Potential habitat dependence in deposition rate of presumptive annual lines in shells of the bivalve Protothaca staminea

Although repeating depositional features in shells of bivalve molluscs continue to be used in ecology and paleoecology to age individual specimens and thereby provide a potentially powerful tool for geochronological reconstruction of past events, few studies actually test the basic assumptions of growth line analysis: (1) that line deposition is truly periodic and (2) that the length of the period is one year. Recapture of marked individuals of the bivalve Protothaca staminea after 12 or 24 months in field plots in two habitats of Mugu Lagoon (California, U.S.A.) suggests a habitat-specific pattern of deposition of the major growth lines previously assumed to be annual. External (surface) and internal (cross-sectional) growth lines were in excellent agreement on any given specimen and showed that each specimen from a muddy-sand environment deposited a single annual line over 12 months, whereas all but one specimen from a clean-sand environment deposited several more major growth lines than predicted from the numbers of years of additional growth. Although more frequent handling or natural disturbance in the higher-energy, clean-sand habitat may explain these differences, the results demand caution in extrapolating tests of annual periodicity in growth line deposition across species or even across habitats for a single species.     

Unpredictability in the facultative association between larvae of Euphilotes enoptes (Lepidoptera: Lycaenidae) and ants

Larvae of the lycaenid butterfly, Euphilotes enoptes , are facultatively associated with ants. To determine whether variation in this association is predictable, I documented the proportion of larvae tended by ants and the identity of those ants over a three-year period in two study areas in central Washington state. Each study area contained numerous patches of Eriogonum compositum , the sole host plant of E. enoptes in this region. Using complete censuses and random subsampling, I estimated the dispersion and densities of E. enoptes larvae occupying each patch. The results of these observations indicated significant spatial and temporal variation in the identities of ants tending larvae and the frequency of ant attendance. In addition, early instar larvae were tended less frequently and by smaller ant species, than fourth instar larvae. Although larvae were aggregated at two spatial scales: individual plants within a patch and individual inflorescences on a plant, attendance rates did not vary consistently with aggregation size at either scale. Similarly, attendance rates were independent of the patch-wide density of larvae. I discuss the potential sources of such unpredictable variation and its implications for the evolution of further specialization in this facultative association.     

Growth response of subalpine fir (Abies lasiocarpa) to climate in the Olympic Mountains, Washington, USA

Growth response of subalpine fir (Abies lasiocarpa) to climate was studied across its local geographical and elevation range in the Olympic Mountains, Washington. A dendroecological analysis of subalpine fir across a range of elevations (1350-1850 m) and annual precipitation (125-350 cm y^?1), was used to compare environmental factors affecting growth. Climate-growth relationships were explored using Pearson product-moment correlation coefficients; partial correlation analysis was used to assess relationships among site chronologies and climatic variables. Radial growth is negatively correlated with winter precipitation at high elevation and wet sites, but not at low and middle elevation dry sites. Growth is positively correlated with current growing season temperature at all sites; however, growth is negatively correlated with previous year August temperature, indicating that climate affects growth in subsequent years. Positive correlations between growth and summer precipitation during the growing season at low and middle elevation dry sites suggest that soil moisture is partially limiting to growth on these sites. If the climate of the Pacific Northwest becomes warmer and drier, then subalpine fir growth may increase at high elevation and wet sites, but may decrease at lower elevation dry sites in the Olympic Mountains. However, the growth response of subalpine fir to potentially rapid climate change will not be uniform because subalpine fir grows over a wide range of topographic features, habitats, and local climates at different geographical scales. A comparison of growth response to current growing season temperature suggests that the temperature-related growth response of subalpine fir is not adequately described by the parabolic curve used in JABOWA-based models.     

Chemical Composition of the Epidermal, Hypodermal, Endodermal and Intervening Cortical Cell Walls of Various Plant Roots

A survey of chemical modifications in the cells of the epidermis,hypodermis, cortical parenchyma and endodermis in roots of 27plant species was performed. Cortical parenchyma walls weregenerally free of modifying substances whereas the walls ofthe epidermis, hypodermis and endodermis were usually modifiedby the presence of lipids, phenols, suberin or lignin. In mostcases, wall-modifying components could be detected within 5mm of the root apex. lipids, phenols, suberin, lignin, ferulic acid, root, epidermis, hypodermis, cortex, endodermis, cell wall     

Gut passage and insect grazer selectivity of lotic diatoms

SUMMARY 1. Grazing experiments were conducted in a small Kentucky stream to determine if epilithic diatom taxa differed in susceptibility to grazing by the larval caddisfly, Neophylax autumnus , and whether ingested taxa varied in digestibility. Live/dead ratios of diatom cells from N. autumnus faecal material were compared with live/dead ratios of cells taken from adjacent epilithic habitats to determine diatom digestibility. Selectivity was studied by comparing relative abundances of diatom taxa on substrata with the relative abundances of these taxa in grazer faecal material.
2. 73% of the diatoms collected from epilithic habitats contained intact chloroplasts and were characterized as live cells. Only 42% of diatoms eliminated in caddisfly faeces were living. The five dominant diatom taxa did not differ in digestibility. Elimination of viable diatoms in grazer faecal material may contribute to diatom drift in streams, and thus supply individuals for downstream recolonization. Failure to document live/dead ratios of diatoms from grazed substrata can lead to overestimation of the digestibility of taxa and misinterpretation of results.
3. Grazers in this investigation were more successful at ingesting large, high-profile diatom taxa (i.e. Cymbella, Meridion and Gomphonema ) and less able to remove small, adnate forms ( Achnanthes minutissima ). Such 'selectivity'may be one mechanism by which A. minutissima maintains dominance in this system.     

Ecological niche conservation and differentiation in the wood-feeding cockroaches, Cryptocercus, in the United States

The distribution of a species is determined by a set of ecological correlates (biotic and abiotic factors) and historical phenomena that define and constrain its ecological niche. By examining the geographical projection of the ecological niche, it is possible to identify areas that are suitable for the species ecologically but that are not actually inhabited. Previous studies have used such discords to anticipate the existence of undescribed species; the absence of species may also result from historical barriers that have prevented colonization of the area. Thus, discords between potential and realized niches of a species may result from aspects of the species’s biology (e.g. lack of dispersal ability, specific habitat requirements) or from barriers to dispersal. In this study, we developed ecological niche models of the five Nearctic species of wood‐feeding cockroaches in the genus Cryptocercus. One species, C. clevelandi, occurs in the western USA and four (C. darwini, C. garciai, C. punctulatus, C. wrighti) are distributed in the Appalachian Mountains in the eastern USA. In general, modelled niches for all species showed ecological restriction relative to the total range of available conditions. Niche models for the eastern USA species predicted the geographical distributions of the other eastern USA species, with the exception of C. garciai, suggesting general overall conservatism in ecological characteristics. Some interspecific differences in modelled niches were also apparent. In general, C. clevelandi was predicted to occupy cooler and drier areas relative to the eastern USA species; among the eastern USA species, C. darwini and C. garciai were predicted to inhabit warmer areas relative to C. punctulatus and C. wrighti. Interspecific differences and similarities were consistent with the phylogenetic relationships among Cryptocercus species previously reported. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90 , 457–466.