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- Borrvall, Charlotte, 1973-
(author)
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Biodiversity and Species Extinctions in Model Food Webs
- 2006
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 3. |
- Borrvall, Charlotte, 1973-, et al.
(author)
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Biodiversity Decreases the risk of Collapse
- 2005
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In: Systems shocks - Systems resilience, The 2000 Abisko Workshop,2000. - London : World Scientific. ; , s. 209-
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Conference paper (peer-reviewed)
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| 4. |
- Borrvall, Charlotte, et al.
(author)
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Biodiversity lessens the risk of cascading extinction in model food webs
- 2000
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In: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 3:2, s. 131-136
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Borrvall, Charlotte, et al.
(author)
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Early onset of secondary extinctions in ecological communities following the loss of top predators
- 2006
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In: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 9:4, s. 435-442
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Journal article (peer-reviewed)abstract
- 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.
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| 6. |
- Borrvall, Charlotte, 1973-
(author)
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The response of model food webs to species loss : secondary extinctions and relaxation times
- 2000
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Licentiate thesis (other academic/artistic)abstract
- Current destruction and fragmentation of natural habitats is predicted to cause great future losses in biodiversity (Pimm et al. 1995; Hughes et al. 1997; Sala et al. 2000). Due to complex interactions between species in food webs, loss of a species can set up a cascade of secondary extinctions (Paine 1966; Estes & Palmisano 1974; Pace et al. 1999).In this thesis we have studied the effects of species deletion in model food webs with different degrees of structural redundancy, measured as number of species per functional group. We found that the risk of secondary species extinction, as well as the fraction of secondary extinctions, decreases with increasing number of species per functional group. Hence, species-rich communities seem to be more stable than species-poor ones as they experience proportionally less secondary extinctions. Further, it is shown that more redundant food webs often have longer relaxation times (time elapsed from initial species loss until the new community composition is reached) compared with less redundant webs. This indicates that species-rich communities are more stable because in those communities there is more time available for dispersal between local communities and hence time for recolonizations to take place. Thus, redundancy seems to enhance food web stability. This lends additional support to the hypothesis (Walker 1992; Naeem 1998) and empirical result (McGrady-Steed et al. 1997) that high biodiversity can be seen as an insurance against radical ecosystem changes even though species may have largely redundant functional roles.The degree of stability of decaying food webs was further found to depend on the type of species being lost from the community. Some studies indicate that extinctions following habitat fragmentation are disproportionately experienced by species at high trophic levels (discussed by Holt et al. 1999). Although species at higher trophic levels might be more prone to extinction than species at lower trophic levels, this study indicates that the loss of species from lower trophic levels causes greater risks of losing additional species.Thus, when it comes to conservational efforts our results show that it might not be enough only to care about those species tending to go extinct in the first place but also take into account what happens as a consequence of the loss of other less extinction-prone species.
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| 7. |
- Ebenman, Bo, et al.
(author)
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Community viability analysis : the response of ecological communities to species loss.
- 2004
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In: Ecology. - : Wiley. - 0012-9658. ; 85:9, s. 2591-2600
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Journal article (peer-reviewed)abstract
- 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.
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| 8. |
- Kaneryd, Linda, et al.
(author)
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Species-rich ecosystems are vulnerable to cascading extinctions in an increasingly variable world
- 2012
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In: Ecology and Evolution. - : Wiley-Blackwell. - 2045-7758. ; 2:4, s. 858-874
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Journal article (peer-reviewed)abstract
- Global warming leads to increased intensity and frequency of weather extremes. Such increased environmental variability might in turn result in increased variation in the demographic rates of interacting species with potentially important consequences for the dynamics of food webs. Using a theoretical approach, we here explore the response of food webs to a highly variable environment.We investigate how species richness and correlation in the responses of species to environmental fluctuations affect the risk of extinction cascades. We find that the risk of extinction cascades increases with increasing species richness, especially when correlation among species is low. Initial extinctions of primary producer species unleash bottom-up extinction cascades, especially in webs with specialist consumers. In this sense, species-rich ecosystems are less robust to increasing levels of environmental variability than species-poor ones. Our study thus suggests that highly speciesrich ecosystems such as coral reefs and tropical rainforests might be particularly vulnerable to increased climate variability.
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| 9. |
- Petchey, Owen L., et al.
(author)
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Trophically Unique Species Are Vulnerable to Cascading Extinction
- 2008
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In: American Naturalist. - : University of Chicago Press. - 0003-0147 .- 1537-5323. ; 171:5, s. 568-579
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Journal article (peer-reviewed)abstract
- 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.
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