Dormancy and other seed strategies

So dispersal is predicted to increase or diminish in response to a number of selective pressures and there is growing evidence in favour of those predictions. Seed dormancy has a similarly rich theoretical history. Cohen (1966) imagined an annual plant living in a temporally varying environment. Seeds can either germinate or remain dormant to the next year, in which case they suffer a mortality cost. He then asked what proportion of germination maximizes long-term population growth rate (a non-ESS approach). Unsurprisingly, as the probability of total reproductive failure in any year increases, so does the optimal dormant fraction of seeds. Dormancy, like dispersal, is therefore favoured by environments that vary in time. The parallels do not end there: dormancy is also favoured by competition between occupants of a patch, and especially if the occupants are sibs, just like dispersal (e.g. Ellner 1987).

There is evidence to connect temporal heterogeneity with dormancy and competition. Pake and Venable (1996) quantified dormancy in winter annuals of the Sonoran desert, and found that species with higher temporal variation in reproductive success had lower germination fractions. Hyatt and Evans (1998) found a weak but significant negative association between family size and germination fraction in the desert mustard Lesquerella fendleri, consistent with the idea that increased levels of sibling competition select for increased levels of dormancy.

There is, however, yet another seed characteristic that can be selected for under the same pressures: seed size. Larger seeds are generally favoured under adverse circumstances for germination, such as competition and unsuitable weather. Therefore plants are actually faced with (at least) three alternative seed strategies to cope with kin competition, crowding, and hetereogeneity: dispersal, dormancy, or larger seeds. Venable and Brown (1988), in a wonderful paper that unifies the theory on these three different life history characteristics, have explored whether these alternative strategies actually have different fitness consequences in the face of environmental variability. All three can work as bet hedging strategies to spread risk. All reduce the year-to-year variation in fitness at the expense of reducing average (arithmetic mean) fitness: dormancy increases fitness in unfavourable years, hence reduces fitness variance across years, but at the cost of reduced population growth rate in favourable years and reduced total germination. Dispersal reduces the spatial variance in fitness but at the cost of mortality or fecundity. Seed size lowers the temporal variance in fitness because it improves fitness in unfavourable years, but decreases seed yield in favourable years because large seeds cost more than small seeds. Suppose now that in a variable environment, there is a single optimum balance of the mean and variance in fitness. Increases in the level of one seed characteristic, say dispersal, will demand reductions in one or both of the others. There should therefore be a trade-off between these three variables, one not governed by a constraint but one due to selection.

In their model, Venable and Brown consider what levels of germination rate, seed yield per germinating seed, and the probability of dispersal maximize long-term fitness gain. The model considers five variables and how they affect selection for dispersal, dormancy, or seed size. The results are understandable if one views dispersal as a way of escaping in space, dormancy as escaping in time, and seed size as local endurance (roughing it).

First, consider variation in the number of habitat patches. Increasing this creates selection for increased dispersal, decreased dormancy, and reduction in seed size. The reason is simple: the opportunity to escape in space is increased. As a correlated response, the other two traits decrease. Second, consider decreases in the likelihood of favourable conditions. This always increases seed size, initially decreases dormancy but later increases it again, and initially increases dispersal but later decreases it. Increasingly unfavourable conditions naturally favour endurance strategies. Dispersal is initially favoured if there are only a few unfavourable patches, but less so when nearly the entire habitat is unfavourable. Then conditions favour dormancy instead. Third, localized dispersal creates selection for reduced dispersal and as a result more dormancy and larger seeds, a very intuitive result. Fourth, if patch suitability is similar in space, this selects for decreased dispersal, increased dormancy, and increased seed size. The effectiveness of dispersal as a means for escaping unfavourable conditions is reduced. Finally, if patch suitabilities are similar in time, this decreases dispersal and dormancy but increases seed size. It is obviously not possible to escape in space, so dispersal should not be favoured here.

Hence not only do seed size, seed dormancy, and seed dispersal trade-off against each other, but the optimal balance between them can evolve in response to subtle changes in the nature of the environment. Comparisons between different species, particularly among the well-described British flora, provide excellent evidence that these seed properties trade-off against each other. Rees (1993, 1997) has compared dispersal measures, seed size, and dormancy propensity in several datasets. All three traits are negatively related to each other. Although there have been few explicit tests of the environmental correlates, the theoretical predictions match anecdotal observations on these traits. For example, in many island plants, not only has dispersal decreased but seed size has increased (see Carlquist 1965, 1974). Indeed the largest seed of any plant, the Coco-de-mer, is an endemic of the Seychelles islands (see Edwards etal. 2002). These observations fit the predictions about the number of habitat patches. Seed dormancy is reputed to be common in desert plants, where the probability of favourable years (rain) is especially low. In general, however, there is a great opportunity for these predictions to be tested by long-term experiments that either manipulate or at least measure these variables in a quantifiable fashion.

Dispersal and dormancy are both costly traits and this realization set in motion two initially independent research programmes. Both traits are exciting because both are favoured by the same broad sets of environmental variables, and, at least in plants, need to be considered in concert. Dispersal in both plants and animals impinges on numerous other life history traits (seed size, fecundity), and perhaps it is time that both should rightly take their place among the core of life history theory. In addition, dispersal and dormancy evolution provide a clear link between life histories and other aspects of ecology and evolution.

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