BIO226 : Predation/Competition : Coevolution : Lesson

Glossary terms that are important in this lesson: Aggressive mimicry, alkaloid, allopatric, aposematic, clade, cladistics, cladogram, convergence, crypsis, ectoparasite, elaiosome, endoparasite, facultative, mast, monophyletic, mycorrhizae, obligate, outgroup, phenolic, polymorphism, preadaptations, secondary compounds, symbiosis, sympatric, tannins, terpenoid, trophic, virulence.

I. Relationships Among Species

1. Introduction
   • Life table schedules of survival and fecundity determine population growth rates
   • Physical factors: similar stresses lead to similar solutions; convergence
   • Biological factors: interactions lead to coevolution and diversity

2. Types of interactions between species
   • Consumer-resource: predation, herbivory, parasitism
     • Biological communities are organized in food chains: energy and nutrients move up the chain
     • Populations are controlled from below by resources, from above by consumers
     • Detritivores do not directly affect the rate of supply of their food

   • Mutualism
     • Both species benefit in a complementary interaction
     • Examples: pollinators, mycorrhizae, lichen

   • Competition between species
     • Indirect through mutual effects on shared resources
     • Direct through antagonistic behavior: territoriality
     • Apparent competition: indirect through predation and disease
     • Examples: birds, bees, moths, sponges, bacteria

   • Predator proficiency
     • Individual hunters are similar in size to prey - cheetah
     • Cooperative hunters can subdue larger prey - wolf, hyena, army ants
     • Filter feeders consume many small items - blue whale
     • Parasites feed on hosts but don't kill them - virus, bacteria, protozoa, worms
     • Parasitoids consume their hosts - flies and wasps
     • Herbivores can act like predators or parasites
     • Specialized for food
       • Jaws and teeth
       • Digestive systems

3. Prey escape
   • Early detection and swift movement - deer, antelope
   • Thorns and chemicals - plants
   • Spines, armor, chemicals - porcupine, armadillo, bombardier beetle
   • Crypsis and warning coloration
     • Camouflage: look like something else - hawkmoth
     • Toxic and aposematic - black and a bright color - monarch butterfly
     • Trade-off: defenses are costly to manufacture and maintain
   • Batesian mimicry
     • Unpalatable models and palatable mimics - monarch and viceroy
     • Works when mimics are rare relative to models - predators learn to avoid both
     • Polymorphism: female swallowtails mimic several models
   • Müllerian mimicry
     • Several unpalatable species resemble one another - Rocky Mountain bumblebees and wasps

4. Parasites
   • Parasites adapted to host: host provides environment and nutrition
   • Host immune defense and parasite counterdefense - chemical crypsis
   • Examples: Ascaris, Schistosoma, Plasmodium

5. Herbivory and plant defenses
   • Most plant tissues have low nutritional value
   • Secondary compounds are toxic
   • Structural defenses: spines, hairs, tough seed coats, sticky gums and resins
   • Protein content of food is important for herbivore growth, development, and survival
   • Biochemistry
     • Tannins: combine with proteins and inhibit digestion - insects produce chemicals which separate the tannins and proteins
     • Secondary compounds: insects detoxify them - bruchids, tobacco hornworm
     • Plants and herbivores with similar biochemical specializations associate into plant defense guilds
     • Plant defenses may be induced by herbivore damage - Tetranychus

6. Mutualism: both species benefit
   • Trophic mutualism
     • Species complement each other in obtaining energy and nutrients
     • Examples: lichen, mycorrhizae, Rhizobium-plants, bacteria-cows, leaf-cutting ants
   • Defensive mutualism
     • Example: cleaning fish and shrimp; predatory mimics
     • Ants and acacia - obligate mutualism with adaptations of each species which strengthens the interaction
   • Dispersive mutualism
     • Pollination
       • Plants provide pollen, nectar, oils, fragrances which attract animals
       • Animals transfer pollen from plant to plant
       • Some species pairs are highly specialized: Stanhopea and Eulaema
     • Variations
       • Empty flowers
       • Dispersal: mites in Heliconius, anther smuts, aggressive mimicry
     • Seed dispersal
       • Fruits entice animals to eat seeds
       • Seeds are defecated at a distance from the parent plant
       • Some animals hoard seeds and those not eaten germinate
       • Some plants produce a large mast crop of seeds in some years and no seeds in others - seed predators cannot maintain a population density high enough to consume all seeds in all years

