Hybridization in agricultural weeds: A review from ecological,evolutionary, and management perspectives

Agricultural weeds frequently hybridize with each other or with related crop species. Some hybrid weeds exhibit heterosis (hybrid vigor), which may be stabilized through mechanisms like genome duplication or vegetative reproduction. Even when heterosis is not stabilized, hybridization events diversify weed gene pools and often enable adaptive introgression. Consequently, hybridization may promote weed evolution and exacerbate weed–crop competition. However, hybridization does not always increase weediness. Even when viable and fertile, hybrid weeds sometimes prove unsuccessful in crop fields. This review provides an overview of weed hybridization and its management implications. We describe intrinsic and extrinsic factors that influence hybrid fitness in agroecosystems. We also survey the rapidly growing literature on crop–weed hybridization and the link between hybridization and invasiveness. These topics are increasingly relevant in this era of genetic tools for crop improvement, intensive and simplified cropping systems, and globalized trade. The review concludes with suggested research priorities, including hybridization in the context of climate change, plant–insect interactions, and redesigned weed management programs. From a weed management perspective, hybridization is one of many reasons that researchers and land managers must diversify their weed control toolkits.     

A comparative study of germination ecology of four Papaver taxa

BACKGROUND AND AIMS: Comparative studies of closely related taxa can increase understanding of adaptations and changes in seed dormancy and germination preferences in an evolutionary perspective. For such studies, a method to describe and compare the performance of taxa in a general way is needed. The germination ecology of four Papaver taxa was studied with the aim of describing and comparing their responses to different seasonal temperature regimes. METHODS: Germination of Papaver argemone, P. rhoeas, P. dubium ssp. dubium and P. dubium ssp. lecoqii was investigated in three different artificial climates over 2.5 years. Seeds were collected in southern Sweden, and samples from different populations were used as replicates of taxa. KEY RESULTS: Despite substantial intra-taxa variation, there were clear taxon-specific responses. Most germination occurred in the warmest climate. In general, the warmer the climate the more germination occurred in autumn instead of spring. Papaver argemone, phylogenetically most distant from the other taxa, was, in contrast to the others, restricted to germinating only at lower temperatures. CONCLUSIONS: Seed dormancy and germination may be described by dormancy pattern, germination preferences and dormancy strength. The general dormancy pattern was a common feature for these taxa and therefore probably an evolutionary conservative character. Germination preferences varied between taxa, resulting in different temperature optima and intervals for germination, and dormancy strength was to some extent taxon-specific, but highly variable. The dormancy pattern explained how the taxa can perform as winter annuals in warmer climates, but mainly as summer annuals in colder climates. Hence, there is no need to interpret the within-taxon temporal differences in seedling emergence as local adaptations. In the field, an entire seed cohort will not germinate during a single season. Instead, emergence will be distributed over several seasons, regardless of local climate, weather and soil cultivation methods.     

Sheep grazing vs. cutting: regeneration and soil nutrient exploitation of the grassland weed Rumex obtusifolius

Broad-leaved dock (Rumex obtusifolius L., Polygonaceae) is an agronomically important perennial weed causing decreases in pasture yields and fodder quality. Non-chemical control measures for dock are often limited to frequent pulling and cutting, additionally it is usually avoided by grazing animals. Here, the regeneration of R. obtusifolius in a Rumex-infested grassland grazed by a sheep breed that explicitly feeds on dock (Ovis aries L. cv. East Prussian Skudden) was compared to cutting. Therefore, regeneration of 90 R. obtusifolius plants of three different size classes was monitored in three plots during three grazing and cutting cycles. Plant height and number of fruit-stands of regrown R. obtusifolius was significantly lower, number of leaves significantly higher after grazing than after cutting, while plant diameter was unaffected. Initially medium and large-sized plants (>40 cm diameter) were significantly more sensitive to grazing or cutting than initially smaller sized plants. Soil nitrate and ammonium concentrations in the vicinity of R. obtusifolius correlated with some regrowth parameters but were not affected by grazing or cutting. Sheep-grazed grassland communities comprised fewer legumes (p = 0.002), more grasses (p = 0.010) and fewer sward gaps (p = 0.025) than cut grassland. At the end of the experiment, abundance of R. obtusifolius in sheep grazed plots was lower than in cut plots (p = 0.089) suggesting that regrowth potential of this weed was depleted by continuous grazing and higher sward density. In conclusion, these data suggest that sheep could be considered in grassland management schemes to both directly and indirectly control Rumex infestations.     

