Hemispheric asymmetries in biodiversity: a serious matter for ecology

[FIRST PARAGRAPH] Penguins have been receiving a lot of bad press lately. They are considered somehow counter, spare, strange. Unlike most plant and animal groups, they do not show a peak of species richness towards the equator and a decline towards the poles. This more conventional spatial pattern is conveniently known as the latitudinal diversity gradient because of the strong covariance of richness and other measures of biodiversity that it describes. It is one of the most venerable, well-documented, and controversial large-scale patterns in macroecology (Willig et al. 2003). Equatorial peaks in species richness have characterised the planet since the Devonian (408–362 million years ago) (Crame 2001) and are typical of a wide range of both terrestrial and marine plants and animals (Gaston 1996; Willig et al. 2003). Despite the fact that this pattern has been documented since the late 1700s, sustained interest in both the regularity of the pattern and its likely underlying mechanisms is relatively modern. The realisation that human activity is posing substantial threats to biodiversity has quickened the pace of this interest (Willig et al. 2003). Where the peaks in richness lie (biodiversity hotspots), how these peaks relate to centres of endemism (areas that support large numbers of species that occur nowhere else), and how these patterns are likely to change through time, especially in the face of major environmental change, are major concerns. Without such knowledge, conservation is unlikely to succeed

A hierarchy of factors influence discontinuous gas exchange in the grasshopper Paracinema tricolor (Orthoptera: Acrididae)

The evolutionary origin and maintenance of discontinuous gas exchange (DGE) in tracheate arthropods are poorly understood and highly controversial. We investigated prioritization of abiotic factors in the gas exchange control cascade by examining oxygen, water and haemolymph pH regulation in the grasshopper Paracinema tricolor. Using a full-factorial design, grasshoppers were acclimated to hypoxic or hyperoxic (5% O2, 40% O2) gas conditions, or dehydrated or hydrated, whereafter their CO2 release was measured under a range of O2 and relative humidity (RH) conditions (5%, 21%, 40% O2 and 5%, 60%, 90% RH). DGE was significantly less common in grasshoppers acclimated to dehydrating conditions compared with the other acclimations (hypoxia, 98%; hyperoxia, 100%; hydrated, 100%; dehydrated, 67%). Acclimation to dehydrating conditions resulted in a significant decrease in haemolymph pH from 7.0±0.3 to 6.6±0.1 (mean ± s.d., P=0.018) and also significantly increased the open (O)-phase duration under 5% O2 treatment conditions (5% O2, 44.1±29.3 min; 40% O2, 15.8±8.0 min; 5% RH, 17.8±1.3 min; 60% RH, 24.0±9.7 min; 90% RH, 20.6±8.9 min). The observed acidosis could potentially explain the extension of the O-phase under low RH conditions, when it would perhaps seem more useful to reduce the O-phase to lower respiratory water loss. The results confirm that DGE occurrence and modulation are affected by multiple abiotic factors. A hierarchical framework for abiotic factors influencing DGE is proposed in which the following stressors are prioritized in decreasing order of importance: oxygen supply, CO2 excretion and pH modulation, oxidative damage protection and water savings

Endemicity biases nestedness metrics: a demonstration, explanation and solution

Nestedness is frequently investigated to understand complex patterns of species occurrences. Temperature (T) is commonly used for comparisons of nestedness of different-sized datasets. However, the assumptions made for the standardization of this metric have not been fully explored, particularly the effects of endemicity. Here we show that T incorrectly indicates an increase in nestedness with the addition of non-nested endemics to matrices that are not perfectly nested - a consequence of standardizing matrix size by the product of species and sites. This problem is common both to test matrices and to real matrices that are typically subjected to nestedness analyses. The latter are often characterized by substantial numbers of endemics and by variation in the numbers of endemics in different taxa. Standardizing by occupancy resolves this problem, which is demonstrated using a derivative of discrepancy, d1. A small modification to T, such that it standardizes matrices by occupancy, would resolve the current problems with this nestedness metric.Ctr Invas Bio

Rapid cold-hardening in a Karoo beetle, Afrinus sp.

