Effects of forest edges on herbivory in a New Zealand mistletoe,

This study examined how forest edges influenced leaf and floral herbivory, as well as seed predation, in a native New Zealand mistletoe species, Alepis flavida. Plants growing on forest edges and in forest interior were compared, and effects of plant size and the neighbouring conspecific plant community were also examined. Leaf herbivory by possums was significantly greater on forest edges than in forest interior in a year of high possum damage, but not in a year with low damage levels. Insect leaf herbivory did not differ between forest edges and interior. Although equal numbers of plants on edges v. interior experienced some floral damage by a specialist caterpillar, there were significantly higher levels of damage on plants growing in the forest interior than on forest edges. Plants with floral damage were larger than plants without damage, and distance to neighbouring mistletoe plants was positively correlated with amount of floral damage, but only for plants in the interior. Significantly greater numbers of plants on edges than in the interior exhibited seed predation by the same specialist caterpillar that caused floral damage, suggesting greater fruit abortion rates in the interior. Amounts of seed damage were inversely correlated with plant size. Forest edges had much stronger effects on leaf herbivory by possums, as well as floral herbivory and seed predation, than did plant size or the neighbouring plant community.     

Insect herbivory on mangrove leaves in North Queensland*

Estimates of leaf damage by insect herbivores are presented for 25 species of mangrove plants, comprising canopy and understorey species. Leaf area loss was highly variable among the species sampled, with means ranging from 0.3 to 35.0% of expanded leaf area. There was also great variability amongst leaves within species, and the mean coefficient of variation for leaf loss from the 25 species was 266%. Of the 12 species sampled at more than one site in North Queensland, eight exhibited small, but significant, between-site differences in herbivory. In general, it did not appear that height in the canopy influenced herbivory. For the dominant mangrove forest community type at Missionary Bay, an estimated mean of 2.1% of leaf production, or 11 g m^-2 per year, entered the direct grazing pathway. This very low figure is compared with estimates from other studies on mangrove forests and estimates from a variety of Australian terrestrial forests.     

Variable strength of top‐down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO event

Abstract Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top‐down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’Nothofagus pumilio forests in the northern Patagonian Andes, Argentina. The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size. The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird‐initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above‐normal herbivory, and also support the view that large‐scale climatic events influence the strength of trophic cascades.     

Variation in Herbivore Damage to Invasive and Native Woody Plant Species in Open Forest Vegetation on Mahé, Seychelles

Enemy release of introduced plants and variation in herbivore pressure in relation to community diversity are presently discussed as factors that affect plant species invasiveness or habitat invasibility. So far few data are available on this topic and the results are inconclusive. We compared leaf herbivory between native and invasive woody plants on Mahé, the main island of the tropical Seychelles. We further investigated variation in leaf herbivory on three abundant invasive species along an altitudinal gradient (50–550 m a.s.l.). The median percentage of leaves affected by herbivores was significantly higher in native species (50%) than in invasive species (27%). In addition, the species suffering from the highest leaf area loss were native to the Seychelles. These results are consistent with the enemy release hypothesis (ERH). While the invasive species showed significant and mostly consistent variation in the amount of leaf damage between sites, this variation was not related to general altitudinal trends in diversity but rather to local variation in habitat structure and diversity. Our results indicate that in the Seychelles invasive woody plants profit from herbivore release relative to the native species and that the amount of herbivory, and therefore its effect on species invasiveness or habitat invasibility, may be dependent on local community structure and composition.     

Patterns of leaf-pathogen infection in the understory of a Mexican rain forest: incidence, spatiotemporal variation, and mechanisms of infection

This study assessed the levels of damage by leaf pathogens and their variability in terms of host species, space (four mature forest sites) and season of the year (dry and rainy), and the mechanisms of infection in the understory of the Los Tuxtlas tropical rain forest. Sixty-five percent of the species surveyed in the dry season (N = 49) and 64.9% of those surveyed in the rainy season (N = 57) were damaged by fungi. Leaf area damaged per plant, on average, was <1% (range: 0.25-20.52%). There was considerable variation in the degree of infection among species, but not among sites and seasons. The survey showed that 43% of the leaves were damaged by herbivores and pathogens concurrently, 16% showed damage by insect herbivory alone, and only 1.4% of the sampled leaves showed damage by pathogens alone. Pathogenicity assays experimentally confirmed that the predominant mechanism of fungal establishment was wounding, such as that caused by herbivory (or other similar sources), and only rarely did infection occur through direct contact (without wounds). The results revealed the omnipresence of leaf fungal infection, although with low damage per plant, and the importance of herbivorous insects in the facilitation of fungal infection in tropical understory plants.     

