How does ontogeny in a Eucalyptus species affect patterns of herbivory by Brushtail Possums?

Visit, consume and quit: Patch quality affects the three stages of foraging

Foraging is a three-stage process during which animals visit patches, consume food and quit. Foraging theory exploring relative patch quality has mostly focused on patch use and quitting decisions, ignoring the first crucial step for any forager: finding food. Yet, the decision to visit a patch is just as important as the decision to quit, as quitting theories can only be used if animals visit patches in the first place. Therefore, to better understand the foraging process and predict its outcomes, it is necessary to explore its three stages together. We used the common brushtail possum (Trichosurus vulpecula) as a model to investigate foraging decisions in response to food varying in quality. In particular, we tested whether patch nutritional quality affected the following: (1) patch visits; (2) behaviours at the patch during a foraging visit; and (3) patch quitting decisions (quantified using giving up density-GUD). Free-ranging possums were presented with diets varying in nitrogen content and concomitantly volatile organic compound (VOC) composition at feeding stations in the wild. We found that possums were able to distinguish between different quality foods from afar, despite the location of the diets changed daily. Possums used VOC (i.e. odour cues) emitted by the diets to find and select patches from a distance. High-quality diets with higher protein and lower fibre were visited more often and for longer. Possums spent more time foraging on diets high in nutritional content, resulting in lower GUDs. Our study provides important quantitative evidence that foraging efficiency plays out during all the three stages of the foraging process (i.e. visit, consume and quit), and demonstrates the significance of considering all these stages together in future studies and foraging models. Sensory cues such as food odours play a critical role in helping foragers, including mammalian herbivores, find high-quality food. This allows foragers to make quick, accurate and important decisions about food patches well before patch quitting decisions come into play.

Evidence that divergent selection shapes a developmental cline in a forest tree species complex

BACKGROUND AND AIMS

Evolutionary change in developmental trajectories (heterochrony) is a major mechanism of adaptation in plants and animals. However, there are few detailed studies of the variation in the timing of developmental events among wild populations. We here aimed to identify the climatic drivers and measure selection shaping a genetic-based developmental cline among populations of an endemic tree species complex on the island of Tasmania.

METHODS

Seed lots from 38 native provenances encompassing the clinal transition from the heteroblastic Eucalyptus tenuiramis to the homoblastic Eucalyptus risdonii were grown in a common-garden field trial in southern Tasmania for 20 years. We used 27 climatic variables to model the provenance variation in vegetative juvenility as assessed at age 5 years. A phenotypic selection analysis was used to measure the fitness consequences of variation in vegetative juvenility based on its impact on the survival and reproductive capacity of survivors at age 20 years.

KEY RESULTS

Significant provenance divergence in vegetative juvenility was shown to be associated with home-site aridity, with the retention of juvenile foliage increasing with increasing aridity. Our results indicated that climate change may lead to different directions of selection across the geographic range of the complex, and in our mesic field site demonstrated that total directional selection within phenotypically variable provenances was in favour of reduced vegetative juvenility.

CONCLUSIONS

We provide evidence that heteroblasty is adaptive and argue that, in assessing the impacts of rapid global change, developmental plasticity and heterochrony are underappreciated processes which can contribute to populations of long-lived organisms, such as trees, persisting and ultimately adapting to environmental change.

A major trade-off between structural and photosynthetic investments operative across plant and needle ages in three Mediterranean pines

