How to ‘catch’ floating populations? Research and the fixing of migration in space and time

Abstract

The recent ‘mobilities turn’ in the social sciences suggests that research always fixes mobility somehow in time and space, in order to understand it. Migration, being a form of mobility, has to be fixed on certain times and spaces in order to make it possible to define and research it. Being a geographer, I build on this insight in the first part of the article to re-read the critiques on post-war methodological nationalism. Reflecting on my own past research practice, I argue in the second part that leaving behind the methodological nationalism paradigm does not mean leaving behind the necessity to fix migration in time and space, on either the ontological or the practical methodological level. New ways of fixing are chosen and, out of recurring ontological and methodological choices, new migration stereotypes are developing such as what might be called ‘methodological ruralism’ in the case of Romanian migration.     

Where can introduced populations learn their tricks? Searching for the geographical source of a species introduction to the Galápagos archipelago

We seek to understand the potential genetic impact of introduced species in an ecosystem where conservation efforts and pressure from development co-occur: the Galápagos Archipelago. To date, the introduced weevil Galapaganus h. howdenae is found only in Santa Cruz Island; however it has recently expanded its range beyond the Agricultural Zone into the highlands. We are focusing on comparisons between continental and introduced populations to clarify whether the current genetic patterns in introduced populations can reflect recovery from a past bottleneck and predict its invasive potential. A second objective is to narrow down the geographic source of the introduction and confirm their origin from a single introduction pulse. We found comparable microsatellite genetic diversity featuring many island-specific alleles, but reduced mitochondrial haplotype diversity in the islands. Additionally, both markers showed signals of population expansion and homogeneously distributed variation in the introduced range, in contrast to structured continental populations. The addition of mitochondrial sequences to the microsatellite dataset in the Approximate Bayesian Computation analyses (Diyabc) favors a single introduction pulse from the Northern Manabí area; a scenario that is in agreement with the haplotype network. An origin from higher elevation habitats provides clues about the environmental amplitude of the naturalized populations. However, given that a large percentage of the microsatellite variation in island populations is due to new island alleles, the answer to the question “where can introduced populations learn their tricks?” could be: right at their new location.     

Roles of dispersal, stochasticity, and nonlinear dynamics in the spatial structuring of seasonal natural enemy–victim populations

Natural enemy–victim systems may exhibit a range of dynamic space–time patterns. We used a theoretical framework to study spatiotemporal structuring in a transient natural enemy–victim system subject to differential rates of dispersal, stochastic forcing, and nonlinear dynamics. Highly mobile natural enemies that attacked less mobile victims were locally spatially segregated from each other when governed by approximate linear dynamics. In contrast, in nonlinear dynamical systems, such as cyclic populations, interacting species achieved local aggregation with each other regardless of dispersal rates, and aggregation was enhanced specifically when highly mobile enemies attacked less mobile victims. These patterns of spatial aggregation held under varying levels of stochastic forcing. This work thus shows a range of dynamic spatial patterns in interacting-species models, and how spatial aggregation between natural enemies and victims can be achieved in locally unstable populations that are linked through dispersal.     

Why is the choice of future climate scenarios for species distribution modelling important?

Species distribution models (SDMs) are common tools for assessing the potential impact of climate change on species ranges. Uncertainty in SDM output occurs due to differences among alternate models, species characteristics and scenarios of future climate. While considerable effort is being devoted to identifying and quantifying the first two sources of variation, a greater understanding of climate scenarios and how they affect SDM output is also needed. Climate models are complex tools: variability occurs among alternate simulations, and no single 'best' model exists. The selection of climate scenarios for impacts assessments should not be undertaken arbitrarily - strengths and weakness of different climate models should be considered. In this paper, we provide bioclimatic modellers with an overview of emissions scenarios and climate models, discuss uncertainty surrounding projections of future climate and suggest steps that can be taken to reduce and communicate climate scenario-related uncertainty in assessments of future species responses to climate change.     

Do vegetation patch spatial patterns disrupt the spatial organization of plant species?

