No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Assessing connectivity in fragmented landscape : from behavioural ecology to biological conservation
Abstract
In fragmented landscapes, movements between habitat patches play a crucial role in population persistence by allowing gene flow, rescue effect possibilities, linking different habitats for reproduction or resources acquisition, ... Although the importance of these movements, the landscape structure between patches (i.e. matrix) is too frequently considered as a homogeneous non-habitat of minor interest and behavioural adaptations in response to fragmentation are rarely taken into account. To investigate the importance of the landscape influence on movements, three complementary approaches are used: 1) experimental studies are performed to assess behavioural and physiological responses of common toads when they are confronted to different land uses. Land uses are thus characterised par the cost of their crossing for the focus species. 2) This work also aims to decipher the rules of movement in heterogeneous landscapes and to highlight behavioural differences according to the originated landscape (adaptations). 3) On the basis of modelling, the third part of this work intends to make predictions at the landscape scale
... Stevens et al. (2006) and Pittman and Semlitsch (2013) hypothesize that juveniles are primarily guided by aversion to water through a move-away process, until the individual reaches a favorable environment, where a random walk is adopted. However, recent studies have shown that juveniles also express an inherited migratory direction, thus improving migration success (Miaud et al., 2005;Janin, 2010). ...
... Safeguarding connectivity requires a better understanding of the orientation mechanisms involved in migration, and the potential inheritance of migration direction in juveniles. The putative negative influence of pesticides on orientation mechanisms must also be investigated (Janin, 2010). ...
Movement, both within an individual's home range and at the scale of dispersal, is a fundamental aspect of an animal's life. The field of movement ecology has established a conceptual framework to analyze the lifetime movement of an organism, offering a sound basis for conservation actions since the movement range of many species has been altered by habitat fragmentation and degradation. An organism's lifetime movement is organized around three main functions—exploitation, exploration, and relocation—which are associated with specific behavioral mechanisms and spatio-temporal scales. The movement ecology framework is a valuable tool as applied to amphibians, as managing these spatially structured populations requires in-depth knowledge of the behavioral mechanisms that determine movement. In terms of exploitation, these animals have a complex lifecycle, which involves migrating between different types of habitat, thus requiring them to cross a landscape matrix that may be more or less inhospitable. In terms of exploration and relocation, between-pond movements within the pond archipelagoes of a given population are frequent and strongly contribute to population resilience. Relocation also occurs at a larger scale, through long-distance dispersal to colonize new patches, exposing the individuals to unknown environments. Each function, at each scale, involves specific interactions between individual motivation (phenotype dependence) and environmental quality (context dependence) that determine decision-making and fitness outputs. Long-term exposure to local selective pressures can lead to differentiation in coping types that could be considered as Evolutionarily Significant Units (ESUs) for conservation. At the scale of a patch, the optimal direction of migration can be inherited, thus allowing the optimization of migration routes for juveniles. At the regional scale, a dispersal syndrome resulting in a greater propensity for boldness and exploration could be a response to unpredictable breeding sites or the high benefits of colonizing a rich habitat. Greater knowledge about such behavioral adaptations to specific situations would allow more targeted development of conservation measures or help to stop the spread of invasive species. The evolutionary context of movement behavior is thus of primary interest in designing effective conservation actions in a changing world.
For amphibians that breed in temporary pools, juvenile emigration is an important life-history movement linking the aquatic habitat of larvae to the surrounding upland habitats occupied by maturing animals and adults. However, little is known regarding the habitat preferences and sensitivity to disturbance of newly metamorphosed amphibians. We examined whether selection occurs for closed-canopy forest conditions during emigration by using pitfall traps and drift fences to sample naturally occurring populations of wood frogs (Rana sylvatica) and spotted salamanders (Ambystoma maculatum) along recently created forest--clearcut edges in central Maine. Habitat preferences during emigration were further investigated via an experimental approach that quantified movements of newly metamorphosed wood frogs cultured in artificial pools located along an abrupt forest-power line edge. Among natural populations, the abundance of juveniles and adults of both species declined sharply across a gradient running from relatively mature forest-interior habitat (70-90 yr old) to recently clearcut habitat (2-11 yr old). Similarly, in the power line experiment, juvenile wood frogs showed an emigration preference for closed-canopy habitat immediately upon metamorphosis, with the highest capture rates occurring in microhabitats characterized by dense foliage in both the understory and canopy layers. Our results suggest populations of wood frogs, spotted salamanders, and other amphibian species breeding in temporary pools are likely to benefit from efforts to maintain connectivity between upland forest habitats and aquatic breeding sites. Seminatural field experiments provide a valuable manipulative approach for studying the potential effects of habitat fragmentation on populations of migratory amphibians.
Our objective was to determine how green frogs (Rana clamitans) are affected by multiple exposures to a sublethal level of the carbamate insecticide, carbaryl, in outdoor ponds. Tadpoles were added to 1,000-l ponds at a low or high density which were exposed to carbaryl 0, 1, 2, or 3 times. Length of the larval period, mass, developmental stage, tadpole survival, and proportion metamorphosed were used to determine treatment effects. The frequency of dosing affected the proportion of green frogs that reached metamorphosis and the developmental stage of tadpoles. Generally, exposure to carbaryl increased rates of metamorphosis and development. The effect of the frequency of carbaryl exposure on development varied with the density treatment; the majority of metamorphs and the most developed tadpoles came from high-density ponds exposed to carbaryl 3 times. This interaction suggests that exposure to carbaryl later in the larval period stimulated metamorphosis, directly or indirectly, under high-density conditions. Our study indicates that exposure to a contaminant can lead to early initiation of metamorphosis and that natural biotic factors can mediate the effects of a contaminant in the environment.
