Nutrient-rich environments select for plants that allocate more resources to growth than to defense.
Some plant species allocate far more resources to defense than others. Because defenses are costly to produce, you might think that plants growing in nutrient-rich habitats should be the best defended. However, consider the counterargument: Plants growing on nutrient-poor soils might invest heavily in defense because they cannot afford to replace tissues lost to herbivores. Such plants would grow relatively slowly because their resources are being used to build defenses rather than to produce new leaves and roots. By contrast, plants from nutrient-rich habitats would favor growth over defense because they can more readily replace lost or damaged tissues.
This situation is an example of a trade-off: Something is gained and, at the same time, something is lost. You can’t have your cake and eat it, too—you have to make a choice. With plants, there can be a trade-off between growth and defense in which allocating resources for one means that those resources cannot be allocated for the other.
A recent study in the Amazon rain forest has focused on a possible trade-off between growth and defense. Researchers compared species found on nutrient-rich clay soils with closely related species found on nutrient-poor sandy soils. They found that species from the nutrient-rich clay soils invested less heavily in defenses than did species from the sandy sites. Because the clay-soil species divert fewer resources to defense, they grow faster in either soil type compared to sandy-soil species—as long as they are grown under a net that excludes herbivores (Fig. 32.17). However, when grown out in the open, each species grows best in the soil type on which it is naturally found.
FIG. 32.17 A trade-off between growth and defense. Plants from nutrient-rich clay habitats can overcome herbivory by growing fast, whereas plants from low-nutrient sandy habitats grow slowly and invest more in defense. The photos show that, without defenses, clay-soil plants are prone to being eaten by herbivores.
The clay-soil species grow poorly on sandy soils when not protected by a net because, without ample soil nutrients available, they are unable to replace tissues consumed by herbivores. Conversely, when grown on the nutrient-rich soils, species from the sandy habitats are poor competitors because they allocate resources that could have been used for growth to defensive compounds and structures. What we see is a classic example of a trade-off.
Quick Check 4 Does the trade-off between growth and defense favor a single species that dominates in all soil types or different species specialized for each habitat?
Quick Check 4 Answer
The trade-off between growth and defense is a diversifying force that favors different species that are specialized for each habitat, rather than a single species that dominates across all soil types.