Plant defense, herbivory, and the growth of Cordia alliodora trees and their symbiotic Azteca ant colonies

The effects of herbivory on plant fitness are integrated over a plant??s lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant?Cplant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant??s growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.     

Fluctuating asymmetry,leaf thickness and herbivory in <Emphasis Type="Italic">Tibouchina granulosa</Emphasis>: an altitudinal gradient analysis

Gradient of stressful conditions affect plant physiological and morphological traits. Previous studies have shown that plants located at higher altitudes might exhibit higher levels of both fluctuating asymmetry and leaf thickness. Although it is expected that higher fluctuating asymmetry levels should be accompanied by higher leaf consumption by herbivores, lower herbivory could be expected for elevated leaf thickness. Aiming to investigate this contradiction our objective was to determine the effects of altitude on fluctuating asymmetry and leaf thickness, and evaluate the importance of these two morphological traits on herbivory levels of Tibouchina granulosa Cogn. (Melastomatecea) in Brazilian Atlantic Forest. The study was conducted in southern Brazil, along a continuous altitudinal gradient raging from 1275 to 1950 m, where we measured fluctuating asymmetry, leaf thickness and herbivory from leaves of 29 individuals of T. granulosa. There was a positive effect of altitude on both fluctuating asymmetry and leaf thickness but only fluctuating asymmetry was related to herbivore. Our results suggest that as altitude increases plants face more stressful conditions, leading to higher fluctuating asymmetry. This may lead to a higher nutritional quality of leaves and herbivores may use leaf asymmetry as a cue for plant quality. The lack of a relationship between leaf thickness and herbivory gives us evidence that, in the studied location, leaf thickness is not primarily used as plant defense and probably has other functions related, for example, to water, solar radiation, and nutrient stresses. These results may be considered a baseline for the understanding on how altitudinal stress and potential herbivory pressure influence plant populations.     

Effects of above- and belowground herbivory on growth,pollination, and reproduction in cucumber

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant–herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.     

Comparison of Herbivores and Herbivory in the Canopy and Understory for Two Tropical Tree Species1

The Janzen–Connell model of tropical forest tree diversity predicts that seedlings and young trees growing close to conspecific adults should experience higher levels of damage and mortality from herbivorous insects, with the adult trees acting as either an attractant or source of the herbivores. Previous research in a seasonal forest showed that this pattern of distance‐dependent herbivory occurred in the early wet season during the peak of new leaf production. I hypothesized that distance‐dependent herbivory may occur at this time because the new foliage in the canopy attracts high numbers of herbivores that are limited to feeding on young leaves. As a consequence, seedlings and saplings growing close to these adults are more likely to be discovered and damaged by these herbivores. In the late wet season, when there is little leaf production in the canopy, leaf damage is spread more evenly throughout the forest and distance dependence disappears. I tested three predictions based on this hypothesis: (1) the same species of insect herbivores attack young and adult trees of a given plant species; (2) herbivore densities increase on adult trees during leaf production; and (3) herbivore densities in the understory rise during the course of the wet season. Censuses were conducted on adults and saplings of two tree species, raribea asterolepis and Alseis blackiana. Adults and saplings of both species had largely the same suite of chewing herbivore species. On adults of Q. asterolepis, the density of chewing herbivores increased 6–10 times during leaf production, but there was no increase in herbivore density on adults of A. blackiana. Herbivore densities increased 4.5 times on A. blackiana saplings and 8.9 times on Q. asterolepis saplings during the wet season, but there were no clear trends on the adults of either species. These results suggest that the potential of adult trees as a source of herbivores on saplings depends on the value of new leaves to a tree species' herbivores, which may differ across tree species.     

