Biotic resistance via granivory: establishment by invasive, naturalized, and native asters reflects generalist preference

Escape from specialist natural enemies is frequently invoked to explain exotic plant invasions, but little attention has been paid to how generalist consumers in the recipient range may influence invasion. We examined how seed preferences of the widespread generalist granivore Peromyscus maniculatus related to recruitment of the strongly invasive exotic Centaurea stoebe and several weakly invasive exotics and natives by conducting laboratory feeding trials and seed addition experiments in the field. Laboratory feeding trials showed that P. maniculatus avoided consuming seeds of C. stoebe relative to the 12 other species tested, even when seeds of alternative species were 53-94% smaller than those of C. stoebe. Seed addition experiments conducted in and out of rodent exclosures revealed that weakly invasive exotics experienced relatively greater release from seed predation than C. stoebe, although this was not the case for natives. Seed mass explained 81% of the variation in recruitment associated with rodent exclusion for natives and weak invaders, with larger-seeded species benefiting most from protection from granivores. However, recruitment of C. stoebe was unaffected by rodent exclusion, even though the regression model predicted seeds of correspondingly large mass should experience substantial predation. These combined laboratory and field results suggest that generalist granivores can be an important biological filter in plant communities and that species-specific seed attributes that determine seed predation may help to explain variation in native plant recruitment and the success of exotic species invasions.     

Seed predation by Neotropical rain forest mammals increases diversity in seedling recruitment

Seed dispersal and seedling recruitment (the transition of seeds to seedlings) set the spatiotemporal distribution of new individuals in plant communities. Many terrestrial rain forest mammals consume post-dispersal seeds and seedlings, often inflicting density-dependent mortality. In part because of density-dependent mortality, diversity often increases during seedling recruitment, making it a critical stage for species coexistence. We determined how mammalian predators, adult tree abundance, and seed mass interact to affect seedling recruitment in a western Amazonian rain forest. We used exclosures that were selectively permeable to three size classes of mammals: mice and spiny rats (weighing <1 kg), medium-sized rodents (1-12 kg), and large mammals (20-200 kg). Into each exclosure, we placed seeds of 13 tree species and one canopy liana, which varied by an order of magnitude in adult abundance and seed mass. We followed the fates of the seeds and resulting seedlings for at least 17 months. We assessed the effect of each mammalian size class on seed survival, seedling survival and growth, and the density and diversity of the seedlings that survived to the end of the experiment. Surprisingly, large mammals had no detectable effect at any stage of seedling recruitment. In contrast, small- and medium-sized mammals significantly reduced seed survival, seedling survival, and seedling density. Furthermore, predation by small mammals increased species richness on a per-stem basis. This increase in diversity resulted from their disproportionately intense predation on common species and large-seeded species. Small mammals thereby generated a rare-species advantage in seedling recruitment, the critical ingredient for frequency dependence. Predation by small (and to a lesser extent, medium-sized) mammals on seeds and seedlings significantly increases tree species diversity in tropical forests. This is the first long-term study to dissect the effects of various mammalian predators on the recruitment of a diverse set of tree species.     

Pre-dispersal seed predation: the role of fruit abortion and selective oviposition

Oviposition sites of phytophagous insects should correlate with plant traits that maximize survival of the progeny. Plants, on the other hand, should benefit from traits and developmental patterns that complicate oviposition decisions. In the antagonistic interaction between plant and pre-dispersal seed predator the time lag between egg laying and seed development may allow for abortion of fruits in plants, potentially reducing fitness loss through predation. We studied the perennial herb Lathvrus vernus and the beetle pre-dispersal seed predator Bruchus atomarius in Sweden to determine the fitness consequences of nonrandom fruit abortion in the plant and oviposition patterns of the beetle. The beetle had a sophisticated ability to locate fruits with high probability of retention, partly by fruit position and phenology but also by some additional unidentified cue. Mortality of eggs was density dependent, but still the egg-laying pattern was clumped. We found no defensive strategy in the plant; instead the predictable fruit abortion pattern was associated with decreased plant fitness. We discuss how interactions may pose simultaneous selection pressures on plant and insect traits and how life history traits and other selective forces may shape the adaptive outcome of the interaction in plant and insect, respectively.     

