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Species extinctions in food webs: local and regional processes
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology . Linköping University, The Institute of Technology.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2009. , 45 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1291
Keyword [en]
Extinction, food web, metacommunity, dispersal, species loss, migration, habitat fragmentation, connectance
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-51815ISBN: 978-91-7393-480-0 (print)OAI: oai:DiVA.org:liu-51815DiVA: diva2:277505
Public defence
2009-12-18, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2009-11-24 Created: 2009-11-19 Last updated: 2009-11-24Bibliographically approved
List of papers
1. Species loss and secondary extinctions in simple and complex model communities
Open this publication in new window or tab >>Species loss and secondary extinctions in simple and complex model communities
2006 (English)In: Journal of Animal Ecology, ISSN 0021-8790, E-ISSN 1365-2656, Vol. 75, no 1, 239-246 p.Article in journal (Refereed) Published
Abstract [en]
  1. The loss of a species from an ecological community can trigger a cascade of secondary extinctions. Here we investigate how the complexity (connectance) of model communities affects their response to species loss. Using dynamic analysis based on a global criterion of persistence (permanence) and topological analysis we investigate the extent of secondary extinctions following the loss of different kinds of species.
  2. We show that complex communities are, on average, more resistant to species loss than simple communities: the number of secondary extinctions decreases with increasing connectance. However, complex communities are more vulnerable to loss of top predators than simple communities.
  3. The loss of highly connected species (species with many links to other species) and species at low trophic levels triggers, on average, the largest number of secondary extinctions. The effect of the connectivity of a species is strongest in webs with low connectance.
  4. Most secondary extinctions are due to direct bottom-up effects: consumers go extinct when their resources are lost. Secondary extinctions due to trophic cascades and disruption of predator-mediated coexistence also occur. Secondary extinctions due to disruption of predator-mediated coexistence are more common in complex communities than in simple communities, while bottom-up and top-down extinction cascades are more common in simple communities.
  5. Topological analysis of the response of communities to species loss always predicts a lower number of secondary extinctions than dynamic analysis, especially in food webs with high connectance.
Place, publisher, year, edition, pages
Wiley InterScience, 2006
Keyword
Cascading extinction, connectance, food web, keystone species, resistance
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-37000 (URN)10.1111/j.1365-2656.2006.01041.x (DOI)000235043700024 ()33326 (Local ID)33326 (Archive number)33326 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-04-18Bibliographically approved
2. Trophically Unique Species Are Vulnerable to Cascading Extinction
Open this publication in new window or tab >>Trophically Unique Species Are Vulnerable to Cascading Extinction
2008 (English)In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 171, no 5, 568-579 p.Article in journal (Refereed) Published
Abstract [en]

Understanding which species might become extinct and the consequences of such loss is critical. One consequence is a cascade of further, secondary extinctions. While a significant amount is known about the types of communities and species that suffer secondary extinctions, little is known about the consequences of secondary extinctions for biodiversity. Here we examine the effect of these secondary extinctions on trophic diversity, the range of trophic roles played by the species in a community. Our analyses of natural and model food webs show that secondary extinctions cause loss of trophic diversity greater than that expected from chance, a result that is robust to variation in food web structure, distribution of interactions strengths, functional response, and adaptive foraging. Greater than expected loss of trophic diversity occurs because more trophically unique species are more vulnerable to secondary extinction. This is not a straightforward consequence of these species having few links with others but is a complex function of how direct and indirect interactions affect species persistence. A positive correlation between a species’ extinction probability and the importance of its loss defines high-risk species and should make their conservation a priority.

Place, publisher, year, edition, pages
University of Chicago Press, 2008
Keyword
biodiversity, redundancy, stability, food webs, species deletions
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-11923 (URN)10.1086/587068 (DOI)000255212900004 ()
Note

Original publication: Owen L. Petchey, Anna Eklöf, Charlotte Borrvall and Bo Ebenman, Trophically Unique Species Are Vulnerable to Cascading Extinction, 2008, American Naturalist, (171), 5, 568-579. http://dx.doi.org/10.1086/587068. Copyright © 2008. University of Chicago Press. All rights reserved

Available from: 2008-05-28 Created: 2008-05-28 Last updated: 2017-04-19Bibliographically approved
3. Spatial aspects of food webs
Open this publication in new window or tab >>Spatial aspects of food webs
Show others...
2005 (English)In: Dynamic Food Webs: Multispecies Assemblages, Ecosystem Development and Environmental Change / [ed] P.C. deRuiter, V. Wolters & J.C. Moore, London, UK: Elsevier, 2005, Vol. 3, 463-469 p.Conference paper, Published paper (Refereed)
Abstract [en]

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.

Place, publisher, year, edition, pages
London, UK: Elsevier, 2005
Series
Theoretical Ecology Series, ISSN 1875-306X
National Category
Ecology
Identifiers
urn:nbn:se:liu:diva-31067 (URN)16789 (Local ID)9780120884582 (ISBN)0120884585 (ISBN)16789 (Archive number)16789 (OAI)
Conference
Food Web Symposium 2003, Giessen, Germany, 13-16 November 2003
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2015-09-14Bibliographically approved
4. Cascading extinctions in spatially coupled food webs
Open this publication in new window or tab >>Cascading extinctions in spatially coupled food webs
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keyword
Cascading extinction, dispersal, food web connectance, habitat destruction, metacommunity, spatial structure, species interactions, species loss
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-51955 (URN)
Available from: 2009-11-24 Created: 2009-11-24 Last updated: 2009-11-24Bibliographically approved
5. Effects of dispersal on local extinctions in multi-trophic metacommunities
Open this publication in new window or tab >>Effects of dispersal on local extinctions in multi-trophic metacommunities
(English)Manuscript (preprint) (Other academic)
Abstract [en]

As a result of habitat destruction many ecological communities have a fragmented distribution and are built up of partially isolated local communities connected through dispersal of interacting species. The dynamics of such metacommunities is governed both by local processes (interactions among species coexisting within habitat patches) and regional processes (movement of species among habitat patches). Earlier theoretical work on simple metacommunities have mainly focused on the positive effects of space and dispersal for the coexistence of interacting species and hence for local and regional species diversity. However, it is plausible that dispersal might also pose some kind of risk to the dispersing individuals. Here we explore how such risks might affect the dynamics of metacommunities. We develop spatially and dynamically explicit models to investigate how the trophic structure (connectance) of local communities, the spatial structure of the metacommunity and the dispersal characteristics of species affect species extinction risks. Species extinction risks in these open communities are measured relative to the extinction risks in closed communities (i.e. no dispersal). We show that the introduction of dispersal among initially closed local communities might lead to increased probability of local species extinction. The effects of dispersal depend on migration rate, movement pattern of individuals and the density of patches in the landscape. Specifically, when dispersal involves a risk, high migration rates, global dispersal and low patch density will all lead to increased probability of local species extinctions. Furthermore, the trophic structure of local communities plays a significant role in the response of metacommunities to changes in the regional processes.

Keyword
Dispersal, dispersal risk, extinction, food web connectance, metacommunity, migration rate, spatial structure
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-51956 (URN)
Available from: 2009-11-24 Created: 2009-11-24 Last updated: 2009-11-24Bibliographically approved

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