Juvenile dispersal is important for the persistence of .amphibian populations. Previous studies have observed nonrandom orientation in juvenile amphibians emigrating from breeding ponds; however, the environmental cues associated with these movements are not well understood. We examined the emigration behavior of recently metamorphosed juveniles of three pond-breeding amphibian species from three woodland ponds. We found that juvenile small-mouthed salamanders (Ambystoma texanum (Matthes, 1855)), American toads (Bufo americanus Holbrook, 1836), and wood frogs (Rana sylvatica LeConte, 1825) exhibited nonrandom orientation upon exiting the breeding ponds. Furthermore, we found a positive relationship between captures of juvenile small-mouthed salamanders and wood frogs and width of the surrounding forest habitat, indicating that these species are selecting areas with broader forested habitat upon exiting the breeding ponds. Our results indicate that migrating juvenile amphibians may rely on direct environmental cues because the orientation of small-mouthed salamanders and wood frogs was influenced by width of the surrounding forested habitat. These observations support previous studies suggesting that maintaining forest habitat, along at least a portion of breeding ponds, is important for the persistence of amphibian populations.
—Some amphibians are able to orient toward habitat features, but it is not always clear whether (1) these animals can directly detect the habitat toward which they are moving (e.g., scenting water from a wetland); or (2) they are detecting,an indirect cue that is consistently correlated with the location of suitable habitat (e.g., the location of sunrise). In 2004, we translocated 400 Rana sylvatica tadpoles from an isolated population on Sears Island in Maine, where emerging metamorphs travel northeast towards a forested wetland. We placed study animals in arrays consisting of a central artificial pool, with a circular drift fence at 0.2 m from,the pool’s edge,to assess orientation,of metamorphs,at emergence,and a similar fence at 5 m to assess orientation postemergence. Arrays were placed at 10 m and 50 m from a forested wetland, with the wetland cue to the southwest (i.e., the opposite direction of the wetland at Sears Island). Rana sylvatica exhibited significant orientation toward the northeast at the 0.2 m fence, indicating that emerging metamorphs,retained,the same,directionality,as at the site where,they were,hatched. A significant,result at the 5 m fence indicated,that animals,continued,to head,toward,the northeast. These results suggest,that the population,of Rana sylvatica,on Sears Island may,rely on indirect cues for orientation. Relying on indirect cues offers less adaptability to changes in habitat such as breeding site loss or road construction, and thus could lead frogs into ecological traps. Juvenile and,adult amphibians,move,up to several
Habitat loss and sometimes habitat fragmentation per se affect species survival, reproduction, dispersal, abundance and distribution. However, understanding the independent effects of fragmentation (i.e., landscape configuration) has been limited because it is frequently confounded with landscape composition (i.e., habitat amount). We assess the independent effects of landscape composition and configuration on the occurrence of northern flying squirrels (Glaucomys sabrinus) in New Brunswick, Canada after controlling for local site conditions. We measured landscape structure using an ‘‘organism- based’’ approach; landscape structure was characterized quantitatively using a spatially explicit local- scale distribution model for northern flying squirrels.
Flying squirrels occurred more frequently in old forest, at sites with mixed coniferous–deciduous microhabitat composition and greater amounts of habitat cover at the neighbourhood (within home- range) scale. Squirrels were less likely to occur at sites surrounded by greater proportions of non-habitat matrix (non-treed or early seral open areas). The occurrence of flying squirrels was not strongly correlated with patch size or edge contrast. We detected no interaction between the effects of patch size and habitat composition. Landscape composition was clearly a more important predictor of flying squirrel distribution than configuration. We conclude that management practices that maximize the amount of old forest cover, maintain diverse tree species composition and minimize the creation of open areas should enhance the conservation value of landscapes for northern flying squirrels. Manipulating landscape pattern though forest management likely has limited use in mitigating the negative influence of habitat loss on this species.
Reliable prediction of metapopulation persistence in fragmented landscapes has become a priority in conservation biology,
with ongoing destruction of habitat confining increasing numbers of species into networks of small patches. A spatially realistic
metapopulation model, which includes the first-order effects of patch area and isolation on extinction and colonization, has
been tested. The distribution of an endangered butterfly was successfully predicted on the basis of parameter values estimated
for a well-studied congeneric species. This modeling approach can be a practical tool in the study and conservation of species
in highly fragmented landscapes.
The quality of the habitat patch in which individuals reside may influence demographic processes, thus affecting social organization. We manipulated the risk of predation and food availability to test the hypothesis that high patch quality decreased the propensity to disperse, increased the likelihood of social units becoming groups, and increased overall group size in prairie voles (Microtus ochrogaster). Prairie voles are socially monogamous, at least in part of their range, and they display varied social organization within a population, including groups (containing a breeding pair and at least 1 additional adult of either sex), male–female pairs, and single females. Our results indicated that the likelihood of dispersal from high-quality patches was significantly less than from lowquality patches. Dispersers also were significantly more likely to settle in similar or higher quality patches than the ones in which they were originally released. These patterns were primarily due to the dispersal of young males. Although the proportion of social units that were groups appeared to decrease with lower patch quality, the difference in social organization among patch types was not statistically significant. The total number of founding voles and founding males per social unit residing in the highest quality patches at the end of the study were significantly greater than in the lowest quality patches. Thus, under the conditions of our experiment, patch quality affected dispersal and group size but not the tendency to form groups.
Diets of both species studied were basically composed of algae and detritus. For Pelodytes punctatus, other food types exhibited minor contributions to the diet. For Bufo bufo, phanerogams reached considerable proportions too and the frequency of animals may be considered noteworthy if related to other species in the area. Morphologically they seem to be both conditioned to bottom dwelling rather than to the use of water column.
Nineteen adult toads equipped with transmitters were followed during and after the breeding period (January to September) in order to estimate their home range area and habitat use in a landscape of cereal agriculture. Median home range area was 0.5 ha (range 0.1-11 ha) during the breeding season (January-April). Home range areas increase during the post-breeding season (April-September) e.g., median minimum concave polygon of 4.1 ha. No relation between home range and toads' body size was observed. Six habitat types were described in the studied area. The proportion of habitats used (i.e. proportion of the pooled positions of 11 toads during the post-breeding season in each habitat) was significantly different from the proportion of available habitats: while crops represented 85% of the available habitat, only 43% of the toads' positions were recorded in this type of habitat. A compositional analysis of habitat preference was performed. At both landscape and individual home range levels, the toads preferred the stone embankments and ditches above all, while the crops were the least preferred habitat. This study highlights the role of habitat linkage and marginal habitats for the persistence of amphibian populations in intensive agricultural and arid landscapes.
