Enhance Population Growth Research Articles

Biological and Statistical Errors Make Inferences Circumspect: Response to Bender and Weisenberger

ABSTRACT Bender and Weisenberger (2005) reported that desert bighorn sheep (Ovis canadensis) on San Andres National Wildlife Refuge (SANWR), New Mexico, USA, were primarily limited by rainfall. However, they failed to mention, or were unaware, that persistent long‐term predator control was used to enhance population growth at SANWR. Additionally, lamb:female ratios were collected throughout the year, rather than dates typically associated with assessing recruitment, and therefore influence of precipitation on lamb recruitment was unknown. Finally, model predictions forwarded by Bender and Weisenberger (2005), that carrying capacity of SANWR is zero when annual rainfall is <28.2 cm, were not supported by data, nor were their model results properly interpreted. The coefficient of determination value of 88.9% for the relationship between population size and current year's precipitation was primarily a function of serial correlation between successive years in population data, with current year's precipitation accounting for only 3.8% of this value. This suggests that precipitation was a weak predictor of population increase. These errors in concert make biological inferences reported in Bender and Weisenberger (2005) of limited value.

Culture adaptation of embryonic stem cells echoes germ cell malignancy

Teratocarcinomas are a subset of tumours that result from the neoplastic transformation of primordial germ cells. Such germ cell tumours (GCT) are histologically heterogeneous, reflecting a capacity for differentiation (pluripotency) of their embryonal carcinoma (EC) stem cells. However, malignant evolution of these tumours may ultimately correlate with a decrease in pluripotency, because this would tend to increase the propensity of EC cells for self-renewal. Human embryonic stem (ES) cells, derived from early blastocysts, closely resemble EC cells and, on prolonged culture in vitro, acquire progressive genetic changes that show striking similarity to those seen in GCT (e.g. gain of material from chromosome 12). In parallel, these abnormal ES cells show enhanced population growth rates and plating efficiencies, indicative of their adaptation to culture conditions. Understanding the mechanisms that drive such culture adaptation of ES cells may also provide insights into the development and progression of GCT.

Female Fecundity Is Dependent on Substrate, Rather Than Male Abundance, in the Wind‐Pollinated, Dioecious Understory Palm Chamaedorea radicalis

ABSTRACTWe examined the impact of substrate, population density, and sexual composition on female reproductive success in harvested populations of the tropical understory palm Chamaedorea radicalis. Leaf harvest for the international floral greenery trade does not kill palms, but has been shown to reduce flowering frequency and has been projected to reduce population growth. Because C. radicalis is dioecious and wind‐pollinated, a reduction in flowering density from leaf harvest or other anthropogenic factors may lower pollen movement between flowering conspecifics and lower fruit production. Such reduced fecundity in harvested populations can contribute to an Allee effect, where the per capita rate of population growth declines at low density. We tested for these effects by sampling C. radicalis along transects in ten populations, spanning a range of densities, during May (flowering peak) and August (fruiting peak) 2002. We applied path analysis using structural equation modeling (SEM) to test a set of hypothesized relationships between substrate, female size, several population parameters, and female fecundity. SEM models revealed that female fecundity was not dependent on any measure of population density or sexual composition, providing no evidence for an Allee effect. The strongest model (R2= 92.9%) related C. radicalis reproductive output only to substrate, female size, flower number, and proportional fruit set. This model revealed that palms on rock outcrops were larger, produced more flowers, and had higher proportional fruit set than palms on the forest floor. This finding suggests that protection of females on outcrops should enhance population growth and viability.

Evidence for biochemical limitation of population growth and reproduction of the rotifer Keratella quadrata fed with freshwater protists

The biochemical factors that determine the food quality of protists for rotifers are poorly understood. We evaluated population growth rates and egg production of the rotifer Keratella quadrata fed with four protist species growing on either an algal or a bacterial diet. The cryptomonad Cryptomonas phaseolus, considered as a good quality prey, and assays without prey served as controls. Population growth rates and egg numbers of K. quadrata were correlated with single biochemical compounds (fatty acids, amino acids and sterols) of the protists. Feeding on the alga C. phaseolus or the algivorous ciliates resulted in enhanced population growth rates and high egg production by K. quadrata, whereas feeding on bacterivores supported only moderate egg production but no population growth. The rotifers' egg production was correlated with the protist biochemical composition, including polyunsaturated fatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanol and the amino acid leucine. No significant relationships were observed between population growth rates of the rotifers and the protists' biochemistry, suggesting that population growth and reproduction of K. quadrata may have different nutritional requirements. To our knowledge, this is one of the first studies to analyze a large variety of biochemical compounds to determine the food quality of protists for a zooplankton predator and the first study to analyze the biochemical quality of protists to a rotifer species.

