Experimental evidence that livestock grazing intensity affects the activity of a generalist predator

Grazing by domestic ungulates has substantial impacts on ecosystem structure and composition. In grasslands of the northern hemisphere, livestock grazing limits populations of small mammals, which are a main food source for a variety of vertebrate predators. However, no experimental studies have described the impact of livestock grazing on vertebrate predators. We experimentally manipulated sheep and cattle grazing intensity in the Scottish uplands to test its impact on a relatively abundant small mammal, the field vole (Microtus agrestis), and its archetypal generalist predator, the red fox (Vulpes vulpes). We demonstrate that ungulate grazing had a strong consistent negative impact on both vole densities and indices of fox activity. Ungulate grazing did not substantially affect the relationship between fox activity and vole densities. However, the data suggested that, as grazing intensity increased i) fox activity indices tended to be higher when vole densities were low, and ii) the relationship between fox activity and vole density was weaker. All these patterns are surprising given the relative small scale of our experiment compared to large red fox territories in upland habitats of Britain, and suggest that domestic grazing intensity causes a strong response in the activity of generalist predators important for their conservation in grassland ecosystems.     

Learning affects prey selection in larvae of a generalist coccinellid predator

Under natural conditions, generalist predatory insects have to cope with a variety of potential prey species that are not all equally suitable. Under these circumstances, learning may be adaptive if it allows adjustment to variations in resource quality and availability. Under laboratory conditions, we examined the learning ability and memory in the prey selection process of larvae of the predatory coccinellid Coleomegilla maculata ssp. lengi Timberlake (Coleoptera: Coccinellidae). Using choice tests, we studied prey rejection behaviour of C. maculata fourth instars towards prey of different quality and we also tested the influence of hunger and prior experience with other food types on the prey rejection behaviour of coccinellid larvae. Coleomegilla maculata larvae gradually changed their behaviour and rejected low‐quality hosts more frequently, whereas high‐quality hosts were nearly always accepted. After 48 h, the learned behaviour appeared to be partially forgotten. Hunger and experience with other food types prior to the test had little effect on the gradual change of behaviour but the quality of the food ingested influenced the initial level of prey rejection. Our results demonstrate that (1) C. maculata larvae can adjust their prey selection behaviour with experience to reject progressively less suitable prey, and (2) previous experience with other prey types can influence their initial preference.     

Effects of landscape structure and land-use intensity on similarity of plant and animal communities

Aim Species richness in itself is not always sufficient to evaluate land management strategies for nature conservation. The exchange of species between local communities may be affected by landscape structure and land‐use intensity. Thus, species turnover, and its inverse, community similarity, may be useful measures of landscape integrity from a diversity perspective. Location A European transect from France to Estonia. Methods We measured the similarity of plant, bird, wild bee, true bug, carabid beetle, hoverfly and spider communities sampled along gradients in landscape composition (e.g. total availability of semi‐natural habitat), landscape configuration (e.g. fragmentation) and land‐use intensity (e.g. pesticide loads). Results Total availability of semi‐natural habitats had little effect on community similarity, except for bird communities, which were more homogeneous in more natural landscapes. Bee communities, in contrast, were less similar in landscapes with higher percentages of semi‐natural habitats. Increased landscape fragmentation decreased similarity of true bug communities, while plant communities showed a nonlinear, U‐shaped response. More intense land use, specifically increased pesticide burden, led to a homogenization of bee, bug and spider communities within sites. In these cases, habitat fragmentation interacted with pesticide load. Hoverfly and carabid beetle community similarity was differentially affected by higher pesticide levels: for carabid beetles similarity decreased, while for hoverflies we observed a U‐shaped relationship. Main conclusions Our study demonstrates the effects of landscape composition, configuration and land‐use intensity on the similarity of communities. It indicates reduced exchange of species between communities in landscapes dominated by agricultural activities. Taxonomic groups differed in their responses to environmental drivers and using but one group as an indicator for ‘biodiversity’ as such would thus not be advisable.     