Aquatic and terrestrial amphibians integrate acoustic, magnetic, mechanical, olfactory and visual directional information into a redundant–multisensory orientation system. The sensory information is processed to accomplish homing following active or passive displacement by either path integration, beaconing, pilotage, compass orientation or true navigation. There is evidence for two independent compass systems, a time-compensated compass based on celestial cues and a light-dependent magnetic inclination compass. Beaconing along acoustic or olfactory gradients emanating from the home site, as well as pilotage along fixed visual landmarks also form an important part in the behaviour of many species. True navigation has been shown in only one species, the aquatic salamander Notophthalmus viridescens. Evidence on the nature of the navigational map obtained so far is compatible with the magnetic map hypothesis.
Triturus alpestris has been found to be adept at homing, and previous studies have found that its orientation behaviour during breeding migration can be explained, at least in part, by the use of olfactory cues from the breeding pond. Nonetheless, the use of chemical cues from the pond probably does not support a guidance strategy when the newts are far from the pond. Hence, T. alpestris may rely on other orientation cues. In this study, alpine newts were tested in a circular arena to determine the sensory cues used to locate breeding ponds. Animals were collected from a permanent pond situated in northern Spain, taken to the experimental site 9020 m distant, and tested for orientation under a variety of conditions (i.e., orientation under a clear night sky, orientation under dark conditions, and orientation under a clear night sky in the presence of an altered geomagnetic field). The possibility of a non-homing directional bias was also tested. Newts chose a compass course in the direction of their breeding pond only when the ambient geomagnetic field and the celestial cues were simultaneously available. Conversely, animals failed to orient only when celestial cues or the geomagnetic field were the sole orientation cues available.
It has been proposed that inbreeding contributes to the decline and
eventual extinction of small and isolated populations,. There is ample
evidence of fitness reduction due to inbreeding (inbreeding depression)
in captivity and from a few experimental, and observational field
studies,, but no field studies on natural populations have been
conducted to test the proposed effect on extinction. It has been argued
that in natural populations the impact of inbreeding depression on
population survival will be insignificant in comparison to that of
demographic and environmental stochasticity,. We have now studied the
effect of inbreeding on local extinction in a large metapopulation of
the Glanville fritillary butterfly (Melitaea cinxia). We found that
extinction risk increased significantly with decreasing heterozygosity,
an indication of inbreeding, even after accounting for the effects of
the relevant ecological factors. Larval survival, adult longevity and
egg-hatching rate were found to be adversely affected by inbreeding and
appear to be the fitness components underlying the relationship between
inbreeding and extinction. To our knowledge, this is the first
demonstration of an effect of inbreeding on the extinction of natural
populations. Our results are particularly relevant to the increasing
number of species with small local populations due to habitat loss and
fragmentation.
1. Capturing the relative influence of landscape composition and configuration in real landscapes remains a challenge. Cost-distance modelling provides an interesting approach to the assessment of landscape complexity in a functional way. However, resistances allotted to landscape elements in cost-distance modelling frequently remain defined on the basis of expert advice. To overcome this weakness, we computed resistance coefficients without a priori knowledge through a calibration/validation method enabling us to test the impact of the matrix heterogeneity on the occurrence of the common toad Bufo bufo, the cycles of which imply migrations between complementary habitats.
Agricultural intensification poses a serious threat to biodiversity as a consequence of increased land-use intensity, decreased landscape heterogeneity and reduced habitat diversity. Although there is interest in the preservation of total species richness of an agricultural landscape (gamma diversity), the effects of intensification have been assessed primarily by species richness at a local scale (alpha diversity). This ignores species richness between local communities (beta diversity), which is an important component of total species richness.
In this study, measures of land-use intensity, landscape structure and habitat diversity were related to gamma, alpha and beta diversity of wild bees (Apoidea), carabid beetles (Carabidae), hoverflies (Syrphidae), true bugs (Heteroptera) and spiders (Araneae) within 16 local communities in 24 temperate European agricultural landscapes.
The total landscape species richness of all groups was most strongly affected by increased proximity of semi-natural habitat patches. Bees also decreased in landscapes with a high intensity of farmland management, demonstrating additive effects of both factors.
Separating total species diversity into components, the decrease in total species richness could be attributed primarily to a decrease in species diversity between local communities. Species richness of the local communities of all investigated groups decreased with increasing land-use intensity and, in the case of spiders, decreasing proximity of the semi-natural habitat patches.
The effect of increased habitat diversity appeared to be of secondary importance to total species richness but caused a shift in the relative contribution of alpha and beta diversity towards the latter.
Synthesis and applications. This study demonstrates that the effects of agricultural change operate at a landscape level and that examining species diversity at a local level fails to explain the total species richness of an agricultural landscape. The coincidence of patterns of beta diversity with those of gamma diversity emphasizes that such information is of crucial importance for the implementation and evaluation of restoration programmes aiming to restore sustainable countryside diversity. As local extinction processes in highly fragmented landscapes shape biodiversity, priority should be given to the conservation of diverse agricultural landscape remnants in Europe.
Summary 1. An animal's perceptual range defines the spatial extent of the landscape for which information is available to make movement decisions. Ecologists studying and model- ling animal dispersal have commonly assumed that individual movements arise from a predefined set of local decision rules operating within a static isotropic (i.e. circular) perceptual range. 2. We discuss how this assumption fails to recognize the potential for plasticity in perceptual ranges and present a conceptual model that illustrates how anisotropic perceptual ranges can arise from animal orientation to environmental stimuli. 3. Using model simulations we show how perceptual distance (i.e. greatest Euclidean distance at which habitat patches can be perceived), horizon (i.e. panoramic view or angular degrees of the landscape perceived) and breadth (i.e. areal extent of the land- scape perceived), change as a function of increased strength of a hypothetical stimuli. 4. Our results show that anisotropic perceptual ranges can differ substantially from tra- ditional, isotropic perceptual ranges in all three aspects depending on the strength of the stimuli and nature in which it interacts with other elements of the landscape. 5. We highlight the implications of these findings for modelling animal movements in ecological landscapes with the hope that context-dependent perceptual ranges are considered in future studies.
