Abundance Of Generalist Predators Research Articles

Understanding stakeholder perceptions on the impacts of gamebird releasing on or near UK protected sites

Conservation of protected sites involves stakeholders with a variety of perspectives and values of how land should be managed. Understanding the perceptions of different stakeholders are key to considering changes to conservation practises and to inform conflict strategies. Non-native gamebird release for recreational shooting in the UK includes associated land management which can have positive and negative impacts on species and habitats. We aimed to understand land manger perceptions of the effects from non-native gamebird release and associated management on native biodiversity within or adjacent to protected sites across the UK. We designed an online questionnaire which was completed by 104 managers of protected sites, including nature reserve staff, farmers and gamekeepers. Overall, respondents reported more positive effects related to estate management, and more negative effects related to the impacts of gamebirds on species and habitats, matching the conclusions of recent ecological evidence reviews. However, there was a marked difference in how the perceptions of different stakeholders matched the available ecological evidence: respondents employed by conservation NGOs, consultancy, government, and non-departmental bodies were more likely to perceive negative impacts, whereas respondents from farming and game shooting estates were more likely to perceive positive impacts. Despite pronounced differences in perceptions between stakeholder groups, there was broad cross-group agreement on the overall positive effect of hedgerow creation and management, and the overall negative impact of increases in generalist predator abundance and predation pressure on native wildlife linked to gamebird releases. Although our findings highlight differences between stakeholders in perceptions of the impacts of gamebird release and management, we use our results to highlight possible social mechanisms surrounding the perceptions of non-native gamebird releasing, opportunities for conflict management and potential needs for collaboration to further explore perceptions and help influence conservation action.

Sown Covers Enhance the Diversity and Abundance of Ground-Dwelling Predators in Mediterranean Pear Orchards

Intensive agriculture has a strong impact on the structure of arthropod communities in soil. Sown covers can contribute to their conservation, especially for generalist predators such as spiders and predatory beetles. The aim of this research was to assess the effect of cover crop management on the abundance and diversity of ground-dwelling arthropods. For this purpose, a three-year experiment was conducted in a pear orchard that was divided into three blocks with two plots each: one with a sown cover of mixed plants from different families, including Boraginaceae, Asteraceae, Apiaceae, Brassicaceae, and Fabaceae, and the other with no cover, in which any spontaneous plants were periodically removed without using herbicides. The abundance of ground-dwelling arthropods was sampled using pitfall traps. The sown cover increased the overall richness of arthropods. Additionally, spiders from the families Lycosidae and Linyphiidae, as well as beetles from the families Carabidae and Staphylinidae, were more abundant in the sown cover. Conversely, detritivores such as collembolans and beetles from the family Tenebrionidae were less abundant in the cover. The abundance of ants was not affected by the type of cover. The sown cover increased the diversity of arthropods in the crop, as well as the abundance of generalist predators.

Tree species richness around urban red maples reduces pest density but does not enhance biological control

Urban trees often host greater insect pest abundance than trees in rural forests. This may be due, in part, to differences in tree diversity and canopy cover between these settings. Urban trees are often planted in isolation or monoculture, which favors pest accumulation. Gloomy scale, Melanaspis tenebricosa Comstock, is a pest of urban red maples (Acer rubrum L.) that is abundant where impervious surfaces dominate the local landscape. Increasing tree diversity and canopy cover around urban red maples may reduce gloomy scale abundance by supporting natural enemy communities. We investigated the effect that surrounding tree species richness and tree canopy cover had on gloomy scale abundance, natural enemy abundance, and biological control in red maple trees in Raleigh, NC, USA. We collected scales and natural enemies from red maples that spanned a gradient of tree species richness, canopy cover, and impervious surface values. We also measured gloomy scale parasitism and predation of sentinel prey in red maple canopies. Greater tree species richness and canopy cover were associated with lower gloomy scale density. Red maples in diverse settings also hosted fewer scales per natural enemy. Parasitoids were less common in maples in diverse settings, but generalist predator abundance was unaffected by tree diversity. Finally, tree species richness and canopy cover did not increase biological control of scales or sentinel prey. Our findings suggest that higher tree diversity and greater canopy cover may reduce gloomy scale density, but this is not entirely explained by the effects of natural enemies and biological control.

Exclusion of Mediterranean ant species enhances biological control of the invasive mealybug Delottococcus aberiae in citrus.

