|Laboratory 4: Biotic Adaptations -
Anti-Herbivory and Anti-Predation
Introduction: Anti-herbivore and Anti-predator Adaptations
Despite the fact that plants provide the primary energy source for the rest of the ecosystem and that studies have shown that herbivores are a major factor in determining the evolution and distribution of plant species, casual observation may mask this important interaction. In areas of sufficient rainfall, the landscape is green and lush, often with little evidence of the impact of herbivores. Sudden defoliation of forests by herbivores such as gypsy moth caterpillars, the ravaging of brushlands by locusts or the denuding of areas of by grazing animals are rare occurrences in natural ecosystems. The primary reason for this is that plants have evolved a variety of defenses against being eaten. These defenses limit the number of species of animals that can survive on a particular species of plant and in turn have selected for increasing specialization in grazing animals. This relationship has probably been important in increasing the diversity of animals.
(A) Chemical Defenses
The plant world's main line of defense consists of a variety of chemical weapons. Very widespread among plants are a number of chemicals that appear to serve no physiological function for the plant but rather act as potent insecticides or insect repellents. Among these are alkaloids, quinones, essential oils, glycosides, calcium oxalate crystals, and silicates. Long before humans synthesized insecticides such as DDT, Malathion, etc., we learned to extract from chrysanthemums pyrethrin, a powerful insecticide that is relatively harmless to mammals. Some plants have also adjusted the nutrient content of their leaves to make them less nutritious for animals. Mature oak leaves for example are very nutrient poor and also high in tannins which bind with proteins. For this reason few herbivorous insects can survive on a diet of mature oak leaves, in contrast young oak leaves are often eaten readily by many insect herbivores. A few others, such as nettles, have stinging hairs that deter feeding or even touching!
Many insects have evolved ways of overcoming chemical defenses of particular plant species. This is usually done either by detoxifying the noxious compounds or storing them in tissues where they will not have toxic effect. Stored plant toxins are often used for the insect's own defense. For instance plants in the milkweed family often contain cardiac glycosides, potent vertebrate heart toxins that cause vomiting in lower concentrations. Many insects that specialize on milkweeds store the toxins and are thus distasteful to vertebrate predators such as birds and rodents. These insects frequently "advertise" their distastefulness by bright warning coloration which many vertebrates seem to remember. In this way, birds that have gotten sick from eating these insects can easily learn to avoid them in the future. Several examples of insects that feed on milkweeds are on display in the lab. Notice their bright color patterns. Similar relationships have evolved between insects and nightshades, passion flowers, pipevines, and violets to name a few.
Besides using toxins produced by plants, animals also rely on their own ability to synthesize noxious chemicals. Skunks and many insects produce vile-smelling compounds that deter predators from coming near or taking a bite. Others produce potent toxins that are used both for subduing prey and protection from predators. Examples include the venoms of snakes, scorpions, wasps, bees, and a number of molluscs. Amphibians such as frogs, toads and salamanders often have glands that produce toxic substances. The toxin of poison arrow frogs is used by indigenous people of South America to poison the tips of their arrows. Many of these animals that possess toxic compounds are also characterized by warning coloration.
(B) Physical Defenses
Many plants rely on physical defenses to deter herbivores. The stout spines of many tropical trees and shrubs as well those of many desert shrubs are effective in deterring feeding of large grazing animals such as deer and antelopes. Spines of cacti and other succulents help them to protect their valuable water reserves from consumption by animals. In areas where cacti such as prickly pear are common ranchers sometimes burn the spines off the plants so that their cattle will feed on them. The finer hairs found on the leaves and shrubs of many plants often help to deter feeding by insects. Plant breeders frequently select for hairy plants when trying to develop crop varieties that are resistant to insect attack. What other advantages might hairiness provide to a plant besides insect resistance?
Animals too may have modified hairs that deter predation. Examples include spines on bodies of butterfly and moth larvae, barbed hairs on abdomens of tarantulas that can be kicked off in the face of a predator, and spines of sea urchins and porcupines.
