Sub-lethal effects of disturbance on a predatory net-spinning caddisfly

1. The sub-lethal effects of hydrologic disturbances on stream invertebrates are poorly understood, but integral to some models of how disturbances influence population and community dynamics. Carnivorous larvae of a net-spinning caddisfly, Plectrocnemia conspersa , have a strong predatory impact in some streams. Their silken nets, however, are vulnerable to high flow disturbance and the consequent destruction of nets could reduce predatory impacts and have life history consequences.
2. In a laboratory experiment, we manipulated the frequency of disturbances that destroyed the nets of P. conspersa , in the presence and absence of potential prey. Animals were housed individually and each trial lasted 8 days. We estimated net size, cumulative mass of silk produced, net allocation (net mass expressed as a proportion of body mass), per capita prey consumption and growth or mass loss of larvae.
3. In the absence of prey, increased disturbance frequency was accompanied by increased loss of body mass, a reduction of net size and an increase in the cumulative mass of silk produced. At the highest disturbance frequency, larvae eventually gave up producing nets. The ratio of net mass to body mass decreased with increasing disturbance, suggesting a trade-off in the allocation of resources, with a decreasing proportion of resources available for foraging. In the presence of prey, increased disturbance frequency was accompanied by a reduction in per capita prey consumption. Although foraging success offset the costs of silk production, growth rate decreased with increasing disturbance and could eventually lead to reduced body size and fecundity of adults.
4. These sub-lethal effects suggest that hydrologic disturbances could impose metabolic costs and reduce foraging efficiency of this predator. Thus, disturbances may reduce predator impact on prey populations and reduce predator population size without any direct mortality or loss of individuals.     

The effects of density-dependent offspring mortality on the synchrony of reproduction

Mortality rates often depend on the size of a population. Using ideal free theory to model the optimal timing of reproduction in model populations, I considered how the specific relationship between density-dependent offspring mortality and population size affects the optimal temporal distribution of reproduction. The results suggest that the specific form of the relationship between density-dependent mortality and the number of offspring produced determines the degree to which reproduction within a population is synchronous. Specifically, reproductive synchrony decreases as density-dependent mortality becomes increasingly inversely related to the number of offspring produced and is highest when density-dependent mortality is directly density-dependent. These findings support the suggestion that predation pressure selects for greater reproductive synchrony in species where mortality is directly density-dependent, but does not affect the timing of reproduction in species with density-independent rates of mortality. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of a rust infection on fitness components in a natural population of Tolumnia variegata (Orchidaceae)

The effects of a rust infection (Sphenosphora saphena, Basidiomycetes) on several fitness components of the orchid Tolumnia (Oncidium) variegata were quantified in a subtropical moist forest of Puerto Rico. Infected and uninfected plants of two size categories were observed and manipulated by increasing natural levels of fruit production to determine the effects of infection on short and long-term sexual reproduction, subsequent vegetative and reproductive growth, and mortality. Under artificially high levels of fruit production, infection had no effect on short or long-term reproduction through male or female function except for a modest decline in seed viability in small plants. Under natural levels of fruit production, infection was only related to a reduction in the number of leaves per shoot. At artificially high levels of fruit production, infection reduced leaf length, leaf width, and the number of live shoots. Survival was not associated with rust occurrence. The rust appears to have minor ecological impact on this orchid population.     

Biology of the sand-smelt (Atherina presbyter Valenciennes) around Fawley power station

Past studies of fish impingement at Fawley power station have shown that sand-smelts, Atherina presbyter Valenciennes, are susceptible to impingement and may be a useful species for examining effects of impingement mortalities on population size and structure. A 21 month study of populations in the vicinity of Fawley power station was carried out to obtain data on the biology of the species for future population dynamics modelling studies. Aspects included in the study were growth, sex ratio, reproduction, survival and mortality rates and diet. Differences in apparent and true rates of growth indicated some size-selective mortality towards younger age groups, possibly caused by the power station. A seasonallyoscillating von Bertalanffy growth equation is given to predict mean length-at-age. Most female sand-smelts (73%) matured at the end of their first year of life, fecundities ranging on average from 1394 eggs per female for 1+ group fish to 6872 for III+ fish. The sex ratio was even (1 : 1). An annual survival rate of 10–8% was estimated for mature fish, and the consequent age structure of the population indicates that the bulk of egg production lies with the smaller (1+ group) fish. Since smaller fish are generally more vulnerable to impingement this factor may render populations sensitive to impingement mortalities. Gut content analysis indicated that sand-smelts probably actively select zooplankters from the general plankton when young and, once larger than 104 mm in length, increasingly feed upon natant macrofaunal species.     

