Resource Availability Hypothesis Research Articles

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.

Preliminary exploration of herbal tea products based on traditional knowledge and hypotheses concerning herbal tea selection: a case study in Southwest Guizhou, China.

Herbal tea usually refers to "beverage plants that do not belong to the genus Camellia", and it holds a significant historical legacy as a traditional beverage among specific regions and ethnic groups. In light of this, our research aims to investigate and analyze the traditional knowledge pertaining to herbal tea plants used by local people in the QianxinanBuyi and Miao Autonomous Prefecture, Guizhou Province. We also initiated preliminary efforts to create tea products from herbal tea leaves using various processing techniques. Additionally, we attempted to test hypotheses to elucidate how local people select herbal tea plants. Data related to the use of herbal tea plants in this study were collected through semi-structured interviews and participatory observations in four villages in Qianxinan. Quantitative indicators, including the relative frequency of citation (RFC) and the relative importance (RI) value, were calculated, and the availability of plants was also evaluated. General linear model was performed to examine the relationship between the frequency of citation and resource availability, as well as the correlation between the relative frequency of citation and the relative importance, to test both the resource availability hypothesis and the versatility hypothesis. Centella asiatica tea was processed using techniques from green tea, black tea and white tea, with a preliminary sensory evaluation conducted. A total of 114 plant species were documented as being used for herbal teas by local residents, representing 60 families and 104 genera. Of these, 61% of herbal tea plants were found growing in the wild, and 11 species were exotic plants. The family with the highest number of species was Asteraceae (20 species). The study identified 33 major medicinal functions of herbal tea, with clearing heat-toxin and diuresis being the most common functions. General linear model revealed a strong correlation (correlation coefficient of 0.72, p < 0.001) between the frequency of citation and plant availability, as well as a significant correlation (correlation coefficient of 0.63, p < 0.001) between RFC and RI. Under different processing conditions, the characteristics of Centella asiatica tea exhibited variations and were found to be suitable for consumption. The consumption of herbal tea serves as a preventive measure against common ailments for local residents. The resource availability hypothesis, diversification hypothesis and the versatility hypothesis were shown to provide some insight into "how and why local communities select plants for use." Exotic herbal tea plants in the study area also possess valuable therapeutic properties. The processing and production of Centella asiatica herbal tea products hold promising prospects.

Resource availability and plant age drive defense against herbivory in salt marshes

The resource‐availability hypothesis (RAH) and the intraspecific RAH (RAHintra), posit that resources, (i.e. nutrients) control plant antiherbivore defenses. Both hypotheses predict that in low‐resource environments, plant growth is slow, and constitutive defense is high. In high‐resource environments, however, the RAH predicts that plant growth is fast, and constitutive defense is low, whereas the RAHintra predicts that increased resources attract more herbivores, and this intensified grazing pressure leads to high constitutive defense. Salt marshes are nutrient‐limited ecosystems threatened by eutrophication and chronic herbivory, yet we know little about how these stressors shape saltmarsh plant antiherbivore defenses, which influence trophic interactions and ecosystem resilience. We manipulated resource availability via nutrient addition and herbivory via the marsh periwinkle Littoraria irrorata, on the saltmarsh foundation species Spartina alterniflora, in mesocosms. Because plant age can also influence trait variation, we measured traits in both original and clonally‐grown new stems. Feeding assays then evaluated how treatments and plant age affected subsequent Littoraria consumption of Spartina. Nutrient addition stimulated growth, while decreasing defensive traits (e.g. fiber and silica content), following the RAH. Herbivory enhanced belowground production and increased stem diameter, yet did not induce defensive traits, contrary to our expectations. Herbivory plus nutrients increased Spartina biomass and reduced phenolics, a defensive trait, further supporting the RAH. Regardless of treatment, clonally‐grown new stems had greater variation in measured traits. Despite altered traits, however, treatments and plant age did not affect Littoraria consumption. Our results support the RAH and part of the RAHintra and suggest: 1) nutrient availability is a primary driver of plant trait change and 2) plant age controls the magnitude of trait variation in Spartina. Further, our findings indicate that eutrophic conditions may not always increase top‒down control by herbivores, and in some instances can enhance saltmarsh resilience against sea‐level rise via stimulated Spartina biomass production.

