Attractiveness of single and multiple species flower patches to beneficial insects in agroecosystems

The provision of floral resources for the enhancement of beneficial insect populations has shown promise as a strategy to enhance biological control and pollination in agroecosystems. One approach involves the provision of a single flower species while a second involves the multiple flower species, but the two have never been compared experimentally. Here we examine the influence of single and multiple species flower treatments on the abundance and foraging behaviour of key beneficial insects in two agricultural agroecosystems (broccoli and lucerne crops). The five flower treatments comprised buckwheat only, phacelia only, a simple mixture of buckwheat and phacelia, a complex mixture of buckwheat, phacelia and a commercial seed blend or the existing crop as a control. The abundance of bumble‐bees (Bombus hortorum) and honey bees (Apis mellifera) was highest in the three treatments that contained phacelia, while hoverfly (Melanostoma fasciatum) numbers were high in all four flower treatments. Bumble‐bees and honey bees probed almost exclusively phacelia flowers, even when provided with a choice of other flower species in the simple and complex mixture treatments. In contrast, hoverflies probed the flowers of all plant species in single and multiple species treatments, with no apparent difference in acceptance. However, in mixture treatments, the majority of individual bumble‐bees, honey bees and hoverflies probed the flowers from only one species, despite the presence of alternative flower species. Our results illustrate how an appreciation of insect floral attractiveness can be used to customise the species composition of floral patches to potentially maximise biological control and pollination in targeted agroecosystems.     

Noncrop flowering plants restore top‐down herbivore control in agricultural fields

Herbivore populations are regulated by bottom‐up control through food availability and quality and by top‐down control through natural enemies. Intensive agricultural monocultures provide abundant food to specialized herbivores and at the same time negatively impact natural enemies because monocultures are depauperate in carbohydrate food sources required by many natural enemies. As a consequence, herbivores are released from both types of control. Diversifying intensive cropping systems with flowering plants that provide nutritional resources to natural enemies may enhance top‐down control and contribute to natural herbivore regulation. We analyzed how noncrop flowering plants planted as “companion plants” inside cabbage (Brassica oleracea) fields and as margins along the fields affect the plant–herbivore–parasitoid–predator food web. We combined molecular analyses quantifying parasitism of herbivore eggs and larvae with molecular predator gut content analysis and a comprehensive predator community assessment. Planting cornflowers (Centaurea cynanus), which have been shown to attract and selectively benefit Microplitis mediator, a larval parasitoid of the cabbage moth Mamestra brassicae, between the cabbage heads shifted the balance between trophic levels. Companion plants significantly increased parasitism of herbivores by larval parasitoids and predation on herbivore eggs. They furthermore significantly affected predator species richness. These effects were present despite the different treatments being close relative to the parasitoids’ mobility. These findings demonstrate that habitat manipulation can restore top‐down herbivore control in intensive crops if the right resources are added. This is important because increased natural control reduces the need for pesticide input in intensive agricultural settings, with cascading positive effects on general biodiversity and the environment. Companion plants thus increase biodiversity both directly, by introducing new habitats and resources for other species, and indirectly by reducing mortality of nontarget species due to pesticides.     

Molecular identification of predation by carabid beetles on exotic and native slugs in a strawberry agroecosystem

Generalist predators have the capacity to exert significant pressure on prey populations. However, integrating them into biological control programs relies on a detailed understanding of their foraging behavior and the levels of trophic connectedness with pest species. Carabid beetles are important predators of slugs, pests of agricultural, floricultural and horticultural crops worldwide, but these interactions have been rarely studied outside the Western Palearctic ecozone. Diagnostic molecular gut-content analysis was used to examine the strength of trophic pathways between a community of carabid beetles and two slug species, the exotic Deroceras reticulatum and native Deroceras laeve, in strawberry agroecosystems. Strawberries were grown according to standard horticultural practices for central Kentucky, following traditional bare ground planting or with the addition of detrital subsidies, to quantify the impact of habitat management on the abundance of pests and the strength of these trophic pathways. Following laboratory characterization of species-specific molecular markers targeting both Deroceras species, carabid beetles collected from a strawberry agroecosystem were screened for slug DNA. Field collections revealed important food web pathways existed between Harpalus pensylvanicus and D. reticulatum, with 7.2% screening positive for these prey yet none screening positive for D. laeve. In contrast, Chlaenius tricolor was found to feed on both slug species in the field, with 16% screening positive for both Deroceras. Despite below normal rainfall limiting slug densities in the field, the results presented here reveal the potential importance of carabid beetles in slug population dynamics in the Nearctic.     

