Canadian Perspectives: Life-cycle Types in the Arctic
General information and editorial notes
News and Notes:
It might be expected that the cold temperatures and marked seasonality of polar regions would limit the types of life cycles that are possible in the arthropods living there. One expectation might be that flexibility or opportunism would be prevalent, because it allows growth and development whenever conditions are favourable, so that a species can take advantage of every window of opportunity for development. On the other hand, many insect life cycles involve strict temporal programmes, especially in cold and seasonal environments. There, developmental programmes restrict adult emergence to suitable times of year or enable larval stages to coincide with temporally limited food resources.
Information especially from the Canadian arctic, where many arthropod life cycles are known in general terms, can be used to assess the relative contributions of flexibility or opportunism and of fixity or programming. Depending on taxon, zone, habitat, and food, life cycles in arctic regions take a variety of forms including relatively complex ones, showing that there are many degrees of flexibility. For example, the fact that a life cycle is programmed does not mean that it is precisely controlled every step of the way. In particular, many insect life cycles are more or less flexible in some of their stages, but mechanisms exist to restore seasonal synchrony at or before critical times of year.
Therefore, it is essential to distinguish patterns of the life cycle as a whole from features of individual stages, even though more information exists about single components because it is easier to collect. For example, high arctic mosquitoes are strictly univoltine and overwinter only as drying- and freezing-resistant eggs in diapause. The position of seasonal emergence nevertheless differs widely from one season to the next, depending on weather, and from one pond to the next, depending on temperatures in a given habitat, because the rates of larval and pupal development are regulated by temperature. Thus, development is flexible locally (at least within a season), but the life cycle as a whole is very closely constrained.
Data for arctic Canada and elsewhere show that several species of springtails and mites, and a few insects, indeed appear to have fully flexible life cycles, as suggested by such things as shorter life cycles where conditions are warmer, winter activity, overlapping generations, lack of seasonal synchrony, winter activity, lack of a fixed overwintering stage and lack of build-up of fat for winter. These species live in relatively well-buffered soil habitats.
Nevertheless, a larger number of species show programmed components. A particularly common pattern is that immature development is flexible (again, typically in habitats that are relatively well buffered, such as the soil or shallow pools), but adult emergence into less stable terrestrial or aerial habitats is very closely controlled, ensuring early emergence and rapid reproduction. For example, high arctic pond chironomids, which spend several years in the larval stage, emerge only if all growth has been completed the previous year, resulting in the earliest possible start in spring and hence the longest possible season for reproduction before winter returns. A few other arthropods develop from egg to adult over a single season but the adult life span is long and eggs are deposited over a number of seasons.
Other species have life cycles that are relatively closely programmed throughout. For example, the high arctic geometrid moth Psychophora sabini appears to moult only once each year, gaining the next instar in spring before beginning to feed. An alternative strategy is to develop extremely rapidly, as in univoltine species that are surprisingly well represented in the arctic. These species complete the whole life cycle in a single season from a fixed overwintering stage. The overwintering stage is characteristic of the taxon, including the egg (e.g. Aedes mosquitoes), larva (e.g. the tundra pool caddisfly Sphagnophylax meiops) or adult (e.g. bumble bees).
On a longer time frame, a feature of many species is some life-cycle variability among individuals, leading to differences in the numbers of years per generation, for example. Such variability among individuals experiencing similar conditions appears to be "insurance" (which is a programmed response) against the uncertainty of environmental events, rather than "flexibility" to take advantage of possible developmental opportunities. Short-term variability prevents a generation late in the year or in an especially cold year that would be likely to fail (as demonstrated in high arctic chironomids); long-term variability reflects the spreading of risk in uncertain environments (such as prolonged diapause in a variety of arctic flies, sawflies, moths and other species).
Mechanisms by which these arctic life cycles are adjusted seasonally therefore include the occurrence, placement, duration, variability and continuity of both development and reproduction, with various ways to accelerate, delay and adjust the life cycle. Some mechanisms depend on durations that can be altered gradually, as for larval growth rates. Others trigger switches between distinct alternatives such as diapause and non-diapause.
In arctic regions with short, cool summers, many species promote the earliest possible emergence. At least some of these responses depend on a fixed overwintering resting or diapause stage, typically the prepupa, in which larvae accumulate at the end of the previous season, as in arctic chironomids and crane flies. Several species develop very rapidly on account of low temperature thresholds for growth, high growth rates even at relatively low temperatures, a relatively short pre-moult period, and an abbreviated adult stage, as demonstrated in arctic moths, bugs, mosquitoes, psyllids, leaf beetles, sawflies and so on.
