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Biodiversity and Species Extinctions in Model Food Webs
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology . Linköping University, The Institute of Technology.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many of the earth’s ecosystems are experiencing large species losses due to human impacts such as habitat destruction and fragmentation, climate change, species invasions, pollution, and overfishing. Due to the complex interactions between species in food webs the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. The complexity of ecological systems makes it difficult to study them empirically. The systems often consist of large species numbers with lots of interactions between species. Investigating ecological communities within a theoretical approach, using mathematical models and computer simulations, is an alternative or a complement to experimental studies. This thesis is a collection of theoretical studies. We use model food webs in order to explore how biodiversity (species number) affects the response of communities to species loss (Paper I-III) and to environmental variability (Paper IV).

In paper I and II we investigate the risk of secondary extinctions following deletion of one species. It is shown that resistance against additional species extinctions increases with redundancy (number of species per functional group) (Paper I) in the absence of competition between basal species but decreases with redundancy in the presence of competition between basal species (Paper II). It is further shown that food webs with low redundancy run the risk of losing a greater proportion of species following a species deletion in a deterministic environment but when demographic stochasticity is included the benefits of redundancy are largely lost (Paper II). This finding implies that in the construction of nature reserves the advantages of redundancy for conservation of communities may be lost if the reserves are small in size. Additionally, food webs show higher risks of further extinctions after the loss of basal species and herbivores than after the loss of top predators (Paper I and II).

Secondary extinctions caused by a primary extinction and mediated through direct and indirect effects, are likely to occur with a time delay since the manifestation of indirect effects can take long time to appear. In paper III we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby implying that secondary extinctions caused by the loss of top predators are less likely to be balanced by dispersal than secondary extinctions caused by the loss of other species. As top predators are vulnerable to human-induced disturbances on ecosystems in the first place, our results suggest that conservation of top predators should be a priority. Moreover, in most cases time to secondary extinction is shown to increase with species richness indicating the decay of ecological communities to be slower in species-rich than in species-poor communities.

Apart from the human-induced disturbances that often force species towards extinction the environment is also, to a smaller or larger extent, varying over time in a natural way. Such environmental stochasticity influences the dynamics of populations. In paper IV we compare the responses of food webs of different sizes to environmental stochasticity. Species-rich webs are found to be more sensitive to environmental stochasticity. Particularly, species-rich webs lose a greater proportion of species than species-poor webs and they also begin losing species faster than species-poor webs. However, once one species is lost time to final extinction is longer in species-rich webs than in species-poor webs. We also find that the results differ depending on whether species respond similarly to environmental fluctuations or whether they are totally uncorrelated in their response. For a given species richness, communities with uncorrelated species responses run a considerable higher risk of losing a fixed proportion of species compared with communities with correlated species responses.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2006.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1015
Keyword [en]
biodiversity, food webs, redundancy, relaxation time, stability, stochasticity, species deletion, species extinction
National Category
Biological Sciences
Identifiers
URN: urn:nbn:se:liu:diva-6660ISBN: 91-85523-86-0 (print)OAI: oai:DiVA.org:liu-6660DiVA: diva2:21928
Public defence
2006-05-19, Sal Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2006-06-08 Created: 2006-06-08
List of papers
1. Biodiversity lessens the risk of cascading extinction in model food webs
Open this publication in new window or tab >>Biodiversity lessens the risk of cascading extinction in model food webs
2000 (English)In: Ecology Letters, ISSN 1461-023X , Vol. 3, no 2, 131-136 p.Article in journal (Refereed) Published
Abstract [en]

Due to the complex interactions between species in food webs, the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. We found that the risk of additional species extinction, following the loss of one species in model food webs, decreases with the number of species per functional group. For a given number of species per functional group, the risk of further extinctions is highest when an autotroph is removed and lowest when a top predator is removed. In addition, stability decreases when the distribution of interaction strengths in the webs is changed from equal to skew (few strong and many weak links). We also found that omnivory appears to stabilize model food webs. Our results indicate that high biodiversity may serve as an insurance against radical ecosystem changes.

Keyword
Biodiversity, extinction • food webs, functional groups, redundancy, resistance, species deletion, stability
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13855 (URN)10.1046/j.1461-0248.2000.00130.x (DOI)
Available from: 2006-06-08 Created: 2006-06-08
2. Community viability analysis: the response of ecological communities to species loss.
Open this publication in new window or tab >>Community viability analysis: the response of ecological communities to species loss.
2004 (English)In: Ecology, ISSN 0012-9658, Vol. 85, no 9, 2591-2600 p.Article in journal (Refereed) Published
Abstract [en]

The loss of a species from an ecological community can set up a cascade of secondary extinctions that in the worst case could lead to the collapse of the community. Both deterministic and stochastic mechanisms may be involved in such secondary extinctions. To investigate the extent of secondary extinctions in ecological communities following the loss of a species, we here develop a community viability analysis. We introduce a measure called the “quasi-collapse risk” that is defined as the probability that the number of species in a community falls below some defined value within a fixed period of time following the loss of a species. We develop deterministic and stochastic methods for finding post-extinction communities. We use these methods to investigate the relationship between diversity (species richness) and quasi-collapse risks in model communities. It is shown that, in a deterministic context, communities with more species within trophic levels have a larger fraction of species remaining in post-extinction communities. This benefit of species richness is to a large extent lost in the presence of demographic stochasticity. The reason for this is a negative relationship between population density and species diversity. We also show that communities become increasingly triangular in shape as secondary extinctions take place, due to greater extinction risk of species at higher trophic levels. We argue that this new approach holds some promise for identifying fragile ecosystems and keystone species.

Keyword
collapse risk, community viability analysis, demographic stochasticity, individual-based models, permanence, redundancy, secondary extinctions, species diversity, species loss
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13856 (URN)10.1890/03-8018 (DOI)
Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2009-05-08
3. Early onset of secondary extinctions in ecological communities following the loss of top predators
Open this publication in new window or tab >>Early onset of secondary extinctions in ecological communities following the loss of top predators
2006 (English)In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 9, no 4, 435-442 p.Article in journal (Refereed) Published
Abstract [en]

The large vulnerability of top predators to human-induced disturbances on ecosystems is a matter of growing concern. Because top predators often exert strong influence on their prey populations their extinction can have far-reaching consequences for the structure and functioning of ecosystems. It has, for example, been observed that the local loss of a predator can trigger a cascade of secondary extinctions. However, the time lags involved in such secondary extinctions remain unexplored. Here we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. Moreover, in most cases time to secondary extinction increases with increasing species richness. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby. The implications of these results for community persistence and conservation priorities are discussed.

Keyword
Ecological community, food web, relaxation time, secondary extinction, species interactions, species loss, species richness, top predators, trophic level
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13857 (URN)10.1111/j.1461-0248.2006.00893.x (DOI)
Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2017-12-13
4. Biodiversity and persistence of ecological communities in a stochastic environment
Open this publication in new window or tab >>Biodiversity and persistence of ecological communities in a stochastic environment
Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-13858 (URN)
Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2010-01-13

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Borrvall, Charlotte

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