Plant genetic identity of foundation tree species and their hybrids affects a litter-dwelling generalist predator

The effects of plant genetics on predators, especially those not living on the plant itself, are rarely studied and poorly understood. Therefore, we investigated the effect of plant hybridization and genotype on litter-dwelling spiders. Using an 18-year-old cottonwood common garden, we recorded agelenid sheet-web density associated with the litter layers of replicated genotypes of three tree cross types: Populus fremontii, Populus angustifolia, and their F₁ hybrids. We surveyed 118 trees for agelenid litter webs at two distances from the trees (0–100 and 100–200 cm from trunk) and measured litter depth as a potential mechanism of web density patterns. Five major results emerged: web density within a 1-m radius of P. angustifolia was approximately three times higher than within a 1-m radius of P. fremontii, with F₁ hybrids having intermediate densities; web density responded to P. angustifolia and F₁ hybrid genotypes as indicated by a significant genotype × distance interaction, with some genotypes exhibiting a strong decline in web density with distance, while others did not; P. angustifolia litter layers were deeper than those of P. fremontii at both distance classes, and litter depth among P. angustifolia genotypes differed up to 300 %; cross type and genotype influenced web density via their effects on litter depth, and these effects were influenced by distance; web density was more sensitive to the effects of tree cross type than genotype. By influencing generalist predators, plant hybridization and genotype may indirectly impact trophic interactions such as intraguild predation, possibly affecting trophic cascades and ecosystem processes.     

Local predator composition and landscape affects biological control of aphids in alfalfa fields

The biological control service supplied to croplands is a result of the predator community present within a focal crop, which is likely influenced by surrounding landscape composition and configuration. In this study, using cage experiments in two regions near Santiago, we determined if predator communities supplied a significant biological control service in alfalfa fields, examined how the abundance of exotic and native coccinellids, as well as other key predator groups, influenced biological control of aphids and measured how landscape composition and heterogeneity at three spatial scales influenced this service. We found that predators significantly suppressed aphid populations in both regions, but the relative importance of predators versus landscape variables on biological control differed between regions. In the region where predators were abundant, biological control was higher and related to the abundance of native coccinellids and syrphids, highlighting the importance of native species as providers of crucial ecological services. In the region where predators were not abundant, biological control was lower, and it was related to landscape composition, being positively associated with the abundance of woodlots and urban habitats, and negatively associated with fruit crops in the landscape. Therefore, landscape effects on biological control service may be weaker than local factors, and only become important when local predator abundance is low.     

Continuous monitoring of predator control operations at landscape scale

Summary   Exotic predators are considered pests to wildlife and agriculture, requiring predator-control programs. Effective monitoring of predator-control operations is essential to justify their considerable cost, but often impossible in practice. The difficulties are especially severe if the target species is small and wide-ranging, and the area to be protected is inaccessible and/or extensive. A convenient model predator of this type, the feral Ferret ( Mustela furo ), is subject to control on farmland in New Zealand. We monitored the distribution of Ferrets over 2400 ha in the central North Island, before and throughout a standard control operation by professional trappers. We used 24 units of a new automated monitoring device, the Scentinel, set in a grid at 1 per 100 ha. Over 11 weeks (11 February to 29 April 2005, 1718 trap nights), we recorded 1559 visits by small mammals, including 198 by Ferrets. By the end of the 4th week, Ferrets had been detected at 17 of 24 sites. Removal of Ferrets from the study area by contractors began during the 5th week, and was reflected in significant declines in the number of Ferret visits recorded per day ( P  = 0.008) and the number of sites visited ( P  = 0.021). Analysis of our extensive repeat-survey data by site-occupancy methods confirmed these trends in greater detail, while also allowing for variation in detectability.     