Increasing enemy biodiversity strengthens herbivore suppression on two plant species

Concern over biodiversity loss, especially at higher trophic levels, has led to a surge in studies investigating how changes in natural enemy diversity affect community and ecosystem functioning. These studies have found that increasing enemy diversity can strengthen, weaken, and not affect prey suppression, demonstrating that multi-enemy effects on prey are context-dependent. Here we ask how one factor, plant species identity, influences multi-enemy effects on prey. We focused on two plant species of agricultural importance, potato (Solanum tuberosum), and collards (Brassica oleracea L.). These species share a common herbivorous pest, the green peach aphid (Myzus persicae), but vary in structural and chemical traits that affect aphid reproductive rates and which may also influence inter-enemy interactions. In a large-scale field experiment, overall prey exploitation varied dramatically among the plant species, with enemies reducing aphid populations by approximately 94% on potatoes and approximately 62% on collards. Increasing enemy diversity similarly strengthened aphid suppression on both plants, however, and there was no evidence that plant species identity significantly altered the relationship between enemy diversity and prey suppression. Microcosm experiments suggested that, on both collards and potatoes, intraspecific competition among natural enemies exceeded interspecific competition. Enemy species showed consistent and significant differences in where they foraged on the plants, and enemies in the low-diversity treatment tended to spend less time foraging than enemies in the high-diversity treatment. These data suggest that increasing enemy diversity may strengthen aphid suppression because interspecific differences in where enemies forage on the plant allow for greater resource partitioning. Further, these functional benefits of diversity appear to be robust to changes in plant species identity.     

Dangerous liaisons disperse the Mediterranean dwarf palm: fleshy-pulp defensive role against seed predators

We chose the interaction between the Mediterranean dwarf palm (Chamaerops humilis) and its major seed disperser, the Eurasian badger (Meles meles), to evaluate the hypothesis that endozoochory is characterized by a mixture of conflicting and overlapping interests, with the capacity of being positive or negative for plant fitness. For instance, because of the potential protective role against invertebrate seed predators of C. humilis pulp, we expected that badger-ingested diaspores (i.e., seeds without pulp) would show lower survival than control seeds with the pulp attached. Conversely, due to the possible germination inhibitory function of C. humilis fruit pulp, it was also likely that badger-ingested seeds germinate in higher proportion than control seeds. We evaluated our predictions by carrying out a field sowing and monitoring it over two years. We estimated several stage-specific transition probabilities as well as the cumulative probability of seedling recruitment under different treatments. Our experimental results revealed the multifunctionality of C. humilis fruit pulp and that fruit ingestion by badgers had conflicting outcomes for the palm. As predicted, seed survival was much lower and seed predation by invertebrates much higher for badger-ingested than for control seeds, suggesting a defensive role of C. humilis ripe fruit pulp. All early-emerged seedlings came from badger-ingested seeds, suggesting an inhibitory function of fruit pulp. Though we did not find an effect of removal from the maternal environment on most components of fitness, seedling survival for badger-ingested seeds was higher away from than beneath conspecifics. Badgers imposed a sizeable short-term fitness cost to C. humilis and therefore could be categorized as a "dangerous liaison." Nonetheless, because of the high mobility of the badger, its dispersal service appears paramount given the severe fragmentation and isolation of most C. humilis populations across the highly humanized Mediterranean basin. Our study thus illustrates the necessity of assessing concurrently direct and indirect effects of plant-disperser interactions at different stages of the plant life cycle and recruitment process.     

Seed trait changes in dispersers' guts and consequences for germination and seedling growth

