Bird predation on herbivorous insects: indirect effects on sugar maple saplings

Insectivorous birds have been shown to have direct effects on abundances of herbivorous arthropods, but few studies have tested the indirect effects of birds on plant performance through consumption of herbivorous insects. In a 3-year study at the Hubbard Brook Experimental Forest, New Hampshire, we tested whether bird predation indirectly affects leaf herbivory levels and leaf and shoot biomass production of understory sugar maple (Acer saccharum) saplings. Trees were randomly assigned to one of four treatments: an insecticide application to reduce herbivory levels, exclosures that prevented bird access, addition of Lepidoptera larvae, and controls. Trees sprayed with an insecticide supported significantly fewer Lepidoptera larvae than other treatments throughout the study. Also, trees in exclosures supported more Lepidoptera larvae than controls during one count each year, and pooled across all counts during the second year. As predicted, the mean proportion of leaf area consumed varied significantly among treatments and was least in the insecticide treatment, followed by controls, exclosures, and Lepidoptera additions. Significant differences among treatments in herbivory levels, however, did not lead to differences in leaf or shoot biomass production. Thus, bird predation decreased Lepidoptera abundances and decreased herbivory levels, but did not increase biomass production during the following year. Over 85% of the herbivores in our study were Homoptera nymphs that were not folivorous and are not important bird prey items, potentially dampening the indirect effects of bird predation on biomass production. A comparison of these results with previous studies suggests that the indirect effects of bird predation on plant biomass production may depend on the plant species, abundance and composition of the herbivore community, and primary productivity of the ecosystem.     

Spatial heterogeneity in the relative impacts of foliar quality and predation pressure on red oak, Quercus rubra, arthropod communities

Predation pressure and resource availability often interact in structuring herbivore communities, with their relative influence varying in space and time. The operation of multiple ecological pressures and guild-specific herbivore responses may combine to override simple predictions of how the roles of plant quality and predation pressure vary in space. For 2 years at the Coweeta LTER in the Southern Appalachian Mountains, we conducted a bird exclosure experiment on red oak (Quercus rubra) saplings to investigate the effects of bird predation on red oak arthropod communities. We established bird exclosures at six sites along an elevational gradient and estimated variation in foliar nitrogen and bird predation pressure along this gradient. Foliar nitrogen concentrations increased with elevation while our index of bird predation pressure was variable across sites. Greater arthropod densities were detected inside exclosures; however, this result was mainly driven by the response of phloem feeders which were much more prevalent inside exclosures than on control trees. There was little evidence for an effect of bird predation on the other arthropod guilds. Consequently, there was no evidence of a trophic cascade either in terms of leaf damage or tree growth. Finally, we found more variation in arthropod density among trees within sites than variation in arthropod density among sites, indicating the importance of micro-site variation in structuring arthropod communities.     

Contrasting cascades: insectivorous birds increase pine but not parasitic mistletoe growth

1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on herbivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator-predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemipterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding caterpillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds.     

Variable strength of top‐down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO event

Abstract Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top‐down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’Nothofagus pumilio forests in the northern Patagonian Andes, Argentina. The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size. The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird‐initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above‐normal herbivory, and also support the view that large‐scale climatic events influence the strength of trophic cascades.     

Effect of fragmentation on predation pressure of insect herbivores in a north temperate deciduous forest ecosystem

1. Forest fragmentation affects many ecosystem processes by spatially altering relationships among organisms. Herbivory by arthropods is an important ecosystem processes in forests that fragmentation alters by changing relationships among herbivores, their predators, and their hosts. The relative importance of these factors remains unclear. 2. It was tested whether the exclusion of vertebrate predators affected the arthropod abundance or amounts of herbivory in a fragmented, deciduous forest landscape in southern Quebec. Differences in the abundance of arthropod herbivores and amounts of herbivory in forest patches with different landscape characteristics (small isolated patches versus large connected ones), on sugar maple saplings with or without exclosures that restricted access by large vertebrate predators were measured. 3. Saplings protected from predators with exclosures had greater abundances of all arthropods (herbivores and invertebrate predators) than those without, indicating potential top‐down effects of vertebrate predators on arthropods. Analysis of effect sizes between exclosure treatments and controls suggests the magnitude of predation effects may be affected by fragmentation. 4. Strong top‐down effects of predators on arthropods, and weak effects of fragmentation on predation or amounts of herbivory were found. As a result, herbivory may be regulated by factors other than vertebrate predation in this system.     

