Resource Concentration Hypothesis Research Articles

Diet specialization mediates drivers of Cucurbita herbivory in a semi-arid agroecosystem

Herbivory is a major fitness pressure for plants and a key driver of crop losses in agroecosystems. Dense monocultures are expected to favor specialist herbivorous insects, particularly those who primarily consume crop species; yet, levels and types of herbivory are not uniform within regional cropping systems. It is essential to determine which local and regional ecological factors drive variation in herbivory in order to support functional agroecosystems that rely less on chemical inputs. Crops in the genus Cucurbita host a suite of both generalist and specialist herbivores that inflict significant damage, yet little is known about the relative contribution of these herbivores to variation in herbivory and how local- and landscape-scale Cucurbita resource concentrations, management practices, and natural enemies mediate this relationship. In this study, we tested whether three foundational ecological hypotheses influenced Cucurbita herbivory across 20 pumpkin fields in the semi-arid Southern High Plains Region of Texas. We used generalized linear mixed models and confirmatory path analysis to assess whether the Density-dependent Herbivory Hypothesis, Resource Concentration Hypothesis, or the Natural Enemies Hypothesis, could explain variation in Cucurbita herbivory and insect dynamics in the context of conventional agronomic practices. We found that herbivory increased over time, indicating that herbivores were causing sustained damage throughout the growing season. We also found that fields with higher local Cucurbita resources had lower herbivory, suggesting a resource dilution effect. Natural enemy communities were more abundant and taxonomically rich in sites with greater generalist herbivore abundance, though predator abundance declined over time, indicating that late-season crop fields are most at risk given high herbivory and low natural enemy-based control. Our findings also suggest that while local resource availability may drive the abundance and richness of arthropod communities, additional agronomic and phenological information is needed to anticipate herbivory risk in an agriculturally dominated landscape.

Insect herbivory on woody broadleaf seedlings along a subtropical elevational gradient supports the resource concentration hypothesis.

Theories of plant-herbivore interactions hold that seedlings are more vulnerable to herbivory in warmer and more stable climates at lower elevations. Hypotheses of plant apparency, resource concentration, and resource availability have been proposed to explain variability in leaf herbivory. However, seasonal differences in the effects of these hypotheses on leaf herbivory on seedlings remain unclear. We evaluated the three herbivory hypotheses by comparing the percentage and frequency of leaf herbivory in understory broadleaf seedlings in a subtropical forest in May (spring) and October (autumn) along an elevational gradient (290-1370 m a.s.l.). In total, we measured 2890 leaves across 696 seedlings belonging to 95 species and used beta regressions to test the effects of plant apparency (e.g., leaf area, seedling height), resource concentration (e.g., plant species diversity), and resource availability (e.g., canopy openness, soil available N and P) on leaf herbivory. Seedlings exhibited unimodal patterns of leaf herbivory along elevation, with drivers of leaf herbivory varying by the month. Variation in the frequency of leaf herbivory was best explained by the resource concentration hypothesis (e.g., plant species diversity) in both months, and herbivory was lower on seedlings in sites with higher plant diversity. Plant apparency hypothesis (e.g., leaf area, seedling height) was weakly supported only in spring, and the evidence for resource availability hypothesis (e.g., canopy openness, soil nutrients) was mixed. This study supports the resource concentration hypothesis and reveals the importance of seasonal difference on understanding leaf herbivory patterns and the drivers of plant diversity in subtropical forests.

Tree identity and canopy openness mediate oil palm biodiversity enrichment effects on insect herbivory and pollination.

