Metabolism growth development and size

Individuals respond to temperature essentially in the manner shown in Figure 2.1a: impaired function and ultimately death at the upper and lower extremes (discussed in Sections 2.3.4 and 2.3.6), with a functional range between the extremes, within which there is an optimum. This is accounted for, in part, simply by changes in metabolic effectiveness. For each 10°C rise in temperature, for example, the rate of biological enzymatic processes often roughly doubles, and thus appears as an exponential curve on a plot of rate against temperature (Figure 2.3). The increase is brought about because high temperature increases the speed of molecular movement and speeds up chemical reactions. The factor by which a reaction changes over a 10°C range is referred to as a Q10: a rough doubling means that Q10 ~ 2.

For an ecologist, however, effects on individual chemical reactions are likely to be less important than effects on rates of growth (increases in mass), on rates of development (progression through lifecycle stages) and on final body size, since, as we shall discuss much more fully in Chapter 4, these tend to drive the core ecological activities of survival, reproduction and movement. And when we plot rates of growth and development of whole organisms against temperature, there is quite commonly an extended range over which there are, at most, only slight deviations from linearity (Figure 2.4).

When the relationship between day-degree concept growth or development is effectively linear, the temperatures experienced by an organism can be summarized in a single very useful value, the number of 'day-degrees'. For instance, Figure 2.4c shows that at 15°C (5.1°C above a development threshold of 9.9°C) the predatory mite, Amblyseius californicus, took 24.22 days to develop (i.e. the proportion of its total development achieved each day was 0.041 (= 1/24.22)), but it took only 8.18 days to develop at 25°C (15.1°C above the same threshold). At both temperatures, therefore, development required 123.5 day-degrees (or, more properly, 'day-degrees above threshold'), i.e. 24.22 X 5.1 = 123.5, and 8.18 X 15.1 = 123.5. This is also the requirement for development in the mite at other temperatures within the nonlethal range. Such organisms cannot be said to require a certain length of time for development. What they require is a combination of time and temperature, often referred to as 'physiological time'.

Together, the rates of growth and development determine the final size of an organism. For instance, for a given rate of growth, a faster rate of development will lead to smaller final size. Hence, if the responses of growth and development to variations in temperature are not the same, temperature will also affect final size. In fact, development usually increases more rapidly with temperature than does growth, such that, for a very wide range of organisms, final size tends to decrease with rearing temperature: the 'temperature-size rule' (see Atkinson et al., 2003). An example for single-celled protists (72 data sets from marine, brackish and freshwater habitats) is shown in Figure 2.5: for each 1°C increase in temperature, final cell volume decreased by roughly 2.5%.

These effects of temperature on growth, development and size may be of practical rather than simply scientific importance. Increasingly, ecologists are called upon to predict. We may wish to know what the consequences would be, say, of a 2°C rise in temperature resulting from global warming (see Section 2.9.2). Or we may wish to understand the role of temperature in seasonal, interannual and geographic variations in the productivity of, for example, marine ecosystems (Blackford et al., 2004). We cannot afford to assume exponential relationships with temperature if they are really linear, nor to ignore the effects of changes in organism size on their role in ecological communities.

Motivated, perhaps, by this need to be able to extrapolate from the known to the unknown, and also simply by a wish to discover fundamental organizing principles governing the world exponential effects of temperature on metabolic reactions effectively linear effects on rates of growth and development temperature-size rule

'universal temperature dependence'?

(a)

1.0

y = 0.072x -

0.32

0.8

R2= 0.64

"a

0.6

_

m i te

0.4

_

rat

o

0.2

CD

0.0

O

-0.2

-

I I

0.18

Figure 2.5 The temperature-size rule (final size decreases with increasing temperature) illustrated in protists (65 data sets combined). The horizontal scale measures temperature as a deviation from 15°C. The vertical scale measures standardized size: the difference between the cell volume observed and the cell volume at 15°C, divided by cell volume at 15°C. The slope of the mean regression line, which must pass through the point (0,0), was -0.025 (SE, 0.004); the cell volume decreased by 2.5% for every 1°C rise in rearing temperature. (After Atkinson et al., 2003.)

Temperature (°C)

te rat

Hi 0.15

0.05

Temperature (°C)

Figure 2.4 Effectively linear relationships between rates of growth and development and temperature. (a) Growth of the protist Strombidinopsis multiauris. (After Montagnes et al., 2003.) (b) Egg development in the beetle Oulema duftschmidi. (After Severini et al., 2003.) (c) Egg to adult development in the mite Amblyseius californicus. (After Hart et al., 2002.) The vertical scales in (b) and (c) represent the proportion of total development achieved in 1 day at the temperature concerned.

Temperature (°C)

Figure 2.4 Effectively linear relationships between rates of growth and development and temperature. (a) Growth of the protist Strombidinopsis multiauris. (After Montagnes et al., 2003.) (b) Egg development in the beetle Oulema duftschmidi. (After Severini et al., 2003.) (c) Egg to adult development in the mite Amblyseius californicus. (After Hart et al., 2002.) The vertical scales in (b) and (c) represent the proportion of total development achieved in 1 day at the temperature concerned.

Figure 2.5 The temperature-size rule (final size decreases with increasing temperature) illustrated in protists (65 data sets combined). The horizontal scale measures temperature as a deviation from 15°C. The vertical scale measures standardized size: the difference between the cell volume observed and the cell volume at 15°C, divided by cell volume at 15°C. The slope of the mean regression line, which must pass through the point (0,0), was -0.025 (SE, 0.004); the cell volume decreased by 2.5% for every 1°C rise in rearing temperature. (After Atkinson et al., 2003.)

around us, there have been attempts to uncover universal rules of temperature dependence, for metabolism itself and for development rates, linking all organisms by scaling such dependences with aspects of body size (Gillooly et al., 2001, 2002). Others have suggested that such generalizations may be oversimplified, stressing for example that characteristics of whole organisms, like growth and development rates, are determined not only by the temperature dependence of individual chemical reactions, but also by those of the availability of resources, their rate of diffusion from the environment to metabolizing tissues, and so on (Rombough, 2003; Clarke, 2004). It may be that there is room for coexistence between broad-sweep generalizations at the grand scale and the more complex relationships at the level of individual species that these generalizations subsume.

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment