Oecologia

Basic photosynthetic and water relations characteristics in relation to soil water availability were investigated in Amaranthus palmeri, a common C4 summer annual of the Sonoran Desert. Under conditions of high soil water availability, photosynthetic capacity exceeded 70 μmol CO2 m-2 s-1 at a leaf temperature optimum of 42°C, and photosynthesis was not light saturated at an irradiance of 2.0 mmol m-2 s-1 (400–700 nm). Leaves of A. palmeri exhibited diaheliotropic movements, allowing them to take advantage of their high photosynthetic capacity. In response to a long term drought cycle, both photosynthetic rate and leaf conductance to water vapor decreased, reaching minima at an approximate leaf water potential of-2.9 MPa. Active leaf osmotic adjustment appeared to play an important role in allowing leaves to maintain gas exchange activities down to these low leaf water potentials. The photosynthetic rate became light saturated at the lower leaf water potentials. Although the two parameters decreased in concert, the decreased photosynthetic rate was not due to increased stomatal diffusion limitations, since intercellular CO2 concentrations remained constant over the range of leaf water potentials. Instead it appeared that during the drought cycle, both intrinsic photosynthetic capacity and leaf protein content decreased as well. These results suggested a coordination of gas exchange parameters during long term drought such that the intercellular CO2 concentration remained constant.

The use of multiple host-plant species by populations of insect herbivores can result from a variety of possible ecological and behavioral mechanisms. An understanding of the foraging mechanisms determining polyphagy in relation to local ecological conditions is therefore essential to understanding the evolutionary ecology of polyphagy. Here, we evaluate patterns of host-plant use by the polyphagous caterpillar Grammia geneura (Lepidoptera: Arctiidae) in relation to host-plant availability and foraging tactics of individuals. Field surveys of caterpillar feeding and plant abundance carried out across several sites, seasons, and years showed that: (1) G. geneura consistently preferred forbs to grasses and woody plants, (2) forb-feeding was opportunistic, supporting the idea that caterpillars sample locally available host-plants, and (3) there were consistent patterns of host-plant use that were not explained by host-plant availability (electivity). An independent set of 7-h observations of 11 caterpillars showed that electivity for a subset of caterpillar-host associations could be explained by variation in the probability of initiating feeding and the average duration of feeding bouts on different hosts but not by variation in the probability of encountering different hosts, thus providing a behavioral basis for the observed variation in host-plant use. The use of detailed foraging tactics by larvae to explain host-plant use at the population level is a novel contribution of this study.

An energy budget, based entirely on field data, has been constructed for a community of three New Zealand alpine grasshopper species, Paprides nitidus Hutton, Sigaus australis (Hutton) and Brachaspis nivalis (Hutton) (Acrididae). The consumption results are compared with a previous gravimetric ingestion rate study of the same field populations. Jointly, the two studies demonstrate some very specific adaptations between grasshopper energetics and feeding behaviour. Several features of the present study are: It is concluded from survey data that New Zealand alpine grasshoppers are rarely likely to consume more than 6% of annual primary production, and that in the majority of grasslands, their consumption is unlikely to exceed 1–2%. However, percent consumption is shown to be an inadequate measure of their impact on grasslands.

Recently Poulin (1991), using published data on Canadian freshwater fishes (60 species, three families), concluded that there is no difference in parasite species numbers between solitary and social species. Nor could he associate parasite diversity to host size, age or range of distribution. Analysing Poulin's data with somewhat differing methods I reached different conclusions. Recognising that reported parasite species number per host species is a function of research effort (also noted by Poulin), I use residuals of the regression model as a measure of standardized parasite species number. First, it turned out that solitary and social salmonids differ in parasite faunal richness. That is, solitary species harbour a smaller diversity than species that school outside the breeding season. Second, host distribution range clearly correlates with the richness of total parasite fauna of the species in the three families. This observation was recently confirmed for North American freshwater fish species by Chandler and Cabana (1991). Third, in Percidae host size and age are also correlated with parasite species diversity.