Effect of food concentration on the carbon balance of Bosmina longirostris (Crustacea: Cladocera)

SUMMARY. 1. The carbon balance of the small cladoceran, Bosmina longirostris , was examined at four food concentrations (0,05, 0.10, 0.25, 2.50mg C 1-⁻¹) based on long-term growth experiments.
2. At birth, B. longirostris allocated about 60% of assimilation to body growth at all food concentrations. However, allocation to body growth decreased with age and was less than 5% after the fourth instar at the lowest food concentration. The proportion allocated to reproduction increased with increasing food concentration, but was relatively constant among different adult instars.
3. Assimilation rate increased with increasing food concentration and increasing body size: the mean rate from birth to 15 days was 6.6 times higher at the highest food concentration than at the lowest food concentration. However, with decreasing food concentration and increasing body size, a higher proportion of the assimilated carbon was lost as respiration. As a result, the net production rate reached a plateau at intermediate body size and was 9.1 times higher at the highest food concentration than at the lowest food concentration.
4. Comparison with previous studies revealed that B. longirostris is more efficient than Daphnia with regard to carbon production, especially at low food conditions, if the death rate is negligible.     

The effects of feeding regime on the growth and reproduction of the medicinal leech Hirudo medicinalis

1. The feeding frequency, the size of meals, the number of meals required to attain reproductive maturity and the number of meals taken between iteroparous reproductive bouts were determined in the laboratory under optimal conditions for the medicinal leech Hirudo medicinalis fed exclusively on mammalian (bovine) blood. In addition the number of bouts of reproduction and the numbers of cocoons and hatchlings per cocoon produced were determined.
2. The average time for H. medicinalis to reach reproductive maturity at 20°C was 289 days, at an average wet biomass of 8143 mg with two–nine separate bouts of cocoon production. The number of meals to first reproduction was 8.9 (mean meal size of 3066.7 mg), with a significant correlation between total mass of blood ingested and the numbers of reproductive bouts and number of cocoons produced. Mean lifetime cocoon production per individual was 12.43, with 3.9 hatchlings per cocoon.
3. The significant positive relationships between ingestion, fecundity and developmental rate observed support the hypothesis that declining abundances of field populations of H. medicinalis are the result of lower available energy for growth, reflecting leeches now feeding predominantly on amphibian blood of lower energetic value than mammalian blood.     

The effects of feeding regime on the growth and reproduction of the medicinal leech Hirudo medicinalis

1. The feeding frequency, the size of meals, the number of meals required to attain reproductive maturity and the number of meals taken between iteroparous reproductive bouts were determined in the laboratory under optimal conditions for the medicinal leech Hirudo medicinalis fed exclusively on mammalian (bovine) blood. In addition the number of bouts of reproduction and the numbers of cocoons and hatchlings per cocoon produced were determined.
2. The average time for H. medicinalis to reach reproductive maturity at 20°C was 289 days, at an average wet biomass of 8143 mg with two–nine separate bouts of cocoon production. The number of meals to first reproduction was 8.9 (mean meal size of 3066.7 mg), with a significant correlation between total mass of blood ingested and the numbers of reproductive bouts and number of cocoons produced. Mean lifetime cocoon production per individual was 12.43, with 3.9 hatchlings per cocoon.
3. The significant positive relationships between ingestion, fecundity and developmental rate observed support the hypothesis that declining abundances of field populations of H. medicinalis are the result of lower available energy for growth, reflecting leeches now feeding predominantly on amphibian blood of lower energetic value than mammalian blood.     

