Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality

Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden.
Journal of Animal Ecology (Impact Factor: 4.73). 03/2010; 79(4):871-8. DOI: 10.1111/j.1365-2656.2010.01679.x
Source: PubMed

ABSTRACT 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.

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Available from: Karin A Nilsson, Jun 17, 2015
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    ABSTRACT: This thesis reports the results from the analysis of size-structured population models that were carried out to address several hypotheses on predator-prey community structure and dynamics. These hypotheses (described in the general introduction, Chapter I) are relevant in the context of collapsed marine predator populations in general and for the case of collapsed Atlantic cod (Gadus morhua) populations in particular. Chapter II reveals the importance of accounting for size- rather than age- structure in fish community dynamics by showing in two examples the potential for a positive response in stage-specific biomass with increased mortality. The first example concerns a model of a size-structured zooplanktivore (consumer), like herring or capelin, which is foraged upon by cod and marine mammal predators. Rather than competitive effects, there is facilitation from marine mammals on cod, due to the differences in prey-size preferences of these predators. Increasing mortality of the large size-range of the prey species (by mammal predation) results in an increase in biomass of small individuals, thus increasing the food availability for a predator preferring the small size range (cod). In the second example the interaction between marine mammal predators and cod fisheries is modeled. Both marine mammals and fisheries target cod, but with preferences for different size classes. Contrary to intuition, the predation on juvenile cod by marine mammals may benefit fisheries by increasing the yield with respect to a situation without marine mammals. In Chapter III two possible predator-prey community scenarios are analyzed that may result from the occurrence of two ontogenetic niche shifts during predator life history. In both scenarios the predator-prey dynamics show cohort cycles, without overlap between subsequent generations. This dynamic behavior is caused by the size-dependent energetic efficiency of individuals (smaller cohorts outcompeting larger cohorts) and the reproduction that is modeled as a discrete yearly event. In the first scenario predator population dynamics is regulated by growth in the intermediate life stage (i.e. the size class preceding the predatory life stage). In this case top-down control of the prey population by the predator turns out to be impossible. For the second scenario the intermediate resource is assumed to be at a constant level (i.e. feedback from foraging by the predator is considered negligible). In this case the predator can exert top-down control on the prey species and even drive it to extinction at high resource productivity levels. Accounting for competition between prey and early life stage predators does not qualitatively change the results in either of the scenarios. Including competition has only a quantitative influence on the potential for coexistence, in the sense that it negatively affects the range of resource productivity levels allowing for coexistence of predator and prey. But in neither of the two scenarios we find the occurrence of alternative stable states. The results presented in Chapter IV show how competition between two size- structured consumer species, reflecting Baltic herring and sprat, affect community structure. One species (herring) has a larger maximum size, larger maturation size and requires lower resource densities for persistence than the second species. Notwithstanding the ability of the larger species to depress the shared resource to lower levels and its accessibility to an exclusive resource, coexistence is possible over a large range of resource productivities. Moreover, because of the smaller size at maturation of the second species, it is even able to exclude the first, larger and competitively superior species under particular conditions. In Chapter V an important assumption in the model as presented in Chapter III is revisited, regarding the form of the ontogenetic niche shift to predation. This chapter shows the results from the analysis of a second model, in which the predator loses access to the intermediate resource when switching to predation and thus exhibits a complete niche shift. As in Chapter III, the dynamics are strongly cohort- driven, which means that the predator population consists of a single cohort. Because of the complete diet shift, the predator becomes completely dependent on the fish prey, which creates a tight link between the timing in presence of predators switching to foraging on prey and the presence of prey with sizes suitable for predation. This model results in the occurrence of alternative stable states with and without predators due to the density dependent growth in the predator population, which can result in either a positive or a negative effect on persistence. Due to the positive effect persistence is possible when the predator population is at high density, but recovery from low densities is impossible. The second major result from the model with complete shift is the occurrence of an effect that is called ‘the invader strikes back’ phenomenon. Due to the negative effect