As the extent of oil palm (Elaeis guineensis) cultivation has expanded at the expense of tropical rainforests, enriching conventional large-scale oil palm plantations with native trees has been proposed as a strategy for restoring biodiversity and ecosystem function. However, how tree enrichment affects insect-mediated ecosystem functions is unknown. We investigated impacts on insect herbivory and pollination in the fourth year of a plantation-scale, long-term oil palm biodiversity enrichment experiment in Jambi, Sumatra, Indonesia. Within 48 plots systematically varying in size (25-1600 m2 ) and planted tree species richness (one to six species), we collected response data on vegetation structure, understory insect abundances, and pollinator and herbivore activity on chili plants (Capsicum annuum), which served as indicators of insect-mediated ecosystem functions. We examined the independent effects of plot size, tree species richness, and tree identity on these response variables, using the linear model for random partitions design. The experimental treatments were most associated with vegetation structure: tree identity mattered, as the species Peronema canescens strongly decreased (by approximately one standard deviation) both canopy openness and understory vegetation cover; whereas tree richness only decreased understory flower density. Further, the smallest plots had the lowest understory flower density and richness, presumably because of lower light availability and colonization rates, respectively. Enrichment influenced herbivorous insects and natural enemies in the understory to a lesser extent: both groups had higher abundances in plots with two enrichment species planted, possibly because higher associated tree mortality created more habitat, while herbivores decreased with increasing tree species richness, in line with the resource concentration hypothesis. Linking relationships in structural equation models showed that the negative association between P. canescens and understory vegetation cover was mediated through canopy openness. Likewise, canopy openness mediated increases in herbivore and pollinator insect abundances. Higher pollinator visitation increased phytometer yield, while impacts of insect herbivores on yield were not apparent. Our results demonstrate that even at an early stage, different levels of ecological restoration influence insect-mediated ecosystem functions, mainly through canopy openness. These findings suggest that maintaining some canopy gaps while enrichment plots develop may be beneficial for increasing habitat heterogeneity and insect-mediated ecosystem functions.

Effects of plant diversity and leaf traits on insect herbivory in plantation and natural forests

Conversion from natural forest to timber plantation is an important cause of pristine forests loss. These changes alter community structure and cause a decrease in the diversity of plant communities. Leaf functional traits, which are an important defensive strategy, might also be influenced by such alterations. Altogether, shifts from natural forest to timber plantation may lead to profound changes in plant-plant and plant-animal interactions, including herbivory and the relative abundance of different feeding guilds, an important aspect of forest ecosystem function and community dynamics. In this study, we assessed insect herbivore damage in saplings of four native tree species in the understory of natural forests and two types of ca. 80-year-old timber plantations. We measured leaf damage by the most common feeding guilds in over 9000 leaves from 200 saplings. We first compared herbivory between natural forest and plantations, and we then analyzed the drivers of herbivory and of the two most common guilds – chewers and miners – using several indices of forest structure and leaf traits as predictors. Total herbivory did not change among forest types, but it showed some variation in two of the four species. Herbivory decreased with increasing species richness and increased with increasing basal area of conspecific and heterospecific trees. Leaf damage caused by chewers increased with leaf thickness, while damage caused by miners showed the opposite relationship. On an ecological level, our findings support expectations from the resource concentration hypothesis and on the role of leaf traits in mediating the impact of insect feeding guilds on leaf herbivory. On an applied level, these results suggest that timber plantations can support similar levels of ecosystem processes than natural forests decades after their establishment. Allowing the regeneration of a diverse understory in plantation forests should be implemented to favor ecosystem dynamics close to those of the original forests.

Hypotheses and lessons from a native moth outbreak in a low‐diversity, tropical rainforest