Density and the commitment of apical meristems to clonal growth and reproduction in Hieracium pilosella

Summary We examined responses to population density in the commitment of apical meristems to reproduction and clonal growth in a rosette-forming, stoloniferous herb (Hieracium pilosella). Despite close physiological coupling between the evocation of the terminal inflorescence bud and the development of one or more axillary buds into stolons, the allocation of meristems was extremely plastic.Genets at the higher sowing densities showed density-dependent mortality consistent with self-thinning along a-3/2 trajectory. The probability of inflorescence evocation and associated stolon development was negatively dependent on surviving density. The proportinal distribution of primary stolons amongst genets became strikingly more unequal (expressed as the Gini coefficient) with increasing density. Clonal growth was resolved into the number of primary stolons per stoloniferous genet and the extent of stolon branching (i.e. number of apices per primary stolon); both showed strongly negative density-dependence. Reproduction, expressed as the mean number of flowering capitula per stoloniferous genet, declined 15-fold with increasing density; although theoretically expected to be unity, greater values resulted from capitulum production by attached secondary rosettes and lower values reflected the increasing abortion rate of inflorescence buds with increasing density.Both the total number of apices produced per unit area and the corresponding number of reproductive apices were maximal at intermediate surviving densities (700–1,000 m^-2). The balance between reproductive and clonal growth may be expressed as the probability of an apical meristem producing a capitulum, that also peaked sharply at intermediate density. This finding does not conform with linear models that predict a shift from vegetative growth to sexual reproduction with increasing population density.     

The effect of temperature on the response of Daphnia to toxic cyanobacteria

1. In a set of life‐table experiments, cohorts of neonate Daphnia pulex were exposed to a toxic strain of Anabaena affinis and A. flos ‐ aquae , and to a pure cyanobacterial toxin (anatoxin‐a), at 12 or 14, 19, and 25 °C. The fecundity and survival of individual animals were assessed at 1‐, 2‐ or 3‐day intervals, depending on the temperature, through to the fifth brood of the control cohort. The sensitivity of D . pulex to the cyanobacteria and the toxin at each temperature was measured by determining its finite population growth rate (λ) in an experimental treatment as a fraction of that in a control treatment. Tests with three concentrations of cyanobacteria (1.0, 2.5 and 5.0 μg mL^–1) and one concentration of anatoxin‐a (1 μg mL^–1), and with two clones of D. pulex , showed a consistent and statistically significant pattern of increasing sensitivity at higher temperatures.
2. Anabaena affinis affected both survivorship and fecundity, while A. flos ‐ aquae and its toxin, anatoxin‐a, primarily affected fecundity. Presence of cyanobacteria affected brood size, brood number, time to first reproduction and interclutch interval. Temperature affected time to first reproduction and interclutch interval at all concentrations of cyanobacteria. Brood number and brood size were little affected by temperature except at the highest concentrations of cyanobacteria. Increasing the concentration of A. flos ‐ aquae affected demographic parameters, especially at the lower temperatures, while increasing the concentration of A. affinis had less effect.
3. The study suggests that increasing water temperatures in natural systems should exacerbate the inhibitory effect of toxic cyanobacteria on daphniid population growth rates.     

A true rate of population growth--Lotka's intrinsic rate of natural increase revisited.

In this paper, Lotka's intrinsic rate of current population growth is evaluated. A new method of computing the net reproduction rate and a new rate of population growth are proposed. The proposed rate is the rate of growth of the female population per woman per year. The rate is positive, equal to zero, or negative as a population is increasing, remaining stationary, or decreasing. The rate for the 1987 U.S. white female population was R = -0.0037. This means that the white population was decreasing in 1987 and was losing 3.7 females for every 1000 women per year.     

Secondary production of Chironomidae (Diptera) in a north temperate stream

SUMMARY 1. Secondary production of chironomids (Diptera: Chironomidae) in a third-order northern Indiana stream was estimated using species-specific, and in most cases cohort-specific, life-history data from the field.
2. Chironomid life-history patterns were diverse, ranging from one to four generations per year in addition to asynchronous development. Cohort production intervals (CPI) for all taxa ranged from 56 to 266 days. CPI for cohorts of conspecifics differed up to 3-fold.
3. Annual secondary production was estimated using the instantaneous growth method or the size—frequency method depending on whether cohorts were distinguishable or not. Total annual chironomid production was 29 700 mg dry mass m⁻², which is the highest value reported for chironomids from a north temperate stream.
4. Eighty per cent of the total chironomid production was attributed to five species: Diamesa nivoriunda (33.9%), Cricotopus bicinctus (16.6%), Pagastia sp. (10.2%), C. trifascia (9.7%) and Orthodadius obumbratus (9.6%).
5. The annual P:B ratio ranged from 4.7 to 21.9. Thus, high secondary production was due to high mean annual standing stocks and not to rapid biomass turnover.     