The effectiveness of a frugivore as a disperser of a plant is greatly determined by how fruits and seeds are handled in its mouth and its digestive tract. Although a number of studies have investigated the effect of avian ingestion on germination, we still know very little about the modifications to seeds during ingestion and the specific consequences on plant fitness. Here we investigate for the first time the different mechanisms by which germination patterns of seeds are modified following ingestion by frugivores. Specifically, we examine changes in seed mass, water content, permeability, seed coat thickness, texture, and resistance in two common Mediterranean fleshy-fruited plants, Phillyrea angustifolia and Myrtus communis, after ingestion by Eurasian Blackbirds, Turdus merula. We found a number of differences between the plant species: Phillyrea seeds lost mass, mainly due to water loss, and had thinner coats after gut passage, but Myrtus seeds did not. Seeds of both species showed increased permeability, while Myrtus seeds in particular became less resistant to breakage. No quantifiable changes in seed coat texture were detected in either species, although this trait was partly associated with differences in germination rate in Phillyrea. High intraspecific plant variation was found for most seed traits measured. Seed passage through birds' guts sped up germination in both species, especially in Myrtus. Increased permeability in seeds of both species following ingestion resulted in a higher germination rate. Moreover, seeds with thick coats (and in the case of Phillyrea, harder coats) germinated at a slower rate and produced seedlings that also grew more slowly, indicating a cost of coat thickness and/or hardness for seedling emergence. Results obtained here contribute to explaining the great heterogeneity in germination responses among and within plant species and the large variety of factors, both intrinsic and extrinsic to the plants, that influence such responses.     

Evaluating the impacts of multiple generalist fungal pathogens on temperate tree seedling survival

Host-specific mortality driven by natural enemies is a widely discussed mechanism for explaining plant diversity. In principle, populations of plant species can be regulated by distinct host-specific natural enemies that have weak or nonexistent effects on heterospecific competitors, preventing any single species from becoming dominant and thus promoting diversity. Two of the first steps in exploring the role of natural enemies in diversity regulation are to (1) identify potential enemies and (2) evaluate their levels of host specificity by determining if interactions between any one host and its enemy have equivalent survival impacts on co-occurring host species. We developed a bioinformatics framework to evaluate impacts of potential pathogens on seedling survival, for both single and multiple infections. Importantly, we consider scenarios not only if there are specialist pathogens for each plant, but also when generalist pathogens have differential effects on multiple host species, and when co-infection has species-specific effects. We then applied this analytical framework to a field experiment using molecular techniques to detect potential fungal pathogens on co-occurring tree seedling hosts. Combinatorial complexity created by 160 plant-fungus interactions was reduced to eight combinations that affect seedling survival. Potential fungal pathogens had broad host ranges, but seedling species were each regulated by different combinations of fungi or by generalist fungi that had differential effects on multiple plant species. Soil moisture can have the potential to shift the nature of the interactions in some plant-fungal combinations from neutral to detrimental. Reassessing the assumption of single-enemy-single-host interactions broadens the mechanisms through which natural enemies can influence plant diversity.     

Predispersal seed herbivores, not pollinators, exert selection on floral traits via female fitness

Herbivores that oviposit in flowers of animal-pollinated plants depend on pollinators for seed production and are therefore expected to choose flowers that attract pollinators. This provides a mechanism by which seed herbivores and pollinators could impose conflicting selection on floral traits. We measured phenotypic selection on floral traits of Lobelia siphilitica (Lobeliaceae) via female fitness to determine the relative strength of selection by pollinators and a specialist predispersal seed herbivore. We were able to attribute selection on flowering phenology to the herbivores. However, no selection could be attributed to pollinators, resulting in no conflicting selection on floral traits. Unlike pollinators, whose preference for certain floral traits does not always translate into higher fitness, any discrimination by seed herbivores is likely to decrease fitness of the preferred floral phenotype. Thus predispersal seed herbivores may be a significant agent of selection on floral traits.     

Population regulation by enemies of the grass Brachypodium sylvaticum: demography in native and invaded ranges

The enemy-release hypothesis (ERH) states that species become more successful in their introduced range than in their native range because they leave behind natural enemies in their native range and are thus "released" from enemy pressures in their introduced range. The ERH is popularly cited to explain the invasive properties of many species and is the underpinning of biological control. We tested the prediction that plant populations are more strongly regulated by natural enemies (herbivores and pathogens) in their native range than in their introduced range with enemy-removal experiments using pesticides. These experiments were replicated at multiple sites in both the native and invaded ranges of the grass Brachypodium sylvaticum. In support of the ERH, enemies consistently regulated populations in the native range. There were more tillers and more seeds produced in treated vs. untreated plots in the native range, and few seedlings survived in the native range. Contrary to the ERH, total measured leaf damage was similar in both ranges, though the enemies that caused it differed. There was more damage by generalist mollusks and pathogens in the native range, and more damage by generalist insect herbivores in the invaded range. Demographic analysis showed that population growth rates were lower in the native range than in the invaded range, and that sexually produced seedlings constituted a smaller fraction of the total in the native range. Our removal experiment showed that enemies regulate plant populations in their native range and suggest that generalist enemies, not just specialists, are important for population regulation.     