Leaf compensatory growth as a tolerance strategy to resist herbivory in Pancratium sickenbergeri

We examined the effects of leaf herbivory by the dorcas gazelle, Gazella dorcas, on the compensatory growth of the geophyte Pancratium sickenbergeri (Amaryllidaceae) in the Negev desert, Israel. In three populations exposed to different levels of herbivory, we removed different amounts of photosynthetic leaf area from plants in five clipping treatments: 0, 25, 50%-dispersed over all leaves, 50%-entire area of half the leaves, 100%. The population with the lowest level of herbivory showed the lowest relative regrowth rate after clipping. In the population with a constantly high level of herbivory, plants in intermediate-clipping treatments overcompensated in leaf area after clipping. For all the populations, clipped plants produce more new leaves than unclipped plants. In the population with the highest level of herbivory, clipping treatments did not have a significant effect on the number of fruits per plant. In addition, we did not find a trade-off between investments in growth and reproduction in this population. Our results indicated that, in the desert lily, herbivores may select for plant mechanisms that compensate after damage as a tolerant strategy to maintain fitness.     

Deciduous leaf drop reduces insect herbivory

Deciduous leaf fall is thought to be an adaptation that allows plants living in seasonal environments to reduce water loss and damage during unfavorable periods while increasing photosynthetic rates during favorable periods. Observations of natural variation in leaf shedding suggest that deciduous leaf fall may also allow plants to reduce herbivory. I tested this hypothesis by experimentally manipulating leaf retention for Quercus lobata and observing natural rates of herbivory. Quercus lobata is primarily deciduous although individuals show considerable natural variation in leaf retention. Oak saplings with no leaves through winter experienced reduced attack by cynipid gall makers the following spring. This pattern was consistent with the positive correlation between natural leaf persistence and gall numbers. These cynipids do not overwinter on the leaves that trees retain through winter, although they appear to use persistent leaves as oviposition cues. If these results are general for woody plants in continental temperate habitats, they suggest that an important and unrecognized consequence of deciduous leaf shedding may be a reduction in herbivore damage, and that this effect should be included in models of deciduous and evergreen behavior.     

Canopy Herbivory and Insect Herbivore Diversity in a Dry Forest–Savanna Transition in Brazil

This study aimed to compare canopy herbivore diversity and resultant insect damage to vegetation in two distinct and adjacent ecosystems, specifically a dry forest ecosystem and a cerrado (savanna) ecosystem that occur together in an abrupt transition zone in southeastern Brazil. In the dry forest, the canopy was reached using a single rope climbing technique, whereas the shorter canopy of the cerrado was assessed using a 7 m ladder. Insect specimens were collected by beating the foliage, and 20 representative leaves were collected to calculate the specific leaf mass (SLM) and leaf area loss through herbivory. Also, we collected ten soil samples from each habitat to determine soil nutrient content. We sampled 118 herbivorous insects from ten families, mostly in dry forest trees (96 individuals belonging to 31 species). A higher abundance of chewing and sap-sucking insects were observed in dry forest trees than in cerrado trees. The same pattern was observed for the richness of chewers, with a higher degree of diversity of chewers found in dry forest trees than in cerrado trees. Herbivorous insects were not affected by SLM regardless of guild and habitat. However, we observed a negative correlation between the herbivory rate and the specific leaf mass (SLM). The cerrado trees showed a higher SLM and lower herbivory rates than trees occurring in the dry forest. These results suggest that herbivory rates in the transition dry forest–cerrado may be driven by soil nutrient content, which is thought to influence leaf sclerophylly.
Abstract in Portuguese is available at http://www.blackwell-synergy.com/loi/btp .     

