| 1. |
- Åkesson, Anna, 1985-
(författare)
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Disturbances in food webs : Importance of species interactions from an ecological and evolutionary perspective
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biodiversity loss is occurring globally at an unprecedented pace. This is not only followed by ethical concerns; it also affects all levels of an ecosystem, with wide-spread implications for ecosystem functioning, services and human well-being. The severe extinction risk for many species is a result of human activities, such as habitat destruction and land-use change, overexploitation and introduction of invasive species. During the past decades, climate change has additionally become an important human-induced driver, causing biodiversity loss and altered species interactions.To mitigate the negative impact on ecosystems, we need to understand how the species building up the systems respond to disturbances. Several structural properties, both at species- and network-level, are known to affect species vulnerability. At the species-level, a species position in the food web, as well as its distribution of prey items, are important factors. At the network-level, diversity and structure of feeding interactions are important measurements related to stability. Additionally, species may both directly and indirectly affect other species, as species are entangled in complex network structures. The loss of a single species could set in motion a cascade of secondary extinctions that may not be predictable based on species’ performance in isolation. Particularly, disturbances of primary producers have a high risk to propagate and augment through the network. Moreover, species interactions have the potential to affect several other ecological processes. For example, altered environmental conditions force species to disperse to more suitable habitats, or to stay and adapt to the new local condition. Such processes can be significantly altered by species interactions. Another example concerns ecosystem service provisioning. Even if a species not being a service provider goes extinct, the event can via direct and indirect effects cause secondary extinctions of service providing species, causing loss of the services.Despite the recognition of the importance of a network context and of species interactions, such aspects are in many cases modeled in a simplified manner or not considered. In this thesis, I use mathematical models to study how species embedded in an ecological network respond to disturbances. I have two primary focus areas. First, I analyze how the interplay between evolution, dispersal and species interactions affect how species respond to climatic change. In paper I, I studied the effects of these eco-evolutionary processes under increasing temperatures, using empirically-motivated parameterizations of a suite of community models with increasing ecological interaction complexity. Second, I analyze how several structural properties affect species persistence following a disturbance. In Paper II, I focus on how network-level properties as well as species-level properties affect consumer species persistence following basal level disturbances. In Paper III, I disentangle the most influential characteristics of groups of basal species, causing a negative impact on consumer species when disturbed. I use the Serengeti savanna food web as case study. In Paper IV, I connect ecosystem services to the species providing them, and analyze how ecosystem service provisioning is affected by anthropogenic threats. I use the Baltic Sea as a case study.In summary, the results of this thesis underscore the importance of studying disturbances within a network context. Species interactions highly influenced the eco-evolutionary dynamics in Paper I, and mitigated some of the negative impacts following climatic change. Several structural network properties, both of the species being disturbed and of the species being affected, influenced species’ vulnerability following disturbance in Paper II-IV. The interplay between species influenced how disturbances percolated through the network. Moreover, Paper IV found that indirect effects mediated by the network of species interactions were of substantial importance for how anthropogenic threats are affecting ecosystem service delivery. in this thesis, I have developed novel methods, as well as extended and showcased new applications for existing ones. As such, this thesis has a broad applicability and expands our basic understanding of the interplay between ecological and evolutionary processes, as well as our understanding of the mechanisms behind how networks of interacting species are affected by disturbances. Further, the results have important implications for conservation efforts.
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| 2. |
- Eklöf, Anna, 1976-
(författare)
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Cascading extinctions in food webs : local and regional processes
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ecological communities all over the world are loosing biodiversity due to different kinds of human activities and there is an urgent need of understanding how those losses affect the function of the ecosystems on which we all depend. The community's response to species losses is likely to depend on both the structure of the local community as well as its interactions with surrounding communities. Also the characteristics of the species going extinct do affect how the community structure changes. The main purpose of this thesis is to study how local population dynamics and regional processes in food webs affect ecological communities' response to species loss, especially the risk of cascading extinctions.In Paper I we use a set of model food webs with different shapes and connectance to look at how the structure of the community affects its resistance to species loss. We also investigate how the resistance is affected by which species, according to trophic level and connectivity, that is lost initially. What we find is that food webs with lower connectance seem to be more vulnerable than more connected communities. The loss of a species at low trophic level and / or with high connectivity triggers the on average highest number of secondary extinctions. We also discuss about the structure of the post extinction community and compare our analysis with topological studies.In paper Il we use as set of metacommunities with different connectances and different number of patches and we vary the dispersal distances and migration rates. The aim of this paper is to investigate how web connectance of local communities, number of habitat patches and dispersal patterns affects a metacommunity's response to the global loss of a species. We find that asynchrony among patch dynamics may arise from relatively low rates of migration, and that the inclusion of space significantly reduces the risk of global cascading extinctions. It is shown that communities with sparsely connected food webs are the most sensitive to species loss, but also that they are particularly well stabilised by the introduction of space. In agreement with theoretical studies of non-spatial habitats, species at the highest trophic level are the most vulnerable to secondary extinction.Paper III is a book chapter that emerged from a working group during a food web symposium in Giessen, Germany 2003. It is dealing with ideas of how to look at spatial aspects of food webs.