How does temporal variability affect predictions of weed population numbers?

1. Population models that are used to predict weed population dynamics or the impact of control measures on weed abundance typically ignore temporal variability in life-history parameters and control measures, and utilize mean arithmetic population growth rates to predict population abundance.
2. We demonstrate that the persistence of weeds in a stochastically varying environment depends on the geometric mean population growth rate being greater than zero, rather than the arithmetic mean population growth rate being greater than zero.
3. In a stochastically varying environment we show that temporal variability in fecundity, germination and survivorship will tend to decrease population size, relative to predictions based on arithmetic means. Conversely, variability in competitive effects and weed control will tend to increase population size, relative to predictions based on arithmetic mean values. The distinction between these two sets of parameters is that increases in the former will increase population growth rate, whereas increases in the latter will decrease it.
4. We argue that population models based on arithmetic mean population growth rates will tend to over-estimate population size. Numerical simulations indicate that this bias may be considerable.
5. Since short-term studies cannot, in general, estimate the geometric mean growth rate of a population we suggest several approaches for estimating the degree of bias in the predictions of models owing to the effects of variability. Accounting for such variability is necessary since current models for the dynamics of weed populations are based on arithmetic mean measures of population growth and hence likely to be biased.     

Survival of Ralstonia solanacearum in Soil under Irrigated Rice Culture and Aquatic Weeds

The survival of Ralstonia solanacearum in naturally infested sandy loam soil under irrigated rice culture was investigated in Sankhu village (1400 m above sea level) in central Nepal. The experimental plot had a previous history of bacterial wilt and a range of 1.5 × 10⁴–3 × 10⁴ colony-forming units (CFU) per g soil was present. The survival of R. solanacearum was monitored in roots of naturally growing aquatic weeds in the rice plot and in soil before and after rice harvest. The incidence of the bacterial infection in the weeds, Dopatrium sp. and Monochoria vaginalis , were 57.5 and 10%, respectively. The bacterial population detected in soil before rice harvest was 1.5 × 10⁴ CFU per g soil whereas a range of 7.5 × 10²–1.5 × 10³ CFU per g was detected after the rice harvest. Biovar typing of R. solanacearum isolated from potato plants, potato tubers, aquatic weeds, and the soil from the experimental plot yielded the diverse biovars 2 A, 3 and 4. This is the first report of the survival of these biovars in soil, which was under continuous flow of irrigation water for 3 months during rice culture.     

Weed macaques: The evolutionary implications of macaque feeding ecology

Patterns of feeding ecology among the living macaques conform poorly with recognized phyletic distinctions within the genus because there is an important ecological division which cross-cuts phyletic groupings. This division, between weed species and non-weed species, is based on the differing abilities of macaques to tolerate and even prosper in close association with human settlements. Based on available information about their ecology in the wild, we tentatively assign macaque species to these two categories. Finally, we consider the implications of our argument for scenarios of the initial spread of the macaques.     

Biodiversity matters in a changing world

It is now widely accepted that the climate of our planet is changing, but it is still hard to predict the consequences of these changes on ecosystems. The impact is worst at the poles, with scientists concerned that impacts at lower latitudes will follow suit. Canada has a great responsibility and potential for studying the effects of climate changes on the ecological dynamics, given its geographical location and its scientific leadership in this field. The 5th annual meeting of the Canadian Society for Ecology and Evolution was held in the International Year of Biodiversity, to share recent advances in a wide variety of disciplines ranging from molecular biology to behavioural ecology, and to integrate them into a general view that will help us preserve biodiversity and limit the impact of climate change on ecosystems.     

Biological control: Premises,ecological input and Mimosa pigra in the wetlands of Australia's Top End

The adverse impacts of weeds on natural ecosystems, together with the inadequate outcomes from treating weeds as a symptom, have escalated interest in finding efficacious control methods. With the aim of protecting wetlands from invasive weeds, this contribution uses the woody shrub Mimosa pigra L. (mimosa) as a case history to examine the methodology of classical biological control and the reasons for the widely accepted 75% failure rate. Overall objectives are for all biocontrol agents to have the opportunity to fully express their potential and to insure that limited resources are spent wisely on attainable weed control. The three main conclusions were that (1) the premises on which biocontrol is based has restricted advancement of this method; (2) monitoring is the logical first step to improving the selection of agents and release sites; and (3) it may be more cost-effective to introduce fewer agents that have undergone agent/plant and host/home range pre- and post-release ecological studies. Weed control may remain elusive unless advantage is taken of every beneficial result. Innovative assistance to agents and the integration of different control methods may preserve a role for weed biological control and has the potential to be of great importance for future weed management. It is proposed that the climate in the Top End of the Northern Territory and the lack of competition on the floodplains has contributed to mimosa's invasiveness. Climate may also underly the difficulties faced by agents. Agents appear unable to impart effective control in the dry season because of low numbers which relates to mimosa's poor growth; nor in the wet season, when the impact from high numbers of agents is outstripped by mimosa's growth.     

