Jacques A. Deere

Critical thermal limits depend on methodological context

A full-factorial study of the effects of rates of temperature change and start temperatures was undertaken for both upper and lower critical thermal limits (CTLs) using the tsetse fly, Glossina pallidipes. Results show that rates of temperature change and start temperatures have highly significant effects on CTLs, although the duration of the experiment also has a major effect. Contrary to a widely held expectation, slower rates of temperature change (i.e. longer experimental duration) resulted in poorer thermal tolerance at both high and low temperatures. Thus, across treatments, a negative relationship existed between duration and upper CTL while a positive relationship existed between duration and lower CTL. Most importantly, for predicting tsetse distribution, G. pallidipes suffer loss of function at less severe temperatures under the most ecologically relevant experimental conditions for upper (0.06°C min−1; 35°C start temperature) and lower CTL (0.06°C min−1; 24°C start temperature). This suggests that the functional thermal range of G. pallidipes in the wild may be much narrower than previously suspected, approximately 20–40°C, and highlights their sensitivity to even moderate temperature variation. These effects are explained by limited plasticity of CTLs in this species over short time scales. The results of the present study have broad implications for understanding temperature tolerance in these and other terrestrial arthropods.

Testing the Beneficial Acclimation Hypothesis and Its Alternatives for Locomotor Performance

The beneficial acclimation hypothesis (BAH) is controversial. While physiological work all but assumes that the BAH is true, recent studies have shown that support for the BAH is typically wanting. The latter have been criticized for assessing the benefits of developmental plasticity rather than acclimation. Here we examine the BAH within a strong inference framework for five congeneric species of ameronothroid oribatid mites that occupy marine to terrestrial habitats. We do so by assessing responses of maximum speed, optimum temperature, and performance breadth, measured from −10°C to 35°C, to four treatment temperatures (0°, 5°, 10°, and 15°C). We show that the BAH and its alternatives often make similar empirical predictions. Weak beneficial acclimation is characteristic of one of the more marine species. In the other two upper‐shore and marine species, evidence exists for deleterious acclimation and the colder‐is‐better hypothesis. In the two fully terrestrial species, there is no plasticity. Lack of plasticity is beneficial when cue reliability is low or costs of plasticity are high, and the former seems plausible in terrestrial habitats. However, weak plasticity in the upper‐shore/marine species and the absence of plasticity in the terrestrial species might also be a consequence of phylogenetic constraint.

Directional Evolution of the Slope of the Metabolic Rate–Temperature Relationship Is Correlated with Climate

The evolution of metabolic rate–temperature (MR‐T) reaction norms is of fundamental importance to physiological ecology. Metabolic cold adaptation (MCA) predicts that populations or species from cooler environments will have either a higher metabolic rate at a common temperature or steeper MR‐T relationships, indicating greater sensitivity of respiratory metabolism to temperature. Support for MCA has been found in some insect species by comparing species or populations differing in latitude. However, the generality of these findings are contentious, with most studies either unable to account for phenotypic plasticity or the evolutionary relatedness of species or populations. Hence, the importance of MCA is vigorously debated from both evolutionary and ecological perspectives. Furthermore, few species, particularly from tropical environments, have been shown to differ in MR‐T sensitivity along altitudinal temperature gradients. Here, using four populations of tsetse flies (Glossina pallidipes, Diptera: Glossinidae) from thermally distinct geographic regions, we test the hypothesis that there is evolved variation in MR‐T relationships to cold climates. We found that a high‐altitude equatorial population from a cool habitat has a steeper MR‐T reaction norm. By contrast, other populations from warmer environments in East Africa do not differ with respect to their MR‐T reaction norms. Squared‐change parsimony analyses, based on the combined mitochondrial 16S rDNA ribosomal subunit and cytochrome c oxidase subunit I (COI), support the hypothesis of adaptive differentiation of MR‐T reaction norms in the cool‐climate population. Seasonal adjustments or laboratory‐temperature‐induced phenotypic plasticity changed the intercept of the reaction norm rather than the slope, and thus the observed intraspecific variation in slopes of MR‐T reaction norms could not be accounted for by phenotypic plasticity. These results therefore suggest evolutionary adaptation of MR‐T reaction norms to cool climates (<22°C) in tsetse and provide novel support for MCA within an insect species.