Environmental drivers of body size in North American bats

Abstract Bergmann's rule—which posits that larger animals live in colder areas—is thought to influence variation in body size within species across space and time, but evidence for this claim is mixed. We used Bayesian hierarchical models to test four competing hypotheses for spatiotemporal variation in body size within 20 bat species across North America: (1) the heat conservation hypothesis, which posits that increased body size facilitates body heat conservation (and which is the traditional explanation for the mechanism underlying Bergmann's rule); (2) the heat mortality hypothesis, which posits that increased body size increases susceptibility to acute heat stress; (3) the resource availability hypothesis, which posits that increased body size is enabled in areas with more abundant food; and (4) the starvation resistance hypothesis, which posits that increased body size reduces susceptibility to starvation during acute food shortages. Spatial variation in body mass was most consistently (and negatively) correlated with mean annual temperature, supporting the heat conservation hypothesis. Across time, variation in body mass was most consistently (and positively) correlated with net primary productivity, supporting the resource availability hypothesis. Climate change could influence body size in animals through both changes in mean annual temperature and resource availability. Rapid reductions in body size associated with increasing temperatures have occurred in short‐lived, fecund species, but such reductions will be obscured by changes in resource availability in longer‐lived, less fecund species. Read the free Plain Language Summary for this article on the Journal blog.

Testing the costs of defense against herbivores in ecotypes of Eruca sativa (Brassicaceae)

Abstract Plants of Eruca sativa from a desert population produce less vegetative biomass, flower earlier and possess higher density of trichomes than plants from a more mesic Mediterranean habitat. When eliciting plant defense mechanisms with methyl-jasmonate (MJ), trichome formation was exclusively enhanced in plants of the Mediterranean ecotype. To test the relative costs of defenses, ecologically-important traits were monitored in non-elicited plants of the two ecotypes and when defenses were induced by MJ. Opposing the expected tradeoffs of induced defense, MJ- elicitation significantly delayed bolting and flowering in plants of the desert ecotype but not in the Mediterranean ones. Moreover, MJ did not impose costs on fitness- related traits, i.e. fruit and seed production, nor differences between plants of the two ecotypes were found. It is suggested that early flowering in plants of the desert ecotype provide a mean to escape both the abiotic stressful environment as well herbivory, while induced defense in the Mediterranean ecotype was favored in habitats of more stressful biotic interactions. The results are discussed in light of the optimal defense theory and resource availability hypothesis.

Unveiling the environmental drivers of intraspecific body size variation in terrestrial vertebrates

AbstractAimWhether intraspecific spatial patterns in body size are generalizable across species remains contentious, as well as the mechanisms underlying these patterns. Here we test several hypotheses explaining within‐species body size variation in terrestrial vertebrates including the heat balance, seasonality, resource availability and water conservation hypotheses for ectotherms, and the heat conservation, heat dissipation, starvation resistance and resource availability hypotheses for endotherms.LocationGlobal.Time period1970–2016.Major taxa studiedAmphibians, reptiles, birds and mammals.MethodsWe collected 235,905 body size records for 2,229 species (amphibians = 36; reptiles = 81; birds = 1,545; mammals = 567) and performed a phylogenetic meta‐analysis of intraspecific correlations between body size and environmental variables. We further tested whether correlations differ between migratory and non‐migratory bird and mammal species, and between thermoregulating and thermoconforming ectotherms.ResultsFor bird species, smaller intraspecific body size was associated with higher mean and maximum temperatures and lower resource seasonality. Size–environment relationships followed a similar pattern in resident and migratory birds, but the effect of resource availability on body size was slightly positive only for non‐migratory birds. For mammals, we found that intraspecific body size was smaller with lower resource availability and seasonality, with this pattern being more evident in sedentary than migratory species. No clear size–environment relationships were found for reptiles and amphibians.Main conclusionsWithin‐species body size variation across endotherms is explained by disparate underlying mechanisms for birds and mammals. Heat conservation (Bergmann's rule) and heat dissipation are the dominant processes explaining biogeographic intraspecific body size variation in birds, whereas in mammals, body size clines are mostly explained by the starvation resistance and resource availability hypotheses. Our findings contribute to a better understanding of the mechanisms behind species adaptations to the environment across their geographic distributions.

The resource availability hypothesis (RAH) and cross-cultural patterns: which one explains West African Cochlospermum species’ uses in Benin?

