The Elements of Seasonal Adaptations in Insects
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1 The elements of seasonal adaptations in insects H.V. Danks Biological Survey of Canada (Terrestrial Arthropods), Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario, Canada K1P 6P4 (e-mail: [email protected]) Abstract—The many components of seasonal adaptations in insects are reviewed, especially from the viewpoint of aspects that must be studied in order to understand the structure and pur- poses of the adaptations. Component responses include dispersal, habitat selection, habitat modi- fication, resistance to cold, dryness, and food limitation, trade-offs, diapause, modifications of developmental rate, sensitivity to environmental signals, life-cycle patterns including multiple al- ternatives in one species, and types of variation in phenology and development. Spatial, tempo- ral, and resource elements of the environment are also reviewed, as are environmental signals, supporting the conclusion that further understanding of all of these seasonal responses requires detailed simultaneous study of the natural environments that drive the patterns of response. Résumé—On trouvera ici une revue des multiples composantes des adaptations saisonnières des insectes, particulièrement des aspects à examiner afin de comprendre la structure et les buts de ces adaptations. Ces composantes incluent la dispersion, la sélection d’habitat, la modification d’habitat, la résistance au froid, à la sécheresse et aux restrictions de nourriture, les compromis, la diapause, les modifications du taux de développement, la sensibilité aux signaux environne- mentaux, les patrons de cycles biologiques (y compris les patrons multiples possibles chez une même espèce), ainsi que les types de variation dans la phénologie et le développement. La revue considère aussi les conditions d’espace, de temps et de ressources dans le milieu de même que les signaux environnementaux. En conclusion, la compréhension accrue de ces réponses saison- nières requiert une étude détaillée et simultanée des environnements naturels qui régissent les pa- trons de réponses. [Traduit par la Rédaction] Danks44 Table of contents Developmental modifications 22 Sensitivity to environmental signals 23 Introduction 1 Life-cycle patterns 25 Elements of the habitat 2 Types of variation 26 Spatial framework 2 Conclusions 29 Temporal framework 3 References 30 Resources and limitations 4 Signals 5 Introduction Elements of the response 6 Spatial framework 6 The extensive literature about insect season- Temporal framework 7 ality reveals great diversity and complexity in Resources and limitations 9 the adaptations that withstand seasonal adver- Modification of adverse conditions 9 sity and synchronize development with the sea- Resistance to adversity 10 sons. This paper reviews the basic structure of Cold 10 these adaptations, identifying the elements that Dryness 13 are needed to characterize a given life cycle and Food limitation 16 hence serving too to identify the decisions Trade-offs 17 needed in planning investigations. Treating this Responses in extreme habitats 19 broad subject area in detail would require sev- Control and integration 22 eral books, but the aim here is to provide a Received 25 May 2006. Accepted 12 September 2006. Can. Entomol. 139: 1–44 (2007) © 2007 Entomological Society of Canada 2 Can. Entomol. Vol. 139, 2007 shorter treatment that gives the necessary over- on regional patterns of climate. Such large-scale view and essential details in one place, in the patterns may also have been modified by the his- hope that such a treatment will be of value to a torical processes of glaciation, isolation, and ge- range of readers. The approach therefore is syn- netic drift (e.g., Armbruster et al. 1998; Bossart optic rather than comprehensive, but relevant 1998; Stone et al. 2001). literature, especially reviews and recent papers, Intermediate spatial complexity, such as com- is cited to lead into the wider literature on each plex topography, influences dispersal and thus topic. Specific practical aspects have been interpopulation variation in species with limited treated by Danks (1987a, chap. 14, 1996, capacity for movement (Wishart and Hughes 2000c). 