Delottococcus aberiae is an invasive mealybug that produces severe damage in Spanish citrus. This mealybug has established a mutualistic relationship with native Mediterranean ant species that may limit biological control of this pest. Herein, we evaluated the effect of tending ants on the biological control of D. aberiae. To do this, we compared: (i) the density of D. aberiae, (ii) the density of its natural enemies, and (iii) the damage produced by the mealybug in trees with (control) and without ants (ants excluded with sticky barriers) in two citrus orchards across two consecutive years. Lasius grandis was the most abundant ant species in both orchards and represented more than 95% of the ants tending D. aberiae in control trees. Spiders and lacewings were the most abundant predators observed in mealybug colonies, and the exclusion of mutualistic ants increased their abundance. Moreover, in control trees, ant activity throughout the year was negatively correlated with the relative abundance of predators (number of predators per mealybug). No parasitoids were recovered during field experiments. Ant exclusion reduced the density of D. aberiae and the ratio of damaged fruit at harvest across years and orchards. This work corroborates the previous finding that D. aberiae benefits from its mutualistic relationship with L. grandis, probably because the presence of ants reduced the abundance of generalist predators. This mutualism can be disrupted using physical barriers on on the trunk. Further research should assess other methods of ant control that are more economic and feasible for citrus producers. © 2023 Society of Chemical Industry.

Licorice strips enhance predator-mediated biological control in China's cotton crop.

Habitat management such as field- or farm-level diversification can conserve arthropod natural enemies, enhance biological pest control and lower (or suspend) insecticide use in agricultural crops. These approaches however have been underexploited to manage the aphid Aphis gossypii in cotton cropping systems of Xinjiang, China. In this study, we investigated whether the presence of licorice (Glycyrrhiza uralensis) at the field edge benefits generalist predator abundance and aphid biological control in local cotton crops. Field trials during 2020 and 2021 showed that licorice strips enhanced the in-field abundance of generalist predators (primarily ladybeetles) 2.2-4.3 fold during early season, i.e. initial growth before A. gossypii peak infestation pressure. During peak outbreak conditions in July, treatment fields with licorice strips experienced a respective 17.5-61.2% lowered aphid density and 12.3-14.6-fold higher predator-to-aphid abundance ratio (PAR) than control fields. In late season (aphid decline phase) of either year, cotton fields with licorice strips also attained a respective 4.7-9.9 fold higher PAR. Exclusion cage assays quantitatively assessed predator-mediated A. gossypii biological control and the relative contribution of licorice strips. The biocontrol services index (BSI) was 2.8 times (2020) and 1.4 times (2021) higher at 5m distances from the licorice strip as compared to control fields. Licorice strips in the immediate vicinity of cotton fields benefit generalist arthropod predators and improve aphid biological control throughout the cotton cropping season. These findings help to integrate habitat management within integrated pest management (IPM) frameworks in the biggest cotton production region of China. © 2022 Society of Chemical Industry.

The key role of inter-row vegetation and ants on predation in Mediterranean organic vineyards

Biodiversity-friendly farming practices are taking centre stage, with herbicide use sparking major public debate on human health. Vegetation cover is increasingly used to enhance agroecosystem biodiversity and functions, providing important ecosystem services like biological pest control. However, further information is required on the relationship between vegetation cover, natural enemies, and pest predation. Using a sentinel prey experiment set up in 26 Mediterranean organic vineyards, we analysed the response of generalist predators, and the associated final predation and dynamics under three inter-row management types: bare soil, one out of two inter-rows vegetated and all inter-rows vegetated. Predation on sentinel prey was monitored on the ground and on the vine trunk throughout the daytime and after a night of exposure. Generalist predator abundance was quantified using pitfall traps. Final sentinel prey predation both on vine trunks and on the ground were significantly higher in fully vegetated vineyards, with a shorter prey survival time than in tilled vineyards. Both diurnal and nocturnal predation were significantly related to increased vegetation cover and richness. Moreover, 96 % of the identified predation events involved ants. We demonstrated the key role of vegetation cover in improving the predation function through a spill-over from the inter-row to the grapevine. Contrary to what is commonly observed in agrosystems, we clearly identified ants as the predominant predator both on the ground and on the vine trunk. Our study has important implications for Mediterranean vineyard management, promoting vegetation as a nature-based solution. Sustainable management of vineyard vegetation favouring the natural predator community may contribute to a reduction in pesticide use and fossil fuel consumption.

