Density-dependent Response Research Articles

Effects of Climate and Stock Size on Recruitment and Growth of Pacific Halibut

Abstract This paper compares long-term changes in the recruitment and growth of Pacific halibut Hippoglossus stenolepis with long-term changes in climate and stock size. It appears that environmental variability—both interdecadal and interannual—is responsible for most of the observed variation in Pacific halibut recruitment. The large changes in growth rates that occurred during the 20th century appear to have been density-dependent responses to changes in stock size, with virtually no environmental influence.

Interactions in a tritrophic acarine predator-prey metapopulation system IV: effects of host plant condition on Tetranychus urticae (Acari: Tetranychidae).

Feeding by spider mites can cause severe injury to a host plant and lead to a decreasing per capita growth rate and an increasing per capita emigration rate. Such density-dependent responses to local conditions are important in a metapopulation context because they allow the herbivores to colonize new host plants and thereby prolong the time until regional (metapopulation) extinction. In order to include density-dependent responses of the two-spotted spider mite (Tetranychus urticae) in a realistic metapopulation model, a series of greenhouse experiments was conducted with the purpose to quantify how the condition of bean plants (Phaseolus vulgaris) influences the demographic parameters of T. urticae. Plant age per se reduced the growth rate of the spider mites only slightly, whereas the growth rate declined significantly as the plants were injured by the mites. The relationships between plant condition (expressed by the plant injury index D) and the birth and loss (death + emigration) rates of the mites were quantified so as to predict population growth as a function of D. Maximum per capita growth rate (r) was estimated to be c. 0.21 day(-1). The growth rate is expected to be negative when D exceeds 0.8. When mites were allowed to emigrate to neighbouring plants via bridges, the per capita emigration rate increased almost exponentially with D. The proportion of eggs in the population decreased with D while the numerical ratio between immatures to adults and the sex ratio did not change with D. Overall, immatures and adults constituted 74% and 26%, respectively, of the active mites and c. 46% of the adults were males. The bridges that connected a donor plant with the surrounding recipient plants were responsible for the majority of the emigrations from donor plants. Most mites stopped after having crossed a single bridge, but a few crossed two bridges while none crossed three bridges within 24 h. The significance of the results for biological control is discussed.

Effects of environmental factors on size‐related growth efficiency of perch, Perca fluviatilis

Abstract – Two aspects of size‐dependent growth were addressed using perch, Perca fluviatilis L, from 22 Swedish lakes. Firstly, maximum annual growth decreased linearly with length after the previous year. Secondly, median or realized growth showed a non‐linear pattern, with minimum growth efficiency at intermediate length, probably related to the shift from invertebrate to fish diet. Between‐lake variation in size‐related growth efficiency was better explained by fish community characteristics than by abiotic factors. The growth of most size classes was negatively related to fish biomass, especially to biomass of 100 to 199‐mm perch. Growth efficiency of perch >200 mm was positively related to the proportion of large perch in the community. These size‐ and density‐dependent growth responses suggest that effects of competitive and predatory interactions will often mask the controlling or limiting effects of abiotic factors, such that realized growth of individuals and populations are poorly described by deterministic asymptotic growth models.

Oviposition behaviour and host colony size discrimination in Episyrphus balteatus (Diptera: Syrphidae)

Three bioassays were used to examine the oviposition behaviour of the hoverfly, Episyrphus balteatus (Degeer) in which various stimuli purported to influence host assessment and choice were tested. Episyrphus balteatus failed to exhibit enhanced behavioural responses, in terms of approaches and landings, to artificial leaves with the highest numbers of aphids, suggesting that females are merely responding to the colour of the artificial leaves. A density-dependent oviposition response was reported in a second bioassay using whole bean plants, although there was an asymptotic relationship. Syrphid responses, which were measured by oviposition over a five-day period, were greatest towards those plants with the highest number of aphids. In a final series of trials, gravid female E. balteatus showed a behavioural preference for filter papers treated with the greatest honeydew concentrations (0.26 mg microl(-1) honeydew). More time was spent in the treated areas and syrphids demonstrated more proboscis and ovipositor extensions in these treatments. These bioassays provided an opportunity to isolate some of the individual components of decision-making by female E. balteatus during egg-laying behaviour. A greater understanding of natural enemy behaviour is essential before enhanced control of pest populations in the field can be established.

