Pre-freeze mortality in three species of aphids from sub-Antarctic Marion Island

Understanding the mechanisms by which aphids survive low temperature is fundamental in forecasting the risk of pest outbreaks. Aphids are chill susceptible and die at a temperature close to that at which a small exothermal event is produced. This event, which can be identified using differential scanning calorimetry (DSC), normally occurs at a higher temperature than the supercooling point (SCP) and has been termed a pre-freeze event (PFE). However, it is not known what causes the PFE or whether it signifies the death of the aphid. These questions are addressed here by using a sensitive DSC to quantify the PFE and SCP and to relate these thermal events to the lower lethal temperature (LT₅₀) of sub-Antarctic aphids acclimated to low temperatures. PFEs were observed in each of the 3 species of aphids examined. They occurred over a narrower temperature range and at a higher temperature range than the SCP (−8.2 to −13.8 and −5.6 to −29.8 °C, respectively). Increased acclimation temperature resulted in increased SCPs in Myzus ascalonicus but not in Rhopalosiphum padi. The LT₅₀ reduced by approximately 1 °C from −9.3 to −10.5 °C with reduced acclimation temperature (10–0 °C). The LT₅₀ was close to the temperature at which the PFE occurred but statistically significantly higher than either the PFE or the SCP. In the majority of cases the PFE exotherm occurred well before the main exotherm produced by the bulk of the insect’s body water freezing (SCP). However, in a few cases it occurred at the same temperature or before the super-cooling point making the term, pre-freeze event (PFE), rather misleading. The possible origins of the PFE are discussed.     

Caterpillars benefit from thermal ecosystem engineering by wandering albatrosses on sub-Antarctic Marion Island

Wandering albatrosses (Diomedea exulans) nest on Southern Ocean islands, building elevated nests upon which they incubate eggs and raise chicks, and which the chicks occupy through winter. The nests support high invertebrate biomass, including larvae of the flightless moth Pringleophaga marioni. Here we argue that high biomass of P. marioni in the nests is not associated with nutrient loading as previously suspected, but that higher temperatures in the nests increase growth and feeding rate, and decrease deleterious repeated cold exposure, providing fitness advantages for P. marioni. Thus, wandering albatrosses may be serving as thermal engineers, modifying temperature and therefore enabling better resource use by P. marioni.     

The microenvironment of house mice on Marion Island (sub-Antarctic)

On Marion Island, house mice ( Mus musculus) establish burrow systems that range from unbranched corridors 0.5 m long with a single chamber (in some instances without a chamber) to complexly branched systems extending over an area of up to 4 m² and containing up to four chambers. Total underground area occupied by burrow systems (chambers plus corridors) was from 5 to 23 m² ha^-1, corresponding to burrow-system volumes of 250-1,300 dm³ ha^-1. In autumn, about three-quarters of chambers contained small food caches. Most (87%) entrances to burrow systems faced away from prevailing winds, especially winds that bring snow, hail or rain. Seasonal and diurnal temperature variations in burrows are considerably dampened (daily minimum in burrows seldom drops below 2°C), compared with the air just above the vegetation canopy. Over the whole year, total night-time warmth in a burrow (heat sum, 24,883 degree hours) was 53% greater than at the top of the canopy (16,317 degree hours). Burrows' entrances are generally connected above ground by runways (paths and tunnels through the vegetation). Runways also represent a warmer environment than the air above the canopy during the breeding season at night (13,466 degree hours at the runway surface compared with 11,900 degree hours at the top of the canopy). House mice, which are living close to their physiological limits, temperature-wise, on Marion Island thus evade the worst extremes of the island's climate by constructing burrows and above-ground runways and this is an important factor in their survival.     

Food, reproduction and survival in mice on sub-Antarctic Marion Island

The house mouse Mus musculus is the most widespread introduced mammal on sub-Antarctic islands, where it may alter ecosystem function. Ambient temperature and food availability affect reproduction and survival for mice. It is unclear how these factors influence mouse demography in the sub-Antarctic, and we tested the influence of food experimentally on Marion Island. Using food supplementation trials, we did not alter reproduction or overwinter survival. Alternatively, we argue ongoing climatic change on Marion could increase mouse densities through summer, while increased winter survival may reduce population growth rates the following summer through density dependence. The overall influence of these apposing forces depends on their relative strengths but may limit changes in mouse numbers with ongoing changes in climate in the sub-Antarctic.     

Natural dispersal to sub-Antarctic Marion Island of two arthropod species

Distinguishing between species that are recent natural colonists, recent anthropogenic introductions, or previously unknown, but long-term resident native species, is a challenge for those who manage the conservation of the Antarctic region. Here, we report the discovery of two new arthropod species on sub-Antarctic Marion Island—Nabis capsiformis Germar (Heteroptera: Nabidae) and Tetragnatha sp. (Araneomorphae: Tetragnathidae). On the basis of their habitat use, dispersal abilities, historic biodiversity survey records, and limited information on genetic diversity, we conclude that the colonization events were natural.     