Acceleration of Exotic Plant Invasion in a Forested Ecosystem by a Generalist Herbivore

Abstract: The successful invasion of exotic plants is often attributed to the absence of coevolved enemies in the introduced range (i.e., the enemy release hypothesis). Nevertheless, several components of this hypothesis, including the role of generalist herbivores, remain relatively unexplored. We used repeated censuses of exclosures and paired controls to investigate the role of a generalist herbivore, white‐tailed deer (Odocoileus virginianus), in the invasion of 3 exotic plant species (Microstegium vimineum, Alliaria petiolata, and Berberis thunbergii) in eastern hemlock (Tsuga canadensis) forests in New Jersey and Pennsylvania (U.S.A.). This work was conducted in 10 eastern hemlock (T. canadensis) forests that spanned gradients in deer density and in the severity of canopy disturbance caused by an introduced insect pest, the hemlock woolly adelgid (Adelges tsugae). We used maximum likelihood estimation and information theoretics to quantify the strength of evidence for alternative models of the influence of deer density and its interaction with the severity of canopy disturbance on exotic plant abundance. Our results were consistent with the enemy release hypothesis in that exotic plants gained a competitive advantage in the presence of generalist herbivores in the introduced range. The abundance of all 3 exotic plants increased significantly more in the control plots than in the paired exclosures. For all species, the inclusion of canopy disturbance parameters resulted in models with substantially greater support than the deer density only models. Our results suggest that white‐tailed deer herbivory can accelerate the invasion of exotic plants and that canopy disturbance can interact with herbivory to magnify the impact. In addition, our results provide compelling evidence of nonlinear relationships between deer density and the impact of herbivory on exotic species abundance. These findings highlight the important role of herbivore density in determining impacts on plant abundance and provide evidence of the operation of multiple mechanisms in exotic plant invasion.     

Flying foxes cease to function as seed dispersers long before they become rare

Rare species play limited ecological roles, but particular behavioral traits may predispose species to become functionally extinct before becoming rare. Flying foxes (Pteropodid fruit bats) are important dispersers of large seeds, but their effectiveness is hypothesized to depend on high population density that induces aggressive interactions. In a Pacific archipelago, we quantified the proportion of seeds that flying foxes dispersed beyond the fruiting canopy, across a range of sites that differed in flying fox abundance. We found the relationship between ecological function (seed dispersal) and flying fox abundance was nonlinear and consistent with the hypothesis. For most trees in sites below a threshold abundance of flying foxes, flying foxes dispersed < 1% of the seeds they handled. Above the threshold, dispersal away from trees increased to 58% as animal abundance approximately doubled. Hence, flying foxes may cease to be effective seed dispersers long before becoming rare. As many species' populations decline worldwide, identifying those with threshold relationships is an important precursor to preservation of ecologically effective densities.     

Postdispersal seed predation limits the abundance of a long-lived perennial forb (Lithospermum ruderale)

Loss of seeds to consumers is common in plant communities, but the degree to which these losses influence plant abundance or population growth is often unclear. This is particularly the case for postdispersal seed predation by rodents, as most studies of rodent seed predation have focused on the sources of spatiotemporal variation in seed loss but not quantified the population consequences of this loss. In previous work we showed that seed predation by deer mice (Peromyscus maniculatus) substantially reduced seedling recruitment and establishment of Lithospermum ruderale (Boraginaceae), a long-lived perennial forb. To shed light on how rodent seed predation and the near-term effects on plant recruitment might influence longer-term patterns of L. ruderale population growth, we combined experimental results with demographic data in stage-based population models. Model outputs revealed that rodent seed predation had a significant impact on L. ruderale population growth rate (lambda). With the removal of postdispersal seed predation, the projected population growth rates increased between 0.06 and 0.12, depending on site (mean deltalambda across sites = 0.08). Seed predation shifted the projected stable stage distribution of populations from one with a high proportion of young plants to one in which larger adult size classes dominate. Elasticities of vital rates also changed, with germination and growth of seedlings and young plants becoming more important with the removal of seed predation. Simulations varying the magnitude of seed predation pressure while holding other vital rates constant showed that seed predation could lower lambda even if only 40% of available seeds were consumed. These results demonstrate that rodent granivory can be a potent force limiting the abundance of a long-lived perennial forb.     