The mechanisms that structure plant diversity and generate long-range correlated spatial patterns have important implications for the conservation of fragmented landscapes. The ability to disperse and persist influences a plant species’ capacity for spatial organization, which can play a critical role in structuring plant diversity in metacommunities. This study examined the spatial patterns of species diversity within a network of patches in Cabo de Gata Natural Park, southeastern Spain. The objectives were to understand how the spatial heterogeneity of species composition (beta diversity) varies in a structured landscape, and how the long-range spatial autocorrelation of plant species is affected by the spatial configuration of patches.The mechanisms underlying the spatial distribution of plants acted at two scales. Between patches, spatial variation in species distributions was greater than that expected based on spatial randomization, which indicated that movement among patches was restricted. Within patches, diffusion processes reduced spatial variability in species distributions, and the effect was more prominent in large patches. Small patch size negatively influenced the long-range spatial autocorrelation of characteristic species, whereas inter-patch distance had a stronger effect on species frequency than it had on the disruption of spatial organized patterns.The long-range spatial autocorrelation was evaluated based on the dispersal abilities of the species. Among the 106 species evaluated, 39% of the woody species, 17% of the forbs, and 12% of the grasses exhibited disrupted long-range spatial autocorrelation where patches were small. The species that are more vulnerable to the effects of fragmentation tended to be those that have restricted dispersal, such as those that have short-range dispersal (atelechoric), e.g., Phlomis purpurea, Cistus albidus, Teucrium pseudochamaepytis, Brachypodium retusum, and the ballistic species, Genista spartioides. Helianthemum almeriense is another vulnerable species that has actively restricted dispersal (antitelechory), which is common in arid regions. Wind dispersers such as Launaea lanifera were less vulnerable to the effects of fragmentation. Long-distance dispersers whose persistence depends on facilitative interactions with other individuals, e.g., allogamous species such as Thymus hyemalis, Ballota hirsuta, and Anthyllis cytisoides, exhibit disrupted long-range spatial autocorrelation when patch size is reduced.     

How does herbivory affect individuals and populations of the perennial herb Paeonia officinalis?

In this study we quantified variability in foliage herbivory and pre-dispersal seed predation and its effects on plant performance and demography in populations of a rare and protected perennial herb, Paeonia officinalis. An individual-based survey was performed during four years in four populations, which contained plants in both open habitat and woodland. We detected marked spatial and temporal variation among and within populations in foliage herbivory (by insects) and pre-dispersal seed predation (by insects, rodents and Roe Deer). Foliage herbivory decreased with plant demographic stages in open habitats, from seedlings to reproductive individuals, but no significant trend was detected in woodland habitats. This may be due to different demographic origin of larger vegetative plants in this habitat. Depending on demographic stage, herbivory was higher in open habitats or not significantly different between habitats. This suggests differences in herbivore abundance in different habitats within sites. Pre-dispersal seed predation remained weak and did not depend on habitat. We did not detect any consequence of foliage herbivory on seedling mortality and individual growth in our study. Our results illustrate the need to investigate plant-herbivore interactions over several years in distinct populations in order to more accurately evaluate herbivore impact on plant population dynamics.     

Role of local culture,religion, and human attitudes in the conservation of sacred populations of a threatened ‘pest’ species

Indigenous belief systems and informal institutions that result in the conservation of wild species or sites exemplify biocultural conservation. The erosion of cultural beliefs and practices can have adverse, often severe, consequences for biodiversity. We explored the relationships among informal institutions, religion, and human attitudes toward sacred populations of a threatened, endemic species, Sclater’s monkey (Cercopithecus sclateri), in two communities in southeastern Nigeria. Due to habitat loss and hunting pressure across the species’ range, monkeys in these two sites live alongside people, raid farms and gardens, and are commonly viewed as pests. Using structured (n = 410) and semi-structured (n = 21) interviews, we examined factors influencing residents’ views of the monkeys, mechanisms affecting adherence to social taboos against harming monkeys, and implications for conservation. Our analyses revealed that most residents, particularly those from one community, women, and farmers, held negative opinions of the monkeys. Crop and garden raiding by monkeys had the most adverse effect on people’s attitudes. Although the adoption of Christianity weakened residents’ views regarding the no-killing taboos, continued adherence to the taboos was particularly influenced by supernatural retribution in one site and community disapproval in the other. Only one community widely conferred symbolic importance on the monkeys. Such site differences illustrate the value of local cultural understanding in conservation. Pre-intervention studies of this nature allow for the development of locally and culturally sensitive conservation programs, as well as better-informed assessments of what interventions are most likely to be effective.     

Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

Background and AimsTheory predicts that outcrossing should be more prevalent among perennials than annuals, a pattern confirmed by comparative evidence from diverse angiosperm families. However, intraspecific comparisons between annual and perennial populations are few because such variation is uncommon among flowering plants. Here, we test the hypothesis that perennial populations outcross more than annual populations by investigating Incarvillea sinensis, a wide-ranging insect-pollinated herb native to China. The occurrence of both allopatric and sympatric populations allows us to examine the stability of mating system differences between life histories under varying ecological conditions.MethodsWe estimated outcrossing rates and biparental inbreeding in 16 allopatric and five sympatric populations in which both life histories coexisted using 20 microsatellite loci. In each population we measured height, branch number, corolla size, tube length and herkogamy for ~30 individuals. In a sympatric population, we recorded daily flower number, pollinator visitation and the fruit and seed set of annual and perennial plants.Key ResultsAs predicted, outcrossing rates (t) were considerably higher in perennial (mean = 0.76) than annual (mean = 0.09) populations. This difference in mating system was also maintained at sympatric sites where plants grew intermixed. In both allopatric and sympatric populations the degree of herkogamy was consistently larger in outcrossing than selfing plants. Perennials were more branched, with more and larger flowers than in annuals. In a sympatric population, annuals had a significantly higher fruit and seed set than perennials.ConclusionsGenetically based differences in herkogamy between annuals and perennials appear to play a key role in governing outcrossing rates in populations, regardless of variation in local ecological conditions. The maintenance of mating system and life history trait differentiation between perennial and annual populations of I. sinensis probably results from correlated evolution in response to local environmental conditions.     

Geographical separation of two Ulex species at three spatial scales: does competition limit species' ranges?

It is a common assumption that species' ranges are limited by their physiological tolerances to climatic factors, Biotic factors, such as competition, are rarely considered. We investigated the distributions of Ulex minor and U. gallii at three spatial scales from geographic ranges to individual heaths - to examine whether the species are negatively associated, as predicted by the hypothesis that the ranges of the species are limited by competition with each other. Distribution maps for the British Isles and France (100 400 km² survey units) show the two species have largely separated, but slightly overlapping ranges. A region of range overlap on the heaths of Dorset, southern England was mapped using 4 ha survey squares. There was strong negative association between the species, and the heaths could be divided into zones where one species was dominant. There was some indication of edaphic differences between the U. minor -dominated zones and the U. gallii zones. The few heaths where the species co-occurred were surveyed using 4 m² quadrats placed along transects. Usually one species was widespread over the heath, while the other occurred in patches. The species were strongly negatively associated in all transects. Therefore, the two species showed strong negative associations at three mapping scales. Apparent co-occurrences detected at one spatial scale largely disappeared when species were mapped at finer scales, emphasising the fractal nature of distributions. This provides evidence that the distributions of the two species are not independent and that they cannot coexist, and therefore that their ranges are limited by competition. Over their ranges, competitive superiority is probably determined by the climate. At the range boundaries in the region of overlap, climate is not important, but other physical factors such as edaphic conditions may determine the outcome of competition.     

The poor health of deep-water species in the context of fishing activity and a warming climate: will populations of Molva species rebuild or collapse?

Many deep-water fish populations, being K-selected species, have little resilience to overexploitation and may be at serious risk of depletion as a consequence. Sea warming represents an additional threat. In this study, the condition, or health, of several populations of common ling (Molva molva), blue ling (Molva dypterygia) and Mediterranean or Spanish ling (Molva macrophthalma) inhabiting different areas in the North Atlantic and the Mediterranean was evaluated, to shed light on the challenges these deep-water species are facing in the context of fishing activity and a warming climate. The data on the condition of Molva populations which are analysed here have been complemented with data on abundance and, for the southernmost species (Mediterranean ling), with two other health indicators (parasitism and hepato-somatic index). Despite some exceptions (e.g., common ling in Icelandic waters), this study shows that the condition of many populations of Molva species in the northeastern Atlantic and the Mediterranean Sea has worsened, a trend which, in recent decades, has usually been found to be accompanied by a decline in their abundance. In addition, the poor health status of most populations of common ling, blue ling and Mediterranean ling considered in this analysis points to a lower sustainability of these populations in the future. Overall, the health status and abundance of Molva populations in the northeastern Atlantic and the Mediterranean suggest that only some populations located in the North Atlantic may be able to rebuild, whereas the populations in southern North Atlantic and the Mediterranean, which are probably most at risk from sea warming, are facing serious difficulties in doing so. In the context of fisheries and global warming, this study's results strongly indicate that management bodies need to consider the health status of many of the populations of Molva species, particularly in southern European waters, before implementing their decisions.     

Effects of spatial scale and sample size in GPS-based species distribution models: are the best models trivial for red deer management?

