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Data from: Deer-mediated changes in environment compound the direct impacts of herbivory on understory plant communities

1. In forests of eastern North America, white-tailed deer (Odocoileus virginianus) can directly affect, via herbivory, the presence, abundance, and reproductive success of many plant species. In addition, deer indirectly influence understory communities by altering environmental conditions. 2. To examine how deer indirectly influence understory plants via environmental modification, we sampled vegetation and environmental variables in- and outside deer exclosures (10-20 years old) located in temperate forests in northern Wisconsin and the Upper Peninsula of Michigan, USA. We assessed how excluding deer affected understory community composition and structure, the soil and light environment, and relationships between direct and indirect effects using non-metric multidimensional scaling (NMDS), mixed linear models and non-parametric multiplicative regression (NPMR). 3. Excluding deer altered sapling communities and several aspects of the understory environment. Excluding deer from plots with lower overstory basal area increased sapling abundance, decreasing the amount of light available to groundlayer plants. Exclusion also reduced soil compaction and the thickness of the soil E horizon. 4. The composition of understory communities and the frequencies of various species and groups covaried in apparent response to the environmental factors affected by exclusion. In several common species and groups, E horizon thickness, compaction, openness, and/or total (sapling and overstory) basal area were significant predictors of plant frequency. 5. Complementary analyses revealed that deer exclusion also altered the frequency distributions of several species and groups across environmental space. Synthesis: Deer alter many facets of the understory environment, such as light availability, soil compaction and depth of the soil E horizon, which, in turn, appear to mediate variation in plant communities. Those environmental modifications likely compound direct impacts of herbivory as drivers of understory community change, having important implications for forest composition. Thus, we suggest a reexamination of the common assumption that understory community shifts stem primarily from tissue removal.

Data from: Herbivory and pollen limitation at the upper elevational range limit of two forest understory plants of eastern North America

Studies of species' range limits focus most often on abiotic factors, although the strength of biotic interactions might also vary along environmental gradients and have strong demographic effects. For example, pollinator abundance might decrease at range limits due to harsh environmental conditions, and reduced plant density can reduce attractiveness to pollinators and increase or decrease herbivory. We tested for variation in the strength of pollen limitation and herbivory by ungulates along a gradient leading to the upper elevational range limits of Trillium erectum (Melanthiaceae) and Erythronium americanum (Liliaceae) in Mont-Mégantic National Park, Québec, Canada. In T. erectum, pollen limitation was higher at the range limit, but seed set decreased only slightly with elevation and only in one of two years. In contrast, herbivory of T. erectum increased from <10% at low elevations to >60% at the upper elevational range limit. In E. americanum, we found no evidence of pollen limitation despite a significant decrease in seed set with elevation, and herbivory was low across the entire gradient. Overall, our results demonstrate the potential for relatively strong negative interactions (herbivory) and weak positive interactions (pollination) at plant range edges, although this was clearly species-specific. To the extent that these interactions have important demographic consequences – highly likely for herbivory on Trillium, based on previous studies – such interactions might play a role in determining plant species' range limits along putatively climatic gradients.

Data from: Gypsy moth herbivory induced volatiles and reduced parasite attachment to cranberry hosts

Interactions between species can have cascading effects that shape subsequent interactions. For example, herbivory can induce plant defenses that affect subsequent interactions with herbivores, pathogens, mycorrhizae, and pollinators. Parasitic plants are present in most ecosystems, and play important roles in structuring communities. However, the effects of host herbivory on parasitic plants, and the potential mechanisms underlying such effects, are not well known. We conducted a greenhouse study to ask whether gypsy moth (Lymantria dispar) damage, host cultivar, and their interaction affected preference of the stem parasite dodder (Cuscuta spp.) on cranberry hosts (Vaccinium macrocarpum). We then assessed the mechanisms that could underlie such effects by measuring induced changes in phytohormones and secondary compounds. We found that damage by gypsy moths delayed dodder attachment by approximately 0.3 days when dodder stems were added 2 days after damage, and reduced attachment by more than 50% when dodder stems were added 1 week after host plant damage. Gypsy moth damage significantly increased jasmonic acid (JA) levels, total volatile emissions, and the flavonol, quercetin aglycone, suggesting possible mechanisms underlying variation in dodder ability to locate or attach to hosts. Dodder preference also differed between cranberry cultivars, with the highest attachment on the cultivar that had significantly lower levels of total volatile emissions and total phenolic acids, suggesting that volatile composition and phenolics may mediate dodder preference. Our results indicate that herbivory can reduce subsequent attachment by a highly damaging parasitic plant, demonstrating the potential importance of early damage for shaping subsequent species interactions.

Data from: Population-level consequences of herbivory, changing climate and source-sink dynamics on a long-lived invasive shrub

Long-lived plant species are highly valued environmentally, economically, and socially, but can also cause substantial harm as invaders. Realistic demographic predictions can guide management decisions, and are particularly valuable for long-lived species where population response times can be long. Long-lived species are also challenging, given population dynamics can be affected by factors as diverse as herbivory, climate, and dispersal. We developed a matrix model to evaluate the effects of herbivory by a leaf-feeding biological control agent released in Australia against a long-lived invasive shrub (mesquite, Leguminoseae: Prosopis spp.). The stage-structured, density-dependent model used an annual time step and 10 climatically diverse years of field data. Mesquite population demography is sensitive to source–sink dynamics as most seeds are consumed and redistributed spatially by livestock. In addition, individual mesquite plants, because they are long lived, experience natural climate variation that cycles over decadal scales, as well as anthropogenic climate change. The model therefore explicitly considered the effects of both net dispersal and climate variation. Herbivory strongly regulated mesquite populations through reduced growth and fertility, but additional mortality of older plants will be required to reach management goals within a reasonable time frame. Growth and survival of seeds and seedlings were correlated with daily soil moisture. As a result, population dynamics were sensitive to rainfall scenario, but population response times were typically slow (20–800 years to reach equilibrium or extinction) due to adult longevity. Equilibrium population densities were expected to remain 5% higher, and be more dynamic, if historical multi-decadal climate patterns persist, the effect being dampened by herbivory suppressing seed production irrespective of preceding rainfall. Dense infestations were unlikely to form under a drier climate, and required net dispersal under the current climate. Seed input wasn't required to form dense infestations under a wetter climate. Each factor we considered (ongoing herbivory, changing climate, and source–sink dynamics) has a strong bearing on how this invasive species should be managed, highlighting the need for considering both ecological context (in this case, source–sink dynamics) and the effect of climate variability at relevant temporal scales (daily, multi-decadal, and anthropogenic) when deriving management recommendations for long-lived species.

Data from: From the individual to the landscape and back: Time-varying effects of climate and herbivory on tree sapling growth at distribution limits

1. As herbivory can modulate climate-induced shifts in species distribution, disentangling the relative importance of herbivory and climate on plant growth can help to predict and manage future changes in vegetation, such as those occurring at treeline areas. 2. An individual-based hierarchical Bayesian time-series model (Individual-Based Model; IBM) was developed to estimate the time-varying impact of climate and herbivory on individual pine-sapling height growth in woodland and treeline ecosystems of Southern Europe during a 16-year period. The performance of the IBM was compared to a Linear Mixed-Effects (LME) model to test for potential inferential effects when individual variability is marginalized. Time-varying models were also compared to constant parameter approaches. 3. Model fitting and posterior predictive checking suggests a better statistical performance of individual-scale, time-varying inference. LME modeling overestimated herbivory effects and underestimated environmental stochastic effects, and model validation indeed suggested severe overfitting in the LME model relative to the IBM strategy. These results reveal a potential failure of common aggregation strategies to correctly resolve the effects of climate and herbivory variability at the individual scale. Moreover, ignoring the time-varying nature of the effects may preclude the correct estimation of the temporal scale of climate and herbivory impacts. 4. In general, we found stronger individual- and time-averaged effects of summer precipitation relative to the effects of herbivory, particularly at the treelines. Also, individual pines responses showed effects of the same sign more consistently in the case of precipitation. This suggests that precipitation is more pervasive at the population level, while herbivory act as a spatially aggregating force through individual-level damage. 5. Synthesis. Our results suggest that accounting for individual and temporal variability in ecological inference greatly improve the assessment of the relative importance of climate and herbivory on species distribution shifts. Strong effects of precipitation at the treeline could allow tree upward expansion, although increasing ungulate populations and associated browsing damages might limit positive climatic responses of pine-saplings in the near future. In this context, the spatially heterogeneous effect exerted by herbivory could result in diverse vegetation structures in ecotones, adding a new dimension to the predictions on climate-driven vegetation shifts.

