Food selection by beavers

Summary Multidimensional contingency tables provide a powerful new statistical tool for analyzing interactions among several variables in an ecological study. This technique is applied to a study of selectivity in tree-cutting by two colonies of beavers in central Massachusetts.At the Blue Heron Cove colony, selection depends jointly on genus and diamter. Beavers cut birch of all diameters available but avoid large diameter maples, pines, and oaks. These beavers are choosy generalists: they show clear preferences in cutting various genera of trees for food yet they cut substantial numbers of trees of non-preferred genera. At the Tamplin Road Pond colony, discrete sites of concentrated cutting activity differ in genera and diameters of trees selected. Trees of all diameters are cut at one site close to water, small diameter trees are selected at two other sites farther from water. Ironwood is preferred at one site but selected against at two other sites. This difference between sites in generic selectivity has two plausible explanations: (1) tree species differ in nutritional and other chemical value between sites, (2) beavers sample trees of the various species present at some sites in order to assess the value of such sites as foraging areas.     

Seasonal and year-to-year differences in food selection by beavers

Summary Selection of trees for food by a colony of beavers in central Massachusetts was studied from September 1972 through April 1974. The beavers exhibited both seasonal and year-to-year differences in preference for certain genera. Pine was selected against during fall but not spring, and there was a switch in preference from birch during fall 1972 to oak and witch hazel in fall 1973. These differences may partly reflect greater seasonal stability in concentrations of stored nutrients in coniferous tree bark than in deciduous tree bark, and greater year-to-year stability in bark concentrations of stored nutrients in non-mast-seeding species than in mast-seeding species (1972 was a mast year for oaks in central Massachusetts).     

Foraging decisions of North American beaver (<Emphasis Type="Italic">Castor canadensis</Emphasis>) are shaped by energy constraints and predation risk

Foraging herbivores have to trade-off between energy requirements and predator avoidance. We aimed to study the relative roles of these factors in beavers (Castor canadensis) when foraging on land. We hypothesized that beavers were able to assess the risk of predation by using two main cues: the distance from the water and the presence or absence of predator odors. First, we studied the food selection of beavers in relation to distance from the water in natural settings. The transects were made at beaver ponds, and the diameter, species, and distance from the shore of intact and beaver-cut trees were recorded. Secondly, we placed rows of aspen sticks (Populus tremula) perpendicular to the shore around beaver ponds, and treated each row with a neutral, alien, or wolf odor. We found that aspen, downy birch (Betula pubescens), and speckled alder (Alnus incana) were the preferred tree species. More of these species were cut close to the shore, and cut trees were smaller further away from the shore, except in the case of aspen. In the experiment, most of the aspen sticks were taken close to the shore. As predicted, beavers took less aspen sticks in rows treated with wolf odor than water. As the predator odor did not affect the foraging distance from the shore, it is likely that our observation that foraging was the most intense close to shore is due to energetic constraints. However, predation risk probably affects the decision whether to forage on the land in the first place.     

Effect of nitrogen fertilization upon the secondary chemistry and nutritional value of quaking aspen (Populus tremuloides Michx.) leaves for the large aspen tortrix (Choristoneura conflictana (Walker))

Summary We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.     

Long-term persistence of aspen – a key host for many threatened species – is endangered in old-growth conservation areas in Finland

Large, dead and dying European aspens (Populus tremula L.) host many threatened species in Fennoscandian boreal forests. Large aspen trees have mostly disappeared and are being harvested from the managed forests that cover 95% of the forest area in Finland. Due to the small area protected (4.1%), the aspen-associated species may encounter major difficulties in the protected areas if aspen trees disappear due to natural forest succession. The availability of aspens was assessed in the old-growth conservation area network in eastern Finland. We mapped all the living and dead aspens in 15 protected old-growth forests. The total number of counted trees was 32 903 individuals. Current amounts of living (2.7 m³/ha) and especially dead aspens (2.8 m³/ha) in the protected areas were higher than in the surrounding managed forests (1.1 and 0.1 m³/ha for living and dead trees, respectively). However, while saplings (dbh<5 cm) occur in most of the areas (12 individuals/ha on average) they survive poorly and young aspen cohorts (5 cm <dbh<15 cm) are lacking or are very rare. The most likely reason for the poor sapling survival is high browsing pressure by the mammalian herbivores, especially the moose. The moose population has increased many times in Finland during the past decades. The poor regeneration of aspens implies that the value of the old-growth conservation areas for aspen-associated species will face a serious bottleneck within a few decades when the currently middle-aged tree cohorts disappear. If the current high browsing pressure and lack of natural disturbances continue the obligatory aspen-associated species may disappear both locally and regionally from the network of the protected areas.     

