Using Phlox as an example of an annual plant has, to a certain extent, been misleading, because the group of seedlings developing in 1 year is a true cohort: it derives entirely from seed set by adults in the previous year. Seeds that do not germinate in 1 year will not survive till the next. In most 'annual' plants this is not the case. Instead, seeds accumulate in the soil in a buried seed bank. At any one time, therefore, seeds of a variety of ages are likely to occur together in the seed bank, and when they germinate the seedlings will also be of varying ages (age being the length of time since the seed was first produced). The formation of something comparable to a seed bank is rarer amongst animals, but there are
Figure 4.9 Survivorship curves (lx, where l0 = 1000) for the sand-dune annual plant Erophila verna monitored at three densities: high (initially 55 or more seedlings per 0.01 m2 plot); medium (15-30 seedlings per plot); and low (1-2 seedlings per plot). The horizontal scale (plant age) is standardized to take account of the fact that each curve is the average of several cohorts, which lasted different lengths of time (around 70 days on average). (After Symonides, 1983.)
examples to be seen amongst the eggs of nematodes, mosquitoes and fairy shrimps, the gemmules of sponges and the statocysts of bryozoans.
Note that species commonly referred to as 'annual', but with a seed bank (or animal equivalent), are not strictly annual species at all, even if they progress from germination to reproduction within 1 year, since some of the seeds destined to germinate each year will already be more than 12 months old. All we can do, though, is bear this fact in mind, and note that it is just one example of real organisms spoiling our attempts to fit them neatly into clear-cut categories.
As a general rule, dormant seeds, which enter and make a significant contribution to seed banks, are more common in annuals and other shortlived plant species than they are in longer lived species, such that short-lived species tend to predominate in buried seed banks, even when most of the established plants above them belong to much longer lived species. Certainly, the species composition of seed banks and the mature vegetation above may be very different (Figure 4.10).
Annual species with seed banks are not the only ones for which the term annual is, strictly speaking, inappropriate. For example, there are many annual plant species living in deserts that are far from seasonal in their appearance. They have a substantial buried seed bank, with germination occurring on rare occasions after substantial rainfall. Subsequent development is usually rapid, so that the period from germination to seed production is short. Such plants are best described as semelparous ephemerals.
Figure 4.10 Species recovered from the seed bank, from seedlings and from mature vegetation in a coastal grassland site on the western coast of Finland. Seven species groups (GR1-GR7) are defined on the basis of whether they were found in only one, two, or all three stages. GR3 (seed bank and seedlings only) is an unreliable group of species that are mostly incompletely identified; in GR5 there are many species difficult to identify as seedlings that may more properly belong to GR1. None the less, the marked difference in composition, especially between the seed bank and the mature vegetation, is readily apparent. (After Jutila, 2003.)
A simple annual label also fails to fit species where the majority of individuals in each generation are annual, but where a small number postpone reproduction until their second summer. This applies, for example, to the terrestrial isopod Philoscia muscorum living in northeast England (Sunderland et al., 1976). Approximately 90% of females bred only in the first summer after they were born; the other 10% bred only in their second summer. In some other species, the difference in numbers between those that reproduce 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.
Was this article helpful?