Population ecology on an environmental gradient: Cakile edentula on a sand dune

Summary A naturally-occurring sand dune population of the annual plant Cakile edentula (Brassicaceae) was studied for two years. The plants grew along an environmental gradient stretching from open sand beach (seaward) to densely vegetated dunes (landward). Survivorship and reproductive output were estimated from plants in permanent quadrats. The dispersal of seeds was documented by sifting fruits from the sand substrate at different seasons.Seedlings germinate in April and May and begin flowering in July and August. They may continue to flower until October unless destroyed by autumn winds or heavy frost. Although seaward plants germinate approximately a month later than landward plants, they grow more rapidly and by September may be two orders of magnitude larger than landward plants (dry weight of vegetative parts 6.86±3.97 g compared to 0.029±0.006 g; dry weight of fruits 5.92±4.27 g compared to 0.016±0.005 g; mean with 95% CI). In both years, seedling survivorship and mature plant reproductive output declined significantly with distance landward. The large plants at the seaward end of the gradient produced most of the fruits (144 and 278 fruits per capita in 1975 and 1976 respectively) but a large proportion of these were moved landward by wind and waves during the winter. Thus, at the seaward end of the gradient, the main influx on individuals was from reproduction, and the main loss of individuals was from dispersal landward during the winter. The small plants at the landward end of the gradient produced few fruits (1.8 and 1.2 fruits per capita in 1975 and 1976 respectively), and mortality greatly exceeded this reproductive output. Thus, at the landward end of the gradient, the main loss of individuals was through seedling mortality, but this was balanced by a large annual influx of individuals from the seaward end of the gradient. Plants at the landward end of the gradient therefore exist only because of annual dispersal of seeds landward. Most seeds produced at the seaward end of the gradient disperse from an area of good habitat (high survivorship and high reproductive output) to an area of poor habitat (low survivorship and low reproductive output).     

Population dynamics in Oxalis acetosella: the significance of sexual reproduction in a clonal, cleistogamous forest herb

I studied demography in three populations of the clonal, cleistogamous herb Oxalis acetosella during three growing seasons, to assess the impact of seedling recruitment relative to ramet recruitment on its population dynamics. I followed each plant in permanent plots, and stage-classified it according to origin (sexually or vegetatively derived) and reproductive status. I calculated between-year transition probabilities, and sexual and vegetative fecundities, and used them in projection matrix analyses to simulate the development of populations and obtain the elasticity of each life-history event.
In general, non-flowering adults was the most numerous stage class and flowering new ramets the rarest, whereas the proportions of the other stage classes varied among populations and years. The negative correlation between plant density and survival was not very strong, and mortality seemed to be largely density-independent. Simulated population growth rates (λ) indicated that one of the populations is declining and the other two are growing, although slowly. The highest elasticities were usually those of stasis in adult plants, whereas elasticities of other matrix entries were low and highly variable. λ was positively correlated with the elasticities of seedling production and survival but negatively correlated with the elasticity of adult survival, indicating that seedling recruitment is more favourable for population growth than is adult stasis. There were no correlations between λ on the one hand and the elasticities of ramet production and survival on the other.
The results suggest that recruitment from seedlings is important for the growth and long-term maintenance of O. acetosella populations, as a complement to the rather low vegetative propagation. In this context cleistogamy is an adaptive strategy, maximizing total seed output.     

Population dynamics of Aurelia aurita (Scyphozoa) from an isolated brackish lake, with particular reference to sexual reproduction

