Several plant-herbivore hypotheses are based on the assumption that plants cannot simultaneously allocate resources to growth and defence. We studied seasonal patterns in allocation to growth and putatively defensive compounds by monitoring several chemical and physical traits in the leaves of mountain birch from early June (budburst) to late September (leaf senescence). We found significant seasonal changes in all measured characteristics, both in terms of concentrations (mg g) and amounts (mg leaf). Changes were very rapid in the spring, slow in the middle of the season, and there was another period of fast changes in the senescing leaves. Co-occurring changes in physical leaf traits and concentrations of several compounds indicated a seasonal decline in foliage suitability for herbivores. Concentrations of protein and free amino acids declined through the growing season whereas individual sugars showed variable seasonal patterns. The seasonal trends of phenolic groups differed drastically: concentrations of soluble proanthocyanidins increased through the season, whereas cell wall-bound proanthocyanidins, gallotannins and flavonoid glycosides declined after an initial increase in young leaves. We failed to find proof that the seasonal accumulation of phenolics would have been seriously compromised by leaf or shoot growth, as assumed by the growth/differentiation balance hypothesis and the protein competition model hypothesis. On the contrary, there was a steady increase in the total amount of phenolics per leaf even during the most active leaf growth.
Understanding the early evolution of aposematic (warning) coloration has been a challenge for scientists, as a new conspicuous morph in a population of cryptic insects would have a high predation risk and would probably die out before local predators learnt to avoid it. Fisher presented the idea of aggregation benefit through the survival of related individuals; however, his theory has been strongly debated as the mechanisms that favour grouping have never been explored experimentally with the incorporation of detectability costs. Here we create a comprehensive 'novel world' experiment with the great tit (Parus major) as a predator to explore simultaneously the predation-related benefits and costs for aposematic aggregated prey, manipulating both group size and signal strength. Our results show that grouping would have been highly beneficial for the first aposematic prey individuals surrounded by naive predators, because (1) detectability risk increased only asymptotically with group size; (2) additional detectability costs due to conspicuous signals were marginal in groups; (3) even naive predators deserted the group after detecting unpalatability (dilution effect); and (4) avoidance learning of signal was faster in groups. None of these mechanisms require kin selection.
The quality of tree leaves as food for herbivores changes rapidly especially during the spring and early summer. However, whether the quality of an individual tree in relation to other trees in the population changes during the growing season and between years is less clear. We studied the seasonal and annual stability of chemical and physical traits affecting leaf quality for herbivores. Rankings of trees in terms of the contents of two major groups of phenolics in their leaves, hydrolyzable tannins and proanthocyanidins (condensed tannins), were very stable from the early spring to the end of the growing season. There were also strong positive within-season correlations in the levels of some other groups of phenolics in the leaves (kaempferol glycosides, myricetin glycosides and p-coumaroylquinic acid derivatives). The contents of individual sugars and the sum content of protein-bound amino acids showed patterns of seasonal consistency in mature leaves, but not in young developing leaves. The seasonal correlations in leaf water content and toughness were also strongest in mature leaves. The correlations between two years at corresponding times of the growing season were strongly positive for the major groups of phenolics throughout the season, but were more variable for the contents of proteins and some sugars. Leaf toughness and water content showed strong positive correlations in mature leaves. Despite the consistency of tree ranking in terms of leaf phenolics, the relative resistance status of trees may, however, change during a growing season because there was a negative correlation between the content of hydrolyzable tannins (early-season resistance compounds) in leaves early in the season and the content of proanthocyanidins (late-season resistance compounds) late in the season, and vice versa. Thus, assuming that phenolics affect herbivore preference and performance, different plants may suffer damage at different times of the growing season, and the overall variation between trees in the fitness consequences may be low. In addition, the adaptation of herbivorous insects to mountain birch foliage in general, as well as to specific tree individuals, may be constrained by variation in the relative resistance status of the trees.