Thermal sensitivity of oxygen uptake of Diptera from sub-Antarctic South Georgia and Marion Island

 Oxygen uptake of Paractora dreuxi (apterous) from Marion Island, which lies to the north of the Antarctic Polar Front, was compared to that of P. trichosterna (macropterous) and Antrops truncipennis (apterous) from South Georgia, which lies within the Antarctic Polar Front, over the range of temperatures experienced by these insects in their microhabitats. No differences in the slopes of log metabolic rate on temperature were found between the larvae of the two Paractora species, but the slope of the regression of log metabolic rate on temperature was steeper in the adults of P. trichosterna than in those of P. dreuxi. Therefore, metabolic cold adaptation was not found in P. trichosterna compared to P. dreuxi. However, some evidence for temperature compensation in A. truncipennis was found, although this could not be considered an adaptation. The difference in the thermal sensitivity of metabolic rate of the adults of the Paractora species is ascribed to differences in their life history strategies. Paractora trichosterna is a winged species in which retention of a thermal sensitivity similar to that of its larvae may facilitate resource location and so enhance fitness. On the other hand, the loss of flight in P. dreuxi may have allowed a reduction in thermal sensitivity that could mean a reduction in respiratory water loss at higher temperatures. Received: 12 December 1995/Accepted: 13 March 1996     

Estimated Impact of feral house mice on sub-Antarctic invertebrates at Marion Island

Summary The energy metabolism of feral house mice Mus musculus was established on sub-Antarctic Marion Island, using the doubly-labelled water turnover technique. Mean water influx was 565 ml kg^-1 day^-1 and mean CO₂ production was 5.41 ml g^-1 h^-1, i.e. 3375 kJ kg^-1 day^-1. From the energy content of the main items (Lepidoptera larvae, Curculionidae) in the diet of the mice it was estimated that the dry mass of food consumed was 3.5 g mouse^-1 day^-1. The overall impact of mice on invertebrates, based on mean mouse density and the mean percentage invertebrates in the diet, was estimated at 108 g ha^-1 day^-1 or 39.4 kg ha^-1 y^-1 (dry mass). Greatest predation pressure was on larvae of the flightless moth Pringleophaga marioni: 65 g ha^-1 day^-1 or 23.7 kg ha^-1 y^-1. Insect biomass is lower on Marion Island than on nearby Prince Edward Island, which is mouse-free. It is suggested that populations of certain insects on Marion Island are depressed by the alien mice.     

Genetic evidence confirms the origin of the house mouse on sub-Antarctic Marion Island

Biological invasions and climate change are two of the largest threats to biodiversity, and this is especially true for island ecosystems that have largely evolved in isolation. The house mouse is considered to have been introduced to sub-Antarctic Marion Island by sealers in the early 1800s. It is currently widespread across the island and has a large impact on the indigenous biota. To date, little information is available on genetic aspects of biological invasions in the sub-Antarctic. Ten specimens of the house mouse were collected from two geographically separated localities on Marion Island. Sequences of the mitochondrial DNA control region revealed only two haplotypes, separated by a single site change. More importantly, these haplotypes are shared between the eastern and western side of Marion Island. By comparing our sequences to data available on GenBank, we provide evidence that house mice on Marion Island is Mus musculus domesticus (Rutty 1772), and most closely related to haplotypes characterizing this species from Denmark, Sweden, Finland, and northern Germany.     

Phenotypic plasticity of thermal tolerances in five oribatid mite species from sub-Antarctic Marion Island

The extent to which phenotypic plasticity might mediate short-term responses to environmental change is controversial. Nonetheless, theoretical work has made the prediction that plasticity should be common, especially in predictably variable environments by comparison with those that are either stable or unpredictable. Here we examine these predictions by comparing the phenotypic plasticity of thermal tolerances (supercooling point (SCP), lower lethal temperature (LLT), upper lethal temperature (ULT)), following acclimation at either 0, 5, 10 or 15 degrees C, for seven days, of five, closely-related ameronothroid mite species. These species occupy marine and terrestrial habitats, which differ in their predictability, on sub-Antarctic Marion Island. All of the species showed some evidence of pre-freeze mortality (SCPs -9 to -23 degrees C; LLTs -3 to -15 degrees C), though methodological effects might have contributed to the difference between the SCPs and LLTs, and the species are therefore considered moderately chill tolerant. ULTs varied between 36 degrees C and 41 degrees C. Acclimation effects on SCP and LLT were typically stronger in the marine than in the terrestrial species, in keeping with the prediction of strong acclimation responses in species from predictably variable environments, but weaker responses in species from unpredictable environments. The converse was found for ULT. These findings demonstrate that acclimation responses vary among traits in the same species. Moreover, they suggest that there is merit in assessing the predictability of changes in high and low environmental temperatures separately.     