BackgroundCochlospermum tinctorium and C. planchonii are two wide edible plants of sub-Saharan countries, e.g., Benin, widely used as food, medicine, dye, handicraft, etc. Unfortunately, the uncontrolled harvest of their rootstocks exposes them to local extension risk. To enhance knowledge on the determinants of their uses in Benin, this study aimed to (i) assess the use forms, use values, abundance and perceived spatiotemporal dynamics, (ii) determine how does local abundance or cultural patterns affect the use of Cochlospermum species, and (iii) assess local harvesting modes and conservation management practices.MethodsIn total, 756 Dialog Partners through 27 ethnic groups were questioned countrywide using semi-structured interviews. Questions refer to local taxonomy, specific uses, organs sought, harvesting modes and local conservation strategies; afterward, local abundance of each species was assessed. Ethnobotanical indicators were analyzed through citation frequencies to obtain quantitative data. Comparison tests and statistical analyses were performed using R program.ResultsC. tinctorium and C. planchonii are locally well known and involved into 83 specific uses, grouped into ten categories of which medicinal use was the main. The use values of C. planchonii (0.10 ± 0.19) and C. tinctorium (0.23 ± 0.20) varied significantly between the ethnic groups, and only C. tinctorium showed index of commercially value above 75% for some ethnic groups. The current abundance, about 84 and 97 tufts.ha−1, respectively, for C. planchonii and C. tinctorium was perceived with a decline of 81.09% (C. tinctorium) and 73.7% (C. planchonii) of informants. Moreover, the Spearman’s correlation and Kruskal–Wallis tests performed revealed that the use values of C. tinctorium and C. planchonii were significantly correlated on the one hand with their local abundance and on the other hand with the investigated ethnic groups. About 42.3% of women produced the powder as principal activity, while more than 57% produced it mainly at the end of farm work. However, the conservation management was practiced by very few informants and consisted of partial harvesting of rootstocks (41.8%, only in southern Benin), and fallowing of harvesting areas (3.97%, only in northern Benin).ConclusionFacing the declining abundance and increasing market demand for rootstock powder of Cochlospermum species, existing local conservation strategies should be promoted and the domestication process should be initiated for sustainable management of these important wild edible plants before these important resources disappear completely in the wild.

Leaf N:P stoichiometry overrides the effect of individual nutrient content on insect herbivore population dynamics in a Tibetan alpine grassland

Based on resource availability hypothesis, increasing exogenous nutrient inputs may favour herbivores by improving quality and quantity of host plants. Such plant-herbivore interactions have been demonstrated under nitrogen (N) addition, but less examined in the context of simultaneous inputs of N and phosphorus (P). Here, we conducted a multiple-nutrient addition experiment (Control, N addition alone, P addition alone, both N and P additions) in a Tibetan alpine grassland, to explore how N addition interacts with P to affect the larval density of the grassland caterpillar Gynaephora menyuanensis. We showed that P addition alone did not obviously affect the larval density, but P suppressed N-induced increases in larval density when N and P were simultaneously added. We further found that changes in leaf N:P of host plants induced by nutrient additions regulated larval density more strongly than leaf N or P content. Both increased plant productivity and shifted species composition under nutrient additions contributed less to the larval density. Interactive effects of N and P suggest that the simultaneous inputs of multiple nutrients may boost the carrying capacity of grasslands and reduce the risk of herbivorous insect outbreaks. The work also reveals that the resource availability hypothesis does not well predict the response of insect herbivore to all nutrient inputs, implying the necessity of incorporating N:P stoichiometry in this hypothesis.

Aspen Leaves as a "Chemical Landscape" for Fungal Endophyte Diversity-Effects of Nitrogen Addition.

