Annual Populations Research Articles

Land use effects on “spring annual” herbs in rare non‐forest ecosystems of New Zealand

We examined trends over six consecutive spring seasons in three “spring annual” herbs, Myosurus minimus subsp. novae‐zelandiae (Ranunculaceae), Ceratocephalapungens (Ranunculaceae), and Myosotis pygmaea var. minutiflora (Boraginaceae), in terms of their habitats and site land uses in several rare, non‐forest ecosystems. On dry hillslopes, saline soils, and turfs of ephemeral wetlands (including coastal turfs), Ceratocephala and Myosotis declined where sites were managed for conservation by removal of mammalian herbivores. Some populations of Myosurus declined only marginally or remained stable despite high cover of ruderal and weedy, herbaceous exotic plants. Loss of monitored populations during the study was offset by discoveries of additional populations of all three taxa. Statistical models of population trends suggest that in some instances spring annual populations benefit from farm animal and rabbit (Oryctolagus cuniculus cuniculus) disturbance of their habitats perhaps by 1) suppressing transitions to taller vegetation and retarding competition of invasive exotic plants and 2) maintaining nutrient supplementation from faeces lost with the extinction of ground‐dwelling birds. Frequent seeds of all three taxa preserved in moa coprolites from several sites in Central Otago point to 1) the herbs’ probable non‐rarity in pre‐human times and 2) dispersal mutualisms and mediation of community ground cover by extinct ratites.

Managing arable weeds for biodiversity

As a result of the recent intensification of crop production, the abundance and diversity of UK arable weeds adapted to cultivated land have declined, with an associated reduction in farmland birds. A number of questions need to be addressed when considering how these declines can be reversed. Firstly, can the delivery of crop production and biodiversity be reconciled by spatially separating cropping from designated wildlife areas? A number of subsidised environmental schemes in the UK take this approach and are focused on establishing vegetation cover on uncropped land. However, because of the lack of regular disturbance in these habitats, they are dominated by perennials and they therefore have limited potential for promoting the recovery of annual weed populations. A number of farmland bird species also rely on the provision of resources in field centres, and it is therefore likely that the recovery of their populations will rely on weed management options targeted at the cropped areas of the field. This raises two further questions. Firstly, is it possible to identify beneficial weed species that are relatively poor competitors with the crop and also have biodiversity value? Secondly, are the tools available to manage these species at acceptable levels while controlling pernicious weeds? A number of approaches are being employed to answer these questions, including predicting yield loss from weed competition models and exploiting herbicide selectivity. The further development of these tools is crucial if farmer opposition to managing weeds in crops is to be overcome.

HUMAN EASTERN EQUINE ENCEPHALITIS IN MASSACHUSETTS: PREDICTIVE INDICATORS FROM MOSQUITOES COLLECTED AT 10 LONG-TERM TRAP SITES, 1979–2004

Human eastern equine encephalitis (EEE) is a life-threatening mosquito-borne disease. To determine whether mosquito abundance and EEE virus infection rates are associated with human EEE disease, we evaluated retrospectively a total of 592,637 mosquitoes and onset dates for 20 confirmed human cases over 26 years in Massachusetts. Annual Culiseta melanura populations at 10 defined sites decreased over the study period (P = 0.002). Weekly infection rates and number of infected Culiseta melanura captured per trap night were positively associated EEE cases (P < 0.023 and P < 0.001, respectively), whereas abundance was not (P = 0.077). The infection rate for Culiseta melanura of 0.39 per 1,000 tested mosquitoes identified human cases with a sensitivity of 0.87, a specificity of 0.82, a positive predictive value of 0.14, and a negative predictive value of 0.995. Timely mosquito testing and infection rate calculation are critical for disease risk estimation and outbreak control efforts.