Similar responses of insect herbivores to leaf fluctuating asymmetry

Fluctuating asymmetry (FA) represents small, random variation from symmetry and it has been used as an indicator of plant quality and susceptibility to herbivory. In this study, the effects of FA on the responses of distinct herbivore species belonging to several guilds were examined along an environmental gradient in south Florida. This approach was chosen because it relies on a multi-species approach to the study of fluctuating asymmetry and patterns of herbivory between and within plants along an environmental gradient of salinity and plant stress. To examine differences in FA between and within plant communities, seven plant species were investigated. Four of these plants were coastal species and three species occurred in upland communities. Levels of FA were assessed before herbivory and plants were followed for the whole herbivory season in 2006. Coastal plants exhibited significantly higher salt concentration, higher percentage of asymmetric leaves and higher asymmetry levels than upland plants. Herbivore abundance varied widely amongst the seven species studied, but quantitative syntheses of our results indicated significant and positive responses of insect herbivores to leaf asymmetry: insects were 25.11% more abundant on more asymmetric plants and stronger effects of asymmetry were observed for leaf miners compared to gall-formers. As demonstrated by other recent studies, FA might be used as a reliable stress indicator, leading to similar responses of insect herbivores to variation in leaf symmetry.     

Does insect herbivory on oak depend on the diversity of tree stands?

Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance.     

Delayed local responses of downy birch to damage by leafminers and leafrollers

Recent findings suggest that impacts of endemic herbivory on forest ecosystems over the long term may exceed impacts of herbivore outbreaks. However, responses of trees to minor and local damage imposed by small arthropod herbivores, especially by those mining or skeletonising individual leaves, remain poorly understood. We studied the delayed effects of injuries by several leafmining and leafrolling insects on the performance of downy birch shoots. Insect feeding did not affect survival of shoots or survival of individual axillary buds in long shoots. In the year following the damage, shoots produced an average of 13.8% more biomass than undamaged shoots of the same tree. The magnitude of this effect increased with an increase in the leaf area injured during the previous year, but it did not differ among four localities in subarctic and boreo‐nemoral forests, between herbivore feeding guilds, or among herbivores imposing damage in early, mid and late summer. We also found that herbivores attacked the next‐year foliage produced by damaged shoots less frequently than they attacked the next‐year foliage produced by undamaged shoots of the same tree. Thus, our study demonstrated delayed local compensatory growth and increased antiherbivore defence in downy birch shoots following local damage by insect feeding. We suggest that this pattern reflects evolutionary adaptations of plants to permanently acting minor, dispersed and spatially unpredictable damage imposed by endemic herbivory. Local responses are less costly and represent a more sustainable strategy to maintain plant fitness under low levels of herbivory than constitutive resistance or systemic responses.     

ANALYSIS OF GENOTYPIC DIFFERENCES IN DEVELOPMENTAL STABILITY IN ANNONA CHERIMOLA

The genetic basis of developmental stability, measured as asymmetry (fluctuating asymmetry in leaves), was analyzed in leaves and flowers of cherimoya (Annona cherimola Mill) and atemoya (A. cherimola × A. squamosa). The individuals analyzed belonged to a controlled collection of cultivars (clones) that had previously been characterized by means of isozymes. We used a nested design to analyze the differences in asymmetry at several sampling levels: individual leaves and flowers, individual trees, and genotypes. The clonal repeatability of developmental stability was not significantly different from zero, thus suggesting the absence of heritability of the asymmetry for leaves and flowers under these environmental conditions. No relationship between asymmetry and individual heterozygosity was found, but leaf fluctuating asymmetry was significantly related to particular isozymic genes. Petal and leaf size showed a phenotypically plastic response to the exposure zone of the tree (mainly due to light). Leaf fluctuating asymmetry also showed such a plastic response. No significant correlation was found between asymmetry and any pomological characters (some of these being fitness related). Finally, the hybrid species (atemoya) did not show larger developmental instability than did the parental species (cherimoya). All these data show that cherimoya asymmetry reveals the random nature of developmental noise, with developmental stability for leaves being possibly related to specific chromosome regions, but with weak evidence for genotypic differences in developmental stability.     