Salmon subsidize an escape from a size spectrum

A general rule in ecology is that the abundance of species or individuals in communities sharing a common energy source decreases with increasing body size. However, external energy inputs in the form of resource subsidies can modify this size spectrum relationship. Here, we provide the first test of how a marine resource subsidy can affect size spectra of terrestrial communities, based on energy derived from Pacific salmon carcasses affecting a forest soil community beside streams in western Canada. Using both species-based and individual approaches, we found size structuring in this forest soil community, and transient community-wide doubling of standing biomass in response to energy pulses from Pacific salmon carcasses. One group of species were clear outliers in the middle of the size spectrum relationship: larval calliphorid and dryomyzid flies, which specialize on salmon carcasses, and which showed a tenfold increase in biomass in their size class when salmon were available. Thus, salmon subsidize their escape from the size spectrum. These results suggest that using a size-based perspective of resource subsidies can provide new insights into the structure and functioning of food webs.     

Climate change will alter amphibian‐mediated nutrient pathways: evidence from Rana temporaria tadpoles in experimental ponds

相似文献   

Beaver activity increases aquatic subsidies to terrestrial consumers

相似文献   

Trophic overlap between fish and riparian spiders: potential impacts of an invasive fish on terrestrial consumers

Studies on resource sharing and partitioning generally consider species that occur in the same habitat. However, subsidies between linked habitats, such as streams and riparian zones, create potential for competition between populations which never directly interact. Evidence suggests that the abundance of riparian consumers declines after fish invasion and a subsequent increase in resource sharing of emerging insects. However, diet overlap has not been investigated. Here, we examine the trophic niche of native fish, invasive fish, and native spiders in South Africa using stable isotope analysis. We compared spider abundance and diet at upstream fishless and downstream fish sites and quantified niche overlap with invasive and native fish. Spider abundance was consistently higher at upstream fishless sites compared with paired downstream fish sites, suggesting that the fish reduced aquatic resource availability to riparian consumers. Spiders incorporated more aquatic than terrestrial insects in their diet, with aquatic insects accounting for 45–90% of spider mass. In three of four invaded trout rivers, we found that the average proportion of aquatic resources in web‐building spider diet was higher at fishless sites compared to fish sites. The probability of web‐building and ground spiders overlapping into the trophic niche of invasive brown and rainbow trout was as high as 26 and 51%, respectively. In contrast, the probability of spiders overlapping into the trophic niche of native fish was always less than 5%. Our results suggest that spiders share resources with invasive fish. In contrast, spiders had a low probability of trophic overlap with native fish indicating that the traits of invaders may be important in determining their influence on ecosystem subsidies. We have added to the growing body of evidence that invaders can have cross‐ecosystem impacts and demonstrated that this can be due to niche overlap.     

Cross‐ecosystem effects of a large terrestrial herbivore on stream ecosystem functioning

