Latitude, seed predation and seed mass

Aim We set out to test the hypothesis that rates of pre‐ and post‐dispersal seed predation would be higher towards the tropics, across a broad range of species from around the world. We also aimed to quantify the slope and predictive power of the relationship between seed mass and latitude both within and across species. Methods Seed mass, pre‐dispersal seed predation and post‐dispersal seed removal data were compiled from the literature. Wherever possible, these data were combined with information regarding the latitude at which the data were collected. Analyses were performed using both cross‐species and phylogenetic regressions. Results Contrary to expectations, we found no significant relationship between seed predation and latitude (log₁₀ proportion of seeds surviving predispersal seed predation vs. latitude, P = 0.63; R² = 0.02; n = 122 species: log₁₀ proportion of seeds remaining after postdispersal seed removal vs. latitude, P = 0.54; R² = 0.02; n = 205 species). These relationships remained non‐significant after variation because of seed mass was accounted for. We also found a very substantial (R² = 0.21) relationship between seed mass and latitude across 2706 species, with seed mass being significantly higher towards the tropics. Within‐species seed mass decline with latitude was significant, but only about two‐sevenths, as rapid as the cross‐species decline with latitude. Results of phylogenetic analyses were very similar to cross‐species analyses. We also demonstrated a positive relationship between seed mass and development time across ten species from dry sclerophyll woodland in Sydney (P < 0.001; R² = 0.77; Standardized Major Axis slope = 0.14). These data lend support to the hypothesis that growing period might affect the maximum attainable seed mass in a given environment. Main conclusions There was no evidence that seed predation is higher towards the tropics. The strong relationship between seed mass and latitude shown here had been observed in previous studies, but had not previously been quantified at a global scale. There was a tenfold reduction in mean seed mass for every c. 23° moved towards the poles, despite a wide range of seed mass within each latitude.     

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.     

Critical phases in the seed development of common juniper (Juniperus communis)

Common juniper (Juniperus communis L.) populations in northwest European lowlands are currently declining in size and number. An important cause of this decline is a lack of natural regeneration. Low seed viability seems to be one of the main bottlenecks in this process. Previous research revealed a negative relation between seed viability and both temperature and nitrogen deposition. Additionally, the seeds of common juniper have a variable ripening time, which possibly influences seed viability. However, the underlying mechanisms remain unresolved. In order to elucidate this puzzle, it is important to understand in which phases of seed production the main defects are situated, together with the influence of ripening time. In this study, we compared seed viability of populations with and without successful recruitment. We examined three seed phases: (i) gamete development; (ii) fertilisation and early‐embryo development; and (iii) late‐embryo development. After the first two phases, we found no difference in the percentage viable seeds between populations with or without recruitment. After late‐embryo development, populations without recruitment showed a significantly lower percentage of viable seeds. These results suggest that late‐embryo development is a bottleneck in seed development. However, the complex interaction between seed viability and ripening time suggest that the causes should be in the second seed phase, as the accelerated development of male and female gametophytes may disturb the male–female synchrony for successful mating.     

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。     

Deceptive fruits reduce seed predation by insects inPistacia terebinthus L. (Anacardiaceae)

Summary The production of seedless fruits (parthenocarpy) is not yet understood from an evolutionary viewpoint, even though it is taxonomically widespread. Here I present a case in which parthenocarpy reduces the incidence of seed predation by insects. At least the first generation of chalcidoid wasps that oviposit in the fruits ofPistacia terebinthus L. (Anacardiaceae) cannot discriminate among viable and inviable fruits, allocating energy and time to oviposition on fruits that are not suitable for larval development.     

The effect of deceptive fruits on predispersal seed predation by birds in Pistacia lentiscus

Deceptive fruits (fruits containing empty seeds and seedless fruits) inthe dioecious shrub Pistacia lentiscus L. (Anacardiaceae)remain attached to the plant throughout the dispersal season. This studyaddresses the hypothesis that deceptive fruits may contribute to parentalfitness by reducing predispersal seed predation by granivorous birds becausethese fruits represent handling costs to predators and they prefer to forageselectively on shrubs with filled fruits. This study was run in two populationsof P. lentiscus and finds that deceptive fruits reduceseedpredation by granivorous birds but only in the population that presented higherP. lentiscus female density, and a higher variability inthe percentage of deceptive fruits among P. lentiscusfemales. Under this scenario, the benefits of flying to another female might behigher than those of remaining in the same female searching filled seeds. Bycontrast, the costs of flying to other females, when they are scarce and have asimilar percentage of deceptive fruits may be higher than the benefits.     

