Biological Survey of Canada (Terrestrial Arthropods), Canadian Museum of Nature,
P.O. Box 3443, Station D, Ottawa, ON K1P 6P4
General information and editorial notes
News and Notes:
Canada occupies about 10 million km², and as might be expected different parts have different climates, with considerable differences from south to north and from east to west. Nevertheless, Canada’s basic climate is a northern, continental one with hot summers and cold winters. Most major Canadian cities have average annual temperatures only 1 to 5°C above freezing, and winters much colder than that.
The potential social impacts of climate change will be greatest in southern Canada, influencing dominant elements of the Canadian economy including agriculture, forestry, and fisheries, and therefore probably attracting more attention than changes in natural environments. But a focus on change in the north is also appropriate not just because there are huge tracts of uniform habitat like arctic, boreal forest, and transition zones that will be influenced by warming trends, but in particular because the available climate models predict that temperature changes will be much greater in northern Canada than in the south.
Global temperatures have been increasing. The trend in Canada is similar, about 0.4°C in the 50 or 60 years since reliable country-wide records began, compared with 0.5° globally. Nevertheless, there is a great deal of year-to-year variation, so that on relatively short time frames it is difficult to see either the climatic trends or the possible insect responses.
Warming of the country is projected to continue. Nevertheless, there are important geographical and seasonal differences. For example, by 2050 the arctic is expected to have warmed by 5 degrees C or so, while most of the country including the boreal forest will see only about 2 or 3 degrees of increase.
These forecasts are by no means precise. Data about trends are extrapolated by making assumptions about future levels of greenhouse gases such as carbon dioxide and sulphates. Complex models incorporate these and other data (including incoming and outgoing radiation, global circulation patterns, clouds, precipitation, snow and ice, wind, mountains, ocean currents, sea ice, volcanic activity, etc.) to make quantitative predictions, so the process is inexact. Indeed, some models predict up to twice the rise of temperature that is customarily cited.
All models confirm that the effects will vary in location and in degree, and among seasons of the year. In Canada, for example, changes are forecast to be smaller in summer than in winter. Quantitative predictions are also available for Canada about the effects of climatic warming on physical elements such as coastline submergence, permafrost thaw, forest fire impacts, river flows, sensitivity to erosion, degradation of northern soils, and peatland changes. However, there is not much detail about expected responses at the biological level, except for the usual generalizations that the ranges of animals and plants will extend northward, for example, or that grassland will eventually replace forest in the warmer, drier centre of the continent.
Looking at northern insects in this context is instructive because the arctic arthropod fauna is surprisingly numerous with at least a few thousand species north of tree line, although they belong mainly to a limited number of taxa that do well under these harsh conditions. Insects that eat decaying materials (especially in water and soil) and parasites like lice on the skin of birds and mammals are most common, reflecting the warmest habitats and the availability of food. Because the fauna is taxonomically and ecologically distinctive, climatic warming might be expected to bring some dramatic changes.
Because of the effects of warming by sunshine, uneven distribution of snow driven by the wind, the presence of permafrost, and so on, temperatures in insect habitats are more diverse and variable than the air temperatures used in climate models. Therefore, insects are affected by various patterns of climate, not just by the mean temperatures that are most easily cited and modelled. For example, different degrees of severity, seasonality, unpredictability and variability are associated with different climates, and these will change in different ways as climate changes.
Severity reflects persistent conditions that limit life, such as low summer temperatures and very low winter temperatures, as well as dryness, for example. The severity of conditions for growth can be estimated from the number of day-degrees accumulated in a season as well as from mean temperatures. Coping with severity requires insect resistance, specific microhabitat selection, and so on. The high arctic currently has very few day-degrees for growth; summer temperatures are so low that the predicted summer temperature increase will double the mean July temperature.
Seasonality reflects the fact that conditions are intermittently favourable on an annual time frame. Insects have to time the life cycle to limit development and activity to appropriate times of year, sometimes in a very narrow window when the season is very short, but also if foodplants are suitable for only a short period.
Unpredictability reflects the short-term pattern of conditions, such as the expected range of relevant temperatures in a given month, and hence the need for adaptations to sudden temperature changes. The impact of this range depends also on the mean temperature. For example, in cold High Arctic sites the likelihood of frost in July (the warmest month) may reach 90%.
Variability shows how variable are such features as summer temperature from year to year. Such patterns establish the extent of mortality that might be associated with fixed patterns such as emergence at a given time of year.
Examining these more complex climate patterns still falls far short of allowing us to interpret what will happen to insects when the climate changes, because arthropods live in microhabitats that, especially when warmed by sunshine, can be extremely favourable compared to the air above them. Some arctic soil and plant habitats have temperatures commonly 10°C and up to 30°C or more higher than ambient air temperatures. Shallow ponds stay 10°C or more above ambient during the High Arctic summer when air temperatures are only 4 or 5°C.
Of course, warming trends in climate should permit additional species to survive farther north or augment populations of existing species, because a dramatic effect might be expected from even a modest increase in temperature when current temperatures operate so close to the limits for life for insects and the effective growing season is so short. However, the expectation for insects that there will be more generations per year, more stable and persistent populations, and addition of species currently prohibited by low heat sums is too simple.
For example, diapause may limit development to one generation per year, and even if temperatures rise time may be “wasted” to prevent development that could not be completed before winter. Then there will not be any sort of “linear” increase in development according to temperature, even though much of the literature and oversimplified experimentation makes this assumption. Summers may be longer, but this change will not necessarily improve conditions for insects because the microhabitat temperatures and moisture relationships important to insects do not all coincide with changes in mean ambient temperature. Winters may be shorter but they will still be very cold, while concomitant changes in insulating snow cover will have other effects on these organisms.