Morphometric measurement selection: an invertebrate case study based on weevils from sub-Antarctic Marion Island

A character-selection procedure initially applied in vertebrates (viverrid carnivores and murid rodents), but with a potential, more general application, was used to select appropriate characters for a morphometric investigation of weevils on Marion Island. An initial set of 23 linear measurements, adopted from a previous morphometric study, was subjected to cluster and ordination procedures to summarize patterns of correlations between measurements. Criteria were developed for the selection of representative measurements within cluster analysis-generated sub-clusters, after the exclusion of redundant measurements. This reduced the 23 initial variables to a final set of 15 measurements. The general grouping of variables was broadly consistent across all weevil species examined. Apart from economizing, by reducing the number of characters that have to be measured for subsequent analyses, the procedure also provides a way to adequately represent the phenotype, and to investigate morphological integration.     

Acclimation effects on thermal tolerances of springtails from sub-Antarctic Marion Island: indigenous and invasive species

Collembola are abundant and functionally significant arthropods in sub-Antarctic terrestrial ecosystems, and their importance has increased as a consequence of the many invasive alien species that have been introduced to the region. It has also been predicted that current and future climate change will favour alien over indigenous species as a consequence of more favourable responses to warming in the former. It is therefore surprising that little is known about the environmental physiology of sub-Antarctic springtails and that few studies have explicitly tested the hypothesis that invasive species will outperform indigenous ones under warmer conditions. Here we present thermal tolerance data on three invasive (Pogonognathellus flavescens, Isotomurus cf. palustris, Ceratophysella denticulata) and two indigenous (Cryptopygus antarcticus, Tullbergia bisetosa) species of springtails from Marion Island, explicitly testing the idea that consistent differences exist between the indigenous and invasive species both in their absolute limits and the ways in which they respond to acclimation (at temperatures from 0 to 20 degrees C). Phenotypic plasticity is the first in a series of ways in which organisms might respond to altered environments. Using a poorly explored, but highly appropriate technique, we demonstrate that in these species the crystallization temperature (Tc) is equal to the lower lethal temperature. We also show that cooling rate (1 degree C min(-1); 0.1 degrees C min(-1); 0.5 degrees C h(-1) from 5 to -1 degrees C followed by 0.1 degrees C min(-1)) has little effect on Tc. The indigenous species typically have low Tcs (c. -20 to -13 degrees C depending on the acclimation temperature), whilst those of the invasive species tend to be higher (c. -12 to -6 degrees C) at the lower acclimation temperatures. However, Ceratophysella denticulata is an exception with a low Tc (c. -20 to -18 degrees C), and in P. flavescens acclimation to 20 degrees C results in a pronounced decline in Tc. In general, the invasive and alien species do not differ substantially in acclimation effects on Tc (with the exception of the strong response in P. flavescens). Upper lethal temperatures (ULT50) are typically higher in the invasive (33-37 degrees C) than in the indigenous (30-33 degrees C) species and the response to acclimation differs among the two groups. The indigenous species show either a weak response to acclimation or ULT50 declines with increasing acclimation temperature, whereas in the invasive species ULT50 increases with acclimation temperature. These findings support the hypothesis that many invasive species will be favoured by climate change (warming and drying) at Marion Island. Moreover, manipulative field experiments have shown abundance changes in the indigenous and invasive springtail species in the direction predicted by the physiological data.     

Demographic responses of house mice to density and temperature on sub-Antarctic Marion Island

Recent changes in the climate of the sub-Antarctic may influence the number of house mouse (Mus musculus sensu lato) living on islands in the region. An increase in mouse numbers, as conditions became milder, could amplify the effects of climate change on native prey species. However, we have no direct evidence of the influence of climate on mouse numbers in the sub-Antarctic. We, therefore, assessed demographic trends in the mouse population on Marion Island between 1991 and 2001. Both the climate and mouse numbers were relatively stable during our study. Mice, however, increased their reproductive output in years when ambient temperatures were relatively high. Moreover, reduced reproductive output followed high densities at the onset of a breeding season, implying density-dependent limitation. We conclude that both temperature and density limited the increase in numbers during the summer breeding season. Major die-offs during winter probably limit population size and explain the relative stability in numbers across the 10 years of our study.     