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| 3. |
- Brose, Ulrich, et al.
(författare)
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Predicting the consequences of species lossusing size-structured biodiversity approaches
- 2017
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Ingår i: Biological Reviews. - : Wiley-Blackwell. - 1464-7931 .- 1469-185X. ; 92:2, s. 684-697
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Forskningsöversikt (refereegranskat)abstract
- Understanding the consequences of species loss in complex ecological communities is one of the great challenges in current biodiversity research. For a long time, this topic has been addressed by traditional biodiversity experiments. Most of these approaches treat species as trait-free, taxonomic units characterizing communities only by species number without accounting for species traits. However, extinctions do not occur at random as there is a clear correlation between extinction risk and species traits. In this review, we assume that large species will be most threatened by extinction and use novel allometric and size-spectrum concepts that include body mass as a primary species trait at the levels of populations and individuals, respectively, to re-assess three classic debates on the relationships between biodiversity and (i) food-web structural complexity, (ii) community dynamic stability, and (iii) ecosystem functioning. Contrasting current expectations, size-structured approaches suggest that the loss of large species, that typically exploit most resource species, may lead to future food webs that are less interwoven and more structured by chains of interactions and compartments. The disruption of natural body-mass distributions maintaining food-web stability may trigger avalanches of secondary extinctions and strong trophic cascades with expected knock-on effects on the functionality of the ecosystems. Therefore, we argue that it is crucial to take into account body size as a species trait when analysing the consequences of biodiversity loss for natural ecosystems. Applying size-structured approaches provides an integrative ecological concept that enables a better understanding of each species' unique role across communities and the causes and consequences of biodiversity loss.
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| 4. |
- Brose, Ulrich, et al.
(författare)
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Spatial aspects of food webs
- 2005
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Ingår i: Dynamic Food Webs. - London, UK : Elsevier. - 9780120884582 - 0120884585 ; , s. 463-469
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Konferensbidrag (refereegranskat)abstract
- Aspects of spatial scale have until recently been largely ignored in empirical and theoretical food web studies (e.g., Cohen & Briand 1984, Martinez 1992, but see Bengtsson et al. 2002, Bengtsson & Berg, this book). Most ecologists tend to conceptualize and represent food webs as static representations of communities, depicting a community assemblage as sampled at a particular point in time, or highly aggregated trophic group composites over broader scales of time and space (Polis et al. 1996). Moreover, most researchers depict potential food webs, which contain all species sampled and all potential trophic links based on literature reviews, several sampling events, or laboratory feeding trials. In reality, however, not all these potential feeding links are realized as not all species co-occur, and not all samples in space or time can contain all species (Schoenly & Cohen 1991), hence, yielding a variance of food web architecture in space (Brose et al. 2004). In recent years, food web ecologists have recognized that food webs are open systems – that are influence by processes in adjacent systems – and spatially heterogeneous (Polis et al. 1996). This influence of adjacent systems can be bottom-up, due to allochthonous inputs of resources (Polis & Strong 1996, Huxel & McCann 1998, Mulder & De Zwart 2003), or top-down due to the regular or irregular presence of top predators (e.g., Post et al. 2000, Scheu 2001). However, without a clear understanding of the size of a system and a definition of its boundaries it is not possible to judge if flows are internal or driven by adjacent systems. Similarly, the importance of allochthony is only assessable when the balance of inputs and outputs are known relative to the scale and throughputs within the system itself. At the largest scale of the food web – the home range of a predator such as wolf, lion, shark or eagle of roughly 50 km2 to 300 km2 –the balance of inputs and outputs caused by wind and movement of water may be small compared to the total trophic flows within the home range of the large predator (Cousins 1990). Acknowledging these issues of space, Polis et al (1996) argued that progress toward the next phase of food web studies would require addressing spatial and temporal processes. Here, we present a conceptual framework with some nuclei about the role of space in food web ecology. Although we primarily address spatial aspects, this framework is linked to a more general concept of spatio-temporal scales of ecological research.