Species distribution models (SDMs) are popular in conservation and management of a wide array of taxa. Often parameterized with coarse GIS-based environmental maps, they perform well in macro-ecological settings but it is debated if the models can predict distribution within broadly suitable “known” habitats of interest to local managers. We parameterized SDMs with GIS-derived environmental variables and location data from 82 GPS-collared female red deer (Cervus elaphus) from two study areas in Norway. Candidate GLM models were fitted to address the effect of spatial scale (landscape vs. home range), sample size, and transferability between study areas, with respect to predictability (AUC) and explained variance (Generalized R ² and deviance). The landscape level SDM captured variation in deer distribution well and performed best on all diagnostic measures of model quality, caused mainly by a trivial effect of avoidance of non-habitat (barren mountains). The home range level SDMs were far less predictable and explained comparatively little variation in space use. Landscape scale models stabilized at the low sample size of 5–10 individuals and were highly transferrable between study areas implying a low degree of individual variation in habitat selection at this scale. It is important to have realistic expectations of SDMs derived from digital elevation models and coarse habitat maps. They do perform well in highlighting potential habitat on a landscape scale, but often miss nuances necessary to predict more fine-scaled distribution of wildlife populations. Currently, there seems to be a trade-off between model quality and usefulness in local management.     

What is the role of disturbance in catalyzing spatial shifts in forest composition and tree species biomass under climate change?

Mounting evidence suggests that climate change will cause shifts of tree species range and abundance (biomass). Abundance changes under climate change are likely to occur prior to a detectable range shift. Disturbances are expected to directly affect tree species abundance and composition, and could profoundly influence tree species spatial distribution within a geographical region. However, how multiple disturbance regimes will interact with changing climate to alter the spatial distribution of species abundance remains unclear. We simulated such forest demographic processes using a forest landscape succession and disturbance model (LANDIS-II) parameterized with forest inventory data in the northeastern United States. Our study incorporated climate change under a high-emission future and disturbance regimes varying with gradients of intensities and spatial extents. The results suggest that disturbances catalyze changes in tree species abundance and composition under a changing climate, but the effects of disturbances differ by intensity and extent. Moderate disturbances and large extent disturbances have limited effects, while high-intensity disturbances accelerate changes by removing cohorts of mid- and late-successional species, creating opportunities for early-successional species. High-intensity disturbances result in the northern movement of early-successional species and the southern movement of late-successional species abundances. Our study is among the first to systematically investigate how disturbance extent and intensity interact to determine the spatial distribution of changes in species abundance and forest composition.     

What drives invasibility? A multi‐model inference test and spatial modelling of alien plant species richness patterns in northern Portugal

Understanding and predicting biological invasions can focus either on traits that favour species invasiveness or on features of the receiving communities, habitats or landscapes that promote their invasibility. Here, we address invasibility at the regional scale, testing whether some habitats and landscapes are more invasible than others by fitting models that relate alien plant species richness to various environmental predictors. We use a multi‐model information‐theoretic approach to assess invasibility by modelling spatial and ecological patterns of alien invasion in landscape mosaics, and by testing competing hypotheses of environmental factors that may control invasibility. Because invasibility may be mediated by particular characteristics of invasiveness, we classified alien species according to their C‐S‐R plant strategies. We illustrate this approach with a set of 86 alien species in northern Portugal. We first focus on predictors influencing species richness and expressing invasibility, and then evaluate whether distinct plant strategies respond to the same or different groups of environmental predictors. We confirmed climate as a primary determinant of alien invasions, and as a primary environmental gradient determining landscape invasibility. The effects of secondary gradients were detected only when the area was sub‐sampled according to predictions based on the primary gradient. Then, multiple predictor types influenced patterns of alien species richness, with some types (landscape composition, topography and fire regime) prevailing over others. Alien species richness responded most strongly to extreme land management regimes, suggesting that intermediate disturbance induces biotic resistance by favouring native species richness. Land‐use intensification facilitated alien invasion, whereas conservation areas hosted few invaders, highlighting the importance of ecosystem stability in preventing invasions. Plants with different strategies exhibited different responses to environmental gradients, particularly when the variations of the primary gradient were narrowed by sub‐sampling. Such differential responses of plant strategies suggest using distinct control and eradication approaches for different areas and alien plant groups.     