Amphibians are frequently characterized as having limited dispersal abilities, strong site fidelity and spatially disjunct breeding habitat. As such, pond-breeding species are often alleged to form metapopulations. Amphibian species worldwide appear to be suffering population level declines caused, at least in part, by the degradation and fragmentation of habitat and the intervening areas between habitat patches. If the simplification of amphibians occupying metapopulations is accurate, then a regionally based conservation strategy, informed by metapopulation theory, is a powerful tool to estimate the isolation and extinction risk of ponds or populations. However, to date no attempt to assess the class-wide generalization of amphibian populations as metapopulations has been made. We reviewed the literature on amphibians as metapopulations (53 journal articles or theses) and amphibian dispersal (166 journal articles or theses for 53 anuran species and 37 salamander species) to evaluate whether the conditions for metapopulation structure had been tested, whether pond isolation was based only on the assumption of limited dispersal, and whether amphibian dispersal was uniformly limited. We found that in the majority of cases (74%) the assumptions of the metapopulation paradigm were not tested. Breeding patch isolation via limited dispersal and/or strong site fidelity was the most frequently implicated or tested metapopulation condition, however we found strong evidence that amphibian dispersal is not as uniformly limited as is often thought. The frequency distribution of maximum movements for anurans and salamanders was well described by an inverse power law. This relationship predicts that distances beneath 11–13 and 8–9 km, respectively, are in a range that they may receive one emigrating individual. Populations isolated by distances approaching this range are perhaps more likely to exhibit metapopulation structure than less isolated populations. Those studies that covered larger areas also tended to report longer maximum movement distances – a pattern with implications for the design of mark-recapture studies. Caution should be exercised in the application of the metapopulation approach to amphibian population conservation. Some amphibian populations are structured as metapopulations – but not all.
Landscape modification and habitat fragmentation are key drivers of global species loss. Their effects may be understood by focusing on: (1) individual species and the processes threatening them, and (2) human-perceived landscape patterns and their correlation with species and assemblages. Individual species may decline as a result of interacting exogenous and endogenous threats, including habitat loss, habitat degradation, habitat isolation, changes in the biology, behaviour, and interactions of species, as well as additional, stochastic threats. Human-perceived landscape patterns that are frequently correlated with species assemblages include the amount and structure of native vegetation, the prevalence of anthropogenic edges, the degree of landscape connectivity, and the structure and heterogeneity of modified areas. Extinction cascades are particularly likely to occur in landscapes with low native vegetation cover, low landscape connectivity, degraded native vegetation and intensive land use in modified areas, especially if keystone species or entire functional groups of species are lost. This review (1) demonstrates that species-oriented and pattern-oriented approaches to understanding the ecology of modified landscapes are highly complementary, (2) clarifies the links between a wide range of interconnected themes, and (3) provides clear and consistent terminology. Tangible research and management priorities are outlined that are likely to benefit the conservation of native species in modified landscapes around the world.
We assessed the effect of plant patch shape and surrounding vegetation on the density, emigration, and immigration of predatory coccinellids, and on the density of their aphid prey Brevicoryne brassicae (L.). Between spring 1997 and fall 1999, we set up square and I-shaped patches of Brassica oleracea Plenck surrounded by Medicago sativa L. or Allium porrum L. Medicago sativa is frequently used by coccinellids, whereas A. porrum is not. We used a factorial (2 x 2) randomized block design, and evaluated the density of coccinellids and aphids every 10 d. We also evaluated emigration and immigration of adult coccinellids through mark-recapture experiments. We quantified the population increase of aphids, and the final live mass of plants. All insects were more abundant in patches surrounded by A. porrum than in those surrounded by M. sativa, and coccinellids were occasionally more dense in square patches than in I-shaped ones. Coccinellids emigrated less from square patches, either surrounded by A. porrum or M. sativa, and immigrated more to patches surrounded by A. porrum. Aphids showed a higher population increase, and plants of B. oleracea ended up being heavier in patches surrounded by A. porrum, particularly in I-shaped patches. Surrounding vegetation and plant patch shape seem to have a direct effect on the density of coccinellids by modifying their immigration and emigration patterns, but also seem to have an indirect effect by changing plant growth and its effect on herbivore recruitment.
As understanding of the processes and mechanisms of bird navigation has increased, older conceptual frameworks and terminology have become inadequate. Terminology used to refer to orientation and navigation behaviour have acquired varied and divergent meanings. Here I present a unified set of terminology that I hope will help clarify thinking and communication in the field.
The geometry of habitat patches may affect population dynamics due to differences in edge-to-area ratios for patches of different sizes and shapes. We conducted a field experiment replicated over 2 years employing four square (40 by 40 m) and four rectangular (16 by 100 m) habitat patches of equal size (1,600 m2) to determine the effects of contrasting shapes of habitat patches on population dynamics of meadow voles (Microtus pennsylvanicus). We tested the predictions that dispersal rates would be higher, home ranges larger, and population densities lower in rectangular patches compared to square patches. The number of dispersers, but not dispersal rates, was greater in rectangular patches than in square patches only when densities of voles were low. Home ranges were of equal area but different shape in the contrasting shapes of patches. Population density, recruitment, body mass of dispersers, body mass of residents, survival, and age structure were largely unaffected by differences in patch shape. Thus, plasticity of behavior (e.g., changes in shape of home range) appears to have prevented differences in population dynamics between the two patch shapes. We conclude that patch shape does not markedly affect the population dynamics of the meadow vole, and that this species appears to be an edge-tolerant species.
We demonstrate that, after correcting for the effects of size, the wing lengths, wing widths and thoracic weights of the forest damselfly Calopteryx maculata differ between populations along forested streams, and those along streams through pasture. Pasture landscapes can be considered as fragmented forest landscapes; forest landscapes are continuous. In the fragmented landscapes some C. maculata fly across intervening pasture to reach foraging sites in forest. We propose that there is morphological plasticity within the species that is revealed through the landscape process of habitat fragmentation and that there is micro-scale selection within the fragmented landscapes for individuals that are better suited to make these flights.