Herbivore and pathogen effects on tree growth are additive,but mediated by tree diversity and plant traits

Herbivores and fungal pathogens are key drivers of plant community composition and functioning. The effects of herbivores and pathogens are mediated by the diversity and functional characteristics of their host plants. However, the combined effects of herbivory and pathogen damage, and their consequences for plant performance, have not yet been addressed in the context of biodiversity–ecosystem functioning research. We analyzed the relationships between herbivory, fungal pathogen damage and their effects on tree growth in a large‐scale forest‐biodiversity experiment. Moreover, we tested whether variation in leaf trait and climatic niche characteristics among tree species influenced these relationships. We found significant positive effects of herbivory on pathogen damage, and vice versa. These effects were attenuated by tree species richness—because herbivory increased and pathogen damage decreased with increasing richness—and were most pronounced for species with soft leaves and narrow climatic niches. However, herbivory and pathogens had contrasting, independent effects on tree growth, with pathogens decreasing and herbivory increasing growth. The positive herbivory effects indicate that trees might be able to (over‐)compensate for local damage at the level of the whole tree. Nevertheless, we found a dependence of these effects on richness, leaf traits and climatic niche characteristics of the tree species. This could mean that the ability for compensation is influenced by both biodiversity loss and tree species identity—including effects of larger‐scale climatic adaptations that have been rarely considered in this context. Our results suggest that herbivory and pathogens have additive but contrasting effects on tree growth. Considering effects of both herbivory and pathogens may thus help to better understand the net effects of damage on tree performance in communities differing in diversity. Moreover, our study shows how species richness and species characteristics (leaf traits and climatic niches) can modify tree growth responses to leaf damage under real‐world conditions.     

Interannual changes in folivory and bird insectivory along a natural productivity gradient in northern Patagonian forests

Trophic regulation models suggest that the magnitude of herbivory and predation (top-down forces) should vary predictably with habitat productivity. Theory also indicates that temporal abiotic variation and within-trophic level heterogeneity both affect trophic dynamics, but few studies addressed how these factors interact over broad-scale environmental gradients. Here we document herbivory from leaf-feeding insects along a natural rainfall/productivity gradient in Nothofagus pumilio forests of northern Patagonia, Argentina, and evaluate the impact of insectivorous birds on foliar damage experienced by tree saplings at each end of the gradient. The study ran over three years (1997–2000) comprising a severe drought (1998–1999), which allowed us to test how climatic events alter top-down forces. Foliar damage tended to increase towards the xeric, least productive forests. However, we found a predictable change of insect guild prevalence across the forest gradient. Leaf miners accounted for the greater damage recorded in xeric sites, whereas leaf chewers dominated in the more humid and productive forests. Interannual folivory patterns depended strongly on the feeding guild and forest site. Whereas leaf-miner damage decreased during the drought in xeric sites, chewer damage increased after the drought in the wettest site. Excluding birds did not affect leaf damage from miners, but generally increased chewer herbivory on hydric and xeric forest saplings. Indirect effects elicited by bird exclusion became most significant after the drought, when total folivory levels were higher. Thus, interannual abiotic heterogeneity markedly influenced the amount of folivory and strength of top-down control observed across the forest gradient. Moreover, our results suggest that spatial turnovers between major feeding guilds may need be considered to predict the dynamics of insect herbivory along environmental gradients.     

亚热带季风常绿阔叶林原始林和次生林植物虫食的研究

亚热带季风常绿阔叶林原始林的日间最高气温、昼夜温差和林间光强较次生林低。原始林叶片受虫食的Ⅱ、Ⅲ和Ⅳ级的比例分别为16.3％、15.4％和12.2％;次生林相应等级比例分别为21.4％,24.3％。和17.7％。两林地植物老叶和成热叶受虫食的频度较幼叶的高。叶片受虫食与光强有关,与叶片含氮量没有直接关系。     

Potential impacts of climate change on patterns of insect herbivory on understorey plant species: A transplant experiment

Species‐specific responses to climate change will lead to changes in species interactions across multiple trophic levels. Interactions between plants and their insect herbivores, in particular, may become increasingly disrupted if mobile herbivores respond more rapidly to climatic change than their associated host plants. We present a multispecies transplant experiment aimed at assessing potential climatic impacts on patterns of leaf herbivory. Four shrubby understorey plant species were transplanted outside their native range into a climate 2.5°C warmer in annual mean temperature. After 12 months, we assessed the types and amount of herbivore leaf damage, compared with plants transplanted to a control site within their native range. The overall amount of foliage loss to herbivores ranged from approximately 3–10% across species and sites, a range consistent with most estimates of leaf loss in other studies. The most common types of leaf damage were sucking and chewing and this pattern was consistent for all four plant species at all sites. There were no significant differences in levels and patterns of herbivory between control and warm sites for three out of four plant species. This suggests that with moderate climate warming, most herbivory will continue to be dominated by chewers and suckers, and that the overall level of foliage loss will be similar to that experienced presently.     

