Reproduction and perennation of Chondrilla juncea L. (skeleton weed) in the Western Australian wheatbelt

Survivorship of first-year plants of Chondrilla juncea was considerably higher on a deep yellow sand than on a yellow duplex soil. Mortality on the latter soil was associated with drying of the Al horizon during early summer. No plants produced seeds in their first year of growth on the duplex soil. However, during both years of the study, first-year individuals of both forms of C. juncea produced viable seeds when growing on the deep sand. Higher population growth rates could be expected on deep sands, owing to a combination of earlier reproduction and greater survivorship through the first summer. It is suggested that whilst the entire wheatbelt might be climatically suitable for C. juncea, the invasive potential of this weed might be restricted by edaphic characteristics over large areas.     

Populational dynamics of three annual species of alpine plants in the Rocky Mountains

Summary Survivorship, fecundity, and seed bank size were measured over two years in alpine populations of the annuals, Koenigia islandica, Polygonum confertiflorum, and P. douglasii. The major loss of individuals from all populations occurred between seed dispersal and germination. Survival of vegetative plants to maturity was high in all species, usually above 80%, and the average number of seeds produced per plant was less than 10 in all species in both years. Seed banks existed for all three species but were small since more than 80% of the viable seeds in the soil germinated in the spring. Both survival and fecundity were negatively correlated with density in P. confertiflorum. These patterns of population dynamics are similar to those found in many annuals of temperate environments.     

Survivorship and fecundity of the radiate and non-radiate morphs of Groundsel,Senecio vulgaris L., raised in pure stand and mixture

British populations of Senecio vulgaris frequently contain two common capitulum morphs (radiate and non-radiate) and one rare intermediate morph. The radiate morph shows a higher maternal rate of intermorph outcrossing than the non-radiate morph and due to the ‘cost of outcrossing’ should decline in frequency, ultimately to be lost from a population. To determine whether the radiate morph exhibits some inherent advantage in fitness to offset the ‘cost of outcrossing’, a comparison was made of the survivorship and fecundity of the radiate and non-radiate morphs raised in pure stands and 1 : 1 mixture at three planting dates (autumn 1983, and spring and autumn 1984). Plants in stands established in spring 1984 were harvested in late August 1984, while plants in stands established in autumn overwintered before being harvested the following summer. In spring planted stands, the two morphs exhibited equivalent survivorships, while the fecundity of the non-radiate morph tended to be greater than the radiate morph. In autumn planted stands, survivorship and Net Reproductive Output (survivorship × fecundity) of the non-radiate morph was greater than that of the radiate morph in mixture, and also in pure stands established in 1983. In no instance was the Net Reproductive Output of either morph significantly greater in mixture than in pure stand. Density had a contrasting effect on morph survivorship and fecundity in the spring and autumn 1984 planted stands. Whereas, in spring stands, fecundity was subject to compensating density dependent regulation while survivorship was density independent, the opposite trend was observed in autumn planted stands. It is concluded that under the conditions of the experiments, the radiate morph exhibited no fitness advantage which might offset the inherent disadvantage it suffers in natural polymorphic populations due to the ‘cost of outcrossing’.     

VARIABLE SELECTION ON THE TIMING OF GERMINATION IN COLLINSIA VERNA (SCROPHULARIACEAE)

Natural selection on the timing of seed germination was investigated in a natural population of the winter annual Collinsia verna (Scrophulariaceae) for two years. The goal was to quantify 1) the importance of the timing of seed germination to life history evolution in this population and 2) variation in selection in time and space. During fall germination, seedlings were assigned to cohorts on the basis of their dates of germination. Growth, survivorship, and reproduction were censused throughout both years. Selection on the timing of germination was quantified using linear and quadratic regressions during three ecologically important periods in the life cycle, using the techniques of Lande and Arnold (1983) and Arnold and Wade (1984a, 1984b). Comparisons were made between years and on two spatial scales within years. Overall, selection favored early-germinating plants in the first year. The primary determinant of the relationship of the timing of germination to fitness was fecundity selection, rather than viability selection on seedlings. Fecundity selection was respondible for from 54% to 80% of the change in the mean time of germination. Significant disruptive selection characterized the second field season, again mediated mainly through fecundity selection. There was also temporal and spatial heterogeneity in selection on this character. Transects and quadrats differed significantly in the direction and magnitude of natural selection. In addition, the direction of selection changed between episodes for the transects. The results illustrate the importance of the timing of germination to life-history evolution in this annual plant and the complex action of natural selection on this character.     