AbstractOutbreaks of defoliating insects in low‐diversity tropical forests occur infrequently but provide valuable insights about outbreak ecology in temperate environments and in general. We investigated an extensive outbreak of the endemic koa moth (Scotorythra paludicola), which defoliated endemic koa trees (Acacia koa) over a third of their range on Hawai‘i Island during 2013 and 2014. At Hakalau Forest National Wildlife Refuge, we observed the dynamics of the outbreak and its effects on host trees, nutrient cycling, and insectivorous consumers in reforestation stands of densely planted koa and in natural forest stands of mixed koa and ‘ōhi‘a (Metrosideros polymorpha). Contrary to predictions of the resource concentration hypothesis, caterpillar biomass and defoliation severity were greater in the natural forest sites, where koa density was relatively low. Caterpillars preferentially consumed the most palatable koa foliage type (phyllodes), and koa initially refoliated with the least palatable foliage type (true leaves). Lightly defoliated small trees refoliated more quickly than did heavily defoliated ones but the opposite was true for large trees, which also produced a greater proportion of phyllodes. Mortality was greatest for heavily defoliated small koa. Caterpillar frass caused larger increases in soil nitrogen (N) than phosphorus (P) availability, with the greatest N increases in fine‐textured soils. Foliar N increased in alien grasses under koa canopies compared to grasses away from koa and to native woody understory species. Bird activity was influenced by ‘ōhi‘a flower abundance and the severity of koa defoliation; birds switched to outbreaking caterpillar prey, and they gained weight during the outbreak. Bat foraging times decreased during the outbreak, apparently because they became satiated quickly each night. Parasitoid wasps increased with caterpillar abundance but had little influence on outbreak dynamics. Reducing alien grass cover and increasing tree diversity would likely reduce the impacts of insect outbreaks and similar perturbations to native forests.

A Multi-Faceted Approach to Understand How Resource Diversity Can Mediate the Coexistence of Cryptic Marine Nematode Species

Based on the principle of competitive exclusion, species occupying the same ecological niche cannot stably coexist due to strong interspecific competition for resources. Niche diversification, for instance through resource partitioning, may alleviate competition. Here, we investigate the effects of resource diversity on foraging behavior, fitness and interspecific interactions of four cryptic bacterivorous nematode species (Pm I–IV) of the Litoditis marina species complex with sympatric field distributions. Three resource (bacteria) diversity levels (low, medium, high) were used as food treatments and compared to a treatment with only Escherichia coli as food. Differences in taxis to food existed between the cryptic species and between bacterial mixtures of different diversity: all the cryptic species except Pm I showed higher attraction toward medium-diversity food. Furthermore, the cryptic species of L. marina generally exhibited higher fitness on a more diverse food resource. Resource diversity also impacted the interspecific interactions between the cryptic species. Our results show that resource diversity can alter the interspecific interactions among the cryptic species of L. marina, indicating that competitive equilibria between species are very context-dependent. Although a considerable body of evidence supports the hypotheses (e.g., “variance-in-edibility” hypothesis and the “dilution hypothesis” or “resource concentration hypothesis”) which predict a negative impact on consumers when resource diversity is increased, the benefits of a diverse resource can outweigh these disadvantages by offering a more complete and/or complementary range of nutritional resources as suggested by the “balanced diet” hypothesis.

Nest Volume and Distance between Nests Do Not Affect Population Size or Species Richness of the Termitophilous Corotocini Fauna

Termite nests act as shelters for a wide variety of invertebrates. We studied the Corotocini termitophile fauna (Staphylinidae, Aleocharinae) in nine Nasutitermes corniger nests and nine N. ephratae nests to test the resource size (larger nests should host more social-parasite species than smaller ones) and resource concentration (nests occurring closer to each other should host a greater richness and variety of social parasites) hypotheses. In total, we found 358 Corotocini individuals. Regression analysis did not reveal a relationship between species richness or population size of termitophiles and nest volume. Thus, we rejected the resource size hypotheses as a driver of termitophile diversity. Neither did distance between nests explain the richness of their fauna; thus, the resource concentration hypothesis was also rejected. In conclusion, we suggest that biological aspects, such as dispersal dynamics and species traits likely explain the diversity of termitophiles.

Does the “Enemies Hypothesis” operate by enhancing natural enemy evenness?