Population dynamics of annuals in perennial grassland controlled by ants and environmental stochasticity

AbstractQuestions:In a system of five annual plant species restricted to nest‐mounds of the antLasiusflavusin a perennial grassland: 1. Are the population dynamics influenced by ant disturbance? 2. Is the survival of the annuals at the scale of the whole grassland possible under the observed conditions of disturbance dynamics? 3. Which phases in the annuals’ life cycle and patch types contribute most to population growth?Location:Borec hill, northern Czechia, 50°31’ N, 13°59’ E, 446 m a.s.l.Methods:Local population dynamics of the annuals were analysed separately for five patch types that differed in the proportion of bare soil. Vitality rates were assessed directly in the field, but also in a garden experiment, during 2000–2001 and 2001–2002. Population dynamics at the scale of the whole grassland was analysed with a megamatrix approach, combining patch dynamics of the nest‐mounds with patch‐specific population dynamics. Contributions of different phases and patch types to growth rate were estimated by elasticity analysis.Results:Nest‐mounds differed in the percentage of bare soil. Increasing moss cover significantly reduced germination and seed production of all studied annuals and decreased their population growth rates (λ). Although successional processes dominated over ant disturbance, populations of all species could survive well (λ≫ 1) in the grassland according to the 2000–2001 megamatrix dynamics. Based on the dynamics from the following period, two species would not survive in a long‐term perspective due to random environmental variation. Whereas the A‐A transition (adult plants originating from adults of the previous year) had the highest elasticity under open conditions and ‘good period’ demography, the importance of persistent seeds increased under reverse conditions. This, however, differed among species.Conclusions:Ant‐disturbance was shown to be critical for the population survival of five annual species in the studied grassland. The fate of the annual populations in the grassland system also depends on random environmental variation, which may override the effect of ant activity.

Population dynamics of annuals in perennial grassland controlled by ants and environmental stochasticity

Questions: In a system of five annual plant species restricted to nest-mounds of the ant Lasius flavus in a perennial grassland: 1. Are the population dynamics influenced by ant disturbance? 2. Is the survival of the annuals at the scale of the whole grassland possible under the observed conditions of disturbance dynamics? 3. Which phases in the annualsʼ life cycle and patch types contribute most to population growth? Location: Borec hill, northern Czechia, 50°31' N, 13°59' E, 446 m a.s.l. Methods: Local population dynamics of the annuals were analysed separately for five patch types that differed in the proportion of bare soil. Vitality rates were assessed directly in the field, but also in a garden experiment, during 2000-2001 and 2001-2002. Population dynamics at the scale of the whole grassland was analysed with a megamatrix approach, combining patch dynamics of the nest-mounds with patch-specific population dynamics. Contributions of different phases and patch types to growth rate were estimated by elasticity analysis. Results: Nest-mounds differed in the percentage of bare soil. Increasing moss cover significantly reduced germination and seed production of all studied annuals and decreased their population growth rates (λ). Although successional processes dominated over ant disturbance, populations of all species could survive well (λ >> 1) in the grassland according to the 20002001 megamatrix dynamics. Based on the dynamics from the following period, two species would not survive in a long-term perspective due to random environmental variation. Whereas the A-A transition (adult plants originating from adults of the previous year) had the highest elasticity under open conditions and ʻgood periodʼ demography, the importance of persistent seeds increased under reverse conditions. This, however, differed among species. Conclusions: Ant-disturbance was shown to be critical for the population survival of five annual species in the studied grassland. The fate of the annual populations in the grassland system also depends on random environmental variation, which may override the effect of ant activity.

REDUCED RODENT BIODIVERSITY DESTABILIZES PLANT POPULATIONS

We tested the hypothesis that species loss at one trophic level will reduce the temporal stability of populations at other trophic levels. We examined the temporal stability of annual plant populations on plots that experimentally manipulated the functional diversity of seed-eating rodent consumers. Experimental reduction of rodent functional diversity destabilized populations of small-seeded plants but had less consistent effects on larger-seeded species. Small-seeded species also exhibited a greater number of years of zero abundance. Thus, experimental reduction of rodent functional diversity resulted in lower plant diversity. The decline in the temporal stability of small-seeded plants likely resulted from increased interspecific competition by large-seeded plants. These results demonstrate that the loss of species at one trophic level can lead to reduced richness at lower trophic levels via competition and reduced temporal stability.

