Regulating Population Dynamics Research Articles

Early establishment of vertebrate trophic interactions: Food web structure in Middle to Late Devonian fish assemblages with exceptional fossilization

In past and present ecosystems, trophic interactions determine material and energy transfers among species, regulating population dynamics and community stability. Food web studies in past ecosystems are helpful to assess the persistence of ecosystem structure throughout geological times and to explore the existence of general principles of food web assembly. We determined and compared the trophic structure of two Devonian fish assemblages [(1) the Escuminac assemblage (ca. 380 Ma), Miguasha, eastern Canada and (2) the Lode assemblage (ca. 390 Ma), Straupe, Latvia] with a closer look at the Escuminac assemblage. Both localities are representative of Middle to Late Devonian aquatic vertebrate assemblages in terms of taxonomic richness (ca. 20 species), phylogenetic diversity (all major groups of lower vertebrates) and palaeoenvironment (palaeoestuaries). Fossil food web structures were assessed using different kinds of direct (i.e. digestive contents and bite marks in fossils) and indirect (e.g. ecomoiphological measurements, stratigraphic species co-occurrences) indicators. First, the relationships between predator and prey body size established for the Escuminac fishes are comparable to those of recent aquatic ecosystems, highlighting a consistency of aquatic food web structure across geological time. Second, non-metric dimensional scaling on ecomorphological variables and cluster analysis showed a common pattern of functional groups for both fish assemblages; top predators, predators, primary and secondary consumers were identified. We conclude that Devonian communities were organized in multiple trophic levels and that size-based feeding interactions were established early in vertebrate history.

Interactions between soil‐dwelling arthropod predators and Ixodes scapularis under laboratory and field conditions

AbstractThe primary vector for Lyme disease and several other medically significant tick‐borne diseases, Ixodes scapularis (blacklegged tick), spends over 95% of its two‐year life cycle exposed to the soil environment. However, the effect of soil‐dwelling arthropod predator populations on I. scapularis survival either in the laboratory or under field conditions is unclear. We collected soil‐dwelling obligate arthropod predators from the field, representing 13 taxonomic families, and investigated whether they would target two I. scapularis life stages (nymph/engorged larva) under laboratory conditions. After 48 h, 30.6% of the predator species targeted I. scapularis nymphs, while 41.3% targeted engorged larvae. One spider species, Schizocosa ocreata, targeted both I. scapularis life stages reliably under laboratory conditions. We used this species to test whether an arthropod predator would affect I. scapularis survival in field microcosms which contained alternative prey items and a complex soil environment. We also investigated the direct and indirect effects of surface litter on I. scapularis survival by removing the leaf litter layer from half of the microcosms. Predator addition reduced the number of nymphs recovered from the microcosms after 21 d by 32.9%. Litter removal also had a negative effect on tick survival, and interacted weakly with the addition of a predator further decreasing tick survival. Additionally, we found that when leaf litter was removed, I. scapularis survival was positively correlated with the organic matter content of the soil within the microcosms. Naturally occurring arthropod predators may play an important role in regulating population dynamics of I. scapularis and show potential as biological control agents for use in integrated pest management protocols.

A native predator utilising the invasive brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae) as a food source

ABSTRACTThe nest provisions of a native predator species Bicyrtes quadrifasciata (Say) (Hymenoptera: Crabronidae) were surveyed and examined for the presence of nymphs of Halyomorpha halys, which is an invasive pest of many economically important agricultural crops in the U.S.A. We found 96% of the nest provisions of B. quadrifasciata were H. halys indicating the potential of B. quadrifasciata in biocontrol of this pest. Potential of B. quadrifasciata in regulating population dynamics of H. halys is discussed.

