Читать книгу Ecology - Michael Begon - Страница 95

There is also often no programmed senescence of whole modular organisms – they appear to have perpetual somatic youth (see Thomas (2013) for a review of senescence, and its avoidance, in plants). Even in trees that accumulate their dead stem tissues, or gorgonian corals that accumulate old calcified branches, death often results from becoming too big or succumbing to disease rather than from programmed senescence. We see evidence of this in Figure 4.2, which shows how rates of mortality and birth vary with age in a wide variety of organisms. It is a figure to which we will return several times in this chapter. For now, we can note that there are a number examples there, indeed of trees and a gorgonian coral, in which there is no evidence of the increases in mortality at older ages that we see in organisms that senesce, like ourselves.

Figure 4.2 Compilation of patterns of mortality (survivorship) and reproduction from across the plant and animal kingdoms from reproductive maturity to the age where only 5% of the adult population is still alive. To emphasise variations in pattern, mortality and fertility are scaled relative to their means. Survivorship 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).