Root (1973) observed fewer pests in weedy Brassica oleracea plantings, and suggested this could be because greater plant diversity bolstered resources for natural enemies and strengthened herbivore suppression (i.e., the Enemies Hypothesis) and/or disrupted host-location by specialist herbivores (i.e., the Resource Concentration Hypothesis). These proposed mechanisms have been exceptionally influential in efforts to manage agroecosystems to promote conservation biological control, yet their relative importance remains unclear. Because direct (plant-driven) and indirect (enemy-mediated) drivers of herbivore pressure are difficult to isolate experimentally, we compiled a multi-year observational dataset tracking arthropod pest and predator densities in B. oleracea plantings on over 50 farms varying in weed cover within fields as well as in habitat diversity in surrounding landscapes. We then used structural equation models to evaluate direct and indirect interaction links consistent with the Enemies and Resource Concentration Hypotheses, while also considering landscape context and enemy community structure. We predicted that non-crop habitat would limit herbivores indirectly by supporting evenly distributed communities of natural enemies. Our results revealed that weedy vegetation indeed benefitted predator evenness and thus indirectly correlated with enhanced predator densities, but instead of limiting herbivores, natural enemies seemed to track both aphids and caterpillars. Landscape diversity had no effect on predator evenness, yet appeared to benefit predator density, while having contrasting direct effects on aphids (positive) and caterpillars (negative). We found no evidence for direct negative links between weed cover and herbivores that would indicate an important role for the Resource Concentration Hypothesis. Instead, we found indirect links between weedy plants and natural enemy evenness across our network of field sites, suggesting partial support for the Enemies Hypothesis. More generally, we suggest that greater consideration of predator-predator interactions and local biodiversity effects could help explain inconsistent relationships between landscape diversity and herbivore suppression.

Resource segregation at fine spatial scales explains Karner blue butterfly (Lycaeides melissa samuelis) distribution

The resource concentration hypothesis predicts that herbivorous insect density scales positively with plant density because insects are better able to identify, and remain longer in, patches with denser plant resources. While some studies support this hypothesis, others do not. Different explanations have been proposed for this discrepancy, including variation in insect dispersal ability and diet breadth. We test the resource concentration hypothesis using the Karner blue butterfly (Lycaeides melissa samuelis), a specialist that relies on wild blue lupine (Lupinus perennis) as its sole host plant. We extended this hypothesis to test whether Karner blue density also scales positively with nectar plant resources. Our findings did not support the resource concentration hypothesis and demonstrate that the spatial segregation of nectar and host plant resources relative to each other can influence the location and abundance of Karner blues on the landscape. This is because the location of resources relative to each other influences the energy and time butterflies expend for flight activity, and thereby influences resource acquisition. During early summer when first brood Karner blues emerge, nectar and host plants were spatially segregated, and Karner blue density peaked at intermediate densities of nectar and host plants occurring at ratios approximately equal to 1:1. During late summer, we found no significant relationships between second brood Karner blues and nectar plants or host plants when there was no correlation between nectar and host plants. Conservation practitioners of specialist insects with low vagility can strategically manage the distribution of plant resources to minimize insect time and energy expenditure and promote resource acquisition for all of an insect’s life stages.

Contrasting effects of host crops and crop diversity on the abundance and parasitism of a specialist herbivore in agricultural landscapes

Landscape complexity affects herbivores in agroecosystems, but consequences on pest control services are variable. Carryover effects of landscape composition in previous years on herbivore control may be important, but have been seldom assessed. Understanding landscape complexity effects at different temporal and spatial scales is important to improve sustainable pest control services. We examined the effect of agricultural landscape complexity (e.g., the percentage of semi-natural habitats) on cereal leaf beetle (CLB), Oulema melanopus L., and its parasitism by Tetrastichus julis (Walker). From 2014 to 2015, we assessed CLB abundance and parasitism in 54 fields along a gradient of landscape complexity (2–70% of semi-natural habitats) in southern Alberta. We used generalized linear models to test the effects of percentage of crops and semi-natural habitats, and landscape and crop diversity on CLB abundance and parasitism at 0.5, 1, 1.5 and 2 km spatial scales. CLB abundance decreased with higher crop diversity at the 0.5 km scale and increased with CLB host crops in the current and previous years at multiple scales, supporting the resource concentration hypothesis. CLB parasitism increased with CLB abundance and in landscapes with increased canola and alfalfa during a year of low CLB abundance. CLB abundance had contrasting associations with semi-natural habitats: positive with woodlands and negative with pastures. Our study suggests that crop diversity reduces the abundance of this specialist pest in agricultural landscapes likely by a dual effect of reducing host crop area and increasing habitats with resources for parasitoids.