Dynamics of a harvested moose population in a variable environment

1. Population size, calves per female, female mean age and adult sex ratio of a moose ( Alces alces ) population in Vefsn, northern Norway were reconstructed from 1967 to 1993 using cohort analysis and catch-at-age data from 96% (6752) of all individuals harvested.
2. The dynamics of the population were influenced mainly by density-dependent harvesting, stochastic variation in climate and intrinsic variation in the age-structure of the female segment of the population.
3. A time delay in the assignment of hunting permits in relation to population size increased fluctuations in population size.
4. Selective harvesting of calves and yearlings increased the mean age of adult females in the population, and, because fecundity in moose is strongly age-specific, the number of calves per female concordantly increased. However, after years with high recruitment, the adult mean age decreased as large cohorts entered the adult age-groups. This age-structure effect generated cycles in the rate of recruitment to the population and fluctuations introduced time-lags in the population dynamics.
5. An inverse relationship between recruitment rate and population density, mediated by a density-dependent decrease in female body condition, could potentially have constituted a regulatory mechanism in the dynamics of the population, but this effect was counteracted by a density-dependent increase in the mean age of adult females.
6. Stochastic variation in winter snow depth and summer temperature had delayed effects on recruitment rate and in turn population growth rate, apparently through effects on female body condition before conception.     

Increasing model structural complexity inhibits the growth of initial condition errors

One source of inaccuracy in non-linear deterministic ecological models is the growth of initial condition errors. A size-resolved pelagic ecosystem model is used to investigate the effects of changing model structural complexity on the growth rate of initial condition errors. Structural complexity is altered by (1) changing the number of biological size-classes; and (2) changing prey size-ranges which changes the number of linkages for the same number of size-classes. Ensembles of model runs with tiny variations in initial conditions are undertaken and member divergence used to estimate ensemble spread (a measure of the growth of initial condition errors). Increasing prey ranges and therefore the number of linkages greatly reduced the rate of growth of initial condition errors, but ecosystem behaviour is also altered, restricting the generality of the result. At more than 123 size-classes, increasing the number of size-classes while not changing either the model equations or parameters does not alter ecosystem behaviour for over 200 days. In this case, increasing structural complexity through increasing the number of size-classes did not alter the growth of initial condition errors for the first 30 days of the simulations, but afterwards reduced error growth. There are many advantages of parsimonious ecological models with small numbers of classes and linkages, but they are more likely to suffer from the growth of initial condition errors than structurally complex models.     

Consequences of Premature Weaning on the Reproduction of Mothers and Offspring in Laboratory Mice

A decrease of energy expenditure per offspring is expected to benefit the mother by increasing the number of offspring she can produce, and to inflict costs upon the young by decreasing their reproductive value. To evaluate the benefit-cost functions of shortened suckling times in house mice, the young were separated from their mothers prior to natural weaning. During two following litters of mothers and young, time of birth and numbers born were recorded. The impact of the resulting effects on the lifetime reproduction of mothers and young were evaluated by model calculations assuming population equilibrium, exponential population growth or exponential growth limited by population breakdowns.     

Effects of habitat size and quality on equilibrium density and extinction time of Sorex araneus populations

1. The effects of changes in habitat size and quality on the expected population density and the expected time to extinction of Sorex araneus are studied by means of mathematical models that incorporate demographic stochasticity.
2. Habitat size is characterized by the number of territories, while habitat quality is represented by the expected number of offspring produced during the lifetime of an individual.
3. The expected population density of S. araneus is shown to be mainly influenced by the habitat size. The expected time to extinction of S. araneus populations due to demographic stochasticity, on the other hand, is much more affected by the habitat quality.
4. In a more general setting we demonstrate that, irrespective of the actual species under consideration, the likelihood of extinction as a consequence of demographic stochasticity is more effectively countered by increasing the reproductive success and survival of individuals then by increasing total population size.     

Effect of food concentration on growth, reproduction and survivorship of Bosmina longirostris (Cladocera)

SUMMARY. 1. Body growth, reproduction and survivorship of Bosmina longirostris were monitored at four food concentrations (0.05, 0.10, 0,25 and 2.50 ing Cl⁻¹) to examine the extent to which food concentration affects these life-history parameters.
2. Food concentration had a significant effect on growth, and most of the reproductive parameters (size and age at maturation, brood size, instar duration and egg development time). More than 60% of animals died before maturation at the lowest food concentration, although most animals survived until maturation at other food concentrations.
3. Change in life-history parameters resulted in a decrease in rate of population growth ( r ) from 0.310 to 0.020 day⁻¹ with decreasing food concentration, showing that this cladoceran is highly food-limited in many lakes.
4. Importance of the effect of food concentration on egg development time was emphasized in relation to analysis of natural populations.     