We propose a c‐sample non‐parametric test W on a circle based on uniform scores. The test can be regarded as an extension of Wheeler and Watson's test (1964). This test is constructed with the help of a variant of a c‐sample test for von Mises populations under the shift‐type of alternatives. We indicate the relationship of the W‐test with a test given by Mardia (1970). This result solves the problem of critical values of this test and of approximations to the null distribution of W. The asymptotic distribution of this test and of its parametric competitor test S are obtained. Bahadur's efficiency of the W‐test relative to the S‐test is derived and some important particular cases are considered. The tests are applied to a numerical example.
The use of a magnetic compass plays a significant role in the daily and seasonal movements of numerous organisms; however among amphibians, magnetic compass orientation has been convincingly demonstrated only in eastern red-spotted newts, Notophthalmus viridescens, with some indirect evidence coming from several species of bufonid toads. In this study, larval bullfrogs, Rana catesbeiana, were trained in outdoor tanks with two different Y-axis (shore/deep water) directions and then tested in an indoor arena in one of four symmetrical alignments of an earth-strength magnetic field. The tadpoles oriented bimodally along the correct magnetic direction of the Y-axis experienced during training, demonstrating that this anuran species is able to learn and orient along the Y-axis by sensing the geomagnetic field.
This paper examines the usage and measurement of “landscape connectivity” in 33 recent studies. Connectivity is defined as the degree to which a landscape facilitates or impedes movement of organisms among resource patches. However, connectivity is actually used in a variety of ways in the literature. This has led to confusion and lack of clarity related to (1) function vs structure, (2) patch isolation vs landscape connectivity and, (3) corridors vs connectivity. We suggest the term connectivity should be reserved for its original purpose. We highlight nine studies; these include modeling studies that actually measured connectivity in accordance with the definition, and empirical studies that measured key components of connectivity. We found that measurements of connectivity provide results that can be interpreted as recommending habitat fragmentation to enhance landscape connectivity. We discuss reasons for this misleading conclusion, and suggest a new way of quantifying connectivity, which avoids this problem. We also recommend a method for reducing sampling intensity in landscape-scale empirical studies of connectivity.
Many wild reptile species are threatened by habitat loss. However, the way in which changes in landscape patterns influence intraspecific ecological processes is not completely understood. Boa constrictor occidentalis is an endangered species and has a special conservation value since it is endemic of dry forests in the Gran Chaco region. Because the Gran Chaco is largely threatened due to habitat loss it is necessary to know how landscape changes influence this species. Therefore, we evaluated the effects of forest loss and landscape composition on the reproductive life-history parameters. Landscape changes were assessed by analyzing satellite imagery and reproductive parameters were determined by ultrasound images of the reproductive structures. The obtained results indicate that habitat loss may affect body condition, clutch size and testicular volume of the Argentine boa constrictor. We also found that the spatial pattern of vegetation influences the distribution of females and males in the landscape. Matting aggregations are scarce in shrublands. Therefore, our study shows that forest loss could enhance vulnerability to extirpation through constraints placed on reproduction. We encourage resource managers to evaluate sensitive reproductive life-history parameters as well as habitat deterioration to asses the conservation status of the populations of the Argentine boa constrictor. Since the Gran Chaco forest, a key habitat to the species’ reproduction, is largely threatened, strong conservation action is needed to halt and reverse forest loss in this region.
Metapopulation models assume that local population size (in a habitat patch) and therefore local extinction probability, is a function of patch size, and that interpatch movement rate and therefore recolonization of local extinctions is a function of both patch size and patch isolation. We hypothesized that the predictive power of models that relate patch immigration rate to patch size and isolation will be reduced when spatial structure in the landscape matrix (the nonhabitat portion of the landscape) affects organism movement through the landscape. We used a simulation model to evaluate this hypothesis for three different aspects of matrix spatial structure (contrast among matrix cover types, number of matrix cover types, and grain of matrix spatial pattern) and for two different types of movement behavior, representing a specialist and a generalist species. We tested the hy- pothesis for one aspect of matrix structure (number of matrix cover types) in a translocation field study of the eastern chipmunk (Tamias striatus, a habitat specialist) and the white- footed mouse (Peromyscus leucopus, a habitat generalist). When the matrix was composed of a small number of cover types, patch size and isolation accounted for up to 75% of the variation in patch immigration rate in the simulation study, and for 61% of the variation in interpatch movement in the field study. However, when the matrix was composed of a large number of cover types, the amount of explained variation dropped to as little as 33% for the simulation study and to 17% in the field study. Our results suggest that patch characteristics, such as patch size and isolation, may be poor predictors of interpatch movement when the landscape matrix is heterogeneous and when the organism responds to boundaries between different matrix cover types. These results imply that habitat patch- based models, such as those based on current metapopulation theory, will perform poorly in these situations.
Evidence is mounting that evolutionary change can occur rapidly and may be an important means by which species escape extinction in the face of global change. Consequently, biologists need to incorporate evolutionary thinking into management decisions in conservation and restoration ecology. Here, we review the genetic and demographic properties that influence the ability of populations to adapt to rapidly changing selective pressures. To illustrate how evolutionary thinking can influence conservation and restoration strategies, we compare the potential of two California plant communities (vernal pools and blue oak woodlands) to evolve in response to global change. We then suggest ways in which restoration biologists can manipulate the genetic architecture of target populations to increase their ability to adapt to changing conditions. While there may not be any universal rules regarding the adaptive potential of species, an understanding of the various processes involved in microevolution will increase the short- and long-term success of conservation and restoration efforts.