The annual cycle of abundance, growth and sexual reproductionof the scyphomedusa Aurelia aurita was examined over a 12 monthperiod in an isolated, shallow, brackish, man-made body of water:Horsea Lake, England. Strobilation started in December and ephyraewere present through June in varying numbers, suggesting a 7month period of near-continuous recruitment. Horsea Lake hasa dense A.aurita population, with the peak abundance of 24.9m^–3 occurring in mid-May. Medusae (>10 mm) were presentin the water column from March through January. Maximum belldiameter of 105 mm was observed in late September. However,medusa growth was very poor (G ranged between –0.03 and0.07 day^–1 over a 12 month period), and monthly mean belldiameters were typically <50 mm. For most of the year, theabundant A.aurita population was most likely severely food limitedas a result of a numerically and species-poor mesozooplanktoncommunity dominated by the calanoid copepods Acartia margalefiand Acartia tonsa. Nevertheless, the very small A.aurita medusaewere still able to attain sexual maturity and reproduce. Ripefemale medusae carrying fertilized eggs and planula larvae werefirst observed in May and June, although the main period ofplanula larva production was September-December. In November,all females were ripe, the smallest being 19–20 mm belldiameter. At both Horsea Lake and a nearby productive estuary,Southampton Water, planula larva production increased linearlywith wet weight, although compared with the larger A.auritamedusae from Southampton Water, reproductive output at HorseaLake was low. Aurelia aurita in Horsea Lake showed plasticityin its population characteristics in response to environmentalconditions, with both fecundity and size to maturity varyingon a seasonal basis. Medusae appeared to partition the availablefood resources into either somatic growth when food was abundantor reproductive effort when food was scarce.     

Population dynamics in a cyclic environment: consequences of cyclic food abundance on tawny owl reproduction and survival

1. Understanding which factors regulate population dynamics may help us to understand how a population would respond to environmental change, and why some populations are declining.
2. In southern Finland, vole abundance shows a three-phased cycle of low, increase and decrease phases, but these have been fading out in recent years. During five such cycles (1981–1995), all tawny owls Strix aluco were censused in a 250-km² study area, and their reproduction and survival were monitored.
3. Males and females showed similar dynamics, but experienced breeders recruited more offspring and had higher survival than first breeders. Offspring recruitment, but not survival of breeding individuals varied in accordance with vole abundance.
4. The population's numerical response to prey abundance was primarily due to first-breeding individuals entering the population in the increase phase when immigration was the highest. First-breeding birds were younger, but experienced breeders were older in more favourable vole years.
5. A stage-specific matrix population model integrating survival and fecundity showed that, despite obvious variation in fecundity between vole cycle phases, this variation had limited importance for overall tawny owl population dynamics, but that the survival of experienced breeders during the low phase is most important for population growth.
6. Model and data agreed that the vole cycle drives the dynamics of this avian predator by limiting the recruitment of new breeders during the low phase. Population dynamics hence differ not only from the classic example of the species in a more temperate region in the UK where the number of territories is stable across years, but also from the dynamics of other avian vole predators in Fennoscandia where the recurring crash in vole abundance drastically lowers adult survival thereby creating vacancies.     

Population genetic structure of Titanotrichum oldhamii (Gesneriaceae), a subtropical bulbiliferous plant with mixed sexual and asexual reproduction

• Background and Aims Titanotrichum oldhamii is a monotypicgenus distributed in Taiwan, adjacent regions of China and theRyukyu Isands of Japan. Its conservation status is vulnerableas most populations are small and widely scattered. Titanotrichumhas a mixed system of reproduction with vegetative bulbils andseeds. The aim of this study was to understand the populationgenetic structure of Titanotrichum in relation to its specificreproductive behaviour and to determine possible implicationsfor conservation strategies. • Methods After an extensive inventory of most wild populationsof Titanotrichum in East Asia, samples from 25 populations withinits major distribution were carried out utilizing RAPD and inter-SSRmolecular fingerprinting analysis. • Key Results The findings support the conclusion thatmany populations reproduce predominantly asexually but thatsome genetic variation still exists within populations. However,significant amounts of variation exist between populations,perhaps reflecting population differentiation by drift. Thispartitioning of genetic diversity indicates that the level ofinter-population gene exchange is extremely low. These findingsare consistent with field observations of very limited seedproduction. The Chinese populations are similar to those ofNorthern Taiwan, while the Ryukyu populations fall within therange of variation of the north-central Taiwan populations.The Taiwanese populations are relatively variable and differentiationbetween north, east and south Taiwan is evident. • Conclusions The distribution of Titanotrichum seems tobe consistent with a former land connection between China, Taiwanand the Ryukyu Islands at a glacial maximum during the Quaternary,followed by progressive fragmentation of the populations. North-centralTaiwan is the centre of genetic diversity, possibly due to theproximity of the former land bridge between the regions, togetherwith the variety of suitable habitats in north Taiwan. The significanceof these findings for conservation is discussed.     