Traits of butterfly communities change from specialist to generalist characteristics with increasing land-use intensity

Land-use intensification leads to species loss and shifts in community composition, but only few studies examine how these dynamics affect ecological and life-history traits. We thus investigated whether ecological and life-history traits differ between butterfly communities of grasslands with different land-use intensity. We conducted butterfly transect surveys in 137 grassland sites in three regions of Germany and compiled 10 species-specific ecological and life-history traits from the literature. These traits are associated with food plant specialisation, dispersal, distribution, reproduction and development. We calculated a land-use intensity gradient based on the amount of fertilise mowing frequency and grazing intensity. We analysed differences of traits characteristics between butterfly communities along the land-use intensity gradient in a fourth-corner analysis, thus considering correlations between traits. Six ecological and life-history traits changed from characteristics associated with specialists to such associated with generalists with increasing land-use intensity. These traits characteristics in intensified grasslands were: high dispersal propensity, large distribution range, low population density, more than one generation per year, hibernation in a more advanced developmental stage and a long flight period. The functional homogenisation of the butterfly communities with changes from specialist to generalist trait characteristics with increasing land-use intensity may have severe consequences for ecosystem functioning and services.     

Consequences of the size structure of fish populations for their effects on a generalist avian predator

Size-structured interspecific interactions can shift between predation and competition, depending on ontogenetic changes in size relationships. I examined the effects of common carp (Cyprinus carpio), an omnivorous fish, on the reproductive success of the red-necked grebe (Podiceps grisegena), an avian gape-limited predator, along a fish size gradient created by stocking distinct age-cohorts in seminatural ponds. Young-of-the-year (0+) carp were an essential food source for young grebes. Only adult birds were able to consume 1-year-old (1+) fish, while 2-year-old (2+) fish attained a size refuge from grebes. Amphibian larvae were the principal alternative prey to fish, followed by macroinvertebrates, but the abundance of both dramatically decreased along the carp size gradient. Fledging success was 2.8 times greater in ponds with 0+ versus 1+ carp; in ponds with 1+ carp, chicks received on average 2.6–3 times less prey biomass from their parents, and over 1/3 of broods suffered total failure. Breeding birds avoided settling on 2+ ponds. These results show that changes in prey fish size structure can account for shifts from positive trophic effects on the avian predator to a negative impact on the predator’s alternative resources. However, competition did not fully explain the decrease in grebe food resources in the presence of large fish, as carp and grebes overlapped little in diet. In experimental cages, 1+ carp totally eliminated young larvae of amphibians palatable to fish. In field conditions, breeding adults of palatable taxa avoided ponds with 1+ and older carp. Non-trophic interactions such as habitat selection by amphibians or macroinvertebrates to avoid large fish may provide an indirect mechanism strengthening the adverse bottom-up effects of fish on birds.     

Quantifying predator dependence in the functional response of generalist predators

A long‐standing debate concerns how functional responses are best described. Theory suggests that ratio dependence is consistent with many food web patterns left unexplained by the simplest prey‐dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and then only so in specialist predators, which are rare in nature. Here we develop an approach to simultaneously estimate the prey‐specific attack rates and predator‐specific interference (facilitation) rates of predators interacting with arbitrary numbers of prey and predator species in the field. We apply the approach to surveys and experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for predator dependence that is poorly described by the ratio dependent model over manipulated and natural ranges of species abundances. It also indicates how, for generalist predators, even the qualitative nature of predator dependence can be prey‐specific.     

Diel activity pattern and predation rate of the generalist predator Dicyphus hesperus

Abstract This study examined the diel activity pattern and the effect of diel activity pattern on predation rate and prey finding of Dicyphus hesperus Knight (Heteroptera: Miridae). To determine the diel activity pattern of D. hesperus, starved females were placed on tomato leaflets Lycopersicon esculentum Mill. (Solanaceae) under zero, low, or high light intensities at 02:00, 08:00, and 14:00 h, respectively, and the amount of time spent walking or resting during a 30‐min interval was recorded. Predation rates of D. hesperus females on Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) egg patches were determined under either a L16:D8 (long day) or L8:D16 (short day) diel period. Egg patches were removed from D. hesperus females after either 8 or 16 h of dark or 16 or 8 h of light, and the number of eggs consumed was counted. Dicyphus hesperus females spent more time searching for prey at night than during the day. Females ate eggs at a higher rate during the night than during the day. Overall, D. hesperus females had higher predation rates when reared under a long day diel cycle compared with females reared under a short day diel cycle. More females reared at the L16:D8 diel cycle found the egg patch during the night than during the day. There was no difference in egg patch finding between night and day for females reared at L8:D16. Overall, L16:D8 reared females found more egg patches than females reared at L8:D16. Therefore, D. hesperus females are more active and find and consume prey at a higher rate at night than day.     