Plant quality and predation risk mediated by plant ontogeny: consequences for herbivores and plants

Bottom‐up and top‐down impacts on herbivores can be influenced by plant productivity, structural complexity, vigor and size. Although these traits are likely to vary with plant development, the influence of plant ontogeny on the relative importance of plant quality (i.e. bottom‐up forces) and predation risk (i.e. top‐down forces) has been the focus of little previous investigation. We evaluated the role of plant ontogeny for the relative importance of bottom‐up and top‐down forces on insect herbivore abundance, species richness, and species diversity attacking the tropical tree Casearia nitida. We also quantified the cascading effects on herbivory, growth and reproduction of this plant species. Plant quality traits (nitrogen and phenolic compounds) were assessed in saplings and reproductive trees. Bottom‐up forces were manipulated by fertilizing plants from both ontogenetic stages. Top‐down forces were manipulated by excluding insectivorous birds from saplings and reproductive trees. Plant ontogeny influenced foliage quality in terms of total phenolics, which were in greater concentration in reproductive trees than in saplings; however, it did not influence bottom‐up forces as modified by fertilization. Bird exclusion increased herbivore density with the same magnitude on both stages. Ontogeny influenced species diversity, which was greater in reproductive trees than in saplings, and also influenced treatment impacts on species richness and diversity. Although top‐down forces increased herbivory equally on plants of each ontogenetic stage, the two stages showed different overcompensation responses to increased damage: caged saplings produced greater leaf biomass than non‐caged saplings, whereas caged trees increased in height proportionally more than non‐caged trees. In sum, plant ontogeny influenced the impact of bird predation on herbivore density, species richness, and species diversity, and the growth variables affected by increased damage in caged plants. We suggest that plant ontogeny can contribute to some extent to the influence of plant quality and the third trophic level on herbivores in this system.     

The direct and indirect effects of insectivory by birds in two contrasting Neotropical forests

A goal among community ecologists is to predict when and where trophic cascades occur. For example, several studies have shown that forest birds can limit arthropod abundances on trees, but indirect effects of bird predation (i.e. decreased arthropod damage to trees) are not always observed and their context is not well understood. Because productivity is one factor that is expected to influence trophic cascades, we compared the extent to which birds indirectly limit herbivore damage to trees in two lowland Neotropical forests that differed in seasonality of leaf production and rainfall. We compared the effects of bird predation on local arthropod densities and on damage to foliage through a controlled experiment using bird exclosures in the canopy and understory of two forests. We found that birds decreased local arthropod densities and leaf damage in the canopy of the drier site during periods of high leaf production, but not in the wetter forest where leaf production was low and sporadic throughout the year. Birds had no effect on arthropod abundances and leaf damage in the understory where leaf production and turnover rates were low. In support of these experimental interpretations, although we observed that arthropod densities were similar at the two sites, bird densities and the rate at which birds captured arthropods were greater at the drier, seasonally productive site. The influence of top-down predation by birds in limiting herbivorous insects appears to be conditional and most important when the production and turnover of leaves are comparatively high.Electronic Supplementary Material Supplementary material is available for this article at     

Effects of bird predation on canopy arthropods in wandoo Eucalyptus wandoo woodland

