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Effects of size-dependent predation and competition on population and community dynamics
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Most animals grow substantially during their lifetime and change in competitive ability, predatory capacity and their susceptibility to predation as they grow. This thesis addresses the implications of this on regulation and dynamics within populations as well as between population interactions.

In size-structured populations either reproduction or maturation may be more limiting. If juveniles are competitively superior, the competitive bottleneck will be in the adults and reproduction will be limiting. Mortality will in this case result in overcompensation in juvenile biomass through increased reproduction. Compensation in biomass was demonstrated in Daphnia pulex populations subjected to size-independent mortality, where juvenile biomass did not decrease when a substantial harvest was imposed due to increase per capita fecundity. This supported that juveniles were superior competitors and that population cycles seen in Daphnia are juvenile-driven.

Compensatory responses in biomass may lead to that predators facilitate eachothers existence by feeding on a common prey, a phenomenon coined emergent facilitation. In an experimental test of the mechanism behind emergent facilitation it was demonstrated that the invertebrate predator Bythotrephes longimanus was favoured by thinning of its prey Holopedium gibberum. The thinning mimicked fish predation and targeted large individuals while Bythotrephes preferrs small prey.

Size dependent predation also occurs within populations, i.e. cannibalism, were large individuals feed on smaller conspecifics. Two populations of the common guppy (Poecilia reticulata) originating from different environments were demonstrated to differ in cannibalistic degree. Cannibalism was also affected by the presence of refuges and females and juveniles from one population were better adapted to structural complexity than the other.

The effects of these differences in cannibalism on population regulation and dynamics were studied in long term population experiments. Both populations were regulated by cannibalism in the absence of refuges, and displayed cannibal-driven cycles with suppression of recruitment and high population variability. The presence of refuges decreased density dependence and population variability and harvesting of large females in the absence of refuges led to population extinctions in the more cannibalistic population. The less cannibalistic population had higher population biomass and stronger density-dependence in the presence of refuges. When refuges were present, cohort competition increased and cycles with short periodicity were seen.

Large individuals were not only cannibals, but could successfully prey on other species. Small and large guppies were allowed to invade resident populations of Heterandria formosa. Small invaders failed while large invaders succeeded as predation from large invaders broke up the competitive bottleneck that the resident population imposed on juveniles of the invader. 

Place, publisher, year, edition, pages
Umeå: Institutionen för ekologi, miljö och geovetenskap. Umeå universitet , 2010. , 34 p.
Keyword [en]
size-structure, cannibalism, resource competition, predation, emergent facilitation, population regulation, population dynamics, overcompensation, density-dependence, cycles
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:umu:diva-35686OAI: oai:DiVA.org:umu-35686DiVA: diva2:349285
Public defence
2010-10-15, Kemi- biologihuset, KB3B1, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2010-09-24 Created: 2010-08-31 Last updated: 2014-01-27Bibliographically approved
List of papers
1. Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality
Open this publication in new window or tab >>Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality
2010 (English)In: Journal of Animal Ecology, ISSN 0021-8790, E-ISSN 1365-2656, Vol. 79, no 4, 871-878 p.Article in journal (Refereed) Published
Abstract [en]

1. Recent theory suggests that compensation or even overcompensation in stage-specific biomass can arise in response to increased mortality. Which stage that will show compensation depends on whether maturation or reproduction is the more limiting process in the population. Size-structured theory also provides a strong link between the type of regulation and the expected population dynamics as both depend on size/stage specific competitive ability.

2. We imposed a size-independent mortality on a consumer-resource system with Daphnia pulex feeding on Scenedesmus obtusiusculus to asses the compensatory responses in Daphnia populations. We also extended an existing stage-structured biomass model by including several juvenile stages to test whether this extension affected the qualitative results of the existing model.

3. We found complete compensation in juvenile biomass and total population fecundity in response to harvesting. The compensation in fecundity was caused by both a higher proportion of fecund females and a larger clutch size under increased mortality. We did not detect any difference in resource levels between treatments.

4. The model results showed that both stages of juveniles have to be superior to adults in terms of resource competition for the compensatory response to take place in juvenile biomass.

5. The results are all in correspondence with that the regulating process within the population was reproduction. From this we also conclude that juveniles were superior competitors to adults, which has implications for population dynamics and the kind of cohort cycles seen in Daphnia populations.

6. The compensatory responses demonstrated in this experiment have major implications for community dynamics and are potentially present in any organisms with food-dependent growth or development.