Consequences of Repeated Defoliation on Belowground Bud Banks of Carex brevicuspis (Cyperaceae) in the Dongting Lake Wetlands, China

Despite the predominant role of bud banks in the regeneration of clonal macrophyte populations, few studies have examined the way in which clonal macrophytes adjust the demographic features of bud banks to regulate population dynamics in response to defoliation in wetlands. We investigated the density and composition of bud banks under repeated defoliation in the wetland sedge Carex brevicuspis C. B. Clarke in the Dongting Lake wetlands, China. The density and biomass of rhizome buds and shoots did not decrease significantly in response to repeated defoliation over two consecutive years. The composition of bud banks, which consisted of long and short rhizome buds, also did not change significantly in response to repeated defoliation. Nevertheless, the ramet height and the shoot, root, and rhizome mass of C. brevicuspis declined significantly under repeated defoliation. Our findings suggest that bud banks are a conservative reproductive strategy that enables C. brevicuspis to tolerate a certain amount of defoliation. The maintenance of large bud banks after repeated defoliation may enable C. brevicuspis populations to regenerate and persist in disturbed habitats. However, bud bank density of C. brevicuspis might decline in the long term because the amount of carbon stored in rhizome buds and plants is reduced by frequent defoliation.

A mast-seeding desert shrub regulates population dynamics and behavior of its heteromyid dispersers.

Granivorous rodent populations in deserts are primarily regulated through precipitation‐driven resource pulses rather than pulses associated with mast‐seeding, a pattern more common in mesic habitats. We studied heteromyid responses to mast‐seeding in the desert shrub blackbrush (Coleogyne ramosissima), a regionally dominant species in the Mojave–Great Basin Desert transition zone. In a 5‐year study at Arches National Park, Utah, USA, we quantified spatiotemporal variation in seed resources in mast and intermast years in blackbrush‐dominated and mixed desert vegetation and measured responses of Dipodomys ordii (Ord's kangaroo rat) and Perognathus flavescens (plains pocket mouse). In blackbrush‐dominated vegetation, blackbrush seeds comprised >79% of seed production in a mast year, but 0% in the first postmast year. Kangaroo rat abundance in blackbrush‐dominated vegetation was highest in the mast year, declined sharply at the end of the first postmast summer, and then remained at low levels for 3 years. Pocket mouse abundance was not as strongly associated with blackbrush seed production. In mixed desert vegetation, kangaroo rat abundance was higher and more uniform through time. Kangaroo rats excluded the smaller pocket mice from resource‐rich patches including a pipeline disturbance and also moved their home range centers closer to this disturbance in a year of low blackbrush seed production. Home range size for kangaroo rats was unrelated to seed resource density in the mast year, but resource‐poor home ranges were larger (P < 0.001) in the first postmast year, when resources were limiting. Blackbrush seeds are higher in protein and fat but lower in carbohydrates than the more highly preferred seeds of Indian ricegrass (Achnatherum hymenoides) and have similar energy value per unit of handling time. Kangaroo rats cached seeds of these two species in similar spatial configurations, implying that they were equally valued as stored food resources. Blackbrush mast is a key resource regulating populations of kangaroo rats in this ecosystem.

Discovery, Prevalence, and Persistence of Novel Circular Single-Stranded DNA Viruses in the Ctenophores Mnemiopsis leidyi and Beroe ovata.

Gelatinous zooplankton, such as ctenophores and jellyfish, are important components of marine and brackish ecosystems and play critical roles in aquatic biogeochemistry. As voracious predators of plankton, ctenophores have key positions in aquatic food webs and are often successful invaders when introduced to new areas. Gelatinous zooplankton have strong impacts on ecosystem services, particularly in coastal environments. However, little is known about the factors responsible for regulating population dynamics of gelatinous organisms, including biological interactions that may contribute to bloom demise. Ctenophores are known to contain specific bacterial communities and a variety of invertebrate parasites and symbionts; however, no previous studies have examined the presence of viruses in these organisms. Building upon recent studies demonstrating a diversity of single-stranded DNA viruses that encode a replication initiator protein (Rep) in aquatic invertebrates, this study explored the presence of circular, Rep-encoding single-stranded DNA (CRESS-DNA) viruses in the ctenophores Mnemiopsis leidyi and Beroe ovata collected from the Skidaway River Estuary and Savannah River in Georgia, USA. Using rolling circle amplification followed by restriction enzyme digestion, this study provides the first evidence of viruses in ctenophores. Investigation of four CRESS-DNA viruses over an 8-month period using PCR demonstrated temporal trends in viral prevalence and indicated that some of the viruses may persist in ctenophore populations throughout the year. Although future work needs to examine the ecological roles of these ctenophore-associated viruses, this study indicates that viral infection may play a role in population dynamics of gelatinous zooplankton.