Density-dependent recruitment after winter disturbance on tidal flats by the lugworm Arenicola marina

The polychaete Arenicola marina is abundant and widespread on intertidal sand flats in Konigshafen (island of Sylt, North Sea). Juveniles overwinter in subtidal channels and then colonize the upper tidal zone above the range of the adults. In the summers of 1995 and 1997, after a mild and a moderate winter, a distinct nursery belt fringing the shoreline was apparent. The severe winter of 1995/96 changed this pattern. Population size of the adults was halved, and in summer 1996 the juveniles were no longer restricted to an upper intertidal belt and settled over a wide range of tidal flats where adults were diminished. This spatial expansion of recruits is assumed to be a density-dependent response. An overcompensation by one-third of the previous population size did not carry over into the next year. Compared to some other species of the tidal-flat fauna, the disturbance effect by the severe winter on the lugworm population was small and brief.

Calf Production, Calf Survival, and Recruitment of Muskoxen on Banks Island during a Period of Changing Population Density from 1986-99

Population estimates for muskoxen (Ovibos moschatus) (age > 1 year) on Banks Island increased from 29 168 (SE 2104) in 1985 to a peak of 64 608 (SE 2009) in 1994 and then declined to 45 833 (SE 1938) in 1998. From 1986 to 1999, annual sex and age classification surveys of muskoxen were conducted during summer. We estimated calf production (number of calves per 100 females aged 2 years or more), calf survival, and recruitment (number of yearlings per 100 females aged 2 years or more). Calf production ranged from 31.3 to 56.3 and was similar between periods of increasing and decreasing density (mean = 42.3 vs. 40.8). Calf survival ranged from 23% to 83% and was generally higher while density was increasing than during its decline (mean = 60 vs. 45). Survival at a given density was lower following the 1994 peak in density. Recruitment ranged from 10.0 to 41.7 and was higher (p = 0.06) during the period of increasing density than during the decline (mean = 28.0 vs. 17.2). Calf survival and recruitment were lowest following two consecutive severe winters, but animal density explained more of the variation in survival and recruitment than did late-winter snow depth. There was a positive relationship between the proportion of sedge (Carex spp., Eriophorum scheuchzeri) in the summer diet and calf survival and recruitment. Patterns of calf survival and recruitment plotted against density were consistent with those modelling a density-dependent relationship. Our results suggest that severe weather alone cannot explain the fluctuations in the population dynamics of Banks Island muskoxen and that underlying density-dependent responses acting upon calf survival and recruitment offer an alternative explanation.

Dynamics of reintroduction in an indigenous large ungulate: the wood bison of northern Canada

AbstractWe document the recolonization of an indigenous large herbivore into its historic range. Eighteen wood bison (Bison bison athabascae) were reintroduced into the Mackenzie Bison Sanctuary of the Northwest Territories, Canada, in 1963. The population subsequently increased in number and range, peaking at about 2400 in 1989; numbers were estimated at about 1900 in 1998. Recolonization occurred through a series of increases in local areas followed by pulses of dispersal and range expansion. This pattern was originally described for exotic species' introductions. Differences in diet and overwinter survival of calves over the bison's range suggest that intraspecific competition for food provided the stimulus for range expansion. For a conservation strategy, the reintroduction of animals into several independent sites in their historic range would facilitate recolonization and achieve a faster spread than a reintroduction into one site followed by waiting for the population to spread as a result of its own density dependent responses.

EFFECTS OF FOOD SUPPLEMENTATION ON THE SOCIAL ORGANIZATION OF PRAIRIE VOLES (MICROTUS OCHROGASTER)

Mammalian social organization can vary over ecological time. We experimentally manipulated food resources in enclosed populations of prairie voles (Microtus ochrogaster) to test the hypothesis that food quality influences the potential for group formation. During each field season, populations were started by releasing 5 pairs of prairie voles into each of 8 0.1-ha enclosures. Populations were monitored for 18-19 weeks during each field season. One-half of the enclosures received supplemental food, and the other one-half were unsupplemented controls. Density of voles increased throughout each field season. There were significant increases through time in philopatry and number of groups in both the food-supplemented and unsupplemented treatments, but there were no differences between treatments. Groups formed early in the season, apparently before the need for thermoregulatory benefits. Results are consistent with the hypothesis that the social organization of prairie voles is not flexible in response to changes in food quality but that formation of groups might be a density-dependent response.