from density dependent growth in the predator population, conditions at low predator population densities allow for predator invasion into the system, but establishment is impossible because of the population feedback on the environment. In Chapter VI the model from Chapter III is used to test four hypotheses that are often postulated as explanations for the lack of recovery of marine predators following population collapses. These hypotheses are tested both in a scenario without top-down control as well as in a scenario with top-down control. Additional mortality during predator eggs can be as high as 98% mortality during the egg stage before the predator population goes extinct. In the top-down scenario increasing the additional mortality of predator eggs leads to biomass overcompensation and increases the top-down control that the predator imposes on the prey population, resulting in lower coexistence potential than without additional mortality in the egg stage of the predator. When cannibalism is included in the predator and resource productivity is high, predators become larger than without cannibalism (especially in the no-top- down scenario). In the top-down scenario cannibalism relaxes the top-down control on the prey population and increases the potential for coexistence. Additional mortality of adult prey did not qualitatively change the outcomes of the model with respect to the baseline dynamics. Additional mortality of large predators results in predator extinction for high resource productivity and the occurrence of bistability in the scenario without top- down control. Bistability occurs between a coexistence state and a state in which the predator was absent from the system, due to the beneficial effect of slow development under high mortality conditions for large predators. In the scenario with top-down control, additional mortality of large predators results merely in a reduction of the potential for persistence but does not lead to dynamics that are qualitatively different from the baseline dynamics. Together, the results presented in this thesis illustrate the importance of population size-structure in shaping community dynamics of predator-prey systems. The general discussion, Chapter VII, elaborates further on the implications of the size-dependent modeling of individual energetic processes as well as trophic interaction networks.
    12/2012, Degree: PhD, Supervisor: André M. de Roos
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    ABSTRACT: Patterns in biomass production are determined by resource input (productivity) and trophic transfer efficiency. At fixed resource input, variation in consumer biomass production has been related to food quality, metabolic type and diversity among species. In contrast, intraspecific variation in individual body size because of ontogenetic development, which characterizes the overwhelming majority of taxa, has been largely neglected. Here we show experimentally in a long-term multigenerational study that reallocating constant resource input in a two-stage consumer system from an equal resource delivery to juveniles and adults to an adult-biased resource delivery is sufficient to cause more than a doubling of total consumer biomass. We discuss how such changes in consumer stage-specific resource allocation affect the likelihood for alternative stable states in harvested populations as a consequence of stage-specific overcompensation in consumer biomass and thereby the risk of catastrophic collapses in exploited populations.
    Nature Communications 03/2015; 6:6441. DOI:10.1038/ncomms7441 · 10.74 Impact Factor
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    ABSTRACT: 1. Size-dependent interactions and habitat complexity have been identified as important factors affecting the persistence of intraguild predation (IGP) systems. Habitat complexity has been suggested to promote intraguild (IG) prey and intraguild predator coexistence through weakening trophic interactions particularly the predation link. 2. Here, we experimentally investigate the effects of habitat complexity on coexistence and invasion success of differently sized IG-predators in a size-structured IGP system consisting of the IG-predator Poecilia reticulata and a resident Heterandria formosa IG-prey population. The experiments included medium-long and long-term invasion experiments, predator—prey experiments and competition experiments to elucidate the mechanisms underlying the effect of prey refuges. 3. Habitat complexity did not promote the coexistence of IG-predator and IG-prey, although the predation link was substantially weakened. However, the presence of habitat structure affected the invasion success of large IG-predators negatively and the invasion success of small IG-predators positively. The effect of refuges on size-dependent invasion success could be related to a major decrease in the IG-predator's capture rate and a shift in the size distribution of IG-predator juveniles. 4. In summary, habitat complexity had two main effects: (i) the predation link was diminished, resulting in a more competition driven system and (ii) the overall competitive abilities of the two species were equalized, but coexistence was not promoted. Our results suggest that in a size-structured IGP system, individual level mechanisms may gain in importance over species level mechanisms in the presence of habitat complexity.
    Journal of Animal Ecology 01/2013; 82(1):55-63. DOI:10.1111/j.1365-2656.2012.02032.x · 4.73 Impact Factor