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Species loss and secondary extinctions in simple and complex model communities
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 .
2006 (English)In: Journal of Animal Ecology, ISSN 0021-8790, 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. Vol. 75, no 1, 239-246 p.
Keyword [en]
Cascading extinction, connectance, food web, keystone species, resistance
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-37000DOI: 10.1111/j.1365-2656.2006.01041.xLocal ID: 33326OAI: diva2:257849
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2009-11-24Bibliographically approved
In thesis
1. Species extinctions in food webs: local and regional processes
Open this publication in new window or tab >>Species extinctions in food webs: local and regional processes
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.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1291
Extinction, food web, metacommunity, dispersal, species loss, migration, habitat fragmentation, connectance
National Category
Natural Sciences
urn:nbn:se:liu:diva-51815 (URN)978-91-7393-480-0 (ISBN)
Public defence
2009-12-18, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2009-11-24 Created: 2009-11-19 Last updated: 2009-11-24Bibliographically approved

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