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Adaptive rewiring aggravates the effects of species loss in ecosystems
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology. Linköping University, The Institute of Technology.
2015 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, 8412Article in journal (Other academic) Published
Abstract [en]

Loss of one species in an ecosystem can trigger extinctions of other dependent species. For instance, specialist predators will go extinct following the loss of their only prey unless they can change their diet. It has therefore been suggested that an ability of consumers to rewire to novel prey should mitigate the consequences of species loss by reducing the risk of cascading extinction. Using a new modelling approach on natural and computer-generated food webs we find that, on the contrary, rewiring often aggravates the effects of species loss. This is because rewiring can lead to overexploitation of resources, which eventually causes extinction cascades. Such a scenario is particularly likely if prey species cannot escape predation when rare and if predators are efficient in exploiting novel prey. Indeed, rewiring is a two-edged sword; it might be advantageous for individual predators in the short term, yet harmful for long-term system persistence.

Place, publisher, year, edition, pages
Nature Publishing Group, 2015. Vol. 6, 8412
Keyword [en]
Resistance, extinction risk, secondary extinction cascades, environmental variation, stochastic, response diversity, functional responses
National Category
Biological Sciences
Identifiers
URN: urn:nbn:se:liu:diva-108905DOI: 10.1038/ncomms9412ISI: 000363138400004OAI: oai:DiVA.org:liu-108905DiVA: diva2:733828
Note

Funding text: Linkoping University.

The original titel of this article was Adaptive rewiring aggravates the effects of species loss in ecological networks.

Available from: 2014-07-11 Created: 2014-07-11 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Extinctions in Ecological Communities: direct and indirect effects of perturbation on biodiversity
Open this publication in new window or tab >>Extinctions in Ecological Communities: direct and indirect effects of perturbation on biodiversity
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the dawning of what may become Earth’s 6th mass extinction the topic of this thesis, understanding extinction processes and what determines the magnitude of species loss, has become only too relevant. The number of known extinctions (~850) during the last centuries translates to extinction rates elevated above the background rate, matching those of previous mass extinction events. The main drivers of these extinctions have been human land use, introduction of exotic species and overexploitation. Under continued anthropogenic pressure and climate change, the current extinction rates are predicted to increase tenfold.

Large perturbations, such as the extinction drivers mentioned above, affects species directly, causing a change in their abundance. As species are not isolated, but connected to each other through a multitude of interactions, the change in abundance of one species can in turn affect others. Thus, in addition to the direct effect, a perturbation can affect a species indirectly through the ecological network in which the species is embedded. With this thesis, I wish to contribute to our basic understanding of these indirect effects and the role they play in determining the magnitude of species loss. All the studies included here are so called in silico experiments, using mathematical models to describe ecological communities and computer simulations to observe the response of these communities to perturbation.

When a perturbation is severe enough, a species will be driven to extinction. The loss of a species from a system is in itself a large perturbation, and may result in further extinctions, so called secondary extinctions. The traits of the species initially lost, can be a potential predictor of the magnitude of secondary species loss. In Paper I of this thesis, I show that when making such predictions, it is important to incorporate temporally dynamic species interactions and abundances, in order not to underestimate the importance of certain species, such as top predators.

I further show that species traits alone are not particularly good predictors of secondary extinction risk (Paper I), but that in combination with community level properties they are (Paper II). Indeed, there seems to be an interaction such that the specific property making a community prone to secondary species loss, depends on what kind of species was lost in the primary extinction. As different types of perturbation put different types of species at risk of (primary) extinction, this means that the specific property making a community prone to secondary species loss, will depend on the type of perturbation the community is subjected to.

