Tillage compromises weed seed predator activity across developmental stages

Granivorous ground beetles (Coleoptera: Carabidae) are ubiquitous throughout temperate agricultural systems, and reduce weed seedbanks. However, trade-offs may exist between tillage frequency and ecosystem services of invertebrate seed predators, especially those in the larval stages, which have relatively poor resistance to disturbance. While much research has focused on adult activity patterns and the conservation biocontrol services they provide, almost nothing is known about carabid biology and habitat requirements during larval stages, despite the fact that adult recruitment is determined by factors that promote larval survival. We present data on larval and adult Harpalus pennsylvanicus Dej., a common weed seed predator across North America, from two experiments examining its activity density across tillage and cover-cropping treatments in organic tomato systems. Larvae emerged 4–6 weeks after the adult activity peak, and larval activity density was up to 10 times higher in no-till crop environments than in cultivated areas. After a long disturbance interval, seasonal cultivation had no effect on foraging activity of adults, but reduced larval activity density in both experiments. Additionally, larvae positively correlated with living weed biomass in no-till treatments, suggesting the importance of plant-based resources in oviposition site choice. Compared with adults, larvae are relatively immobile and vulnerable to disturbance; thus, weed management strategies that rely on frequent cultivation may undermine the ecosystem services provided by granivorous insects.     

Habitat effects on second-order predation of the seed predator Harpalus rufipes and implications for weed seedbank management

Seed predators provide a valuable ecosystem service to farmers by reducing densities of weed seeds, and, in turn, densities of weed seedlings they must manage. The predominant invertebrate weed seed predator in Maine, USA, agroecosystems is the carabid beetle Harpalus rufipes DeGeer. Pitfall trapping has shown that H. rufipes prefers sites with vegetative cover to fallow sites, preference speculated to be driven by predator avoidance behavior. To test this hypothesis, ‘second-order predation assays’ were developed, in which live H. rufipes prey were presented to second-order predators. Field experiments were conducted to determine foremost if H. rufipes was subject to second-order predation, and secondly, whether (a) vegetative cover affords H. rufipes protection from second-order predators, and (b) high rates of second-order predation correspond with decreased invertebrate seed predation rates. Two 72-h experiments were conducted (mid August and September 2012) at crop and non-crop sites across a 28 ha diversified farm in Stillwater, ME, USA.Second-order predation was 2.8% per day. Based on images from motion-sensing cameras, H. rufipes’ predators included birds and small mammals. Neither a relationship between second-order predation and vegetative treatment, nor an empirical relationship between second-order predation and invertebrate seed predation were detected. However, a simulation model predicted that 2.8% per day second-order predation could increase the number of seeds entering the seedbank by more than 17% annually. Additionally, complex habitats supported higher rates of second-order predation than did simple habitats.     

Comparing seed removal rates in actively and passively restored tropical moist forests

High rates of seed removal can impede forest recovery, but tropical seed removal studies are few and mainly from the neotropics. Little is known about the comparative influences of active restoration (i.e. planting) and passive restoration (i.e. protection of natural regrowth) on seed removal. We conducted an evaluation of seed removal in grasslands, natural forests (tropical moist semideciduous forest), and actively (21‐, 17‐, 16‐, 11‐, 8‐, and 6‐year‐old) and passively (21‐year‐old) restored forests in Kibale National Park, Uganda. We wanted to compare the effect of vegetation type, time since restoration and restoration actions (i.e. active vs. passive) on removal of seeds of five animal‐dispersed tree species during wet and dry seasons. Seeds were either fully exposed or placed in closed mesh cages or under a mesh roof. We used differential removal rates between these treatments to attribute seed removal to different animal taxa. Seed removal rate (percentage of seed removed over a 4‐day period) was highest in passively restored forests, compared with actively restored forests, grasslands, and natural forests. We detected no significant relationship between time since restoration and seed removal rates within actively restored sites. Seed removal rate from roofed treatments was not significantly different from removal from open treatments but was significantly higher than removal from closed treatments, which we interpret as reflecting the greater effect of small mammals versus insects. Smaller seeds tended to be removed at a greater rate than larger seeds. We discuss the implications of these findings for forest regeneration.     