Data from: Costs and tradeoffs of resistance and tolerance to belowground herbivory in potato

1. The success of sustainable crop production depends on our ability to select or create varieties that can allocate resources to both growth and defence. However, breeding efforts have emphasized increases in yields but have partially neglected defence traits against pests. Estimating the costs of multiple defences against tuber herbivores and the tradeoffs among them, as well as understanding the relationship between yield and multiple defences is still unknown but relevant to both basic and applied ecology. 2. Using twenty commercial potato varieties available in Colombia and the tuber herbivore Tecia solanivora, we tested whether high yielding varieties show a reduction in three types of defence: constitutive and induced resistance, as well as tolerance. Specifically, we determined (1) the costs in terms of yield of all three defences, (2) the possible tradeoffs among them, and (3) if oviposition preference was related to the expression of these defences. 3. We detected no costs in terms of yield of constitutive and induced resistance to tuber damage. We did, however, find evidence of costs of being able to tolerate tuber herbivory. While we found no tradeoffs among any of the estimated defences, there was a positive correlation between aboveground compensatory growth and tolerance in terms of tuber production, suggesting that after damage there are no shifts in the allocation of resources from aboveground to belowground biomass. Finally, we found that females laid more eggs on those varieties with the lowest level of constitutive resistance. 4. Synthesis and applications. Our findings suggest that in potatoes, breeding for higher yields has not caused any reduction in constitutive or induced resistance to tuber damage. This is not the case for tolerance where those varieties with higher yields are also less likely to tolerate tuber damage. Given the high incidence of tuber pests in Colombia, selecting for higher tolerance could allow for high productivity in the presence of herbivores. Finding mechanisms to decouple the tolerance response from yield should be a new priority in potato breeding in Colombia to guarantee a higher yield in both the presence and absence of herbivores.

Data from: Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long-standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long-lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant-herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.

Data from: Self-fertilization and herbivory in a rare alpine plant in California, Claytonia megarhiza (Montiaceae)

Reproduction in alpine habitats is challenging because of the short growing season, low temperatures, and high winds. This predicts alternative strategies for sexual reproduction in plants: compensatory measures such as larger floral displays and greater floral longevity to attract scarce pollinators and maintain outcrossing, or high levels of autonomous self-fertilization to assure reproduction in the absence of reliable pollinators. Here, we assessed the roles of animals (crawling insects, flying insects, and vertebrates) on the reproductive success of Claytonia megarhiza (A. Gray) S. Watson (alpine spring beauty). We measured fruit set and leaf herbivory while excluding animals from individual plants at a single site in Yosemite National Park, California. We found that plants were capable of setting fruit in the absence of pollinators and that, in the presence of animals, there was a 42% reduction in fruit set and a 159% increase in leaf damage. This suggests that Claytonia megarhiza may reproduce primarily by self-fertilization, and that herbivory may limit the reproductive success of this species near its southern range edge in California.

