Predator–prey interactions between Dugesia gonocephala and free-living nematodes

1. Three groups of laboratory experiments clarified the role of nematodes as a potential food resource for the triclad Dugesia gonocephala. The first group measured the functional response of adult D. gonocephala feeding on juvenile or adult Caenorhabditis elegans. The feeding rates of D. gonocephala on adult and juvenile C. elegans followed a type II functional response. The maximum number of adult nematodes and juvenile nematodes eaten by a single D. gonocephala individual within 3 h was 94 and 197 nematodes, respectively. 2. A second group of microcosm experiments investigated the effect of D. gonocephala on the density and the vertical distribution of a nematode community in fine sand. The following treatments were performed: (i) microcosms with 400 nematodes and (ii) microcosms with 400 nematodes and one D. gonocephala. After 5 days, nematodes as a group, as well as the dominant species Tobrilus pellucidus and Trischistoma monohystera, showed no significant difference in vertical patterns between the treatments with and without D. gonocephala. 3. The third group of experiments determined whether grain size of the sediment (sand, fine gravel and coarse gravel) altered the ability of D. gonocephala to consume adult C. elegans. Sand and fine gravel reduced the predation effectiveness of D. gonocephala by 100%, whereas the predator consumed nematodes in coarse gravel (19 nematodes within 3 h).     

Geographical and within-population variation in the globeflower–globeflower fly interaction: the costs and benefits of rearing pollinator’s larvae

Interspecific interactions can vary within and among populations and geographical locations, and this variation can influence the nature of the interaction (e.g. mutualistic versus antagonistic) and its evolutionary stability. Globeflowers are exclusively pollinated by flies whose larvae feed only on their seeds. Here we document geographical variability in costs and benefits in globeflowers in sustaining their pollinating flies throughout the range of this arctic-alpine European plant over several years. A total of 1,710 flower heads from 38 populations were analysed for their carpel, egg and seed contents. Individual and population analyses control for the confounding influences of variation in both: (1) population traits, such as fly density and egg distribution among flower heads; and (2) individuals traits, such as carpel and egg numbers per flower head. Despite considerable variation in ecological conditions and pollinator densities across populations, large proportions (range 33–58%) of seeds are released after predation, with a benefit-to-cost ratio of 3, indicating that the mutualism is stable over the whole globeflower geographical range. The stability of the mutualistic interaction relies on density-dependent competition among larvae co-developing in a flower head. This competition is revealed by a sharp decrease in the number of seeds eaten per larva with increasing larval number, and is intensified by non-uniform egg distribution among globeflowers within a population. Carpel number is highly variable across globeflowers (range 10–69), and flies lay more eggs in large flowers. Most plants within a population contribute to the rearing of pollinators, but some pay more than others. Large globeflowers lose more seed to pollinator larvae, but also release more seed than smaller plants. The apparent alignment of interests between fly and plant (positive relationship between numbers of seeds released and destroyed) is shown to hide a conflict of interest found when flower size is controlled for.     

Following the initiation and development of individual leaf primordia at the level of the shoot apical meristem: the case of distichous phyllotaxis in Begonia

BACKGROUND AND AIMS: By using the technique of replicas of a developing apex it is possible to obtain a direct measure of phyllotactic parameters (plastochrone and platochronic ratio) involved in the initiation of two successive primordia at the level of the SAM. The goal of this study is to compare, in a real time setting, the value of phyllotactic parameters in distichous systems using Begonia as a case study, with the value of the same parameters in spiral phyllotactic systems. METHODS: To determine the real-time sequence of events at the level of the SAM, replicas were made of the developing apex at different intervals using previously described techniques. Impression moulds were made at 24-h intervals. The following phyllotactic parameters were measured: plastochrone, angle of divergence, plastochrone ratio and ratio between the diameter of the leaf and the apex. RESULTS: The time between the appearance of two successive leaves is 15-20 d. The average value of the plastochrone ratio (R) is 1.3, and the ratio of the leaf to the diameter of the apex (Gamma) is 2.5. The angle of divergence varies from 165 masculine to 180 masculine. The speed of advection of the primordium from the apex, varies from 0.28 to 0.37 microm d(-1). CONCLUSIONS: The speed of advection of primordia in Begonia is lower than that of Anagalis. This is not in accordance with theoretical simulations that predict the opposite. In Begonia, the plastochrone ratio does not reflect the real time of appearance of two successive primordia. The time separating the appearance of two primordia is not directly related to the distance of these two primordia from the centre of the apex but is related instead to the enlargement of leaves.     