Seed dynamics of resprouting shrubs in grassy woodlands: Seed rain,predators and seed loss constrain recruitment potential

Abstract Measuring the fate of seeds between seed production and seedling establishment is critical in understanding mechanisms of recruitment limitation of plants. We examined seed fates to better understand the recruitment dynamics of four resprouting shrubs from two families (Fabaceae and Epacridaceae) in temperate grassy woodlands. We tested whether: (i) pre‐dispersal seed predation affected seed rain; (ii) post‐dispersal seed predation limited seed bank accumulation; (iii) the size of the seed bank was related to seed size; and (iv) viable seeds accumulated in the soil after seed rain. There was a distinct difference in seed production per plant between plant families with the legumes producing significantly more seeds per individual than the epacrids. Seed viability ranged from 43% to 81% and all viable had seed or fruit coat dormancy broken by heat or scarification. Pre‐dispersal predation by Lepidopteran larvae removed a large proportion of seed from the legume seed rain but not the epacrids. Four species of ants (Notoncus ectatomoides, Pheidole sp., Rhytidoponera tasmaniensis and Iridomyrmex purpureus) were major post‐dispersal seed removers. Overall, a greater percentage of Hardenbergia (38%) and Pultenaea (59%) seeds were removed than the fleshy fruits of Lissanthe (14%) or Melichrus (0%). Seed bank sizes were small (<15 seeds m^?2) relative to the seed rain and no significant accumulation of seed in the soil was detected. Lack of accumulation was attributed to seed predation as seed decay was considered unlikely and no seed germination was observed in our study sites. Our study suggests that seed predation is a key factor contributing to seed‐limited recruitment in grassy woodland shrubs by reducing the number of seeds stored in the soil.     

Extrafloral nectaries as a deterrent mechanism against seed predators in the chemically protected weed Crotalaria pallida (Leguminosae)

Abstract In several plants, extrafloral nectaries (EFN) are located close to the reproductive structures, suggesting that ants may act as a defence against specialized seed predators that overcome chemical defences. Alternatively, ants may also deter herbivores in a generalized manner, thereby protecting the whole plant. In this work, we examined the relationship between the chemically protected weed Crotalaria pallida Ait. (Leguminosae) that bears EFN, its specialized seed predator, the larvae of the arctiid moth Utetheisa ornatrix L. (Arctiidae) and ants. We tested two hypotheses related to the type of deterrence caused by ants. The Seed Predator Deterrence Hypothesis predicts that ant deterrence is directed primarily towards herbivores that destroy seeds and other reproductive structures, without attacking herbivores on vegetative structures. The General Deterrence Hypothesis states that ants are general in their effects, equally deterring herbivores in vegetative and reproductive structures. Our results supported the predictions of the Seed Predator Deterrence Hypothesis, namely, that (i) ant activity on EFN was related to the vulnerability of reproductive structures to attack by U. ornatrix; (ii) ant patrolling was restricted almost entirely to racemes; (iii) ants removed termites used as baits more frequently on racemes than on leaves; and (iv) U. ornatrix larvae were often expulsed from the racemes. These results indicate that EFN can act as another deterrent mechanism in chemically protected plants by promoting the expulsion of specialist seed predators.     

Inferring species interactions from ecological survey data: A mechanistic approach to predict quantitative food webs of seed feeding by carabid beetles

1. Ecological networks are valuable for ecosystem analysis but their use is often limited by a lack of data because many types of ecological interaction, for example, predation, are short‐lived and difficult to observe or detect. While there are different methods for inferring the presence of interactions, they have rarely been used to predict the interaction strengths that are required to construct weighted, or quantitative, ecological networks.
2. Here, we develop a trait‐based approach suitable for inferring weighted networks, that is, with varying interaction strengths. We developed the method for seed‐feeding carabid ground beetles (Coleoptera: Carabidae) although the principles can be applied to other species and types of interaction.
3. Using existing literature data from experimental seed‐feeding trials, we predicted a per‐individual interaction cost index based on carabid and seed size. This was scaled up to the population level to create inferred weighted networks using the abundance of carabids and seeds from empirical samples and energetic intake rates of carabids from the literature. From these weighted networks, we also derived a novel measure of expected predation pressure per seed type per network.
4. This method was applied to existing ecological survey data from 255 arable fields with carabid data from pitfall traps and plant seeds from seed rain traps. Analysis of these inferred networks led to testable hypotheses about how network structure and predation pressure varied among fields.
5. Inferred networks are valuable because (a) they provide null models for the structuring of food webs to test against empirical species interaction data, for example, DNA analysis of carabid gut regurgitates and (b) they allow weighted networks to be constructed whenever we can estimate interactions between species and have ecological census data available. This permits ecological network analysis even at times and in places when interactions were not directly assessed.