Data from: Effect of insect herbivory on plant community dynamics under contrasting water availability levels

1. Diversity of plant communities is impacted by multiple global change drivers but also by altered biotic interactions with antagonist and mutualist organisms that can potentially affect species coexistence. 2. With a two-year outdoor mesocosm experiment in realistic mesic grassland communities, we explored the role of insect herbivory in impacting plant community dynamics under contrasting levels of water availability simulating altered rainfall regimes. We selected a grasshopper species (Calliptamus italicus) feeding predominantly on forbs while avoiding grasses. 3. High water availability reduced species coexistence, boosting productivity while decreasing individual plant survival. At the community level, herbivores were not able to promote species coexistence but asymmetrically influenced grasses and forbs, reducing forb biomass under high water availability. 4. Herbivores shaped individual plant responses to both abiotic conditions and individual-neighbours' interactions. Herbivores influenced focal plant survival by altering the effect of neighbouring plants, mitigating the negative effect of high neighbour biomass at low water availability and exacerbating it at high level of water availability. 5. Synthesis: Altered rainfall has the capacity to change the relative strength of the plant-plant interactions and also to determine the effects of herbivores on grassland communities. The complexity of the interactions between plants and herbivores and the observed context-dependence indicate the need to incorporate multiple biotic and abiotic drivers to fully understand the mechanisms underlying plant dynamics and species coexistence in a changing world.

Data from: Ants on Clerodendrum infortunatum: disentangling effects of larceny and herbivory

Nectar larcenists extract nectar from flowers without pollinating them. A reasonable expectation is that any form of nectar larceny should have a detrimental effect on the plant's reproductive success. However, studies reveal an entire range of effects, from highly negative to highly positive. This variation in effect may be partly explained by additional, unmeasured, effects of nectar larcenists on plants. In a study system where two ant species Tapinoma melanocephalum (Fabr.) and Trichomyrmex destructor (Jerd.) act as nectar larcenists, we examined the effect of larceny on the female reproductive success of Clerodendrum infortunatum Gaertn. in rain forest fragments of the Western Ghats, India. This was done through a combination of field observations and a series of field experiments looking at the effects of excluding ants from inflorescences. We found that T. destructor reduces fruit set considerably. Rather than this being a consequence of nectar larceny, however, our experiments show that the negative effect arises instead from the herbivorous behaviour of the ant. At a population level, both ant species prefer edges over interiors of forest patches, spatially concentrating the interaction zone to forest edges. Simultaneously considering multiple ecological interactions and disentangling their relative contributions might explain the large variation across species in the observed effect of larceny. The overall population effect of nectar larceny and herbivory is likely to depend on the spatial structuring of plants and ants.

Data from: Herbivory promotes dental disparification and macroevolutionary dynamics in grunters (Teleostei: Terapontidae), a freshwater adaptive radiation

Trophic shifts into new adaptive zones have played major (although often conflicting) roles in reshaping the evolutionary trajectories of many lineages. We analyze data on diet, tooth, and oral morphology and relate these traits to phenotypic disparification and lineage diversification rates across the ecologically diverse Terapontidae, a family of Australasian fishes. In contrast to carnivores and most omnivores, which have retained relatively simple, ancestral caniniform tooth shapes, herbivorous terapontids appear to have evolved a variety of novel tooth shapes at significantly faster rates to meet the demands of plant-based diets. The evolution of herbivory prompted major disparification, significantly expanding the terapontid adaptive phenotypic continuum into an entirely novel functional morphospace. There was minimal support for our hypothesis of faster overall rates of integrated tooth shape, spacing, and jaw biomechanical evolution in herbivorous terapontids in their entirety, compared with other trophic strategies. There was, however, considerable support for accelerated disparification within a diverse freshwater clade containing a range of specialized freshwater herbivores. While the evolutionary transition to herbivorous diets has played a central role in terapontid phenotypic diversification by pushing herbivores toward novel fitness peaks, there was little support for herbivory driving significantly higher lineage diversification compared with background rates across the family.

opencc-zeroDec 2013View details →

Data from: Density dependence, precipitation, and biological control agent herbivory influence landscape-scale dynamics of the invasive Eurasian plant Linaria dalmatica