Hybrid aspen is not preferred by the large poplar borer (Saperda carcharias)

An industrially valuable tree, aspen, suffers from several fungal diseases and insect pests such as the large poplar borer (Saperda carcharias). The role of the beetle as a pest of aspen and hybrid aspen was investigated in five aspen and three hybrid aspen stands in Finland. Approximately 50% of the trees in the study had signs of the large poplar borer. Of these trees, 5% had insect exit holes. Approximately 70% of the trees with larval galleries had only one or two larval galleries. The trees with larval galleries were on average 2 m shorter than those without larval galleries. No significant difference could be detected in the diameter growth between these two groups. The proportion of decay was greater in hybrid aspen (27%) than in aspen (14%). These results show that the large poplar borer is an important pest of both aspen and hybrid aspen in Finland. No significant difference was observed between the aspen and hybrid aspen stands in the number of trees with larval galleries. Hybrid aspen therefore does not appear to be more sensitive to damage caused by the large poplar borer. Thus, a change from hybrid aspen to regular aspen in aspen cultivation is unnecessary, presuming that healthy saplings and an optimal rotation time are used.     

Dynamics of food resources for beavers in settlements colonized and abandoned several times

The state of food resources for Eurasian beavers (Castor fiber Linnaeus, 1758) after several cycles of habitat colonization and abandonment has been analyzed in three beaver settlements on small rivers of Novgorod oblast. It has been shown that tree stands surrounding the settlements differ in composition and degree of damage but are similar in that they contain no forage for beavers: aspen is absent, and birch trees are large and require great efforts to cut them down and utilize. The undergrowth and understory also contain no aspen, while abundant willows and birch are represented by young, thin-stemmed plants unevenly distributed over the area. Tree species prevailing in abandoned settlements are not favored by beavers, implying that the animals must have expended extra energy for foraging. Supposedly, populations of beavers on small rivers in the southern taiga subzone, in habitats modified by their activities, will maintain their abundance far below the maximum possible level, depending on the rate of forage plant regrowth against the background of deteriorating habitat quality.     

Differential responses of tiger swallowtail subspecies to secondary metabolites from tulip tree and quaking aspen

Summary Two subspecies of the eastern tiger swallowtail butterfly, Papilio glaucus, exhibit reciprocal inabilities to survive and grow on each other's preferred foodplant. P. g. canadensis R. & J. performs well on quaking aspen (Populus tremuloides Michx.) but not on tulip tree (Liriodendron tulipifera L.); P. g. glaucus L. performs well on tulip tree but not on quaking aspen. This study was designed to test the hypothesis that secondary metabolites in tulip tree and quaking aspen are responsible for these differential utilization abilities. We extracted and fractionated leaf constituents into different chemical classes, applied them to a mutually acceptable diet (black cherry, Prunus serotina, leaves), and bioassayed them against neonate larvae (survival) and penultimate instar larvae (survival, growth, digestibility and conversion efficiencies). For each plant species, one fraction in particular showed activity against the unadapted subspecies. One tulip tree fraction dramatically reduced survival of P. g. canadensis neonates, and reduced consumption rates, growth rates, and ECI's of fourth instar larvae. The tulip tree constituents most likely responsible for these effects are sesquiterpene lactones. One quaking aspen fraction greatly lowered survival of P. g. glaucus neonates, and decreased survival, consumption rates, growth rates and ECD's of fourth instar larvae. The compounds responsible for these results are probably simple phenols or phenolic glycosides. Surprisingly, P. g. glaucus and P. g. canadensis showed slightly poorer performance on the active tulip tree and quaking aspen fractions, respectively, indicating that even adapted insects incur a metabolic cost in the processing of their host's phytochemicals.     