A few species eliminate generations or stages (adult feeding, mating, and even the adult stage itself) that are present in their temperate relatives, and although such traits are also known in certain temperate species they are more common in arctic regions. For example, telescoped development and modified mating in the arctic are known in groups that include aphids, bumble bees, black flies and chironomids, and parthenogenesis is known in mayflies, caddisflies and bugs, and in flies of various families.
Rapid development is possible even in cold regions through relatively rapid metabolism at a given temperature. However, although temperature adaptation has been demonstrated in several species of arctic arthropods it is by no means universal, because many other functions besides the rate of respiration may constrain species in these conditions. Much more commonly, arctic species increase body temperature by choosing specific, relatively warm, microhabitats. Many species — including butterflies and flies and even immature stages such as caterpillars of the moth Gynaephora — have closely adapted basking behaviours to increase body temperatures in sunshine.
There are also mechanisms to delay rather than accelerate the life cycle; such delays may be necessary to ensure that feeding stages coincide with habitats or food supplies that are of low quality, unreliable, or seasonally restricted, or to avoid producing a generation or stage so close to winter that it would be certain to fail. These mechanisms include slow development or reproduction, and interpolation of resting stages.
Diapause, the resting stage characteristic of temperate regions, does occur in arctic species, including some species of chironomid midges, crane flies, mosquitoes, black flies and stoneflies, and this finding is especially significant because the number of studies designed to detect it is so small, given the difficulty of working in the arctic with very limited laboratory facilities.
Both directly controlled and cued development help to adjust arctic life cycles. Many species use temperature cues (as opposed to simple regulation of faster development at higher temperatures) to control development, and one or more key temperature thresholds govern seasonal emergence of adults in various species. In high arctic chironomids, temperature requirements rise steadily for successive components of spring emergence: 1°C for larval activity, 4–5°C for pupation, and 7°C for adult emergence.
Temperature, then, is especially important to arctic arthropods, and is commonly used to regulate the life cycle. Conversely, photoperiod becomes much less relevant than temperature toward the poles: indeed, temperature alone is used by chironomid species in the high arctic to govern adult emergence, including its daily pattern.
In summary, the life cycles of arctic arthropods are diverse, including both flexible development (as in some soil arthropods), and relatively closely governed systems (such as a single moult each spring). However, the life cycles of most species do not show one or the other extreme but instead combine elements of flexibility and programming at different stages, depending on the species. In particular, even species with life cycles that appear to be largely flexible have programmed elements at the season or stage when timing is most significant, such as the emergence of the adult in spring. In addition, some degree of insurance or risk spreading, rather than precise and universal seasonal or annual coincidence, is achieved through variability.
Moreover, as already noted, arctic life cycles are clearly correlated with the habitats and microhabitats of individual species. For example, forms such as mites and springtails that spend their whole lives in soil tend to have less structured life cycles, because conditions there are buffered. Again, species that live on warm-blooded vertebrate hosts in the winter, such as some fleas and lice, show little life-cycle programming.
On the other hand, most insect species that spend their larval stages in the buffered habitats of soil or water have aerial adults that emerge into more seasonal and rigorous conditions above ground. Their life cycles tend to be seasonally constrained, and adult emergence is closely programmed. Such a finding for species that emerge into aerial habitats re-emphasizes the need to look at whole life cycles and not simply for flexible larval growth, for example.
Evidence from arctic arthropods, then, coincides with the lessons from the more abundant information that is available for temperate regions in confirming that flexibility and opportunism are by no means the only way to cope with highly seasonal or unpredictable environments. Certainly there is room for selective opportunism. However, in most habitats including extreme ones a successful life cycle normally cannot be maintained merely through simple, unstructured, opportunistic responses to the environment. Such a conclusion suggests that life cycles evolve not so much to ensure that growth can be fitted in (or even that adverse conditions can be avoided) but rather so that critical stages of the life cycle, and notably reproduction, coincide with conditions that are favourable. Even in the arctic, specific adaptations for life-cycle timing are generally required, not just flexible opportunism.
[For detailed information and additional references, see especially Danks, H.V. 1999. Life cycles in polar arthropods – flexible or programmed? European Journal of Entomology 96: 83-102.]
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