1. Resource availability and natural enemies are among the most commonly cited mechanisms affecting competitive ability of invasive plants, but their simultaneous effects on plant dynamics are seldom evaluated in the field. Understanding how endogenous and exogenous factors affect invasive plant abundance is essential when evaluating the impact of classical weed biological control agents because misinterpretations of the mechanisms regulating plant demography may bias inference of herbivore impact. 2. In this study we report results from a citizen-science monitoring program initiated to evaluate the effects of the stem–mining weevil Mecinus janthiniformis on the Eurasian invasive weed Dalmatian toadflax Linaria dalmatica. We used a discrete model of population dynamics to evaluate the relative importance of endogenous and exogenous processes affecting changes in ramet density of L. dalmatica at 38 spatially replicated sites in Idaho, USA. 3. Analysis of per capita population growth rates based on ramet density indicated the presence of direct density dependence, which was most likely due to intraspecific competition. Changes in ramet density were not influenced by the abundance of the resident plant community. 4. Precipitation had a strong, positive effect on ramet density of L. dalmatica while the abundance of M. janthiniformis had a weaker, but significant, negative effect after accounting for the density dependence. There was no support for an interactive effect between precipitation and herbivory. 5. Synthesis and applications. Our results indicate that biological control is an important factor affecting weed population growth at the landscape scale, but they also suggest that biological control impact may vary considerably on local infestations due to site-specific variation in rainfall and density-dependent processes. We recommend that invasive plant management strategies integrate precipitation and biological control agent monitoring into their programs to estimate expected biological control efficacy. Alternative control methods should be prioritized in areas where herbivore impact is expected to be low.

Data from: Leaf herbivory imposes fitness costs mediated by hummingbird and insect pollinators

Plant responses induced by herbivore damage can provide fitness benefits, but can also have important costs due to altered interactions with mutualist pollinators. We examined the effects of plant responses to herbivory in a hummingbird-pollinated distylous shrub, Palicourea angustifolia. Through a series of field experiments we investigated whether damage from foliar herbivores leads to a reduction in fruit set, influences floral visitation, or alters floral traits that may influence pollinator preference or pollinator efficiency. Foliar herbivory by a generalist grasshopper led to reduced fruit set in branches that were directly damaged as well as in adjacent undamaged branches on the same plant. Furthermore, herbivory resulted in reduced floral visitation from two common hummingbird species and two bee species. An investigation into the potential mechanisms behind reduced floral visitation in induced plants showed that foliar herbivore damage resulted in shorter styles and lower nectar volumes. This reduction in style length could reduce pollen deposition between different floral morphs that is required for optimal pollination in a distylous plant. We did not detect any differences in the volatile blends released by damaged and undamaged branches, suggesting that foliar herbivore-induced changes in floral morphology and rewards, and not volatile blends, are the primary mechanism mediating changes in visitation. Our results provide novel mechanisms for how plant responses induced by foliar herbivores can lead to ecological costs.

opencc-zeroDec 2013View details →

Data from: Native plant diversity increases herbivory to non-natives

There is often an inverse relationship between the diversity of a plant community and the invasibility of that community by non-native plants. Native herbivores that colonize novel plants may contribute to diversity–invasibility relationships by limiting the relative success of non-native plants. Here, we show that, in large collections of non-native oak trees at sites across the USA, non-native oaks introduced to regions with greater oak species richness accumulated greater leaf damage than in regions with low oak richness. Underlying this trend was the ability of herbivores to exploit non-native plants that were close relatives to their native host. In diverse oak communities, non-native trees were on average more closely related to native trees and received greater leaf damage than those in depauperate oak communities. Because insect herbivores colonize non-native plants that are similar to their native hosts, in communities with greater native plant diversity, non-natives experience greater herbivory.

opencc-zeroSep 2019View details →

Data from: Cryptic petal colouration decreases floral apparency and herbivory in nocturnally closing daisies

