| 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. |
- Eklöf, Anna, 1976-, et al.
(författare)
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Cascading extinctions in spatially coupled food webs
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The spatial structure of ecological communities as well as the dynamics and structure of local communities can be expected to have important consequences for the long-term persistence of metacommunities, that is, their resistance to different kind of perturbations. The aim of the present work is to investigate how web connectance of local communities and number of local habitat patches affects a metacommunity’s response to the global loss of a species. We find that the inclusion of space significantly reduces the risk of global and local 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 stabilized 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, although they often take the longest time to die out. This is particularly pronounced in spatial habitats, where the top predators appear to be the least well adapted to exploit the stabilizing properties of space.
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| 4. |
- Eklöf, Anna, 1976-
(författare)
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Species extinctions in food webs : local and regional processes
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Loss of biodiversity is one of the most severe threats to the ecosystems of the world. The major causes behind the high population and species extinction rates are anthropogenic activities such as overharvesting of natural populations, pollution, climate change and destruction and fragmentation of natural habitats. There is an urgent need of understanding how these species losses affect the ecological structure and functioning of our ecosystems. Ecological communities exist in a landscape but the spatial aspects of community dynamics have until recently to large extent been ignored. However, 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 can cope with species loss. The overall goal of the present work has been to investigate how both local and regional processes affect ecosystem stability, in the context of preserved biodiversity and maintained ecosystem functioning. The focus is particularly on how these processes effects ecosystem’s response to species loss. To accomplish this goal I have formulated and analyzed mathematical models of ecological communities. We start by analyzing the local processes (Paper I and II) and continue by adding the regional processes (Paper III, IV and V).In Paper I we analyze dynamical models of ecological communities of different complexity (connectance) to investigate how the structure of the communities affects their resistance to species loss. We also investigate how the resistance is affected by the characteristics, like trophic level and connectivity, of the initially lost species. We find that complex communities are more resistant to species loss than simple communities. The loss of species at low trophic levels and/or with high connectivity (many links to other species) triggers, on average, the highest number of secondary extinctions. We also investigate the structure of the post-extinction community. Moreover, we compare our dynamical analysis with results from topological analysis to evaluate the importance of incorporating dynamics when assessing the risk and extent of cascading extinctions.The characteristics of a species, like its trophic position and connectivity (number of ingoing and outgoing trophic links) will affect the consequences of its loss as well as its own vulnerability to secondary extinction. In Paper II we characterize the species according to their trophic/ecological uniqueness, a new measure of species characteristic we develop in this paper. A species that has no prey or predators in common with any other species in the community will have a high tropic uniqueness. Here we examine the effect of secondary extinctions on an ecological community’s trophic diversity, the range of different trophic roles played by the species in a community. We find that secondary extinctions cause loss of trophic diversity greater than expected from chance. This occurs because more tropically unique species are more vulnerable to secondary extinctions.In Paper III, IV and V we expand the analysis to also include the spatial dimension. Paper III is a book chapter discussing spatial aspects of food webs. In Paper IV we analyze how metacommunities (a set of local communities in the landscape connected by species dispersal) respond to species loss and how this response is affected by the structure of the local communities and the number of patches in the metacommunity. We find that the inclusion of space reduces the risk of global and local extinctions and that lowly connected communities are more sensitive to species loss.In Paper V we investigate how the trophic structure of the local communities, the spatial structure of the landscape and the dispersal patterns of species affect the risk of local extinctions in the metacommunity. We find that the pattern of dispersal can have large effects on local diversity. Dispersal rate as well as dispersal distance are important: low dispersal rates and localized dispersal decrease the risk of local and global extinctions while high dispersal rates and global dispersal increase the risk. We also show that the structure of the local communities plays a significant role for the effects of dispersal on the dynamics of the metacommunity. The species that are most affected by the introduction of the spatial dimension are the top predators.
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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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