     

Seed transfer among bird-dispersed trees and its consequences for post-dispersal seed fate

We investigated seed transfer, i.e. the seed movement away from a source canopy to areas beneath heterospecific canopies, among the ornithochorous tree species Taxus baccata, Ilex aquifolium and Crataegus monogyna in temperate secondary forests in NW Spain, by analysing the composition of multispecific seed rain beneath the canopy of each species, at four sites and for 2 years. To evaluate the consequences on seed fate, we estimated predation by rodents in manipulated seed rains, representing variable levels of relative proportion and total density for combinations of a preferred species paired with a less-preferred species. Seed rain under Taxus canopies was dominated by Taxus seeds, which occurred in low proportion under heterospecific canopies. Ilex seeds dominated the areas under Ilex but accounted for 20–40% of seeds under heterospecific trees. Crataegus seeds were not dominant in any of the microhabitats. The probability of being deposited beneath a heterospecific canopy was much higher for Ilex and Crataegus than for Taxus. The effects of seed rain composition on post-dispersal seed predation were species-specific. Taxus seeds experienced lower predation when occurring in a background of seeds dominated by heterospecific, Ilex or Crataegus, seeds. Crataegus seeds escaped predation more successfully in high-density patches, independently of seed clump composition. Predation on Ilex seeds was independent to both the density and the composition of seed clump. Seed transfer among heterospecific tree species may contribute to shape the template of propagule abundances from which forest will develop, by generating seed combinations favourable to escape from predation.     

Observation of epigeic predators and predation on artificial prey in a cereal field

Predation activity of different epigeic predators on artificially supplied prey was studied in a cereal field during several 24 h periods from spring to early summer 1993. Carabids were the most frequently observed predators. Among themPoecilus cupreus was the most voracious species during day-time, andAgonum muelleri during night-time. Predation rates were highest between 10.00 and 18.00 h and lowest between 02.00 and 10.00 h. Linear regressions showed significantly positive relations between temperature and predation rates, and between predation and activity density of epigeic predators. The latter relation was stronger when only activity density of predators which were observed to consume baits was included. Quadrat sampling and pitfall sampling showed that with the former only a small proportion of bait-consuming predators were caught whereas with the latter method this proportion was high.     

Seed predators exert selection on the subindividual variation of seed size

Subindividual variation among repeated organs in plants constitutes an overlooked level of variation in phenotypic selection studies, despite being a major component of phenotypic variation. Animals that interact with plants could be selective agents on subindividual variation. This study examines selective pressures exerted during post‐dispersal seed predation and germination on the subindividual variation of seed size in hawthorn (Crataegus monogyna). With a seed offering experiment and a germination test, we estimated phenotypic selection differentials for average and subindividual variation of seed size due to seed predation and germination. Seed size affects germination, growth rate and the probability of an individual seed of escaping predation. Longer seeds showed higher germination rates, but this did not result in significant selection on phenotypes of the maternal trees. On the other hand, seed predators avoided wider seeds, and by doing so exerted phenotypic selection on adult average and subindividual variation of seed size. The detected selection on subindividual variation suggests that the levels of phenotypic variation within individual plants may be, at least partly, the adaptive consequence of animal‐mediated selection.     

Introduction of mammalian seed predators and the loss of an endemic flightless bird impair seed dispersal of the New Zealand tree Elaeocarpus dentatus

Understanding the mutualistic services provided by species is critical when considering both the consequences of their loss or the benefits of their reintroduction. Like many other Pacific islands, New Zealand seed dispersal networks have been changed by both significant losses of large frugivorous birds and the introduction of invasive mammals. These changes are particularly concerning when important dispersers remain unidentified. We tested the impact of frugivore declines and invasive seed predators on seed dispersal for an endemic tree, hinau Elaeocarpus dentatus, by comparing seed dispersal and predation rates on the mainland of New Zealand with offshore sanctuary islands with higher bird and lower mammal numbers. We used cameras and seed traps to measure predation and dispersal from the ground and canopy, respectively. We found that canopy fruit handling rates (an index of dispersal quantity) were poor even on island sanctuaries (only 14% of seeds captured below parent trees on islands had passed through a bird), which suggests that hinau may be adapted for ground‐based dispersal by flightless birds. Ground‐based dispersal of hinau was low on the New Zealand mainland compared to sanctuary islands (4% of seeds dispersed on the mainland vs. 76% dispersed on islands), due to low frugivore numbers. A flightless endemic rail (Gallirallus australis) conducted the majority of ground‐based fruit removal on islands. Despite being threatened, this rail is controversial in restoration projects because of its predatory impacts on native fauna. Our study demonstrates the importance of testing which species perform important mutualistic services, rather than simply relying on logical assumptions.     