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| 5. |
- Eklöf, Anna, 1976-, et al.
(författare)
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Networks, Ecological
- 2012. - 1
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Ingår i: Encyclopedia of Theoretical Ecology. - : University of California Press. - 9780520269651 - 9780520951785 ; , s. 470-478
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Bokkapitel (refereegranskat)
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| 6. |
- Eklöf, Anna, 1976-, et al.
(författare)
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Relevance of evolutionary history for food web structure
- 2012
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Ingår i: Proceedings of the Royal Society of London. Biological Sciences. - : The Royal Society Publishing. - 0962-8452 .- 1471-2954. ; 279:1733, s. 1588-1596
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Tidskriftsartikel (refereegranskat)abstract
- Explaining the structure of ecosystems is one of the great challenges of ecology. Simple models for foodweb structure aim at disentangling the complexity of ecological interaction networks and detect the main forces that are responsible for their shape. Trophic interactions are influenced by species traits, which in turn are largely determined by evolutionary history. Closely related species are more likely to share similar traits, such as body size, feeding mode and habitat preference than distant ones. Here, we present a theoretical framework for analysing whether evolutionary history—represented by taxonomic classification—provides valuable information on food web structure. In doing so, we measure which taxonomic ranks better explain species interactions. Our analysis is based on partitioning of the species into taxonomic units. For each partition, we compute the likelihood that a probabilistic model for food web structurere produces the data using this information. We find that taxonomic partitions produce significantly higher likelihoods than expected at random. Marginal likelihoods (Bayes factors) are used to perform model selection among taxonomic ranks. We show that food webs are best explained by the coarser taxonomic ranks (kingdom to class). Our methods provide a way to explicitly include evolutionary history in models for food web structure.
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| 7. |
- Eklöf, Anna, 1976-, et al.
(författare)
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Secondary extinctions in food webs : a Bayesian network approach
- 2013
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Ingår i: Methods in Ecology and Evolution. - : Wiley-Blackwell. - 2041-210X. ; 4:8, s. 760-770
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Tidskriftsartikel (refereegranskat)abstract
- Ecological communities are composed of populations connected in tangled networks of ecological interactions. Therefore, the extinction of a species can reverberate through the network and cause other (possibly distantly connected) species to go extinct as well. The study of these secondary extinctions is a fertile area of research in ecological network theory.However, to facilitate practical applications, several improvements to the current analytical approaches are needed. In particular, we need to consider that (i) species have different ‘a priori’ probabilities of extinction, (ii) disturbances can simultaneously affect several species, and (iii) extinction risk of consumers likely grows with resource loss. All these points can be included in dynamical models, which are, however, difficult to parameterize.Here we advance the study of secondary extinctions with Bayesian networks. We show how this approach can account for different extinction responses using binary – where each resource has the same importance – and quantitative data – where resources are weighted by their importance. We simulate ecological networks using a popular dynamical model (the Allometric Trophic Network model) and use it to test our method.We find that the Bayesian network model captures the majority of the secondary extinctions produced by the dynamical model and that consumers’ responses to species loss are best modelled using a nonlinear sigmoid function. We also show that an approach based exclusively on food web structure loses power when species at higher trophic levels are preferentially lost. Because the loss of apex predators is unfortunately widespread, the results highlight a serious limitation of studies on network robustness.
