Effects of consumption on consumers

The beneficial effects that food has on individual predators are not difficult to imagine. Generally speaking, an increase in the amount of food consumed leads to increased rates of growth, development and birth, and decreased rates of mortality. This, after all, is implicit in any discussion of intraspecific competition amongst consumers (see Chapter 5): high densities, implying small amounts of food per individual, lead to low growth rates, high death rates, and so on. Similarly, many of the effects of migration previously considered (see Chapter 6) reflect the responses of individual consumers to the distribution of food availability. However, there are a number of ways in which the relationships between consumption rate and consumer benefit can be more complicated than they initially appear. In the first place, all animals require a certain amount of food simply for maintenance and unless this threshold is exceeded the animal will be unable to grow or reproduce, and will therefore be unable to contribute to future generations. In other words, low consumption rates, rather than leading to a small benefit to the consumer, simply alter the rate at which the consumer starves to death.

At the other extreme, the birth, growth and survival rates of individual consumers cannot be expected to rise indefinitely as food availability is increased. Rather, the consumers become satiated. Consumption rate eventually reaches a plateau, where it becomes independent of the amount of food available, and benefit to consumers therefore also reaches a plateau. Thus, there is a limit to the amount that a particular consumer population can eat, a limit to the amount of harm that it can do to its prey population at that time, and a limit to the extent by which the consumer population can increase in size. This is discussed more fully in Section 10.4.

The most striking example of whole populations of consumers being satiated simultaneously is provided by the many plant species that have mast years. These are occasional years in which there is synchronous production of a large volume of seed, often across a large geographic area, with a dearth of seeds produced in the years in between (Herrera et al., 1998; Koenig & Knops, 1998; Kelly et al., 2000). This is seen particularly often in tree species that suffer generally high intensities of seed predation (Silvertown, 1980) and it is therefore especially significant that the chances of escaping seed predation are likely to be much higher in mast years than in other years. Masting seems to be especially common in the New Zealand flora (Kelly, 1994) where it has also been reported for tussock grass species (Figure 9.10). The individual predators of seeds are satiated in mast years, and the populations of predators cannot increase in abundance rapidly enough to exploit the glut. This consumers often need to exceed a threshold of consumption consumers may become satiated mast years and the satiation of seed predators

Figure 9.10 The flowering rate for five species of tussock grass (Chionochloa) between 1973 and 1996 in Fiordland National Park, New Zealand. Mast years are highly synchronized in the five species, seemingly in response to high temperatures in the previous season, when flowering is induced. (After McKone et al., 1998.)

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Figure 9.11 Insect predation on florets of Chionochloa pallens in mast (n = 3) and nonmast years (n = 7) from 1988 to 1997 at Mount Hutt, New Zealand. A mast year is defined here as one with greater than 10 times as many florets produced per tussock than in the previous year. The significant difference in insect damage supports the hypothesis that the function of masting is to satiate seed predators. (After McKone et al., 1998.)

Figure 9.11 Insect predation on florets of Chionochloa pallens in mast (n = 3) and nonmast years (n = 7) from 1988 to 1997 at Mount Hutt, New Zealand. A mast year is defined here as one with greater than 10 times as many florets produced per tussock than in the previous year. The significant difference in insect damage supports the hypothesis that the function of masting is to satiate seed predators. (After McKone et al., 1998.)

is illustrated in Figure 9.11 where the percentage of florets of the grass Chionochloa pallens attacked by insects remains below 20% in mast years but ranges up to 80% or more in nonmast years. The fact that C. pallens and four other species of Chionochloa show strong synchrony in masting is likely to result in an increased benefit to each species in terms of escaping seed predation in mast years.

On the other hand, the production of a mast crop makes great demands on the internal resources of a plant. A spruce tree in a mast year averages 38% less annual growth than in other years, and the annual ring increment in forest trees may be reduced by as much during a mast year as by a heavy attack of defoliating caterpillars. The years of seed famine are therefore essentially years of plant recovery.