Phylogenetic similarity and structure of Agaricomycotina communities across a forested landscape

The Agaricomycotina are a phylogenetically diverse group of fungi that includes both saprotrophic and mycorrhizal species, and that form species – rich communities in forest ecosystems. Most species are infrequently observed, and this hampers assessment of the role that environmental heterogeneity plays in determining local community composition and in driving β‐diversity. We used a combination of phenetic (TRFLP) and phylogenetic approaches [Unifrac and Net Relatedness Index (NRI)] to examine the compositional and phylogenetic similarity of Agaricomycotina communities in forest floor and surface soil of three widely distributed temperate upland forest ecosystems (one, xeric oak – dominated and two, mesic sugar maple dominated). Generally, forest floor and soil communities had similar phylogenetic diversity, but there was little overlap of species or evolutionary lineages between these two horizons. Forest floor communities were dominated by saprotrophic species, and were compositionally and phylogenetically similar in all three ecosystems. Mycorrhizal species represented 30% to 90% of soil community diversity, and these communities differed compositionally and phylogenetically between ecosystems. Estimates of NRI revealed significant phylogenetic clustering in both the forest floor and soil communities of only the xeric oak‐dominated forest ecosystem, and may indicate that this ecosystem acts as a habitat filter. Our results suggest that environmental heterogeneity strongly influences the phylogenetic β‐diversity of soil inhabiting Agaricomycotina communities, but has only a small influence on forest floor β‐diversity. Moreover, our results suggest that the strength of community assembly processes, such as habitat filtering, may differ between temperate forest ecosystems.     

The rise of a marine generalist predator and the fall of beta diversity

Determining the importance of physical and biological drivers in shaping biodiversity in diverse ecosystems remains a global challenge. Advancements have been made towards this end in large marine ecosystems with several studies suggesting environmental forcing as the primary driver. However, both empirical and theoretical studies point to additional drivers of changes in diversity involving trophic interactions and, in particular, predation. Moreover, a more integrated but less common approach to the assessment of biodiversity changes involves analyses of spatial β diversity, whereas most studies to date assess only changes in species richness (α diversity). Recent research has established that when cod, a dominant generalist predator, was overfished and collapsed in a northwest Atlantic food web, spatial β diversity increased; that is, the spatial structure of the fish assemblage became increasingly heterogeneous. If cod were to recover, would this situation be reversible, given the inherent complexity and non‐linear dynamics that typify such systems? A dramatic increase of cod in an ecologically similar large marine ecosystem may provide an answer. Here we show that spatial β diversity of fish assemblages in the Barents Sea decreased with increasing cod abundance, while decadal scale changes in temperature did not play a significant role. These findings indicate a reversibility of the fish assemblage structure in response to changing levels of an apex predator and highlight the frequently overlooked importance of trophic interactions in determining large‐scale biodiversity patterns. As increased cod abundance was largely driven by changes in fisheries management, our study also shows that management policies and practices, particularly those involving apex predators, can have a strong effect in shaping spatial diversity patterns, and one should not restrict the focus to effects of climate change alone.     

Predation and reproduction of the generalist predator Nabis pseudoferus preying on Tuta absoluta

The damsel bug, Nabis pseudoferus Remane (Hemiptera: Namidae), is a generalist predator of small arthropods, including key insect pests of vegetable crops. In this study, we characterized the predation and development of various N. pseudoferus life stages when preying on the eggs and first- and fourth-instar nymphs of the invasive South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae). These findings were compared to those of the bug’s common prey, cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae). Of the life stages tested, females showed the highest predation rate on all tested prey, due to their high longevity, large body, and great energy requirements for reproduction. The predator’s oviposition rate was clearly influenced by prey type, and was highest when feeding on fourth instars of T. absoluta. Considering the zoophytophagy of most of the life stages of other mirid species of tomato, and the lower propensity of N. pseudoferus to feed on plants, these results show that this species can be considered as a key indigenous natural enemy for sustainable pest control packages against T. absoluta in newly invaded areas.     