Sex, secondary compounds and asymmetry. Effects on plant–herbivore interaction in a dioecious shrub

We analysed the links between herbivory, anthraquinone content and developmental instability of leaves in Rhamnus alpinus, taking into account possible effects of sexual dimorphism. The amount of leaf loss caused by herbivores averaged 3%, rarely exceeding 25%. Leaf losses were evenly distributed in the shrubs, with highest variability among leaves of the same shoot, thus hiding possible shrub, sex or population effects. This pattern of herbivory implies a shifting of caterpillars from one leaf to another before consuming all readily available material. We suggest that this behaviour might be triggered by a short-term change in leaf palatability by means of an increase in the production of secondary compounds. Supporting this hypothesis, we have found a higher anthraquinone content in damaged leaves compared with undamaged ones. The leaves of male plants exhibited a higher concentration of anthraquinones than those of females, which contrasts with classic hypotheses. We relate this to the lower rate of biomass increase in males, which should allow them to allocate more resources to defence. Leaves showed fluctuating asymmetry (FA), but we did not find any relationship between the degree of asymmetry and sex, herbivory or anthraquinone content at any level considered. Therefore, FA cannot be considered as an indicator of susceptibility to damage by herbivores or of the ability to induce the production of defensive compounds in R. alpinus.     

Synchronous leaf production and herbivory in juveniles of Gustavia superba

Summary Synchronous leaf production has been proposed as a mechanism to reduce herbivore damage to young leaves by satiating herbivores. To test this hypothesis, I measured leaf production, leaf survivorship, and herbivore damage on juveniles of Gustavia superba (H.B.K.) Berg (Lecythidaceae), in two sites in Central Panama. Leaves were produced throughout the year, but there were peaks in leaf production at the beginning of the wet scason. Plants that produced leaves synchronously with conspecifics received significantly less damage than plants that produced leaves out of synchrony, and high levels of leaf damage were correlated with shorter leaf lifetimes. These data suggest that plant phenology can influence risks of herbivory.     

Order of herbivore arrival on wild cabbage populations influences subsequent arthropod community development

In plant–arthropod associations, the first herbivores to colonise a plant may directly or indirectly affect community assembly on that particular plant. Whether the order of arrival of different arthropod species further modulates community assembly and affects plant fitness remains unclear. Using wild Brassica oleracea plants in the field, we manipulated the order of arrival of early‐season herbivores that belong to different feeding guilds, namely the aphid Brevicoryne brassicae and caterpillars of Plutella xylostella. We investigated the effect of herbivore identity and order of arrival on community assembly on two B. oleracea plant populations during two growth seasons. For this perennial plant, we evaluated whether foliar herbivory also affected herbivore communities on the flowers and if these interactions affected plant seed production. Aphid infestation caused an increase in parasitoid abundance, but caterpillars modulated these effects, depending on the order of herbivore infestation and plant population. In the second growth season, when plants flowered, the order of infestation of leaves with aphids and caterpillars more strongly affected abundance of herbivores feeding on the flowers than those feeding on leaves. Infestation with caterpillars followed by aphids caused an increase in flower‐feeding herbivores compared to the reversed order of infestation in one plant population, whereas the opposite effects were observed for the other plant population. The impact on plant seed set in the first reproductive year was limited. Our work shows that the identity and arrival order of early season herbivores may have long‐term consequences for community composition on individual plants and that these patterns may vary among plant populations. We discuss how these community processes may affect plant fitness and speculate on the implications for evolution of plant defences.     

Variability in leaf traits,insect herbivory and herbivore performance within and among individuals of four broad-leaved tree species

Individual plants may vary in their suitability as hosts for insect herbivores. The adaptive deme formation hypothesis predicts that this variability will lead to the fine-scale adaptation of herbivorous insects to host individuals. We studied individual and temporal variation in the quality of leaves of the tree species ash, lime, common oak, and sycamore in the field as food for herbivores. We determined herbivore attack and leaf consumption and performance of the generalist caterpillars of Spodoptera littoralis in the laboratory. We further assessed the concentrations of carbon, nitrogen and water in the leaves.All measures of leaf tissue quality varied among and within individuals for all tree species. The level of herbivory differed among the tree individuals in lime, oak and sycamore, but not in ash. Within host individuals, differences in herbivory between the upper and lower crown layer varied in direction and magnitude depending on tree species. In feeding experiments, herbivore performance also varied among and within tree individuals. However, variation in palatability was not consistently related to the leaf traits measured or to herbivory levels in the field. The ranking of individuals with respect to the quality of leaf tissue for herbivorous insects varied between years in lime and oak. Thus, trees of both species might present moving targets for herbivores which prevents fine-scale adaptations. In contrast, among individuals of ash and sycamore the pattern of insect performance remained constant over 2 years. These species may be more suitable hosts for the formation of adapted demes in herbivores.     

