Ecology. Michael Begon
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in their first or second years is so slight that the description annual–biennial is most appropriate. In short, it is clear that annual life cycles merge into more complex ones without any sharp discontinuity.
APPLICATION 4.1 Seed banks and the restoration of forested wetlands
Changes to farming practices mean that there are an increasing number of cases of agricultural land being abandoned. Whenever this happens, there is an understandable hope that the natural habitat that had been destroyed to make way for farming can be restored. One example is the swampland dominated by bald cypress trees, Taxodium distichum, lying along the rivers and streams of the Gulf coastal floodplain of North America, running from south‐eastern Texas north and east to the Atlantic Ocean, and including a study site along the Cache River in southern Illinois (Middleton, 2003). Agricultural development expanded there in the 1950s, and by the late 1980s only about half of the forested swampland remained. However, the process has been halted and reversed, in part by a crash in the soybean market, and the emphasis now is on plans to restore the original swamplands, with the provision of habitat for hunting being a particular commercial driver, though there are Nature Preserve areas where hunting is not allowed.
In fact, restoration has proved difficult. The original species‐rich forests supported up to 60 or so species of trees, shrubs and vines, many of them with seeds dispersed in the seasonal floods, but the forests developing following agricultural abandonment tend to be dominated by a few species with wind‐dispersed seeds. An important question, therefore, is what potential seed banks have in promoting more natural restoration. To address this, Middleton (2003) assayed the seeds from nine sites in intact bald cypress swamps and 51 sites in the area that had been farmed for between one and 50 years. She found not only that there was no relationship for the dominant swamp species between the length of time farming had been practiced and seed abundance – but actually, the seeds of many of those species, including bald cypress itself, were absent from the seed banks altogether in both the farmed and intact sites. Instead these were composed mostly of seeds from large numbers of herbaceous species. It seems, therefore, that in this case seed banks can have little part to play in habitat restoration, and that abandonment alone, even of land that has been farmed for a relatively short time, offers little prospect of a return to natural habitat. Rather, the short‐lived seeds of the dominant woody species of bald cypress swamps are likely to return and promote successful restoration only if flood pulsing across the landscape is re‐engineered, reconnecting pristine to abandoned sites.
4.5 Dormancy
migration in time
We will discuss in Chapter 6 how an organism gains in fitness by dispersing its progeny ‘elsewhere’, as long as the progeny are more likely to leave descendants than if they remained undispersed. Similarly, an organism gains in fitness by delaying its arrival 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