In the insect rapid cold-hardening response, survival at subzero temperatures is greatly improved by a brief pre-exposure at a milder temperature. It is predicted that insects with minimal cold tolerance capabilities living in variable environments should use rapid cold-hardening to survive sudden cold snaps. This is tested in Afrinus sp., a beetle that lives in an exposed habitat on rock outcrops in the Karoo Desert, South Africa, where microclimate temperatures drop infrequently to below freezing. Afrinus sp. shows a significant rapid cold-hardening response: survival of a 2-h exposure to -6.5°C is much improved after pre-exposure to -2°C, to 0°C with a 2-h return to the rearing temperature, and to 40°C, but not after pre-exposure to 0°C. Little is known about the mechanism of the rapid cold-hardening response, although the data suggest that rapid coldhardening may be mediated via several different mechanisms.Centre of Excellence for Invasion Biolog

Quantification of intra-regional propagule movements in the Antarctic

Management of non-native species introductions is a conservation priority in the Antarctic region. However, despite the recognised importance of intra-regional propagule transfer, the majority of studies have focused on inter-regional pathways (i.e. from outside of the Antarctic region). Here we quantify the number of seeds carried by expeditioners who have visited sub-Antarctic Marion Island. We recorded 420 seeds from 225 items of clothing, with seeds found on 52% of the items and soil on 45% of them. The median number of seeds for field-based and station-based personnel was 20.5 and 3 per person, respectively. Waterproof trousers and socks, particularly those of field workers, carry the greatest number of propagules (for field workers, medians of 5 and 6.5, respectively) and therefore should be the focus of intra-regional management interventions. Amongst the seeds found entrained within clothing several were from species which are widespread aliens in the Antarctic region including Agrostis stolonifera, Poa annua and Sagina procumbens, and indigenous zoochorous species (Acaena magellanica, Uncinia compacta) were also well represented. The present data provide quantitative evidence in support of previous, largely hypothetical concerns about the risks of intra-regional propagule transfer in the Antarctic

Inertia in physiological traits: Embryonopsis halticella caterpillars (Yponomeutidae) across the Antarctic Polar Frontal Zone

Geographic variation is characteristic of many physiological traits at the population and species levels. However, several recent studies have suggested that population-level variation is either limited or that it is mostly a consequence of phenotypic plasticity. Here we show that there is considerable physiological inertia in cold hardiness, upper thermal tolerance limits and desiccation resistance in caterpillars of the sub-Antarctic moth Embryonopsis halticella Eaton, such that populations fromtwo climatically different islands are physiologically very similar. Both populations are moderately chill tolerant, with no difference in the supercooling points of caterpillars (-17 to -20°C). Within their host plants caterpillars of both populations freeze at substantially higher, and statistically equivalent temperatures (-9.5 to -11.5°C). The populations also have similar upper lethal limits (38°C),and survival times of dry conditions (6–170 h depending on mass). The previously inexplicably low freezing point of caterpillars at the climatically less severe Marion Island seems likely a consequence of physiological inertia given that the freezing point of caterpillars within their hosts is only a few degrees below absolute minima at the older, and colder, Heard Island. Lack of adaptive geographic variation in physiological traits has consequences for models of range limits, and highlights the importance of exploring phenotypic plasticity as a response to climatic variation.Centre of Excellence for Invasion Biolog

Geographic variation and plasticity in climate stress resistance among southern African populations of Ceratitis capitate (Weidemann) (Diptera: Tephritidae)

Traits of thermal sensitivity or performance are typically the focus of species distribution modelling. Among-population trait variation, trait plasticity, population connectedness and the possible climatic covariation thereof are seldom accounted for. Here, we examine multiple climate stress resistance traits, and the plasticity thereof, for a globally invasive agricultural pest insect, the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). We also accounted for body size and population genetic connectivity among distinct populations from diverse bioclimatic regions across southern Africa. Desiccation resistance, starvation resistance, and critical thermal minimum (CTmin) and maximum (CTmax) of C. capitata varied between populations. For thermal tolerance traits, patterns of flexibility in response to thermal acclimation were suggestive of beneficial acclimation, but this was not the case for desiccation or starvation resistance. Population differences in measured traits were larger than those associated with acclimation, even though gene flow was high. Desiccation resistance was weakly but positively affected by growing degree-days. There was also a weak positive relationship between CTmin and temperature seasonality, but CTmax was weakly but negatively affected by the same bioclimatic variable. Our results suggest that the invasive potential of C. capitata may be supported by adaptation of tolerance traits to local bioclimatic conditions