Comparative suitability of binocular observation, burrow counting and excavation for the quantification of the mangrove fiddler crab Uca annulipes (H. Milne Edwards)

Insect emergence from three different sections of a small mountain brook in Germany was studied: a free-flowing section (`brook'), a section impounded by beavers (`beaver pond') and a beaver dam. The three sections had very different faunas with that of the dam being more similar to that of the brook than that of the pond. The heterogeneity of the fauna was greatest in the dam and lowest in the pond. Median emergence density in the dam was 443 specimens m^–2 d^–1. This was 3.2 times higher than median emergence density in the brook section and 5.5 times higher than median emergence density in the pond section. In particular, the density of emerging Trichoptera (especially filter-feeding species) was increased. Mean annual number of emerging EPTC species was 32.3/trap (779 cm² ground area) in the dam, 18.4/trap in the brook and 11.0/trap in the pond. Emerging biomass was highest in the dam (18.0 – 26.7 g m^–2 yr^–1; 5.0 times higher than in the free flowing section and 5.4 times higher than in the pond section). The composition of a recently broken beaver dam is described by measuring all pieces of wood used in building the dam and analysing sediment samples. The dam consisted of a framework of wood pieces almost entirely of a diameter <5 cm, in which cavities are partly filled with fine sediment (mean organic content = 20%). The `inner surface' of the dam was 2.09 times higher than the dam's ground area and 1.63 times higher than the dam's surface. Beavers, therefore, significantly increase the heterogeneity of faunal composition and productivity of small brooks.     

叶损伤诱导兴安落叶松针叶中10种酚酸的变化

酚酸是一类重要次生抗虫物质.为研究损伤及昆虫取食诱导对兴安落叶松针叶内酚酸含量的影响,采用3种不同程度剪叶或落叶松毛虫幼虫取食处理兴安落叶松幼树,以高效液相色谱技术测定兴安落叶松健康针叶中酚酸含量.结果表明：与对照相比,处理后1 d,剪叶或昆虫取食4枝50%针叶处理的兴安落叶松幼苗健康针叶中,除阿魏酸无显著差异外,苯甲酸、咖啡酸、绿原酸、水杨酸、苯乙酸、肉桂酸、香草酸、丁香酸和没食子酸9种酚酸均差异显著;4枝75%针叶处理的10种酚酸含量均发生显著变化.说明剪叶及虫害50%、75%针叶处理均达到诱导阈值,能显著诱导兴安落叶松化学防御.在损伤程度相同情况下,处理1 d时,剪叶4枝50%、75%诱导的咖啡酸、苯乙酸、肉桂酸、香草酸和没食子酸的含量显著高于虫害诱导处理;5 d时,剪叶4枝50%、75%诱导处理的这5种酚酸含量显著低于虫害诱导处理;10 d时,两种方法诱导的酚酸含量差异不显著.说明剪叶诱导处理的酚酸含量变化比昆虫取食处理迅速,且诱导强度与剪叶程度相关.采用适当处理诱导针叶中酚酸含量的变化来增强兴安落叶松对害虫的防御能力是可行的.     

Bark structure and mode of canopy regeneration in trees of Melia azedarach L.

Summary The bark texture of Melia azedarach L. changes from smooth to furrowed as trees age. In trees that were cut down, those with smooth bark sprouted below the cut from suppressed buds; trees with thick, furrowed bark sprouted at the edge of the cut surface from adventitious buds. The trees that had thin, furrowed bark sprouted mainly at the edge of the cut from adventitious buds, but sometimes also from suppressed buds in cracks. The relationship between sprouting pattern and tree architecture are discussed.     

Selective foraging on woody plant species by the Eurasian beaver (Castor fiber) in Telemark, Norway

Beavers Castor spp. are generalist herbivores, feeding on the bark, shoots and leaves of woody plants, terrestrial herbs and forbs, ferns and aquatic vegetation. As central-place foragers, beavers move out from water to select and cut trees and vegetation, and then transport it back to their refuge. These terrestrial forays are energetically costly; therefore, beavers should concentrate their foraging activity near the central place and increase the degree of selectivity for specific sizes or species of food with increasing distance from the water. The aim of this study was to test the predictions of the central place and the optimal foraging theories for the food selection of the Eurasian beaver Castor fiber , and show the foraging preferences of the focal species in the boreal conifer forest zone of Europe. Foraging intensity by beavers and the abundance of woody species were surveyed in transects positioned randomly at seven beaver territories. In compliance with the central-place foraging theory, the foraging intensity declined with increasing distance from the river. Beavers fed preferentially on willows ( Salix ), rowan ( Sorbus ) and birches ( Betula ), although alders ( Alnus ) dominated their diet. Size selectivity showed similar patterns to previous North American studies, which were also carried out in habitats with predominantly small saplings. The probability of selection of small saplings dropped as distance increased, which is consistent with the predictions of optimal foraging models that larger prey items are more likely to be favoured with increasing provisioning distance.     