Concentration of glucosinolates in relation to habitat and insect herbivory for the native crucifer Cardamine cordifolia

Understanding plant-insect interactions requires further data on herbivory in relation to the variation in concentration of characteristic secondary compounds. We report here analyses of the glucosinolate contents for a native perennial, montane crucifer Cardamine cordifolia in relation to: (a) plant characteristics; (b) insect herbivory; and (c) habitat. The only pattern of variation of glucosinolate content with leaf characteristics found was an inverse correlaton between leaf weight and total isothiocyanate-yielding glucosinolates (IYG) in shaded plants. There was a highly significant, negative relationship between total IYG and leaf damage by insects, particularly in typical shaded habitats. Higher insect-caused damage on denser, smaller leaves of plants from the driest soils was observed. Additionally, plants occurring in sun-exposed habitats from the beginning of the growing season, both naturally and experimentally, had similar (or lower) concentrations of total IYG, and were significantly more damaged by insects, than those in the more usual shaded habitats. The experimental removal of shade cover in mid-season resulted in significantly elevated quantities of total IYG in the first year, with a relaxation of that stress-induced response in the second year. We suggest that the insect herbivore guild on Cardamine cordifolia responds to concentration and composition of glucosinolates and exerts its greatest pressure on plants with lower concentrations. Differential herbivory, consumption mediated in part by glucosinolate concentration, appears to contribute to microhabitat occurrence of C. cordifolia.     

Microclimate and tree community linked to differences in lepidopteran larval communities between forest fragments and continuous forest

Aim  We aim to assess the impact of forest fragmentation on lepidopteran larval community and study the associations of microclimate and tree community with lepidopteran assemblage.
Location  Kibale National Park, Uganda.
Methods  We investigated the effects of forest fragmentation on leaf herbivory, density of lepidopteran caterpillars, species richness and diversity as well as the composition of herbivorous lepidopteran larval community. Microclimate, size of the fragment, distance to the continuous forest, and tree diversity were studied as possible explanatory factors. We sampled 10 Neoboutonia macrocalyx Pax. (Euphorbiaceae) trees in each fragment during dry and rainy season, total of four times, in a year to cover the seasonal variation.
Results  The rates of herbivory, total larval density and species richness were significantly lower in the forest fragments than in the continuous forest but species diversity expressed as Fisher's alpha did not differ. The dominance structure and community composition of the larval communities in the fragments was different from that of the continuous forest. None of the differences we observed were related to the fragment area or distance to the continuous forest. Instead, we found an indication of association between the herbivore and the tree communities. The fragments had significantly lower humidity during most of the day and higher temperature during the afternoons (14–17 h), which might partially explain the differences in lepidopteran larval communities.
Main conclusions  Decreased larval density and species richness as well as differences in the community composition and structure all highlight the importance of large continuous forest areas for maintaining larval biodiversity.     

The Seedling Community of Tropical Rain Forest Edges and Its Interaction with Herbivores and Pathogens1

Edge effects alter biotic interactions and forest regeneration. We investigated whether edge creation affected the seedling community and its interactions with herbivores and leaf‐fungal pathogens. In forest edges and interior sites in Chiapas, Mexico, we counted all woody seedlings and species (10–100 cm tall) present in 1‐m² plots, measured their size (height and leaf number), and examined them for the occurrence of herbivory and pathogen damage. We investigated relationships between levels of damage and size, species richness and density. Species composition and biotic damage varied greatly among sites and habitats (edge vs interior). Late‐successional species dominated the community, but richness was lower in interior sites and species similarity was greater among edges than among interiors. Nearly all species (95%) present at edges and interiors showed herbivory damage, whereas 76 percent of the species in edge plots and 68 percent in interior plots showed pathogen damage. Although leaf area damaged by herbivores was similar between habitats (average 9.2%), pathogen damage was three times greater in edge plots (1.85%) than in interior plots (0.57%). Size was positively related to biotic damage at both habitats. Relationships between herbivory and pathogen damage and between pathogen damage and leaf number were significant only for edges. Biotic damage was not related to density or species richness. Overall, plant community structure was similar between habitats, but biotic damage was enhanced at edges. Thus, disease spread at edges may arise as a threat to tropical rain forest vegetation.     