A comparison of seed banks across a sand dune successional gradient at Lake Michigan dunes (Indiana,USA)

In habitats where disturbance is frequent, seed banks are important for the regeneration of vegetation. Sand dune systems are dynamic habitats in which sand movement provides intermittent disturbance. As succession proceeds from bare sand to forest, the disturbance decreases. At Indiana Dunes National Lakeshore, we examined the seed banks of three habitat types across a successional gradient: foredunes, secondary dunes, and oak savanna. There were differences among the types of species that germinated from each of the habitats. The mean seed bank density increased across the successional gradient by habitat, from 376 to 433 to 968 seeds m⁻², but with foredune and secondary dune seed bank densities being significantly lower than the savanna seed bank density. The number of seeds germinated was significantly correlated with soil organic carbon, demonstrating for this primary successional sequence that seed density increases with stage and age. The seed bank had much lower species richness than that of the aboveground vegetation across all habitats. Among sites within a habitat type, the similarity of species germinated from the seed banks was very low, illustrating the variability of the seed bank even in similar habitat types. These results suggest that restoration of these habitats cannot rely on seed banks alone.     

Population dynamics of Lernaea cyprinacea (Crustacea: Copepoda) on four cyprinid species

A total of 217 Barbus graellsii, 54 Barbus haasi, 417 Cyprinus carpio and 85 Leuciscus cephalus captured at 9 sampling stations in the central basin of the Llobregat River (NE Spain) were studied for Lernaea cyprinacea parasitism. Prevalence (B. graellsii 39.2, B. haasi 31.5, C. carpio 8.4, L. cephalus 48.2), intensity of infection (B. graellsii range 1 to 39, mean 4.6; B. haasi range 1 to 7, mean 2.6; C. carpio range 1 to 16, mean 3.1; L. cephalus range 1 to 8, mean 2.4), and abundance (B. graellsii 1.80; B. haasi 0.83; C. carpio 0.26; L. cephalus 1.15) varied with the fish species studied. Correlations between abundance and host size were positive and significant for B. graellsii and L. cephalus. Correlation was positive but non-significant for B. haasi. Correlation between intensity and fish size was positive but not significant for B. graellsii, C. carpio, and L. cephalus. This is the first record of L. cyprinacea infecting B. graellsii and B. haasi.     

Population dynamics of Panonychus osmanthi (Acari: Tetranychidae) on two Osmanthus species

Panonychus osmanthi is a non-diapausing species of spider mite that superficially resembles P. citri. It infests Osmanthus species, which are evergreen roadside and garden trees. The population dynamics of P. osmanthi were studied on Osmanthus aurantiacus and O.×fortunei during a three-year period. Seasonal changes in P. osmanthi populations were fundamentally the same in each year, although their density differed greatly from year to year. TheP. osmanthi population was bimodal, with one peak in spring (May–June) and another in winter (November–January). Populations abruptly declined after the spring peak. Predators showed a delayed density-dependent response to changes in spider mites from spring to summer, whereas in autumn and winter, predators were few because they had entered diapause. To determine the effect of predators on the rapid decline of spider mites just after the spring peak, the predators were removed by treating the trees with a synthetic pyrethroid. As a result, spider mite density did not decline after the spring peak and remained at a high level during the June-August period when spider mite density is usually very low. This suggests that predators play an important role in the drastic decline of P. osmanthi density just after the spring peak. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

Self-compatibility in high arctic and alpine areas is regarded as an adaptation to low pollinator abundance. However, high genetic variability as a consequence of outcrossing is, with regard to population persistence, favorable in highly stochastic environments such as tundra habitats. To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predominant outcrosser Saxifraga oppositifolia in ten populations at two different elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limitation at high compared with low elevation thus could not be demonstrated in this experiment. Hand-crossings yielded equal mean proportion seed set at both elevations, and so did hand-selfings. This constant pattern of the breeding system in S. oppositifolia indicates selective factors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play important roles. However, female reproductive offer in terms of ovule number per flower did not change from low to high elevation. Since neither increased pollinator limitation nor increased seed set in selfed flowers was found at high compared with low elevation, the prerequisites for testing the hypothesis were not given. This study contradicts the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially outcrossing species like S. oppositifolia. Received: 28 April 1997 / Accepted: 20 October 1997     

Dideoxy fingerprinting (ddE): a rapid and efficient screen for the presence of mutations.

