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Interactions between populations Evolution Biol. 4974/5974 D. F. Tomback Interactions Between Populations and Species Biol 4974/5974 Evolution Learning goals Know and understand: • The basic ecological interactions between one or more populations or species that lead to evolution. • How species exert selection pressure on other species in ecological interactions. • The three evolutionary consequences of competition: Resource partitioning, character displacement, and competitive exclusion. • The evolutionary consequences of predator-prey relationships. • The kinds of mutualistic interactions, and the evolutionary consequences of mutualism. Basic interactions Basic ecological interactions between populations that can lead to evolution: • Competition • Predation • Symbiosis and mutualism These interactions can be affected by relative population sizes of species, which change over time. Many of these interactions are coevolutionary—each species exerts selection on the other. Fig. 20.1B Fluctuations in populations of the Great Tit in Holland between 1912 and 1943. 1 Interactions between populations Evolution Biol. 4974/5974 D. F. Tomback Outcomes of competition Resource partitioning: When two species or more species compete for the same resources, they evolve to specialize in foraging to minimize competition. Fig. 20.1 a Character displacement: Resource partitioning can lead to changes in morphology related to foraging. (Grant, P.R. 1986) • Beak dimensions reflect range of resources available. • They will change as resources shift. Fig. 20.2 Outcomes of competition Competitive exclusion: The Competitive Exclusion Principle states that no two species can coexist indefinitely when the same resource limits both species. • This implies that two species cannot occupy the same ecological niche. • Study reveals that for similar species there are often subtle differences that allow them to coexist. • E.g., work of G. F. Gauss on two species of Paramecium in the laboratory. Fig. 20.3 2 Interactions between populations Evolution Biol. 4974/5974 D. F. Tomback Predation overview Predation describes the foraging behavior of consumers—i.e., predators. --animals catching and eating other animals for food. But, predation is also used in a general way: • Herbivores consume plant parts, such as leaves or stems. • Granivores are “seed predators,” because they consume seeds. • Parasitoids are insects that lay their eggs in host organisms, which are consumed alive by larvae. • Parasites consume tissue or nutrients from living hosts. www.thebigzoo.org USFWS: snowy owl Predation overview Ecuadorian stick insects www.insects.org Predators have morphology and behavior adapted to catch prey. Their teeth and digestive tracts are adapted to their prey. • E.g., cheetahs run fast. • E.g., spiders weave webs. Prey species are adapted to avoid predators. • Cryptically-colored (use camouflage) • Mimic surroundings • Travel in large groups; have acute senses. Plants and seeds are adapted to discourage herbivores. • May have chemical compounds that deter predators (e.g., tannins in acorns). • Deter herbivores with tough leaves, thorns, or prickles. www.waynesword.palomar.edu Predator-prey cycles Close dependence of predator and prey, e.g., snowshoe hare-Canada lynx interaction. • The prey is a partial cycle ahead of the predator cycle. • Evolutionary “arms race”. R.E. Ricklefs. 2008. The Economy of Nature, 5th ed. 3 Interactions between populations Symbiosis and mutualism Symbiosis: Close physical association between species, usually coevolved. Trophic mutualism: Species evolved to share energy and nutrients. • lichens: fungi and cyanobacteria (symbiosis). • leaf cutter ants: ants and fungi. • Rhizobium and root nodule plants: nitrogenfixing association (symbiosis). Defensive mutualisms: One species receives food or shelter from the other species; in return, it defends the partner against predators, herbivores, or parasites. • Cleaning symbioses: “cleaner” shrimp and “cleanee” fish in the tropics • Acacia-ants and ant-acacias: ants defend the plant and obtain food and shelter (neotropics). Evolution Biol. 4974/5974 D. F. Tomback Figure 20.14 www.waynesword.palomar.edu Mutualism Dispersive mutualisms: Involve pollination or seed dispersal mutualisms. • Hummingbirds pollinate plants: Some relationships are very general and some specialized (tropical). • Yucca moth and yucca plant: Female moths carry pollen, enter a flower, cut ovary with ovipositor and deposit 1-15 eggs; deposit pollen on stigma to set seeds as food for larvae. Loss of seeds <30%. Obligate. • Clark’s nutcracker and whitebark pine: Cones do not release seeds; nutcrackers harvest seeds and cache them in the ground for future food; remaining seeds germinate and produce seedlings. Figure 20.16 Study questions • What are the three ecological interactions between populations that can lead to evolution? • These interactions can be coevolved—what does that mean? • Competition as a process can result in resource partitioning, competitive exclusion, or character displacement. Explain how these outcomes result from evolutionary interactions. • How is the concept of predator used to refer to consumer interactions? • Explain how predators and prey may coevolve in an evolutionary “arms race.” • Explain how mutualists may coevolve together. What are the different categories of mutualism? 4