Crop production in much of the western United States has caused extensive fragmentation of shrubsteppe and other grasslands and has increased regional levels of disturbance through repeated tillage of crop fields and applications of agrochemicals. Although there have been numerous studies of the effects of fragmentation on arthropods in forests, old fields, and cropland, there have been very few studies of fragmentation effects on grassland arthropods. I conducted a two-year study in the Columbia Basin region of the U.S. Pacific Northwest to determine the effects of fragmentation and regional disturbance on shrubsteppe communities. I sampled plants and arthropods in 20 shrubsteppe fragments associated with annual crops, 20 fragments associated with perennial crops, and 20 large sections of shrubsteppe distant from crop fields. Fragments were corners of crop irrigation circles, and only sites that were undisturbed, with an apparent high coverage of shrubs, were included in the study. Results indicated no significant differences in coverages of shrubs, perennial grasses, total perennial forbs, total annual/biennial forbs, and species richness of grasses and shrubs between fragments and large sections of shrubsteppe. Mean percent coverage of the annual grass Bromus tectorum was 18.6% greater in small fragments than in large sections of shrubsteppe distant from crop fields, and mean coverage of bare ground was 22.9% higher and coverage of biological soil crusts was 41.5% lower in the fragments than in large shrubsteppe sections. Shrubsteppe fragments had significantly lower abundances of obligate herbivores, omnivores, total herbivores, obligate predators, total predators, plant detritivores, total detritivores, and butterflies, as well as fewer individuals of numerous other taxa within these trophic groups. Fragmentation had very little effect on species richness within trophic groups, probably because fragments were not that isolated from each other and large sections of shrubsteppe. Shrubsteppe communities that were next to highly disturbed annual crops had greater coverages of total annual/biennial forbs, and lower coverages of total perennial forbs and biological soil crusts when compared to shrubsteppe near perennial crops. Abundances of obligate herbivores, omnivores, total predators, plant detritivores, vertebrate-detritus feeders, and bees were similar in shrubsteppe near annual crops and near perennial crops. However, within these trophic groups, shrubsteppe near annual crops supported fewer obligate shrub-feeding grasshoppers, herbivorous ground beetles, harvestmen, antlions, darkling beetles, and butterflies than did shrubsteppe near perennial crops. The results suggest that a reduction in resources within fragments and regional disturbances caused by the production of annual crops were responsible for changes in arthropod trophic structure in fragments. Changes in trophic structure were probably not due to reductions in habitat heterogeneity or habitat isolation because plant community structure was similar in fragmented and unfragmented shrubsteppe, and fragments were not isolated from large sections of shrubsteppe.
Post-metamorphic dispersal in the common frog Rana temporaria (Amphibia, Anura) was studied with a combina- tion of field (pit-fall traps) and laboratory (arena, artificial crossing) experiments. In the first studied population, the breeding place was surrounded by lines of fence-pitfall traps allowing capture of dispersing froglets. Dispersal was at random on the edge of the pond, but oriented in the most favorable terrestrial habitat at 10 m from the edge. Froglets of this population were then tested in orientation arena built on the University campus, where they also dispersed at random. The two other studied populations reproduced at each side (north and south) of a lake. Froglets from each population were tested in similar orientation arena, where they did not dispersed at random but to the north and south direction respectively. In the laboratory we crossed males and females originated from these two populations. Re- sulting crossed froglets exhibited variable dispersal patterns, which significantly differed from those observed with their respective parents. These results argued for an at least partly genetic control of emigration direction in these two frog populations, that we interpreted as the result of directional selection due to landscape change during the XXth century.
THE Blackcap, Sylvia atricapilla, a widespread Palearctic migratory bird, rarely wintered in Britain until the 1950s. The winter population has since increased to several thousand birds1,2. Ringing indicates that these are not British Blackcaps forestalling migration, but birds breeding in Continental Europe reaching Britain on a novel westerly migration route3,4. The proportion of north-western migrants among Blackcaps ringed in parts of Germany and Austria has increased from 0% before 1960 to currently 7–10%5–7. We bred British wintering Blackcaps in captivity and determined the migratory direction of their offspring. Here we report that these birds migrate west-northwest in autumn, a direction genetically distinct from the British breeding population and the predominantly southwestern migratory population of west–central Europe. The novel route must have evolved within the past 30 years with selection favouring birds wintering some 1,500 km further north than most of their conspecifics. To our knowledge, this is the first case in any vertebrate in which a drastic and recent evolutionary change of behaviour has been documented and its genetic basis established.
Experiments were carried out to investigate whether Iberian green frog tadpoles Pelophylax perezi (formerly Rana perezi) are able of using the geomagnetic field for y-axis orientation (i.e. orientation toward and away from shore). Tadpoles were trained outdoor for 5 d, in two differ- ent training configurations: (i) a training tank aligned along the mag- netic north-south axis, with shore facing south, and (ii) a training tank aligned along the magnetic east-west axis, with shore located east, and similar to the shore-deep water axis ('y-axis') found in their home stream, which flows from south to north. After training, tadpoles were individually tested for magnetic orientation in a water-filled circular outdoor arena surrounded by a pair of orthogonally aligned cube-sur- face-coils used to alter the alignment of the earth's magnetic field. Tad- poles held in the east-west training tank oriented towards shore, indicating that they were able to distinguish between the shoreward and waterward direction along the y-axis. Tadpoles trained in the tank that was aligned along the north-south axis showed bimodal magnetic compass orientation along the shore-deep water magnetic axis. These findings provide evidence for the use of magnetic compass cues for y-axis orientation by P. perezi tadpoles.
Suggests that isolation selects for non-movement.
Keywords:Amphipoda;behaviour;orientation;heterozygosity
1. Species would be expected to shift northwards in response to current climate warming, but many are failing to do so because of fragmentation of breeding habitats. Dispersal is important for colonisation and an individual-based spatially explicit model was developed to investigate impacts of habitat availability on the evolution of dispersal in expanding populations. Model output was compared with field data from the speckled wood butterfly Pararge aegeria, which currently is expanding its range in Britain.
It has been increasingly recognized that landscape matrices are an important factor determining patch connectivity and hence the population size of organisms living in highly fragmented landscapes. However, most previous studies estimated the effect of matrix heterogeneity using prior information regarding dispersal or habitat preferences of a focal organism. Here we estimated matrix resistance of harvest mice in agricultural landscapes using a novel pattern-oriented modeling with Bayesian estimation and no prior information, and then conducted model validation using data sets independent from those used for model construction. First, we investigated the distribution patterns of harvest mice for approximately 400 habitat patches, and estimated matrix resistance for different matrix types using statistical models incorporating patch size, patch environment, and patch connectivity. We used Bayesian estimation with a Markov chain Monte Carlo algorithm, and searched for appropriate matrix resistance that best explained the distribution pattern. Patch connectivity as well as patch quality was an important determinant of local population size for the harvest mice. Moreover, matrix resistance was far from uniform, with rice and crop fields exhibiting low resistance and forests, creeks, roads and residential areas showing much higher resistance. The deviance explained by this model (heterogeneous matrix model) was much larger than that obtained by the model with no consideration of matrix heterogeneity (homogeneous matrix model). Second, we obtained distribution data from five additional landscapes that were more fragmented than that used for model construction, and used them for model validation. The heterogeneous matrix model well predicted the population size for four out of five landscapes. In contrast, the homogeneous model considerably overestimated population sizes in all cases. Our approach is widely applicable to species living in fragmented landscapes, especially those for which prior information regarding movement or dispersal is difficult to obtain.