Herbivory-induced stress: Leaf developmental instability is caused by herbivore damage in early stages of leaf development

Herbivory is a major source of plant stress and its effects can be severe, decreasing plant fitness, or subtle, affecting the development of leaves by influencing the normal pattern of growth and expansion of leaf blades. Fluctuating asymmetry (FA) analysis is recognized as a measure of plant stress, and can be used to evaluate subtle effects of herbivory on the imperfect growth of bilaterally symmetrical traits, such as leaves. One general issue is that authors usually consider FA as an indicator of stress, which can attract herbivores (plant stress hypothesis), and studies showing that herbivores themselves affect leaf symmetry (herbivory-induced stress hypothesis) are scarce, with mixed results. Here, we investigated the relationship between herbivory by thrips and leaf FA in Banisteriopsis malifolia and Heteropterys escallonifolia (Malpighiaceae). Pseudophilothrips obscuricornis is a free-living, non-pest, sucking species that feeds mainly on leaf buds. We hypothesized that herbivory by thrips in the early stages of leaf development would provoke increased FA levels in mature leaves. The results showed that thrips herbivory rate was low, affecting barely more than 1% of the leaf blade. Nonetheless, thrips-attacked leaves of B. malifolia and H. escallonifolia presented increases of 15 and 27% in leaf asymmetry, respectively, compared to uninjured leaves, corroborating the herbivory-induced stress hypothesis. Since herbivory by thrips in leaf buds was related to significant increases in the stress of mature leaves, we assume that under these circumstances, FA can be used as a biomarker for plant stress following herbivory damage. To be useful as a biomarker of stress, FA in plants must be investigated with caution, taking into account the natural history of the herbivore species and timing of leaf damage.     

Reproductive ecology of distylous Palicourea padifolia (Rubiaceae) in a tropical montane cloud forest. II. Attracting and rewarding mutualistic and antagonistic visitors

By definition, the floral morphs of distylous plants differ in floral architecture. Yet, because cross-pollination is necessary for reproductive success in both morphs, they should not differ in attributes that contribute to attracting and rewarding floral visitors. Floral and vegetative attributes that function in distylous polymorphism in hummingbird-pollinated Palicourea padifolia (Rubiaceae) and the responses of pollinators and insect herbivores to the resources offered by both morphs were investigated. The performance of each morph along multiple stages of the reproductive cycle, from inflorescence and nectar production to fruit production, was surveyed, and pollinator behavior and nectar standing crops were then observed. Costs associated with such attractiveness were also evaluated in terms of herbivore attack and of plant reproductive fitness (female function) as a function of leaf herbivory. The number of inflorescences, floral buds, open flowers, and ripe fruits offered by either floral morph were similar, but short-styled plants almost doubled the number of developing fruits of long-styled plants. Long-styled flowers produced higher nectar volumes and accumulated more nectar over time than short-styled flowers. Measures of nectar standing crop and data on pollinator behavior suggest that hummingbirds respond to this morph-specific scheduling of nectar production. Lastly, long-styled plants suffered a higher herbivore attack and lost more leaf area over time than those with short-styled flowers. Herbivory was negatively correlated with fruit number and fruit mass, and long-styled plants set significantly less fruit mass than short-styled plants. The results suggest that pollinators and herbivores may exert selective pressures on floral and vegetative traits that could also influence gender function.     