Abiotic and biotic factors may shape the mycobiome communities in plants directly but also indirectly by modifying the quality of host plants as a substrate. We hypothesized that nitrogen fertilization (N) would determine the quality of aspen (Populus tremula) leaves as a substrate for the endophytic fungi, and that by subjecting the plants to N, we could manipulate the concentrations of positive (nutritious) and negative (antifungal) chemicals in leaves, thus changing the internal “chemical landscape” for the fungi. We expected that this would lead to changes in the fungal community composition, in line with the predictions of heterogeneity–diversity relationship and resource availability hypotheses. To test this, we conducted a greenhouse study where aspen plants were subjected to N treatment. The chemical status of the leaves was confirmed using GC/MS (114 metabolites, including amino acids and sugars), LC/MS (11 phenolics), and UV-spectrometry (antifungal condensed tannins, CTs), and the endophytic communities were characterized using culture-dependent sequencing. We found that N treatment reduced foliar concentrations of CT precursor catechin but not that of CTs. Nitrogen treatment also increased the concentrations of the amino acids and reduced the concentration of some sugars. We introduced beetle herbivores (H) as a second treatment but found no rapid changes in chemical traits nor strong effect on the diversity of endophytes induced by herbivores. A few rare fungi were associated with and potentially vectored by the beetle herbivores. Our findings indicate that in a controlled environment, the externally induced changes did not strongly alter endophyte diversity in aspen leaves.

Soil nitrogen mediates the effect of climatic distance on herbivory rates in a tropical herb

AbstractThe center‐periphery hypothesis predicts that species are most abundant at the center of their distribution range. Differential herbivory rates between center and periphery populations can explain this variation in species abundance. However, if the geographic center of a species distribution coincides with its ecological optimum, the resource availability hypothesis predicts higher herbivory rates and tolerances at the center compared to the periphery. Biogeographical studies on herbivory have treated these two mechanisms separately, limiting our mechanistic understanding of the role of herbivory in shaping species range limits. We analyzed the role of resource availability on herbivory variation from center to periphery using data collected across the distribution of Thunberbia atacorensis, a range‐limited species of West Africa. We used two types of distances: geographic distance (the distance from each plot to the geographic center of all populations) and climatic distance (the distance from each plot to the preferendum of the species). We found no increase in herbivory toward the periphery of the climatic and geographic ranges. However, herbivory rates increased with soil nitrogen. Soil nitrogen decreased from the center to the periphery of the climatic range. Phylogenetic diversity and competition from surrounding plants did not affect herbivory rates. Our study provides insights into how nutrient limitation can shape species center‐periphery distribution by altering spatial variation in herbivory rates.

Siliceous and non‐nutritious: Nitrogen limitation increases anti‐herbivore silicon defences in a model grass

Abstract Silicon (Si) accumulation alleviates a diverse array of environmental stresses in many plants, including conferring physical resistance against insect herbivores. It has been hypothesised that grasses, in particular, utilise ‘low metabolic cost’ Si for structural and defensive roles under nutrient limitation. While carbon (C) concentrations often negatively correlate with Si concentrations, the relationship between nitrogen (N) status and Si is more variable. Moreover, the impacts of N limitation on constitutive physical Si defences (e.g. silica and prickle cells) against herbivores are unknown. We determined how N limitation affected Si deposition in the model grass Brachypodium distachyon and how changes in these constitutive defences impacted insect herbivore Helicoverpa armigera growth rates. We used scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometry in conjunction with X‐ray mapping (XRM) to quantify physical structures on leaves and determine Si deposition patterns. We also determined how N limitation and Si supply impacted the jasmonic acid (JA) pathway, the master regulator of induced defences against arthropod herbivores. N limitation reduced shoot growth by over 40%, but increased root mass (+21%), leaf Si concentrations (+50%) and the density of silica (+28%) and flattened prickle (+76%) cells. Energy‐dispersive X‐ray spectroscopy and XRM established that Si was being deposited in these structures, together with hooked prickle cells and macro‐hairs. Herbivore relative growth rates (RGR) were more than 115% lower in Si‐supplied plants compared to plants without Si supply and negatively correlated with leaf Si concentration and silica cell density. RGR was further reduced by N limitation and positively correlated with leaf N concentrations. Increases in JA concentrations following induction of the JA pathway were at least doubled by N limitation. Synthesis. Si accumulation and deposition were highly regulated by N availability, with N limitation promoting both constitutive Si physical defences and induction of the JA defensive pathway, in line with the resource availability hypothesis. These results indicate that grasses use ‘low‐cost Si’ when resources are limited and suggest that plant productivity may benefit from optimising conventional fertilisers and Si fertilisation.