Place, publisher, year, edition, pages
John Wiley & Sons, Inc, 2010
Keyword
biomass compensation, biomass model, compensatory response, Daphnia pulex, harvesting, population dynamics, population regulation, regulatory mechanisms, size dependence, stage structure
Identifiers
urn:nbn:se:umu:diva-35812 (URN)10.1111/j.1365-2656.2010.01679.x (DOI)000278399300017 ()
Available from: 2010-09-21 Created: 2010-09-06 Last updated: 2017-12-12Bibliographically approved
2. Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey
Open this publication in new window or tab >>Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey
2011 (English)In: Journal of Animal Ecology, ISSN 0021-8790, E-ISSN 1365-2656, Vol. 80, no 3, 615-621 p.Article in journal (Refereed) Published
Abstract [en]

1. Recent theoretical insights have shown that predator species may help each other to persist by size-selective foraging on a shared prey. By feeding on a certain prey stage, a predator may induce a compensatory response in another stage of the same prey species, thereby favouring other predators; a phenomenon referred to as emergent facilitation.

2. To test whether emergent facilitation may occur in a natural system, we performed an enclosure experiment where we mimicked fish predation by selectively removing large zooplankton and subsequently following the response of the invertebrate predator Bythotrephes longimanus.

3. Positive responses to harvest were observed in the biomass of juvenile individuals of the dominant zooplankton Holopedium gibberum and in Bythotrephes densities. Hence, by removing large prey, we increased the biomass of small prey, i.e. stage-specific biomass overcompensation was present in the juvenile stage of Holopedium. This favoured Bythotrephes, which preferentially feed on small Holopedium.

4. We argue that the stage-specific overcompensation occurred as a result of increased per capita fecundity of adult Holopedium and as a result of competitive release following harvest. If shown to be common, emergent facilitation may be a major mechanism behind observed predator extinctions and patterns of predator invasions.

Place, publisher, year, edition, pages
John Wiley & Sons, 2011
Keyword
Bythotrephes, coexistence, facilitation, harvest, Holopedium, overcompensation, regulation, reproduction, size-structure, stage-specific predation
Identifiers
urn:nbn:se:umu:diva-36128 (URN)10.1111/j.1365-2656.2011.01810.x (DOI)
Available from: 2010-09-21 Created: 2010-09-17 Last updated: 2017-12-12Bibliographically approved
3. Guppy populations differ in cannibalistic degree and adaption to structural environments
Open this publication in new window or tab >>Guppy populations differ in cannibalistic degree and adaption to structural environments
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigated the degree of cannibalism in guppy (Poecilia reticulata) populations originating from high and low predation environments in Trinidad and how cannibalism was affected by the presence of refuges. Females from two populations were allowed to feed on juveniles from two populations in aquaria trials. The cannibalism was size-dependent and varied depending on both juvenile and female origin. Low predation females were more efficient cannibals and low predation juveniles were better at avoiding cannibalism compared to high predation guppies when no refuges were present. The addition of refuges decreased cannibalism when females from the low predation population were feeding on juveniles from the high predation population. In contrast, cannibalism increased with the addition of refuges for all other combinations. The high predation females were also superior cannibals and the high predation juveniles were better at escaping cannibalism than the low predation guppies with refuges present. We discuss whether the differences in cannibalism and response to refuge addition relate to predation induced habitat shifts and differences in the guppies’ natural environment.

Keyword
Cannibalism, size-dependence, habitat use, structural heterogeneity, predation
Identifiers
urn:nbn:se:umu:diva-36129 (URN)
Available from: 2010-09-21 Created: 2010-09-17 Last updated: 2010-11-10Bibliographically approved
4. Cannibalism and resource competition determine population structure and regulation in experimental guppy populations
Open this publication in new window or tab >>Cannibalism and resource competition determine population structure and regulation in experimental guppy populations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Cannibalism and intraspecific resource competition, separately or in combination, can be important factors in regulating populations. Cannibalistic voracity and size of offspring have been shown to be two important factors in determining the extent to which cannibalism can regulate a population. We investigated guppy populations originating from two different environments and that differ in cannibalistic voracity and size of offspring. We investigated mechanisms behind density independent and density dependent relationships by varying the availability of juvenile refuges and harvesting of large females.  One population (Turure) had a lower total biomass than the other population (Quare) and showed stronger density dependence in the treatments without refuges. In contrast, the Quare population showed a stronger density dependence when refuges were present. We suggest that the presence of refuges decreased cannibalism and increased competition leading to stronger density dependence through decreased fecundity of females and juvenile growth and survival. Harvest of large females decreased cannibalism but decreased reproductive output even more which led to extinctions of the Turure populations when no refuges were present. When refuges were present harvesting had only a small effect related to that few females grew large enough to be harvested.