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Linear-scaling parallel algorithms for the first principles treatment of metals

Orbital-free kinetic energy density functionals provide the means to accurately simulate the behavior of simple sp-band metals, on a length scale currently unparalleled by other ab initio methods. We present some technical aspects of the efficient parallel implementation of kinetic energy functionals, including a functional with a recently developed density-dependent response kernel that provides a good description of metal surfaces. We further illustrate the ability of this new functional to treat finite metallic systems by examining the metal-insulator transition in a 2-dimensional array of metal quantum dots.

Functional response and switching behavior of young-of-the-year piscivorous bluefish

Young-of-the-year (YOY) bluefish, Pomatomus saltatrix (L.) impose significant mortality on recently metamorphosed striped bass, Morone saxatilis (W.) in the Hudson River estuary. Field observations indicate that bluefish selectivity on striped bass increases with increasing striped bass density suggesting a density-dependent feeding response. Functional response and switching experiments were conducted to aid in determining the mechanism for this response. The type of functional response exhibited by bluefish under several prey treatments was examined. These were striped bass alone, Atlantic silversides, Menidia menidia (L.) alone, striped bass with a background density of alternative prey, and striped bass in the presence of eelgrass Zostera marina (L.). The functional response of bluefish to increasing prey density was more similar to a type I or II model than a type III model under all treatments. Switching was examined by providing bluefish both striped bass and silversides at five different ratios with and without a prey refuge, eelgrass. Bluefish did not exhibit a type III functional response or switching behavior suggesting that these mechanisms do not explain the observed density-dependent selectivity pattern in the field. Switching experiments did suggest that the presence of prey refuge can determine selectivity by YOY bluefish in the field. The abundance of submerged aquatic vegetation and other prey refuge may be important in controlling the predatory impact of bluefish on striped bass in the Hudson River.

Population dynamics of Tetranychus kanzawai (Acari: Tetranychidae) on hydrangea.

The seasonal occurrence of Tetranychus kanzawai Kishida populations on hydrangea (Hydrangea macrophylla) was studied at two different localities in Ibaraki, Japan, during a three-year period. There were two types of seasonal population trends: one with a population peak from May to June, and the other with the spring peak in June and the autumn peak in September-October. Each year the populations on hydrangea plants abruptly declined just after the spring peak. Predators showed a delayed density-dependent numerical response. The population crashed even in the absence of predators, suggesting that the predators had nothing to do with the June decline. Furthermore, the rate of development from larva to adult and the fecundity in adult females on detached hydrangea leaves decreased markedly just prior to the abrupt decline in density in June. Consequently, seasonal changes in plant quality (perhaps influenced by secondary compounds) seem to contribute to the drastic decline of T. kanzawai density on hydrangea in June.

Resource Limitation in Seasonal Environments

A model applying to species that are dependent on different resources during breeding and during non-breeding (over-wintering) periods is considered. The model is used to make both intra- and inter-species comparisons, predicting the difference in population size in varying as compared with unchanging environments, expressed in terms of demographic parameters. Different populations of the same species can respond differently depending on context: populations in which numbers are primarily restricted by the availability of resources necessary for over-winter survival are typically more vulnerable to environmental fluctuations than populations restricted by resources required for reproduction. Although species that have low fecundity and are short-lived are most vulnerable overall, species that have low fecundity but are long-lived tire most sensitive to the season of the limiting bottleneck. For species with a high basic fecundity the season of most concern depends on seasonal differences in density dependence responses. Thus one needs to know whether populations are limited by scramble competition or by territorial behaviour, for example, and in which season. One cannot evade the importance of identifying the limiting resource when assessing the vulnerability of populations to environmental change.

Impact of age-0 bluefish (Pomatomus saltatrix) predation on age-0 fishes in the Hudson River estuary: evidence for density-dependent loss of juvenile striped bass (Morone saxatilis)

We measured bluefish (Pomatomus saltatrix) weights, densities, and prey sizes during the summers of 1992 and 1993 and diets over a 4-year period (1990-1993) in the Hudson River estuary. This information was used to estimate the loss of young-of-the-year (YOY) striped bass (Morone saxatilis) resulting from YOY bluefish predation. We then compared this predation mortality with the total loss of striped bass in the system. Data from sampling surveys conducted since the mid-1970's were used to examine relationships between bluefish abundance and striped bass recruitment levels. YOY striped bass, bay anchovy (Anchoa mitchilli), Atlantic silverside (Menidia menidia), and Alosa spp. dominated YOY bluefish diets. There were ontogenetic and interannual differences in YOY bluefish diets. Bluefish avoided striped bass at low densities but selected for them at high densities, suggesting a density-dependent feeding response. In the early summer of 1993, bluefish predation accounted for 50-100% of the total estimated loss of YOY striped bass. A significant negative correlation exists between the relative magnitude of striped bass recruitment and bluefish abundance. We conclude that YOY bluefish are important predators of estuarine fish and can have a substantial impact on their recruitment.