Bet Hedging via Seed Banking in Desert Evening Primroses (Oenothera, Onagraceae): Demographic Evidence from Natural Populations

Bet hedging is one solution to the problem of an unpredictably variable environment: fitness in the average environment is sacrificed in favor of lower variation in fitness if this leads to higher long-run stochastic mean fitness. While bet hedging is an important concept in evolutionary ecology, empirical evidence that it occurs is scant. Here we evaluate whether bet hedging occurs via seed banking in natural populations of two species of desert evening primroses (Oenothera, Onagraceae), one annual and one perennial. Four years of data on plants and 3 years of data on seeds yielded two transitions for the entire life cycle. One year was exceptionally dry, leading to reproductive failure in the sample areas, and the other was above average in precipitation, leading to reproductive success in four of five populations. Stochastic simulations of population growth revealed patterns indicative of bet hedging via seed banking, particularly in the annual populations: variance in fitness and fitness in the average environment were lower with seed banking than without, whereas long-run stochastic mean fitness was higher with seed banking than without across a wide range of probabilities of the wet year. This represents a novel, unusually rigorous demonstration of bet hedging from field data.

Correlation between the soil seed bank and weed populations in maize fields

Annual weed populations establish every year from persistent seed banks in the soil. This 3 year study investigated the relationship between the number of weed seeds in the soil seed bank and the resultant populations of major broadleaf and grass weeds in 30 maize fields. After planting the crop, 1 m2 areas were protected from the pre‐emergence herbicide application. Soil samples were collected soon after spraying to a depth of 100 mm and the weed seeds therein were enumerated. The emerged weed seedlings in the field sampling areas were counted over the following 8 weeks. Up to 67 broadleaf species and five grass weeds were identified, although not all were found at every site and some were specific to a region or soil type. For the most abundant weeds in the field plots, on average 2.1–8.2% of the seeds of the broadleaf species and 6.2–11.9% of the seeds of the grass weeds in the soil seed bank emerged in any one year, depending on the species. Overall, the results showed a strong linear relationship between the seed numbers in the soil and the seedling numbers in the field for all the grasses and for most broadleaf weeds. For some species, like Trifolium repens, only a weak relationship was observed. In the case of Chenopodium album, which had the largest seed bank, there was evidence of asymptotic behavior, with seedling emergence leveling off at high seed numbers. An estimate of the soil seed bank combined with knowledge of the germination and behavior of specific weed species would thus have good potential for predicting future weed infestations in maize fields.

Reproduction strategies of Macrocystis pyrifera (Phaeophyta) in Southern Chile: The importance of population dynamics

Macrocystis pyrifera is an ecologically dominant species along the temperate Northern and Southern Pacific Coast of America, showing some similarities and differences at population and community level. In general, this kelp is reported to be reproductive all year round. Annual populations present in wave-protected areas of southern Chile suggest that the reproductive strategies of this population can be different. In this study we explore the reproductive strategies of annual M. pyrifera present in wave-protected areas and perennial populations encountered in exposed areas of southern Chile (41∘S). Our results show that M. pyrifera present in wave-exposed locations has a reproductive strategy that is similar to populations in the northern hemisphere. These populations reproduce all year round and their strategy is to produce high numbers of sporophylls and ensure that most of them (over 90%) become sporogenous. On the other hand, the protected populations with an annual life cycle, produce more spores per area of sorus. In a few months, they are able to produce sufficient propagules to recolonize areas before the adult plants disappear in autumn.

The implication of stubble tillage for weed population dynamics in organic farming

SummaryIn practical farming, early and shallow stubble tillage is carried out post‐harvest to stimulate germination of freshly ripened crop and weed seeds, to kill the resulting seedlings and hence to reduce the input into the soil seedbank. Additionally, it aims at reducing perennial weeds by mechanical damage. In this paper, field experiments and laboratory studies are presented which show that stubble tillage can reduce perennial weeds. However, it had a variable effect on annual weeds. After 5 years of experimentation, no effect of stubble tillage was seen on the aboveground vegetation. In contrast, the soil seedbank of the control was roughly doubled where the stubble had been left uncultivated until autumn ploughing. These results indicate that practical experience which assumes that stubble tillage reduces annual weed populations may be correct, despite the fact that in other published studies stubble tillage exerted no control on annual weeds or had a variable effect. This will have practical application in organic arable production systems.