European bird declines: Do we need to rethink approaches to the management of abundant generalist predators?

Abstract Bird species are declining across Europe. Current European policy, that is, the Birds and Habitats Directives, focus on habitat management as a way of halting the declines. This paper explores the role of predation in causing bird population declines and asks if we need to reconsider our approach to the management of generalist predators. We analysed bird population trends and distribution changes across Europe, Britain and Ireland, reflecting an increasing gradient of generalist predator abundance (principally red fox Vulpes vulpes and species of the family Corvidae). We tested if ground‐nesting bird species, considered more vulnerable to predation, were in greater decline compared to other nesting strategies. We also compared Annex I designated species to non‐designated species as a proxy for habitat management. We found that across Europe, 74% of ground‐nesting bird species were in decline, compared to 41% of other species. This was especially evident in Britain, where the pattern was 66% compared to 31%, and in Ireland, 71% compared to 20%. Ground‐nesting species were significantly more likely to be declining than other species. These patterns are consistent with the idea that population declines are at least partially related to the increased abundance of generalist predators. In Britain, ground‐nesting species were less likely to be in decline if covered by Annex I designation. However, in Europe and Ireland, Annex I status did not mitigate the effect of nesting strategy. Policy implications. Current legislation is clearly insufficient to prevent widespread declines in ground‐nesting birds, and this is the case across Europe, in Britain and Ireland. Ignoring the role of generalist predators in modern landscapes may lead to further declines and losses. We urgently need large‐scale experiments to establish causality in the impact of generalist predators on ground‐nesting birds in different landscapes. If we value our ground‐nesting bird species, consideration needs to be given to the control of widespread generalist predators, at least until landscapes are restored.

Effect of Herbage Density, Height and Age on Nutrient and Invertebrate Generalist Predator Abundance in Permanent and Temporary Pastures

The aim of this research was to assess differences in the quantity and quality of herbage and invertebrate generalist predator abundance among permanent and temporary pastures. Two permanent pastures and four temporary ley pastures (either one year or two years since being sown) were monitored weekly for 10 weeks in the spring. Permanent pastures included a diverse range of native UK grass species, and temporary ley pastures were predominantly perennial ryegrass (Lolium perenne) with or without white clover (Trifolium repens). Weekly measurements of herbage height (in centimeters), herbage cover (fresh and dry matter in kg per hectare) and herbage density (fresh and dry matter in kg per hectare per centimeter) were obtained for each field, along with lycosid spider and carabid beetle abundance. Weekly pasture samples were used to obtain nutrient concentrations of dry matter, crude protein, neutral detergent fibre (NDF), acid detergent fibre (ADF), ash, oil, sugars, digestible organic matter in the dry matter (DOMD) and metabolisable energy (ME) in the herbage as a measure of forage quality for grazing or harvesting. A linear mixed model was used to assess the effect of sward age, herbage density and height on herbage production, nutrient concentrations and invertebrate abundance. Although this study showed that permanent pastures were associated with lower nutrient concentrations of crude protein, ash, oil and ME compared to younger and predominantly perennial ryegrass pastures, the older pastures were associated with higher carabid numbers. Furthermore, permanent pastures had a higher density of dry matter herbage compared to younger pastures, and more dense and taller swards were associated with higher lycosid numbers. The study suggests that within pastures of 3 to 20 cm height, increasing the height and density of swards increases both ME and oil concentrations in herbage, therefore enhancing forage nutrient quality. Older and more permanent pastures can be beneficial for plant and invertebrate generalist predator populations, and still provide a useful source of nutrients for forage production.

Identification of flower functional traits affecting abundance of generalist predators in perennial multiple species wildflower strips

In agricultural fields, wildflower strips can be sown to enhance conservation biological control of insect pests. However, issues remain regarding the composition of flower mixtures to effectively attract and support large communities of natural enemies. Trait-based approaches are promising for this purpose. In the present study, conducted in an agricultural field of Belgium in 2014 and 2015, 15 flower mixtures were considered to explore the relation between the abundance of trapped generalist predators (i.e. lacewings [Neuroptera: Chrysopidae], ladybeetles [Coleoptera: Coccinellidae] and hoverflies [Diptera: Syrphidae]) and the community-weighted means of seven flower traits. Through a redundancy analysis, it was found that the presence/absence of flower ultra-violet pattern and the morphology of the corolla (that determines the accessibility of floral resources) were the traits that significantly affected the abundance of the generalist predators in the flower mixtures. The ladybeetles Harmonia axyridis and Propylea quatuordecimpunctata as well as the lacewings Chrysoperla carnea were more abundant in mixtures with a high cover of flowers showing an ultra-violet pattern, while the opposite was observed for the ladybeetle Coccinella septempunctata. As for hoverflies, Episyrphus balteatus and Eupeodes corollae were more abundant in mixtures with a high cover of flowers with open nectar. These results bring new knowledge regarding how a range of natural enemy species reacts to flower cues in diversified plant communities and should help in elaborating flower mixtures that enhance conservation biological control.

