Emergence of some caddisflies (Trichoptera) from a wooded stream in southern Ontario

Twenty-eight species of nine families of caddisflies (Trichoptera) were identified in 170 samples taken over an 8-month period from five emergence traps placed on a second-order, forested, cold-stenothermal stream on the Niagara Escarpment, Ontario, Canada.A mean of 980.9 caddisflies m^–2 of streambed was obtained over the entire sampling period. Eleven common species accounted for 92.8% of the total emergence with specific proportions ranging from 23.8% (Wormaldia moesta) to 0.11 % (Rhyacophila sp.). The use of various kinds of traps in other studies and their effects on the detection of species composition and abundance are discussed and compared with the present study.Although the distributions of all the common species were invariant over time, four species showed low to high degrees of patchiness in the streambed; the other seven common species were uniformly distributed. However, a large residual variance suggested a subtle mechanism of microhabitat selection by the larvae and (or) pupae, not detectable by even the small emergence traps used.Both sexes of 15 species, only males of 4 and only females of 9 species were collected. Eight of the eleven common species showed significant departures from a balanced sex ratio and five exhibited a protandry of from 1 to 3 weeks. Neither this study nor others have been able to establish a predictable pattern of sex ratios in Trichoptera.The emergence periods and patterns of the eleven common species are described and compared with other studies. Of these common species, one emerged in the spring, seven during the summer and three during the late summer or early fall. Ten species had a short emergence period with a distinct peak and a significantly skewed pattern. One species,Parapsyche apicalis, exhibited a prolonged emergence period, no distinct peak and a significantly platykurtotic² pattern. With the exception ofLepidostoma sp. A, the emergence patterns of the common species were unimodal.     

Primary production in Rattlesnake Springs,a cold desert spring-stream

Primary productivity and respiration were measured in Rattlesnake Springs, Washington, using the upstream-downstream diel pH-CO₂ curve and harvest methods.Daily P_g and P_n rates averaged 8.7 and 0.9, 0.6 and 0.3, and 9.3 and 1.2 g C m^–2 d^–1 for periphyton, watercress, and total community, respectively. Average photosynthetic efficiencies (%, P _n Lt^–1) were approximately 0.22 and 0.07 for periphyton and watercress, respectively. Annual community P_g was 2 700 g C m^–2 a^–1 and was highest for periphyton (2 526 g C m^–2 a^–1). Periphyton P_n (356 g C m^–2 a^–1) exceeded that of watercress (87 g C m^–2 a^–1). Community R was 2 257 g C m^–2 a^–1, and was highest for periphyton (2 170 g C m² a^–1).Desert streams appear to be enigmas in terms of their relationship between autotrophy and heterotrophy and their ability to be net importers or exporters of organic matter. The fact that they can be autotrophic and net importers of organic matter is probably related to the characteristic flash-flooding of desert streams, and emphasizes the necessity of examining these systems over more than a single annual cycle.     

Competition between laboratory populations of green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

Intra- and interspecific competition between laboratory populations of four green leafhoppers, Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature species N. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs. N. nigropictus), a tropical and a temperature (N. virescens vs. N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs. N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between two Nephotettix species. Between N. virescens and N. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. Between N. virescens and N. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population of N. virescens persisted at a slightly higher density over the population of N. cincticeps. Between the rice-feeding N. virescens and the grass-inhabiting N. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between two Nephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species.     

Breeding phenology,variation in reproductive effort and offspring size in a tropical population of the woodlouse Porcellionides pruinosus

Summary Most species of woodlice in temperate habitats have discrete breeding seasons. It is hypothesised that breeding synchronises with favourable environmental conditions to maximise offspring growth and survivorship (Willows 1984). We measured the breeding phenology of a species introduced to a tropical environment, primarily to consider the assumption that life histories in the tropics will differ fundamentally from those in temperate habitats. In addition to breeding phenology we considered variation in reproductive effort between individual females and the division of this effort between the size and number of young.A continuous breeding phenology was observed in a synanthropic population of Porcellionides pruinosus within the tropics. Reproductive effort varied between months, showed a weak relationship with female size and was independent of female fecundity. Female sizefecundity relationships varied between samples and when the proportion of reproductive females was high size-fecundity slopes were steeper than at other times. Mean offspring size varied between months and there was a wide range in offspring size within broods. Offspring size was not related to female body mass, reproductive effort or fecundity; consequently brood mass increased linearly with an increase in fecundity. Increased reproductive effort goes into more rather than larger offspring.We propose that the continuous breeding in this population was the result of the constant presence of an environmental cue to reproduction evolved in temperate habitats. Continuous breeding is not necessarily equivocal to high individual reproductive success even though overall population growth may be rapid. However, variation in reproductive effort suggests that individuals respond to current environmental conditions on short time scales.     

Infidelity of leafcutting ants to host plants: resource heterogeneity or defense induction?

Summary Leafcutting ants have strong among- and within-plant preferences, and generally abandon plants long before they are completely defoliated. Two tropical deciduous forest tree species preferred by the leafcutting ant Atta colombica were studied to determine how variation in resource quality affects ant selectivity and partial defoliation of plants. Significant differences in palatability and leaf characteristics of Spondias mombin and Bursera simaruba were found among trees and among leaf types within trees, but not among branches within trees. No short-term responses to experimental defoliation of up to 50% of total canopy were found in either species. Leaf nutrient and poisture content were positively correlated, and phenolic content negatively correlated, with the palatability of Spondias mombin, a species containing hydrolyzable tannins. Leaf moisture and phenolic content were both positively correlated with the palatability of Bursera simaruba, which contains predominantly condensed tannins. The results suggest that variation in leaf quality among and within plants is at least a partial explanation for ant selectivity and partial defoliation of preferred species. There is no evidence that rapidly induced changes in plant chemistry affect ant decisions to abandon these plants. Instead, it appears likely that ants abandon plants once high-quality leaf patches are exhausted. Quantitative variation in leaf nutrients, moisture, and secondary chemicals all appear to contribute to ant preferences for individuals and tissues of highly palatable plants.     