One of the predicted main drivers of future species extinction is climate change. If the local climate becomes adverse, a species can either migrate to new and better areas or stay and evolve. Both these processes will be important in determining the magnitude of species loss under climate change. However, migration and evolution do not occur in vacuum – the biotic community in which these processes play out may modulate their effect on biodiversity. In paper III, I show that the strength of competition between species modulates the effect of both dispersal and evolution on the magnitude of species loss under climate change. The three-way interaction between interspecific competition, evolution and dispersal, creates a complex pattern of biodiversity responses, in which both evolution and dispersal can either increase or decrease the magnitude of species loss. Thus, when species interactions are incorporated, it is clear that even though migration and evolution may alleviate the impact of climate change for some species, they may indirectly aggravate the situation for others.

In Paper III, the aspect of climate change incorporated in the model is an increase in mean annual temperature. But climate change is also predicted to increase environmental variability. Paper IV shows that species-rich communities are more sensitive to high environmental variability than species-poor ones. The smaller population sizes in the species-rich communities increased the extinction risk connected to population fluctuations driven by the variable environment. Hence, systems such as tropical forests and coral reefs are predicted to be particularly sensitive to the increased variability that may follow with climate change.

In Paper IV, primary extinctions of primary producers result in extinction cascades of consumer species, when they lose their prey. However, in reality a consumer species might be able to switch to another prey, and such flexibility has both been observed and suggested as a potential rescue mechanism. But what is beneficial for an individual predator in the short-term can become detrimental to the ecological community in the long-term. Paper V shows that consumer flexibility often led to consumers continuously overexploiting their new prey, in the worst case to the point of system collapse. Thus, the suggested rescue mechanism aggravated the effect of initial species loss, rather than ameliorating it.

Overall, the research presented here, underscores the importance of including population dynamics and biotic interactions when studying the effects of perturbation on biodiversity. Many of the results are complex, hard to foresee or even counter-intuitive, arising from the indirect effects of the perturbation being translated through the living web of species interactions.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 60 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1609
National Category
Other Natural Sciences
Identifiers
urn:nbn:se:liu:diva-108906 (URN)10.3384/diss.diva-108906 (DOI)978-91-7519-278-9 (ISBN)
Public defence
2014-08-29, Schrödinger, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-07-11 Created: 2014-07-11 Last updated: 2017-04-19Bibliographically approved
2. Structure and Stability of Ecological Networks: The role of dynamic dimensionality and species variability in resource use
Open this publication in new window or tab >>Structure and Stability of Ecological Networks: The role of dynamic dimensionality and species variability in resource use
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main focus of this thesis is on the response of ecological communities to environmental variability and species loss. My approach is theoretical; I use mathematical models of networks where species population dynamics are described by ordinary differential equations. A common theme of the papers in my thesis is variation – variable link structure (Paper I) and within-species variation in resource use (Paper III and IV). To explore how such variation affect the stability of ecological communities in variable environments, I use numerical methods evaluating for example community persistence (the proportion of species surviving over time; Paper I, II and IV). I also develop a new method for quantifying the dynamical dimensionality of an ecological community and investigate its effect on community persistence in stochastic environments (Paper II). Moreover, if we are to gain trustworthy model output, it is of course of major importance to create study systems that reflect the structures of natural systems. To this end, I also study highly resolved, individual based empirical food web data sets (Paper III, IV).

In Paper I, the effects of adaptive rewiring induced by resource loss on the persistence of ecological networks is investigated. Loss of one species in an ecosystem can trigger extinctions of other dependent species. For instance, specialist predators will go extinct following the loss of their only prey unless they can change their diet. It has therefore been suggested that an ability of consumers to rewire to novel prey should mitigate the consequences of species loss by reducing the risk of cascading extinction. Using a new modelling approach on natural and computer-generated food webs I find that, on the contrary, rewiring often aggravates the effects of species loss. This is because rewiring can lead to overexploitation of resources, which eventually causes extinction cascades. Such a scenario is particularly likely if prey species cannot escape predation when rare and if predators are efficient in exploiting novel prey. Indeed, rewiring is a two-edged sword; it might be advantageous for individual predators in the short term, yet harmful for long-term system persistence.