Decline in Cervical Cancer Incidence and Mortality in New South Wales in Relation to Control Activities (Australia)

To examine time trends in cervical cancer incidence and mortality in NSW women aged yen20 years in relation to important health service initiatives and programs. Data on cervical cancer incidence and mortality were obtained from the NSW Central Cancer Registry for 1972-2001, and corresponding annual populations obtained from the Australian Bureau of Statistics. Direct age-standardised rates in the yen20 year population were calculated using the 2001 NSW census population as standard. Proportional reductions in incidence and mortality since 1972-1974 were also calculated and related to key health service factors and to published NSW 5-year cervical cancer relative survival for similar periods. Declines in cervical cancer incidence (-10%) and mortality (-20%), and increased degree-of-spread specific survival following the introduction of universal health care in 1975 suggest effects of greater access to Pap screening, earlier access to diagnosis and treatment services, and improved effectiveness of treatment. Incidence plateaued during the 1980s, but mortality fell further (-7%) due to an increased proportion of localised cancers (without change to degree-of-spread specific survival). The 1980s mortality reduction was a consequence of earlier diagnosis and/or secondary prevention, not improved treatment effectiveness or reduced incidence. A marked and sustained incidence decline to 2001 (-35%) occurred after the introduction of the NSW Cervical Screening Program in 1992. This was followed 3 years later by a sustained mortality decline (-20%). During the 1990s survival across all degrees of spread remained unchanged and the mortality reduction was due entirely to reduction in incidence. The substantial reduction of cervical cancer incidence and mortality in NSW over the last 3 decades is associated with important health service interventions that relate to control of cervical cancer, particularly the implementation of a population-based organised cervical screening program.

Pleiotropic Quantitative Trait Loci Contribute to Population Divergence in Traits Associated With Life-History Variation in Mimulus guttatus

Evolutionary biologists seek to understand the genetic basis for multivariate phenotypic divergence. We constructed an F2 mapping population (N = 539) between two distinct populations of Mimulus guttatus. We measured 20 floral, vegetative, and life-history characters on parents and F1 and F2 hybrids in a common garden experiment. We employed multitrait composite interval mapping to determine the number, effect, and degree of pleiotropy in quantitative trait loci (QTL) affecting divergence in floral, vegetative, and life-history characters. We detected 16 QTL affecting floral traits; 7 affecting vegetative traits; and 5 affecting selected floral, vegetative, and life-history traits. Floral and vegetative traits are clearly polygenic. We detected a few major QTL, with all remaining QTL of small effect. Most detected QTL are pleiotropic, implying that the evolutionary shift between these annual and perennial populations is constrained. We also compared the genetic architecture controlling floral trait divergence both within (our intraspecific study) and between species, on the basis of a previously published analysis of M. guttatus and M. nasutus. Eleven of our 16 floral QTL map to approximately the same location in the interspecific map based on shared, collinear markers, implying that there may be a shared genetic basis for floral divergence within and among species of Mimulus.

Effect of Spring Tillage Sequence on Summer Annual Weeds in Vegetable Row Crop Rotations

The effects of spring tillage sequence on summer annual weed populations were evaluated over two cycles of a 3-yr crop rotation of snap beans, sweet corn, and winter wheat. Continuous no-till (N) planting of vegetable crops each spring (NNNN) reduced summer annual weed density 63 to 86% compared to that of continuous conventional tillage (CCCC), depending upon site and herbicide level. Hairy nightshade populations were reduced by 88 to 96% when spring tillage was eliminated from the crop rotation. The effects of the NNNN spring tillage sequence on weed density were similar at two sites even though the crop rotations at the two sites began with different crops. The rotational tillage sequence of NCNC at the East site, in a crop rotation that began with corn, reduced summer annual weed density by 46 to 51% compared to that of continuous conventional tillage and planting (CCCC) at low and medium herbicide rates, respectively. In contrast, the tillage sequence of CNCN in the same crop rotation and at the same site increased weed density by 80% compared to that of CCCC at a low herbicide rate. The effects of the NCNC and CNCN rotational tillage sequences on weed density were reversed at the West site, and was probably caused by pairing sweet corn with conventional tillage rather than no tillage. The reduction in summer annual weed density caused by reduced spring tillage frequency did not significantly increase crop yields.