Seasonality of insect herbivory on the leaves of Neoboutonia macrocalyx in the Kibale National Park, Uganda

The seasonality of herbivory on the leaves of Neoboutonia macrocalyx Pax. in Kibale Forest National Park, Uganda, was studied. A total of 2929 fallen leaves was collected during 15 months under randomly-selected trees in three different habitats; natural forest and two selectively cut forest sites. The percentage of leaf area eaten and leaf size were estimated. Leaf herbivory was highly seasonal and correlated with rainfall in the previous 2 months, but less than 100 mm monthly rainfall had no effect. There was no correlation between leaf size and rainfall. Although Kibale Forest has two wet seasons, insect feeding on leaves had only one peak during the major rainy season from September to December. Three to four months after peak herbivory, leaves had very low rates of insect damage. Habitat had only a small effect on the amount of insect feeding. The sampling time accounted for 71% of variation in leaf herbivory. New leaves were formed continuously year-round. The constant leaf production by Neoboutonia trees may be an adaptation to escape generalist herbivorous insects which might be synchronized with the major wet season when the leaf flush of the most other deciduous species occurs. Thus, the availability of fresh leaves is not acting as a regulating factor in seasonality of Neoboutonia herbivory.     

The alkaloidal responses of wild tobacco to real and simulated herbivory

Summary I compared the induced alkaloidal response in undamaged leaves of plants subjected to herbivory by the larvae of Manduca sexta and to different simulations of this herbivory; all herbivory treatments removed similar amounts of leaf mass. Although larval feeding induced a significant increase (2.2x) in alkaloid concentrations compared to undamaged plants, the alkaloid responses to larval feeding were significantly lower than the responses to an herbivory simulation (4x controls) which involved removing the same amount of leaf area from the same positions on the leaf, over a similar time period. Moreover, another herbivory simulation, identical in amount of leaf mass removed and duration of damage to the larval feeding, but without regard to spatial array of leaf damage, resulted in an alkaloidal response (5.5x controls) higher still than the previous herbivory simulation. In a second experiment the importance of leaf vein damage on the induced alkaloidal response was examined. Here, leaf removal that involved cutting leaf tissues from between secondary veins before removing the midrib, resulted in alkaloidal responses that were significantly lower (1.7x controls) than responses from leaf removal that involved cutting both veins and midribs along with the intervein tissues (2.6x controls). Vein damage alone did not produce a significant response. These results indicate that herbivory is difficult to simulate: that how a leaf is damaged can be as important as the magnitude of leaf damage in determining a plant's response to damage.     

Differential Responses of Herbivores and Herbivory to Management in Temperate European Beech

Forest management not only affects biodiversity but also might alter ecosystem processes mediated by the organisms, i.e. herbivory the removal of plant biomass by plant-eating insects and other arthropod groups. Aiming at revealing general relationships between forest management and herbivory we investigated aboveground arthropod herbivory in 105 plots dominated by European beech in three different regions in Germany in the sun-exposed canopy of mature beech trees and on beech saplings in the understorey. We separately assessed damage by different guilds of herbivores, i.e. chewing, sucking and scraping herbivores, gall-forming insects and mites, and leaf-mining insects. We asked whether herbivory differs among different forest management regimes (unmanaged, uneven-aged managed, even-aged managed) and among age-classes within even-aged forests. We further tested for consistency of relationships between regions, strata and herbivore guilds. On average, almost 80% of beech leaves showed herbivory damage, and about 6% of leaf area was consumed. Chewing damage was most common, whereas leaf sucking and scraping damage were very rare. Damage was generally greater in the canopy than in the understorey, in particular for chewing and scraping damage, and the occurrence of mines. There was little difference in herbivory among differently managed forests and the effects of management on damage differed among regions, strata and damage types. Covariates such as wood volume, tree density and plant diversity weakly influenced herbivory, and effects differed between herbivory types. We conclude that despite of the relatively low number of species attacking beech; arthropod herbivory on beech is generally high. We further conclude that responses of herbivory to forest management are multifaceted and environmental factors such as forest structure variables affecting in particular microclimatic conditions are more likely to explain the variability in herbivory among beech forest plots.