After Pope et al. (2000).
counting births
Counting births can be more difficult even than counting individuals. The formation of the zygote is often regarded as the starting point in the life of an individual. But it is a stage that is often hidden and extremely hard to study. We simply do not know, for most animals and plants, how many embryos die before ‘birth’, though in the rabbit at least 50% of embryos are thought to die in the womb, and in many higher plants it seems that about 50% of embryos abort before the seed is fully grown and mature. Hence, it is almost always impossible in practice to treat the start of life as the time of birth. In birds we may use the moment that an egg hatches; in mammals, perhaps, when an individual starts to be supported outside the mother as a suckling; and in plants we may use the germination of a seed as the birth of a seedling, although it is really only the moment at which a developed embryo restarts into growth after a period of dormancy. We need to remember that often half or more of a population will have died before they can be recorded as born!
counting deaths
Counting deaths poses as many problems. Dead bodies do not linger long in nature. Only the skeletons of large animals persist long after death. Seedlings may be counted and mapped one day and gone without trace the next. Mice, voles and soft‐bodied animals such as caterpillars and worms are digested by predators or rapidly removed by scavengers or decomposers. They leave no carcasses to be counted and no evidence of the cause of death. Capture–recapture methods can go a long way towards estimating deaths from the loss of marked individuals from a population (they are probably used as often to measure survival as abundance), but even here it is often impossible to distinguish loss through death and loss through emigration.
4.4 Life cycles
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
