelling Norway rats (Rattus norvegicus)

Movement of individuals promotes colonization of new areas, gene flow among local populations, and has implications for the spread of infectious agents and the control of pest species. Wild Norway rats ( Rattus norvegicus ) are common in highly urbanized areas but surprisingly little is known of their population structure. We sampled individuals from 11 locations within Baltimore, Maryland, to characterize the genetic structure and extent of gene flow between areas within the city. Clustering methods and a neighbour-joining tree based on pairwise genetic distances supported an east–west division in the inner city, and a third cluster comprised of historically more recent sites. Most individuals (~95%) were assigned to their area of capture, indicating strong site fidelity. Moreover, the axial dispersal distance of rats (62 m) fell within typical alley length. Several rats were assigned to areas 2–11.5 km away, indicating some, albeit infrequent, long-distance movement within the city. Although individual movement appears to be limited (30–150 m), locations up to 1.7 km are comprised of relatives. Moderate F _ST, differentiation between identified clusters, and high allelic diversity indicate that regular gene flow, either via recruitment or migration, has prevented isolation. Therefore, ecology of commensal rodents in urban areas and life-history characteristics of Norway rats likely counteract many expected effects of isolation or founder events. An understanding of levels of connectivity of rat populations inhabiting urban areas provides information about the spatial scale at which populations of rats may spread disease, invade new areas, or be eradicated from an existing area without reinvasion.