EFFECTS OF HYBRIDIZATION AND INBREEDING ON FITNESS IN PHLOX

The effects of interpopulation hybridization, and self-fertilization and immigration on fitness in Phlox drummondii were analyzed in 5 natural field sites. Germination rates, survivorship to flowering, fecundity, and net reproductive rates (R_o) were determined for planted populations of natives, hybrids, aliens, and the products of one and two generations of self-fertilization. At all sites, seed germination was 36% for natives, 35% for hybrids and 28% for aliens. Survivorship for natives was 50%, compared to 49% for hybrids and 41% for aliens. The mean fecundity was 34, 35, and 39 seeds per plant for natives, hybrids, and aliens, respectively. The R_o of natives averaged 5.2 vs. 6.4 for hybrids and 4.2 aliens. At all the sites, germination averaged 25% for self-1 plants and 19% for self-2 plants compared to 24% for open-pollinated controls. Survivorship progressed from 50% in the controls to 46% in self-1 and 44% in self-2 plants. Seed production per plant averaged 35% in the control vs. 32% in the self-1 and 25% in the self-2 plants. The mean R_o of the control was 3.8 vs. 3.3 in the self-1 and 2.14 in the self-2 plants. Our results demonstrate that the genetic variable may have a substantial effect on plant fitness in the field.     

SEED BANK,SURVIVORSHIP, AND SIZE DISTRIBUTION OF A NEBRASKA POPULATION OF IMPATIENS CAPENSIS (BALSAMINACEAE)

The growth and survivorship of a population of Impatiens capensis were followed in eastern Nebraska. Soil cores were taken within the population to estimate the size of the seed bank. The number of seeds in the soil remained relatively constant from April through October, with a mean of 200 seeds/m². The seed bank size corresponded to observed seedling densities in the following years. Twenty permanent plots were established within the population and censused for three years, 1983–1985. Seedling recruitment to the population was similar in 1983 and 1984 but 30% higher in 1985. The seedling densities of plots were uncorrelated from year to year. Survivorship in 1983 followed a Deevey Type I pattern. Almost one-third of the population survived until chasmogamous reproduction. Plant size (stem height) was measured monthly on all individuals during 1983. The frequency distribution of stem height was positively skewed as long as growth was exponential. When the growth rate slowed, the distribution became more normal.     

Consequences of mite feeding injury to beans on the fecundity and survivorship of the two-spotted spider mite (Acari: Tetranychidae)

Fecundity and survival of the two-spotted spider mite,Tetranychus urticae Koch, were examined on bean (Phaseolus vulgaris L.) plants that had been subjected to mite feeding injury in the laboratory. Different numbers ofT. urticae were restricted on the first two leaves of young bean plants, and spider-mite fecundity and survivorship was assayed on the third leaf. Each plant received four recently enclosed females, one female from each of four mite lineages. Using changes in the ratio of root mass to shoot mass of bean plants as a continuous measure of plant stress from spider-mite feeding, fecundity was positively related to stress for three out of four experiments. In two out of four experiments, survival of females was also positively related to stress, but reached an asymptote at slight or moderate stress levels. No evidence for induced resistance in beans was found. Mite lineage and the interaction between lineage and stress affected female survival but not fecundity. The implications of these results for understanding spider-mite outbreaks are discussed.     

Effects of reproductive mode on demography and life history in Arabis fecunda (Brassicaceae)

Life history theory predicts that trade-offs between growth and reproduction should be dictated by a population's mortality schedule. We tested this prediction with Arabis fecunda, a short-lived perennial that occurs in many different habitats in southwest Montana. Individuals produce either or both axillary or terminal inflorescences. Axillary-flowering plants are usually iteroparous and have smaller reproductive bouts, while terminal-flowering plants have larger reproductive bouts, and tend to be semelparous. We recorded size and fecundity of A. fecunda individuals from 1989 to 1993 in three different habitats. There was great variation in demographic and life history traits among the populations. A wide range of life history strategies among populations of A. fecunda is achieved through different proportions of axillary- and terminal-flowering plants. Arabis fecunda demonstrated a lower recruitment rate, higher survivorship, slower growth, and lower annual fecundity at the low-elevation site compared to the high-elevation site. At the low-elevation site population size was more stable, and elasticity analysis of matrix projection models indicated that adult survivorship was the most important demographic parameter contributing to population growth. This association of life history characters conforms to theoretical predictions.     

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).     