Invasive competitor and native seed predators contribute to rarity of the narrow endemic Astragalus sinuatus Piper

The conservation of rare plant species hinges on our ability to identify the underlying mechanisms that limit rare plant populations. Theory on rarity suggests that both predispersal seed predation and competition can be important mechanisms influencing abundance and/or distribution of rare plant populations. Yet few studies have tested these interactions, and those that have evaluated each mechanism independently. Astragalus sinuatus Piper (Whited's milkvetch) is a narrow endemic plant species restricted to eight populations within a 10-km2 area in eastern Washington. We used experimental and observational methods to test the effects of native insect predispersal seed predators and an invasive grass (Bromus tectorum L. [cheatgrass]) on seed set and population density of A. sinuatus. We quantified per capita seed production and pod predation rates across four sites and among four years. Seed predation rates were high across four sites (66-82%) and all years (65-82%). Experimental reduction of predispersal seed predators significantly increased per capita seed set of A. sinuatus (164-345%) at two experimental sites. Concurrently, two seed addition experiments demonstrated the effect of seed loss and presence of B. tectorum on seedling recruitment and establishment of A. sinuatus over four growing seasons. In the first seed addition experiment, we found no difference in recruitment and establishment between low (40) and high (120) seed addition levels. In the second addition experiment (one level of addition; 40 seeds), we found that recruitment and survivorship increased 200% in plots where B. tectorum was removed compared to plots where B. tectorum was present. Thus, seed addition had no impact in the presence of B. tectorum; conversely, in the absence of B. tectorum, seed addition was highly effective at increasing population numbers. Results suggest that, in areas where B. tectorum is present, recruitment is site limited, and it is seed limited when B. tectorum is absent. We recommend that managers reduce B. tectorum in an effort to increase population growth of A. sinuatus; in areas where B. tectorum is absent, short-term reduction of insect predators should be considered as a strategy to increase population growth of this rare species.     

Effects of different pretreatments on germination of Prunus serotina seed sources

Establishing intensive plantations of fast growing hardwood tree species that have high market values in the forest industry can narrow the gap between Turkey's demand and the supply of quality hardwood products. Black cherry (P. serotina Ehrh.) is a fast growing hardwood species with a high market value. Introducing and intensively growing black cherry (BC) in Turkey may significantly reduce the country's quality wood shortage. Adequate seed germination constitutes the first essential step for successful establishments. In this paper effects of different pretreatments, including artificial and natural stratification, on the seeds of different BC seed sources (SSs) were studied. Pretreatments had substantial effects on the dormancy breaking and germination behaviours of the SSs. Consecutive periods of complex warm and cold artificial stratification regimes longer than 90 days or natural stratification (where seeds were assumed to be naturally exposed to this complexity) resulted in best dormancy breaking and, in turn, germination among all pretreatments. Deeper dormancy and reduced germination rates of some BC seeds as the altitude of the source increases might suggest an ecological adaptive strategy of the species. BC may have deeper morphophysiological dormancy than is commonly believed. Seed size may have a positive effect on seed germination.     

