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We have noted already that counting the numbers in a population provides only an outline sketch, and one key reason for this is that virtually all organisms go through a number of stages in their lives, each with their own birth and death rates, responses to other organisms, resources and conditions, and so on. Hence, we need to understand the sequences of events that occur in those organisms’ life cycles. A highly simplified, generalised life history is outlined in Figure 4.6a. It comprises birth, followed by a prereproductive period, a period of reproduction, perhaps a postreproductive period, and then death as a result of senescence (though of course other forms of mortality may intervene at any time). The variety of life cycles is also summarised diagrammatically in Figure 4.6, although there are many life cycles that defy this simple classification. Some organisms fit several or many generations within a single year, some have just one generation each year (annuals), and others have a life cycle extended over several or many years. For all organisms, though, a period of growth occurs before there is any reproduction, and growth usually slows down (and in some cases stops altogether) when reproduction starts.

Figure 4.6 Schematic life histories for unitary organisms. (a) An outline life history for a unitary organism. Time passes along the horizontal axis, which is divided into different phases. Reproductive output is plotted on the vertical axis. The figures below (b–f) are variations on this basic theme (technical terms explained in the text). (b) A semelparous annual species. (c) An iteroparous annual species. (d) A long‐lived iteroparous species with seasonal breeding (that may indeed live much longer than suggested in the figure). (e) A long‐lived species with continuous breeding (that may again live much longer than suggested in the figure). (f) A semelparous species living longer than a year, where the prereproductive phase may be a little over one year (a biennial species, breeding in its second year) or longer, often much longer than this (as shown).

semelparous and iteroparous life cycles

Whatever the length of their life cycle, species may, broadly, be either semelparous or iteroparous (often referred to by plant scientists as monocarpic and polycarpic). In semelparous species, individuals have only a single, distinct period of reproductive output in their lives. Prior to this they have largely ceased to grow, during it they invest little or nothing in survival that might take them to future reproductive events, and after it they die. By contrast, in iteroparous species, an individual normally experiences several or many such reproductive events, which may in fact merge into a single extended period of reproductive activity. During each period of reproductive activity, however, the individual continues to invest in future survival and possibly growth, and it therefore has a reasonable chance of surviving beyond each bout of reproduction to reproduce again.

For example, many annual plants are semelparous (Figure 4.6b): they have a sudden burst of flowering and seed set, and then they die. This is commonly the case among the weeds of arable crops. Other annuals, such as groundsel (Senecio vulgaris), are iteroparous (Figure 4.6c): they continue to grow and produce new flowers and seeds through the season until they are killed by the first lethal frost of winter. They die with their buds on.

the variety of life cycles

There is also a marked seasonal rhythm in the lives of many long‐lived iteroparous plants and animals, especially in their reproductive activity, with a period of reproduction once per year (Figure 4.6d). Mating (or the flowering of plants) is commonly triggered by the length of the photoperiod (see Section 2.3.7), synchronising birth, egg hatch or seed ripening with the time that seasonal resources are likely to be abundant. Here, though, unlike annual species, the generations overlap and individuals of a range of ages breed side by side. The population is maintained in part by survival of adults and in part by new births.

In wet equatorial regions, on the other hand, where there is very little seasonal variation in temperature and rainfall and scarcely any variation in photoperiod, we find species of plants that are in flower and fruit throughout the year – and continuously breeding species of animal that subsist on this resource (Figure 4.6e). There are several species of fig (Ficus), for instance, that bear fruit continuously and form a reliable year‐round food supply for birds and primates. In more seasonal climates, humans are unusual in also breeding continuously throughout the year, though numbers of other species, cockroaches, for example, do so in the stable environments that humans have created.

Amongst long‐lived (i.e. longer than annual) semelparous plants (Figure 4.6f), some are strictly biennial. Each individual takes two summers and the intervening winter to develop, but has only a single reproductive phase, in its second summer. An example is the white sweet clover, Melilotus alba (Klemow & Raynal, 1981). In New York State, USA this has relatively high mortality during the first growing season (whilst seedlings are developing into established plants), followed by much lower mortality until the end of the second summer, when the plants flower and survivorship decreases rapidly. No plants survive to a third summer. Thus, there is an overlap of two generations at most. A more typical example of a semelparous species with overlapping generations is the composite Grindelia lanceolata, which may flower in its third, fourth or fifth years. But whenever an individual does flower, it dies soon after.

A well‐known example of a semelparous animal with overlapping generations (Figure 4.6f) is the Pacific salmon Oncorhynchus nerka. Salmon are spawned in rivers. They spend the first phase of their juvenile life in fresh water and then migrate to the sea, often travelling thousands of miles. At maturity they return to the stream in which they were hatched. Some mature and return to reproduce after only two years at sea; others mature more slowly and return after three, four or five years. At the time of reproduction the population of salmon is composed of overlapping generations of individuals. But all are semelparous: they lay their eggs and then die; their bout of reproduction is terminal.

There are even more dramatic examples of species that have a long life but reproduce just once. Many species of bamboo form dense clones of shoots that remain vegetative for many years: up to 100 years in some species. The whole population of shoots, from the same and sometimes different clones, then flowers simultaneously in a mass suicidal orgy. Even when shoots have become physically separated from each other, the parts still flower synchronously.

seed banks, ephemerals and other not‐quite‐annuals

For many annual plants, that description is itself, in an important sense, misleading, because their 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). This is just one example of real organisms spoiling our attempts to fit them neatly into clear‐cut categories. The formation of something comparable to a seed bank is rarer amongst animals, but there are examples to be seen amongst the eggs of nematodes, mosquitoes and fairy shrimps, the gemmules of sponges and the statocysts of bryozoans.

the species composition of seed banks

As a general rule, dormant seeds, which enter and make a significant contribution to seed banks, are more common in annuals and other short‐lived 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 4.7; see also Application 4.1).

Figure 4.7 The composition of seed banks can be very different from the vegetation above them. 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 (numbers of species shown below the group numbers). 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. Nonetheless, the marked difference in composition, especially between the seed bank and the mature vegetation, is readily apparent.

Source: After Jutila (2003).

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.

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 north‐east 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.