As well as illustrating the potential importance of predator satiation, the example of masting highlights a further point relating to timescales. The seed predators are unable to extract the maximum benefit from (or do the maximum harm to) the mast crop because their generation times are too long. A hypothetical seed predator population that could pass through several generations during a season would be able to increase exponentially and explosively on the mast crop and destroy it. Generally speaking, consumers with relatively short generation times tend to closely track fluctuations in the quantity or abundance of their food or a consumer's numerical response is limited by its generation time ...

Figure 9.12 Fluctuations in the fruit production of Asphodelus (■) and the number of Capsodes nymphs (•) and adults (a) at a study site in the Negev desert, Israel. (After Ayal, 1994.)

prey, whereas consumers with relatively long generation times take longer to respond to increases in prey abundance, and longer to recover when reduced to low densities.

The same phenomenon occurs in desert communities, where year-to-year variations in precipitation can be both considerable and unpredictable, leading to similar year-to-year variation in the productivity of many desert plants. In the rare years of high productivity, herbivores are typically at low abundance following one or more years of low plant productivity. Thus, the herbivores are likely to be satiated in such years, allowing plant populations to add considerably to their reserves, perhaps by augmenting their buried seed banks or their underground storage organs (Ayal, 1994). The example of fruit production by Asphodelus ramosus in the Negev desert in Israel in shown in Figure 9.12. The mirid bug, Capsodes infus-catus, feeds on Asphodelus, exhibiting a particular preference for the developing flowers and young fruits. Potentially, therefore, it can have a profoundly harmful effect on the plant's fruit production. But it only passes through one generation per year. Hence, its abundance tends never to match that of its host plant (Figure 9.12). In 1988 and 1991, fruit production was high but mirid abundance was relatively low: the reproductive output of the mirids was therefore high (3.7 and 3.5 nymphs per adult, respectively), but the proportion of fruits damaged was relatively low (0.78 and 0.66). In 1989 and 1992, on the other hand, when fruit production had dropped to much lower levels, the proportion of fruits damaged was much higher (0.98 and 0.87) and the reproductive output was lower (0.30 nymphs per adult in 1989; unknown in 1992). This suggests that herbivorous insects, at least, may have a limited ability to affect plant population dynamics in desert communities, but that the potential is much greater for the dynamics of herbivorous insects to be affected by their food plants (Ayal, 1994).

Chapter 3 stressed that the quantity of food consumed may be less important than its quality. In fact, food quality, which has both positive aspects (like the concentrations of nutrients) and negative aspects (like the concentrations of toxins), can only sensibly be defined in terms of the effects of the food on the animal that eats it; and this is particularly pertinent in the case of herbivores. For instance, we saw in Figure 9.8 how even in the presence of predatory spiders, enhanced food quality led to increased survivorship of grasshoppers. Along similar lines, Sinclair (1975) examined the effects of grass quality (protein content) on the survival of wildebeest in the Serengeti of Tanzania. Despite selecting protein-rich plant material (Figure 9.13a), the wildebeest consumed food in the dry season that contained well below the level of protein necessary even for maintenance (5-6% of crude protein); and to judge by the depleted fat reserves of dead males (Figure 9.13b), this was an important cause of mortality. Moreover, it is highly relevant that the protein requirements of females during late pregnancy and lactation (December-May in the wildebeest) are three to four times higher than the normal. It is therefore clear that the shortage of high-quality food (and not just food shortage per se) can have a drastic effect on the growth, survival and fecundity of a consumer. In the case of herbivores especially, it is possible for an animal to be apparently surrounded by its food whilst still experiencing a food shortage. We can see the problem if we imagine that we ourselves are provided with a perfectly balanced diet - diluted in an enormous swimming pool. The pool contains everything we need, and we can see it there before us, but we may very well starve to death before we can drink enough water to extract enough nutrients to sustain ourselves. In a similar fashion, herbivores may frequently be confronted with a pool of available nitrogen that is so dilute that they have difficulty processing enough material to extract what they need. Outbreaks of herbivorous insects may then be associated with rare elevations in the concentration of available nitrogen in their food plants (see Section 3.7.1), perhaps associated with unusually dry or, conversely, unusually waterlogged conditions (White, 1993). Consumers obviously need to acquire resources - but, to benefit from them fully they need to acquire them in appropriate quantities and in an appropriate form. The behavioral strategies that have evolved in the face of the pressures to do this are the main topic of the next two sections.

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