Tracking the role of alternative prey in soybean aphid predation by Orius insidiosus: a molecular approach

The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is a pest of soybeans in Asia, and in recent years has caused extensive damage to soybeans in North America. Within these agroecosystems, generalist predators form an important component of the assemblage of natural enemies, and can exert significant pressure on prey populations. These food webs are complex and molecular gut-content analyses offer nondisruptive approaches for examining trophic linkages in the field. We describe the development of a molecular detection system to examine the feeding behaviour of Orius insidiosus (Hemiptera: Anthocoridae) upon soybean aphids, an alternative prey item, Neohydatothrips variabilis (Thysanoptera: Thripidae), and an intraguild prey species, Harmonia axyridis (Coleoptera: Coccinellidae). Specific primer pairs were designed to target prey and were used to examine key trophic connections within this soybean food web. In total, 32% of O. insidiosus were found to have preyed upon A. glycines, but disproportionately high consumption occurred early in the season, when aphid densities were low. The intensity of early season predation indicates that O. insidiosus are important biological control agents of A. glycines, although data suggest that N. variabilis constitute a significant proportion of the diet of these generalist predators. No Orius were found to contain DNA of H. axyridis, suggesting intraguild predation upon these important late-season predators during 2005 was low. In their entirety, these results implicate O. insidiosus as a valuable natural enemy of A. glycines in this soybean agroecosystem.     

Web-site selection strategies of linyphiid spiders in alfalfa: implications for biological control

Site-specific foraging can enhance the ability of generalist predators to provide effective and sustainable levels of pest control in agroecosystems. This can result from increased growth rates, higher population densities, and improved capture frequencies of pests at high prey density microsites. We tested the hypothesis that linyphiid spiders would exhibit microhabitat-specific web-site selection strategies in alfalfa. This was predicted to result in high prey densities at web-sites compared to paired non-web-sites through direct, or indirect, selection of prey-rich habitats. A total of 22,242 potential prey items were collected from mini-sticky traps located at 896 microsites. Web-centered mini-sticky traps on the ground, representative of Erigone autumnalis Emerton (Araneae: Linyphiidae) webs, captured similar numbers of potential prey as paired non-web-centered traps nearby. However, aerial sticky traps at web-sites of Bathyphantes pallidus (Banks) (Araneae: Linyphiidae) contained significantly more Diptera and Empoasca fabae (Harris) (Homoptera: Cicadellidae) than paired non-web-centered sticky-traps. Prey activity-densities also varied between web-sites of E. autumnalis and B. pallidus. Diptera were dominant at aerial microsites of B. pallidus whilst Collembola were abundant on ground-based traps of E. autumnalis. These results suggest that in alfalfa, the pressure for selecting prey-rich web-sites by erigonine spiders is low, but B. pallidus exhibits a selective web-location strategy targeted towards high quality dipteran prey. These sites also captured large numbers of E. fabae, a major pest of alfalfa, thus implicating aerial-based linyphiines as valuable predators in biological control.     

Insect-resistant biotech crops and their impacts on beneficial arthropods

With a projected population of 10 billion by 2050, an immediate priority for agriculture is to achieve increased crop yields in a sustainable and cost-effective way. The concept of using a transgenic approach was realized in the mid-1990s with the commercial introduction of genetically modified (GM) crops. By 2010, the global value of the seed alone was US $11.2 billion, with commercial biotech maize, soya bean grain and cotton valued at approximately US $150 billion. In recent years, it has become evident that insect-resistant crops expressing δ-endotoxin genes from Bacillus thuringiensis have made a significant beneficial impact on global agriculture, not least in terms of pest reduction and improved quality. However, because of the potential for pest populations to evolve resistance, and owing to lack of effective control of homopteran pests, alternative strategies are being developed. Some of these are based on Bacillus spp. or other insect pathogens, while others are based on the use of plant- and animal-derived genes. However, if such approaches are to play a useful role in crop protection, it is desirable that they do not have a negative impact on beneficial organisms at higher trophic levels thus affecting the functioning of the agro-ecosystem. This widely held concern over the ecological impacts of GM crops has led to the extensive examination of the potential effects of a range of transgene proteins on non-target and beneficial insects. The findings to date with respect to both commercial and experimental GM crops expressing anti-insect genes are discussed here, with particular emphasis on insect predators and parasitoids.     

Does maternal care evolve through egg recognition or directed territoriality?