A surplus of seeds: High rates of post-dispersal seed predation in a flooded grassland in monsoonal Australia

Abstract Oryza meridionalis is an annual emergent wetland grass which produces between 26 and 260 kg seeds ha^-1 annually. Seed shed occurs at the end of the wet season, when the plains are usually still partially flooded. The juvenile recruitment of key native vertebrate species, such as the Magpie Goose (Anseranus semipalmata) and the Dusky Plains Rat (Rattus colletti), coincides with seed shed. This study investigated predation of O. meridionalis seeds at two sites on the South Alligator River floodplain in monsoonal Australia. The effects of inundation and the presence of a background density of seeds on seed removal were investigated by stratified sampling with respect to position down the topographic slope, to include the ‘dry’ floodplain margin, ‘damp’Oryza zone, and ‘flooded’Oryza zone. The effect of seed lot size on the proportion of seed removed was also investigated, and exclosures were used to identify the principal predator group. The proportion of seeds removed was not affected by the presence/absence of a background of seeds nor the number of seeds placed in experimental ‘lots’. The majority of seeds (75%) was consumed by vertebrate predators (most likely the abundant Dusky Plains Rat). Inundation afforded some protection from predation. Despite high losses of seeds exposed to predators, O. meridionalis is an abundant and widespread species on these floodplains, possibly because of the protection from predation afforded by inundation to those seeds which are shed into the water column. It is likely that there is a complex interaction between topography, rainfall and predator and prey relationships, which ultimately determines the importance of seed removal for the maintenance of populations of O. meridionalis. These high losses of seeds to predation have implications for wetland rehabilitation where seed broadcasting is proposed.     

The role of pre- and post-dispersal seed predation in determining total seed loss

Most seed predation studies focus on either pre- or post-dispersal predation and may therefore underestimate the role of predation in regulating plant populations. We therefore estimated total seed predation of an invasive tree, mesquite (Leguminoseae: Prosopis spp.), by examining the entire seed pool from tree to seed bank. The spatio-temporal dynamics of total seed predation was examined by sampling across its Australian distribution and through time. The main predator was a host-specialist multivoltine beetle, Algarobius prosopis L. (Bruchidae), previously introduced as a biocontrol agent. Seed predation exceeded 20% in all seed stages (in pods on and off the tree, and seeds within woody endocarps (capsules) and free seeds on and in the ground) but was consistently highest in capsules on the ground (up to 90%). Pre-dispersal predation contributed little. Total seed predation rates were primarily determined by predation rates on the most persistent seed stage, in this case fallen pods if only pods are considered and seeds in capsules for the total seed pool. This pattern was consistent across the surveyed taxa, regions, years and seasonally. Predation rate was relatively unaffected by seed density, potentially because densities were always low (<150 seeds m⁻²). Average total seed predation within a region reached 55%, but we conclude that any population regulation of mesquite by seed predation will principally be through reduced seed bank persistence. Our results highlight the need to consider the entire seed pool, especially the often cryptic and overlooked long-lived stages, when determining seed loss to predation and its likely population consequences.     

The effect of samara wing presence on predation of Acer pseudoplatanus (Sapindaceae) seeds on the ground

The key selective pressure shaping the morphology of samaras is seen as enhancing primary wind-borne dispersal from the parent plant to the ground. However, the consequences of the samara wing of primarily wind-dispersed tree species for post-dispersal processes has not been well studied. We explored whether the presence of this wing in Acer pseudoplatanus either deters or promotes predation after dispersal, either by increasing the time and energy required to predate the seed or by increasing the seed's visibility to predators. We found that wing-removed fruits were preferred, suggesting that the presence of samaras makes seed handling more expensive for granivores. Further, we found that fewer seeds were consumed from treatments that contained the most winged seeds, thus there was no evidence of the samaras making seed finding easier for granivores. We conclude that the presence of the wing may offer an anti-predatory benefit as well as aiding primary dispersal.     