Importance of intercropping for biodiversity conservation

Traditional there are two strategies to handle pest problems in crop production, either dependence on non-chemical agricultural practices (such as cultural, mechanical, biological practices etc.) or reliance on existing natural pest control mechanisms. Intercropping is a cultural non-chemical agricultural practice where two or more crops are grown on the same field in a year with different cropping patterns. In this multiple cropping system, biodiversity and pest suppression are increased. Biodiversity can restore the natural elements of agro ecosystem because almost all favorable elements of natural enemies are available in diversified agro ecosystem. Energy intensive modern technology in agriculture is one of the vital causes for loss of biodiversity. In intercropping system biological pest control method can be ensured with higher level of crop diversity instead of energy intensive agriculture. Intercropping provides different benefits on pest management with two available hypotheses or mechanism. One of the hypotheses is the ‘resource concentration hypothesis’ and another is the ‘natural enemies hypothesis’. Intercropping, directly and indirectly, influences to increase biodiversity which results in reduction of pest densities in crop fields. As a result, less expense for use of pesticide is required and finally higher yield also add some financial benefits. Intercropping system utilizes inherent ability of plant to protect pests. Therefore further knowledge about genotypic crop diversity, diversity of natural enemies, chemically-mediated mechanisms of Volatile Organic Compounds (VOCs) will be effective for further improvement of intercropping system for greater benefits.

Spatial and temporal patterns of a pulsed resource dynamically drive the distribution of specialist herbivores

Patterns and drivers of the spatio-temporal distribution of herbivores are key elements of their ecological and evolutionary impacts on plant populations. Herbivore spatial distributions may be influenced by increased (RCH: resource concentration hypothesis) or decreased (RDH: resource dilution hypothesis) resource densities, but the effect of temporal variations in resource densities on such distributions remains poorly documented. We used a survey of a masting tree species and its seed predators in Southeastern France to address the effect of a host’s pulsed resource on the spatio-temporal distributions of highly specialized insect herbivores feeding on seeds. Variations in both resource and seed predator densities were assessed by estimating seed production and seed infestation rates in focus trees during 10 consecutive years. We found increasing seed infestation rates with decreasing host tree densities in years of low seed production, indicating a RDH pattern of seed predators. However, such pattern was not persistent in years of high seed production during which seed infestation rates did not depend on host tree densities. We showed that temporal variations in resource density can lead to transience of seed predator spatial distribution. This study highlights how predictions of plant-herbivore interactions in natural ecosystems may rely on temporal components underlying RCH and RDH hypotheses.

Traits of litter-dwelling forest arthropod predators and detritivores covary spatially with traits of their resources.

The functional trait approach proposes that relating traits of organisms within a community to variation in abiotic and biotic characteristics of their environment will provide insight on the mechanisms of community assembly. As traits at a given trophic level might act as filters for the selection of traits at another trophic level, we hypothesized that traits of consumers and of their resources covary in space. We evaluated complementary predictions about top‐down (negative) and bottom‐up (positive) trait covariation in a detrital food web. Additionally, we tested whether positive trait covariation was better explained by the Resource Concentration Hypothesis (i.e., most commonly represented trait values attract abundant consumers) or the Resource Specialization Hypothesis (i.e., resource diversity increases niche availability for the consumers). Macroarthopods were collected with pitfall traps over two summers in three forested sites of southern Quebec in 110 plots that varied in tree species composition. Six feeding traits of consumers (detritivores and predators) and six palatability traits of their resources (leaf litter and prey) were matched to assess spatial covariation. Trait matches included consumer biting force/resource toughness, detritivore mandibular gape/leaf thickness, predator/prey body size ratio, etc. Our results demonstrate for the first time a covariation between feeding traits of detritivores and palatability traits of leaf litter (31–34%), and between feeding traits of litter‐dwelling predators and palatability traits of potential prey (38–44%). The observed positive covariation supports both the Resource Concentration Hypothesis and Resource Specialization Hypothesis. Spatial covariation of consumer and resource traits provides a new tool to partially predict the structure of the detrital food web. Nonetheless, top‐down regulation remains difficult to confirm. Further research on top‐down processes will be undoubtedly necessary to refine our capacity to interpret the effect of biotic interactions on co‐distribution.

