+ Chapter 17: Predation and Herbivory (and a bit of Chapter 20) Robert E. Ricklefs The Economy of Nature, Fifth Edition

The rabbit/myxoma story

Interacting populations evolve in response to each other 3

Evolution of Resistance in Rabbits Decline in lethality of the myxoma virus in Australia resulted from evolutionary responses in both the rabbit and the virus populations: genetic factors conferring resistance to the disease existed in the rabbit population prior to introduction of the myxoma virus: the myxoma epidemic exerted strong selective pressure for resistance eventually most of the surviving rabbit population consisted of resistant animals 4

Evolution of Hypovirulence in Myxoma Virus Decline in lethality of the myxoma virus in Australia resulted from evolutionary responses in both the rabbit and the virus populations: less virulent strains of virus became more prevalent following initial introduction of the virus to Australia: virus strains that didn’t kill their hosts were more readily dispersed to new hosts (mosquitoes bite only living rabbits) 5

The Rabbit-Myxoma System Today Left alone, the rabbit-myxoma system in Australia would probably evolve to an equilibrial state of benign, endemic disease, as in South America: pest management specialists continue to introduce new, virulent strains to control the rabbit population Contagious diseases spread through the atmosphere or water are less likely to evolve hypovirulence, as they are not dependent on their hosts for dispersal. 6

RABBITS AND MYXOMA … … is an example of a predator (the virus) and prey (the rabbits).

Prickly Pear cactus were also introduced into Australia. Like rabbits, they quickly spread over the continent. A predator of the cactus was introduced. The cactus moth. The cactus only survived in areas where the moth was absent.

Comparing cactus before (a) and after (b) the moth introduction.

The cactus is an example of predator prey interactions. Do predators limit prey population growth? Do prey limit predator population growth? The balance between the two depends on their adaptations. Some adaptations were already found in species. Some adaptations are a result of predator/prey interactions.

All life forms are both consumers and victims of consumers. There are many consumer-resource interactions: Predator-prey Herbivore-plant Parasite-host Producers Consumers Predator; Parasite; Parasitoid: Herbivore; Detritivore

+ Some Definitions Predators catch individuals and consume them, removing them from the prey population. Parasites consume parts of a living prey organism, or host: parasites may be external or internal a parasite may negatively affect the host but does not directly remove it from the population 11

+ More Definitions Parasitoids consume the living tissues of their hosts, eventually killing them: parasitoids combine traits of parasites and predators Herbivores eat whole plants or parts of plants: may act as predators (eating whole plants) or as parasites (eating parts of plants): grazers eat grasses and herbaceous vegetation browsers eat woody vegetation 12

+ Detritivores occupy a special niche. Detritivores consume dead organic material, the wastes of other species: have no direct affect on populations that produce these resources: do not affect the abundance of their food supplies do not influence the evolution of their resources are important in the recycling of nutrients within ecosystems 14

An example of a parasitoid wasp. This was is laying its egg in the caterpillar. The egg will develop into larvae. The larvae will consume the caterpillar as it grows. A combination of predation, and parasitism.

Predators have adaptations for exploiting their prey. This lion has adaptations to capture fast prey. This whale is a filter feeder. Spiders make webs to subdue prey.

Even predator adaptations take practice!

Predators and prey are different sizes, and this can pose problems. If a prey item is too small – it may be too hard to handle. Imagine trying to capture mice with your hands. If a prey item is too large – the predator may not be able to subdue. Imagine trying to tackle a elephant to eat. Blue whales weigh many tons, but eat tiny shrimp (use of filters).

+ Form and Function Match Diet Form and function of predators are closely tied to diet: vertebrate teeth are adapted to dietary items: horses have upper and lower incisors used for cutting fibrous stems of grasses, flat-surfaced molars for grinding deer lack upper incisors, simply grasping and tearing vegetation, but also grinding it carnivores have well-developed canines and knifelike premolars to secure and cut prey 16

+ A predator’s form and function are closely tied to its diet. (a) upper incisors are used to cut plant material; (b) flat-surfaced molars for grinding plant material; (c) knifelike premolars secure prey and tear flesh 17

+ More Predator Adaptations The variety of predator adaptations is remarkable: consider grasping and tearing functions: forelegs for many vertebrates feet and hooked bills in birds distensible jaws in snakes digestive systems also reflect diet: plant eaters feature elongated digestive tracts with fermentation chambers to digest long, fibrous molecules comprising plant structural elements 18

+ Distensible jaws: shift the articulation of the jaw with the skull from the quadrate bone to the supratemporal 19

+ Digestive tracts of consumers are adapted to their diets. Digestive organs of herbivores > carnivores 20

Large carnivores tend to pursue large prey. Size of prey consumed is related to size of predator.

What about the prey? How much energy do you have available for growth? If you are predated upon, your growth rates are affected.

Prey have adaptations to avoid being consumed. Hiding If a predator can’t see you, it can’t eat you. Evolution of cryptic coloration. Escaping If you can outrun your predator, it can’t eat you. Evolution of speed or maneuverability. Active defense mechanisms Animals with poison glands. Plants with thorns, toxic substances.

+ Crypsis and Warning Coloration Through crypsis, animals blend with their backgrounds; such animals: are typically palatable or edible match color, texture of bark, twigs, or leaves are not concealed, but mistaken for inedible objects by would-be predators Behaviors of cryptic organisms must correspond to their appearances. 23

+ Cryptic appearances (a) mantid; (b) stick insect; (c) lantern fly 24

Chemical defenses. The production of chemicals which repel potential predators. Notice the colors of this insect.