II. Evolutionary Responses and Coevolution

1. Introduction
   • Interacting populations tend to evolve in response to characteristics which affect evolutionary fitness
   • Antagonistic interactions lead to adaptations at another's expense
   • Mutualistic interactions tend to lead to stable arrangements of complementary adaptations that promote the interaction

2. Evolutionary relationships between antagonists
   • Myxoma virus and the rabbit
     • Release of a few pairs in Australia in 1859 resulted in hundreds of millions and a threat to the country
     • Myxoma virus from South America was introduced and spread by mosquito; high initial mortality declined rapidly
       • Resistant alleles in rabbits increased in frequency
       • Less virulent strains of the virus spread more easily and became predominant

   • Parasite-host
     • David Pimentel grew Musca domestica and Nasonia vitripennis together
     • Housefly evolved resistance to the wasp parasitoid
       • Higher productivity of flies, lower population density of wasps and moderate oscillations compared to results with evolutionarily "naive" flies

   • Genetics in plant-pathogen systems
     • Assume each population contains genetic variability for traits that influence interactions
     • Interaction between individual genes for virulence in wheat rust with genes for resistance in wheat
     • Genetic interaction between scale insects and ponderosa pine

3. Evolutionary equilibria
   • Selective adaptations in prey populations should increase as predation rate increases
   • Selective adaptations in predator populations should increase as exploitation rate decreases
   • Interacting populations reach an evolutionary steady state when rates balance

4. Genetic variation in competitive ability
   • Competitive ability has a genetic basis and can evolve in laboratory populations
   • Examples: flour beetles, fruit flies, house flies and blowflies
   • Sparse populations can evolve interspecific competitive ability more rapidly than dense populations

5. Character displacement
   • Theory: if resources are sufficiently varied, competitors should diverge and specialize
   • Hypotheses
     • Differences among species evolved as a result of their interaction
     • Differences evolved in response to different resources in different places and remained when the species subsequently overlapped in their ranges
   • Evidence
     • Character displacement occurs where species are sympatric
     • Example: ground finches (Geospiza) in the Galapagos - bird beaks
     • Coexistence depends on some ecological differences between species

6. Coevolution - reciprocal evolutionary responses
   • Hypotheses
     • Pairs of populations undergo reciprocal evolution
     • Only compatible species co-occur but their similar traits did not evolve in response to each other
     • Species adaptations to each other enhance community productivity and resistance to perturbation
   • Ants and aphids
     • Ants protect aphids and leafhoppers from predators
     • Ants harvest nutritious honeydew from aphids and leafhoppers
     • Difficult to show that adaptations resulted from reciprocal interactions
   • Ants and seed dispersal
     • Plants in the family Proteaceae produce seeds with fleshy, edible attached elaiosomes
     • Foraging ants eat the elaiosomes and discard seeds in underground nests
     • After fires, discarded seeds in underground nests germinate
     • Development of the elaiosome may have resulted from the interaction with ants but ants' discarding the seeds underground probably developed independently of the plant
   • Herbivores and chemical defenses
     • Furanocoumarins in the umbellifers and their insect herbivores
     • Insect collections
       • More insect species on plants with both LFC and AFC
       • Insect herbivores on AFC/LFC plants were extreme diet specialists
       • Specialists were abundant but generalists were rare
     • Inferences can be used for phylogenetic reconstruction
   • Yucca moth and the yucca
     • Obligate mutualism between Yucca and Tegeticula
     • Several highly specialized characteristics of Tegeticula are primitive features of the family - preadaptations

7. Summary
   • Interactions among species are major sources of selection and evolutionary response
   • Populations respond to interactions with many other species - diffuse coevolution
   • Strong coevolutionary interactions may be limited to pairs of species

When you have completed this lesson, go on to Review Questions