K. 2005. Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores. Á/ Oikos 111: 221 Á/234.Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g (1 ), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully-grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing-induced changes in food quality.
Document VersionPublisher's PDF, also known as Version of record Citation for published version (APA):Leimu, R., Riipi, M., & Staerk, D. (2005). Food preference and performance of the larvae of a specialist herbivore: variation among and within host-plant populations. Acta Oecologica, 28,[325][326][327][328][329][330] AbstractSpecialist herbivores are suggested to be unaffected by or attracted to the defense compounds of their host-plants, and can even prefer higher levels of certain chemicals. Abrostola asclepiadis is a specialist herbivore whose larvae feed on the leaves of Vincetoxicum hirundinaria, which contains toxic alkaloids and is unpalatable to most generalist herbivores. The food choice, leaf consumption and growth of A. asclepiadis larvae were studied to determine whether there is variation among and within host-plant populations in their suitability for this specialist herbivore. There was significant variation in food preference and leaf consumption among host-plant populations, but no differences were found in larval growth and feeding on different host-plant populations. A. asclepiadis larvae preferred host-plant populations with higher alkaloid concentrations, but did not consume more leaf material from plants originating from such populations in a no-choice experiment. There was also some variation in food preference of larvae among host-plant individuals belonging to the same population, suggesting that there was variability in leaf chemistry also within populations. Such variation in larval preference among host-plant genotypes and populations may create potential for coevolutionary dynamics in a spatial mosaic.
Abstract:We studied whether the larval performance of four sawfly species feeding on mountain birch (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti) leaves at different times of the growing season was related to the same chemical and physical leaf traits. In the three phenologically earliest species, larval growth was related to the same set of leaf traits. In contrast, consumption of leaf material was related to different leaf traits in all of the four species. Leaf suitability for herbivore growth and tree resistance to herbivore consumption were thus not always determined by the same leaf traits. Leaf water content and toughness had the strongest associations with larval performance. Phenolics, which are often considered the main resistance compounds in deciduous trees, explained only a small part of the variance in leaf consumption, but some groups of phenolics were negatively related to larval growth. In conclusion, the same mountain birch trees were not resistant to all of the studied sawflies in terms of leaf consumption, but low leaf suitability, which was related to the same set of leaf traits in the case of three sawfly species, may serve birch resistance, e.g., by exposing larvae to natural enemies for a longer period. Résumé :Les auteurs ont examiné si la performance des larves de quatre espèces de tenthrèdes qui se nourrissent sur les feuilles du bouleau pubescent (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti) à différents moments de la saison de croissance est reliée aux mêmes caractéristiques physiques et chimiques des feuilles. Chez les trois espèces les plus précoces du point de vue phénologique, la croissance des larves était reliée au même groupe de caractéristi-ques des feuilles. Par contre, la consommation de matériel foliaire était reliée à différentes caractéristiques des feuilles chez chacune des quatre espèces. La qualité nutritive des feuilles pour la croissance des herbivores et la résistance des arbres à la consommation par les herbivores n'étaient par conséquent pas toujours déterminées par les mêmes traits foliaires. La teneur en eau des feuilles et leur coriacité étaient le plus étroitement reliées à la performance des larves. Les composés phénoliques qui sont souvent considérés comme responsables de la résistance chez les feuillus expliquaient seulement une faible proportion de la variance dans la consommation des feuilles. Par contre, certains groupes de composés phénoliques étaient négativement reliés à la croissance des larves. En conclusion, les mêmes bouleaux n'étaient pas résistants à toutes les espèces de tenthrèdes étudiées en terme de consommation des feuilles, mais la faible qualité nutritive des feuilles, qui était reliée au même groupe de traits foliaires dans le cas de trois espèces de tenthrèdes, pourrait contribuer à la résistance du bouleau, par exemple, en exposant les larves à des ennemis naturels pour une plus longue période.[Traduit par la Rédaction] Riipi et al. 198
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