Conditions in the high arctic provide a cautionary example. In the Canadian western high arctic relatively small islands lie in the Arctic Ocean. Climate warming is likely to melt permanent or seasonal sea ice to expose more open ocean. In turn, this open ocean will probably lead to increased cloudiness in summer. The resulting reduction in sunshine would make insect microhabitats cooler and thus almost certainly more than offset any increase in mean air temperature. Indeed, the insect fauna of the northwestern arctic is already much less rich than the east because of the greater summer cloud cover. Such changes are driven by global circulation patterns (cf. climate models) but also by more local effects such as island size, relief and the extent of sea ice in summer.
Moreover, temperature is not the only constraint. The high arctic is a polar desert with extremely limited annual precipitation, where moisture as well as cold constrains life. Changes in moisture regimes that come with climate change affect the suitability of habitats for insects through the amount and seasonal supply of moisture itself, not only by the duration of sunshine controlled by cloudiness. But the effects of climatic warming on moisture regimes are difficult to predict at the local level.
Another complication of understanding climate-change effects on insects is that individual arctic species are engaged in surprisingly complex food webs. Herbivorous insects are much reduced in more severe sites, where insects drop out much faster than the plants on which they could feed. Change would therefore be expected to generate changes in the dynamics of herbivores and pollinators, but we have only general indications of what might happen. Again, most arctic plants are low and clump forming and rely for seasonal growth or seed development on sunshine, and so would be affected in some of the same ways as insects by changes in cloud cover.
In addition, many insects characteristically respond to changed environments not by new adaptations but by movement to or from areas that are newly or no longer suitable. The short life cycles and the potential for aerial dispersal of many insects allows a rapid response to environmental changes, and insects of various kinds disperse into arctic areas from sources many hundreds of kilometers away. Changes will reflect naturally and human-caused introduced species and the dynamics of colonization, not just a change in local climates.
Northern insects could be used to measure climate change Such measurements would have to be relatively simple, because for the arctic fauna in particular it is not possible to sample continuously or even regularly in such remote places.
Some conspicuous species (butterflies, mosquitoes, and bumble bees) that are well known taxonomically would lend themselves to use as markers of abundance or range.
Because many arctic species are linked with other organisms, it may be possible to use interspecific ratios, such as herbivores compared to plants, to indicate ecosystem structure, and hence to reveal correlated or discordant responses of different taxa to environmental change. Aquatic species are better represented in more northern zones compared to terrestrial species, reflecting the fact that shallow waters are especially favourable habitats in the arctic because they warm up rapidly by solar heating of the bottom. Their proportion might decline as climates warm up and support additional terrestrial species.
Detailed assessments of change might also be possible by summarizing the composition of the fauna, because particular taxa increase proportionally and others decrease as climates become more severe or ameliorate. For example, in the arctic, taxa such as the Order Diptera, the family Chironomidae and certain insect genera are well represented whereas many others are not. Therefore, the relative representation of selected groups (which can be assessed by a relatively modest local faunal inventory) reflects environmental severity and could be monitored to detect long-term changes.
In summary, thinking about insects in northern Canada in the context of climate change gives a number of clues as to what might happen. First, matters will be much more complex than is suggested by simple proposals such as the invasion of more species into higher latitudes, because the effects will vary widely among species. Insects survive by multiple adaptations throughout the life cycle, involving co-ordinated physiological, structural, ecological and timing features – in the arctic for example especially by cold hardiness, microhabitat choice, life-cycle timing, and energy budgeting. There is no one-to-one relationship with any simple temperature statistic, and by the same token the major constraint for a given insect species may not be the most conspicuous one. Indeed, the need for early spring development and reproduction outweighs the need for additional protection from cold in many arctic species, which therefore overwinter in early thawing but relatively exposed sites. Some predicted effects of reduced winter snow cover would not apply to these species.
The importance of microhabitats likewise is undervalued by most temperature-change models, because sunshine rather than air temperature is most important in raising the temperature of insect microhabitats at northern latitudes. Increases in air temperature can even have counter-intuitive effects, as when reduction of sea ice reduces solar warming of insect habitats.
Although we see such possibilities in extreme form in parts of northern Canada, the complexity thereby revealed does suggest caution in extrapolating the effects of climate change for insects anywhere in the world.
Selected references for the information on insects here include the following reviews that also cite wider literature:
Danks, H.V. 2006. Insect adaptations to cold and changing environments. The Canadian Entomologist 138: 1-23;
Danks, H.V. 2004. Seasonal adaptations in arctic insects. Integrative and Comparative Biology 44: 85–94;
Danks, H.V. 1999. Life cycles in polar arthropods – flexible or programmed? European Journal of Entomology 96: 83-102;
Danks, H.V. 1993. Patterns of diversity in the Canadian insect fauna. pp. 51-74 in Ball, G.E. and H.V. Danks (Eds.). Systematics and entomology: diversity, distribution, adaptation and application. Mem. ent. Soc. Can. 165. 272 pp. ;
Danks, H.V. 1992. Arctic insects as indicators of environmental change. Arctic 45(2): 159-166;
Danks, H.V. 1990. Arctic insects: instructive diversity. pp. 444-470, Vol. II in C.R. Harington (Ed.), Canada's missing dimension: Science and history in the Canadian arctic islands. Canadian Museum of Nature, Ottawa. 2 vols, 855 pp.
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