Constraint and competition in assemblages: a cross continental and modeling approach for ants

The mechanisms leading to structure in local assemblages are controversial. On the one hand, assemblage structure is thought to be the outcome of local interactions determined by the properties of species and their responses to the local environment. Alternatively, this structure has been shown to be an emergent property of assemblages of identical individuals or of random sampling of a regional assemblage. In ants at baits, a combination of environmental stress and interspecific competition is widely held to lead to a unimodal relationship between the abundance of dominant ants and species richness. It is thought that in comparatively adverse environments, both abundance and richness are low. As habitats become more favorable, abundance increases until the abundance of dominant ants is so high that they exclude those that are subordinate and so depress richness. Here we demonstrate empirically that this relationship is remarkably similar across three continents. Using a null model approach, we then show that the ascending part of the relationship is largely constrained to take this form not simply as a consequence of stress but also as a result of the shape of abundance frequency distributions. While the form of the species-abundance frequency distribution can also produce the descending part of the relationship, interspecific competition might lead to it too. Scatter about the relationship, which is generally not discussed in the literature, may well be a consequence of resource availability and environmental patchiness. Our results draw attention to the significance of regional processes in structuring ant assemblages

Water loss in insects: An environmental change perspective

In the context of global environmental change much of the focus has been on changing temperatures. However, patterns of rainfall and water availability have also been changing and are expected to continue doing so. In consequence, understanding the responses of insects to water availability is important, especially because it has a pronounced influence on insect activity, distribution patterns, and species richness. Here we therefore provide a critical review of key questions that either are being or need to be addressed in this field. First, an overview of insect behavioural responses to changing humidity conditions and the mechanisms underlying sensing of humidity variation is provided. The primary sensors in insects belong to the temperature receptor protein superfamily of cation channels. Temperature-activated transient receptor potential ion channels, or thermoTRPs, respond to a diverse range of stimuli and may be a primary integrator of sensory information, such as environmental temperature and moisture. Next we touch briefly on the components of water loss, drawing attention to a new, universal model of the water costs of gas exchange and its implications for responses to a warming, and in places drying, world. We also provide an overview of new understanding of the role of the sub-elytral chamber for water conservation, and developments in understanding of the role of cuticular hydrocarbons in preventing water loss. Because of an increasing focus on the molecular basis of responses to dehydration stress we touch briefly on this area, drawing attention to the role of sugars, heat shock proteins, aquaporins, and LEA proteins. Next we consider phenotypic plasticity or acclimation responses in insect water balance after initial exposures to altered humidity, temperature or nutrition. Although beneficial acclimation has been demonstrated in several instances, this is not always the case. Laboratory studies show that responses to selection for enhanced ability to survive water stress do evolve and that genetic variation for traits underlying such responses does exist in many species. However, in others, especially tropical, typically narrowly distributed species, this appears not to be the case. Using the above information we then demonstrate that habitat alteration, climate change, biological invasions, pollution and overexploitation are likely to be having considerable effects on insect populations mediated through physiological responses (or the lack thereof) to water stress, and that these effects may often be non-intuitive

The abundance structure of Azorella selago Hook. f. on sub-Antarctic Marion Island: testing the peak and tail hypothesis

Understanding the spatial distribution of organisms and the factors underlying it are key questions in ecology. Two competing hypotheses exist about the form of spatial variation in abundance. The abundant centre hypothesis suggests that abundance is highest in the centre of a species’ range and declines towards the range margins. By contrast, the peak and tail spatial pattern in abundance posits that several high abundance areas exist across a species range. Here, we test these competing hypotheses by surveying the abundance of the keystone plant species Azorella selago Hook. f. (Apiaceae) across sub-Antarctic Marion Island on a regular spatial grid. We also examine several factors that might explain variation in abundance. Azorella selago occurs between ca. 30 and 850 m above sea level, with sharp discontinuities in abundance at ca. 30 m and at 667 m a.s.l. The survey and analyses revealed a complex abundance structure with patches of high abundance alternating with areas of low abundance or absence, providing support for the peak and tail hypothesis, but with some support for the abundant centre idea too. Variation in abundance was best explained by a model including the negative effects of elevation and of closed vegetation. Our work provides support for the peak and tail pattern of spatial variation in abundance, which has profound importance for understanding the mechanisms underlying the spatial distribution of abundance and other macroecological regularities