Modeling effects of increased larval mortality on bay anchovy population dynamics in the mesohaline Chesapeake Bay:evidence for compensatory reserve

We applied an individual-based population model to examine the potential compensatory scope of the bay anchovy Anchoa mitchilli in Chesapeake Bay. Model simulations were analyzed to estimate: (1) how losses of individuals in different life stages affect year-class production; (2) the changes needed in individual processes and, simultaneously in multiple processes, to offset a 50% increase in larval-stage mortality; and (3) population responses to increasing larval-stage mortality under conditions of presumed high compensatory potential. We hypothesize that, in response to lower densities, the bay anchovy population could compensate for increased larval mortality through increased growth rates, increased allocation of energy to reproduction, or reduced predation mortality as predators target other species. Simulation results indicate that late-larval and juvenile bay anchovy may be able to consume a significant fraction of their zooplankton prey, suggesting that anchovy is at abundances that can cause density-dependent growth in the Chesapeake Bay. However, density-dependent effects on prey resources alone had a limited buffering effect against a 50% reduction in larval-stage survival. The potential effect of losses of larvae on future production of a year class depended upon when during the larval stage individuals are removed from the population. Modeled alone, large changes in spawning intensity (no, of batches and eggs per batch), egg survival, or mortality of juveniles and adults were required to offset increased larval mortality. When all processes were varied simultaneously, much smaller changes were required. Under a high compensation scenario, there was a strong dome-shaped response in adult production potential to increased larval mortality, such that highest adult production occurred when survival rate of larvae was reduced by as much as 60%. While the information presently available to examine density-dependent population responses in bay anchovy is limiting, the modeled results indicate that the bay anchovy population in Chesapeake Bay potentially can regulate its abundance through simultaneous shifts in processes believed to be sensitive to population density.

Optimal Floating and Queuing Strategies: Consequences for Density Dependence and Habitat Loss

Field studies of many vertebrates show that some individuals (floaters) do not defend territories even when there is space for them to do so. We show that the evolutionarily stable strategy (ESS) for the threshold territory quality at which floating takes place is that which maximizes the size of the floating population (but not the total population, breeding population, or reproductive output). The ESS is solved separately for two assumptions: whether individuals wait to occupy a single territory or multiple territories and whether queuing rules are strict or if all waiting individuals are equally likely to obtain the next territory. The four combinations of these assumptions all give the same evolutionarily stable population size of both floaters and breeders. At the ESS, only territories with expected lifetime reproductive success (LRS) exceeding 1 should be occupied, which introduces a limit to ideal habitat selection. The behavioral decision to float alters the shape of the density-dependent response, reduces the equilibrium population size, and affects the response of the population to habitat loss. Specifically, the floater: breeder ratio is directly related to average breeding habitat quality, and the floater population size will decrease more than the breeding population size if better than average quality habitat is lost.

Histological assessment of variations in sexual maturity of cod (Gadus morhuaL.) at the Flemish Cap (north-west Atlantic)

A histological method is used to detect the proportion of both adult and recruit females at the beginning of gonad development, in summer, a few months after spawning. At this time it is diYcult to make a visual diagnosis of the sexual maturity stage. The proportion of females with postovulatory follicles is used to determine the length and age at maturity in the year of sampling, and the proportion of females in the cortical alveoli and/or vitellogenic stages is used to estimate the same parameter for the following year. Variations in age and length at maturity during the period 1992‐1995 were analysed by comparing the respective maturity curves of both the ‘‘previous spawners’’ and the ‘‘next-year spawners’’ females. A drastic decrease in both the age and the length at maturity is observed in this period and it seems likely that the age at maturity of female cod became fixed at around age 3 at the end of the period analysed. The possibility that this could be a density-dependent response of the stock at the Flemish Cap is discussed. ? 1998 International Council for the Exploration of the Sea

Association of the Opiine Parasitoid Diachasmimorpha tryoni (Hymenoptera: Braconidae) with the Lantana Gall Fly (Diptera: Tephritidae) on Kauai