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| 8. |
- Gudmundson, Sara, 1985-
(författare)
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Species Responses to Environmental Fluctuations : impacts of food web interactions and noise color
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Species constantly experience changes in their environmental conditions owing to natural or human induces reasons. Understanding how species respond to these fluctuations are important for ecology, especially given the ongoing climate change. Empirical studies have shown that species respond differently to the same disturbance. However, our knowledge of what create these differences in the environmental response is limited and in most cases based on studies focusing on single species. In this thesis, I have taken a theoretical approach and used dynamical models to investigate how the population dynamics of species are affected by species-species interactions and environmental fluctuations. In the first paper (Paper I) I investigated how a species respond to environmental fluctuations when isolated or embedded in a food web. The study showed that species-species interactions had an effect in temporally positively autocorrelated environments (red noise) but not in uncorrelated environments (white noise). This was owing to species following their equilibrium densities in red environments which in turn enabled species-species interactions to come into play. Red environmental variables are more prominent in nature than white. Thus, these results show the importance of using a food web approach when analyzing species response to environmental fluctuations. The most commonly discussed effect of climate change is an elevated mean temperature. This shift is expected to affect the growth rate of many species. However, there is no robust theory of how we should expect species in food webs to respond to a rise in temperature. In the second paper (Paper II) I defined and studied the dynamic rate of food webs(DR) acting analogously to single species growth rate. I found that the higher DR the easier for species population densities to follow their equilibrium over time. Both DR and noise color changed the temporal relationship between the population and the environmental noise. Thus, it is of major importance to take the scale of time into consideration when investigating species response to environmental fluctuations. Another important factor which affect population dynamics is species spatial distributions. Dispersal between subpopulations enable individuals to rescue or prolong the time to extinction for the population seen as a whole. In the third paper (Paper III), I investigated how species in food webs respond to environments that varies both in time and space and compared the results with the one from single species. I found that single species were stabilized by an increased dispersal rate independent of the noise color. Species-species interactions had an effect for some of the species in these landscapes.At red asynchronous noise, one resource species in each food web had a local minimum in stability at low dispersal rate. Here, dispersal decoupled local population dynamics and prevented species from tracking their equilibriums. At high dispersal rates, all resource species and their single species counterparts were stabilized by dispersal as local patch dynamics lost its importance. Environmental noise together with the spatial dimension does seem to explain much of the stability properties of species on our planet. However, natural ecosystems are much more complex and species rich than the food web models I have used so far. Theoreticians have previously had a hard time describing stable complex systems that survive environmental fluctuations. Thus, in my fourth and last project (PaperIV) I investigated how species population dynamics are affected by environmental fluctuations when embedded in larger food webs. These systems were built by connecting food web modules with periodic boundary conditions (PBC). The PBC method has previously helped physicists to understand the nature of waves and particles by removing the edges in systems. I found that food web size does not have to have a negative effect on food web stability. I showed that by removing the destabilizing effect of edges it is possible to describe large stable food webs, more similar to natural ecosystems. Overall, the research presented here give new insights into species responses to environmental fluctuations. They especially highlight the importance of considering both species interactions and environmental noise color when studying population dynamics in a fluctuating environment. A food web approach is necessary when analyzing species population dynamics and planning for conservation actions, especially when studying the effects of climate change on biodiversity.
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| 9. |
- Ohlsson, Mikael, 1983-
(författare)
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The significance of species groups for food web structure and functioning
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In ecosystems across the world, species co-exist, compete, and consume, all while adapting to environmental conditions and disruptions. An important key to the puzzle of understanding how species will respond to changes in the ecosystems, caused by for example climate change, pollution, habitat destruction, and overexploitation, is what current roles species have in a larger context. Species interactions are the basis for many ecological processes, for example, describing who eats whom in food webs. Finding groups of species that have similar interactions can provide insight into what roles species have in a food web, as well as identify core structures and functions of said food webs. Food webs are often based on data aggregates of large areas. Consequently, there is a possibility of blurring local aspects of the food web structure, thus blurring locally realized species roles. In Paper I, I used the group model to analyze local and regional group structures of a food web in the Barents Sea. The group model identifies groups linked to their niche, in which species eat, and are eaten by similar species. I found the large, regional food web diverged from the local group structures, indicating that locally scaled food webs may be required to find more accurately realized species roles. On a local scale, similar group structures were generally spatially clustered and environmentally similar. This was to some extent explained by similarities in species compositions, but more fine-grained patterns related to species identities further impacted the group structures. In essence, the group model is a type of community detection based on stochastic block models. Generated groups contain groups of species with similar sets of prey and predators. Groups are related to both trophic similarity and modularity, but the process itself is, as the name implies, stochastic. Various methodologies to determine a "best fit" group structure out of multiple iterations exist. Arguments can however be made, that discarded, alternative group structures may still hold ecological relevance. In Paper II, I investigated five food webs by