Animals with warning coloration that have some noxious quality are usually learned and avoided by predacious vertebrates. This has lead to selection for color patterns similar to those of noxious species in species that are not in themselves unpalatable. By mimicking distasteful or venomous species these palatable species are also protected from predation. The most familiar type of mimicry, Batesian mimicry, is characterized by a distasteful model and a palatable mimic. The mimic gains from this type of association by lessening its chances of being eaten while the model is at a disadvantage since birds may first encounter a mimic and then feed on a model having associated the color pattern with palatability. The most familiar examples of this kind of mimicry are the mimicking of the monarch butterfly (which feeds on milkweed and stores the toxins of that plant) by the non toxic viceroy butterfly, and also the wasp-like color patterns of many harmless flies. Another type of mimicry that is especially common in the tropics is Mullerian mimicry. In this type of mimicry several noxious species evolve a common warning pattern. In this way predators are more likely to learn to avoid individuals bearing the common warning pattern. Examples of this type of mimicry include the bright patterns of many tropical butterflies and the similar black and yellow patterns of many wasps.
Camouflage is a common anti-predator adaptation in animals. Camouflage patterns range from matching the color of a common background against which an animal is often viewed, to morphological adaptations that allow a particular animal to look like something of no interest to a carnivore i.e. leaves, twigs, stems, thorns, bird droppings, etc. Some animals such as Anolis lizards, squid, and others can quickly alter their color pattern to match a new background. Camouflaged animals often have behavioral adaptations as well. For instance, stick and leaf mimics often remain motionless during the day and feed and move about only at night. Common camouflage patterns include disruptive coloration, color patterns that tend to break up the outline of an object and/or make the eyes less conspicuous (e.g., the stripes of a tiger or a tiger swallowtail butterfly and the black slash across the eye of a California tree frog); and countershading in which the dorsal coloration of an organism is darker than the underside. This is seen in many fish making it difficult for a predator to see them from below when viewed against the bright light of the water's surface.
Perhaps the most unusual form of protective coloration is the frightening "eyespots" on the wings of certain butterflies and moths. These species do not taste bad but rather rely on a sudden display of their large vertebrate-like eyespots to startle predators. Experiments have shown that some birds have an innate fear of such large eye-like patterns. Small eyespots on the other hand are often at the edge of the wings. Birds use small eyespots as cues for the position of the head of prey and generally strike at the head first. Small eyespots thus serve to deflect a birds strike away from vital body parts and allow the butterfly or moth to escape. Some species carry the hoax even further and have evolved a pair of fake antennae which they wiggle to call attention to their bogus head. This is called posture reversal.
(F) Multiple defenses
Interactions between herbivores and plants, and predators and prey are often complex resulting in the evolution of different defensive adaptations. The relationship between the plant Passiflora and their most important herbivores, larvae of butterflies Heliconius provides an excellent example of the kinds of multiple defenses that plants have against their enemies.
Passiflora plants are protected from most herbivores by chemicals such as alkaloids and glycosides in their tissues. Heliconius butterflies are not affected by these toxins, and have come to specialize on Passiflora as their only larval food. Moreover these butterflies appear to store the plant toxins and are thus themselves protected from predators, plus exhibit warning coloration. Members of Passiflora have evolved a number of specific adaptations to lessen the feeding of Heliconius larvae. Among them are:
(1) Hooked trichomes are found on the stems of at least one species P. adenopoda. These serve to puncture the body wall of Heliconius caterpillars and cause them to bleed to death. For the time being at least, P. adenopoda seems to have won the evolutionary contest with Heliconius.
(2) Extra-floral nectaries are found on a number of species of Passiflora. These are thought to provide food for small wasps and ants which either parasitize or prey on Heliconius larvae. Thus Passiflora uses a kind of biological control to combat its enemies in much the same way as humans attempt to use natural enemies to control agricultural pests.
(3) About 2% of Passiflora species have stipules that mimic the eggs of Heliconius butterflies. Unlike most butterflies Heliconius lay conspicuous eggs. These serve as a signal to egg-laying females. In order to avoid competition for food and perhaps cannibalism, female Heliconius butterflies avoid plants that already have eggs on them, and plants with egg-like structures thus receive fewer eggs.