The transformation of Saperda calcarata (Coleoptera: Cerambycidae) into a cellulose digester through the inclusion of fungal enzymes in its diet

Summary The larvae of the aspen borer, Saperda calcarata, which feed on the inner bark and sapwood of living aspen stems, are unable to digest cellulose. However, they can be transformed into cellulose digesters by adding the active cellulase complex of the fungus, Penicillium funiculosum to their diet. S. calcarata larvae are preadapted to exploit the digestive potential of ingested microbial enzymes. We argue that ingested fungal enzymes may be responsible for cellulose digestion in many, perhaps most or even all, cellulose digesting cerambycid beetles.     

Ophiostomatoid fungi isolated from Japanese red pine and their relationships with bark beetles

We isolated ophiostomatoid fungi from bark beetles infesting Pinus densiflora and their galleries at 24 sites in Japan. Twenty-one ophiostomatoid fungi, including species of Ophiostoma, Grosmannia, Ceratocystiopsis, Leptographium, and Pesotum, were identified. Among these, 11 species were either newly recorded in Japan or were previously undescribed species. Some of these fungal species were isolated from several bark beetles, but other species were isolated from only a particular beetle species. Thus, it is suggested that some ophiostomatoid fungi have specific relationships with particular beetle species. In addition, fungus-beetle biplots from redundancy analysis (RDA) summarizing the effects of beetle ecological characteristics suggested that the association patterns between bark beetles and the associated fungi seemed to be related to the niches occupied by the beetles.     

Influence of breeding conditions on longevity and fecundity ofRaphitelus maculatus [Hym.: Pteromalidae] reared under standard laboratory conditions

The rearing ofRaphitelus maculatus Walker, which oviposits through the bark onto beetle larvae has been carry out under standard laboratory conditions of 22 ± 2°C, 60% R.H. and photoperiod 16L∶8D. The host (Phloeotribus scarabaeoides Bernard) has been reared by cutting fresh branches of living olive tree on which they overwinter as adults. The effect of several rearing conditions (feeding conditions) on longevity and fecundity has been tested. The time of parasitoid and host development as well as the periods of emergence from the infested cut branches has been determined. It has been noted that both longevity and fecundity are influenced by the feeding conditions, reaching toca. double value when parasitoids were fully fed. Under standard rearing conditionsR. maculatus completes its life cycle in 17–18 days depending on sex, whereasP. scarabaeoides takes about 48 days. Finally 3 emergence periods has been observed from which the 1^st is the most important and the 3^rd is always the less significantly one, provided that the emerging brood cannot oviposits easier than the parents due to a decrease on host larvae proportion as well as on parasitoid fecundity.      

Vegetative response of Arctostaphylos uva-ursi to experimental cutting and burning

Arctostaphylos uva-ursi uses vegetative resprouting to recover rapidly after disturbances. This study aims to widen knowledge of its regeneration response, so it was subjected to experimental cutting and burning. Fifteen pairs of small circular plots (50 cm diameter) were situated at random in an area where this species is abundant and forms homogeneous carpets with close to 100% cover. One plot of each pair was burned and the other one was cut. Initial biomass was estimated as dry weight from the cut plots. A 50 cm diameter circular sampling quadrat, divided into two concentric circles, was used in order to determine cover recovery and the way in which this occurred (predominantly by colonization from outside or uniformly over the whole surface). Initial recovery was very fast after both disturbances, exceeding 30% cover from the fourth month. Recovery of aboveground biomass was also similar on comparing both types of disturbance, but statistically significant differences with respect to initial values were detected.     