Ants are known to distinguish their own nests and nestmates from all others, using colony-specific odors. Here I show that harvester ants (Pogonomyrmex barbatus) can further distinguish between two kinds of non-nestmates of the same species: neighbors and strangers. Interactions between colonies were thought to depend on the numbers of alien ants that each colony encounters on its territory. The results described here show that such interactions also depend on information about colony identity. Encounters on foraging trails with ants from neighboring colonies, deter foraging more than encounters with ants from distant ones. The history of interactions between particular pairs of colonies may have important effects on intraspecific competition for food.

Unfortunately, the given and family names of author “Mickal Houadria” was incorrectly published in the original.

Tropical rainforest disturbance and conversion are critical drivers of biodiversity loss. A key knowledge gap is understanding the impacts of habitat modification on mechanisms of community assembly, which are predicted to respond differently between taxa and across spatial scales. We use a null model approach to detect trait assembly of species at local- and landscape-scales, and then subdivide communities with different habitat associations and foraging guilds to investigate whether the detection of assembly mechanisms varies between groups. We focus on two indicator taxa, dung beetles and birds, across a disturbance gradient of primary rainforest, selectively logged rainforest, and oil palm plantations in Borneo, Southeast Asia. Random community assembly was predominant for dung beetles across habitats, whereas trait convergence, indicative of environmental filtering, occurred across the disturbance gradient for birds. Assembly patterns at the two spatial scales were similar. Subdividing for habitat association and foraging guild revealed patterns hidden when focusing on the overall community. Dung beetle forest specialists and habitat generalists showed opposing assembly mechanisms in primary forest, community assembly of habitat generalists for both taxa differed with disturbance intensity, and insectivorous birds strongly influenced overall community assembly relative to other guilds. Our study reveals the sensitivity of community assembly mechanisms to anthropogenic disturbance via a shift in the relative contribution of stochastic and deterministic processes. This highlights the need for greater understanding of how habitat modification alters species interactions and the importance of incorporating species’ traits within assessments.

The body temperature of free-ranging Andean toadsBufo spinulosus was measured either directly or radiotelemetrically during two 15-day periods at 3200 m elevation in the Mantaro Valley, Central Perú. All toads attempted to maintain their diurnal sum of body temperature within a narrow range. Consequently thermoregulatory behaviour differed according to cloud cover and precipitation. If the sky was clear, toads emerged from their hiding place and exposed themselves to solar radiation during 3–5 h in the morning. Core temperature increased up to 15° C above the air temperature in shade and reached maximum values of about 32° C. At air temperatures (in sun) exceeding 29° C, toads maintained body temperatures below 32° C by evaporative cooling. Following heliothermic heating during the moring toads retreated to the shade, thereby decreasing body temperature below air temperature. Under overcast sky toads remained exposed during the whole day displaying body temperatures at or slightly above ambient levels. Quantitative models to predict the core temperature of toads under the different weather conditions demonstrated that the substrate temperature was the main energy source accounting for 64.6–77.9% of total variance whereas air temperature was of minor importance (1.5–4.4%). The unexplained variance was probably due to evaporative cooling. The volume of urine stored into the urinary bladder of toads varied diurnally; during basking in the morning hours most bladders contained large volumes of urine, whereas during the afternoon the bladders were mostly empty. The bladder contents probably serve as water reserves during basking when evaporative water loss was high. Toads preferred sites that provided shady hiding places as well as sun-exposed bare soil within a radius of 5 m. However, they frequently changed their centers of activity and moved to other sites in 20–70 m distance after periods of 2–5 days. The helio-and thigmothermic behaviour of the Andean toad permits the maintenance of high core temperature during morning which probably increases the digestion rate and accelerate growth. Evaporative cooling and preference of shady sites were employed to regulate body temperature below the morning levels in response to the constraints of water balance. Periodic changes between thigmothermic behaviour and locomotory activity during the night maintains body temperature above air temperature and prolongs the period of food uptake.