1. Floral apparency is shaped by both mutualistic and antagonistic interactions that can act in opposing ways. Pollinators are expected to select for more visually apparent flowers, but this likely trades off against the potentially severe fitness costs of damage to apparent flowers by floral herbivores. One way in which flowers that close during parts of the day might circumvent this trade-off, is by evolving less visible lower petal surfaces that are inconspicuous to herbivores when flowers are closed. 2. Here we used visual system modelling and herbivory experiments to test whether petal surfaces that are exposed when flowers are closed are cryptically coloured. We collected lower and upper petal surface spectra for 77 Asteraceae species from Namaqualand, South Africa. This included closing species that expose their lower petal surfaces for 5 to 6 daylight hours, and non-closing species that do not expose their lower surfaces. We used these contrasting groups to test the expectation of reduced conspicuousness of lower petal surfaces in closing, but not non-closing species. 3. By modelling reflectance spectra of petal surfaces against a green leaf background in various visual systems we showed 1) that conspicuousness of upper petal surfaces to pollinators and various herbivores were strongly correlated, suggesting the potential for fitness trade-offs between attracting mutualists and antagonists to open flowers, 2) that closing species' lower petal surfaces were less visible to herbivores against a leaf background than those of non-closing species, and 3) that closing species had larger differences between upper and lower petal surface colouration than non-closing species. Behavioural experiments with tortoise herbivores demonstrated that flowers are easily detected when upper surfaces are exposed, but that tortoises were unable to distinguish lower petal surfaces against a leaf background, resulting in reduced flower herbivory. 4. These results are consistent with selection by herbivores for cryptic colouration of lower petal surfaces, and divergence of colouration between lower and upper petal surfaces in species with closing flowers. Visual crypsis of flowers may be an effective anti-herbivory strategy during times when pollinators are inactive, and provides an alternative to chemical defence, which often involves costs to pollination.

Data from: Herbivory and competition of Tibetan steppe vegetation in winter pasture: effects of livestock exclosure and plateau pika reduction

Rangeland degradation has been identified as a serious concern in alpine regions of western China on the Qinghai-Tibetan plateau (QTP). Numerous government-sponsored programs have been initiated, including many that feature long-term grazing prohibitions and some that call for eliminating pastoralism altogether. As well, government programs have long favored eliminating plateau pikas (Ochotona curzoniae), assumed to contribute to degraded conditions. However, vegetation on the QTP evolved in the presence of herbivory, suggesting that deleterious effects from grazing are, to some extent, compensated for by reduced plant-plant competition. We examined the dynamics of common steppe ecosystem species as well as physical indicators of rangeland stress by excluding livestock and reducing pika abundance on experimental plots, and following responses for 4 years. We established 12 fenced livestock exclosures within pastures grazed during winter by local pastoralists, and removed pikas on half of these. We established paired, permanent vegetation plots within and outside exclosures and measured indices of erosion and biomass of common plant species. We observed modest restoration of physical site conditions (reduced bare soil, erosion, greater vegetation cover) with both livestock exclusion and pika reduction. As expected in areas protected from grazing, we observed a reduction in annual productivity of plant species avoided by livestock and assumed to compete poorly when protected from grazing. Contrary to expectation, we observed similar reductions in annual productivity among palatable, perennial graminoids under livestock exclusion. The dominant grass, Stipa purpurea, displayed evidence of density-dependent growth, suggesting that intra-specific competition exerted a regulatory effect on annual production in the absence of grazing. Complete grazing bans on winter pastures in steppe habitats on the QTP may assist in the recovery of highly eroded pastures, but may not increase annual vegetative production.

opencc-zeroDec 2018View details →

Data from: Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

Urban landscapes are characterized by high proportions of impervious surface resulting in higher temperatures than adjacent natural landscapes. In some cities, like those at cooler latitudes, trees may benefit from warmer urban temperatures, but trees in many cities are beset with problems like drought stress and increased herbivory. What drives patterns of urban tree health across urbanization and latitudinal temperature gradients? In natural systems, latitude-herbivory relationships are well-studied, and recent temperate studies have shown that herbivory generally increases with decreasing latitudes (warmer temperatures). However, the applicability of this latitude-herbivory theory in already-warmed urban systems is unknown. In this study, we investigated how the interaction of urbanization, latitudinal warming, and scale insect abundance affected urban tree health. We predicted that trees in warmer, lower latitude cities would be in poorer health at lower levels of urbanization than trees at cooler, higher latitudes due to the interaction of urbanization, latitudinal temperature, and herbivory. To evaluate our predictions, we surveyed the abundance of scale insect herbivores on a single, common tree species (Acer rubrum) in eight US cities spanning 10° of latitude. We estimated urbanization at two extents, a local one that accounted for the direct effects on an individual tree, and a larger one that captured the surrounding urban landscape. We found that urban tree health did not vary with latitudinal temperature but was best predicted by local urbanization and herbivore abundance. We did not observe increased herbivore abundance in warmer, lower latitudes cities, but instead herbivore abundance peaked in the mid latitudes of our study. This study demonstrates that urban landscapes may deviate from classical theory developed in natural systems and reinforces the need for research reconciling ecological patterns in urban landscapes.

