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Assessing connectivity in fragmented landscape : from behavioural ecology to biological conservation

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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

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... 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). ...
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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.
Article
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.
Article
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.
Chapter
Suggests that isolation selects for non-movement.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.