Effects of supplementary feeding on growth and reproduction of three carnivorous plant species in a subarctic environment

1 The effect of supplementary feeding on growth and reproduction of three carnivorous plants species was investigated over a 6‐year period. Pinguicula alpina , P. villosa and P. vulgaris populations growing at two altitudes in a subarctic environment were fed with fruit flies ( Drosophila melanogaster ).
2 Fed plants increased in size relative to control plants during the first years. Subsequently a stable size difference between feeding levels was established. The weight of the over‐wintering part was higher in fed plants than in control plants.
3 The flowering frequency (i.e. proportion of plants in a population carrying flowers) was also higher in the fed plants. The proportion of flowering plants increased in the feeding treatment compared to control plants during more‐or‐less the whole experimental feeding period (5–6 years). Seed production also increased slowly in response to feeding.
4 No feeding effect on nitrogen and phosphorus concentrations in seeds and over‐wintering parts was found.
5 In most characteristics the high‐altitude populations were less responsive than populations growing at low altitude. In the high‐altitude population reproduction failed repeatedly, suggesting that seed output may be more dependent on abiotic factors than on resource availability in cold environments.
6 Pinguicula villosa and P. vulgaris used the new resources the same summer for increased leaf rosette growth and current reproduction, showing these species to be income breeders, while the third species ( P. alpina ) allocated a larger proportion to storage and future reproduction, characteristic of a capital breeder.     

Increasing returns in the life history of Columbian ground squirrels

1. We examined positive associations and trade-offs of maternal and reproductive traits in a population of Columbian ground squirrels, Spermophilus columbianus .
2. Structural size, body condition, mother's personal allocation to body mass during reproduction, and timing of littering were estimated for live-trapped reproductive females that were observed during an 8-year period, and were compared to litter mass, litter size, and average pup mass using path analyses.
3. Mothers exhibited age-structured traits that influenced reproductive patterns. Yearling mothers were significantly smaller, bred later, and had smaller litters than older females. Mothers that gained more body mass during reproduction and older mothers in good body condition that were structurally large had larger litters.
4. Early seasonal timing of littering was an important positive influence on successful reproduction by older mothers only in early breeding seasons and in years when conditions for reproduction were good for all females.
5. The number of offspring that survived to 1 year of age was most strongly associated with litter mass and litter size; date of breeding was of secondary influence, with earlier litters exhibiting greater success.
6. In general, mothers that gained the most in body mass during reproduction were concurrently more successful in weaning large litters (perhaps due to better quality of foraging habitat).
7. In addition to expected reproductive trade-offs, reproduction by Columbian ground squirrels exhibited positive associations of life-history traits that may reflect evolutionary increasing returns.     

Life histories of some species of Chydoridae (Cladocera: Crustacea)

SUMMARY 1. Life history parameters of two species of Chydoridae ( Leydigia leydigi and Disparalona rostrata ) were studied in the field and in the laboratory.
2. For both species the number of juvenile instars was variable and tentatively related to initial size of neonate. Egg volume increased with increasing parent length and with decreasing temperature; possible advantages gained by the offspring are discussed in relation to invertebrate predation.
3. The life history strategy employed by the littoral Chydoridae is compared to that of large and small planktonic cladocerans. Small planktonic cladocerans and the Chydoridae (except the Eurycercinae and the Saycinae) produce large young relative to their size at maturity and mature early. However, unlike the small planktonic cladocerans, growth in the Chydoridae is curtailed after the onset of reproduction and in this they resemble the large planktonic cladocerans. This strategy may be related to vertebrate predation and the presence of vegetation in the habitat.     

How useful are laboratory-determined growth curves for the estimation of size-specific mortalities in Daphnia populations?

The estimation of size-specific mortalities of zooplankton populationsis strongly dependent on the estimation of somatic growth inthe lake Several laboratory growth experiments at differenttemperatures and times were conducted to examine if growth ofthe lake population can be reconstructed from a general-shapedgrowth curve and the size at first reproduction in the lake.It is shown that reconstruction of growth curves is possibleif there are no strong interaction effects of food and temperatureon the age at first reproduction