The secret lives of cannibals: Modelling density-dependent processes that regulate population dynamics in Chaoborus crystallinus

Population regulation is mainly driven by density-dependent processes. In many species, mortality caused by cannibalism is an important density-dependent factor, which has often been overlooked when investigating population dynamics. In this study, we want to show the significance of cannibalism in regulating the population densities of the phantom midge, Chaoborus crystallinus. To this end, an integrative approach was used that combined experimental data with an individual-based population model of C. crystallinus.In laboratory experiments, density-dependent cannibalism rates of first and fourth larval stages preying on first instar larvae were quantified. The model was parameterised with laboratory and mesocosm data and subsequently validated using independent, outdoor semi-field experiments. Thus, population dynamics and structure of C. crystallinus could be accurately simulated at different temperatures and food regimes.The comparison of simulated to measured population dynamics in outdoor mesocosms revealed that cannibalism was mainly responsible for the high overall mortality (>80%) in C. crystallinus populations when using cannibalism rates for the simulations measured in the laboratory. Our results suggest that cannibalism also acts as a density-dependent compensatory mechanism by regulating population dynamics at higher larval densities and reducing population vulnerability at lower larval densities.

A stepwise algorithm to detect significant time lags in ecological time series in terms of autocorrelation functions and ARMA model optimisation of pest population seasonal outbreaks

A central issue in population ecology is to determine the structure of negative feedback-density depend process which regulates population dynamics and seasonal fluctuations. In this work the incidence of population density dependences and seasonality was examined in fruit orchards of three closely related pest species (Adoxophyes orana, Anarsia lineatella and (Grapholita) Grapholitha molesta). Analysis included 13 moth population time series during 2003–2011. Additionally, considering that time lags and seasonality are fundamental characteristics of ecological organisation and pest management, the work aimed to introduce a step wise algorithm to detect significant population feedbacks, moth seasonality and population synchronisation of nearby locations. In the proposed procedure, each population-time series was first analysed on the basis of autocorrelation and partial autocorrelation. Moreover, assuming that each of the ecological variable, observed at successive time points, consist of a stochastic process, autoregressive moving average ARMA(p,q) models and seasonal autoregressive moving average models SARMA(p,q)x(P,Q) S were fitted on data. The Akaike information criteria was further used by the stepwise algorithm for parameter optimization and model improvement. Model construction is accompanied by a presentation of the fitting results and a discussion of the heuristic benchmarks used to assess the forecasting performance of the models. Life cycles of populations belonging to same species appeared to synchronise by terms of their autocorrelation functions. Delayed density dependence and order was in most cases of lag:1 and 2, while lag >3 was not found more frequently as expected by chance. In A. orana and A. lineatella moth species lag = 1 delayed density dependence was significantly more frequent and in particular in nearby locations. However, the structure of the fitted models varied with respect to species and observation region. In some cases, seasonal models were considered to be more accurate in simulating moth population dynamics. Finally, to provide means in forecasting moth emergence and abundance, utile in pest management, the models were trained using 2003–2009 data sets and their forecasting performance were validated for each case using data sets of 2010–2011. In most cases, the constructed stochastic linear autoregressive models simulated the population outbreaks very well. Describing and forecasting stochastic population fluctuations is a basic tenet of theoretical and applied ecology, while detecting the relative roles of exogenous and endogenous mechanisms can partly describe the phenomenological behavior of pest population time series data and improve pest management.