Spring Census of Mid-Continent Sandhill Cranes Using Aerial Infrared Videography

Aerial infrared videography was used to map spatial distributions of nocturnal sandhill crane (Grus canadensis) flocks and determine crane densities within roosts as an alternative to the currently used diurnal photo-corrected ocular transect method to estimate the size of the mid-continental population. The densities determined from samples taken over the course of a night show variability. Densities measured early in the night (2100 to 2300 hrs) were generally lower than those measured in the time period after midnight and up until cranes prepared to depart their roosts before sunrise. This suggests that cranes may be more active early in the night and possibly still settling into their roosts at this time. For this reason, densities and areas measured later at night and into the early morning were used to estimate population size. Our methods estimated that the annual crane populations along the central Platte River in Nebraska were higher than estimates from the ocular transect method; however both methods showed a similar trend with time. Our population size estimates likely were higher because our methodology provided synoptic imagery of crane roosts along the entire study reach when all cranes had returned to the river, and the nocturnal densities were higher than previous estimates using observations from late evening or early morning. In addition to providing a tool for estimating annual population size, infrared videography can be utilized over time to identify spatial changes in the roosting patterns that may occur as a result of riverine management activities.

Precipitation history and ecosystem response to multidecadal precipitation variability in the Mojave Desert region, 1893–2001

Precipitation varied substantially in the Mojave Desert through the 20th century in a manner broadly similar to the other warm North American deserts. Episodes of drought and prolonged dry conditions (1893–1904, ca. 1942–1975, and 1999-present) alternated with relatively wet periods (1905–ca. 1941 and ca. 1976–1998), probably because of global-scale climate fluctuations. These are the El Niño-Southern Oscillation that affects interannual climate and the Pacific Decadal Oscillation that evidently causes decadal-scale variability such as prolonged dry and wet episodes. Studies done in the late 20th century demonstrate that precipitation fluctuations affected populations of perennial vegetation, annuals, and small herbivores. Landscape rephotography reveals that several species, particularly creosote bush, increased in size and density during the ca. 1976–1998 wet period. A brief, intense drought from 1989 to 1991 and the ongoing drought caused widespread mortality of certain species; for example, chenopods and perennial grasses suffered up to 100% mortality. Drought pruning, the shedding of above-ground biomass to reduce carbon allocation, increased substantially during drought. Overall, drought had the greatest influence on the Mojave Desert ecosystem.

Annual plant–shrub interactions along an aridity gradient

Summary Annual plants in semi-arid and arid areas are often closely associated with shrubs. The degree of association largely depends on the balance of negative and positive effects between these contrasting plant life-forms, ranging from interference to facilitation. Since positive interactions are predicted to become less important with increasing rainfall, the interaction balance is expected to shift along aridity gradients. However, this prediction has not been tested on a community level and for different life-history stages across large geographical gradients. Here, we show such changes for annual plant populations and communities in four contrasting sites along a steep climatic gradient, ranging from the arid desert to mesic Mediterranean regions in Israel. Above-ground productivity, richness, seedling density, and seed bank density of the annual plant community, as well as fecundity of annual plant populations, were generally higher under shrubs than in areas between shrubs at the arid end of the gradient, but significantly lower at the humid end. Net effects of shrubs on annuals expressed as relative interaction intensity indicated a steady and consistent shift from net positive or neutral effects in the desert to net negative effects in the mesic part of the gradient. These findings emphasize the usefulness of studies along large-scale gradients encompassing a wide range of environmental conditions for understanding community level interactions among coexisting species.

Pollen–pistil barriers to crossing in maize and teosinte result from incongruity rather than active rejection

Many popcorn strains cannot be fertilized by pollen of dent and flint strains although the reciprocal crosses are successful. Similarly, plants of some annual teosinte populations can fertilize maize but do not accept its pollen. Single genes or gene complexes govern these two unilateral barriers to crossing. Failure of fertilization could reflect active rejection by the pistil of pollen containing a contrasting allele (incompatibility). Alternatively, the pistil could require presence of a matching allele in pollen (congruity). To distinguish between these possibilities genetically, the receptivity to pollen having both alleles was determined. If there is active rejection, heteroallelic pollen would not be accepted; if presence of a matching allele is required, heteroallelic pollen would be accepted. In both the popcorn and teosinte crossing barrier systems, heteroallelic pollen functioned, consistent with the congruity model.