We describe dideoxy fingerprinting (ddF), a hybrid between dideoxy sequencing and SSCP that can detect the presence of single base and other sequence changes in PCR-amplified segments. As implemented herein, ddF involves a Sanger sequencing reaction with one dideoxynucleotide followed by nondenaturing gel electrophoresis. When ddF was used to examine segments of the human factor IX gene, 84 of 84 different mutations were detected with a very low rate of false positive signals. The approximate locations of the sequence changes could be determined from the ddF pattern and samples with different sequence changes had different fingerprints. In addition, large segments could be amplified and rapidly screened by ddF in multiple smaller subsegments. The patterns observed with ddF are instructive in that they suggest an inherent limitation in the detection of certain mutations by SSCP.     

Growth of a human carcinoma (HEp3) in nude mice: rapid and efficient metastasis

Our aim was to identify conditions which would permit the development of spontaneous metastasis of a human tumor in nude mice in a rapid and predictable manner and to explore ways to quantitate metastasis. Using a human squamous carcinoma--HEp3--we determined that invasiveness and metastasis were influenced by the host. HEp3 cells grew very rapidly and without a significant lag period in Balb/c and NIH(S)-II nude mice kept in aseptic conditions; a much longer lag period was observed in NIH-Swiss mice kept in conventional conditions. The HEp3 tumor displayed a highly invasive behavior in N-NIH(S)-II mice, in which it invaded the body wall, gaining access to the peritoneal cavity. Microinvasion was noted in all strains of mice inoculated with HEp3 cells. To prolong survival of the mice until metastases became evident, primary tumors were excised when they weighed 1-2 gm. N-NIH(S)-II and Balb/c nude mice, maintained in germ-free conditions, were most receptive to the development of metastases-lung metastases developed in 80% of these mice. Over 60% of all metastases were present within 4 weeks following the removal of the primary. Only 26% of tumor bearing NIH-Swiss developed lung metastases. Lung metastases were observed in some mice in the absence of local microinvasion, local tumor recurrence, and regardless of the presence of lymph node involvement. They were also noted in mice from which primary tumors were not excised. We compared three methods of lung metastasis detection: histology, detection of tumor cells in the cultures of lung minces, and the measurement of the levels of human urokinase-type plasminogen activator directly in the lysates of lungs. The detection of tumor cells in cultures of lung minces appeared to be the most sensitive of these methods and the determination of enzyme in lung lysates seemed to hold most promise for a quantitative approach. These data indicate that, the type of tumor, as well as the genetic background and the maintenance conditions of the host, have to be carefully selected to ensure the successful outcome of the particular tumor-host interaction being studied. Adherence to these guidelines allowed us, in the case of the HEp3 tumor, to develop a rapid, predictable, and efficient model in which to study factors affecting metastasis of this human tumor.     

Reproductive mechanisms and dynamics of habitat colonization in Microcerotermes biroi (Isoptera: Termitidae)

Abstract. 1. Previous studies on the arboreal termite community in coconut plantations of northern New Guinea showed that Microcerotermes biroi is the most abundant species, despite the fighting superiority of two competitor species Nasutitermes princeps and N.novarumhebridarum. In this study we tested the hypothesis that the success of M.biroi is due to its efficiency at colonizing new habitats following nuptial flights.
2. We demonstrated experimentally the ability of M.biroi colonies to replace their reproductives when removed, or to produce reproductives in satellite nests when isolated from the remainder of the colony. Replacement reproductives were always neotenics, derived from nymphs or workers.
3. Despite the ability of neotenics to differentiate within their home colonies, 84% of field colonies were headed by dealated imagos. This value constitutes a minimum estimate of the proportion of field colonies founded independently by imagos after the nuptial flight.
4. The monitoring of a young plantation during the first 31/2 years of its colonization by arboreal nesting termites revealed its invasion by M.biroi , which colonized 63% of the trees while neither Nasutitermes species appeared.
5. Our results demonstrate that M.biroi is actually a pioneer species, able to invade a new habitat by means of nuptial flights. They outline the importance of each species' reproductive strategy in shaping the arboreal-nesting termite community.     

