Response of grazing snails to phosphorus enrichment of modern stromatolitic microbial communities

1. The effects of added phosphorus (P) on the growth, P and RNA : DNA contents, and survivorship of snails grazing on laminated microbial mats (living ‘stromatolites’) were examined in the Rio Mesquites at Cuatro Ciénegas, Mexico (total P, c. 0.60 μmol L^?1) to test the hypothesis that strong P‐limitation of microautotroph growth produces a stoichiometric constraint on herbivores because of mineral P‐limitation. 2. In a 3‐week experiment performed in summer 2001, addition of phosphorus (+15 μmol L^?1) resulted in a strong decline in stromatolite biomass C : P ratio from very high levels (c. 2300 : 1 by atoms) to moderate levels (c. 550 : 1). The endemic hydrobiid snail Mexithauma quadripaludium responded to P‐enrichment with elevated body P content and higher RNA : DNA ratios, especially for small animals likely to be actively growing. This positive response is consistent with the existence of a stoichiometric constraint on snail growth. 3. In a longer experiment (8 weeks) involving a more moderate P enrichment (+5 μmol L^?1) in summer 2002, P enrichment reduced stromatolite C : P ratio from moderate values in control treatments (c. 750) to very low values (<100 : 1). Snails responded to stromatolite P‐enrichment with increased body P content but, in contrast to the first experiment, with lower RNA : DNA ratio, lower growth rates, and higher mortality. 4. These contrasting results suggest that both very high and very low biomass C : P ratios in stromatolites are detrimental to M. quadripaludium performance, leading us to hypothesise that these herbivores live on a ‘stoichiometric knife edge’.     

The sensory basis of trail following in some lepidopterous larvae: contact chemoreception

ABSTRACT. . Caterpillars of the genus Malacosoma follow trails of the chemical 5-beta-cholestane-3,24-dione, but nothing is known of how they perceive this compound, or more generally about the sensory basis of trail following in caterpillars. By selective ablations of chemosensory organs we show that, in Malacosoma , the trail chemical is perceived by the maxillary palpi. In another lepidopteran species, Yponomeuta cagnagellus , the palpi are needed to discriminate their own trails from a trail of Malacosoma. Malacosoma larvae also lose their specificity for conspecific trails when their palpi are ablated. Volatile cues evidently do not play a role in trail-following behaviour, since neither Malacosoma nor Yponomeuta can orient on a trail covered with fine nylon mesh. These data indicate that for Malacosoma , and probably also for Yponomeuta , contact chemoreception mediated by the maxillary palpi is the primary mode of pheromone perception. The evolution of receptor sensitivity to trail chemicals in caterpillars is discussed.     

Trilobite size-frequency distributions, recognition of instars, and phyletic size changes

Size-frequency analysis of over 5,000 Ordovician trilobites from the Teretiusculus Shales of the Builth inlier, central Wales, has revealed size distributions with counter intuitive shapes. Not only do most species show normal or slightly skewed distributions, despite the preponderance of moults, but there is no evidence of instar peaks. Such features can, however, be explained by reference to steady-state population structures of Recent marine arthropods, in which small individuals often form only a minor proportion of the post-larval population structure. Trilobite steady-state population structures would have differed in detail from species to species, but certain distribution shapes may have been characteristic of particular environments. These findings necessitate a reappraisal of previous work on trilobite size-frequency distributions, survivorship and recognition of instars. The Builth data also show the first clear evidence of phyletic size increase and parallel size changes in trilobites. ▭ Trilobites, size-frequency distributions, steady-state populations, instars, phyletic size changes.     

Changes in a forest bird community during a period of fire and drought near Bega,New South Wales

The bird community of a forested 13 ha plot was studied for 3 years, during which the plot was burnt by wildfire and was severely drought-affected. The combined effects of fire and drought were surprisingly limited. Similar numbers of species were recorded in each year and differences in species composition involved only rare visitors to the plot. Colour-banding indicated population declines in at least six resident species. The greatest decline was that of the Brown Thornbill (Acanthiza pusilla), from 20 pairs to 10 pairs. In general, insectivorous birds were most abundant and widespread on the plot during the first year, before the fire. In contrast, nomadic birds feeding on flowers or fruits were most abundant in the second year, when there was a prolific flowering and fruiting of several important food plants. After the fire, at least ten bird species avoided the burnt ridges and became more concentrated in the unburnt gullies. Most were species associated with dense shrub thickets, the layer of vegetation most affected by the fire. It seems that gullies are major refuge areas in these forests and are the key to the resilience of the bird community to fire and drought.     

Apex marine predator declines ninety percent in association with changing oceanic climate

Three time series of pelagic bird abundance collected in disparate portions of the California Current reveal a 90% decline in Sooty Shearwater (Puffinus griseus) abundance between 1987 and 1994. This decline is negatively correlated with a concurrent rise in sea-surface temperatures; Sooty Shearwaters have declined while sea temperatures have risen. There is a nine-month lag in the response by shearwaters to changing temperatures. The geographical scale of our study demonstrates that the decline of Sooty Shearwaters is not a localized phenomenon, nor can it be ascribed to a short-term distributional shift. The Sooty Shearwater is the numerically dominant species of the California Current System (CCS) in summer (austral winter), with an estimated population in the late 1970s of 5 million individuals. If the observed warming of the waters of the California Current System is an irreversible manifestation of a changing global climate, then the impact upon Sooty Shearwater populations seems likely to be profound.     

Archaeochlus Brundin: a midge out of time (Diptera: Chironomidae)

Abstract. Archaeochlus , a plesiomorphic genus of Podonominae (Chironomidae), is revised and two new species (biko from Namibia and brundini from southwestern Australia) described. The systematic status of the genus and the tribe Boreochlini is reassessed. Biogeographic evidence, sustained by a fossil record, dates the genus at least to the Upper Jurassic. The ecology of early Chironomidae is suggested to be eurythermic and hygropetric rather than cold stenothermic in lotic waters.