Cloning capacity helps seeds of Garcinia xanthochymus counter animal predation

Seed predators have the potential to act as agents of natural selection that influence seed traits and seed fates, which in turn affect the whole plant population dynamic. Accordingly, plants deploy a variety of mechanisms (e.g., resistance and tolerance strategies) to lessen the impact of predation on seed crop or on an individual seed. In this study, we described a novel mechanism, seed cloning strategy, in a tropical plant species in countering animal predation. By conducting field‐ and laboratory‐based germination experiments, we found that both rodent damaged and artificially damaged seed fragments of a large‐seeded tree Garcinia xanthochymus (Clusiaceae) could successfully germinate and establish as seedlings. Tissue culture experiments revealed that G. xanthochymus has no endosperm in seeds, and its seed fragments own strong capacity of differentiation and cloning. Seed damage negatively affected seedling growth and germination, but the seed germination rate was remarkably high. Our study suggests that, seed cloning capacity, adopted by the large‐seeded tree G. xanthochymus may act as a novel strategy counteract for seed predation and would play a significant role in stabilizing the mutualism between plant and animals.     

槲栎种子雨进程中昆虫的捕食特征

为了了解昆虫捕食与槲栎Quercus aliena种子大小和产量的关系以及在栎林更新中的作用,于2008和2009年秋季,分别在暖温带伏牛山系天池山国家森林公园内研究了两个年度槲栎的种子雨过程及昆虫捕食特征。结果表明：（1）种子雨过程从8月中下旬到9月底约经历40 d,存在高峰期且时段较明显,高峰期下落种子量分别占全部种子雨量的78.13%（2008年）和75.91%（2009年）;（2）槲栎的种子雨强度年间存在较大差异,2008年明显小于2009年（两年分别为31.75±16.65粒/m²和51.92±29.26粒/m²）,但2009年的橡子明显比2008年小（两年分别为1.94±0.61 cm³和2.46±0.57 cm³）;（3）种子雨构成比例在两个年份间存在差异,完好种子的量分别为59.05%（2008年）和36.12%（2009年）,虫蛀率在大量结实的2009年显著提高;（4）虫蛀橡子（2.29±0.42 cm³）显著大于完整橡子（1.59±0.32 cm³）,且虫蛀种子中所含虫卵数与种子大小显著正相关,昆虫有选择大种子产卵寄生的偏好。本研究的结果说明,昆虫对槲栎大种子有寄生选择偏好;大部分槲栎种子遭遇象甲虫蛀而降低生命活力,这可能是影响槲栎林更新的主要因素之一。     

Wild flower seed predation by Pterostichus madidus (Carabidae: Coleoptera)

The objective of the study was to investigate the ability of Pterostichus madidus, one of the most abundant carabid beetles in the United Kingdom, to consume wildflower seeds. The plant species chosen are commonly included in wildflower seed mixes sown to enhance the biodiversity of arable field margins, and hence any seed predation could potentially reduce successful seedling establishment. The study combined cafeteria style laboratory feeding experiments with predation studies in an established field margin. In laboratory studies the Pterostichus madidus consumed both unimbibed and imbibed seeds of Centaurea scabiosa, Galium verum, Leucanthemum vulgare and Primula veris. Seed consumption was still evident when Drosophila pupae were also supplied, although there was significant preference for the pupae. Placement of seeds in the field showed high levels of seed consumption, although part of this was attributed to slugs. Carabid beetles may have a significant role as wild flower seed predators and thus may affect establishment of re‐seeded flower‐rich meadows.     

Impact of overgrazing on seed predation by rodents in the Thar desert,northwestern India

We compared the vegetation structure, rodent density and seed loss rate between protected and disturbed sites affected from grazing by cattle, goats and sheep, in the Thar desert of India. A perennial tussocky grassLasiurus sindicus Hent. was largely dominant in the protected site, whileL. sindicus was rare and replaced by undershrub speciesAerva pseudotomentosa Blatt. & Halb. andCrotalaria burbira Buch.-Ham. in the overgrazed site. In the grazed site, plant coverage was low, but the density of rodent burrows and the frequency of rodent captures were significantly high as compared to the protected site. Corresponding with the density of desert rodents, seed predation was significantly higher in the grazed site than in the protected site. These results suggest that overgrazing by large mammals has strong effects on plant succession by altering not only the species composition and abundance of plant community, but also the habitat suitability for seed-eating rodents.     

