Life cycles and the quantification of death and birth

To a large extent, patterns of birth, death and growth are a reflection of the organism's life cycle, of which there are five main types (although there are many life cycles that defy this simple classification). The species are said to be either semelparous or iteroparous (often referred to by plant scientists as monocarpic and polycarpic). Like so many dichotomies in ecology, this one is not clear-cut: some species occupy the continuum between the two extremes. Nonetheless, we may say that in semelparous species, individuals have only a single, distinct period of reproductive output in their lives, prior to which they have largely ceased to grow, during which they invest little or nothing in survival to future reproductive events and after which they therefore die. In iteroparous species, on the other hand, 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 the individual continues to invest in future survival and possibly growth, and  it therefore has a reasonable chance of surviving to reproduce again.

Often, in order to monitor and examine changing patterns of mortality w_ith age or stage, a life table is used. Frequently this allows a survivorship curve to be constructed, which traces the decline in numbers, over time, of a group of newly born or newly emerged individuals – or it can be thought of as a plot of the probability, for a representative newly born individual, of surviving to various ages. Patterns of birth amongst individuals of different ages are often monitored at the same time as life tables are constructed. These patterns are displayed n fecundity schedules.

Life tables, survivorship curves and fecundity schedules are of the utmost importance because they contain the raw material of our 'ecological fact of life'. Without them we have little hope of understanding the Nnow of the species that interest us, and still less hope of predicting the Nfuture. Annual life cycles take approximately 12 months or rather less to complete. Usually, every individual in a population breeds during one particular season of the year, but then dies before the same season in the next year. Generations are therefore said to be discrete, in that each generation is distinguish-able from every other; the only overlap of generations is between breeding adults and their offspring during and immediately after the breeding season. Species with discrete generations need not be annual, since generation lengths other than 1 year are conceivable. In practice, however, most are: the regular annual cycle of climates provides the major pressure in favour of synchrony.

 

PART II

 

Dispersal, Dispersion and Migration in Space and Time

Introduction

 

All organisms in nature are where we find them because they have moved there_'  This is true for even the most apparently sedentary of organisms, such as oysters and redwood trees. Their movements range from the passive transport that affects many plant seeds to the active movement of many mobile animals and their larvae._' The effects of such movements are also varied. In some cases, they aggregate members of a population into clumps; in others they continually redistribute and shuffle them amongst each other and in others they spread the individuals out and 'dilute' their density.

The terms dispersal and migration are used to describe certain aspects of the movement of organisms. Dispersal is most often taken to mean a spreading of individuals away from others (e.g. their parents or siblings), and it may involve active (walking, swimming, flying) or passive movements (carriage in water or wind)_' Dispersal is therefore an appropriate description for several kinds of movements: (i) of plant seeds or starfish larvae away from each other and their parents; (ii) of voles from one area of grassland to another, usually leaving residents behind and being counterbalanced by the dispersal of other voles in the other direction; and (iii) of land birds amongst an archipelago of islands (or aphids amongst a mixed stand of plants) in the search for a suitable habitat._.

Migration is most often taken to mean the mass directional movements of large numbers of a species from one location to another. The term therefore applies to classic migrations (the movements of locust swarms, the intercontinental journeys of birds, the transoceanic movements of eels), but also to less obvious examples tike the to-and-fro movements of shore animals following the tidal cycle. The terms dispersal and migration are both defined for groups of organisms_' However, it is the individual that actually moves. Migration is mass movement_' and an individual can only disperse, literally, if it separates into pieces. Many dispersing organisms (especially plant seeds and many marine larvae) have little or no control over where or how far they travel. They are simply hazarded into the world at large to be carried at the mercy of winds and waves. At the level of the individual, there is no sharp distinction between migration and dispersal.

 

2.   Patterns of distribution: dispersion

The movements of organisms affect the spatial pattern of their distribution (their dispersion) and we can recognize three main patterns of dispersion, although they form part of a continuum.

Random when there is an equal probability of an organism occupying any point in space (irrespective of the position of any others)_' The result is that individuals are unevenly distributed because of chance events.

Regular dispersion (also called, uniform, even or overdispersion) occurs either when an individual has a tendency to avoid all other individuals, or when individuals that are especially close to others die. The result is that individuals are more evenly spaced than expected from chance.

Aggregated dispersion (also called contagious, clumped or underdispersion) occurs either when individuals tend to be attracted to (or are more likely to survive in) particular parts of the environment, or when the presence of one individual attracts_' or gives rise to, another close to it. The result is that individuals are closer together than expected from chance._'

These patterns are defined by the relative positions of the organisms to one another, but how they will appear to an observer or be relevant to the life of another organism depends on how the spatial scale is sampled. For example, consider the distribution of an aphid living on a particular species of tree in a woodland. If the area is sampled with large samples, for example acre or hectare quadrats, the aphids will appear to be aggregated in particular parts of the world, i.e. in woodlands, as opposed to other types of habitat. If our samples are smaller and taken only in woodlands, the aphids will still appear to be aggregated, but now on their host tree species rather than on the trees in general. However, if our samples were still smaller (25 cm², about the size of a leaf) and were taken within the canopy of a single tree, the aphids might appear to be randomly distributed over the tree as a whole. An even smaller quadrat (1 cm2) might detect a regular distribution because individual aphids on a leaf avoid one another.

In practice, the populations of all species are patchily distributed at some scale or another. One of the most important consequences of the patchy distribution of organisms is that we can have quite misleading measures of their effective density. For example, the human density of a country or region is usually calculated as the total number of individuals divided by the total land area. For the contiguous 48 states of the USA, the 196O census would give the population density as 59.94 persons per square mile. But, because most people live in patches, (towns and cities), their effective density is really about 3000 persons per square mile. When we come to consider the role of density in regulating natural populations it becomes vitally important to distinguish average density from effective density: a population may have a very low average density, but the few individuals may be very close together.

It is immensely more difficult, but ecologically even more relevant, to describe the dispersion of organisms on scales that are relevant to the life style of motile organisms. MacArthur and Levins introduced the concept of environmental grain to make this point. For example, the canopy of an oak hickory forest is fine grained (not patchy), from the point of view of a bird like the scarlet tanager which forages indiscriminately in both oaks and hickories, but is coarse grained (patchy) for defoliating insects which attack either oaks or hickories preferentially. Indeed it has been argued that ‘the problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystem science, and marrying basic and applied ecology’ (Levin,1992).

 

 

Dispersal