The Edmondson Egg Ratio Method: An Elegant Approach to Obtaining Birth and Death Rates for Field Populations

Every important process in population ecology ultimately influences either fecundity or survival, so reliable methods for estimating birth and death rates in the field are key to understanding what features of the environment regulate population dynamics. In 1960 W. T. Edmondson, after visiting the Istituto Italiano de Idrobiologia in Pallanza while on sabbatical leave from the University of Washington, published a remarkably simple method for estimating zooplankton instantaneous birth rate, b, based on females’ mean clutch size and egg development time as a function of temperature (Edmondson 1960). He further pointed out that, taken together with population instantaneous rate of change, r, calculated from the population sizes on consecutive dates, instantaneous death rate could be calculated as d = r − b.

The adaptive potential of maternal stress exposure in regulating population dynamics.

Ecologists, evolutionary biologists and biomedical researchers are investing great effort in understanding the impact maternal stress may have on offspring phenotypes. Bian et al. advance this field by providing evidence that density-induced maternal stress programs offspring phenotypes, resulting in direct consequences on their fitness and population dynamics, but doing so in a context-dependent manner. They suggest that intrinsic state alterations induced by maternal stress may be one ecological factor generating delayed density-dependent effects. This research highlights the connection between maternal stress and population dynamics, and the importance of understanding the adaptive potential of such effects in a context-dependent manner.

Predation on artificial caterpillars is higher in countryside than near-natural forest habitat in lowland south-western Costa Rica

Abstract:Predation pressure is essential in regulating population dynamics of herbivorous insects. We used artificial caterpillars (25 × 4 mm) made from brown-or green-coloured plasticine to compare predation pressure between countryside and near-natural rain-forest habitat in the Gulfo Dulce region (Costa Rica). Within each habitat, 162 caterpillars were placed randomly on different substrates along a 1200-m transect and at heights between 0.5 and 2.0 m. Artificial caterpillars were inspected at 24-h intervals for 3 consecutive days. Predation pressure was almost twice as high for countryside (mean attack frequency per capita: 1.11 ± 0.08 SE) compared with rain forest (0.66 ± 0.07 SE). In both habitats arthropods emerged as chief predator group, followed by birds. Attacks by non-volant mammals were very rare and restricted to rain-forest sites. In the countryside, bird attacks were more than four times as common as in forest, indicating a change in their relative importance across habitats.

Shrub cover regulates population dynamics of herbaceous plants at individual shrub scale on the Mongolian steppe

AbstractQuestionsDo shrubs regulate herbaceous plant community by modifying seed and plant distributions at the individual shrub scale? Do the magnitude and extent of effects at this scale change with shrub cover?LocationA desert steppe, Mandalgobi, Mongolia.MethodsWe compared soil properties, seed accumulation and the establishment of plants of three functional types (annual forb, perennial forb and perennial graminoid) between shrub microsites (on mounds vs mound interspace) at sites differing in shrub coverage (high and low).ResultsShrub patches (a legume Caragana microphylla) promoted seed accumulation but inhibited plant establishment on mounds, relative to the mound interspace, for all functional types. However, the effects varied with shrub cover, particularly in the mound interspace, with responses specific to each functional type. The accumulation of soil and annual forb seeds increased with shrub cover at both microsites. Additionally, the density of annual forbs was positively related to shrub cover in the mound interspace. In contrast, the density of perennial forbs and graminoids decreased with increasing shrub cover at both microsites, possibly because of enhanced sand accumulation.ConclusionShrubs shape herbaceous plant community by causing differences at the individual shrub scale in seed and plant distributions. Additionally, we showed that individual shrub effects are shrub cover‐dependent: dense shrub stands enhanced the dominance of annual forbs in spite of the negative effect found at the individual shrub scale, whereas perennial forbs and graminoids were less abundant in denser shrub stands. Shrubs can play an important role in structuring plant communities via shrub cover‐dependent variation at microsites.

