Ecology. Michael Begon
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is plotted on a log scale. The plots are arranged in order of decreasing mortality at the terminal age. Note the marked contrast between organisms like ourselves (top line) that show senescence, where there is a marked increase in mortality in old age, and those like the coral and oak tree in the bottom line where there is no such increase. This is part of a more general variation in the shape of survivorship curves, picked up again in Figure 4.11.
Source: After Jones et al. (2014).
At the modular level, things are quite different. The annual death of the leaves on a deciduous tree is the most dramatic example of senescence – but roots, buds, flowers and the modules of modular animals all pass through phases of youth, middle age, senescence and death. The growth of the individual genet is the combined result of these processes. Figure 4.3, for example, shows that the age structure of leaves of the perennial herb, Wedelia trilobata, a native of central America, is changed dramatically by the application of nitrogen fertiliser. Plants are larger when they are more heavily fertilised, and the rate at which they ‘give birth’ to leaves is greater, but so too is the death rate of those leaves.
Figure 4.3 The growth of a genet reflects the births and deaths of its component modules. (a) Numbers of leaves of plants of Wedelia trilobata (means of six plants), divided into seven‐day age classes, cultivated at low (above) and high (below) nitrogen availability. Bars are SEs. At high nitrogen availability, the plants are not only larger: they also have a much higher proportion of young leaves. (b) The cumulative number of newly produced (above) and dead (below) leaves in the same study. The high proportion of young leaves at high nitrogen (HN) availability seen in (a) is the result of both birth and death rates of leaves being higher. LN, low nitrogen.
Source: After Suarez (2016).
4.2.4 Integration
For many rhizomatous and stoloniferous species, this changing age structure is in turn associated with a changing level to which the connections between individual ramets remain intact. A young ramet may benefit from the nutrients flowing from an older ramet to which it is attached and from which it grew. But the pros and cons of attachment will have changed markedly by the time the daughter is fully established in its own right and the parent has entered a postreproductive phase of senescence – a comment equally applicable to unitary organisms with parental care, like ourselves (Caraco & Kelly, 1991).
The changing benefits and costs of integration have been studied experimentally in the pasture grass Holcus lanatus, by comparing the growth of: (i) ramets that were left with a physiological connection to their parent plant, and in the same pot, so that parent and daughter might compete (competing, connected: CC); (ii) ramets that were left in the same pot so competition was still possible but had their connection severed (competing, not connected: CN); and (iii) ramets that had their connection severed and were repotted in their parent’s soil after the parent had been removed, so no competition was possible (independent plants, neither competing nor connected: NN) (Figure 4.4). These treatments were applied to daughter ramets of various ages, which were then examined after a further eight weeks’ growth. For the youngest daughters, just one week old (Figure 4.4a), connection to the parent significantly enhanced growth (CC > CN), but competition with the parent had no apparent effect (CN ≈ NN). For slightly older daughters, two weeks old (Figure 4.4b), competition with the parent did depress growth (NN > CN), but physiological connection with the parent effectively negated this (CC > CN; CC ≈ NN). For even older daughters, however, the balance shifted further still. Competition with the parent again depressed growth of the daughter (NN > CN), but this time physiological connection to the parent was either not enough to fully overcome this (at four weeks, Figure 4.4c; NN > CC > CN) or eventually appeared to represent a further drain on the daughter’s resources (after eight weeks, Figure 4.4d; NN > CN > CC).
Figure 4.4 Integration within a plant leads to a shifting balance of positive and negative effects between parent and daughter modules as modules age. The growth of daughter ramets of the grass Holcus lanatus, which were initially (a) one week, (b) two weeks, (c) four weeks and (d) eight weeks old, and were then grown on for a further eight weeks. LSD, least significant difference, needs to be exceeded for two means to be significantly different from each other. For further discussion, see text. CC, competing, connected; CN, competing, not connected; NN, independent plants, neither competing nor connected.
Source: After Bullock et al. (1994).
4.3 Counting individuals
If we are going to study birth, death and modular growth seriously, we must quantify them. This means counting individuals and (where appropriate) modules. Indeed, many studies concern themselves not with birth and death but with their consequences – the total number of individuals present and the way these numbers vary with time. Whether organisms are unitary or modular, ecologists face enormous technical problems when they try to count what is happening to populations in nature. A great many ecological questions remain unanswered because of these problems.
what is a population?
It is usual to use the term population to describe a group of individuals of one species under investigation. What actually constitutes a population, though, will vary from species to species and from study to study. In some cases, the boundaries of a population are readily apparent: the sticklebacks occupying a small lake are ‘the stickleback population of the lake’. In other cases, boundaries are determined more by an investigator’s purpose or convenience: it is possible to study the population of lime aphids inhabiting one leaf, one tree, one stand of trees or a whole woodland. In yet other cases – and there are many of these – individuals are distributed continuously over a wide area, and an investigator must define the limits of a population arbitrarily. In such cases, especially, it is often more convenient to consider the density of a population. This is usually defined as ‘numbers per unit area’, but in certain circumstances ‘numbers per leaf’, ‘numbers per host’ or some other measure may be appropriate.
estimating population size
To determine the size of a population, one might imagine that it is possible simply to count individuals, especially for relatively small, isolated habitats like islands