Population dynamics, reproduction, and diet of the lesser long-nosed bat (Leptonycteris curasoae) in Jalisco, Mexico: implications for conservation

We estimate fluctuations in population size and sex ratio, documentbreeding behavior and reproduction, and determine the diet of a population ofthe lesser long-nosed bat, Leptonycteris curasoae, in anisland cave in Chamela Bay, Jalisco, Mexico, with monthly sampling during anannual cycle (October 1999–October 2000). Based on the area of thecave's ceiling and wall covered with L. curasoae inrelation to the potential roost area without them, in 1999 the abundanceincreased from 80% in October to 100% in November and December. In 2000 thepopulation decreased to 80% in January, 50% in February, 30% in March, 20% inApril, 10% in May, 5% in June and July, and less than 1% in August. Thepopulation rapidly increased to 60% in September and to 80% in October.Throughout the year there were significantly more males than females; however,there was significant heterogeneity over months. In September–Novemberthere were more females, but in December–August there were more malespresent. The majority of pregnant and lactating females were observed fromDecember to March and in July, while males were reproductive fromSeptember–January and in May–June. Breeding activity was observed inthe cave in November–December. Twenty-six species of plants were consumedduring the year, based on pollen identification from fecal samples. Bombacaceousspecies were the most important component of the diet from January to May andCactaceae were most important in June–September. Peak abundance and reproductive activitycoincided with peak flower resource availability, which occurred between Octoberand January and in June–July. The year-round presence and reproductiveactivity of L. curasoae at this site throughout the yeardemonstrate that many individuals are annual residents in this area and indicatethe importance of this roosting site. In order to develop a successfulconservation program for L. curasoae, in addition toprotecting migratory corridors and northern maternity roosts, it is equallyimportant to identify and protect areas that function as breeding colonies andyear-round sanctuaries for resident populations in the south.     

Evidence for introgression in azaleas (Rhododendron; Ericaceae): Chloroplast DNA and morphological variation in a hybrid swarm on Stone Mountain,Georgia

Hybridization and introgression have been suggested by previous workers as important factors in the evolution of the southeastern azaleas. However, because of high levels of intraspecific variation and lack of intensive sampling, documentation of this phenomenon has been difficult. We investigated a population of azaleas on Stone Mountain, Georgia, composed of Rhododendron flammeum, R. canescens, and their interspecific hybrids. After analysis of chloroplast DNA, we found evidence of extensive localized cytoplasmic introgression into R. flammeum. Many individuals in the sample population that are morphologically indistinguishable from R. flammeum possess the chloroplast genome of R. canescens. This suggests that some of the observed variation in some populations of R. flammeum may be due to past introgression from R. canescens.     

Population dynamics ofMalacosoma neustria testacea (Lepidoptera: Lasiocampidae): Stabilizing process in a field population

Stabilizing mechanism in population of Malacosoma neustria testacea was investigated in a central part of Japan based on eight year survey. Population fluctuation in each developmental stage in the experimental field was rather small, i.e., 5.9 times in egg and 85.0 times in female adult. Pupal weight negatively correlated with the densities of 5th instar larvae and prepupae (cocoons) and correlation coefficient was highly significant in females. Population density was stabilized by density-dependent dispersal of female moths in preovipositional period. Comparison between fecundity of emerged moths and that of actually oviposited ones in the experimental field suggested that density-dependent dispersal took place as the result of density-dependent size variation, i.e., small-sized female months have higher flying ability. This hypothesis was supported by the experiment in which flying ability of newly emerged female moth was measured. Similar stabilizing mechanism is expected to occur in semelparous or pro-ovigenic insects.