Abstract: As top predators, birds may have significant effects on arthropod abundances and affect the trophic structure of arthropod communities through predation of lower order predators (e.g. spiders) and by competition for prey. We investigated the effects of bird predation on canopy arthropods in south‐western Australia by using plastic bird mesh to exclude insectivorous birds from the foliage of wandoo Eucalyptus wandoo saplings. Exclosure resulted in an increase in the number of herbivorous and predatory arthropods. Total arthropods (with and without ants), spiders, adult Coleoptera, and larval Lepidoptera were significantly more abundant on meshed than unmeshed saplings. All size‐classes of arthropods, taxa grouped, were more abundant on meshed than unmeshed saplings, but with no evidence of a disproportionate increase of the largest arthropods on meshed saplings. All size‐classes of spiders increased in abundance on saplings from which birds were excluded. There were significant differences in the total abundance of arthropods (with and without ants), spiders (Araneae), sucking bugs (Homoptera), adult beetles (Coleoptera), larval moths (Lepidoptera), and wasps and ants (Hymenoptera) for both unmeshed and meshed saplings between sample periods. These seasonal patterns of abundance and differences between sample periods appeared to be determined by seasonal weather patterns, with the lowest numbers associated with drier and hotter conditions in summer and autumn than in winter and spring. The conclusion reached is that eucalypt forest birds have limited effects on temporal variation in canopy arthropod abundances, but depress abundances, and affect the size and trophic composition of the fauna. Given the cascading effects of birds as predators on arthropods, successful conservation management of eucalypt ecosystems, including plantations and revegetation, should be planned to maximize bird numbers and diversity.     

Winter predation by insectivorous birds and consequences for arthropods and plants in summer

Top-down effects of predators can have important consequences for ecosystems. Insectivorous birds frequently have strong predation effects on herbivores and other arthropods, as well as indirect effects on herbivores’ host plants. Diet studies have shown that birds in temperate ecosystems consume arthropods in winter as well as in summer, but experimental studies of bird predation effects have not attempted to quantitatively separate winter predation impacts from those in summer. To understand if winter foraging by insectivorous birds has consequences for arthropods or plants, we performed a meta-analysis of published bird exclusion studies in temperate forest and shrubland habitats. We categorized 85 studies from 41 publications by whether birds were excluded year-round or only in summer, and analyzed arthropod and plant response variables. We also performed a manipulative field experiment in which we used a factorial design to exclude birds from Quercus velutina Lam. saplings in winter and summer, and censused arthropods and herbivore damage in the following growing season. In the meta-analysis, birds had stronger negative effects on herbivores in studies that included winter exclusion, and this effect was not due to study duration. However, this greater predation effect did not translate to a greater impact on plant damage or growth. In the field experiment, winter exclusion did not influence herbivore abundance or their impacts on plants. We have shown that winter feeding by temperate insectivorous birds can have important consequences for insect herbivore populations, but the strength of these effects may vary considerably among ecosystems. A full understanding of the ecological roles of insectivorous birds will require explicit consideration of their foraging in the non-growing season, and we make recommendations for how future studies can address this.     

The direct and indirect effects of insectivory by birds in two contrasting Neotropical forests

A goal among community ecologists is to predict when and where trophic cascades occur. For example, several studies have shown that forest birds can limit arthropod abundances on trees, but indirect effects of bird predation (i.e. decreased arthropod damage to trees) are not always observed and their context is not well understood. Because productivity is one factor that is expected to influence trophic cascades, we compared the extent to which birds indirectly limit herbivore damage to trees in two lowland Neotropical forests that differed in seasonality of leaf production and rainfall. We compared the effects of bird predation on local arthropod densities and on damage to foliage through a controlled experiment using bird exclosures in the canopy and understory of two forests. We found that birds decreased local arthropod densities and leaf damage in the canopy of the drier site during periods of high leaf production, but not in the wetter forest where leaf production was low and sporadic throughout the year. Birds had no effect on arthropod abundances and leaf damage in the understory where leaf production and turnover rates were low. In support of these experimental interpretations, although we observed that arthropod densities were similar at the two sites, bird densities and the rate at which birds captured arthropods were greater at the drier, seasonally productive site. The influence of top-down predation by birds in limiting herbivorous insects appears to be conditional and most important when the production and turnover of leaves are comparatively high. Figure legends were missing in the original article published under Plant Animal Interactions, Oecologia (2005) 143: 106–166. The complete article is repeated here. The online version of the original article can be found at     

Nutrient supply and bird predation additively control insect herbivory and tree growth in two contrasting forest habitats