Combined effects of allelochemical-fed and scarce prey on the generalist insect predator Podisusmaculiventris

1. The simultaneous effects of allelochemicals ingested by herbivorous insect prey and prey scarcity on the performance of a generalist insect predator were examined.
2. Fifth-instar predatory stinkbugs ( Podisus maculiventris : Pentatomidae) were fed caterpillars ( Manduca sexta : Sphingidae) in three prey scarcity treatments: every day (unlimited amount), one caterpillar every third day, one caterpillar every fifth day. The caterpillars were fed either a plain diet or a diet containing rutin, chlorogenic acid and tomatine, which are three of the major allelochemicals in tomato leaves ( Lycopersicon esculentum : Solanaceae), the preferred food of these caterpillars.
3. Food consumed, efficiency of conversion of ingested food to biomass (ECI), biomass gained, stadium duration and relative growth rate (RGR) of predators were negatively affected by prey scarcity. The allelochemicals negatively affected food consumed and ECI.
4. There were prey scarcity by allelochemical interactions for ECI, biomass gained and RGR. For ECI, the allelochemicals had a greater negative impact on the predatory stinkbugs when prey were scarce. When prey diet contained allelochemicals, biomass gained and RGR declined more steeply with increased prey scarcity. There was an allelochemical by predator gender interaction for biomass gained. Allelochemicals had no effect on biomass gained by female stinkbugs, whereas biomass gained declined more steeply with increased prey scarcity for male stinkbugs fed caterpillars containing allelochemicals than for males fed control caterpillars.     

Ontogenetic change of prey preference in the generalist predator Zelus renardii and its influence on predator–predator interactions

1. Two field experiments were conducted to test the hypothesis that the intensity of predation by a generalist predator on two species of prey changes with the developmental stage of the predator. The generalist predator studied was Zelus renardii Kolenati (Hemiptera: Reduviidae) and the prey were the lacewing larva, Chrysoperla carnea Stephens, and the cotton aphid, Aphis gossypii Glover.
2. Zelus renardii and lacewings feed on aphids, thereby acting as potential competitors. In addition, Z. renardii feeds on lacewings. Thus, Z. renardii is an intraguild predator of lacewings.
3. Zelus renardii exhibited changes in prey preferences across developmental stages. The older stages of Z. renardii exerted greater mortality on lacewings and fed on larger lacewing larvae than did the younger stages.
4. Lacewings suppressed aphid population growth strongly. In contrast, none of the stages of Z. renardii was an effective control agent of the cotton aphid.
5. The addition of Z. renardii frequently disrupted the effective control of aphids generated by lacewings. In one of the two replicates of the experiment, the disruption increased with the developmental stage of Z. renardii , paralleling the increase in lacewing mortality.
6. Although the developmental stage of Z. renardii can influence the prevalence of intraguild predation and the intensity of the disruption of the aphid biological control, these experiments have demonstrated that even the youngest instars of Z. renardii can cause substantial lacewing mortality and release aphid populations from regulation.     

Effect of land-use heterogeneity on carabid communities at the landscape scale

Carabid beetle assemblages were studied to assess how diversity and community structure varied along a gradient of land-use. This gradient was composed of six 1 km² quadrats with an increasing proportion of agricultural land reflecting the anthropogenic fragmentation and intensification of landscapes. Carabid species richness and abundance was predicted to peak in the most heterogeneous landscape, in accord with the intermediate disturbance hypothesis (IDH), and then decline as agricultural intensification increased. It was also predicted that the different landscapes would support beetle communities distinct from each other. The IDH was unsupported-in both years of this study carabid species richness and abundance was greatest in the most intensively managed, agricultural sites. Detrended correspondence analysis revealed a clear separation in beetle community structure between forested and open habitats and between different forest types. Canonical correspondence analysis revealed a significant correlation between beetle community structure and the environment, showing distinct beetle assemblages to be significantly associated with specific edaphic and botanical features of the land-use gradient. This study adds to increasing evidence that landscape-scale patterns in land-use significantly affect beetle community structure producing distinct assemblages.