降雨年型变化及竞争对反枝苋和大豆生长的影响

为探索不同降雨年型及栽培方式下外来杂草与本地作物的竞争机制, 为未来全球变化背景下控制外来杂草提供理论依据, 本研究以广泛入侵东北农田生态系统的外来杂草反枝苋(Amaranthus retroflexus)和本地作物大豆(Glycine max)为研究对象, 在遮雨棚内人工模拟正常、欠缺、丰沛三种降雨年型, 采用盆栽实验的方法, 研究两种植物在单种和混种条件下的生长季节动态。结果表明, 降雨丰沛年两种植物的株高和总生物量均大于降雨正常年, 降雨欠缺年则均小于降雨正常年。生长季初期两种植物的根冠比均在降雨欠缺年最高, 说明两种植物均可通过增大根系的生物量分配, 减少地上生物量的分配来适应干旱环境。在三种降雨年型下, 混种时大豆的株高、相对生长速率及总生物量均显著小于单种大豆, 而反枝苋则相反, 尽管有时不显著, 说明种间竞争抑制大豆生长而促进反枝苋的生长, 两种植物之间的竞争是不对称竞争。总的来看, 降雨增加有利于提高大豆的竞争能力, 降雨减少有利于提高反枝苋的竞争能力, 随着生长发育的推移, 这种现象更明显。反枝苋可以在较广的降雨变化范围内保持较高的株高、相对生长速率及生物量, 这很可能是其成为全球范围成功入侵的外来杂草的重要原因之一; 干旱更有利于反枝苋入侵大豆田。     

Forest gap dynamics and the Ising model

The vegetation height in forest ecosystems is spatially heterogeneous. Canopy gaps (sites with low vegetation) are formed by treefalls, and they recover to canopy sites (with high vegetation) either by growth of small trees or by branch extension of surrounding trees. The dynamics of canopy gaps have been studied using a spatial Markov chain with nearest neighbor interaction. (1) If the canopy recovery rate is constant and if the gap formation rate for a site increases exponentially with the number of neighboring gap sites, the equilibrium distribution is the same as the one generated by the Ising model in statistical mechanics. Here, we extend the equivalence to the situation in which both the gap formation and canopy recovery depend on the neighborhood, as shown in recent forest data. (2) We develop a statistical test of whether a given spatial pattern is a random sample from the Ising model. The test is based on the conditional probability of configurations on a partial lattice. We apply the method to vegetation height data from the Ogawa forest reserve, Japan, measured on a 5x5 m grid in 1976, 1981, 1986, and 1991. The spatial pattern of the original forest data deviates significantly from the Ising model. We examine whether a larger sampling distance or the removal of the effects of the topography can reduce this deviation.     

Uncertainty in research about key invasion characteristics limits the evaluation of exotic perennial grasses in natural systems in New South Wales,Australia

Exotic perennial grasses (EPGs) pose a significant risk to native communities globally. With over 2,200 species in Australia, understanding which characteristics enable high threat invasions, and comparing between functionally similar EPGs, can help prioritise species management. We developed a framework of risk and used the literature to rank 21 EPGs considered a threat to plant communities in New South Wales, while also evaluating the reliability of information currently available. Characteristics were scored within five broad categories that distinguish invasiveness: Arrival, Establishment, Persistence, Impact and Distribution. These included aspects of reproductive biology, competitive ability and environmental tolerance. The risk assessment was effective in assessing key characteristics of invasion. EPGs with an economic benefit (trade‐off species) were more likely to have reliable research and frequently ranked as high‐risk invaders in natural habitats due to the overlap of characteristics important in invasion with those considered important in agriculture. Lack of formal scientific research hindered assessment for some species, and some traits had been poorly assessed in the literature. High uncertainty was associated with key characteristics for Establishment, Persistence and Impact. Uncertainty in key characteristics revealed a need for improved integration of less formal research validated by more formal scientific research. This may lead to more informed decisions in the management of EPGs in native habitats and assist in early control of EPGs not yet assessed.     

