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Interaction strength and responses of ecological communities to disturbances
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]

Ecological communities are continuously exposed to natural or anthropogenic disturbances of varied intensity and frequency. The way communities respond to disturbances can depend on various factors, such as number of species, structural characteristics of the community, stability properties, species characteristics and the nature of the disturbance. This thesis is a collection of theoretical studies on how ecological communities respond to different kind of disturbances, mainly in relation to interaction strength between species, a measure of how strongly or weakly species interact with each other.

A major disturbance for natural communities is the loss of a species. Although species extinctions is a natural process in the geological time scale, it has lately been dangerously accelerated due to human activities and interferences. Extinction of a species can have dramatic consequences for the community, can trigger a cascade of secondary species extinction and can even lead to community collapse. In Paper I, we identify species whose loss can trigger a large number of secondary extinctions (namely keystone species), species that are particularly vulnerable to become extinct following the loss of another species and mechanisms behind the sequence of secondary extinctions. We also highlight the fact that the keystone status of a species can be context dependent, that is, it is dependent on the structure of the community where it is embedded.

Although the global extinction of a species is an irreversible process, in cases of local extinction, conservation and restoration plans can include reintroduction of the species to their former location. Such reintroduction or translocation attempts often fail, due to characteristics of the reintroduced species and to changes in the community structure caused by the initial loss of the species (Paper II ). Using model communities we show that this risk of reintroduction failure can be high - even in cases where the initial species loss did not cause any secondary extinctions - and it differs between attempts to reintroduce weakly or strongly interacting species (Paper II ).

Disturbances are not always as profound as species extinction. Human activities and environmental changes can cause small and permanent changes in birth and mortality rates of species and in the strength of interaction between species. Such disturbances can change the stability properties of ecological communities making them more vulnerable to further disturbances. In Paper III, we derive analytical expressions for the sensitivity of resilience to changes in the intrinsic growth rate of species and in the strength of interaction links. We also apply the method to model communities and identify keystone species and links, i.e. species and links whose small disturbance would cause large changes in community resilience. We found that changes in the growth rate of strongly interacting species have a larger impact on resilience than changes in the growth rate of weakly interacting species, which is in line with the main findings of the species deletion study (Paper I ).

Community complexity - mainly expressed as number of species and links - was one of the first community characteristics to be related to stability properties and the way communities respond to disturbances. Many theoretical works support the hypothesis that complexity reduces stability, contradicting intuition, observation and many experimental studies. In Paper IV, we state that this could be, at least partly, due to methodological misconceptions and misinterpretations. We propose a new sampling method for parameterizing model communities and we highlight the importance of feasibility of model communities (all species densities are strictly positive), for a more proper estimation of stability probability of communities with different degrees of complexity.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2006. , 43 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1002
Keyword [en]
Species deletion, secondary extinctions, keystone species, keystone links, species reintroductions, Lotka-Volterra models, local stability, permanence, resilience, sensitivity analysis, diversity, complexity, food webs, competition communities
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-7536ISBN: 91-85497-20-7 (print)OAI: oai:DiVA.org:liu-7536DiVA: diva2:22543
Public defence
2006-03-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2006-10-04 Created: 2006-10-04 Last updated: 2012-11-14
List of papers
1. Keystone species and vulnerable species in ecological communities: strong or weak interactors?
Open this publication in new window or tab >>Keystone species and vulnerable species in ecological communities: strong or weak interactors?
2005 (English)In: Journal of Theoretical Biology, ISSN 0022-5193, Vol. 35, no 1, 95-103 p.Article in journal (Refereed) Published
Abstract [en]

The loss of a species from an ecological community can trigger a cascade of secondary extinctions. The probability of secondary extinction to take place and the number of secondary extinctions are likely to depend on the characteristics of the species that is lost—the strength of its interactions with other species—as well as on the distribution of interaction strengths in the whole community. Analysing the effects of species loss in model communities we found that removal of the following species categories triggered, on average, the largest number of secondary extinctions: (a) rare species interacting strongly with many consumers, (b) abundant basal species interacting weakly with their consumers and (c) abundant intermediate species interacting strongly with many resources. We also found that the keystone status of a species with given characteristics was context dependent, that is, dependent on the structure of the community where it was embedded. Species vulnerable to secondary extinctions were mainly species interacting weakly with their resources and species interacting strongly with their consumers.