The persistence of pond-breeding amphibians in highly fragmented land-scapes may be constrained by the need for connectivity between aquatic breeding sites and suitable terrestrial habitat, an example of landscape complementation. Although migratory ability determines the spatial scale at which landscape complementation operates, the factors influencing migratory success of amphibians, especially of juveniles, are poorly understood. This study is the first to investigate whether juvenile amphibians possess any innate, long-distance orientation mechanisms that might improve their chances of locating suitable terrestrial habitat. I conducted experimental releases of spotted salamanders (Ambystoma maculatum) and American toads (Bufo americanus) from 18 artificial pools in replicate pastures at distances of 5–50 m from the nearest forest edges. Using circular drift fences with pitfall traps, I captured, individually marked, and released metamorphosed salamanders (n 323) and toads (n 203) leaving each pool. Salamanders exhibited nonrandom orientation at nine pools, but at only one was the mean movement direction consistent with the direction to the nearest forest edge. Emigrating salamanders probably responded to microtopographic or other distinct features of each pool, rather than to distant cues. Mi-gratory success was determined by recaptures of marked juveniles at drift fences along the forest edges. I used logistic regression to model probability of recapture and evaluated alternative models using an information-theoretic approach. Migratory success of both species was primarily a function of distance to nearest forest. Of salamanders and toads released from 50-m pools, 15% reached the forest, suggesting that few juvenile amphibians would be able to migrate greater distances across pastures. Breeding sites lacking connec-tivity to suitable terrestrial habitat may be population sinks due to high mortality of juveniles during emigration. Additional research is needed to determine appropriate threshold dis-tances between breeding sites and terrestrial habitat, as well as the potential effectiveness of movement corridors for migrating amphibians.
Habitat loss and fragmentation have led to a widespread increase in the proportion of edge habitat in the landscape. Disturbance-dependent bird species are widely assumed to benefit from these edges. However, anthropogenic edges may concentrate nest predators while retaining habitat cues that birds use to select breeding habitat. This may lead birds to mistakenly select dangerous habitat—a phenomenon known as an ''ecological trap.'' We experimentally demonstrated how habitat shape, and thus amount of edge, can adversely affect nest site selection and reproductive success of a disturbance-dependent bird species, the Indigo Bunting (Passerina cyanea). We did so within a landscape-scale experiment composed of equal-area habitat patches that differed in their amount of edge. Indigo Buntings preferentially selected edgy patches, which contained 50% more edge than more compact rectangular patches. Further, buntings fledged significantly fewer young per pair in edgy patches than in rectangular patches. These results provide the first experimental evidence that edges can function as ecological traps.
Classical metapopulation (CM) theory considers that species persistence in the landscape depends on a turnover of extinction-(re)colonisation of suitable habitat patches at each generation. Therefore, metapopulation dynamics are approximated by binary changes in the state of individual patches. This approach has allowed rigorous mathematical analyses of metapopulation dynamics, establishing links with general laws in ecology or deriving operational concepts in conservation biology. However, a critical review of the literature shows that only a few spatially structured populations function according to CM theory. Moreover, empirical studies of CMs usually focus on species (1) at the margin of their distribution range, (2) with small or extremely small local population sizes and (3) declining in the study area (expanding in one case). Such non-equilibrium population systems are far from the equilibrium world idealised in CM models. Furthermore, empirical comparisons of metapopula-tions in highly fragmented landscapes show that extinction-(re)colonisation dynamics ARTICLE IN PRESS
Summary 1. Habitat edges are thought to explain much of the negative effects arising from hab- itat fragmentation; however, progress has been limited in extrapolating edge effects to different situations because ecologists still do not understand if and how multiple edges interact within fragments. It also remains controversial whether edge effects govern patch-size effects, such as area sensitivity, observed in many migratory songbirds. 2. I examined how multiple edges within fragments may intensify edge responses by investigating spatial distributions of an area-sensitive songbird that breeds in temperate grasslands of North America, the bobolink ( Dolichonyx oryzivorus Linnaeus). I tested whether bobolinks avoid edges and whether avoidance is stronger near two edges (double- edge plots) than near only one edge (single-edge plots). I subsequently linked bobolink distributions to landscape maps that vary in the amount of habitat and degree of frag- mentation to explore some potential implications of multiple edges on patch- and landscape- level distributions. 3. Multiple edges appeared to influence the magnitude of observed edge effects, in which the probability of bobolink occurrence was four times lower in double-edge plots and two times lower in single-edge plots than in the interior of grasslands. Within single- edge plots, the probability of occurrence increased with increasing distance from edge. Within double-edge plots, the probability of occurrence increased as a function of the nearest and next-nearest distances from edges. Multiple edges also appeared to increase the extent of edge effects, or distance of edge influence, which was estimated to be approximately 11-33% greater in double-edge plots than in single-edge plots, depend- ing on the next-nearest distance from edge. 4. Extrapolating local bird distributions to landscape models suggests that edge effects can have strong influences on large-scale distributions and that models incorporating multiple edge effects are different to simple nearest-edge models only in highly frag- mented landscapes, regardless of landscape composition. Furthermore, edge effects can lead to patch-size effects similar to empirical patterns of area sensitivity observed in this species. I conclude that edge effects can be intensified when multiple edges collide, a feature that permeates many fragmented landscapes.