It has been suggested that bottom–up and top–down forces interactively control food web dynamics. While top–down effects would increase with resource availability to plants, bottom–up effects would be stronger under low predator abundance. These predictions, however, have rarely been tested at contrasting sites while keeping the dominant plant species unchanged. Furthermore, few studies have factorially manipulated both types of forces in forest communities. For two years, we evaluated the effects of fertiliser (NPK) addition and bird exclusion on tree growth, leaf traits, insect abundance, and folivory rates in a dry/warm and a wet/cold Nothofagus pumilio forest in Patagonia, Argentina. Overall, we found no interaction between nutrient supply and bird predation, although the strength of bottom–up and top–down forces differed markedly between forest sites. Treatment effects were generally weak in the wet forest, where tree growth rates and insect herbivory were low relative to the dry forest. In the dry forest, fertilisation increased sapling growth, insect abundance and folivory, whereas bird exclusion increased leaf damage and reduced tree growth. In the wet forest, fertilisation enhanced leaf nutrient contents and folivore abundance but not sapling growth, while bird exclusion had little impact on insects or trees. These results imply that factors other than nutrients and birds were important in controlling tree growth and folivore activity in the wet forest. While treatment effect sizes varied widely among feeding guilds, in general, nutrient effects on folivores were stronger than predator effects. We conclude that, within the time‐frame of this study, tree growth and herbivory were additively affected by soil nutrients and predator presence, as bird exclusion effects did not change with elevated folivore activity on fertilised trees. We also show that both top–down and bottom–up cascades were weaker in a forest site characterised by slow‐growing juvenile trees subjected to low folivore pressure.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

Bottom-up trophic cascades caused by moose browsing on a natural enemy of a galling insect on balsam fir

To elucidate the impact of browsing by moose on the indigenous insect community, we investigated the abundance of a specialist insect herbivore (galler; Paradiplosis tumifex) and its natural enemy (inquiline; Dasineura balsamicola) on balsam fir saplings in the field using exclosures in Newfoundland, Canada. The number of current-year shoots on each sapling, but not the density of balsam fir saplings, was significantly higher inside exclosures, where moose had been excluded for 9–12 years, than outside exclosures. Current-year shoots were longer and contained more, but smaller, needles inside than outside exclosures. Moose browsing had a negative impact on the abundance of both the galler and its natural enemy. Gall density was 85.6% and 60.4% lower in 2006 and 2007, respectively, outside than inside exclosures. Similarly, the attack rate of D. balsamicola was 76.5% lower outside than inside exclosures in 2007. The apparent survival rate of P. tumifex larvae was similar outside and inside exclosures due to higher rates of unexplained mortality in galls outside exclosures. Survival of P. tumifex larvae was positively related to shoot length and inversely related to gall density. Only gall density was negatively related to D. balsamicola survival. Our results demonstrate that a large mammal can cause strong bottom-up trophic cascades.     

Plant Diversity Impacts Decomposition and Herbivory via Changes in Aboveground Arthropods

Loss of plant diversity influences essential ecosystem processes as aboveground productivity, and can have cascading effects on the arthropod communities in adjacent trophic levels. However, few studies have examined how those changes in arthropod communities can have additional impacts on ecosystem processes caused by them (e.g. pollination, bioturbation, predation, decomposition, herbivory). Therefore, including arthropod effects in predictions of the impact of plant diversity loss on such ecosystem processes is an important but little studied piece of information. In a grassland biodiversity experiment, we addressed this gap by assessing aboveground decomposer and herbivore communities and linking their abundance and diversity to rates of decomposition and herbivory. Path analyses showed that increasing plant diversity led to higher abundance and diversity of decomposing arthropods through higher plant biomass. Higher species richness of decomposers, in turn, enhanced decomposition. Similarly, species-rich plant communities hosted a higher abundance and diversity of herbivores through elevated plant biomass and C:N ratio, leading to higher herbivory rates. Integrating trophic interactions into the study of biodiversity effects is required to understand the multiple pathways by which biodiversity affects ecosystem functioning.     