Pine (Pinus) species exhibit extensive variation in needle shape and size between juvenile (primary) and adult (secondary) needles (heteroblasty), but few studies have quantified the changes in needle morphological, anatomical and chemical traits upon juvenile-to-adult transition. Mediterranean pines keep juvenile needles longer than most other pines, implying that juvenile needles play a particularly significant role in seedling and sapling establishment in this environment. We studied needle anatomical, morphological and chemical characteristics in juvenile and different-aged adult needles in Mediterranean pines Pinus halepensis Mill., Pinus pinea L. and Pinus nigra J. F. Arnold subsp. salzmannii (Dunal) Franco hypothesizing that needle anatomical modifications upon juvenile-to-adult transition lead to a trade-off between investments in support and photosynthetic tissues, and that analogous changes occur with needle aging albeit to a lower degree. Compared with adult needles, juvenile needles of all species were narrower with 1.6- to 2.4-fold lower leaf dry mass per unit area, and had ~1.4-fold thinner cell walls, but needle nitrogen content per dry mass was similar among plant ages. Juvenile needles also had ~1.5-fold greater mesophyll volume fraction, ~3-fold greater chloroplast volume fraction and ~1.7-fold greater chloroplast exposed to mesophyll exposed surface area ratio, suggesting overall greater photosynthetic activity. Changes in needle traits were similar in aging adult needles, but the magnitude was generally less than the changes upon juvenile to adult transition. In adult needles, the fraction in support tissues scaled positively with known ranking of species tolerance of drought (P. halepensis > P. pinea > P. nigra). Across all species, and needle and plant ages, a negative correlation between volume fractions of mesophyll and structural tissues was observed, manifesting a trade-off between biomass investments in different needle functions. These results demonstrate that within the broad trade-off, juvenile and adult needle morphophysiotypes are separated by varying investments in support and photosynthetic functions. We suggest that the ecological advantage of the juvenile morphophysiotype is maximization of carbon gain of establishing saplings, while adult needle physiognomy enhances environmental stress tolerance of established plants.

Environment vs. Plant Ontogeny: Arthropod Herbivory Patterns on European Beech Leaves along the Vertical Gradient of Temperate Forests in Central Germany

Environmental and leaf trait effects on herbivory are supposed to vary among different feeding guilds. Herbivores also show variability in their preferences for plant ontogenetic stages. Along the vertical forest gradient, environmental conditions change, and trees represent juvenile and adult individuals in the understorey and canopy, respectively. This study was conducted in ten forests sites in Central Germany for the enrichment of canopy research in temperate forests. Arthropod herbivory of different feeding traces was surveyed on leaves of Fagus sylvatica Linnaeus (European beech; Fagaceae) in three strata. Effects of microclimate, leaf traits, and plant ontogenetic stage were analyzed as determining parameters for herbivory. The highest herbivory was caused by exophagous feeding traces. Herbivore attack levels varied along the vertical forest gradient for most feeding traces with distinct patterns. If differences of herbivory levels were present, they only occurred between juvenile and adult F. sylvatica individuals, but not between the lower and upper canopy. In contrast, differences of microclimate and important leaf traits were present between the lower and upper canopy. In conclusion, the plant ontogenetic stage had a stronger effect on herbivory than microclimate or leaf traits along the vertical forest gradient.

Colonization by nitrogen-fixing Frankia bacteria causes short-term increases in herbivore susceptibility in red alder (Alnus rubra) seedlings

Carbon allocation demands from root-nodulating nitrogen-fixing bacteria (NFB) can modulate the host plant's chemical phenotype, with strong bottom-up effects on herbivores. In contrast to well-studied rhizobia, the effects of other important NFB on plant chemistry and herbivory are much less understood. Here, combining field surveys in the Oregon Coast Range, USA with laboratory experiments, we analyzed how N₂-fixing Frankia bacteria influenced plant growth, chemistry, and herbivory on Alnus rubra (red alder) seedlings. In the field, we quantified Frankia nodulation, herbivore damage, and plant size. In the laboratory, we grew seedlings with Frankia (F+), Frankia-free but nitrogen-fertilized (N+), or both uncolonized and unfertilized (F-N-) and assessed growth and leaf chemistry. We further conducted choice trials with black slugs, Arion rufus, a natural red alder herbivore. In the field, Frankia nodulation was significantly positively correlated with herbivory and negatively with seedling height. In contrast, in the lab, F+ as well as N+ seedlings were significantly taller than the F-N- controls. Seedlings from both treatments also had significantly increased leaf protein concentration compared to controls, whereas carbon-based nutritive compounds (carbohydrates) as well as leaf palatability-decreasing condensed tannins, lignin, and fiber were decreased in F+ but not in N+ treatments. In the choice assays, slugs preferred leaf material from F+ seedlings, but the effects were only significant in young leaves. Our study indicates that colonization by Frankia causes short-term ecological costs in terms of susceptibility to herbivory. However, the ubiquity of this symbiosis in natural settings suggests that these costs are outweighed by benefits beyond the seedling stage.