Pollen beetle mortality is increased by ground-dwelling generalist predators but not landscape complexity

Biological control of crop pests by naturally occurring arthropods depends on the entire community of natural enemies, but generalist predators and parasitoids are rarely considered in the same study. Also, the level of biological control in the field is affected by both within-field and landscape scale management. A multi-taxa approach that integrates multiple scales of management is needed to understand drivers for pest mortality. We examined local (weed cover and soil characteristics) and landscape (proportions of semi-natural and oilseed rape habitat) effects on natural enemy communities and biological control of pollen beetles in 15 oilseed rape (OSR) fields in Sweden. We found that agricultural intensification at the local (low weed cover) and landscape scale (low proportion of semi-natural area) increased evenness of generalist predators, but had no effect on the densities of pests and their natural enemies. This suggests that the generalist predators in OSR are well adapted to crop lands, at least within the examined gradient. Increasing OSR in the landscape decreased parasitoid densities and increased pest density, indicating a potential loss of pest control services by specialist natural enemies in landscapes with a high proportion of OSR. Finally, pollen beetle mortality increased with ground-dwelling generalist predator abundance and soil clay content. Parasitism rates did not affect pest mortality, which is interesting as parasitoids have been considered major control agents in OSR. The hypothesis that increasing semi-natural habitat in the landscape enhances natural enemy abundances and species richness in agricultural landscapes was not supported. Local measures targeting generalist predators appear as a reasonable strategy to maximize pollen beetle control.

Environmental correlates of breeding abundance and population change of Eurasian Curlew Numenius arquata in Britain

ABSTRACTCapsule: Across Britain, breeding Eurasian Curlew Numenius arquata are less numerous and have shown greater population declines in areas with more arable farming, woodland cover and higher generalist predator abundance.Aims: We present the first national-scale analysis of the potential drivers of Curlew population change in Britain, which is needed to guide conservation action for this globally near-threatened, declining species.Methods: Breeding Bird Survey data and environmental predictors were used to model variation in Curlew abundance in 1995–99 and 2007–11, and population change between these periods.Results: Arable farming and woodland cover were negatively associated with Curlew abundance and population declines. Curlew abundance was positively associated with extent of protected area coverage and gamebird numbers. Abundance and population change were positively associated with cooler temperatures and higher summer rainfall, but negatively associated with numbers of generalist predators.Conclusions: We found support for the negative effects of intensive agriculture, forestry, increases in generalist predator populations and climate warming on Curlew abundance and population change. Effective site protection and measures to reduce generalist predator abundance may be important conservation measures, together with improving breeding habitat quality in the wider countryside.

Climate change induced molting mismatch? Mountain hare abundance reduced by duration of snow cover and predator abundance

AbstractIn northern latitudes, species that have adapted to snow cover by molting and changing pelage color will be particularly vulnerable to climate change, as predation levels may increase due to a mismatch with background coloration. Here, we investigated the synergistic effect of mammalian generalist predators (red fox Vulpes vulpes and pine marten Martes martes) and the duration of snow cover on the abundance of a winter‐adapted species, the mountain hare (Lepus timidus). We analyzed 12 yr of data from 622 snow tracking lines in southeast Norway along a gradient in annual duration of snow‐covered ground. The mountain hare abundance index was positively related to the generalist predator abundance index, probably due to a combined numerical and functional response of the predators. The mountain hare abundance index was negatively associated with a short duration of snow cover, and this effect was stronger in areas with a high predator abundance index. Hence, we demonstrated a causal link between the mountain hare abundance index, predators, and the duration of snow cover. We expect declining mountain hare densities in the future caused by interactive effects of reduced duration of snow‐covered ground, potentially increasing generalist predator densities and the limited ability of mountain hares to adapt to the climate change‐induced molting mismatch.