Age-dependent bark photosynthesis of aspen twigs

The photosynthetic performance of trembling aspen (Populus tremula L.) twigs and leaves was studied in relation to selected structural features of aspen bark. PFD transmittance of intact periderm was reduced by about 90% in current-year twigs through peridermal thickening. However, because of drastic changes within the bark microstructure, PFD transmittance increased in 1-year-old twig segments up to 26% of the incident PFD. On a unit surface area basis, the chlorophyll content of young twigs (425 mg Chl m^-2) almost reached that of leaves (460 mg Chl m^-2). The chlorophyll content of aspen bark chlorenchyma was clearly age-dependent, even increasing in current-year twigs with advancing internodal age. The low bark chlorophyll a/b ratios (about 2.6 compared with 3.9 in leaves) indicate that bark chloroplasts are shade-adapted. Positive net photosynthesis was not found in aspen twigs, but apparent respiration was distinctly reduced in the light due to light-driven carbon refixation (bark photosynthesis) within the chlorenchymal tissues. Under constant microclimatic conditions, dark respiration rates were strongly correlated with stem-internal CO₂ refixation. In accordance with increasing dark respiration rates, the efficiency of this carbon recycling was generally greater in the metabolically more active, younger twig segments than in older segments; carbon refixation rates reached up to 80% of dark respiration values. At least in young twigs and branches and thus in the light-exposed outer parts of tree crowns, respiratory CO₂ losses by the tree skeleton could efficiently be reduced. Refixation of carbon dioxide may be of great importance for carbon budgets in the environmentally controlled or pathogen-induced leafless states of deciduous aspen trees.     

Patterns in the within-tree distribution of the emerald ash borer Agrilus planipennis (Fairmaire) in young, green-ash plantations of south-western Ontario, Canada

1 The emerald ash borer Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) is a serious exotic pest of ash trees (Fraxinus spp.) in North America, and is responsible for the deaths of millions of trees in Ontario and Michigan. One of the greatest challenges facing the successful management of the pest is the ability to accurately detect its presence in a tree. 2 Observations were made on A. planipennis larval feeding galleries found within 65 young, green‐ash trees cut from plantations in Essex County, Ontario, Canada. The within‐tree distributions of feeding galleries were described in relation to height‐above‐ground, stem diameter, bark thickness and stem aspect. 3 Galleries were not distributed randomly or evenly; minimum boundaries of stem diameter and bark thickness and a maximum boundary of height‐above‐ground were detected. Indications of maximum boundaries for stem diameter and bark thickness were also observed. Galleries were found most often on the south‐west side of the tree. 4 Using the technique of upper boundary regression, we were able to identify significant quadratic relationships between A. planipennis gallery density and stem diameter and bark thickness, as well as a significant negative linear relationship between gallery density and height‐above‐ground. 5 Agrilus planipennis gallery density in newly‐infested trees was lower than in previously‐infested trees, and was observed to peak at smaller stem diameters and bark thicknesses than in previously‐infested trees. 6 Survey teams would increase their probability of detecting new A. planipennis infestations by initiating searches for exit holes and feeding galleries in trunk sections and branches of approximately 7 cm in diameter.     

Bark borer Semanotus japonicus (Col., Cerambycidae) utilization of Japanese cedar Cryptomeria japonica: a delicate balance between a primary and secondary insect

Abstract: The bark borer, Semanotus japonicus (Col., Cerambycidae), attacks living Japanese cedar, Cryptomeria japonica, with newly hatched larvae mainly feeding within the inner bark of the trunk. To understand how S. japonicus is affected by host nutrition and resin flow, newly hatched larvae were introduced into stressed cedar trees. Stress was induced by either heavy pruning, stem cutting (i.e. removing the side branches and top of tree), or girdling. Larval mortality due to resin flow in the 'heavy pruning treatment' and the 'stem cutting treatment' tended to be lower than that in the untreated control cedar trees where all larvae were drowned by resin flow. Parasitism to the larval stage was observed in the stem-cutting trees, not in living trees, suggesting that S. japonicus may avoid parasitism in living cedar trees because few volatiles are produced. In the 'stem-girdling treatment', although more than 90% of the introduced larvae died due to poor nutrition below the girdle, 7.8% of the larvae above the girdle reached the adult stage. The live body weight of the adults collected from above the girdle was similar to those taken from naturally infested cedar trees. These results suggest that S. japonicus requires adequate host nutrition but that larvae are defenceless against high resin flow. Thus, S. japonicus seems to be in a transition state between being primary or secondary with respect to its attack behavior on living cedar trees.     

芳樟穗条性状对扦插成活率的影响

利用温室大棚进行芳樟容器扦插育苗试验。结果表明,芳樟穗条性状对扦插成活率的影响显著或极显著,同一枝条梢部穗条的扦插成活率显著高于中部、下部和基部;穗条长度10 cm、直径0.3 cm、带2片半叶、下切口切为斜面的穗条,扦插成活率高于其他处理。最佳穗条应是同一枝条靠梢部1～2段,长度10 cm左右、直径0.3 cm、带2片半叶、下切口成斜面的穗条。