Herbivore attack in Casearia nitida influenced by plant ontogenetic variation in foliage quality and plant architecture

Traits influencing plant quality as food and/or shelter for herbivores may change during plant ontogeny, and as a consequence, influence the amount of herbivory that plants receive as they develop. In this study, differences in herbivore density and herbivory were evaluated for two ontogenetic stages of the tropical tree Casearia nitida. To assess plant ontogenetic differences in foliage quality as food for herbivores, nutritional and defensive traits were evaluated in saplings and reproductive trees. Predatory arthropods were quantified and the foraging preferences of a parasitoid wasp of the genus Zacremnops were assessed. In addition, survival rates of lepidopteran herbivores (Geometridae) were evaluated experimentally. Herbivore density was three times higher and herbivory was 66% greater in saplings than in reproductive trees. Accordingly, concentrations of total foliar phenolics were higher in reproductive trees than in saplings, whereas leaf toughness, water and nitrogen concentration did not vary between ontogenetic stages. Survival rates of lepidopteran larvae exposed to natural enemies were equivalent in reproductive trees and saplings. Given the greater herbivore density on saplings, equal survival rates implied a greater foraging effort of predators on reproductive trees. Furthermore, observed foraging of parasitoid wasps was restricted to reproductive trees. I propose that herbivore density, and as a consequence, leaf damage were lower in reproductive trees than in saplings due to both traits influencing food quality, and architectural or unmeasured indirect defensive traits influencing foraging preference of natural enemies of herbivores.     

Effects of two types of herbivores on the population dynamics of a perennial herb

The joint effects of multiple herbivores on their shared host plant have received increasing interest recently. The influence of herbivores on population dynamics of their host plants, especially the relative roles of different types of damage, is, however, still poorly understood. Here, we present a modelling approach, including both deterministic and stochastic matrix modelling, to be used in estimating fitness effects of multiple herbivores on perennial plants. We examined the effects and relative roles of two specialist herbivores, a pre-dispersal seed predator, Euphranta connexa, and a leaf-feeding moth, Abrostola asclepiadis, on the population dynamics and long-term fitness of their shared host plant, a long-lived perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). We collected demographic data during 3 years and combined these data with the effects of natural levels of herbivory measured from the same individuals. We found that both seed predation and leaf herbivory reduced population growth of V. hirundinaria, but only very high damage levels changed the growth trend of the vigorously growing study populations from positive to negative. Demographic modelling indicated that seed predation had a greater impact on plant population growth than leaf herbivory. The effect of leaf herbivory was weaker and diminished with increasing level of seed predation. Evaluation of individual fitness components, however, suggested that leaf herbivory contributed more strongly to host plant fitness than seed predation. Our results emphasize that understanding the effects of a particular herbivore on plant population dynamics requires also knowledge on other herbivores present in the system, because the effect of a particular type of herbivory on plant population dynamics is likely to vary according to the intensity of other types of herbivory. Furthermore, evaluating herbivore impact from using individual fitness components does not necessarily reflect the long-term effects on total plant fitness.     

Lagged effects of sawfly leaf herbivory on reproductive organs in cherry trees: Overcompensation in flower production reduces quality of fruits and seeds

Herbivores reduce plant productivity by removing part of the assimilation surface. Also, they can alter plant traits that affect plant–pollinator interactions and reproductive success. The objective of this study was to evaluate the impact of defoliation by sawfly (Caliroa cerasi) larvae on fruit production and quality in three cultivars of sweet cherry (Prunus avium). We hypothesized that the fruit production and quality is reduced as a consequence of changes in the allocation of resources within the plant in response to partial leaf removal during the previous year. Number of flowers per branch meter was higher in infested trees than in non-infested trees, while the number of fruits per branch meter was similar due to fruit abortion in all cultivars. Fruit quality was significantly affected by herbivory in different traits depending on cultivar. Infested Lapins and Van trees had significantly lower soluble solid content than non-infested trees. Titratable acidity was higher and ripening index was lower in infested Bing and Lapins trees than non-infested trees. Infested Van trees also exhibited a significant decrease in equatorial diameter and fresh fruit weight as well as pulp fresh weight and dry weight of seed compared to non-infested trees. Overall our study highlights that the direct impact of herbivores at leaf level has lagged effects on productivity in terms of fruit and seed quality in the year following the leaf damage.     