Successful reintroductions of the endangered long-lived Sargent’s cherry palm, Pseudophoenix sargentii, in the Florida Keys

The 1991–1994 reintroductions of Florida endangered Pseudophoenix sargentii to 13 Florida Keys sites represent a rare example of a successful multi-agency long-term effort to conserve a long-lived palm. To assess reintroduction success, we compared population demographics with and without reintroduced plants and conducted population viability analyses. Since 1991, the wild population has increased 6.4-fold. Survival from 2000–2004 was 94%, growth was positive ( λ = 1.013), and there was no predicted extinction risk. Recent wild population growth is attributed to good seedling recruitment and removing the greatest threats. After 14 years, reintroductions had 43% survival, increased total plants in the wild by 27%, and expanded the species’ distribution. Reintroduced plants had faster maturation rates, improved population age structure, and enhanced population growth ( λ = 1.032). Success varied with transplant year, location, microsite, and original transplant size. Failures in 1991 and at some historic sites emphasize the need for a multi-year, multi-site approach to reintroductions to buffer against stochastic losses. Rockland hammocks and the tops of coastal berms had greatest plant growth and survival. Large transplants had the greatest survival. Because no reintroduced plants are reproductive, transitions between stages are extremely slow, and plants may require >30 years to mature, continued institutional dedication to long-term monitoring will be required to assess whether the populations are self-sustaining. Horticultural expertise and ex situ collections complimented support of land managing agencies for the species’ preservation. These first rare plant reintroductions to Florida State Parks opened avenues for more plant conservation efforts and public interpretation.

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g−1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.

Costs and benefits of Daphnia defense against Chaoborus in nature

To estimate costs and benefits of antipredator defenses in the Chaoborus–Daphnia system, we employed lake enclosures wherein controls (C) had no predators, the predation (P) treatment had freely swimming Chaoborus, and the kairomone (K) treatment predators were sequestered in a mesh tube apart from the Daphnia. Population growth (r) of two Daphnia pulex clones, one responsive (RC) and the other nonresponsive (NRC) to Chaoborus kairomone, was estimated for each predator treatment. Cost of defense was calculated as r(C,RC) – r(K,RC). Benefit was calculated as r(P,RC) – r(P,NRC). Antipredator defenses of Daphnia towards Chaoborus kairomone led to a 32% reduction in population growth in nature. The benefit of the defense, however, was a short-term 68% enhanced population growth by a responsive over a nonresponsive clone in the presence of the actual predation threat. The benefit of the defense exceeded the cost, but cost was nevertheless substantial. Our results verify that the in situ effects of Chaoborus on Daphnia involve direct and indirect impacts.

Stress signaling: coregulation of hemoglobin and male sex determination through a terpenoid signaling pathway in a crustacean

Environmental signals can activate neuro-endocrine cascades that regulate various physiological processes. In the present study, we demonstrate that two responses to environmental stress signaling in the crustacean Daphnia magna--hemoglobin accumulation and male offspring production--are co-elevated by the crustacean terpenoid hormone methyl farnesoate and several synthetic analogs. Potency of the hormones with respect to the induction of both hemoglobin and male offspring was highly correlated, suggesting that both processes are regulated by the same terpenoid signaling pathway. Six clones of the D. pulex/pulicaria species complex that were previously characterized as unable to produce male offspring and five clones that were capable of producing males were evaluated for both hemoglobin induction and male offspring production in response to methyl farnesoate. Four of the five male-producing clones produced both hemoglobin and male offspring in response to the hormone. Five of the six non-male-producing clones produced neither hemoglobin nor males in response to the hormone. These results provide additional evidence that both physiological processes are regulated by the same signaling pathway. Furthermore, the results indicate that the non-male-producing clones are largely defective in some methyl farnesoate signaling component, downstream from methyl farnesoate synthesis but upstream from the genes regulated by the hormone. A likely candidate for the site of the defect is the methyl farnesoate receptor. As a consequence of this defect, non-male-producing clones have lost their responsiveness to environmental signals that are transduced by this endocrine pathway. This defect in signaling would be likely to enhance population growth in stable environments due to the elimination of males from the population, assuming that other processes critical to population growth are not also compromised by this defect.