Mammalian body size is determined by interactions between climate, urbanization, and ecological traits

Anthropogenically-driven climate warming is a hypothesized driver of animal body size reductions. Less understood are effects of other human-caused disturbances on body size, such as urbanization. We compiled 140,499 body size records of over 100 North American mammals to test how climate and human population density, a proxy for urbanization, and their interactions with species traits, impact body size. We tested three hypotheses of body size variation across urbanization gradients: urban heat island effects, habitat fragmentation, and resource availability. Our results demonstrate that both urbanization and temperature influence mammalian body size variation, most often leading to larger individuals, thus supporting the resource availability hypothesis. In addition, life history and other ecological factors play a critical role in mediating the effects of climate and urbanization on body size. Larger mammals and species that utilize thermal buffering are more sensitive to warmer temperatures, while flexibility in activity time appears to be advantageous in urbanized areas. This work highlights the value of using digitized, natural history data to track how human disturbance drives morphological variation.

Soil resource availability, plant defense, and herbivory along a successional gradient in a tropical dry forest

Tropical dry forests (TDFs) usually occur as mosaics at different stages of natural regeneration, along which variations in soil attributes (fertility and texture) can affect plant growth and leaf traits and, consequently, leaf herbivory. In the present study, we used a chronosequence approach to determine the variations in plant growth, leaf defense, and insect herbivory in different successional species groups, following the predictions of the resource availability hypothesis: plants at early successional stages would grow faster, have lower specific leaf mass (SLM), and suffer higher herbivore damage than late successional plants. For this purpose, we measured plant diameter and collected leaves at the end of the rainy season (April) during three consecutive years in three different successional stages (early, intermediate, and late). In total, we sampled 38 plant species, from which 31 species were exclusive to a single successional stage (10 early, 13 intermediate, and eight late species). Our results did not conform to the resource availability hypothesis, as plants growing in early successional stages had higher growth rates and SLM, and lower leaf damage by herbivores. Soil fertility and texture did not affect plant growth or SLM, but leaf damage was negatively affected by SLM. Apart from being a defensive trait, SLM is also involved in the conservation of water and nutrients and photoprotection, and its variation along successional gradients in TDFs conforms to a continuum from conservative traits in hot and dry, early successional stages, to acquisitive traits in more humid late stages. Although other plant defenses must be considered, our three-year study indicates that the resource availability hypothesis, which was formulated for wet forests, should be adapted to deciduous TDFs where water availability is the main environmental filter along succession.

Herbivory and leaf traits of two tree species from different successional stages in a tropical dry forest

ABSTARCT Leaf traits are analyzed as essential drivers for the evolutionary and ecological role of plant defense mechanisms. Plants show leaf trait variation as a response to the diverse environmental conditions, like different successional stages. Those variations can impact leaf herbivory rates and drive changes in the allocation of plant resources. In this study, we aimed at comparing the expression of leaf defenses in established plants over different successional stages in a tropical dry forest to understand how these defenses modify the plant–herbivore interactions based on herbivory rates. We analyzed physical leaf traits (specific leaf area, thickness, and density), nutrient content (N, P, and K), total phenolic compounds, and leaf herbivory, of the native tree species Aspidosperma pyrifolium and Cenostigma pyramidale, in early- and late-successional stage areas. Results showed that the plant investment in defenses varies according to the successional stage and that both species have similar defense strategies, confirming the resource availability hypothesis. Individuals from the early stage adopt a strategy to lower sclerophylly, higher nutrient content and less phenolic compounds, while the late-stage individuals showed an opposite behaviour. For both species in this study, the average percentage of leaf herbivory observed was 40% higher in the early compared to the late-stage area. Our data indicate that plant defenses are tightly coupled to sclerophylly and investments in secondary metabolites, and the environmental conditions of different successional stage drive that plasticity in such leaf traits.

Evolutionary trade-offs in the chemical defense of floral and fruit tissues across genus Cornus.