Quantitative and qualitative shifts in defensive metabolites define chemical defense investment during leaf development in Inga, a genus of tropical trees

Selective pressures imposed by herbivores are often positively correlated with investments that plants make in defense. Research based on the framework of an evolutionary arms race has improved our understanding of why the amount and types of defenses differ between plant species. However, plant species are exposed to different selective pressures during the life of a leaf, such that expanding leaves suffer more damage from herbivores and pathogens than mature leaves. We hypothesize that this differential selective pressure may result in contrasting quantitative and qualitative defense investment in plants exposed to natural selective pressures in the field. To characterize shifts in chemical defenses, we chose six species of Inga, a speciose Neotropical tree genus. Focal species represent diverse chemical, morphological, and developmental defense traits and were collected from a single site in the Amazonian rainforest. Chemical defenses were measured gravimetrically and by characterizing the metabolome of expanding and mature leaves. Quantitative investment in phenolics plus saponins, the major classes of chemical defenses identified in Inga, was greater for expanding than mature leaves (46% and 24% of dry weight, respectively). This supports the theory that, because expanding leaves are under greater selective pressure from herbivores, they rely more upon chemical defense as an antiherbivore strategy than do mature leaves. Qualitatively, mature and expanding leaves were distinct and mature leaves contained more total and unique metabolites. Intraspecific variation was greater for mature leaves than expanding leaves, suggesting that leaf development is canalized. This study provides a snapshot of chemical defense investment in a speciose genus of tropical trees during the short, few-week period of leaf development. Exploring the metabolome through quantitative and qualitative profiling enables a more comprehensive examination of foliar chemical defense investment.

Seedling-herbivore interactions: insights into plant defence and regeneration patterns

BACKGROUND

Herbivores have the power to shape plant evolutionary trajectories, influence the structure and function of vegetation, devastate entire crops, or halt the spread of invasive weeds, and as a consequence, research into plant-herbivore interactions is pivotal to our understanding of plant ecology and evolution. However, the causes and consequences of seedling herbivory have received remarkably little attention, despite the fact that plants tend to be most susceptible to herbivory during establishment, and this damage can alter community composition and structure.

SCOPE

In this Viewpoint article we review why herbivory during early plant ontogeny is important and in so doing introduce an Annals of Botany Special Issue that draws together the latest work on the topic. In a synthesis of the existing literature and a collection of new studies, we examine several linked issues. These include the development and expression of seedling defences and patterns of selection by herbivores, and how seedling selection affects plant establishment and community structure. We then examine how disruption of the seedling-herbivore interaction might affect normal patterns of plant community establishment and discuss how an understanding of patterns of seedling herbivory can aid our attempts to restore semi-natural vegetation. We finish by outlining a number of areas where more research is required. These include a need for a deeper consideration of how endogenous and exogenous factors determine investment in seedling defence, particularly for the very youngest plants, and a better understanding of the phylogenetic and biogeographical patterns of seedling defence. There is also much still be to be done on the mechanisms of seedling selection by herbivores, particularly with respect to the possible involvement of volatile cues. These inter-related issues together inform our understanding of how seedling herbivory affects plant regeneration at a time when anthropogenic change is likely to disrupt this long-established, but all-too-often ignored interaction.

Mechanisms of optimal defense patterns in Nicotiana attenuata: flowering attenuates herbivory-elicited ethylene and jasmonate signaling

To defend themselves against herbivore attack, plants produce secondary metabolites, which are variously inducible and constitutively deployed, presumably to optimize their fitness benefits in light of their fitness costs. Three phytohormones, jasmonates (JA) and their active forms, the JA-isoleucine (JA-Ile) and ethylene (ET), are known to play central roles in the elicitation of induced defenses, but little is known about how this mediation changes over ontogeny. The Optimal Defense Theory (ODT) predicts changes in the costs and benefits of the different types of defenses and has been usefully extrapolated to their modes of deployment. Here we studied whether the herbivore-induced accumulation of JA, JA-Ile and ET changed over ontogeny in Nicotiana attenuata, a native tobacco in which inducible defenses are particularly well studied. Herbivore-elicited ET production changed dramatically during six developmental stages, from rosette through flowering, decreasing with the elongation of the first corollas during flower development. This decrease was largely recovered within a day after flower removal by decapitation. A similar pattern was found for the herbivore-induced accumulation of JA and JA-Ile. These results are consistent with ODT predictions and suggest that the last steps in floral development control the inducibility of at least three plant hormones, optimizing defense-growth tradeoffs.