Density related effects on lifetime fecundity of Heterakis gallinarum in chickens.

BackgroundDensity related effects, both inverse density- and density-dependent, contribute to regulating population dynamics of parasites. We investigated whether density related effects are directly controlling lifetime fecundity of Heterakis gallinarum.MethodsDaily total numbers of H. gallinarum eggs in faeces samples (N = 1365) from chickens (N = 39) were quantified starting from 3 weeks (wk) post-infection (p.i.). The birds were necropsied 8 wk p.i., and intensity and demographic characteristics of infrapopulations were determined. Density related effects on cumulative egg excretion (CEE), lifetime fecundity and worm length were investigated with a segmented regression analysis.ResultsFor CEE, lifetime fecundity and female worm length, we determined highly similar parasite intensity thresholds (52–54 worms), which separated infrapopulations for influences of inverse density- and density dependence. CEE increased as parasite intensity increased up to an intensity of 52 worms. After this threshold, the relationship followed more of a horizontal line indicating impaired worm fecundity at higher parasite intensities. Lifetime fecundity was enhanced linearly in infrapopulations with up to 54 worms, but thereafter decreased gradually with increasing infrapopulation size. Female worm length increased linearly with elevating parasite intensity up to a threshold of 54 worms and thereafter declined with a rate of -0.014 mm for each additional worm. Lifetime fecundity and female worm length did not significantly differ between infrapopulations below and above the thresholds (P > 0.05). Lifetime fecundity was positively associated with the percentage of male worms (r = 0.44; P < 0.001), but negatively with absolute deviation from the theoretically expected sex-ratio in the infrapopulations (r = -0.56; P = 0.005). These relationships were stronger in infrapopulations below the threshold (r = 0.51 and -0.61, respectively), and were not significantly different from zero in the infrapopulations above the threshold (P > 0.05).ConclusionsEgg production of H. gallinarum is regulated by the effects of both inverse density- and density-dependent mechanisms, which result in similar average lifetime fecundity below or above intensity thresholds. In infrapopulations below the intensity thresholds, inverse density dependence effects on lifetime fecundity appear to result partly from sex-ratio fluctuations and impaired mating success of the nematode.

Trophic ecology of mesostigmatan and oribatid mites in harvested and control coniferous and deciduous stands of the boreal mixedwood forest determined using 15N stable isotopes

In boreal forest ecosystems, thick forest floors are a key habitat for microorganisms, micro- and mesofauna that drive soil biochemical processes. While they are the least abundant of soil mesofauna, mesostigmatan mites play a central role in soils by regulating population dynamics of other meso- and microfauna and by affecting nitrogen mineralization rates. The most abundant and diverse oribatid mites are especially important for decomposition processes in boreal forests. In this study, we evaluated the trophic ecology of mesostigmatan and oribatid mites from Canadian boreal forests by using body δ15N signatures as indicators of their trophic position. We extracted mites from the forest floors of control and clearcut stands from coniferous and deciduous stands dominated by white spruce (Picea glauca (Moench) Voss), and trembling aspen (Populus tremuloides Michx.), respectively. The mite values spanned 10.2 δ units in 15N, and were enriched by at least 1.2‰ compared to forest floor (baseline) values. When compiled to known eating habits from published feeding experiments, isotopic values placed the mites in three main trophic guilds, including detritivorous, omnivorous (overlapping with predators), and predator mites. Isotopic nitrogen fractionation within the mites did not seem to be affected by their habitat (spruce vs. aspen), or clearcutting practices. Instead, in this boreal forest environment, the well-defined degree of isotopic fractionation within the food web may be solely dependent on the mite feeding relationships.