Defense investment in plant reproductive structures is relatively understudied compared to the defense of vegetative organs. Here the evolution of chemical defenses in reproductive structures is examined in light of the optimal defense, apparency, and resource availability hypotheses within the genus Cornus using a phylogenetic comparative approach in relation to phenology and native habitat environmental data. Individuals representing 25 Cornus species were tracked for reproductive phenology over a full growing season at the Arnold Arboretum of Harvard University. Floral, fruit, and leaf tissue was sampled to quantify defensive chemistry as well as fruit nutritional traits relevant to bird dispersal. Native habitat environmental characteristics were estimated using locality data from digitized herbarium records coupled with global soil and climate data sets. The evolution of later flowering was correlated with increased floral tannins, and the evolution of later fruiting was correlated with increased total phenolics. Leaves were found to contain the highest tannin activity, while inflorescences contained the highest total flavonoids. Multiple aspects of fruit defensive chemistry were correlated with fruit nutritional traits. Floral and fruit defensive chemistry were evolutionarily correlated with aspects of native habitat temperature, precipitation, and soil characteristics. Results provide tentative support for the apparency hypothesis with respect to both flower and fruit phenology, while relative concentrations of secondary metabolites across organs provide mixed support for the optimal defense hypothesis. The evolution of reproductive defense with native habitat provides, at best, mixed support for the resource availability hypothesis.

Resource availability drives microevolutionary patterns of plant defences

Abstract The resource availability hypothesis (RAH) was formulated to disentangle patterns of variation among species in plant antiherbivore defences. A novel theoretical framework was recently proposed to expand the RAH among populations within species (RAHintra), but unresolved conceptual issues and considerable gaps surrounding the new hypothesis still need to be addressed. Here we tested the RAHintra predictions in a pine tree species using population‐level correlations among plant traits and climate as a proxy of resource availability. We analysed genetic variation among 10 maritime pine populations in growth, constitutive and induced chemical defences and resistance to herbivory in a glasshouse experiment. We used a clonally replicated collection with family structure and accounted for the neutral genetic differentiation with Single Nucleotide Polymorphism (SNPs) data. Most of our results agree with the predictions of the RAHintra. As postulated, growth showed a positive relation with resource availability at population origin. Constitutive investment in phenolics (but not in terpenes) and antiherbivore resistance was also positively related to resource availability at population origin, as theory predicts. Conversely, we did not detect relationships between resource availability and the inducibility of any of the traits. A trade‐off between constitutive investment and inducibility of chemical defences was observed for phenolics, as expected, but not for terpenes. A positive relationship between growth and constitutive resistance was also found likely due to reduced physiological constraints under suitable conditions in the glasshouse experiment. Our study provides evidence and support for the RAHintra, highlighting the strong influence of resource availability in shaping genetically based patterns of local adaptive variation among populations. We detected clinal patterns in phenolic investment along a resource availability gradient, suggesting that population local adaptation to resource availability has more implications for investment in phenolics than for terpenes. Most patterns emerged only after accounting for the neutral processes in the species, not initially conceived in the RAHintra. Neutral differentiation, as a result of non‐adaptive demographic processes and population isolation, may obscure the influence of resource availability as a driver responsible of local adaptation of populations. A free Plain Language Summary can be found within the Supporting Information of this article.

No Difference in Herbivory Preferences Among Ecotypes of Big Bluestem (Andropogon gerardii)

Big bluestem (Andropogon gerardii), a dominant prairie grass, exhibits a wide distribution with several genetically distinct ecotypes. Each ecotype adapts to abiotic and biotic factors within its environment. These adaptations may prove more or less desirable to herbivores in the area. For instance, plants adapted to areas with greater rainfall tend to grow larger than those adapted to drier conditions. Wetter conditions might result in greater herbivore pressure, but according to the Resource Availability Hypothesis, individual plants tend to invest little in herbivore defenses when resources are abundant. Consequently, we hypothesized A. gerardii ecotypes adapted to wetter conditions might be more susceptible to herbivory and that ecotypes adapted to drier conditions might have evolved greater herbivore defense mechanisms. We tested this with feeding preference trials involving grasshoppers and leaves from five ecotypes of A. gerardii that represented plants adapted to wet and dry conditions. Scans of leaves before and after herbivory trials indicated 43 to 78 percent of leaf area remained, with no difference among ecotypes regarding which were favored by herbivores. We also hypothesized that leaves with more tannins would be less preferable to herbivores. This was tested by measuring leaf tannin concentration from five ecotypes of A. gerardii from four common gardens across a precipitation gradient. Leaf tannin concentrations ranged from 0.8 to 2.4% of leaf dry mass and were different among both sites and ecotypes. Site differences caused the biggest difference in leaf tannins, indicating a strong environmental influence on leaf tannin concentration. There was no correlation between tannin concentration and herbivory preference among ecotypes, indicating other factors are related to herbivore defense in A. gerardii.