Alyssum flowers promote biological control of collard pests

Collard greens Brassica oleracea (L.) are often attacked by various pests including whiteflies, aphids and diamondback moth. Hitherto, the main method used to manage these pests in Brazil has been the application of a limited number of registered insecticides. The search for more sustainable pest management strategies is therefore warranted. In this context, the conservation biological control stands out as an appealing alternative. Conservation biological control is achieved, at least in part, by strip-cultivating and/or conserving flowering plants within the agroecosystem. The present study investigates how alyssum flowers Lobularia maritima (L.) could contribute to the attraction of natural enemies and to the management of collard pests. Two field experiments were conducted in different years. Each experiment consisted of two treatments and three replicates, which were set up in a completely randomized design. The treatments were (1) collards alone, and (2) collards + alyssum. We evaluated weekly the population density of natural enemies and pests on both treatments. The results show that the alyssum flowers attractiveness contributed to increase the abundance of generalist predators during both experiments, which in turn translated into a significant reduction of collards pests, especially aphids. Some of the main predators attracted/harbored by alyssum flowers were spiders, coccinellids, syrphids and Orius sp. Finally, strip intercropping alyssum with collards can be an important strategy to manage brassica pests and cope with the limited availability of insecticides registered for this vegetable crop.

Non-crop plant communities conserve spider populations in chili pepper agroecosystems

Habitat management enhances heterogeneity in agroecosystems and also has the potential to increase recruitment of spiders, which can improve the biological control services afforded by these important predators. A paucity of studies has documented the associations of spiders with plant communities or the efficacy of non-crop plants for increasing the density and diversity of spider populations. Here we examined natural associations of spiders with native non-crop plants within Brazilian chili pepper agroecosystems. Following this characterization, a manipulative experiment was undertaken at two locations to identify the effects of non-crop plant strips and borders on spider community structure. The composition of native plants altered the community of spiders. The abundance of these predatory spiders was highest on Asteraceae. Spatial Analyses by Distance Indices (SADIE) determined that there was significant aggregation of spiders in chili pepper fields under these plant management strategies. Although incorporating this habitat management approach reduced the overall cropping area, the corresponding increase in generalist predator abundance could offset costs by improving the natural control of pests. This study revealed associations between native plants and spider communities, and how these help to conserve predator biodiversity. Targeted management of native non-crop plants promote the abundance of natural enemies and enhances biological control in chili pepper agroecosystems.

The effect of an irrigated buckwheat cover crop on grape vine productivity, and beneficial insect and grape pest abundance in southern California

The effect of an irrigated buckwheat cover crop on populations of beneficial insects and grape pests, vine growth, grape yield, and berry quality was investigated over 1year in a commercial organic vineyard in southern California, USA. Buckwheat was grown in the spring and summer with additional irrigation that supplemented prevailing vine watering regimens. Treatments replicated four, three and six times respectively were: (1) buckwheat cover crop with supplemental irrigation between vine rows; (2) supplemental irrigation with no buckwheat cover crop; and (3) control plots with no buckwheat cover crop or supplemental irrigation. Flowering buckwheat was extremely attractive to beneficial insects at the beginning of the trial, resulting in 27 times more insects captured from shake sampling, compared with grape foliage in control plots. Results from sticky trap and visual count data indicated that buckwheat may enhance the abundance of generalist predators at certain times. However, densities of pestiferous leafhoppers on grape leaves in August was significantly higher (129–240% greater) in irrigated buckwheat and irrigated plots lacking buckwheat when compared with control plots. This increase in leafhopper density may be attributed to these pests preferring well-irrigated, vigorously growing vines. Mean cane weight was 222% and 170% greater for vines in irrigated buckwheat and irrigated plots lacking buckwheat, respectively, compared with controls indicating that vine vigor increased with supplemental irrigation. An irrigated buckwheat cover crop increased berry size, on average, by 0.67mm for berries harvested on the side of the row that contained buckwheat, and reduced sugar content of berries by 3.2° Brix, compared with non-irrigated controls. Additionally, the buckwheat cover crop was associated with reduced berry quality because of insect feeding damage. Under drought conditions and water shortages, supplemental irrigation to support cover crops may result in water use penalties. Irrigated cover cropping during summer months may not be a viable pest management option for grape growers in southern California.