The floriphilic katydid, Phaneroptera brevis, is a frequent flower visitor of non-native, flowering forbs

Distribution of consumers in a patch of vegetation can be predicted by resource availability and explained by the resource-concentration and optimal-foraging hypotheses. These hypotheses have not been explored for flower-visiting Orthoptera because they are deemed less economically or ecologically important. Some flower-visiting orthopterans can provide pollination services, which warrants more attention. We studied a Singaporean, floriphilic katydid, Phaneropterabrevis, to investigate the following questions: 1) how frequently does P.brevis visit flowers compared to other flower visitors and 2) what factors predict the abundance of P.brevis? We collected abundance data for P.brevis and other flower-visiting arthropods and quantified seven environmental parameters, including flower abundance and host-plant species richness. We found that P.brevis frequents flowers significantly more often than some common and expected flower visitors such as hoverflies. In line with the prediction of the resource-concentration hypothesis, the abundance of P.brevis was positively correlated with a higher flower abundance. Owing to the limited information on unexpected wild flower visitors and pollinators, especially from the understudied tropics of Southeast Asia, we propose that P.brevis can be a model organism for future studies to answer fundamental questions on flower visitation.

Habitat Management for Pest Management: Limitations and Prospects

AbstractHabitat management is an important strategy for pest control in integrated pest management (IPM). Various categories of habitat management such as trap cropping, intercropping, natural enemy refuges such as ‘beetle banks’, and floral resources for parasitoids and predators, have been used in applied insect ecology for many years. In a broader sense, two mechanisms, the ‘enemies hypothesis’ and the ‘resource concentration hypothesis’ have been identified as acting independently or combined in pest population dynamics. The ‘enemies hypothesis’ directly supports the conservation and enhancement of natural enemies, floral resources such as shelter, nectar, alternative food sources, and pollen (SNAP) to improve conservation biological control. The ‘resource concentration hypothesis’ emphasizes how the host selection behavior of herbivores in a diverse habitat can reduce pest colonization in crops. This review emphasizes the potential of these approaches, as well as possible dis-services, and includes limitations and considerations needed to boost the efficacy of these strategies worldwide.

Galling inducing insects associated with a tropical shrub: The role of resource concentration and species interactions

Gall-forming insects are sophisticated sedentary herbivores that present high level of specifcity with host plant, but their performance can be affected by biotic and abiotic factors. In this study we have tested two predictions: a) plants that have a greater number of conspecifc neighbors have greater richness and abundance of gall-forming insects, and b) interspecifc competition is a force capable of shaping the organization of gall-forming insect communities in super-host plants. We used the Copaifera oblongifolia (Fabaceae)/galling insects’ system to test these predictions. Fieldwork was carried out in areas of Cerrado (Brazilian Savanna) in northern Minas Gerais, Brazil. To test the frst hypothesis, we evaluated with generalized linear mixed models, the effects of the number of conspecifc neighbors on the richness and abundance of galls associated with 67 C. oblongifolia individual plants belonging to two populations. To test the second hypothesis, we used null models to evaluate whether a plant of C. oblongifolia colonized by a species of gall is preferred or avoided by another species of gall. A total of 2901 gall-forming insects belonging to 15 species were collected from the host plant C. oblongifolia. We observed negative relationships between the number of conspecifc neighbors and the abundance and richness of gall-forming insects associated C. oblongifolia. Thus, our data did not support the resource concentration hypothesis. Instead, we used the resource dilution theory to explain the negative relation between resource concentration and frequency of attack by galling insects. Our results also showed that the co-occurrence pattern of gall-forming insects in the host plant did not differ from those expected by chance. Therefore, the structure of the gall-forming insect community associated to single C. oblongifolia plants cannot be attributed to deterministic factors such as interspecifc competition.https://doi.org/10.25260/EA.19.29.1.0.751