Keyword
Species loss; Keystone species; Interaction strength; Community dynamics; Permanence; Secondary extinction
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14053 (URN)10.1016/j.jtbi.2004.12.022 (DOI)
Available from: 2006-10-04 Created: 2006-10-04
2. Prospects and risks of species reintroductions: a community ecological perspective
Open this publication in new window or tab >>Prospects and risks of species reintroductions: a community ecological perspective
(English)Manuscript (Other academic)
Abstract [en]

For many threatened species reintroduction of captive-reared individuals into the wild may offer the only hope of preventing global extinction. However, attempts to reintroduce a species into an ecological community from which it has once gone extinct often fail. This can be due to captivity-induced genetic and behavioural changes in the species itself. Here we argue that also changes in the structure of the community caused by the initial loss of the species can hinder its subsequent reintroduction. Due to the interdependences amongspecies in ecological communities the local extinction of a species leads to changes in the densities and strength of interactions among the remaining species in the community and sometimes even to a cascade of secondary extinctions. Such alterations in the structure of a community caused by the initial extinction of the species might make it impossible for the species to reinvade at a later stage. Here we show, using models of multitrophic communities, that the risk of reintroduction failure can be high and that it differs between species belonging to different trophic levels and between weakly and strongly interacting species. Specifically, there is a high risk of reintroduction failure for primary producer species and weakly interacting consumer species. We also find the risk that the reintroduction will cause new extinctions to be higher in communities where the initial loss triggered secondary extinctions. In the worst case extinctions caused by the reintroduction may lead to the subsequent extinction of the reintroduced species itself. This risk is not negligible implicating that reintroductions might sometimes aggravate the state of local communities.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14054 (URN)
Available from: 2006-10-04 Created: 2006-10-04 Last updated: 2012-11-14
3. Sensitivity analysis at the community level
Open this publication in new window or tab >>Sensitivity analysis at the community level
2006 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Human-induced changes in the birth and mortality rates of species and in the strength of interactions within and between species can lead to changes in the resilience of ecological communities. Here we derive analytical expressions for the sensitivity of community resilience to changes in the intrinsic growth rate of species and in the strengths of interaction links. These new methods are applied on model communities to illustrate how they can be used to identify keystone species and keystone links - keystone species and keystone links in the sensethat small changes in their growth rates and strengths, respectively, will have large effects on the resilience of the community. We find that small changes in the intrinsic growth rates of rare species (e.g. top predators) and strongly interacting species have larger effect on resilience than changes in the growth rates of abundant species and weakly interacting species. Moreover, we find that small changes in the strength of weak interaction links cause larger changes in resilience than changes in the strength of strong links. We believe that thecommunity sensitivity analysis developed here holds some promise for identifying species and links that are critical for the resilience of ecological communities.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14055 (URN)
Available from: 2006-10-04 Created: 2006-10-04 Last updated: 2012-11-14
4. Diversity-stability relation from a methodological point of view
Open this publication in new window or tab >>Diversity-stability relation from a methodological point of view
(English)Manuscript (Other academic)
Abstract [en]

The complexity - stability relation has been a central issue in ecology for a long time. Both theoretical and empirical studies have been investigating whether complexity promotes stability or not, which could be the underlying mechanisms creating a positive or negative cmnplexity-stability relation and which are the structures, characteristics and constraints that would allow complex ecological systems to persist and be stable. In this paper we approach the subject from amethodological point of view. We study the effect of parameterizing the communities in a certain way and illustrate the effect of treating feasibility (i.e. densities of all species are strictly positive) separately from stability. We observe that increasing number of species in the communities requires a more skewed interaction strength distribution toward weak interactions, in order for the communities to theoretically exist. Using model Lotka-Volterra competition communities we illustrate that probability of feasibility decreases with increasing interaction strength and number of species in the community. However, forfeasible systems we find that local stability probability and resilience do not significantly differ between communities with few or many species, in contrast with earlier studies that, did not account for feasibility and concluded that species poor communities had higher probability of being locally stable than species rich communities.

Keyword
Local stability, resilience, feasibility, Lotka-Volterra model, competition community, Central Limit Theorem
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14056 (URN)
Note

Alternative title: Complexity - Stability relation from a methodological point of view

Available from: 2006-10-04 Created: 2006-10-04 Last updated: 2012-11-14

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