creating a solution landscape from their respective alternative group structures. My results showed, that the core group structure remained intact across alternative solutions, while potential changes in the group structure were generally limited to smaller subsets of groups or species. Expanding on the analysis of Paper II, Paper III, accounts for the inherent un-certainty of interactions in food webs. Food webs are based on data sets, potentially covering hundreds of species and thousands of interactions. How-ever, spatial and temporal aspects, and the dynamical nature of whether interactions are realized, can impact the food web structure. Here, I investigated how group structures responded to disturbing interactions (i.e., random removal of different fractions). The key findings showed how in general, core group structures remained intact, and already unstable groups turned increasingly unstable. Species traits distinctly defined group identities, but I found no particular species traits ubiquitously linked to unstable group structures. How species interact is intrinsic to their traits; basic trait-matching constraints must be fulfilled for an interaction to be realized, such as a predator being large enough to eat a specific prey. Traits are however also subject to change, with potentially strong selective pressures from for example environmental change or overexploitation. If traits change sufficiently, species interactions can also change, potentially putting affected species in a new ecological context with new predators, prey, and competitive relationships. Possibly related, the cod population in the Baltic Sea, has failed to recover even after ceasing fishing. In Paper IV, I formulated an eco-evolutionary model, which considered cod’s changed ecological role after the collapse, highlighting how competition with flounder species can contribute to blocking the cod population in the Baltic Sea from recovering. With this thesis, I aimed to improve understanding of how species groups based on interactions relate to food web structure. My results highlight how the group model can generate robust groups, which are generally resilient to even moderate disturbances while providing a coarse-grained representation of species roles in a food web. The spatial context of the food web, with its included species and interactions, needs to be considered to get a more accurate representation of locally realized species roles. I have further modeled how species traits may be altered by eco-evolutionary dynamics under strong selective pressure, with subsequent shifts in ecological roles. These aspects are pivotal in understanding how species in our ecosystems will be affected by today’s multitude of environmental impacts.
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| 10. |
- Årevall, Jonatan, 1986-
(författare)
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Species’ responses to an ever-changing world
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of nature’s most astonishing features is the diversity of life, as more than a million species has been identified and classified. This diversity not only have a intrinsic value, but sustains us humans through a multitude of ecosystem services. Many of these services are critical for our sustenance. In our current age this diversity is threatened however, in what sometimes is referred to the Earth’s 6th mass extinction as the current frequency of species-extinction is estimated to be over a hundred times over the background rate. The largest threat to biodiversity is human land use and overexploitation. However, in the past decades we have seen the addition of another large threat that can also compound to the extinction rates - climate change.In the first two studies (Paper I and Paper II), I show how species’ dispersal ability might affect their response to climate change. In paper I, I found that when the potential new habitat of a species is homogeneously distributed in the landscape most species in the study were able to emigrate to new habitats. There was a larger variation in heterogenic landscapes where the outcome depended on both the dispersal ability of the species combined with how the habitat were specifically arranged in relation to the climatic optimum of the species. From this also follows that it is not only the amount of habitat that is important but also where the habitat is located in relation to other habitat. I also show that both dispersal ability and habitat might be more important predictors of a successful climatic shift of a species than the speed of climate change. In paper II, I use previously collected data of population abundances and dispersal of a butterfly (Pyrgus armoricanus) in Sweden to model its future distribution. Similar to Paper I we see that low habitat availability, as well as heterogeneous configuration together with low dispersal ability negatively impacts its range expansion.In the third study (Paper III) I examine how the stability of species in small food webs is influenced by the autocorrelation of environmental noise and dispersal rates. I also let modeled basal species independently to see if stability differed when modeling species in isolation. I found that not only does the stability of species abundance depend on the environmental noise and dispersal rates, but the stability also changes non-linearly with changes in autocorrelation and dispersal rate. The lowest stability for a species were not necessarily at the lowest dispersal rate but at a low to medium rate. An analysis of the results shows that at least some species seem to have an equilibrium that is not determined by the autocorrelation of environmental noise. The results thus underline complex mechanisms that might influence the abundance and stability of populations.In the fourth study (Paper IV) we generated ecological networks and compared how simulated networks differed, depending on if selection was included or not. Here we used an allometric model were growth rate, mortality and interactions between phenotypes depend on the body size of the phenotypes. We found that while removing implicit traits such as the intraspecific competition between individuals of the same species made the webs unstable and prone to lose many species during the simulation, networks became much more unstable when introducing selection on the body mass trait. The restructuring of networks due to evolution either led to competitive exclusion of species or a race between plants, with an evolutionary pressure to escape predators, and animals to become larger and larger.Overall the research presented here give new insights into how species’ dispersal ability together with landscape configuration and climatic shift might determine the future distributions of species. Not only is the future climate range of a a species important, but also the mechanisms that would affect a successful range shift. Of these mechanisms the dispersal ability and the distribution and availability of habitat in the landscape are the most important. It is also shown that dispersal ability is important to take into account when planning for conservation actions as to identify which potential habitat will better facilitate range expansion.
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