昆虫种子捕食与蒙古栎种子产量和种子大小的关系

为了了解昆虫种子捕食者在栎类种群更新中的作用,于2006年秋季,在黑龙江省伊春市带岭区东方红林场研究了昆虫对蒙古栎Quercus mongolica在参园和东山两个种群的种子蛀食情况及其与蒙古栎种子产量和种子大小的关系。结果表明：(1)在参园和东山两个林分内,蒙古栎种子雨动态非常相似,种子雨成分中完好种子的平均密度仅为3.2±4.1个/m²（参园）和1.7±2.8个/m²（东山）,分别仅占种子产量的4.0％和3.2％,而虫蛀种子和败育种子的比例均在38%以上,以虫蛀种子的比例最高,分别为58.2％和57.7％;(2)柞栎象Curculio arakawai是蛀食蒙古栎种子的主要昆虫种类,在虫蛀种子中所占比例高达96.8％（参园）和97.1％（东山）,且象甲蛀食种子中所含虫卵数与种子大小有关,即种子越大,所含象甲的虫卵数就较多。本研究的结果说明2006年蒙古栎成熟种子多遭遇虫蛀,导致完好种子的数量极低,因而可能成为限制蒙古栎种群更新的重要因素。     

Weed seed availability,carabid intraspecific interference and their interaction drive weed seed consumption by carabid beetles

1. Carabid beetles may play a role in the regulation of arable weeds, when seed predation is directly density‐dependent and consumption increases more than proportionally with seed density.
2. We quantified the effect of seed and carabid density on consumption by two carabid species (Poecilus cupreus and Harpalus affinis) of the weed species Taraxacum officinale and Viola arvensis. For each pairing of carabid and weed species, we quantified per capita seed mortality (PCSM) over 72 h under combinations of low and high densities of carabids and seeds, aiming to assess whether per capita consumption increased more or less than proportionally with seed density.
3. We tested three hypotheses: (i) PCSM is constant as seed availability increases; (ii) for a fixed seed density, PCSM decreases with increasing numbers of carabid conspecifics; and (iii) the response of PCSM changes with the interaction between seed and carabid density.
4. Without intraspecific competition, we detected a seed density‐independent predation effect, which became inverse density‐dependent over time. At similar seed densities, the presence of conspecific carabids did not affect PCSM, although it led to a faster shift towards inverse density dependence in T. officinale.
5. The findings of the present study suggest that a sufficiently large carabid numerical response might lead to direct predation density‐dependence.

     

Co-evolution of seed size and seed predation

Using the evolutionarily stable strategy (ESS) approach in a model for the co-evolution of seed size and seed predation, I show that seed size variation within individual plants is favoured if there is a trade-off in the predator's attack rate for different seed sizes. A single seed size is not evolutionarily stable because a predator that is optimally adapted to one particular seed size cannot prevent invasion by plants with a different seed size. The model generates the following predictions. The ESS consists of a continuous range of seed sizes. Small seeds tend to be attacked more frequently than big seeds. Plants with many resources and plants with low (frequency-independent) juvenile mortality have more variable seeds than plants with few resources and a high juvenile mortality. Seed size variation is higher in fluctuating populations regulated by seed predation alone than in stable populations (partially) regulated by seedling competition. Predator searching behaviour does not directly affect the ESS seed size range, but may have an indirect effect by affecting population stability or the significance of seedling competition as a population regulating mechanism. Moreover, seed size distributions are found to be more skewed in favour of small seeds if predation is spatially non-uniform than if predation is more even. Application of the model to systems of several co-evolving plant and predator species is discussed.     

Pre‐dispersal seed predators and fungi differ in their effect on Luehea seemannii capsule development,seed germination,and dormancy across two Panamanian forests

Pre‐dispersal seed predation can greatly reduce crop size affecting recruitment success. In addition, non‐fatal damage by seed predators may allow infection by fungi responsible for post‐dispersal seed losses. The objectives of this study were (1) to quantify pre‐dispersal seed predation and fungal infection in a Neotropical tree species, Luehea seemannii, that produces dehiscent fruits and wind‐dispersed seeds, and (2) to link pre‐dispersal effects on seed quality to seed survival in the soil. To examine how seed predators and fungi influence seed losses, mesh exclosures, fungicide, and the combination of both treatments were applied to separate branches in the canopy of trees in Gamboa and Parque Natural Metropolitano (PNM), Panama. To determine if treatments affect seed viability and survival in the soil, half of the seeds collected from each treatment were buried for 4 weeks in forest soils and subsequently allowed to germinate before and after the breaking of dormancy. Overall, 24 percent of developing fruit were lost to insect attack. In contrast, fungi infected only 3 percent of seeds at the pre‐dispersal stage. For seeds germinated directly after collection, fungicide significantly increased germination in the wetter site (Gamboa) but decreased germination in the drier site (PNM). The pre‐dispersal insect exclosure treatment increased the fraction of seeds that remained dormant after burial in the soil. This result suggests that exposure to insect predators may cause physical damage to seeds that results in the loss of physical dormancy but does not necessarily increase the susceptibility of seeds to pathogen attack in the soil.     