Experimental manipulation of seed shadows of an Afrotropical tree determines drivers of recruitment

The loss of animals in tropical forests may alter seed dispersal patterns and reduce seedling recruitment of tree species, but direct experimental evidence is scarce. We manipulated dispersal patterns of Manilkara mabokeensis, a monkey-dispersed tree, to assess the extent to which spatial distributions of seeds drive seedling recruitment. Based on the natural seed shadow, we created seed distributions with seeds deposited under the canopy ("no dispersal"), with declining density from the tree ("natural dispersal"), and at uniform densities ("good dispersal"). These distributions mimicked dispersal patterns that could occur with the extirpation of monkeys, low levels of hunting, and high rates of seed dispersal. We monitored seedling emergence and survival for 18 months and recorded the number of leaves and damage to leaves. "Good dispersal" increased seedling survival by 26%, and "no dispersal" decreased survival by 78%, relative to "natural dispersal." Using a mixed-effects survival model, we decoupled the distance and density components of the seed shadow: seedling survival depended on the seed density, but not on the distance from the tree. Although community seedling diversity tended to decrease with longer dispersal distances, we found no conclusive evidence that patterns of seed dispersal influence the diversity of the seedling community. Local seed dispersal does affect seedling recruitment and survival, with better dispersal resulting in higher seedling recruitment; hence the loss of dispersal services that comes with the reduction or extirpation of seed dispersers will decrease regeneration of some tree species.     

土壤种子库研究方法评述

土壤种子库（soil seed bank,SSB）是指存在于土壤表面和土壤中的全部存活种子的总和。开展对土壤种子库的研究,对于分析了解过去地上植被植物群落和预示未来植被发展方向具有重要的指导意义,而对于土壤种子库研究方法的探讨则是开展土壤种子库基础实验研究的基础和支撑。本文从土壤种子库的取样时间、取样方法、取样大小和数量、鉴定方法四个方面归纳了土壤种子库的研究方法,理论分析了同位素应用于土壤种子库中种子年龄结构测定的可行性,以及植冠种子库的研究方法,并探讨了提高土壤种子库中种子萌发率的新方法。综合分析来看：（1）土壤种子库的取样时间、取样方法、取样大小和数量、鉴定方法是土壤种子库研究方法需要关注的四大关键问题,对土壤种子库的实验结果具有显著的直接影响。不同的取样时间代表的土壤种子库的内容和意义不同,取样方法、大小和数量则应根据研究区和研究内容来确定,鉴定方法也应根据具体实验区研究的物种数来考量;（2）理论上在利用同位素来测定土壤种子库中种子年龄结构是可行的,但若想广泛应用于实验中仍需进一步的深入研究;（3）目前对植冠种子库的研究方法也仅仅局限于直接计数法和间接计数法,对植冠种子库的研究方法的探讨则急需加强;（4）同时总结和提出了4种提高土壤种子库中种子萌发率的方法,未来在利用种子萌发法时,应加强注意和应用这4种方法来测定土壤种子库。     

Soil vs. canopy seed storage and plant species coexistence in species-rich Australian shrublands

The fire-prone shrublands of southwestern Australia are renowned for their high plant species diversity and prominence of canopy seed storage (serotiny). We compared species richness, abundance, and life history attributes for soil and canopy seed banks in relation to extant vegetation among four sites with different substrate conditions and high species turnover (50-80%) to identify whether this unusual community-level organization of seed storage might contribute to maintenance of high species richness. Soil seed bank (SSB) densities were low to moderate (233-1435 seeds/m2) compared with densities for other Mediterranean-type vegetation and were lowest for sites with highest canopy seed bank (CSB) species richness and lowest nutrient availability, but not richness or abundance of resprouters. Annuals were infrequent in the lowest nutrient sites, but there was no evidence that small SSB size was due to low seed inputs or a trade-off between seed production/storage and seed size in response to low nutrient availability. Sorensen's similarity between SSB and extant vegetation was 26-43% but increased to 54-57% when the CSB was included, representing levels higher than reported for most other ecosystems. Resprouting species were well represented in both the SSB and CSB, and there was no evidence for lower seed production in resprouters than in non-sprouters overall. The SSB and CSB held no species in common and were characterized by markedly different seed dispersal attributes, with winged or small seeds in the CSB and seeds dispersed by ants, birds, and wind (though none with wings) in the SSB. There was no evidence of spatial differentiation in the distribution of seeds of SSB species between vegetated and open microsites that might facilitate species coexistence, but most woody non-sprouters showed aggregation at scales of 1-2 m, implying limited seed dispersal. High similarity between overall seed bank (SSB + CSB) and extant species composition, high number of resprouting species, and seed dispersal processes before (SSB) and after fire (CSB) leading to differential spatial aggregation of post-fire recruits from the two seed bank types may buffer species composition against rapid change and provide a mechanism for maintaining species coexistence at the local scale.     