THE GENETIC BASIS OF LIFE-HISTORY CHARACTERS IN A POLYCHAETE EXHIBITING PLANKTOTROPHY AND LECITHOTROPHY

The polychaete Streblospio benedicti is unusual in that several field populations consist of individuals that exhibit either planktotrophic or lecithotrophic larval development. Planktotrophy in this species involves production of many small ova that develop into feeding larvae with a two- to three-week planktonic period. Lecithotrophy involves production of fewer, larger ova that develop into nonfeeding larvae that are brooded longer and have a brief planktonic stage. Reciprocal matings were performed to investigate genetic variance components and the correlation structure of life-history traits associated with planktotrophy and lecithotrophy. Our objective was to better understand persistence of this developmental dichotomy in Streblospio benedicti, and among marine invertebrates in general. Substantial additive genetic variation (75–98% of total) was detected for the following characters at first reproduction: female length; position of the first gametogenic setiger and first brood pouch; ovum diameter; three traits related to fecundity (numbers of ova per ovary, larvae per brood pouch, and larvae per brood); median larval planktonic period and the presence of larval swimming setae; but not for total number of brood pouches; larval length; larval feeding; and larval survivorship. Based on the unusual geographic distribution of development modes in this species, we hypothesize that the developmental traits have evolved in allopatry and have only recently come into contact in North Carolina. The high additive contribution to variance observed for many traits may be inflated due to (a) nonrandom breeding in nature, and (b) examination of only one component of an age-structured population at one time. Nuclear interaction variance and maternal variance accounted for 84% of the total variation in larval survivorship. This observation supports other empirical studies and theoretical predictions that nonadditive components of variance will increase in importance in individual traits that are most closely tied to fitness. A network of life-history trait correlations was observed that defines distinct planktotrophic and lecithotrophic trait complexes. Negative genetic correlations were present between fecundity and egg size, between fecundity and position of the first gametes, and between larval survivorship and median planktonic period. Positive genetic correlations were detected between fecundity and female size at first reproduction and between planktonic period and the presence of swimming setae. Intergenerational product-moment correlations were negative for larval length and fecundity, planktonic period and egg size, female size and larval survivorship, and fecundity and larval survivorship. If the genetic correlation structure observed in the laboratory persists in the field, it may constrain responses of individual characters to directional selection, and indirectly perpetuate the dichotomies associated with planktotrophy and lecithotrophy.     

Is the insect or the plant the driving force in the cinnabar moth — Tansy ragwort system?

Summary The interactions between cinnabar moth, Tyria jacobaeae L. and its food plant, tansy ragwort, Senecio jacobaea L. were studied for 4 to 6 years at 9 sites in North America to assess if the herbivore drove the dynamics of the plants or if the plants determined the dynamics of the insects. Cinnabar moth larval density is not closely related to changes in the size and spacing of tansy ragwort plants although high densities of larvae were associated with a high proportion of rosettes the next year. Fluctuating moth populations live in areas where rosettes are small, closely spaced and numerous compared to flowering stem plants. This situation is also associated with greater larval dispersal which may lead to over exploitation of the food supply. The coefficient of variation of both the size of rosettes and the distance between clumps of plants is associated with the coefficient of variation in moth density. This suggests that the plants may be driving the dynamics of the insect populations. The size of the moth egg batches is correlated to the size of the flowering stem plants in the previous year, indicating an adjustment between moth fecundity and food plant size. The conclusion is that environmental factors such as weather and soil type determine to a large degree the characteristics and variation in the plant populations and this in turn controls the dynamics of the insects. The relation of this situation to the biological control of weeds is discussed.     

Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Survivorship throughout the life cycle,a synthesis

Survivorship in Acacia suaveolens was assessed through seedling and adult stages. Moisture stress was found to be the critical factor limiting early seedling survival. Both seedling and adult populations were characterized by periods of low mortality interspersed with pulses of high mortality. A composite survivorship curve for A. suaveolens based on nine sites predicts that some 20–25 years after afire, established plants should disappear from the above-ground flora if another fire does not occur. Fecundity and survivorship data were used to estimate the flux of seed in the soil over time in a hypothetical A. suaveolens population. From this it was predicted that, following establishment of plants after a fire, the seed-bank would rapidly reach a maximum after 6 years and thereafter slowly decline, until after 60 years there would be only as many seeds as there were original parental plants. The situation would vary with predispersal seed predation, seed predation on the soil surface, seed dispersal by ants to ‘unsafe sites’ and the size of the initial seed-bank prior to establishment. Only after a very long inter-fire period would A. suaveolens be eliminated from a site. Elimination of the species is also possible under very frequent fires. A 2–5 year fire-free period is needed for plants to reach maturity and another 6 years are needed to maximize seed input into the soil seed-bank. In addition, seedling recruitment following cool burns is low to non-existent as dormancy is not broken for most seeds in the soil during such burns.     