Bio-physical coupling in the formation of zooplankton and fish aggregations over abrupt topographies

Aggregations of zooplankton, micronekton and fish are frequently observed above seamounts, canyons and shelf breaks. The aggregations are produced by five different mechanisms, all driven by ocean currents. Upwelling is frequently generated when ambient currents impinge on abrupt topographies. Nevertheless, upwelling enhances local production of zooplankton and fish only over large topographies such as shelf breaks, where the residence time of the upwelled water is sufficiently long to allow enrichment in primary production to propagate up the food web and augment the growth of resident animals. Daily accumulations occur over topographies at shallow and intermediate depths when the topography blocks the morning descent of migrating zooplankton. This mechanism is common over seamounts. Two other mechanisms are driven by behavioral response to vertical currents when zooplankton swims vertically in order to maintain depth: accumulations by depth retention against upwelling are common at depths to which migrating zooplankton descend during the day, while depth retention against downwelling seems to operate only in the upper water column (e.g., along topographically generated fronts). The fifth mechanism is driven by the amplification of currents over abrupt topographies. Strong currents enhance population growth of resident animals by augmenting fluxes of suspended food. A feed-rest hypothesis is proposed to explain how site-attached planktivorous fish can benefit from strong currents at sites with ample quiescence shelters in which the fish rest during non-feeding intervals. Four of the above mechanisms generate “trophic focusing”, a process by which prey from immense volumes of flowing water is accumulated (or trapped) in a relatively small confined area. The ensuing subsidy of prey propagates up the food web, supporting aggregations of higher predators, such as fish, marine mammals and fishermen. Abrupt topographies can have an important role in determining trophic interactions in the marine realm.

Bio-physical coupling in the formation of zooplankton and fish aggregations over abrupt topographies

Aggregations of zooplankton, micronekton and fish are frequently observed above seamounts, canyons and shelf breaks. The aggregations are produced by five different mechanisms, all driven by ocean currents. Upwelling is frequently generated when ambient currents impinge on abrupt topographies. Nevertheless, upwelling enhances local production of zooplankton and fish only over large topographies such as shelf breaks, where the residence time of the upwelled water is sufficiently long to allow enrichment in primary production to propagate up the food web and augment the growth of resident animals. Daily accumulations occur over topographies at shallow and intermediate depths when the topography blocks the morning descent of migrating zooplankton. This mechanism is common over seamounts. Two other mechanisms are driven by behavioral response to vertical currents when zooplankton swims vertically in order to maintain depth: accumulations by depth retention against upwelling are common at depths to which migrating zooplankton descend during the day, while depth retention against downwelling seems to operate only in the upper water column (e.g., along topographically generated fronts). The fifth mechanism is driven by the amplification of currents over abrupt topographies. Strong currents enhance population growth of resident animals by augmenting fluxes of suspended food. A feed-rest hypothesis is proposed to explain how site-attached planktivorous fish can benefit from strong currents at sites with ample quiescence shelters in which the fish rest during non-feeding intervals. Four of the above mechanisms generate “trophic focusing”, a process by which prey from immense volumes of flowing water is accumulated (or trapped) in a relatively small confined area. The ensuing subsidy of prey propagates up the food web, supporting aggregations of higher predators, such as fish, marine mammals and fishermen. Abrupt topographies can have an important role in determining trophic interactions in the marine realm.

Novel genes continue to enhance population growth in adders ( Vipera berus)

In 1999, we published a report on how an introduction of 20 males into a severely inbred and isolated population of adders ( Vipera berus) halted its decline towards extinction, induced a profound change in population genetic variability, resulted in a dramatic increase in offspring viability and thus in a rapid increase in numbers. Since the publication of our paper we have received numerous inquiries as to whether we have continued to monitor this population, as this is the case we herein present population demographic data gathered between 1981 and 2003. The population has continued to increase in numbers and in 2003, we collected 39 adult male adders, more than at any time over the 23-year study. Our continued work, hence, lends strong support to the importance of novel genes in enhancing the viability of inbred populations and supports the importance of preserving genetic variability in order to maintain viable wild populations.