Summary 1. I synthesize the understanding of the relationship between landscape structure and animal movement in human-modified landscapes. 2. The variety of landscape structures is first classified into four categories: continuous habitat, patchy habitat with high-quality matrix, patchy habitat with low-quality matrix, and patchy, ephemeral habitat. Using this simplification I group the range of evolved movement parameters into four categories or movement types. I then discuss how these movement types interact with current human-caused landscape changes, and how this often results in non-optimal movement. 3. From this synthesis I develop a hypothesis that predicts the relative importance of the different population-level consequences of these non-optimal movements, for the four movement types. 4. Populations of species that have inhabited landscapes with high habitat cover or patchy landscapes with low-risk matrix should have evolved low boundary responses and moderate to high movement probabilities. These species are predicted to be highly susceptible to increased movement mortality resulting from habitat loss and reduced matrix quality. 5. In contrast, populations of species that evolved in patchy landscapes with high-risk matrix or dynamic patchy landscapes are predicted to be highly susceptible to decreased immigration and colonization success, due to the increasing patch isolation that results from habitat loss. 6. Finally, I discuss three implications of this synthesis: (i) 'least cost path' analysis should not be used for land management decisions without data on actual movement paths and movement risks in the landscape; (ii) 'dispersal ability' is not simply an attribute of a species, but varies strongly with landscape structure such that the relative rankings of species' dispersal abilities can change following landscape alteration; and (iii) the assumption that more mobile species are more resilient to human-caused landscape change is not generally true, but depends on the structure of the landscape where the species evolved.
Summary 1. When assessing the cause of population absence or decline, it is important to under- stand the relative effect of local, regional and global factors. In this study we evaluated the first of these factors for two frog populations. 2. Amphibians are often absent from intensively farmed areas. There could be several rea- sons for this, one of them being the quality of the aquatic habitat available for reproduction. 3. In order to test why common frogs Rana temporaria and moor frogs Rana arvalis are absent from most ponds in the intensively agricultural areas of southern Sweden, we performed a field experiment. Spawn of both species were introduced into 18 ponds sur- rounded by intensively cropped fields. 4. Tadpole performance generally did not differ from that in a set of reference ponds in various other habitat types where one or both of these frog species occurs naturally. 5. In the same experimental ponds and in a number of reference ponds, we also intro- duced tadpoles of the two species into enclosures that protected them from predation and thus increased recapture rate. This experiment revealed that the water quality of farmland ponds is rarely unsuitable for successful frog reproduction. 6. Having measured abiotic and biotic variables in the experimental and reference ponds, we assessed the importance of different parameters to tadpole performance. While farmland ponds generally had higher pH, higher conductivity and higher nitrate and nitrite concentrations than our reference ponds, these factors had no discernible effects on tadpole performance under the ranges found across all pond types. None of the other parameters differed between the two groups of ponds, nor did they have any strong or obvious effects on tadpole performance or survival. 7. Synthesis and applications . The results indicate that water quality alone is not responsible for the scarcity of amphibians in farmland areas of southern Sweden. To understand better the cause of their rarity, future studies should also focus on the qual- ity of the terrestrial habitat surrounding the ponds and the metapopulation structure.
Migratory white-throated sparrows (Zonotrichia albicollis) were exposed to acephate (acetylphosphoramidothioic acid O, S-dimethyl ester), an organophosphorus pesticide, to determine its effects on migratory orientation and behavior. Birds were also exposed to polarizer sheets to determine the mechanism by which acephate may affect migratory orientation. Adult birds exposed to 256 ppm acephate a.i. were not able to establish a preferred migratory orientation and exhibited random activity. All juvenile treatment groups displayed a seasonally correct southward migratory orientation. We hypothesize that acephate may have produced aberrant migratory behavior by affecting the memory of the migratory route and wintering ground. This experiment reveals that an environmentally relevant concentration of a common organophosphorus pesticide can alter migratory orientation, but its effect is markedly different between adult and juvenile sparrows. Results suggest that the survival of free-flying adult passerine migrants may be compromised following organophosphorus pesticide exposure.
1. Theoretical studies of the costs and benefits of migration predict that evolutionary changes in dispersal traits may take place in response to habitat fragmentation.
2. For the butterfly Plebejus argus , we investigated five morphological characters potentially associated with flight ability, in locations that varied in level of habitat fragmentation. The traits were: total mass, relative thorax mass (containing flight muscle), relative abdomen mass (containing reproductive organs), relative wing area and wing aspect ratio. All characters were measured on individuals reared in a common environment.
3. Morphology was related to the level of habitat fragmentation in both limestone and heathland habitats. Total mass increased with decreasing heathland habitat area (over the range 13 000–10 ha). Relative allocation to the thorax increased, while allocation to the abdomen decreased, with declining limestone habitat area (over the range 3·5–0·2 ha). Morphological characters were not significantly correlated with habitat isolation.
4. Significant family effects for total mass, relative thorax mass, relative abdomen mass and (for females only) relative wing area indicate that these traits may have a heritable component, and therefore have the potential to respond to selection acting on flight ability.
5. We suggest that evolutionary changes in life history traits are taking place in response to changes in landscape structure: in P. argus , these traits may be influenced by the effects of mate‐location strategy on emigration rates. Specific changes in traits can be complex, and may vary among species and populations.
6. P. argus had a 50% chance of occurring in heathland fragments of 33 ha, and was present on all heathlands above 50 ha, but population systems may not yet have achieved equilibrium. P. argus is unlikely to undertake major evolutionary changes in response to reduced habitat area in heathlands > 50 ha. Extinctions from smaller heathlands can be explained most plausibly by population/vegetation dynamics. Therefore, evolutionary changes in morphology may be more likely to be symptomatic of populations with altered costs and benefits of migration, rather than to be a direct cause of extinction.
Metapopulation dynamics lead to predictable patterns of habitat occupancy, population density and trophic structure in relation to landscape features such as habitat patch size and isolation. Comparable patterns may occur in behavioural, physiological and life-history traits but remain little studied. In the Glanville fritillary butterfly, females in newly established populations were more mobile than females in old populations. Among females from new populations, mobility decreased with increasing connectivity (decreasing isolation), but in females from old populations mobility increased with connectivity. The [ATP]/[ADP] ratio of flight muscles following controlled activity showed the same pattern as mobility in relation to population age and connectivity, suggesting that physiological differences in flight metabolic performance contribute to the observed variation in mobility. We demonstrate with an evolutionary metapopulation model parameterised for the Glanville fritillary that increasing spatial variation in landscape structure increases variance in mobility among individuals in a metapopulation, supporting the general notion that complex landscape structure maintains life-history variation.