Direct and interactive effects of enemies and mutualists on plant performance: a meta-analysis

Plants engage in multiple, simultaneous interactions with other species; some (enemies) reduce and others (mutualists) enhance plant performance. Moreover, effects of different species may not be independent of one another; for example, enemies may compete, reducing their negative impact on a plant. The magnitudes of positive and negative effects, as well as the frequency of interactive effects and whether they tend to enhance or depress plant performance, have never been comprehensively assessed across the many published studies on plant-enemy and plant-mutualist interactions. We performed a meta-analysis of experiments in which two enemies, two mutualists, or an enemy and a mutualist were manipulated factorially. Specifically, we performed a factorial meta-analysis using the log response ratio. We found that the magnitude of (negative) enemy effects was greater than that of (positive) mutualist effects in isolation, but in the presence of other species, the two effects were of comparable magnitude. Hence studies evaluating single-species effects of mutualists may underestimate the true effects found in natural settings, where multiple interactions are the norm and indirect effects are possible. Enemies did not on average influence the effects on plant performance of other enemies, nor did mutualists influence the effects of mutualists. However, these averages mask significant and large, but positive or negative, interactions in individual studies. In contrast, mutualists ameliorated the negative effects of enemies in a manner that benefited plants; this overall effect was driven by interactions between pathogens and belowground mutualists (bacteria and mycorrhizal fungi). The high frequency of significant interactive effects suggests a widespread potential for diffuse rather than pairwise coevolutionary interactions between plants and their enemies and mutualists. Pollinators and mycorrhizal fungi enhanced plant performance more than did bacterial mutualists. In the greenhouse (but not the field), pathogens reduced plant performance more than did herbivores, pathogens were more damaging to herbaceous than to woody plants, and herbivores were more damaging to crop than to non-crop plants (suggesting evolutionary change in plants or herbivores following crop domestication). We discuss how observed differences in effect size might be confounded with methodological differences among studies.     

Effects of Pioneer Tree Species Hyperabundance on Forest Fragments in Northeastern Brazil

Abstract: Despite many studies on fragmentation of tropical forests, the extent to which plant and animal communities are altered in small, isolated forest fragments remains obscure if not controversial. We examined the hypothesis that fragmentation alters the relative abundance of tree species with different vegetative and reproductive traits. In a fragmented landscape (670 km²) of the Atlantic Forest of northeastern Brazil, we categorized 4056 trees of 182 species by leafing pattern, reproductive phenology, and morphology of seeds and fruit. We calculated relative abundance of traits in 50 1‐ha plots in three types of forest configurations: forest edges, small forest fragments (3.4–83.6 ha), and interior of the largest forest fragment (3500 ha, old growth). Although evergreen species were the most abundant across all configurations, forest edges and small fragments had more deciduous and semideciduous species than interior forest. Edges lacked supra‐annual flowering and fruiting species and had more species and stems with drupes and small seeds than small forest fragments and forest interior areas. In an ordination of species similarity and life‐history traits, the three types of configurations formed clearly segregated clusters. Furthermore, the differences in the taxonomic and functional (i.e., trait‐based) composition of tree assemblages we documented were driven primarily by the higher abundance of pioneer species in the forest edge and small forest fragments. Our work provides strong evidence that long‐term transitions in phenology and seed and fruit morphology of tree functional groups are occurring in fragmented tropical forests. Our results also suggest that edge‐induced shifts in tree assemblages of tropical forests can be larger than previously documented.     

稻田环境与生物多样性实证研究

通过对浙江省安吉县的调查发现，在自然生态环境相似的两个自然村中，以农业为基础产业的禹山坞村稻田及其环境中水栖无脊椎动物、畦畔和沟渠植物的种类和数量多于以工业为主导产业的北村，稻田杂草和秋季稻田天敌的种类和数量少于北村。研究分析表明，稻田及其环境的生物种类和数量与农业生态环境质量的优劣及化肥、农药、除草剂的施用量密切相关。