Effects of Experimental Sediment Resuspension on a Coastal Planktonic Microbial Food Web

Effects of sediment resuspension on microbial food web dynamics in the water column of microcosms consisting of sediment cores with the overlying water have been studied. These microcosms were obtained from Rassower Strom, coastal Southern Baltic, and incubated for 108h under in situ light and temperature conditions. They were resuspended with oscillating grids at intensities that reproduced the range of naturally occurring seston concentrations during resuspension events in these waters.After 12h of resuspension, the abundances of pico- and nanoautotrophs, rhizopods (naked and testate amoebae) and heterotrophic nanoplankton had increased and were significantly positively correlated to seston concentration, indicating that resuspension of benthic representatives directly affected their water column abundances. Similar trends were found for benthic diatoms and ciliates. In addition, resuspension enhanced population growth of pico- and nanoautotrophs and resuspended diatoms over the whole course of the experiment. This enhancement was independent of a direct transport of cells from the sediment and was most likely caused by reduced nitrogen limitation due to resuspension. The abundance of heterotrophic nanoplankton was tightly correlated to those of their main food organisms, pico- and nanoautotrophs and bacteria. It was also significantly increased by resuspension over the whole course of the experiment. This is consistent with decreased bottom-up control of heterotrophic nanoplankton due to resuspension.These results indicate that sediment resuspension may strongly affect the planktonic microbial food web of coastal waters, both by a direct transport of cells from the sediment into the water column and by indirect effects mediated by dissolved nutrients and trophic interactions.

Effects of resuspension and mixing on population dynamics and trophic interactions in a model benthic microbial food web

The effects of resuspension and mixing on the population dynamics and trophic inter- actions in a simple benthic microbial food web were studied during 3 plankton wheel experiments of 9 to 15 d duration. The food web consisted of a mixed bacterial assemblage, the heterotrophic flagel- late Bodo designis, the ciliate Euplotes balteatus and the rhizopod Vannella platypodia. The diatom Amphora coffeaeformis was included to maintain oxygen concentrations. B. designis, E. balteatus, V. platypodia and A. coffeaeformis are hereafter referred to by genus name alone. Population dynamics in 500 ml microcosms on rotating and non-rotating plankton wheels were compared. Final abun- dances of Amphora increased in suspension in all experiments. Resuspension increased initial growth rates and final abundances of Euplotes and Vannella during a whole-community experiment and a community subset experiment during which resuspension effects with and without the top predator Euplotes were compared. During the community subset experiment, suspended Bodo grew faster and reached higher final abundances than non-resuspended Bodo when Euplotes was absent, but experienced higher loss to Euplotes when the ciliate was present. Individual consumption rates of Bodo by Euplotes were estimated to be higher in suspension (5.5 vs 3.6 Bodo Euplotes -1 h -1 ), which could not be explained by higher abundance of Bodo alone. Similar but non-significant trends were found for bacteria. During a third experiment, Euplotes did not benefit from resuspension at low food concentrations. Our results show that resuspension and mixing can enhance population growth of autotrophic and heterotrophic protists, and that the trophic coupling between flagellates and ciliates, and possibly between other microbial food web components, can become closer in suspension when the food supply is sufficient. This suggests that subsidiary energy input by resuspension and mixing may increase the biomass turnover in benthic microbial food webs.

GABA enhances reproduction of the rotifer Brachionus plicatilis Muller: application to mass culture

Based on the results of individual and batch culture experiments in small volumes, we conducted experiments in larger volumes of 100 mL, 1 L and 10 L to determine: (1) at what phase of population growth would gamma-aminobutyric acid (GABA) treatment be most effective; and (2) whether GABA treatment of concentrated rotifers for several hours before mass culture would also be effective. GABA treatment of rotifer cultures at lag phase significantly enhanced population growth, whereas treatment at log phase had a lesser effect, and treatment at stationary phase had no effect. Addition of GABA to rotifer cultures every 2 days hastened population growth until day 8, but resulted in culture collapse thereafter. To reduce the cost of the use of GABA in mass cultures, GABA treatment may be conducted on concentrated rotifers (100 individuals mL−1) before mass culture. GABA treatment of concentrated rotifers for 24 h and 48 h before mass culture resulted in a significantly higher population density compared with their respective controls (no GABA treatment) on day 4, and on days 4 and 6 respectively.

Can artificial brood reduction assist the conservation of yellow‐eyed penguins (Megadyptes antipodes)?