Critical thermal limits and their responses to acclimation in two sub-Antarctic spiders: Myro kerguelenensis and Prinerigone vagans

Despite the relative richness of spider species across the Southern Ocean islands remarkably little information is available on their biology. Here, the critical thermal limits of an indigenous (Myro kerguelenensis, Desidae) and an introduced (Prinerigone vagans, Linyphiidae) spider species from Marion Island were studied after 7–8 days acclimation to 0, 5, 10 and 15°C. Critical thermal minima (CTMin) were low in these species by comparison with other spiders and insects measured to date, and ranged from −6 to −7°C in M. kerguelenensis and from −7 to −8°C in P. vagans. In contrast, critical thermal maxima (CTMax) were similar to other insects on Marion Island (M. kerguelenensis: 35.0–35.6°C; P. vagans: 35.1–36.0°C), although significantly lower than those reported for other spider species in the literature. The magnitude of acclimation responses in CTMax was lower than those in CTMin for both species and this suggests decoupled responses to acclimation. Whilst not conclusive, the results raise several important considerations: that oxygen limitation of thermal tolerance needs to be more widely investigated in terrestrial species, that indigenous and alien species might differ in the nature and extent of their plasticity, and that upper and lower thermal tolerance limits might be decoupled in spiders as is the case in insects.

Eco-Evolutionary Interactions as a Consequence of Selection on a Secondary Sexual Trait

Ecological and evolutionary population changes are often interlinked, complicating the understanding of how each is affected by environmental change. Using a male dimorphic mite as a model system, we studied concurrent changes in the expression of a conditional strategy and in the population in response to harvesting over 15 generations. We found evolutionary divergence in the expression of alternative male reproductive morphs—fighters and defenceless scramblers (sneakers)—caused by the selective harvesting of each male morph. Regardless of which morph was targeted, the direction of evolution of male morph expression in response to harvesting was always towards scramblers, which, in case of the harvesting of scramblers, we attributed to strong ecological feedback (reduced cannibalism opportunities for fighters) within the closed populations. Current evolutionary theory, however, predicts that the frequency of a morph always decreases when selected against: to understand phenotypic trait evolution fully, evolutionary theory would benefit from including ecological interactions, especially if traits have ecological consequences that in turn feedback to their evolutionary trajectory.

Correlative changes in life-history variables in response to environmental change in a model organism.

Global change alters the environment, including increases in the frequency of (un)favorable events and shifts in environmental noise color. However, how these changes impact the dynamics of populations, and whether these can be predicted accurately has been largely unexamined. Here we combine recently developed population modeling approaches and theory in stochastic demography to explore how life history, morphology, and average fitness respond to changes in the frequency of favorable environmental conditions and in the color of environmental noise in a model organism (an acarid mite). We predict that different life-history variables respond correlatively to changes in the environment, and we identify different life-history variables, including lifetime reproductive success, as indicators of average fitness and life-history speed across stochastic environments. Depending on the shape of adult survival rate, generation time can be used as an indicator of the response of populations to stochastic change, as in the deterministic case. This work is a useful step toward understanding population dynamics in stochastic environments, including how stochastic change may shape the evolution of life histories.

Life History Consequences of the Facultative Expression of a Dispersal Life Stage in the Phoretic Bulb Mite (Rhizoglyphus robini)

Life history traits play an important role in population dynamics and correlate, both positively and negatively, with dispersal in a wide range of taxa. Most invertebrate studies on trade-offs between life history traits and dispersal have focused on dispersal via flight, yet much less is known about how life history trade-offs influence species that disperse by other means. In this study, we identify effects of investing in dispersal morphology (dispersal expression) on life history traits in the male dimorphic bulb mite (Rhizoglyphus robini). This species has a facultative juvenile life stage (deutonymph) during which individuals can disperse by phoresy. Further, adult males are either fighters (which kill other mites) or benign scramblers. Here, in an experiment, we investigate the effects of investing in dispersal on size at maturity, sex and male morph ratio, and female lifetime reproductive success. We show that life history traits correlate negatively with the expression of the dispersal stage. Remarkably, all males that expressed the dispersal life stage developed into competitive fighters and none into scramblers. This suggests that alternative, male reproductive strategies and dispersal should not be viewed in isolation but considered concurrently.

