on the scene, so long as the delay increases its chances of leaving descendants. This will often be the case when conditions in the future are likely to be better than those in the present. Thus, a delay in the recruitment of an individual to a population may be regarded as ‘migration in time’.
Organisms generally spend their period of delay in a state of dormancy. This relatively inactive state has the benefit of conserving energy, which can then be used during the period following the delay. In addition, the dormant phase of an organism is often more tolerant of the adverse environmental conditions prevailing during the delay (i.e. tolerant of drought, extremes of temperature, lack of light and so on). Dormancy can be either predictive or consequential (Müller, 1970). Predictive dormancy is initiated in advance of the adverse conditions, and is most often found in predictable, seasonal environments. It is generally referred to as ‘diapause’ in animals, and in plants as ‘innate’ or ‘primary’ dormancy (Harper, 1977). Consequential (or ‘secondary’) dormancy, on the other hand, is initiated in response to the adverse conditions themselves.
4.5.1 Dormancy in animals: diapause
Diapause has been most intensively studied in insects, where examples occur in all developmental stages. The common field grasshopper Chorthippus brunneus is a fairly typical example. This annual species passes through an obligatory diapause in its egg stage, where, in a state of arrested development, it is resistant to the cold winter conditions that would quickly kill the nymphs and adults. In fact, the eggs require a long cold period before development can start again (around five weeks at 0°C, or rather longer at a slightly higher temperature). This ensures that the eggs are not affected by a short, freak period of warm winter weather that might then be followed by normal, dangerous, cold conditions. It also means that there is an enhanced synchronisation of subsequent development in the population as a whole. The grasshoppers ‘migrate in time’ from late summer to the following spring.
the importance of photoperiod
Diapause is also common in species with more than one generation per year. For instance, the fruit‐fly Drosophila obscura passes through four generations per year in England, but enters diapause during only one of them. This facultative diapause shares important features with obligatory diapause: it enhances survivorship during a predictably adverse winter period, and it is experienced by resistant diapause adults with arrested gonadal development and large reserves of stored abdominal fat. In this case, synchronisation is achieved not only during diapause but also prior to it. Emerging adults react to the short daylengths of autumn by laying down fat and entering the diapause state; they recommence development in response to the longer days of spring. Thus, by relying, like many species, on the utterly predictable photoperiod as a cue for seasonal development, D. obscura enters a state of predictive diapause that is confined to those generations that inevitably pass through the adverse conditions.
Consequential dormancy may be expected to evolve in environments that are relatively unpredictable. In such circumstances, there will be a disadvantage in responding to adverse conditions only after they have appeared, but this may be outweighed by the advantages of: (i) responding to favourable conditions immediately after they reappear; and (ii) entering a dormant state only if adverse conditions do appear. Thus, when many mammals enter hibernation, they do so (after an obligatory preparatory phase) in direct response to the adverse conditions. Having achieved ‘resistance’ by virtue of the energy they conserve at a lowered body temperature, and having periodically emerged and monitored their environment, they eventually cease hibernation whenever the adversity disappears.
4.5.2 Dormancy in plants
Seed dormancy is an extremely widespread phenomenon in flowering plants. The young embryo ceases development whilst still attached to the mother plant and enters a phase of suspended activity, usually losing much of its water and becoming dormant in a desiccated condition. In a few species of higher plants, such as some mangroves, a dormant period is absent, but this is very much the exception – almost all seeds are dormant when they are shed from the parent and require special stimuli to return them to an active state (germination).
Dormancy in plants, though, is not confined to seeds. Many species accumulate dormant bud banks analogous to the seed banks produced by other species. In one study of tallgrass prairies in north‐eastern Kansas, USA, for example, it was estimated that more than 99% of new tiller production arose from below‐ground vegetative buds rather than from seed (Benson & Hartnett, 2006); and in another prairie study, in Montana, USA, the differential responses of grass species to fire at different seasons of the year, especially their release by fire from dormancy, were crucial in driving the overall dynamics and community structure (Russell et al., 2015).
Indeed, the very widespread habit of deciduousness is a form of dormancy displayed by many perennial trees and shrubs. Established individuals pass through periods, usually of low temperatures and low light levels, in a leafless state of low metabolic activity.
innate, enforced and induced dormancy
Three types of dormancy have been distinguished:
1 Innate dormancy is a state in which there is an absolute requirement for some special external stimulus to reactivate the process of growth and development. The stimulus may be the presence of water, low temperature, light, photoperiod, fire (see previously) or an appropriate balance of near‐ and far‐red radiation. Seedlings of such species tend to appear in sudden flushes of almost simultaneous germination. Deciduousness is also an example of innate dormancy.
2 Enforced dormancy is a state imposed by external conditions (i.e. it is consequential dormancy). For example, the Missouri goldenrod Solidago missouriensis enters a dormant state when attacked by the beetle Trirhabda canadensis. Eight clones, identified by genetic markers, were followed prior to, during and after a period of severe defoliation. The clones, which varied in extent from 60 to 350 m2 and from 700 to 20 000 rhizomes, failed to produce any above‐ground growth (i.e. they were dormant) in the season following defoliation and had apparently died, but they reappeared 1–10 years after they had disappeared, and six of the eight bounced back strongly within a single season (Figure 4.8). Generally, the progeny of a single plant with enforced dormancy may be dispersed in time over years, decades or even centuries. Seeds of Chenopodium album collected from archaeological excavations have been shown to be viable when 1700 years old (Ødum, 1965).
3 Induced dormancy is a state produced in a seed during a period of enforced dormancy in which it acquires some new requirement before it can germinate. The seeds of many agricultural and horticultural weeds will germinate without a light stimulus when they are released from the parent; but after a period of enforced dormancy they require exposure to light before they will germinate. For a long time it was a puzzle that soil samples taken from the field to the laboratory would quickly generate huge crops of seedlings, although these same seeds had failed to germinate in the field. It was a simple idea of genius that prompted Wesson and Wareing (1969) to collect soil samples from the field at night and bring them to the laboratory in darkness. They obtained large crops of seedlings from the soil only when the samples were exposed to light. This type of induced dormancy is responsible for the accumulation of large populations of seeds in the soil. In nature they germinate only when they are brought to the soil surface by earthworms or other burrowing animals, or by the exposure of soil after a tree falls.