The cost of reproduction in Senecio keniodendron,a giant rosette species of Mt. Kenya

Summary A three year study of Senecio keniodendron (Compositae), a giant rosette species of the alpine zone of Mt. Kenya, demonstrated that individuals which reproduce are more likely to die, and less likely to reproduce in the future if they do survive, than are vegetative individuals of the same size. However, if an individual reproduces, survives and reproduces again, then it produces more seeds during the second reproductive episode than does a plant of the same height reproducing for the first time, because reproduction is followed by production of lateral rosettes, increasing the number of potentially-reproductive rosettes per plant.Slow-growing rosettes are less likely to reproduce than fast-growing rosettes. For rosettes which do reproduce, rosette size and rate of leaf production, measured before reproduction begins, are good predictors of fecundity.     

VARIATION IN POPULATION GROWTH RATE IN THE WOODLAND ANNUAL IMPATIENS PALLIDA (BALSAMINACEAE)

Variation in population growth rate over environmental gradients was determined for the annual Impatiens pallida (Balsaminaceae), by monitoring age- and size-specific survivorship and fecundity in five populations of this summer-flowering and primarily woodland species. All I. pallida seedlings emerged within a few days of each other and a size hierarchy was established within a month, and remained unchanged thereafter. Light and initial seedling density together explained 67% of the variance in mean adult plant size. As a result of differences among populations in the amount of disturbance, survivorship to the time of floral bud production ranged from 39% to 93% of the initial seedling total. Survivorship during the reproductive period was most affected by plant size and soil moisture. For plants surviving to flowering time, the probability of setting seed, the type of seed and the number of seeds produced per plant were significantly positively correlated with plant size. More than 82% of all seeds were produced by plants greater than or 1 m tall. Cleistogamy was the major form of reproduction in 4 out of 5 populations monitored. The net reproductive rate (R₀) differed greatly among populations, ranging from 1.23 to 16.27. Population growth rate increased with increasing resource availability and decreasing disturbance during the growing season. The magnitude of R₀ depended primarily on 1) timing, intensity and frequency of disturbance, 2) length of the reproductive period, and 3) population size structure.     

Anopheles subpictus Grassi: Observations on survivorship and population size using mark-release-recapture and dissection methods

A mark-release-recapture experiment to estimate population survivorship and absolute size was performed with wild-caught An. subpictus adults at the village of Khano-Harni, Lahore District, Punjab Province, Pakistan during September 1978, the end of the monsoon rainy season, when temporal population abundance was maximized. Daily survival rate estimated from the recapture sequence of marked adults was low, males=0.192 and females=0.343. Survivorship for females estimated by several vertical age-grading procedures ranged from 0.347 to 0.628. Both stage- and age-specific life tables were calculated from vertical age-grading data determined by the dilatation method. Female and male population size was estimated byBailey 's modification of theLincoln Index and was found to average 4478.4 and 6106.8, respectively. The bionomics, survivorship and population size of An. subpictus in the Lahore are indicated that this species was probably not important in the transmission of human malaria.     

Restoration of a Forest Understory After the Removal of an Invasive Shrub, Amur Honeysuckle (Lonicera maackii)

The recruitment of native seedlings is often reduced in areas where the invasive Amur honeysuckle (Lonicera maackii) is abundant. To address this recruitment problem, we evaluated the effectiveness of L. maackii eradication methods and restoration efforts using seedlings of six native tree species planted within eradication and unmanipulated (control) plots. Two eradication methods using glyphosate herbicide were evaluated: cut and paint and stem injection with an EZ‐Ject lance. Lonicera maackii density and biomass as well as microenvironmental characteristics were measured to study their effects on seedling growth and survivorship. Mean biomass of Amur honeysuckle was 361 ± 69 kg/ha, and density was 21,380 ± 3,171 plants/ha. Both eradication treatments were effective in killing L. maackii (≥ 94%). The injection treatment was most effective on large L. maackii individuals (>1.5 cm diameter), was 43% faster to apply than cutting and painting and less fatiguing for the operator, decreased operator exposure to herbicide, and minimized impact to nontarget vegetation. Deer browse tree protectors were used on half of the seedlings, but did not affect survivorship or growth. After 3 years, survival of native seedlings was significantly less where L. maackii was left intact (32 ± 3%) compared with the eradication plots (p < 0.002). Seedling survival was significantly different between cut (51 ± 3%) and injected (45 ± 3%) plots. Species had different final percent survival and rates of mortality. Species survival differed greatly by species (in descending order): Fraxinus pennsylvanica > Quercus muehlenbergii ≥ Prunus serotina≥ Juglans nigra > Cercis canadensis > Cornus florida. Survivorship and growth of native seedlings was affected by a severe first‐year drought and by site location. One site exhibited greater spring soil moisture, pH, percent open canopy, and had greater survivorship relative to the other site (55 ± 2 vs. 30 ± 2%). Overall, both L. maackii eradication methods were successful, but restorationists should be aware of the potential for differential survivorship of native seedlings depending on species identity and microenvironmental conditions.     