2001). Certainly, there is much evidence for Most accounts of insect seasonality focus im- heritable variation in life-history traits for popu- mediately on the organisms’ responses, such as lations from different habitats, for example be- photoperiodism. I begin instead with the impor- tween closed woodlands and open landscapes tant environmental context, because unravelling that are warmer (Karlsson and Van Dyck 2005) or the responses requires an understanding of the between habitats at different elevations (Sorensen characteristics and complexity of natural envi- et al. 2005). In particular, because local selection ronments. Spatial, temporal, and resource- favours specific life-cycle adaptation but inter- limited elements of the responses are then out- breeding offsets regional differentiation, knowing lined, including the ways in which insects deal about dispersal, range size, and other factors with seasonal adversity, followed by an account that interact with local habitat features such as of the ways in which the responses are con- growing season and resource distribution helps trolled and integrated. scientists to interpret seasonal adaptations and voltinism (cf. Dennis et al. 2000; Llewellyn et Elements of the habitat al. 2003). Habitat suitability at the local level comprises Habitats vary in space and time on a variety many elements beyond the general effects of of different scales. Small-scale spatial influ- climatic differences. For example, complex in- ences, notably the role of microhabitats, are teractions with host-plant phenology are known. especially important for insects, but they oper- Host plants may or may not coincide with the ate in the context of much larger patterns (for seasons in the same way as their herbivores, example, compare Corbet 1972; Oke 1987; creating differences in suitability from one Bailey et al. 1997). Adaptations are a result of place to the next. The fitness of many herbi- changes not just on seasonal time frames but vores depends on the close synchrony of egg also over longer and shorter intervals, and re- hatch with bud burst (e.g., Chen et al. 2003) or flect too the evenness or predictability of the on the quality of leaves at particular times of changes. year. Leaf quality tends to decline rapidly after Habitats provide both resources (such as spring, and again towards the end of the season. food) and constraints (such as cold winters). Summer leaves vary less, increasing the range Depending on time and place, therefore, insects of developmental durations that are possible in require a resistant stage during adversity and an species that feed during the middle of the sea- active stage during conditions that can be ex- son (cf.Foxet al. 1997; Kause et al. 2001). ploited for development or reproduction. Habi- Smaller scale influences are also known. For tats also differ in how visibly or reliably they example, modelling for one butterfly population provide signals from which current or future showed that locating larvae differently within conditions can be assessed. Consequently, the the habitat can change phenology by up to analysis of insect habitats in the context of life 11 days (Weiss and Weiss 1998). Differences cycles must address a range of elements: spatial among microhabitats are also critical to winter and temporal frameworks, resources and limita- survival. Thus, depending on locality and year, tions, and signal features. survival of the boll weevil Anthonomus grandis Boheman (Curculionidae) overwintering in lit- Spatial framework ter varies from 0% to 100% (Pfrimmer and The spatial complexity of habitats is one tem- Merkl 1981). Differences in survival can de- plate for the evolution of seasonal adaptations. pend on many very small-scale features, such as At the largest scale, the presence and intensity of local vegetation (which holds up snow) and developmental delays such as diapause depend whether it dies back seasonally, and surface © 2007 Entomological Society of Canada Danks 3 depressions (which accumulate water or snow especially dangerous to synchronized cohorts. and may reduce the impact of fires) (Danks Again, their effect depends on the mean tem- 1991b). perature (cf. Table 1). Variability from year to year makes suitable Temporal framework habitats or resources different in amount or in Habitat suitability, driven chiefly by climatic timing from one year to the next, threatening suitability, depends on how far conditions de- species without ways to modify fixed modes of part from those allowing development and on development. The effects of long-term extremes their seasonal pattern, but it depends also on the that differ greatly from the mean depend on the extent of variation both within and among years mean temperature. For example, subfreezing (Danks 1999). Differences in these parameters temperatures can occur in arctic summers (as are exemplified by data for a small number of noted above for the shorter term) and unusual sample sites shown in Table 1. Of course, many heat in subtropical summers (Table 1). The con- more climate types and gradations have been sequences of inter-seasonal variability have sel- recognized, including a range of alpine, desert, dom been quantified but may be more pervasive tropical, and oceanic climates (e.g., Hodkinson than is usually considered. For example, 2005a; Oliver 2005; Turnock and Fields