Safe space in the woods: Mechanistic spatial models for predicting risks of human–bear conflicts in India

Human–wildlife interactions can have negative consequences when they involve large carnivores. Spatial risk modelling could serve as a useful management approach for predicting and pre-emptively mitigating negative interactions. We present a mechanistic modelling framework and examine interactions between humans and sloth bears (Melursus ursinus) in a multi-use forest landscape of central India. We first assessed patterns and determinants of bear distribution across the landscape using indirect sign surveys. At the same spatial scale, we then estimated spatial probabilities of bear attacks on people using information from 675 interviews with local residents, incorporating estimates of distribution probabilities from the previous step. We found the average occupancy probability across 128 grid-cells to be 0.77 (SE = 0.03). Bear occupancy was influenced by terrain ruggedness, forest composition and configuration, vegetation productivity and size of human settlements. The average probability of a bear attack in any given grid-cell was 0.61 (SE = 0.03), mostly determined by bear occurrence patterns, forest cover, terrain ruggedness, and size of human settlements. Using spatial information on people's dependence on forest resources, we identified locations with the highest risk of bear attacks. Our study demonstrates that human attacks by bears—generally believed to be random or incidental—in fact showed deterministic patterns. Our framework can be applied to other scenarios involving human–wildlife conflicts. Based on our findings, we propose that a proactive co-management approach which involves collaboration between wildlife managers and local residents could help better manage human–bear conflicts in central India and elsewhere across the species' range.     

Do early life history characteristics determine species ranges and dispersal of coral reef fishes in the Indo‐Pacific?

The Indo‐Pacific consists of extensive continuous coastlines and many island groups of varying sizes and isolation. The species ranges of coral reef fishes vary enormously from Indo‐Pacific wide to highly endemic. There is also great variation in the early life history characteristics of coral reef fishes ( e.g . pelagic larval durations, spawning strategies and swimming abilities). We use individual‐based models (IBMs) to simulate the dispersal of coral reef fishes in the Indo‐Pacific. The development of dispersal strategies is explored based on ecological and geographical constraints. Simulations are presented for climatic and anthropogenically‐induced events.     

Functional traits,spatial patterns and species associations: what is their combined role in the assembly of wetland plant communities?

Studies of community assembly focus on finding rules that predict which species can become member of a plant community. Within a community, species can be categorized in two ways: functional groups classify species according to their functional traits, whereas generalized guilds group species based on their (co-)occurrence, spatial distribution and abundance patterns. This study searches for community assembly rules by testing for coherence among functional groups and generalized guilds, as well as for correlations between the individual functional traits and assembly features, in two wetland plant communities in South Africa. The classifications of functional groups and generalized guilds were not consistent. However, rhizome internode length was related to fine-scale spatial pattern, suggesting that in systems dominated by clonal species (including wetlands, where recruitment sites are strongly limited) community assembly may be strongly linked to colonization ability. Functional groups do not predict guilds in wetland plant communities, precluding their use as the basis for assembly rules. However, an explicit consideration of clonal strategies and their effect on species’ spatial patterns appears to be important for understanding community assembly in systems dominated by clonal plants.     

Characterization of the spatial distribution of latrines in reintroduced mountain gazelles: do latrines demarcate female group home ranges?

Mountain gazelle Gazella gazella in Saudi Arabia are listed as 'vulnerable' by the IUCN. At present, the species' survival is secured by extensive captive-breeding programmes and reintroductions into protected areas. Two reintroduction attempts (Ibex Reserve and Uruq Bani Ma'Arid protected areas) in Saudi Arabia have been undertaken in the past two decades. Post-monitoring of released individuals is essential for the success of such reintroduction programmes; however, cryptic species like mountain gazelles are extremely difficult to observe directly. As radio-tracking is a cost-intensive and invasive post-monitoring technique, we asked: how can reintroduced or remnant pockets of natural gazelle populations be monitored indirectly ? Here, we propose the use of latrine mapping as an effective, cost-efficient and non-invasive tool to survey the social organization of reintroduced mountain gazelles as an indicator for repatriation success. In this study, we used released radio-collared animals to characterize the spatial distribution of latrines within female group home ranges. Distance to the next latrine, latrine size, as well as numbers of fresh faecal pellet groups per latrine or presence of urination marks were used as dependent variables for step-wise backward multiple regressions and were correlated with various ecological factors. Most dependent variables were correlated with distance or direction from the nearest tree, but not indicative of home-range cores. Only latrine densities were distinctly higher in core areas of female group home ranges, and no pattern of peripheral marking was detected. Hence, latrine density is a good indicator of home-range use in female group home ranges. Mapping latrines and determining latrine densities are therefore the methods of choice to survey mountain gazelle populations.