Tritrophic interactions follow phylogenetic escalation and climatic adaptation

One major goal in plant evolutionary ecology is to address how and why tritrophic interactions mediated by phytochemical plant defences vary across species, space, and time. In this study, we tested three classical hypotheses about plant defences: (i) the resource-availability hypothesis, (ii) the altitudinal/elevational gradient hypothesis and (iii) the defence escalation hypothesis. For this purpose, predatory soil nematodes were challenged to hunt for root herbivores based on volatile cues from damaged or intact roots of 18 Alpine Festuca grass species adapted to distinct climatic niches spanning 2000 meters of elevation. We found that adaptation into harsh, nutrient-limited alpine environments coincided with the production of specific blends of volatiles, highly attractive for nematodes. We also found that recently-diverged taxa exposed to herbivores released higher amounts of volatiles than ancestrally-diverged species. Therefore, our model provides evidence that belowground indirect plant defences associated with tritrophic interactions have evolved under two classical hypotheses in plant ecology. While phylogenetic drivers of volatile emissions point to the defence-escalation hypothesis, plant local adaptation of indirect defences is in line with the resource availability hypothesis.

An empirical test of the mechanistic underpinnings of interference competition

Interspecific competition is an essential element of the evolution of species and can strongly influence the abundance and distribution of species. Where competition interacts with anthropogenic habitat modification, this natural ecosystem process can become a threatening process. Understanding the mechanisms behind competition in such cases is essential for the formulation of cost‐effective management responses for biodiversity conservation. According to the resource availability hypothesis of competition, interference competition limits access to resources by species vulnerable to such aggression and wastes energy in evasive responses. Studying competition is notoriously difficult, however, empirical evidence of the resource availability theory is limited, and there are few published experimental studies showing such an effect at larger scales. We present the results of a controlled and replicated empirical study of interference competition at a landscape scale. We removed an aggressive, overabundant native bird, Manorina melanocephala (noisy miner), whose interference competition is a threatening process for small woodland birds in the highly modified agricultural landscapes of eastern Australia. We monitored foraging and harassment rates of small woodland birds before and after removal to indicate if levels of interference competition changed. Due to unexpected immediate recolonisation, abundance of M. melanocephala in treatment sites declined by only 34% relative to control sites. Twenty‐four bird species displayed aggressive behaviour towards other birds and 41 bird species were victims of aggression. Manorina melanocephala was responsible for 66% of all aggressive interactions. After removal, we recorded a doubling in foraging rate of small woodland birds in treatment compared to control sites. This appears to confirm the resource availability hypothesis of competition. Paradoxically, however, increased foraging was not accompanied by a decline in harassment. Low detection rates of harassment of small woodland birds, combined with the modest reduction in abundance of M. melanocephala, make it difficult to draw firm conclusions about the relationship between rates of harassment and rates of foraging.

Assessing Rensch’s rule in a newt: Roles of primary productivity and conspecific density in interpopulation variation of sexual size dimorphism

AbstractAimMajor efforts have been devoted to understanding the geographic pattern of sexual size dimorphism (SSD). Rensch's rule posits that SSD increases with body size in male‐biased SSD species and decreases with body size in female‐biased SSD species. This pattern, and its inverse, have been mainly explored at the interspecific level, whereas research at the intraspecific level has been largely neglected. Here, we test whether the allometric pattern of SSD in an urodele amphibian conforms to Rensch's rule and evaluate the relative role of four potential mechanisms: sexual selection, fecundity selection, density‐dependent resource availability and differential plasticity.LocationIberian Peninsula.TaxonLissotriton boscai (Amphibia: Caudata: Salamandridae).MethodsWe used original (field‐based) and published data on body size, courtship behaviour and fecundity parameters and a suite of climatic, ecological and genetic (mitochondrial and nuclear markers) predictors to assess these hypotheses.ResultsThe results showed that SSD increased with increasing female mean body size, supporting the inverse Rensch's rule pattern. Primary productivity‐related variables and female density were among the most relevant ecological predictors of SSD after accounting for genetic structure and capture date.Main conclusionsThis study reveals that the interplay between the density‐dependent resource availability hypothesis and the differential plasticity hypothesis explains the inverse Rensch's rule. We discuss how combining biogeographical and experimental approaches can provide alternative interpretations to the classical sexual and fecundity selection hypotheses on the interpopulation variation in SSD.