Does frequency-dependence determine male morph survival in the bulb mite Rhizoglyphus robini?

Alternative reproductive phenotypes (ARPs) represent discrete morphological variation within a single sex; as such ARPs are an excellent study system to investigate the maintenance of phenotypic variation. ARPs are traditionally modelled as a mixture of pure strategies or as a conditional strategy. Most male dimorphisms are controlled by a conditional strategy, where males develop into a particular phenotype as a result of their condition which allows them to reach a certain threshold. Individuals that are unable to reach the threshold of a conditional strategy are considered to ‘make the best of a bad job’; however, these individuals can have their own fitness merits. Given these fitness merits, condition-dependent selection alone is not sufficient to maintain a conditionally determined male dimorphism and other mechanisms, most likely frequency-dependent selection, are required. We studied in an experiment, the male dimorphic bulb mite Rhizoglyphus robini—where males are fighters that can kill other males or benign scramblers—to assess the strength of frequency-dependent survival in a high and low-quality environment. We found that male survival was frequency-dependent in the high-quality environment but not the low-quality environment. In the high-quality environment the survival curves of the two morphs crossed but the direction of frequency-dependence was opposite to what theory predicts.

Evidence for a third male type in a male‐dimorphic model species

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A Modelling Exercise to Show Why Population Models Should Incorporate Distinct Life-Histories of Dispersers

Dispersal is an important form of movement influencing population dynamics, species distribution, and gene flow between populations. In population models, dispersal is often included in a simplified manner by removing a random proportion of the population. Many ecologists now argue that models should be formulated at the level of individuals instead of the population-level. To fully understand the effects of dispersal on natural systems, it is therefore necessary to incorporate individual-level differences in dispersal behaviour in population models. Here we parameterised an integral projection model (IPM), which allows for studying how individual life histories determine population-level processes, using bulb mites, Rhizoglyphus robini, to assess to what extent dispersal expression (frequency of individuals in the dispersal stage) and dispersal probability affect the proportion of dispersers and natal population growth rate. We find that allowing for life-history differences between resident phenotypes and disperser phenotypes shows that multiple combinations of dispersal probability and dispersal expression can produce the same proportion of leaving individuals. Additionally, a given proportion of dispersing individuals results in different natal population growth rates. The results highlight that dispersal life histories, and the frequency with which disperser phenotypes occur in the natal population, significantly affect population-level processes. Thus, biological realism of dispersal population models can be increased by incorporating the typically observed life history differences between resident phenotypes and disperser phenotypes, and we here present a methodology to do so.Dispersal, an important form of movement, influences population dynamics, species distribution, and gene flow between populations. In population models, dispersal is often included in a simplified manner by removing a random proportion of the population. Many ecologists now argue that models should be formulated at the individual-level instead of the population-level to accurately capture how dispersal affects population dynamics. This is especially true for populations exhibiting boom-bust dynamics as these often harbour specialised disperser morphs, life histories or behaviours, as dispersal is essential for persistence. Within a management context, such dynamics play a key role in the stability of populations and in turn extinction risk of species. Here we parameterised an integral projection model, which allows studying how individual life histories determine population-level processes. Using bulb mites (Rhizoglyphus robini), a species that shows boom-bust dynamics, we assess the extent dispersal expression (frequency of disperser morphs) and dispersal probability (probability to emigrate) affect the proportion of dispersers and natal population growth rate. We find that, if residents and dispersers differ in life history, different combinations of dispersal probability and dispersal expression produce the same proportion of leaving dispersers. Additionally, for a given proportion of dispersing individuals different natal population growth rates occur. Dispersal life histories, and frequency of disperser morphs occurring in the natal population, can thus significantly affect population-level processes. It is therefore important for our understanding of boom-bust dynamics to elucidate how dispersal characteristics of individuals relate to population resilience and potential re-establishment for populations after a bust phase.