Ants in citrus: impact on the abundance, species richness, diversity and community structure of predators and parasitoids

Ants can act as plant biotic defenses, however, in agricultural ecosystems they are often associated with outbreaks of honeydew-producing pests mainly due to the protection they offer to the plant feeders in exchange for honeydew. In this interaction ants may alter the abundance, diversity and community structure of predators and parasitoids. In the present study, we conducted ant-exclusion experiments in three commercial citrus orchards, each one dominated by one ant species (Pheidole pallidula, Lasius grandis or Linepithema humile) during two consecutive years. We then compared the abundance, species richness, diversity and community structure of predators and parasitoids between the ant-allowed and ant-excluded treatments. A total of 176,074 natural enemies belonging to 81 taxa were captured and identified. The abundance of the natural enemies showed a species specific response between treatments. When examining functional groups of natural enemies the abundance of generalist predators decreased while that of parasitoids increased in the ant-allowed treatment. The species richness was significantly lower for predators and higher for parasitoids in the ant-allowed treatment. The Shannon diversity index was not different between treatments for predators, whereas parasitoid diversity was significantly higher in the ant-allowed treatment. Finally, the community structure of predators and parasitoids was not significantly different between treatments. These results suggest that ants in citrus are not associated with a dramatic decrease in natural enemy abundance or biodiversity; on the contrary ants were associated with increased parasitoid species richness and diversity. On the other hand, ants negatively affected the abundance of specific natural enemy species, mainly generalist predators. The impact on these predators might explain the higher pest densities associated with ants in citrus.

Ecological engineering of ground cover vegetation promotes biocontrol services in peach orchards

We conducted a 2-year field experiment at two sites in eastern China, examining the effects of the ground cover by Trifolium repens L. on the biocontrol services in peach orchards. The results indicated that compared to those in control areas, the abundances of aphids and Grapholitha molesta decreased, respectively, by 31.4% and 33.3% in Shanghai and by 30.1% and 33.3% in Jiangsu, while the abundance of generalist arthropod predators increased by 116.7% in Shanghai and by 115.8% in Jiangsu in ground cover areas. Compared to that in control areas, the ratio of generalist predator abundance to aphid abundance and to G. molesta abundance increased, respectively, by 260.0% and 384.2% in Shanghai and by 213.3% and 253.1% in Jiangsu in ground cover areas. Our study revealed that the ecological engineering of ground cover by T. repens promoted biocontrol services in peach orchards.

Impact of Bt transgenic cottons and insecticides on target and non-target insect pests, natural enemies and seedcotton yield in India

Genetically engineered cottons expressing ä-endotoxins from Bacillus thuringiensis have been adopted on a large- scale worldwide. Therefore, we studied the efficacy of Bt cottons for the management of bollworms, their effects on non- target insects, and seedcotton yield under insecticide protected and unprotected conditions. Helicoverpa armigera and Earias vittella damage was significantly lower in Bt than in non-Bt cottons, while no significant differences were observed in egg-laying by H. armigera. The populations of major non-target sucking insect pests such as Amrasca biguttula biguttula, Bemisia tabaci, Aphis gossypii, Oxycarenus laetus, Dysdercus koenigii and Nezara viridula and the generalist predators, viz Cheilomenes sexmaculatus, Chrysopa spp., and spiders did not differ significantly between Bt and non-Bt cottons. Insecticide application resulted in resurgence of cotton aphid and whitefly, possibly because of elimination of natural enemies or better growth of plants uder protected conditions. Abundance of bollworms, non-target pests, and generalist predators was significantly greater before insecticide sprays than after insecticide application, except in a few cases. Bollworm damage was lower and seedcotton yields higher in Bt than in non-Bt cottons. The present studies indicated that Bt cotton hybrids are effective for the management of bollworms and yield more, and do not have any adverse effects on the abundance of generalist predators.