Mixed forests to mitigate risk of insect outbreaks

ABSTRACTThe majority of agricultural and forest production systems are typically simplified systems and therefore thought to be sensitive to disturbance. Ecosystems might be less sensitive to disturbance if complexity increases. The resource concentration hypothesis predicts that complexity makes it more difficult for herbivores to find and exploit their food. The natural enemy hypothesis predicts less damage in diverse environments because enemy abundance and diversity remain higher and more stable in environments providing more alternative food sources. We explore the effect of tree diversity in two types of pine stands differing in tree diversity. We ask whether European pine sawfly (Neodiprion sertifer) larval group distribution and larval mortality, imposed by specialist natural enemies, is affected by tree diversity. We find fewer larval groups on pine trees surrounded by non-host trees, indicating that fewer egg-batches were laid. Mortality rates by specialist enemies are not affected by tree diversity but parasitism rates are likely to be higher in larger trees. Herbivores are inhibited by increased tree diversity and density, whereas natural enemies seem to be unaffected, partly increasing the resistance of more diverse forest stands against insect damage. Thus, managing for more mixed forests could mitigate risk of herbivore damage.

Seasonally dependent relationship between insect herbivores and host plant density in Jatropha nana, a tropical perennial herb.

ABSTRACTThe fact that plant spatial aggregation patterns shape insect-herbivore communities in a variety of ways has resulted in a large body of literature on the subject. The landmark resource concentration hypothesis predicts that density of insect herbivores per plant will increase as host plant density increases. I examined this prediction across temporal samplings using Jatropha nana and the associated specialist insect herbivores as a system. Through 12 field samplings, I modelled the effect of host plant density on insect-herbivore loads. The initial samplings (2–3) provided evidence for the resource concentration hypothesis, with insect loads increasing with increasing host plant density, whereas the later samplings (4–5, 7–11) showed the opposite; a resource dilution pattern with a decline of insect loads with increasing host plant density. These patterns also depend on the biology of the herbivores and have important implications on J. nana population dynamics.This article has an associated First Person interview with the first author of the paper.

Nestedness of bird assemblages along an urbanisation gradient in Central India

We censused terrestrial bird assemblages along a five-stage urbanisation gradient (rural forest, industrial zone, peri-urban, suburban and urban habitats) in and around Amravati City, Central India, between January and April 2010–2013. A total of 89 species of birds were recorded, with the highest detected species richness in the rural areas (67 species) and the lowest in the urban one (29 species). The rural habitats had the highest diversity, followed, in a decreasing order, by industrial, peri-urban, suburban and urban areas, supporting Gray's increasing disturbance rather than Connell's intermediate disturbance hypothesis. The densities were, however, highest in the urban habitats, supporting the resource concentration hypothesis. The assemblages were significantly nested, indicating a common origin from the rural forest.

Extending the ‘resource concentration hypothesis’ to the landscape-scale by considering dispersal mortality and fitness costs

Abstract The ‘resource concentration hypothesis’ proposes mechanisms by which polycultures (local plant diversity) can directly reduce densities of insect pests in agriculture. It predicts that herbivore loads are directly reduced because host plant cues are disrupted and herbivore residency times are shorter in polycultures than in monocultures. The hypothesis has been similarly applied to landscape-scale investigations; however, the mechanisms by which diverse landscapes can directly suppress herbivores are likely different than those operating at the polyculture scale and have not been clearly articulated. Here, we discuss how landscape-scale habitat diversity may directly reduce agricultural pest loads by requiring more dispersal activity, which, in turn, can increase mortality and decrease fitness. We also lay out a research agenda and argue that a better understanding of the mechanisms by which landscape-scale habitat diversity affect insect pest populations can lead to better predictions of pest control as an ecosystem service provided by diverse agricultural landscapes.