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire‐prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non‐serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post‐fire communities and ecosystems widespread in the Rocky Mountains.     

凋落物和土壤覆盖对动物取食和搬运辽东栎种子的影响

在六盘山区的华北落叶松人工林,研究了清除凋落物、凋落物覆盖和土壤覆盖(以不清除凋落物直接将种子投放于森林地表为对照)等处理对动物取食和搬运辽东栎种子的影响.结果表明:种子释放3d后,凋落物和土壤覆盖处理种子均具有较高的留存率(分别为10.7％和7.0％);释放14 d后,土壤覆盖处理种子的留存率仍最高(0.7％),但凋落物覆盖处理种子的留存率为0.凋落物和土壤覆盖处理种子的就地取食率很高(分别为45.9％和41.5％);清除凋落物处理种子的就地取食率最低(27.0％),但其被搬运后的取食率最高(49.8％);凋落物覆盖、清除凋落物和土壤覆盖处理种子被动物搬运后的埋藏率均显著高于对照(P＜0.01).种子被动物搬运后集中分布于5 m以内,尤其在＜1 m和l～2m两个距离组的分布频率更高;种子被搬运后取食的平均距离大干埋藏的平均距离,以土壤覆盖和凋落物覆盖处理最大,分别为2.38 m±0.55 m和1.44m±0.26 m.     

Satiation of predispersal seed predators: the importance of considering both plant and seed levels

Plants can reduce the fitness costs of granivory by satiating seed predators. The most common satiation mechanism is the production of large crops, which ensures that a proportion of the seeds survive predation. Nevertheless, satiation of small granivores at the seed level may also exist. Larger seeds would satiate more efficiently, enhancing the probability of seed survival after having been attacked. However, a larger seed size could compromise the efficiency of satiation by means of large crops if there were a negative relationship between seed size and the number of seeds produced by an individual plant. We analyze both types of satiation in the interaction between the holm oak Quercus ilex and the chestnut weevil Curculio elephas. Both crop size and acorn size differed strongly in a sample of 32 trees. Larger crop sizes satiated weevils, and higher proportions of the seeds were not attacked as crop size increased. Larger seeds also satiated weevil larvae, as a larger acorn size increased the likelihood of embryo survival. Seedling size was strongly related to acorn size and was reduced by weevil attack, but seedlings coming from large weeviled acorns were still larger. The number and the size of the acorns produced by individual trees were negatively related. Larger proportions of the crop were infested in oaks producing less numerous crops of larger acorns. However, contrary to expectations, these trees did not satiate more effectively at the seed level either. Effective satiation by larger acorns was precluded by larger multi-infestation rates associated to smaller seed crops, in such a way that the proportion of attacked seeds that survived did not vary among trees with different acorn sizes. These results highlight the need of considering satiation by means of large crops and large seeds in studies of predispersal seed predation. Long-term monitoring on individual oaks will help to assess whether there is a trade-off between the number and the size of the acorns and, if it existed, how it could condition the fitness consequences of both types of satiation.     

杂草种子传播研究进展

种子传播将母株生殖周期的末端与它们后代种群的建立连结了起来,广泛认为,其对植被结构具有深刻的影响。种子传播的整个过程称为种子传播循环。研究表明,杂草种子传播的因子多种多样,包括仅依赖自身来完成的主动传播,以及依赖风、水、动物、人类等外界媒介的被动传播。其中,人类传播杂草种子是影响最广泛的一种,对现代植物的分布格局产生了深刻的影响。杂草种子的传播,对杂草种子库的数量和空间动态影响很大。研究种子传播的主要方法有荧光染料标记法、放射性同位素标记法、稳定同位素分析、分子遗传标记等。结合近几年国内外的研究进展,作者就杂草种子传播对种子库数量和空间动态影响的精确直接研究、杂草种子传播的过程及传播后的命运、杂草种子适应传播的机理、生态控草措施研究、外来杂草入侵蔓延与其种子传播的关系等方面提出了展望。