Attracting pollinators and avoiding herbivores: insects influence plant traits within and across years

Perennial plants interact with herbivores and pollinators across multiple growing seasons, and thus may respond to herbivores and pollinators both within and across years. Joint effects of herbivores and pollinators influence plant traits, but while some of the potential interactions among herbivory, pollination, plant size, and plant reproductive traits have been well studied, others are poorly understood. This is particularly true for perennial plants where effects of herbivores and pollinators may manifest across years. Here, we describe two experiments addressing the reciprocal interactions of plant traits with herbivore damage and pollination across 2 years using the perennial plant Chamerion angustifolium. We measured (1) plant responses to manipulation of damage and pollination in the year of treatment and the subsequent season, (2) damage and pollination responses to manipulation of plant size and flowering traits in the year of treatment, and (3) plant-mediated indirect interactions between herbivores and pollinators. We found that plant traits had little effect on damage and pollination, but damage and pollination affected plant traits in both the treatment year and the subsequent year. We found evidence of indirect effects between leaf herbivores and pollinators in both directions; indirect effects of pollinators on leaf herbivores have not been previously demonstrated. Our results indicate that pollen receipt results in shorter plants with fewer stems but does not change flower number, while leaf herbivory results in taller plants with fewer flowers. Together, herbivory and pollination may contribute to intermediate plant height and plants with fewer stems and flowers in our system.     

Leaf Phenology and Leaf Damage of Saplings in the Luquillo Experimental Forest,Puerto Rico1

Changes in light or water availability can result in synchronous leaf production, concentrating food availability for herbivores of young leaves to only a few months. To determine the importance of food availability on herbivory, leaf phenology and leaf damage were studied in the Luquillo Experimental Forest (LEF) of Puerto Rico. We studied 20 individuals of eight species for two years. Every month, new leaves were marked; the following month, leaf area and area of damage were measured. Over two years, comparison of leaf production and percent herbivory were performed for each species, and for all species taken together. More than 30 percent of the annual leaf production occurred in May and June. Leaf production was associated with an increase in PFD (photon flux density) and was not related to the patterns of rainfall. Although leaf production was synchronous, there were no differences in herbivory between the peak and non‐peak periods of leaf production. Possible explanations for the constant levels of herbivory throughout the year are the presence of a generalist herbivore community, the ability of herbivores to track changes in food availability, or high densities of herbivore predators that control herbivore populations.     

Genetic Constraints and Selection Acting on Tolerance to Herbivory in the Common Morning Glory Ipomoea purpurea

Tolerance to herbivory minimizes the effects of herbivory on plant fitness. In the presence of herbivores, maximal levels of tolerance may be expected to evolve. In many plant species, however, tolerance is found at an intermediate level. Tolerance may be prevented from evolving to a maximal level by genetic constraints or stabilizing selection. We report on a field study of Ipomoea purpurea, the common morning glory, in which we measured three types of costs that may be associated with tolerance and the pattern of selection acting on tolerance to two types of herbivore damage: apical meristem damage and folivory. We used genetic correlations to test for the presence of three types of costs: a trade-off between tolerance and fitness in the absence of herbivore damage, a trade-off between tolerance and resistance, and genetic covariances among tolerance to different types of damage. We found no evidence that tolerance to apical meristem damage or tolerance to folivory was correlated with resistance, although these two types of tolerance were positively correlated with one another. Tolerance to both types of damage involved costs of lower fitness in the absence of herbivory. Selection acting on tolerance to either type of herbivory was not detected at natural levels of herbivory. Selection is expected to act against tolerance at reduced levels of herbivory and favor tolerance at elevated levels of herbivory. In addition, significant correlational selection gradients indicate that the pattern of selection acting on tolerance depends on values of resistance. Although we found no evidence for stabilizing selection, fluctuating selection resulting from fluctuating herbivore loads may be responsible for maintaining tolerance at an intermediate level.