Population biology of the intertidal kelp, Alaria marginata Postels and Ruprecht: A non-fugitive annual

Persistence of annual plant populations requires sufficient seeds and suitable habitat for development and growth each year. Competition with perennials may prevent within site persistence and result in “fugitive” annual populations. Comparisons have been made between the population biology of annual macroalgae and terrestrial plants, but demographic information necessary to make strong comparisons is lacking for most of these algae, and life history differences may make such comparisons questionable. We studied population dynamics of the kelp Alaria marginata to determine if it was an annual and, if so, how populations persisted. This kelp is the dominant macroalga on exposed mid to low rocky intertidal shores along the Big Sur coast of California. Experimental clearings at two sites were used to assess recruitment timing and survivorship. Sporophytes were collected monthly to determine growth and fecundity. Recruitment occurred in late winter to early spring, primarily on geniculate corallines and residual A. marginata holdfasts. Thinning was inversely related to density, and occurred during the February through July growing season as larger thalli rapidly increased in length (up to 1.4 m month − 1) and formed a thick canopy. Sorus development was positively related to size, began as early as March, peaked in late August–October, and decreased as adults were removed by winter surf. Spore release was generally highest (10 8–10 9 spores individual − 11 h − 1) between October and January and associated with high water motion. Survivorship of sporophytes beyond one year was < 1%, showing the populations were annual. Field observations and experiments on effects of canopy clearing, season of clearing, and influence of substrate type on recruitment were done to assess how these annual populations persist. Massive spore production at the onset of fall storms, survival of microscopic stages for 3–4 months facilitated by microhabitat refuges, rapid growth, large size and rapid maturation of sporophytes contributed to persistence. Furthermore, the dense stands with thick canopies may suppress potential competitors via shading and abrasion. Rather than being a fugitive, this combination of growth and life history features enables A. marginata and perhaps other large, annual kelps to maintain perennial populations.

Autotoxicity in Lolium rigidum: analyzing the role of chemically mediated interactions in annual plant populations

Autotoxicity is the chemical influence of one plant on the development of individuals of the same species. Autotoxicity in perennial plants can be a useful mechanism to avoid future conspecific competitors, however, the ecological role of autotoxicity in annuals, if any, remains uncertain. In this paper, we analyse the autotoxic phenomenon in Lolium rigidum, an annual, self-incompatible grass from semi-arid environments suspected to be autotoxic, and we dissert on the mechanisms of intraspecific interference, carrying out density-dependent experiments with additions of conspecific competitors and plant residues. Our experiments show that chemical interference and resource competition are density-dependent processes that act in opposite directions. Inhibitory chemical interference increases at low population densities whereas competition for resources intensifies at high densities. Based on these results, the manuscript theorizes on the ecological role of the autotoxic phenomenon in short-lived plants. We hypothesize that autotoxicity enhances population viability by restraining the development of populations with few individuals, which should decrease the chances of outcross and experience inbreeding depression. If this hypothesis proves true, it could contribute to explain the unusually high within-population diversity encountered in this species, which contrasts with reported values from other similar wind-pollinated, self-incompatible grasses.

Population genetic structure of annual and perennial populations ofZostera marinaL. along the Pacific coast of Baja California and the Gulf of California

The Baja California peninsula represents a biogeographical boundary contributing to regional differentiation among populations of marine animals. We investigated the genetic characteristics of perennial and annual populations of the marine angiosperm, Zostera marina, along the Pacific coast of Baja California and in the Gulf of California, respectively. Populations of Z. marina from five coastal lagoons along the Pacific coast and four sites in the Gulf of California were studied using nine microsatellite loci. Analyses of variance revealed significant interregional differentiation, but no subregional differentiation. Significant spatial differentiation, assessed using theta(ST) values, was observed among all populations within the two regions. Z. marina populations along the Pacific coast are separated by more than 220 km and had the greatest theta(ST) (0.13-0.28) values, suggesting restricted gene flow. In contrast, lower but still significant genetic differentiation was observed among populations within the Gulf of California (theta(ST) = 0.04-0.18), even though populations are separated by more than 250 km. This suggests higher levels of gene flow among Gulf of California populations relative to Pacific coast populations. Direction of gene flow was predominantly southward among Pacific coast populations, whereas no dominant polarity in the Gulf of California populations was observed. The test for isolation by distance (IBD) showed a significant correlation between genetic and geographical distances in Gulf of California populations, but not in Pacific coast populations, perhaps because of shifts in currents during El Niño Southern Oscillation (ENSO) events along the Pacific coast.