Sexual and site differences in calcium consumption by the Malabar Giant Squirrel Ratufa indica

Summary The calcium ingestion of Malabar Giant Squirrels Ratufa indica (Sciuridae) was examined at two sites, Magod and Bhimashankar, in western India. In females at Magod a positive correlation was found between rates of calcium ingestion from food resources and the contribution of those resources to the daily diet. This relationship was true for all females at Magod irre-spective of their reproductive condition. This relationship was neither significant for males at the same site nor for both sexes at Bhimashankar. A differential requirement for calcium between the sexes and the occurrence of higher rates of calcium ingestion or the probable presence of more easily digestible calcium at Bhimashankar are postulated to explain the observed phenomenon. Mature leaves and bark appear to be reliable sources of calcium at these sites.     

Community composition and soil properties in northern Bolivian savanna vegetation

Abstract. Direct gradient analysis (Canonical Correspondence Analysis) of northern Bolivian savanna vegetation revealed correlations between the composition of plant communities and physical and chemical soil properties. Cover/abundance values for 193 species from 27 sites were related to data on eight soil factors. A water-regime variable and the percentage of sand and silt were correlated with the first axis of the species-environment biplot and explained most of the variation in community composition. Along this axis, species and sites of flood-plain vegetation were separated from sites not affected by flooding rivers. The second axis of the biplot was correlated with soil-chemical variables, namely extract-able phosphate, base saturation, organic carbon, pH, and effective cation exchange capacity. Part of the variation in community composition can be explained by these soil nutrient variables. Grassland communities were separated from woody vegetation along the soil nutrient gradient, and floodplain communities of white-water rivers from those of clear-water rivers. The results of the gradient analysis indicate that the soil texture-moisture gradient is the prime factor determining the variation in the floristic composition of the savanna communities examined, and that, in addition, the soil nutrient gradient accounts for some of the variation.     

Stream vegetation in different landscape types

A methodology is proposed for assessing the ecological value of streams in the catchment of the Moravská Dyje River. It is concluded that by using a wide range of parameters that a more objective assessment is achieved than if only one were used. The landscape of the catchment studied contains excessive amounts of nutrients and, in comparison to its natural state, has become too uniform.     

A long-term perspective of dissolved organic carbon transport in Sycamore Creek, Arizona, USA

Dissolved organic carbon (DOC) dynamics were examined over five years (1989–1993) in Sycamore Creek, a Sonoran Desert stream, specifically focusing on DOC concentration in surface and hyporheic waters, and rates of export. In 1989 and 1990, the years of lowest stream discharge (0.08 and 0.04 m³ s^–1 annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgC l^–1 (weighted annual means). In contrast, from 1991 through 1993, a period of increased flow (1.1, 1.2 and 4.3 m³ s^–1), concentration was significantly lower (P<0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgC l^–1. Concentration exhibited little spatial variation between two sampling stations located 6 km apart along the mainstem or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five-year period from a mean rate of only 24 kgC d^–1 in 1990 to 2100 kgC d^–1 in 1993. Ninety percent of DOC was exported by flows greater than 2.8 m³ s^–1, and 50% during flows greater than 27 m³ s^–1; flows of 2.8 and 24 m³ s^–1 occurred only 9 and 1% of the time. The export of organic matter in Sycamore Creek appears to be coupled to El Niño-Southern Oscillation phenomena. The years of highest export, 1991–1993, had El Niño conditions while 1989 and 1990 had medial conditions.     

Effects of eucalyptus afforestation on leaf litter dynamics and macroinvertebrate community structure of streams in Central Portugal

To test the hypothesis whether afforestation with Eucalyptus globulus affects litter dynamics in streams and the structure of macroinvertebrate aquatic communities, we compared streams flowing through eucalyptus and deciduous forests, paying attention to: (i) litterfall dynamics, (ii) accumulation of organic matter, (iii) processing rates of two dominant leaf species: eucalyptus and chestnut, and (iv) macroinvertebrate community structure. The amount of allochthonous inputs was similar in both vegetation types, but the seasonality of litter inputs differed between eucalyptus and natural deciduous forests. Eucalyptus forest streams accumulated more organic matter than deciduous forest streams. Decomposition of both eucalyptus and chestnut leaf litter was higher in streams flowing through deciduous forests. The eucalyptus forest soils were highly hydrophobic resulting in strong seasonal fluctuations in discharge. In autumn the communities of benthic macroinvertebrates of the two stream types were significantly different. Deciduous forest streams contained higher numbers of invertebrates and more taxa than eucalyptus forest streams. Mixed forest streams (streams flowing through eucalyptus forests but bordered by deciduous vegetation) were intermediate between the two other vegetation types in all studied characteristics (accumulation of benthic organic matter, density and diversity of aquatic invertebrates). These results suggest that monocultures of eucalyptus affect low order stream communities. However, the impact may be attenuated if riparian corridors of original vegetation are kept in plantation forestry.