Pre- and postdispersal seed predation by rodents: balance of food and safety

Seed presentation and availability for seed predators changeduring every plant reproductive cycle. We know very little abouthow those changes impinge on both the ability of seed predatorsto impact plant populations and the foraging costs associatedwith seed consumption. Therefore, we conducted several fieldexperiments to evaluate whether wood mice Apodemus sylvaticusbalance food and safety while foraging on Helleborus foetidusseeds during both the pre- and early postdispersal phases ofthe plant reproductive cycle. Both food and safety were keydeterminants of mouse foraging on H. foetidus seeds, thoughtheir roles were not consistent along the plant reproductivecycle. Thus, augmenting ambient food reduced fruit removal bymice during the predispersal phase. During the postdispersalphase, seeds in sheltered microsites experienced higher removalrates than those located in nonsheltered microsites; however,no effect of food augmentation was detected. This apparent reversedrole of food and safety on decision making by mice seemed closelylinked to both the dramatic changes in accessibility and presentationof H. foetidus seeds and the coupled changing foraging costsfaced by mice at different phases of the plant reproductivecycle. For instance, because the cost of foraging for predispersalseeds was higher than for postdispersal seeds, the effect offood augmentation on foraging by wood mice was greater duringthe predispersal phase. Thus, our study illustrates the needof considering differences between pre- and postdispersal seedpredation in the study of granivore rodents and their impacton plant populations.     

食种子动物对三种锦鸡儿属植物繁殖更新的影响

锦鸡儿属(Caragana)植物是黄土高原植被的重要组分,具有固氮、耐旱的特性,对黄土丘陵沟壑区的植被恢复具有重要意义。作者于2002–2003年对该属本地种白毛锦鸡儿(C.licentiana)和甘蒙锦鸡儿(C.opulens)及外来种中间锦鸡儿(C.intermedia)种子被动物取食情况进行了比较研究。2003年白毛锦鸡儿种子散落前的虫害率为88.2%,甘蒙锦鸡儿为29.7%±1.7%,中间锦鸡儿为43.2%±4.8%。种子落地后,白毛锦鸡儿和甘蒙锦鸡儿的种子被小啮齿动物取食的比例在各种微生境下均为100%,且取食速度很快,在种子放置后4d内即全部被取食。中间锦鸡儿种子被取食率则因微生境的不同而不同:在灌丛下被取食率为100%,在半阳坡的开阔草地上为85.0%±10.0%,在种植植物未成功的水平阶地上为31.0%±8.7%,且取食速度相对较慢。研究结果表明,食种子动物的取食大大减少了白毛锦鸡儿和甘蒙锦鸡儿的种子量以及种子传播萌发的机会。但食种子动物可能充当了白毛锦鸡儿4.0%硬实种子的传播者,从而有助于白毛锦鸡儿实现种子途径的更新;中间锦鸡儿种子无论在散落前还是散落后受食种子动物的影响均很小。     

Evidence for pre-dispersal predation of seeds of Tithonia diversifolia by the black-faced canary (Serinus capistratus)

Pre-dispersal predation of seeds of exotic Asteraceae by foraging birds is understudied. Using phenological records and photo-assisted analysis of damages made to seed heads, this paper provides evidence that the black-faced canary Serinus capistratus Finsch and Hartlaub feeds on immature achenes of Tithonia diversifolia (Hemsley) A. Gray and may control its propagule pressure up to 18% of the total number of achenes aged between 20–30 days counted from petal fall.     

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.     

Post‐dispersal seed predation and its relations with seed traits: a thirty‐species‐comparative study

Post‐dispersal seed predation is a key process determining the variability in seed survival in forests, where most seeds are handled by rodents. Seed predation is thought to affect seedling regeneration, colonization ability and spatial distribution of plants. Basic seed traits are the essential factors affecting rodent foraging preferences and thus seed survival and seedling recruitment. Many studies have discussed several seed traits and their effects upon seed predation by rodents. However, the results of those previous studies are usually equivocal, likely because few seed traits and/or plant species tend to be incorporated into these studies. In order to elucidate the relationships between seed predation and seed traits, we surveyed the predation of 48 600 seeds in a natural pine forest, belonging to 30 species, for three consecutive years. The results demonstrated that: (i) seed size and seed coat hardness did not significantly affect seed predation; (ii) total phenolics had a negative effect upon seed predation; (iii) positive effects of nitrogen content upon seed predation were found. From our study, it seems that the better strategy to prevent heavy predation is for plants to produce seeds with higher total phenolics content rather than physical defenses (i.e. hard seed coat) or larger seeds. Additionally, rodent foraging preference may depend more on Nitrogen content than other nutrient content of seeds.