AbstractThe yellow‐eyed penguin is a threatened species endemic to New Zealand and its subantarctic islands. Survival and fledging success vary from year to year partly in response to changing food availability. Mortality in ‘poor food’ years may potentially be reduced if adults are allowed to rear only one of their two chicks. Conservation managers have therefore deliberately reduced broods to one in some recent breeding seasons, but the merits of this policy are controversial. We investigated, by simulation modelling, the conditions under which a policy of artificial brood reduction would enhance population growth. Food supply may be difficult to predict from early‐season indicators so we also considered the effect of forecasting errors. Artificial brood reduction can increase the population growth rate, even in the presence of moderate forecasting errors, given a large enough cost of breeding. However, field studies suggest that the required cost of breeding is unrealistically large and the policy is unlikely to be a useful management tool for yellow‐eyed penguins.

Extinction dynamics, population growth and seed banks : An example using an age-structured annual.

The goal of this paper is to test theoretical predictions about the effects of seed banks on population dynamics and extinction rates in variable environments using simulations based on data from a natural population of the winter annual Collinsia verna. In the simulations, we varied the frequency of demographically good and bad years and the autocorrelation between conditions in consecutive years to examine the impact of seed dormancy on population growth rate, extinction rate and time to extinction. The existence of a seed bank enhanced population growth rates under all environmental regimes except when good years were very frequent, but this enhancement was minimal. In addition, the presence of the seed bank decreased the likelihood of extinctions and increased the time to extinction. The time to extinction was longest when the environmental conditions were most unpredictable.

Multiple mating and its effect on the reproductive success of female Epiphyas postvittana (Lepidoptera: Tortricidae)

Abstract. Multiple mating and its effect on reproductive performance of female Epiphyas postvittana (Walker) moths were studied under controlled conditions. The age at which the moths mated for the first time ranged from the first to the tenth day after emergence, but 71% of first matings were during the first 3 days. The majority (63%) of females had one or two spermatophores in the bursa copulatrix. Some (24%) were found with three to five spermatophores, whereas no successful mating occurred among 13% of individuals. The number of matings was partly dependent on the number of mates available to the female. Between the range of sex ratios of one male to one female and four males to one female maximal mating success occurred at the ratio of three males to one female. Virgin females were capable of egg‐laying, but mating stimulated and accelerated oviposition. Mated individuals laid twice as many eggs as unmated ones. The level of copulatory activity did not influence the longevity of females irrespective of the number of males available to them. Sex ratios with greater than one male to a female improved the reproductive success by marginally increasing fecundity and fertility. It is concluded that multiple mating would enhance population growth, and is of particular benefit to populations with a preponderance of females, as is known to occur naturally in this species.

Influence of host plant condition on population increase ofTetranychus telarius(Linnaeus) (Acarina: Tetranychidae)

Population growth studies were conducted with Tetranychus telarius (Linnaeus) on lima beans in the greenhouse. The capacity of populations to increase was studied in relation to host-plant nutrition, leaf age and plant age. Host-plant nutrition influenced population increase by affecting both longevity and fecundity. Adult female longevity was adversely affected when the mites were confined to plants grown in nutrient solutions deficient in phosphorus, nitrogen or potassium, rather than in full-nutrient solutions. In the host-plant nutrition studies, nitrogen deficiency was most detrimental for survival and phosphorus deficiency least detrimental. Age-specific fecundities were severely reduced when females were confined to plants grown in the deficient solutions. The phosphorus-deficient treatment gave the greatest reduction in total fecundity as well as maximum fecundity attained. This was true for both old and young leaves. The old leaves of the potassium-deficient treatment induced the second greatest reduction in both total and maximum fecundities. The nitrogen-deficient treatment was superior to either mentioned above but significantly inferior to the full-nutrient treatment. The young foliage in the potassium-deficient treatment was somewhat superior to the old foliage in the full-nutrient treatment in both total and maximum fecundities. Mite populations reared on host plants which received a full nutrient supply showed greater net reproduction rates and intrinsic rates of increase. Phosphorus-deficient plants were the most detrimental to these population attributes. The nitrogen-deficient plants and old leaves of the potassium-deficient plants gave intermediate results, while young leaves of the latter gave results comparable to those obtained on old leaves of the full-nutrient treatment. Although leaf age had no effect upon adult survival, young leaves, regardless of nutritional treatment, were more favorable for both total and maximum fecundities. Consequently, young leaves enhanced population growth in comparison to old leaves. Plant age was also found to affect population increase. Fecundity was reduced on older plants but survival was not affected.