Factors regulating population dynamics of the amphipod Ampithoe valida in a eutrophic subtropical coastal lagoon

Year-round investigations were carried out to address the population dynamics of amphipod Ampithoe valida and its potential regulating factors in the Yundang Lagoon (a eutrophic subtropical coastal lagoon in Xiamen City, China) from October 2007 to October 2008. The results show that A. valida population was largely composed of juveniles and reached a peak abundance (12.02×103 ind./m2) in March, but sharply shrunk in July. The monthly variation pattern of A. valida seemed to follow that of Ulva lactuca which is the main food resource for A. valida, indicating a strong influence of food availability on A. valida population dynamics. Life cycle studies in laboratory indicate that A. valida adopted an r -selected life strategy, e.g., rapid growth rate (0.216–0.302 mm/d), consecutive breeding (4–10 times), short reproductive rhythm (9–17 d) and huge fecundity (24–192 eggs or 6–94 juveniles per brood), which contributed to the fast population growth of A. valida from January to March. Although temperature may also be a significant cause, A. valida could grow quickly and show normal reproductive traits (i.e., the timing of sexual differentiation or maturity, the reproductive rhythm, the number of broods all through its lifespan and offspring production) in temperature between 15–30°C. Therefore, the temperature variation in the lagoon water should not account for the sharp variation of A. valida population by affecting the survival, growth and reproduction of the amphipod. However, negative correlation between water temperature and U. lactuca biomass in the lagoon suggested that temperature could have indirectly affected the population dynamics of A. valida by affecting its food availability. The authors conclude that, in the Yundang Lagoon, the population dynamics of A. valida was mainly controlled by the food availability rather than water temperature.

Navigating the Devious Course of Evolution: The Importance of Mechanistic Models for Identifying Eco-Evolutionary Dynamics in Nature

In proposing his genetic feedback mechanism, David Pimentel was one of the first biologists to argue that the reciprocal interplay of ecological and evolutionary dynamics is an important process regulating population dynamics and ultimately affecting community composition. Although the past decade has seen an increase in research activity on these so-called eco-evolutionary dynamics, there remains a conspicuous lack of compelling natural examples of such feedback. Here we argue that this lack may be due to an inherent difficulty in detecting eco-evolutionary dynamics in nature. By examining models of virulence evolution, host resistance evolution, and antigenic evolution, we show that the influence of evolution on ecological dynamics can often be obscured by other ecological processes that yield similar dynamics. We then show, however, that mechanistic models can be used to navigate this, in Pimentel's words, "devious" course of evolution when effectively combined with empirical data. We argue that these models, improving upon Pimentel's original mathematical models, will therefore play an increasingly important role in identifying more subtle, but possibly ubiquitous, eco-evolutionary dynamics in nature. To highlight the importance of identifying these potentially subtle dynamics in nature, we end by considering our ability to anticipate the effect of population control strategies in the presence of these eco-evolutionary feedbacks.

Indirect and mitigated effects of pulsed resources on the population dynamics of a northern rodent