Evolutionary-thinking in agricultural weed management

Agricultural weeds evolve in response to crop cultivation. Nevertheless, the central importance of evolutionary ecology for understanding weed invasion, persistence and management in agroecosystems is not widely acknowledged. This paper calls for more evolutionarily-enlightened weed management, in which management principles are informed by evolutionary biology to prevent or minimize weed adaptation and spread. As a first step, a greater knowledge of the extent, structure and significance of genetic variation within and between weed populations is required to fully assess the potential for weed adaptation. The evolution of resistance to herbicides is a classic example of weed adaptation. Even here, most research focuses on describing the physiological and molecular basis of resistance, rather than conducting studies to better understand the evolutionary dynamics of selection for resistance. We suggest approaches to increase the application of evolutionary-thinking to herbicide resistance research. Weed population dynamics models are increasingly important tools in weed management, yet these models often ignore intrapopulation and interpopulation variability, neglecting the potential for weed adaptation in response to management. Future agricultural weed management can benefit from greater integration of ecological and evolutionary principles to predict the long-term responses of weed populations to changing weed management, agricultural environments and global climate.     

Annual Cycle of Planktothrix agardhii (Gom.) Anagn. & Kom. Nature Population

Changes in abundance, biovolume and morphology of Planktothrix agardhii in a natural population were followed over one year period in shallow fishpond Bílá Lhota (Central Moravia, Czech Republic). The selected environmental parameters (pH, oxygen, temperature, conductivity, nutrients, light) were measured at the surface and at the bottom of the fishpond, together with the Planktothrix abundances and filament morphology – filament length, width, shape, aerotopes (gas vacuoles) formation. The annual cycle of P. agardhii in this hypertrophic fishpond starts in March with the germination of hormogonia and the growth of overwintered filaments. The filament length quickly increases to a maximum length in April. The following summer period can be characterized by filament shortening and by changes in the aerotopes shape. On the other hand the abundance and biomass of P. agardhii is increasing until the maximum in August. Further shortening of filaments, loss of aerotopes and hormogoniae formation is typical for the autumn (October) with the average temperature of 9.4 °C. The population overwinters near the pond bottom in the form of hormogonia (60%) and filaments (40%). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Richard J. Hobbs: how one ecologist has influenced the way we think about restoration ecology

Professor Richard Hobbs has had a profound influence on the development of the discipline of restoration ecology. With more than 300 publications spanning a broad scope of applied ecological sciences, he has collaborated with hundreds of researchers. His sometimes‐provocative insights, balanced by extensive empirical research, will have a lasting impact by encouraging people to think more broadly about the science and practice of ecological restoration. Here, on the eve of his retirement, some of his staff and students, past and present, take a retrospective look at his contributions to restoration ecology both as a scientist and as a mentor.     

OCCURRENCE OF CODIUM FRAGILE SUBSP. TOMENTOSOIDES (CHLOROPHYTA: BRYOPSIDALES) IN MARINE EMBAYMENTS OF SOUTHEASTERN AUSTRALIA

Codium fragile (Sur.) Hariot subsp. tomentosoides (van Goor) Silva, a widespread weedy species, is reported from Australia for the first time. Specimens were initially found in June 1996 in Corner Inlet, Victoria, where the species fouled fishing nets. This invasive chlorophycean alga was subsequently found in large numbers on 30 January 1997 in Port Phillip Bay and on 9 March 1998 in Western Port, Victoria. Presumably the species is native to Japan. The alga formed dense stands on rocky substrata in intertidal or in subtidal regions. In Port Phillip Bay, the species was also found attached to the common blue mussel Mytilus edulis planulatus Lamarck, and in Western Port it was found attached to the mud oyster Ostrea angasi Sowerby. Results of this study suggest that recruitment of juvenile germlings and growth of C. fragile subsp. tomentosoides differs between populations in Port Phillip Bay and those in Western Port.     

A Plague on Your House? Some Impacts of Chromolaena odorata on Timorese Livelihoods

Following its introduction on the island of Timor, the shrub Chromolaena odorata has expanded dramatically across the landscape. A highly flammable but fire tolerant plant species, Chromolaena is an extremely successful plant coloniser of disturbed ground and open savanna woodlands. For Timorese semisubsistence agriculturalists, Chromolaena odorata represents the most recent in a long history of invasive weed species that have periodically covered their lands and challenged their ability to farm and prosper. Already its impact is being felt in a number of domains of socioeconomic life. This paper explores some of these issues and the prospects for future management and control strategies.