The ontogeny of plant defense and herbivory: characterizing general patterns using meta-analysis

Defense against herbivores often changes dramatically as plants develop. Hypotheses based on allocation theory and herbivore selection patterns predict that defense should increase or decrease, respectively, across ontogeny, and previous research partly supports both predictions. Thus, it remains unclear which pattern is more common and what factors contribute to variability among studies. We conducted a meta-analysis of 116 published studies reporting ontogenetic patterns in plant defense traits and herbivory. Patterns varied depending on plant life form (woody, herbaceous, grass), type of herbivore (insect, mollusk, mammal), and type of defense trait (secondary chemistry, physical defense, tolerance). In woody plants, chemical defense increased during the seedling stage, followed by an increase in physical defenses during the vegetative juvenile stage. Mammalian herbivores showed a strong preference for mature compared to juvenile tissues in woody plants. Herbs experienced a significant increase in secondary chemistry across the entire ontogenetic trajectory, although the magnitude of increase was greatest during the seedling stage. Correspondingly, mollusks preferred young compared to older herbs. Future research investigating growth/defense trade-offs, allometry, herbivore selection patterns, and ecological costs would shed light on the mechanisms driving the ontogenetic patterns observed.

Separating ontogenetic and environmental determination of resistance to herbivory in cottonwood

We used narrowleaf cottonwood, Populus angustifolia, and the gall-forming aphid, Pemphigus betae, to determine the extent to which ontogenetic variation in resistance to herbivory is due to endogenous, stable genetic influences. In a three-year common garden trial using ramets propagated from the top, middle, and bottom of mature trees, we found that the resistance of trees to aphids was significantly higher in top vs. bottom source ramets, supporting the hypothesis of a stable, genetically programmed component to aphid resistance. The magnitude of ontogenetically based variation in resistance within an individual tree is comparable to the genetic variation in resistance among narrowleaf cottonwood genotypes or populations found in other studies. These ontogenetic-based findings have the potential to alter ecological interactions and evolutionary trajectories of plant-herbivore interactions.

Frequency and distribution of cyanogenic glycosides in Eucalyptus L'Hérit

In this study approximately 420 of the described species of Eucalyptus were examined for cyanogenesis. Our work has identified an additional 18 cyanogenic species, 12 from living tissues and a further six from herbarium samples. This brings the total of known cyanogenic species to 23, representing approximately 4% of the genus. The taxonomic distribution of the species within the genus is restricted to the subgenus Symphyomyrtus, with only two exceptions. Within Symphyomyrtus, the species are in three closely related sections. The cyanogenic glycoside was found to be predominantly prunasin (1) in the 11 species where this was examined. We conclude that cyanogenesis is plesiomorphic in Symphyomyrtus (i.e. a common basal trait) but has probably arisen independently in the other two subgenera, consistent with recent phylogenetic treatments of the genus. The results of this study have important implications for the selection of trees for plantations to support wildlife, and to preserve genetic diversity.

Development of an Insect Herbivore and its Pupal Parasitoid Reflect Differences in Direct Plant Defense

In nature, plants defend themselves by production of allelochemicals that are toxic to herbivores. There may be considerable genetic variation in the expression of chemical defenses because of various selection pressures. In this study, we examined the development of the small cabbage butterfly, Pieris rapae, and its gregarious pupal ectoparasitoid, Pteromalus puparum, when reared on three wild populations (Kimmeridge, Old Harry, Winspit) of cabbage, Brassica oleracea, and a Brussels sprout cultivar. Wild plant populations were obtained from seeds of plants that grow naturally along the south coast of Dorset, England. Significant differences in concentrations of allelochemicals (glucosinolates) were found in leaves of plants damaged by P. rapae. Total glucosinolate concentrations in Winspit plants, the population with the highest total glucosinolate concentration, were approximately four times higher than in the cultivar, the strain with the lowest total glucosinolate concentration. Pupal mass of P. rapae and adult body mass of Pt. puparum were highest when reared on the cultivar and lowest when developing on Kimmeridge plants, the wild strain with the lowest total glucosinolate concentration. Development of male parasitoids was also more negatively affected than female parasitoids. Our results reveal that plant quality, at least for the development of 'adapted' oligophagous herbivores, such as P. rapae, is not based on total glucosinolate content. The only glucosinolate compound that corresponded with the performance of P. rapae was the indole glucosinolate, neoglucobrassicin. Our results show that performance of ectoparasitoids may closely reflect constraints on the development of the host.