+ Warning Coloration: aposematism Why should a prey item evolve bright colors? It definitely brings attention to you. Black and yellow are the most common colors. Unpalatable animals may acquire noxious chemicals from food or manufacture these chemicals themselves: such animals often warn potential predators with warning coloration or : certain aposematic colorations occur so widely that predators may have evolved innate aversions If an animal eats a brightly colored prey item: It may get sick. It may die. If it lives, it will remember. 25

+ Unpalatable organisms 26

+ Why aren’t all prey unpalatable? Chemical defenses are expensive, requiring large investments of energy and nutrients. Some noxious animals rely on host plants for their noxious defensive chemicals: not all food plants contain such chemicals animals utilizing such chemicals must have their own means to avoid toxic effects 27

+ Batesian Mimicry Certain palatable species mimic unpalatable species (models), benefiting from learning experiences of predators with the models. This relationship has been named Batesian mimicry in honor of discoverer Henry Bates. Experimental studies have demonstrated benefits to the mimic: predators quickly learn to recognize color patterns of unpalatable prey mimics are avoided by such predators 28

+ Harmless mantid (b) and moth (c) evolved to resemble a wasp (a)

+ Müllerian Mimicry Müllerian mimicry occurs among unpalatable species that come to resemble one another: many species may be involved each species is both model and mimic process is efficient because learning by predator with any model benefits all other members of the mimicry complex certain aposematic colors/patterns may be widespread within a particular region 30

+ Costa Rican butterflies and moths

Parasites! Parasites have adaptations to allow them to live in the host. The host has adaptations to fight off parasites. The parasite does not want to kill the host, but disperse its offspring to another host.

+ Parasites have adaptations to ensure their dispersal. Parasites are usually much smaller than their hosts and may live either externally or internally: internal parasites exist in a benign environment: both food and stable conditions are provided by host parasites must deal with a number of challenges: host organisms have mechanisms to detect and destroy parasites parasites must disperse through hostile environments, often via complicated life cycles with multiple hosts, as seen in Plasmodium, the parasite that causes malaria 32

+ Parasite-Host Systems: A Balancing Act The parasite-host interaction represents a balance between parasite virulence and host defenses: immune system of host can recognize and disable parasites but parasites may multiply rapidly before an immune response can be deployed 33

+ Parasites may defeat a host’s immune response. Circumventing the host’s immune system is a common parasite strategy: some parasites suppress the host’s immune system (AIDS virus) other parasites coat themselves with proteins that mimic the host’s own proteins (Schistosoma) some parasites continually coat their surfaces with novel proteins (trypanosomes) 34

+ Cross-Resistance Some parasites elicit an immune response from the host, then coat themselves with host proteins before the immune response is fully mobilized: initial immune response by host may benefit the host later when challenged by related parasites in a phenomenon known as cross-resistance Once an immune response has been elicited, antibodies can persist for a long time, preventing reinfection. 35

Many parasites have complex life cycles. Malaria (Plasmodium) parasitic life cycle.

+ Plants have antiherbivore defenses. Plant-herbivore “warfare” is waged primarily through biochemical means. Full spectrum of plant defenses includes: low nutritional content of plant tissues toxic compounds synthesized by the plants structural defenses: spines and hairs tough seed coats sticky gums and resins 36

Plant adaptations against predation. Nutritional value? It could be as simple as a spine. “Ouchy bush!” It could be as complicated as chemicals. Tannins. Secondary compounds.

+ Spines protect the stems and leaves (a) cholla cactus and (b) prickly pear cactus

+ Digestibility Animals typically select plant food according to its nutrient content: especially important to young animals, which have high demands for protein Some plants deploy compounds that limit the digestibility of their tissues: tannins produced by oaks and other plants interfere with the digestion of proteins some animals can overcome the effect of tannins through production of digestive dispersal agents 38

+ Secondary Compounds Secondary compounds are produced by plants for purposes (typically defensive) other than metabolism. Such compounds can be divided into three major classes: nitrogen compounds (lignin, alkaloids, nonprotein amino acids, cyanogenic glycosides) terpenoids (essential oils, latex, plant resins) phenolics (simple phenols) 39

+ Induced and Constitutive Defenses Constitutive chemical defenses are maintained at high levels in the plant at all times. Induced chemical defenses increase dramatically following an attack: suggests that some chemicals are too expensive to maintain under light grazing pressure plant responses to herbivory can reduce subsequent herbivory 40

+ Herbivores control some plant populations. Examples of control of introduced plant pests by herbivores provides evidence that herbivory can limit plant populations: prickly pear cactus in Australia controlled by introduction of a moth, Cactoblastis Klamath weed in California controlled by introduction of a beetle, Chrysolina 41

+ Effects of Grazers and Browsers on Vegetation Herbivores consume 30-60% of aboveground vegetation in grasslands: demonstrated by use of exclosures limiting access to vegetation by herbivores Occasional outbreaks of tent caterpillars, gypsy moths, and other insects can result in complete defoliation of forest trees. 42

Imagine a plant being eaten, which stimulates plant or chemical production. Mite growth is inhibited if the plant was previously eaten.

Look at the impact of herbivores.

Outbreaks of herbivorous insects can defoliate forests.