The mechanism that facilitates the evolution of maternal care is ambiguous in egg‐laying terrestrial vertebrates: does the ability of mothers to recognize their own eggs lead them under some circumstances to begin providing care or can maternal care evolve from simply being in close proximity to the eggs (e.g. through territorial behaviour)? This question is difficult to answer because in most species, parental care is either absent altogether or present; in only a few species we have the opportunity to observe intraspecific variation in the expression of parental care. We studied a population of long‐tailed skinks (Eutropis longicaudata) in which females have recently evolved maternal care from a noncaring state. Females on Orchid Island, Taiwan, remain with their eggs during incubation and when doing so, actively deter egg predation by egg‐eating snakes (Oligodon formosanus); in all other populations, females lack post‐ovipositional maternal care. Nest‐guarding females on Orchid Island (i) showed antipredator behaviours only in the original nest site in which they laid eggs, even after we removed all of the eggs or substituted them with those of a conspecific; (ii) protect any eggs present inside the original nest site (even when the eggs belong to a conspecific); and (iii) develop this behaviour while gravid (i.e. prior to laying eggs). This supports the hypothesis that long‐tailed skinks cannot recognize their own eggs, suggesting that maternal care is a directed form of territoriality only expressed towards egg‐eating snakes and only during reproduction. Nest guarding is among the most primitive forms of parental care, and the recent evolution of this behaviour in a single population provides insight into one of the mechanisms by which parental care can originate in terrestrial vertebrates.     

Superpredation patterns in four large European raptors

Predatory interactions among top predators, like superpredation or intraguild predation (IGP), can influence community structure. Diurnal raptors occupy high trophic levels in terrestrial food webs, and thus can regulate the presence of mesopredators. We studied superpredation (the killing and eating of another predator) in four large European raptors. We gathered 121 dietary studies, totalling 161,456 prey for the Goshawk Accipiter gentilis L., Golden Eagle Aquila chrysaetos L., Bonelli’s Eagle Aquila fasciata Vieillot, and Eagle Owl Bubo bubo L. Results showed that superpredation: (1) is a widespread interaction in large raptors, but it can vary according to the top predator species; (2) is not an important energetic resource for large raptors, but rather seems mostly related to diet diversification when the main prey decreases; (3) is spatially clustered reflecting habitat heterogeneity, but shows no temporal or large-scale spatial trends; and (4) it is associated with lower breeding success of the top predator species. These findings support the food stress hypothesis as the main driving force behind increases in superpredation and IGP in raptors, with the decrease in breeding performance as a side effect. Superpredation by large raptors deserves future research to understand its effects on mesopredators, because on one hand it might contribute to promote biodiversity, while on the other hand, it can sometimes represent an additional risk for small populations of endangered mesopredators.     

Vertebrate predators have minimal cascading effects on plant production or seed predation in an intact grassland ecosystem

The strength of trophic cascades in terrestrial habitats has been the subject of considerable interest and debate. We conducted an 8-year experiment to determine how exclusion of vertebrate predators, ungulates alone (to control for ungulate exclusion from predator exclusion plots) or none of these animals influenced how strongly a three-species assemblage of rodent consumers affected plant productivity. We also examined whether predator exclusion influenced the magnitude of post-dispersal seed predation by mice. Both ungulates and rodents had strong direct effects on graminoid biomass. However, rodent impacts on plant biomass did not differ across plots with or without predators and/or ungulates. Deer mice removed more seeds from seed depots on predator exclusion plots, suggesting trait-mediated indirect effects of predators, but this short-term behavioural response did not translate into longer-term impacts on seed survival. These results suggest that vertebrate predators do not fundamentally influence primary production or seed survival in our system.     

Temperature,Herbivory and Epibiont Acquisition as Factors Controlling the Distribution and Ecological Role of an Invasive Seaweed

The invasive canopy alga, Codium fragile ssp. tomentosoides, first observed at the Isles of Shoals in 1983, has become the dominant canopy species to 8 m throughout the islands. Codium populations are replacing themselves at most sites in what appears to be a new, climax, canopy species. However, Codium densities have declined in protected Gosport Harbor areas where it first became established. Codium has only slowly expanded its presence in adjacent nearshore subtidal habitats. Recent studies suggest a combination of factors that may be influencing the relative success of populations between habitats. The herbivorous sea slug, Placida dendritica, may be reducing populations in protected areas in spite of predators such as the green crab, Carcinus maenas, while surge may inhibit herbivore buildup in exposed habitats. Temperature instability due to localized, wind-driven upwelling may be slowing the buildup of subtidal Codium populations in nearshore sites. The combination of Codium dominance and the acquisition of increasing epibiont diversity are producing a new, potentially more complex community state than the previous kelp-dominated climax typical of the Gulf of Maine.