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 3.27 define niche boundaries in two dimensions. Resource combinations to one side of line B allow the organisms to thrive - but to the other side of the line the organisms decline.
complementary resources antagonistic resources
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 3.27a-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 specialize 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 labeled, 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. We turn in the next chapter to look in more detail at the most fundamental responses of organisms to those conditions and resources: their patterns of growth, survival and reproduction.
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