Question

Explain the difference between structural and inducible defences. Use a graph showing concentrations of both before and after herbivore attack to discuss the advantages and disadvantages of both strategies, and what the best combinations are for a common and for a rare plant.

Answer 1

Plants and herbivores have co-evolved together for 350 million years. Plants have evolved many defense mechanisms against insect herbivory. Such defenses can be broadly classified into two categories: (1) permanent, constitutive defenses, and (2) temporary, inducible defenses. Both types are achieved through similar means but differ in that constitutive defenses are present before an herbivore attack, while induced defenses are activated only when attacks occur. In addition to constitutive defenses, initiation of specific defense responses to herbivory is an important strategy for plant persistence and survival. Other defensive strategies used by plants include escaping or avoiding herbivores in any time and/or any place, for example by growing in a location where plants are not easily found or accessed by herbivores, or by changing seasonal growth patterns. Another approach diverts herbivores toward eating non-essential parts or enhances the ability of a plant to recover from the damage caused by herbivory. Some plants encourage the presence of natural enemies of herbivores, which in turn protect the plant. Each type of defense can be either constitutive (always present in the plant), or induced (produced in reaction to damage or stress caused by herbivores).

constitutive-plant response to a pathogen that is always present
induced-plant response to a pathogen that is produced in response to the pathogen

Herbivores are dependent on plants for food and have evolved mechanisms to obtain this food despite the evolution of a diverse arsenal of plant defenses. Herbivore adaptations to plant defense have been likened to offensive traits and consist of adaptations that allow increased feeding and use of a host plant. Relationships between herbivores and their host plants often result in reciprocal evolutionary change, called co-evolution.

Defensive structures and chemicals are costly as they require resources that could otherwise be used by plants to maximize growth and reproduction. Many models have been proposed to explore how and why some plants make this investment in defenses against herbivores. Repellent companion planting, defensive live fencing hedges, and "obstructive-repellent" interplanting, with host-plant resistance species as beneficial 'biological control agents' is a technique in biological pest control programs for: organic gardening, wildlife gardening, sustainable gardening, and sustainable landscaping; in organic farming and sustainable agriculture; and in restoration ecology methods for habitat restoration projects.