Keystone mutualism influences forest tree growth at a landscape scale

Interactions between ants and phloem‐feeding herbivores are characterised as a keystone mutualism because they restructure arthropod communities and generate trophic cascades. Keystone interactions in terrestrial food webs are hypothesised to depend on herbivore community structure and bottom‐up effects on plant growth. Here, we tested this prediction at a landscape scale with a long‐term ant‐exclusion experiment on hickory saplings in the context of spatial variation in herbivore community structure and habitat quality. We quantified top‐down effects of ants, herbivore communities as well as abiotic factors impacting hickory shoot growth. We found that ants influenced shoot growth via strong, context‐dependent, compensatory effects, with clear cascading benefits only when phloem‐feeders were present and chewing herbivore abundance was high. By contrast, while several landscape variables predicted hickory growth, they did not mediate the strength of cascading effects of ants. These results suggest that ant/sap‐feeder mutualisms may regulate forest productivity by mediating effects of multiple herbivore guilds.     

Shorebirds, snails, and the amphipod (Corophium volutator) in the upper Bay of Fundy: top–down vs. bottom–up factors, and the influence of compensatory interactions on mudflat ecology

During their annual mid- to late-summer southward migration, Semipalmated Sandpipers (Calidris pusilla) feed intensively on the amphipod Corophium volutator on intertidal mudflats in the Bay of Fundy. Corophium, in turn, feed on diatoms and bacteria. Using a series of bird exclosures and fertilizer addition, we examined top–down and bottom–up effects, and investigated the presence of a trophic cascade in the mudflat community during the period when birds are abundant. Although both top–down and bottom–up forces were present in this system, neither transmitted beyond a single trophic link. Predation by shorebirds, which may be less size-selective than previously thought, reduced Corophium abundance in control plots by approximately 80% relative to exclosures, but most other species were unaffected. Shorebird predation did not result in an increase in diatom abundance, as predicted under the trophic cascade hypothesis. Fertilizer increased diatom abundance, but had no effect on Corophium abundance or bird predation, and little effect on other mudflat invertebrates. The only indirect effect observed was on mud snails (Ilyanassa obsoleta), which, by rapidly responding to changes in diatom abundance, compensated for both bird exclusion and fertilizer addition, and prevented the trophic cascade. This population response by snails, possibly stemming from competition with Corophium, probably contributed to the stability of the community. Our results provide an example of short-term compensation in a simple intertidal community, and highlight the importance of considering direct and indirect effects in community ecological studies. We conclude that while compensatory interactions that block trophic cascades may be more common in more complex ecosystems, they are not restricted to them.     

Light environment and the impacts of foliage quality on herbivorous insect attack and bird predation

Theory predicts that variation in plant traits will modify both the direct interactions between plants and herbivores and the indirect impacts of predators of those herbivores. Light has strong effects on leaf quality, so the impacts of herbivores and predators may differ between plants grown in sun and shade. However, past experiments have often been unable to separate the effects of light environment on plant traits and herbivory from direct effects on herbivores and predators. We first manipulated light availability in an open habitat using a shade cloth pre-treatment to produce oak saplings with different leaf qualities. Leaves on plants exposed to high light were thicker and tougher and had lower nitrogen and water contents, and higher carbon and phenolic contents than leaves on plants under a shade cloth. Then, in the main experiment, we moved all plants to a common shade environment where bird predators were excluded in a factorial design. We measured insect herbivore abundance and leaf damage. Herbivores were significantly more abundant and caused greater leaf damage on sun trees, although these leaf characteristics are usually associated with low-quality food. Bird exclusion did not change herbivore abundance but did increase leaf damage. Contrary to our predictions, the effects of birds did not differ between trees grown in sun and shade conditions. Thus, differences in effects of predators on herbivores and plants between light habitats, when observed, might be due to variation in predator abundance and not bottom-up effects of host plant quality.     

From protégé to nurse plant: establishment of thorny shrubs in grazed temperate woodlands