Metabolic rate, genetic and microclimate variation among springtail populations from sub-Antarctic Marion Island

Measurement of metabolic rates (made at 10°C) of individuals of the springtail Cryptopygus antarcticus travei from six geographically distinct populations on sub-Antarctic Marion Island were combined with mitochondrial DNA (COI) haplotype analysis to examine in parallel both physiological and genetic variation of distinct populations. We found evidence of genetic differentiation among populations and a general indication of long-term isolation with limited gene flow. While we found support for an overall pattern of metabolic rate structure among populations from different geographic locations on the island (mean rate = 0.0009–0.0029 μl O₂ μg⁻¹ h⁻¹ for populations of a mean individual mass of 8–26 μg), we were unable to demonstrate a coherent common pattern between this and genetic variation. However, spatial structure in metabolic rate variation was strongly related to the extent of variability in microclimate among sites, and also showed some indication of a phylogeographic signal. Thus, over the relatively short timescale of Marion Island’s history (<1 million years), the periodic geographic barriers that have driven population differentiation from a molecular perspective may also have resulted in some physiological differentiation of populations.     

DNA barcoding and the documentation of alien species establishment on sub-Antarctic Marion Island

Invasive alien species constitute a substantial conservation challenge in the terrestrial sub-Antarctic. Management plans, for many of the islands in the region, call for the prevention, early detection, and management of such alien species. However, such management may be confounded by difficulties of identification of immatures, especially of holometabolous insects. Here we show how a DNA barcoding approach has helped to overcome such a problem associated with the likely establishment of an alien moth species on Marion Island. The discovery of unidentifiable immatures of a noctuid moth species, 5 km from the research station, suggested that a new moth species had colonized the island. Efforts to identify the larvae by conventional means or by rearing to the adult stage failed. However, sequencing of 617 bp of the mitochondrial cytochrome oxidase subunit I gene, and comparison of the sequence data with sequences on GENBANK and the barcoding of life database enabled us to identify the species as Agrotis ipsilon (Hufnagel), a species of which adults had previously been found regularly at the research station. Discovery of immatures of this species, some distance from the research station, suggests that a population may have established. It is recommended that steps to be taken to eradicate the species from Marion Island.     

Turgidosculum complicatulum on sub-Antarctic Marion Island: carbon acquisition response to climate change

The lichen Turgidosculum complicatulum (formerly Mastodia tesselata) occurs in the shore-zone of Marion Island (sub-Antarctic: 47°S,38°E). Net CO₂ exchange in the lichen is dominated by a strong temperature-dependence of respiration rate. The light/temperature response of photosynthesis is such that under the prevailing climatic regime on the island the lichen, if sufficiently hydrated, would exhibit near-maximal photosynthesis rates for 75% of the photoperiod over the year. A photosynthetic response model predicts that the lichen's net annual carbon acquisition is 3.1 g C g^-1 year^-1 under the current solar radiation and temperature regime at the island. The model predicts that changes in temperature and radiation by the amounts known to have occurred in the past few decades, and even greater changes (temperature increase by up to 2°C, radiation by up to 10%), would negligibly affect the annual amount of carbon acquired provided the thalli remain hydrated. Incorporating hydration/desiccation cycles into the model resulted in a substantial lowering of annual net C exchange. However, attempts to include the increase in aridity known to have occurred at the island since 1971 gave conflicting scenarios for the effect on annual C acquisition, depending on whether atmospheric drying or thallus drying was considered.     

Thermal tolerance limits in six weevil species (Coleoptera, Curculionidae) from sub-Antarctic Marion Island

Supercooling points, lower lethal temperatures, and the effect of short-term exposures to low temperatures were examined during both winter and summer in the adults of six weevil species from three different habitats on Marion Island. Upper lethal limits and the effects of short-term exposure to high temperatures were also examined in summer-acclimatized adult individuals of these species. Bothrometopus elongatus, B. parvulus, B. randi, Ectemnorhinus marioni, and E. similis were freeze tolerant, but had high lower lethal temperatures (−7 to −10°C). Seasonal variation in these parameters was not pronounced. Physical conditions of the habitat appeared to have little effect on cold hardiness parameters because the Ectemnorhinus species occur in very wet habitats, whereas the Bothrometopus species inhabit drier areas. The adults of these weevil species are similar to other high southern latitude insects in that they are freeze tolerant, but with high lower lethal temperatures. In contrast, Palirhoeus eatoni, a supra-littoral species, avoided freezing and had a mean supercooling point of −15.5 ± 0.94°C (SE) in winter and −11.8 ± 0.98°C in summer. Survival of a constant low temperature of −8°C also increased in this species from 6 h in summer to 27 h in winter. It is suggested that this strategy may be a consequence of the osmoregulatory requirements imposed on this species by its supra-littoral habitat. Upper lethal temperatures (31–34°C) corresponded closely with maximum microclimate temperatures in all of the species. This indicates that the pronounced warming, accompanied by the increased insolation that has been recorded at Marion Island, may reduce survival of these species. These effects may be compounded as a consequence of predation by feral house mice on the weevils. Received: 4 February 1997 / Accepted: 3 May 1997