of resources
essential resources
Two resources are said to be essential when neither can substitute for the other. This is denoted in Figure 3.30a by the isoclines running parallel to both axes. They do so because the amount available of one resource defines a maximum possible growth rate, irrespective of the amount of the other resource. This growth rate is achieved unless the amount available of the other resource defines an even lower growth rate. Generally, therefore, growth rate will be determined by the resource in most limited supply. This will be true for nitrogen and potassium as resources in the growth of green plants, and for two host species in the life of a parasite that must alternate between them (see Chapter 12).
perfectly substitutable resources
Two resources are said to be perfectly substitutable when either can wholly replace the other. This will be true for seeds of wheat or barley in the diet of a farmyard chicken, or for zebra and gazelle in the diet of a lion. Note that we do not imply that the two resources are as good as each other. This feature (perfectly substitutable but not necessarily as good as each other) is included in Figure 3.30b by the isoclines having slopes that do not cut both axes at the same distance from the origin. Thus, in Figure 3.30b, in the absence of resource 2, the organism needs relatively little of resource 1, but in the absence of resource 1 it needs a relatively large amount of resource 2.
complementary resources
Substitutable resources are defined as complementary if the isoclines bow inwards towards the origin (Figure 3.30c). This shape means that a species requires less of two resources when taken together than when consumed separately. A good example is human vegetarians combining beans and rice in their diet. The beans are rich in lysine, an essential amino acid poorly represented in rice, whilst rice is rich in sulphur‐containing amino acids that are present only in low abundance in beans.
antagonistic resources
By contrast, a pair of substitutable resources with isoclines that bow away from the origin are defined as antagonistic (Figure 3.30d). The shape indicates that a species requires proportionately more resource to maintain a given rate of increase when two resources are consumed together than when consumed separately. Though probably rare, this could arise, for example, if the resources contain different toxic compounds that act synergistically (more than just additively) on their consumer. For example, d,l‐pipecolic acid and djenkolic acid (two defensive chemicals found in certain seeds) had no significant effect on the growth of the seed‐eating larva of a bruchid beetle if consumed separately, but they had a pronounced effect if taken together (Janzen et al., 1977). Whenever resources are substitutable, whether or not they are perfectly substitutable, the growth rate is determined by their joint availability.
inhibitory resources
Finally, Figure 3.30e illustrates the phenomenon of inhibition at high resource levels for a pair of essential resources: resources that are essential but become damaging when in excess. CO2, water and mineral nutrients such as iron are all required for photosynthesis, but each is lethal in excess. Similarly, light leads to increased growth rates in plants through a broad range of intensities, but can inhibit growth at very high intensities. In such cases, the isoclines form closed curves because growth decreases with an increase in resources at very high levels.
3.8.2 Resource dimensions of the ecological niche
In Chapter 2 we developed the concept of the ecological niche as an n‐dimensional hypervolume. This defines the limits within which a given species can survive and reproduce for a number (n) of environmental factors, including both conditions and resources. Note, therefore, that the zero growth isoclines in Figure 3.30 define niche boundaries in two dimensions. Resource combinations to one side of this zero isocline allow the organisms to thrive – but to the other side of the line the organisms decline.
The resource dimensions of a species’ niche can sometimes be represented in a manner similar to that adopted for conditions, with lower and upper limits within which a species can thrive. Thus, a predator may only be able to detect and handle prey between lower and upper limits of size. For other resources, such as mineral nutrients for plants, there may be a lower limit below which individuals cannot grow and reproduce but an upper limit may not exist (Figure 3.30a–d). However, many resources must be viewed as discrete entities rather than continuous variables. Larvae of butterflies in the genus Heliconius require Passiflora leaves to eat; those of the monarch butterfly specialise on plants in the milkweed family; and various species of animals require nest sites with particular specifications. These resource requirements cannot be arranged along a continuous graph axis labelled, for example, ‘food plant species’. Instead, the food plant or nest‐site dimension of their niches needs to be defined simply by a restricted list of the appropriate resources.
Together, then, conditions and resources define a species’ niche. In the next chapter, we will look in more detail at the most fundamental responses of organisms to those conditions and resources: their patterns of growth, survival and reproduction.
3.9 A metabolic theory of ecology
Resources and conditions are also important insofar as they determine the metabolic rates of individuals, which determine the levels of resources available to those individuals for reproduction, growth and so on, which in turn influence their life histories, their abundance and indeed all of the processes we discuss in subsequent chapters. This perspective has generated interest in a ‘metabolic theory of ecology’ (Brown et al., 2004). At the heart of metabolic theory are the effects on metabolic rate of temperature, and in particular of size (most often, body mass). We examined the effects of temperature in Section 2.3. We turn now to size.
metabolic scaling
The most fundamental point, perhaps, is that life is typically faster for small organisms than it is for large ones – metabolising at greater rates, and maturing and dying sooner. For example, per gram of body mass, a resting mouse metabolises about 20 times faster than an elephant. There are exceptions to this pattern, as we’ll see below, but the more general rule is very widespread. Putting this more formally, we can say that the rate of a metabolic process, Y, varies with size according to the equation
where Y0 is referred to as the normalisation constant, reflecting the typical rate for the metabolic process concerned, M is the organism’s mass and b is the so‐called allometric exponent.
