Can limited dispersal or biotic interaction explain the declining abundance of the whelk,Morula marginalba,at the edge of its range?

The southern range limit of the planktotrophic, intertidal whelk, Morula marginalba, occurs immediately to the north of the south‐east Australian biogeographical barrier and we predict that its range may be limited by dispersal and biotic interactions. The barrier includes several hundred kilometers without suitable habitat and is the convergence zone of major ocean currents. Within the barrier region, along‐shore larval transport is expected to be erratic, with water moving offshore to form long‐lived eddies. Through comparison of central and range edge populations, we test for evidence of recruitment limitation and interspecific competition with the whelk, Haustrum vinosum, which occurs sympatrically at the range edge. We found similar levels of recruitment and allozyme genetic variation within centre and edge populations where H. vinosum was absent, implying that populations are strongly interconnected. Nevertheless, recruitment and abundance of M. marginalba were lower at its extreme range limit, where it occurs with H. vinosum. Indeed, we detected significant inverse correlations between the abundance of these species, although their habitat use was similar and they consumed similar prey. These lines of evidence suggest a restricted recruitment across the barrier limits the range of M. marginalba but emphasize that multiple factors may act to set species range limits. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 849–862.     

The elephant problem–an alternative hypothesis

Published hypotheses to account for habitat changes wrought by elephants begin from the assumption that elephant-forest systems possess a stable equilibrium point. The ‘elephant problem’ is conceived as a displacement of this equilibrium by man. Controversy centres around which human activities caused the dislocation of equilibrium and by which mechanisms these activities resulted in local high densities of the elephant Loxodonta africana. A study on elephant-forest relationships in the Luangwa Valley of Zambia casts doubt upon the basic assumption of these hypotheses and an alternative hypothesis is therefore offered. It begins from the opposite assumption–that there is no attainable natural equilibrium between elephants and forests in eastern and southern Africa. The relationship is viewed instead as a stable limit cycle in which elephants increase while thinning the forest and decline until reaching a low density that allows resurgence of the forest. This in turn triggers an increase of elephants and the cycle repeats. The period of the cycle, if the hypothesis is correct, is in the order of 200 years in the Luangwa Valley. The activities of man can impose an artificial equilibrium on the system such that trees and elephants are trapped at the low density phase of the cycle. When interference is relaxed, as with the conversion of an area to a national park, the cyclic relationship reasserts. The parameters of a system possessing a stable limit cycle need not differ in kind or interrelationship from those of a system with a stable equilibrium. Whether one or other outcome manifests may depend only on the numerical values of the parameters. If the elephant-forest system is characterized by a stable limit cycle the period and amplitude should change along a climatic gradient and may contract to a stable equilibrium in some climatic zones. A set of predictions is offered to facilitate rejection of the hypothesis.     

Response of Tulip Scale Mitochondria to Temperature in Relation to Cold Treatment of the Bulbs

A uniform batch of tulip bulbs were kept in the dark eitherat 2 °C or 18 °C. Sample bulbs were withdrawn at intervalsfrom both treatments and mitochondria were isolated from thescale tissue. Arrhenius plots of succinate and NADH oxidationby the mitochondria were made. Within the range 0–36 °C,mitochondria from uncooled bulbs showed a single transitionpoint at about 16 °C, as did mitochondria from freshly formeddaughter bulbs. In contrast, mitochondria from bulbs kept for 8 weeks or longerat 2 °C showed strong indications of two discontinuitiesin the Arrhenius diagrams within a similar temperature range,one about 10 °C below the other. The results suggest thattulip bulbs are chilling-sensitive, and that alterations inthe activation energies of certain oxidative enzymes in responseto accumulated cold treatment are necessary for metabolism tobe directed towards the growth of normal flowers.     

Home range size and distribution in a wild population of Cape pangolins, Manis temminckii, in north‐west Zimbabwe

The aim of this study was to estimate the size of the home range used by individual Cape pangolins ( Manis temminckii ). The study was conducted in 1991–95 at Sengwa Wildlife Research Institute, Gokwe, Zimbabwe. Radio‐telemetry was used to repeatedly locate study pangolins. Home range area was estimated from the location of the burrows used by each pangolin. It was established that pangolins must be tracked for 85+ days to generate a reliable estimate of home range. The size of home ranges, determined from 1141 tracking days of data for 10 pangolins (3·0–15·8 kg body mass) that were each tracked for at least 85 days, was from 0·17 to 11·07 km². Larger (older) pangolins used more burrows and had larger home ranges than smaller (younger) pangolins. The data indicate that large adult males had larger home ranges than large adult females. Within each sex, the home ranges were adjacent to each other with only slight overlap at the boundaries. There was clear overlap of home ranges between males and females.     

Respiration of 14C-labelled Assimilates in Stems of Willow

Willow cuttings were allowed to assimilate ¹⁴CO₂, and the changein the specific activity of phloem sap, collected as aphid honeydew,was compared with the change in the specific activity of the¹⁴CO₂ given off from the respiration of the labelled translocates.With young shoots (3–5 weeks old), the acceleration inthe specific activities of honeydew and respiratory CO₂ werevery similar. With 2–4-year-old mature stems, however,there was a considerable difference between the accelerationof the two specific activities, that of the honeydew alwaysbeing greater than respiratory CO₂. Studies have also been made on the rate of breakdown of ¹⁴C-labelledtranslocates in isolated and intact young shoots and maturestems. No differences between the isolated and intact stemswere apparent. The results are discussed in relation to possible translocationmechanisms, and the results of other workers in this field ofexperimentation.