Why we have been unable to generalize about bird nest predation

Nest predation is such an important determinant of the reproductive success of many birds, particularly passerines, that its dynamics are of great interest to ecologists and wildlife managers. The large body of literature devoted to assessing these dynamics, however, has not succeeded in producing much consensus as to what makes a nest more or less likely to be depredated. The idea that nest predation increases at habitat edges, for instance, was very popular for a time, but in fact most studies do not find such an ‘edge effect’, and in a few sites nest predation actually decreases towards the edge (Lahti, 2001). Perhaps the nearest we have come to a definitive statement about nest predation is that it is exacerbated by habitat fragmentation at the landscape scale (Donovan et al., 1995). This hypothesis has strong intuitive appeal: the nesting success of forest birds, for instance, seemingly must decline if their habitat is broken up into small pieces surrounded by a matrix known to harbor an abundance of generalist predators. Apparently, we cannot maintain even this moderate and unsurprising generalization. Spanhove et al. (2009) found, among several Afromontane forest fragments, that the lower the edge-to-area ratio (in other words, the larger a patch and less convoluted its shape), the more likely a forestdwelling passerine nest will be ravaged by predators. Thus, their focal bird, the white-starred robin Pogonocichla stellata, might actually benefit from a fragmented forest habitat, with more edge and lower habitat patch sizes. To some extent, at least in the largest patch in the study site, predation rates were highest in the less disturbed interior of the patch, where nesting tended to occur later in the season and under denser canopy and sparse herbaceous cover. Can landscape fragmentation really benefit some bird species? I believe that the answer is yes, but that conservationists and managers ought to regard this point as trivial, and that more productive questions in avian reproductive success lie elsewhere. The reality of our ecological situation, in Kenya’s Taita Hills as elsewhere, is that distinctive habitats for wildlife have been and often continue to be greatly shrunk and subdivided. Species like the whitestarred robin, which are common, have a broad range and inhabit a variety of forest types as well as shrublands, are not the species of concern in this situation. Many species might benefit from fragmentation, in which case we are in the serendipitous situation of having inadvertently managed a landscape in their favor. Managers should, of course, consider the nesting and other habitat requirements of a threatened species before drafting a plan; this goes without saying. The point here is that very few of these plans will ever need to be drawn up for birds that benefit from fragmentation. Nevertheless, studies that buck the general trends in ecological correlates of nest predation are valuable red flags for those who wish to understand the factors underlying the reproductive success of birds. From Spanhove et al. (2009), one can gather four candidate explanations for their surprising result and its apparent inconsistency with other studies: the use of artificial nests in many studies, different life histories or habitat requirements of the prey species, the scarcity of Afrotropical studies, and different nest predator assemblages. Dozens of studies of nest predation, including assessments of edge and other fragmentation effects, have used natural as well as artificial nests, with no clear distinction overall in their findings (Lahti, 2001), and so this factor is unlikely to be key. Nest site and other behavioral and lifehistory traits will certainly be important determinants of reproductive success, but studies during the last 40 years in the temperate zone have already assessed nest predation for a broad array of species with a range of nesting habits and life histories. This brings us to the scarcity of studies of tropical birds, and birds of the Afrotropics in particular. In recent years, nest predation studies on African birds have become more common (e.g. Schaefer et al., 2005; Hanson, Newmark & Stanley, 2007; Kotze & Lawes, 2007; Lloyd, 2007; Boukhriss, Selmi & Nouira 2009), but a tentative conclusion that one might draw from them as a whole is that general ecological correlates of nest predation are not

Do Top-Down Or Bottom-Up Forces Determine Stephanitis Pyrioides Abundance In Urban Landscapes?

This study examined the influence of habitat structural complexity on the collective effects of top-down and bottom-up forces on herbivore abundance in urban landscapes. The persistence and varying complexity of urban landscapes set them apart from ephemeral agroecosystems and natural habitats where the majority of studies have been conducted. Using surveys and manipulative experiments. We explicitly tested the effect of natural enemies (enemies hypothesis), host plant quality, and herbivore movement on the abundance of the specialist insect herbivore, Stephanitis pyrioides, in landscapes of varying structural complexity. This herbivore was extremely abundant in simple landscapes and rare in complex ones. Natural enemies were the major force influencing abundance of S. pyrioides across habitat types. Generalist predators, particularly the spider Anyphaena celer, were more abundant in complex landscapes. Predator abundance was related to greater abundance of alternative prey in those landscapes. Stephanitis pyrioides survival was lower in complex habitats when exposed to endemic natural enemy populations. Laboratory feeding trials confirmed the more abundant predators consumed S. pyrioides. Host plant quality was not a strong force influencing patterns of S. pyrioides abundance. When predators were excluded, adult S. pyrioides survival was greater on azaleas grown in complex habitats, in opposition to the observed pattern of abundance. Similarly, complexity did not affect S. pyrioides immigration and emigration rates. The complexity of urban landscapes affects the strength of top-down forces on herbivorous insect populations by influencing alternative prey and generalist predator abundance. It is possible that habitats can be manipulated to promote the suppressive effects of generalist predators.