Question: Thorny shrubs play keystone roles in grazed ecosystems by defending non‐protected plants against herbivores, but their establishment in grazed ecosystems is poorly understood. Which factors control establishment of recruits of thorny nurse shrubs in grazed temperate woodlands? Location: Ancient grazed temperate woodlands (52°32′N, 6°36′E), The Netherlands. Methods: We surveyed biotic and abiotic factors for saplings of thorny nurse shrubs in plots with and without saplings. To disentangle these factors, we performed a transplantation experiment over two growing seasons with nurse shrub saplings (Prunus spinosa and Crataegus monogyna) planted in two dominant vegetation types – tall unpalatable swards and short grazed lawns – half of them protected from herbivory via exclosures. Results: Plots with shrub saplings had taller surrounding vegetation, higher soil pH and higher soil moisture than plots without saplings. These plots predominantly contained unpalatable sward species, while plots without saplings mainly contained palatable lawn species. After transplantation, sapling survival was higher in exclosures than in the open, and higher in sward exclosures than in lawn exclosures. Sapling growth was higher in swards than in lawns, higher inside than outside exclosures, and higher for Prunus than Crataegus, while browsing on saplings was higher in lawns. Conclusion: Unpalatable swards form essential establishment niches for thorny shrubs in grazed temperate woodlands: they protect against herbivores before thorns fully develop in saplings, and sapling growth is better due to improved micro‐environmental conditions. Once established and thorny, shrub saplings grow out of the protective range of the swards and in turn facilitate tree seedlings, which are essential for long‐term persistence of grazed temperate woodlands. This study shows that nurse plants may start as protégés before becoming facilitators for other plants in a later life stage. This may be common for nurse plants in various ecosystems. We argue that improved understanding of establishment of nurse plants and their constraining factors is crucial for effective conservation and restoration in various ecosystems.     

Maize‐field complexity and farming system influence insectivorous birds’ contribution to arthropod herbivore regulation

The contribution of insectivorous birds to reducing crop damage through suppression of herbivory remains underappreciated, despite their role as cropland arthropod predators. We examined the roles of farming system, crop cover pattern, and structural configuration in influencing assemblage composition of insectivorous birds and their herbivorous arthropod prey across maize fields, and determined how bird exclusion affects crop herbivory levels. To achieve these objectives, we collected data across a sample of organic and conventional small‐scale non‐Bt maize farms in western Kenya. Assessments of abundance, diversity, and richness of insectivorous birds and abundance of their arthropod prey were compared between organic and conventional small‐scale non‐Bt maize on monocultured and inter‐cropped farms. We also employed bird exclusion experiments to assess impacts of bird predation on herbivorous arthropod abundance. Results showed that higher structural heterogeneity supported higher insectivorous bird richness, particularly under organic systems, dense trees, large woodlots, and thick hedgerows. Bird abundance further increased with crop diversity but not in relation to cropping method, hedgerow type, or percent maize cover per se. Conversely, herbivorous arthropod abundance and richness increased on conventional farms and those with higher percent maize cover, but were unaffected by cropping methods, tree, or hedgerow characteristics. Birds’ arthropod prey was more abundant under completely closed experimental plots compared with open or semi‐closed plots, confirming a significant linkage between birds and herbivorous arthropod suppression. In this study, we demonstrate importance of structural heterogeneity in agricultural landscapes, including diverse croplands and on‐farm trees to maximize insectivorous birds’ contribution to reducing crop arthropod herbivory. Abstract in Swahili is available with online material.     

Equivalence in the strength of deer herbivory on above and below ground communities

Herbivores exert a strong influence on the species composition and richness of plant communities, but the magnitude of their effect on belowground communities remains poorly understood. While an increasing number of studies acknowledge the importance of documenting belowground effects of herbivores, very few of these evaluate variation in the strength of the response from aboveground to belowground communities. Our study documents the long-term consequences of sustained deer herbivory for plant and arthropod communities adjacent to 15 exclosures that have been in place since 1996. We hypothesized that herbivory would alter the composition and diversity of communities, but the strength of the effects of herbivory would weaken from plants, to leaf-litter invertebrates, and to belowground microarthropod communities. First, we found that herbivory negatively impacted plant seedling and sapling abundance and performance, reduced the abundance of ants and the taxonomic richness of arthropods in the litter layer and reduced the richness of soil microarthropod communities. Second, in contrast to our hypothesis, the magnitude of effect size did not vary among trophic levels, indicating that effects of deer herbivory cascade from plants to the leaf-litter and soil arthropod communities with equal strength. While much recent research has focused on how specific traits of plants may mediate the effects of herbivory on associated species, our results suggest that indirect effects of herbivory might influence many components of belowground communities.