The persistence of an ecological community in a variable world depends on the strength of environmental variation pushing the community away from equilibrium compared to the strength of the deterministic feedbacks, caused by interactions among and within species, pulling the community towards the equilibrium. However, it is not clear which characteristics of a community that promote its persistence in a variable world. In Paper II, using a modelling approach on natural and computer-generated food webs, I show that community persistence is strongly and positively related to its dynamic dimensionality (DD), as measured by the inverse participation ratio (IPR) of the real part of the eigenvalues of the community matrix. A high DD means that the real parts of the eigenvalues are of similar magnitude and the system will therefore approach equilibrium from all directions at a similar rate. On the other hand, when DD is low, one of the eigenvalues has a large magnitude of the real part compared to  the others and the deterministic forces pulling the system towards  equilibrium is therefore weak in many directions compared to the stochastic forces pushing the system away from the equilibrium. As a consequence the risk of crossing extinction thresholds and boundaries separating basins of attractions increases, and hence persistence decreases, as DD decreases. Given the forecasted increase in climate variability caused by global warming, Paper II suggests that the dynamic dimensionality of ecological systems is likely to become an increasingly important property for their persistence.

In Paper III, I investigate patterns in the size structure of one marine and six running freshwater food webs: that is, how the trophic structure of such ecological networks is governed by the body size of its interacting entities. The data for these food webs are interactions between individuals, including the taxonomic identity and body mass of the prey and the predator. Using these detailed data, I describe how patterns in diet variation and predator variation scales with the body mass of predators or prey, using both a species- and a size-class-based approach. I also compare patterns of size structure derived from analysis of individual-based data with those patterns that result when data are aggregated into species (or size class-based) averages. This comparison shows that analysis based on species averaging can obscure interesting patterns in the size structure of ecological communities. For example, I find that the strength of the relationship between prey body mass and predator body mass is consistently underestimated when species averages are used instead of the individual level data. In some cases, no relationship is found when species averages are used, but when individual-level data are used instead, clear and significant patterns are revealed. These results have potentially important implications for parameterisation of models of ecological communities and hence for predictions concerning their dynamics and response to different kinds of disturbances.

Paper IV continues the analysis of the highly resolved individual-based empirical data set used in Paper III and investigates patterns and effects of within- and between species resource specialisation in ecological communities. Within-species size variation can be considerable. For instance, in fishes and reptiles, where growth is continuous, individuals pass through a wide spectrum of sizes, possibly more than four orders of magnitude, during the independent part of their life cycle. Given that the size of an organism is correlated with many of its fundamental ecological properties, it should come as no surprise that an individual’s size affects the type of prey it can consume and what predators will attack it (Paper III). In Paper IV, I quantify within- and between species differences in predator species’ prey preferences in natural food webs and subsequently explore its consequences for dynamical dimensionality (Paper II) and community stability in stage structured food web models. Among the natural food webs there are webs where species overlap widely in their resource use while the resource use of size-classes within species differs. There are also webs where differences in resource use among species is relatively large and the niches of sizeclasses within species are more similar. Model systems with the former structure are found to have low dynamical dimensionality and to be less stable compared to systems with the latter structure. Thus, although differential resource use among individuals within a species is likely to decrease the intensity of intraspecific competition and favor individuals specializing on less exploited resources it can destabilize the community in which the individuals are embedded.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 38 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1735
National Category
Biological Sciences Environmental Sciences related to Agriculture and Land-use
Identifiers
urn:nbn:se:liu:diva-123970 (URN)10.3384/diss.diva-123970 (DOI)978-91-7685-853-0 (ISBN)
Public defence
2016-02-12, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-01-15 Created: 2016-01-15 Last updated: 2016-01-20Bibliographically approved

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Gilljam, DavidCurtsdotter, AlvaEbenman, Bo

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