Parasitization of the lantana gall fly, Eutreta xanthochaeta Aldrich, by an introduced larval fruit fly parasitoid, Diachasmimorpha tryoni (Cameron), was assessed by measuring total host and parasitoid recruitment in patches of lantana plants growing in 3 distinct ecological habitats on the island of Kauai. Over a 17-wk period, means of 87, 261, and 276 E. xanthochaeta galls were recruited per 10-m2 patches of lantana growing in a lowland pasture (elevation approximately 124 m), a midelevation sugarcane field (elevation approximately 500 m), and an upland forest (elevation approximately 1000 m), respectively. From 42 to 72% of all newly developed galls recruited were lost before reaching maturity from predation (presumably by birds and rodents), damage by the lantana tortricid moth, Crocidosema lantana Busck, or natural abortion inside the gall. Between 28 and 58% of newly recruited galls matured successfully. Percentage parasitism of E. xanthochaeta by D. tryoni varied significantly with ecological habitat: 4.1, 1.2, and 28% of mature E. xanthochaeta galls were parasitized by D. tryoni in the lowland pasture, the midelevation sugarcane field, and the upland forest, respectively. Percentage parasitism did not vary significantly with the number of galls recruited in each patch. Parasitoids emerging from field-collected E. xanthochaeta were significantly larger and had significantly more female-biased sex ratios (1.7:1) than those emerging from their normal rearing hosts, Ceratitis capitata (Wiedemann). Still, D. tryoni may not play a major role in regulating populations of E. xanthochaeta because it lacked a spatial density-dependent response to gall fly populations. Further studies are needed to determine how ecological habitat characteristics influence patterns of exploitation of E. xanthochaeta by D. tryoni.

Searching rate and potential of some natural enemies as bio‐agent against the cotton whitefly, Bemisia tabaci Genn. (Hom., Aleyrodidae)

Abstract: The searching rate of the two aphelinid parasitoids, Eretmocerns mundas Merc. and Prospaltella lutea Masi, and the two predators, Chrysoperla carnea Steph. and Coccinella undecimpunctata L., were evaluated on Bemisia tabaci in the laboratory. Analysis of the obtained data indicated that E. mundus had a high searching rate with relatively low mutual interference in comparison with P. lutea. C. carnea also showed a higher searching rate than C. undecimpunctata. E. mundus exhibited a higher density dependent response to B. tabaci population on Lantana camara than P. lutea in October and December, while P. lutea exhibited the same response in August. Key factor analysis proved that the first instar larvae had the highest rate of mortality among the immature stages. Mortality caused by E. mundus contributed with high fate of total generation mortality (K) in comparison with P. lutea. The same result was obtained in the case of C. carnea against C. undecimpunctata.

Cell density dependent response ofE. Coli cells to weak ELF magnetic fields

The effects of weak magnetic fields of extremely low frequency (ELF) on E. coli K12 AB1157 cells were studied by the method of anomalous viscosity time dependencies (AVTD). E. coli cells at different densities within a range of 5 x 10(5)-10(9) cell/ml were exposed to ELF (sinusoidal, 30 microT peak, 15 min) at a frequency of 9 Hz. A transient effect with maximum 40-120 min after exposure was observed. Kinetics of the per-cell-normalised ELF effects fitted well to a Gaussian distribution for all densities during exposure. A maximum value of these kinetics and a time for this maximum were strongly dependent on the cell density during exposure. These data suggest a cell-to-cell interaction during response to ELF. Both dependencies had three regions close to a plateau within the ranges of 3 x 10(5) - 2 x 10(7) cell/ml, 4 x 10(7) - 2 x 10(8) cell/ml and 4 x 10(8)-10(9) cell/ml and two rather sharp transitions between these plateaus. The effect reached a maximum value at a density of 4 x 10(8) cell/ml. Practically no effect was observed at the lowest density of 3 x 10(5) cell/ml. The data suggested that the ELF effect was mainly caused by a secondary rather than a primary reaction. The filtrates from exposed cells neither induced significant AVTD changes in unexposed cells nor increased the ELF effect when were added to cells before exposure. The data did not provide evidence for significant contribution of stable chemical messengers, but some unstable compounds such as radicals could be involved in the mechanism of cell-to-cell interaction during response to ELF. The results obtained were also in accordance with a model based on an re-emission of secondary photons during resonance fluorescence.