Larval performance of two leafroller species on known and potential hosts

Two univoltine leafroller species,Archips argyrospila Walker andArchips rosana L., were reared from eclosion to adulthood on known or potential host plants. Both species were able to complete development on eight of the ten plants tested, the exceptions being apricot and peach. Rates of development differed among hosts, and indicated a range of host suitability. Differences in female pupal weights were apparent following the different rearing treatments, but these did not correlate with subsequent female fecundity. When development was compared between the two leafroller species,A. rosana had higher survivorship, more rapid development and higher female fecundity than didA. argyrospila, regardless of host. These findings indicate thatA. rosana may successfully exploit a wider host range than doesA. argyrospila. Comparisons of survivorship and developmental rates among hosts for each species of leafroller suggest that both species may have the potential for host-range expansion.     

Size-specific reproductive parameters in red algae: a comparative analysis for two sympatric species from Central Chile

Ahnfeltia durvillaei and Gymnogongrus furcellatus, two intertidal red algae from central Chile, often grow sympatrically and their gametophytic phases are dominant in the field, but recruitment is low. A shortterm comparative analysis of the reproductive behavior of their gametophytes was carried out to determine quantitative levels of reproduction and their degree of functional similarity. Size, rather than age, was evaluated as predictor of fecundity. Both species showed the same qualitative patterns, although maintaining quantitative differences. Reproductive plants were recorded throughout the size range, and fecundity increased continuously and directly with size. A size-independent threshold in reproductive effort was found for each species, and the variance of effort values decreased inversely with size. Vertical, size-specific life tables revealed low dependence of survivorship with size, and reproductive values exhibited a maximum at the same size class for both species. This last pattern, and a negative relationship found between fecundity and survivorship, suggest the existence of costs or trade-offs operating at the phenotypic level, but they would not be supported in a selective context.     

Association with mature plants protects seedlings from predation in an arid grassland shrub,Gutierrezia microcephala

Summary Survivorship of Gutierrezia microcephala (Compositae) seedlings was studied in an undisturbed arid grassland and in experimental plots where various components of the natural vegetation were removed following seed germination. The major causes of seedling mortality were herbivore damage from the specialist grasshopper, Hesperotettix viridis, and drought stress associated with competition from established plants. The relative intensity of these mortality factors varied strongly with seedling size. Large seedlings had higher overall survivorship but were most likely to be killed by defoliation; most small seedlings died of drought stress.In plots where all perennial grasses were removed (leaving established G. microcephala plants), seedling survivorship was 5 times greater than in undisturbed vegetation. Surviorship in plots where both grasses and mature G. microcephala were removed was slightly poorer than in undisturbed vegetation due to a large increase in mortality from defoliation. From May–August, when the herbivore H. viridis was abundant, seedling survivorship was better in the immediate vicinity of mature conspecific plants than in plots lacking mature G. microcephala, both in the presence and absence of perennial grasses. These results provide a counter example to theories predicting that the impact of specialist herbivores on seedling recruitment is greatest in the vicinity of parent plants.     

Increased death of new leaves of coastal banksia (Banksia integrifolia L.f) around ocean sewage outfall sites

Abstract Salt‐spray damage to coastal vegetation is a worldwide phenomenon. Worsening of such damage by surfactants present in sewage disposed of at sea has been observed in Europe and on trees planted around metropolitan beaches in Australia. Whether adverse impacts of sewage occur on the more widespread native vegetation around non‐metropolitan outfalls in Australia is unknown. The present study investigated the survivorship of existing leaves, and of new shoots and leaves, on Banksia integrifolia around three outfall and three non‐outfall sites in spring/summer of 1997‐1998. Survivorship of existing leaves was almost identical at outfall and non‐outfall sites. When plants initiated new shoots over the spring/summer, plants at outfall sites initiated more than plants at non‐outfall sites; but the proportion of these new shoots lost subsequently differed little between outfall and non‐outfall sites. Survivorship of newly initiated leaves on the new shoots differed between outfall and non‐outfall sites, with 65% of the new leaves on average being lost at outfall sites, and 47% being lost at non‐outfall sites by the end of summer. The probability of observing a difference of this magnitude ranged between P= 0.057 and 0.11 (February‐April 1998). This is the first report of a possible adverse impact on an Australian native plant from sewage discharged to the ocean.