Large terrestrial consumers have direct and indirect effects on forest ecosystem function, but few studies have investigated the impacts of terrestrial consumers on freshwater ecosystems. In the Cape Breton Highlands of Nova Scotia, browsing by hyper‐abundant moose following a spruce budworm outbreak has transformed boreal forest into grasslands. We conducted a field study to investigate the potential for cross‐ecosystem effects of hyper‐abundant moose following budworm outbreak on small boreal stream ecosystem structure and function. With our field study, we tested the prediction that watersheds with higher levels of moose‐mediated grasslands in their sub‐basin would have higher stream temperatures, total nitrogen, electrical conductivity, periphyton biomass and macroinvertebrate abundances. While our data supported several of our predictions pertaining to moose impacts on the abiotic variables (i.e. temperature range, total nitrogen, electrical conductivity) we found evidence of variable moose impacts on the benthic community. Specifically, we observed lower relative abundance of predatory invertebrates in streams with high moose impacts compared to streams with low moose impacts in their watersheds but no evidence of moose impacts on the relative abundance of shredders, filterers, gatherers, and grazers. This empirical study fills a key gap in our understanding of spatial ecosystem ecology by providing insight into the effects of large terrestrial consumers across ecosystem boundaries with potential implications for landscape‐scale management of hyper‐abundant ungulates. Given the broad availability and improvement in remote sensing technology, the novel integration of remote sensing and field studies employed here may provide a roadmap for future studies of meta‐ecosystem dynamics.     

Multiple anthropogenic changes cause biodiversity loss through plant invasion

Non‐native invasive plants are a widely acknowledged threat to global biodiversity. However, our understanding of the mechanisms underlying plant invasion, and the relative importance of multiple rather than single drivers, remains poor. Here, we provide a case study using time‐series data to reconstruct patterns of change, and field experiments to test for causality. We show how, over a 50‐year period, a series of unrelated human‐induced changes created highly favorable conditions for the non‐native tree mallow (Lavatera arborea) to turn invasive, causing loss of native vegetation and seabird breeding habitat. The combination of three drivers: human‐introduced disease, climate warming and a fisheries‐mediated increase in seabird populations, removed major constraints on plant population growth, (i.e. grazer control, climatic control, germination opportunity and nutrient limitation). Collectively, these changes created optimal conditions for the rapid expansion of tree mallow. The resulting dramatic impact on both the native vegetation and breeding seabirds, notably puffins (Fratercula arctica), exemplifies how non‐native invasive plant species can transform terrestrial ecosystems. While climate change is regarded as a key factor behind plant invasion, we highlight that multiple rather than single factors may be critical to biodiversity loss.     

Small island biogeography in the Gulf of California: lizards, the subsidized island biogeography hypothesis, and the small island effect

Aim We used insular lizard communities to test the predictions of two hypotheses that attempt to explain patterns of species richness on small islands. We first address the subsidized island biogeography (SIB) hypothesis, which predicts that spatial subsidies may cause insular species richness to deviate from species–area predictions, especially on small islands. Next, we examine the small island effect (SIE), which suggests small islands may not fit the traditional log‐linear species–area curve. Location Islands with arthropodivorous lizard communities throughout the Gulf of California. Methods To evaluate the SIB hypothesis, we first identified subsidized and unsubsidized islands based on surrogate measures of allochthonous productivity (i.e. island size and bird presence). Subsequently, we created species–area curves from previously published lizard species richness and island area data. We used the residuals and slopes from these analyses to compare species richness on subsidized and unsubsidized islands. To test for an SIE, we used breakpoint regression to model the relationship between lizard species richness and island area. We compared results from this model to results from the log‐linear regression model. Results Subsidized islands had a lower slope than unsubsidized islands, and the difference between these groups was significant when small islands were defined as < 1 km². In addition to comparing slopes, we tested for differences in the magnitude of the residuals (from the species–area regression of all islands) for subsidized vs. unsubsidized islands. We found no significant patterns in the residual values for small vs. large islands, or between islands with and without seabirds. The SIE was found to be a slightly better predictor of lizard species richness than the traditional log‐linear model. Main conclusions Predictions of the SIB hypothesis were partially supported by the data. The absence of a significant SIE may be a result of spatial subsidies as explained by the SIB hypothesis and data presented here. We conclude by suggesting potential scenarios to test for interactions between these two small island hypotheses. Future studies considering factors affecting species richness should examine the possible role of spatial subsidies, an SIE, or a synergistic effect of the two in data sets with small islands.