Pulsed resources have significant effects on population and community dynamics in terrestrial ecosystems. Mast seeding is an important resource pulse in deciduous forests; these boom and bust cycles of seed production generate strong lagged population responses by post-dispersal seed predators such as rodents, which then cascade through multiple trophic levels and regulate population dynamics of their predators and prey. However, similar interactions in another major pulsed system, coniferous forests, are inconsistent, and the effects of interannual variation in conifer seed production on many consumer populations are largely unknown. We used large-scale manipulation and intensive monitoring to examine the population dynamics of deer mice (Peromyscus maniculatus) in relation to fall seed production by two northern conifers, white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa). Previous studies have shown that spruce seeds are a preferred food source of mice, while fir seeds are generally avoided if other foods are available. Therefore, we expected that there would be a positive relationship between mouse demography and previous spruce seed production, but no effect of fir mast seeding. Supplementation of a mouse population using spruce seeds indicated that increased fall spruce seed availability can enhance overwinter survival and population densities in the following spring, summer, and fall. However, long-term population monitoring indicated that mouse demography was not positively affected by spruce mast seeding, likely due to strong interspecific competition with the North American red squirrel (Tamiasciurus hudoniscus), a dominant pre-dispersal spruce seed predator. Conversely, we observed an unexpected delayed effect of fir mast seeding, where increased fall fir seed production did not influence overwinter or spring mouse demography, but instead enhanced summer survival, body masses and pregnancy rates of overwintered adults. This led to increased summer population densities and may have been mediated by population responses of invertebrate post-dispersal seed predators to increased fir seed availability. Our results indicate that rodent responses to resource pulses in coniferous forests are more complex than in deciduous environments and reveal previously unobserved direct and indirect consumer-resource dynamics that require further examination. This system is ideal for the large-scale, integrative ecosystem studies that ecologists are encouraged to pursue.

密度制约效应对啮齿动物繁殖的影响

The regulation of population dynamics in rodent,which is closely relevant to strategy-making of pest-controlling,is one of critical issues in ecology. Scholars of domestic and overseas have obtained significant achievements in related field at present. This paper summarized the research progresses of density-dependent on the reproduction of rodents,including sexual organs, sexual maturation, estrus cycle, reproduction index, hormone, and behavior regulation, and also,introduced the research methods and analyzed the prospects of the research area,aimed to provide some useful ideas for the future study.

Effects of Flour Conditioning on Cannibalism of &lt;I&gt;T. castaneum&lt;/I&gt; Eggs and Pupae

Cannibalism is a very important factor regulating population dynamics of the red flour beetle. After several days of feeding, the flour becomes conditioned by the beetles, which can affect rates of cannibalism. Flour conditioning is caused by an accumulation of feces, pheromones, and ethylquinone, which is a repellent produced by the beetles. We determined the effect of five different levels of flour conditioning on cannibalism of red flour beetle eggs and pupae by adult and larval stages. Larvae had the highest rates of egg cannibalism (12 eggs eaten over the 4-d period) followed by female adults (seven eggs consumed). Adult males had the lowest rates of cannibalism with only two eggs consumed. Cannibalism of eggs by females was correlated negatively with the level of flour conditioning. There was no effect of flour conditioning on egg or pupal cannibalism by larvae or adult males. Cannibalism by adult females may decrease as the level of flour conditioning increases because females may spend less time tunneling in highly conditioned flour and more time trying to disperse to other areas that are better for oviposition.

Population variation and individual maximum size in two leech populations: energy extraction from cannibalism or niche widening?

The theory of cannibal dynamics predicts a link between population dynamics and individual life history. In particular, increased individual growth has, in both modeling and empirical studies, been shown to result from a destabilization of population dynamics. We used data from a long-term study of the dynamics of two leech (Erpobdella octoculata) populations to test the hypothesis that maximum size should be higher in a cycling population; one of the study populations exhibited a delayed feedback cycle while the other population showed no sign of cyclicity. A hump-shaped relationship between individual mass of 1-year-old leeches and offspring density the previous year was present in both populations. As predicted from the theory, the maximum mass of individuals was much larger in the fluctuating population. In contrast to predictions, the higher growth rate was not related to energy extraction from cannibalism. Instead, the higher individual mass is suggested to be due to increased availability of resources due to a niche widening with increased individual body mass. The larger individual mass in the fluctuating population was related to a stronger correlation between the densities of 1-year-old individuals and 2-year-old individuals the following year in this population. Although cannibalism was the major mechanism regulating population dynamics, its importance was negligible in terms of providing cannibalizing individuals with energy subsequently increasing their fecundity. Instead, the study identifies a need for theoretical and empirical studies on the largely unstudied interplay between ontogenetic niche shifts and cannibalistic population dynamics.