Identifiers
urn:nbn:se:umu:diva-36130 (URN)
Available from: 2010-09-21 Created: 2010-09-17 Last updated: 2010-09-24Bibliographically approved
5. Refuge availability and within-species differences in cannibalism determine population variability and dynamics
Open this publication in new window or tab >>Refuge availability and within-species differences in cannibalism determine population variability and dynamics
2013 (English)In: Ecosphere, ISSN 2150-8925, E-ISSN 2150-8925, Vol. 4, no 8, 100- p.Article in journal (Refereed) Published
Abstract [en]

Theoretical studies show that both cannibalism and intraspecific resource competition can have major effects on population dynamics. Cannibalistic intensity, offspring size, harvesting and refuge availability are important factors affecting the interplay between cannibalism and competition. We studied two populations of the common guppy (Poecilia reticulata) that differed in their cannibalistic voracity as well as offspring size. We manipulated the availability of refuges for juveniles and harvesting intensity of large adults to investigate how these factors influenced the dynamics of the two populations. Overall population dynamics was mainly affected by the origin of the founder populations and the presence of refuges. The population with a higher cannibalistic propensity and smaller offspring exhibited higher population variability, and the presence of refuges reduced cannibalism and stabilised the dynamics in both populations. Harvest of large cannibalistic females destabilised the dynamics and caused extinctions of several populations without refuges. Both populations displayed cannibal-driven cycles with repression of recruitment when no refuges were present. Cycle periods were shorter with refuges present and the dynamics were more cohort like with synchronised peaks in density of vulnerable juveniles and cannibals. We suggest that increased number of refuging juveniles led to intensified resource competition in the population. The harvest yield was low in the refuge treatments as few females grew large due to resource competition, leading to a small impact of harvesting in these treatments.

Place, publisher, year, edition, pages
Washington: Ecological Society of America, 2013
Keyword
cannibalism, coefficient of variation, competition, extinction, guppy, harvest, Poecilia reticulata, population dynamics, refuges, structural complexity
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-84804 (URN)10.1890/ES13-00105.1 (DOI)000327379400009 ()
Available from: 2014-01-27 Created: 2014-01-20 Last updated: 2017-12-06Bibliographically approved
6. Invasion success depends on invader body size in a size-structured mixed predation-competition system
Open this publication in new window or tab >>Invasion success depends on invader body size in a size-structured mixed predation-competition system
Show others...
2009 (English)In: Journal of Animal Ecology, ISSN 0021-8790, E-ISSN 1365-2656, Vol. 78, no 6, 1152-1162 p.Article in journal (Refereed) Published
Abstract [en]

1. The size of an individual is an important determinant of its trophic position and the type of interactions it engages in with other heterospecific and conspecific individuals. Consequently an individual's ecological role in a community changes with its body size over ontogeny, leading to that trophic interactions between individuals are a size-dependent and ontogenetically variable mixture of competition and predation.

2. Because differently sized individuals thus experience different biotic environments, invasion success may be determined by the body size of the invaders. Invasion outcome may also depend on the productivity of the system as productivity influences the biotic environment.

3. In a laboratory experiment with two poeciliid fishes the body size of the invading individuals and the daily amount of food supplied were manipulated.

4. Large invaders established persistent populations and drove the resident population to extinction in 10 out of 12 cases, while small invaders failed in 10 out of 12 trials. Stable coexistence was virtually absent. Invasion outcome was independent of productivity.

5. Further analyses suggest that small invaders experienced a competitive recruitment bottleneck imposed on them by the resident population. In contrast, large invaders preyed on the juveniles of the resident population. This predation allowed the large invaders to establish successfully by decreasing the resident population densities and thus breaking the bottleneck.

6. The results strongly suggest that the size distribution of invaders affects their ability to invade, an implication so far neglected in life-history omnivory systems. The findings are further in agreement with predictions of life-history omnivory theory, that size-structured interactions demote coexistence along a productivity gradient.

Place, publisher, year, edition, pages
John Wiley & Sons, Inc, 2009
Keyword
coexistence, competitive recruitment bottleneck, life-history omnivory, ontogenetic niche shift, productivity gradient
Identifiers
urn:nbn:se:umu:diva-3122 (URN)10.1111/j.1365-2656.2009.01590.x (DOI)
Available from: 2008-04-29 Created: 2008-04-29 Last updated: 2010-09-24Bibliographically approved

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