Data from: The effect of rhizosphere microbes outweighs host plant genotype in reducing insect herbivory

Rhizosphere microbes affect plant performance, including plant resistance against insect herbivores; yet, a direct comparison of the relative influence of rhizosphere microbes vs. plant genotype on herbivory levels and on metabolites related to defense is lacking. In the crucifer Boechera stricta, we tested the effects of rhizosphere microbes and plant genotype on herbivore resistance, the primary metabolome, and select secondary metabolites. Plant populations differed significantly in the concentrations of 6 glucosinolates (GLS), secondary metabolites known to provide herbivore resistance in the Brassicaceae. The population with lower GLS levels experienced ~60% higher levels of aphid (Aphis spp.) attack; no association was observed between GLS and damage by a second herbivore, flea beetles (Altica spp.). Rhizosphere microbiome (disrupted vs. intact native microbiome) had no effect on plant GLS concentrations. However, aphid number and flea beetle damage were respectively ~3-fold and 7-fold higher among plants grown in the disrupted vs. intact native microbiome treatment. These differences may be attributable to shifts in primary metabolic pathways previously implicated in host defense against herbivores, including increases in pentose and glucoronate interconversion among plants grown with an intact microbiome. Further, native microbiomes with distinct community composition (as estimated from 16s rRNA amplicon sequencing) differed 2-fold in their effect on host plant susceptibility to aphids. The findings suggest that rhizosphere microbes, including distinct native microbiomes, can play a greater role than plant genotype in defense against insect herbivores, and act through metabolic mechanisms independent of plant genotype.

Data from: Interactions between rainfall, fire and herbivory drive resprouter vital rates in a semi-arid ecosystem

1. Global change is threatening ecosystems and biodiversity worldwide, creating a pressing need to understand how climate and disturbance regimes interact and influence the persistence of species. We quantify how three ecosystem drivers– rainfall, fire and herbivory – influence vital rates in the perennial resprouting graminoid, Triodia scariosa, a foundation species of semi-arid Australia. 2. We used an 11 year data set from a fire and herbivore exclosure experiment, to model flowering, post-fire recruitment and the post-fire survival of seedlings and resprouting plants. Regression modelling quantified the effect of rainfall, inter-fire interval, fire type (wildfire or prescribed fire), grazing by herbivores (native and feral) and an interaction between fire type and herbivory on T. scariosa populations. 3. Rainfall, fire and herbivory had significant effects on post-fire recruitment and the survival of seedlings and resprouting plants, including strong interactions between these drivers. Herbivory following wildfire had a minor effect, but in years of below-average rainfall herbivory following prescribed fire had a large effect, reducing the survival of seedlings and resprouting plants by 20% and over 50% respectively, relative to post-fire survival under average rainfall conditions. 4. Variation in rainfall underpinned significant variation in post-fire resprouting and seedling survival, thus we postulate rainfall primarily drives the dynamics of T. scariosa populations. 5. Synthesis. This study highlights the importance of modelling interactions between key ecosystem drivers when predicting how changes in global climate and disturbance regimes influence plant vital rates. Relatively small changes to disturbance regimes can substantially alter population processes, even in perennial resprouting species. This work suggests that conservation of foundation species, such as T. scariosa, will benefit if fire management decisions are better integrated with inter-annual weather forecasts and herbivore management.