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Species Interaction and Biological Diversity Chapter Overview Questions • What determines the number of species in a community? • How can we classify species according to their roles in a community? • How do species interact with one another and control each other’s populations? • How do communities respond to changes in environmental conditions? • Does high species biodiversity increase the stability and sustainability of a community? What is community structure? • Structure or spatial distribution. Is described by four characteristics – Physical appearance: relative sizes, stratification and distribution of populations and species – Species diversity or richness: number of different species – Species abundance: number of individuals of each species – Niche structure: number of ecological niches, how they resemble each other and how they interact COMMUNITY STRUCTURE AND SPECIES DIVERSITY • Biological communities differ in their structure and physical appearance. Figure 7-2 Structure in a Mature Deciduous Forest How do communities differ in physical appearance? • They usually consist of a mosaic of vegetation patches of differing size. Results in: – Sharp edges and – Ecotones (wider and more diffuse transition zones between one community and another) • Edge Effects: caused by differences in physical structures at boundaries and in ecotones – Example: edge area between a forest and open field. It will have a different combination of species than the forest and field interiors. Ecotone Sharp Edge Edge between woodlands and meadow Community Structure • Structure: patterns of spatial distribution of individuals and populations within community and relation of a community to its surroundings – – – – – Random Ordered Clustered Patchiness: graininess in spatial distribution Vertical and horizontal (eg, rain forest is vertical) Community Structure • Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2 Desert plant spacing of Creosote Bush and Ocotillo, Sonoran Desert. Clustered community; school of fish Abundance and Diversity • Abundance: total number of organisms in a biological community • Diversity: measure of number of different species, ecological niches, or genetic variation present in an ecosystem • In general, species diversity decreases, but abundance within species increases as one travels from equator toward poles. Species diversity and abundance • Most species-rich environments are tropical rain forests, coral reefs, deep sea, and large tropical lakes. • Diversity affected by: – Latitude (highest in tropical, lowest in polar) – Depth in aquatic systems (increases from surface to depth of 2000 m, and at sea bottom – Pollution (diversity decreases as pollution increases) General types of species • Native: normally live and thrive in a particular ecosystem • Nonnative: species that migrate or are deliberately or accidentally introduced into an ecosystem by humans. Also known as exotic or alien species • Indicator: serve as early warnings of damage to a community or an ecosystem • Keystone: species whose role is much more important than their abundance suggests Case Study: Species Diversity on Islands • MacArthur and Wilson proposed the species equilibrium model or theory of island biogeography in the 1960’s. • Model projects that at some point the rates of immigration and extinction should reach an equilibrium based on: – Island size – Distance to nearest mainland Introduced or Invasive Species • If introduced species preys upon or competes more successfully with one or more native populations, the whole community can be altered. • Considered to be artificially introduced (I.e., by man) – kudzu, privet, zebra mussels, cane toads, Asian ambrosia beetle Kudzu Chinese Privet Cane toad and distribution in Australia Asian ambrosia beetle and tree damage Zebra mussel and distribution Indicator Species: Biological Smoke Alarms • Species that serve as early warnings of damage to a community or an ecosystem. – Presence or absence of trout species because they are sensitive to temperature and oxygen levels. Indicator Species • Birds are excellent indicators because they are found everywhere and respond quickly to environmental change. • Fish are good indicator species because they need high quality water. • Amphibians live partly on land and partly in water, so can be good indicator species also. Examples of Indicator Species Keystone Species: Major Players • Keystone species help determine the types and numbers of other species in a community thereby helping to sustain it. Figures 7-4 and 7-5 Critical roles of keystone species • Pollination of flowering plant species (bees, bats, humming birds) • Dispersion of seeds by fruit-eating animals • Habitat modification • Predation by top carnivores that helps control the populations of various species • Improving the ability of plant species to obtain soil minerals and water • Efficient recycling of animal wastes Kelp Sea urchins eating kelp (a type of algae) Sea Otter Gopher Tortoise Burrow Long Leaf Pine Ecosystem Gopher Tortoise Habitat modifications of keystone species • Elephants push over, break, or uproot trees, creating forest openings in Africa. • Gopher tortoises dig burrows that provide refuge to more than 360 animal species, including indigo snake, burrowing owl, and Florida mouse. • Bats and birds regenerate deforested areas by depositing plant seeds in their droppings. • Beavers build dams, which produce lakes. This provides habitat for many other animals. • Top predator keystone species exert a stabilizing effect on their ecosystems (wolf, leopard, lion, alligator) • Dung beetles remove, bury, and recycle animal wastes. Competition • Is a kind of antagonistic relationship within a community – Compete for growing space, energy, and specific sites for all life activities • Intraspecific: competition among members of the same species – Territoriality: individuals of same species define an area surrounding their homesite and defend it • Interspecific: competition among members of different species Competitive Exclusion Principle • No two species will occupy the same niche and compete for same resources in same habitat for long – Resource partitioning: response to competition for resources • Different species use different parts of same resource to coexist • Different species use the same resources at different times of day • Different species have different morphologies that allow them to use slightly different resources (beak length vs depth of flower) Resource Partitioning • Each species minimizes competition with the others for food by spending at least half its feeding time in a distinct portion of the spruce tree and by consuming somewhat different insect species. Figure 7-7 Species Interactions • Predation and competition for scarce resources are major factors in evolution and adaptation • Predator: feeds directly upon other living organisms – Includes omnivores, herbivores and carnivores, but not detritivores, scavengers and decomposers • Parasite: feeds on host organism or stealing its resources without killing it • Pathogen: organism that causes disease Mutualism, parasitism and commensalism are types of symbiosis, which is defined as the intimate living together of members of two or more species. Predation • Complex influence on population balance of communities • Is an important factor in evolution – Reduces number of individuals that are slowest, least fit and weakest (populations with predators are in best balance with environment • Prey species, in response to predation, have evolved many specialized protective or defensive adaptations – Coevolution: species exert selective pressure on each other PREDATION • Some prey escape their predators or have outer protection, some are camouflaged, and some use chemicals to repel predators. Figure 7-8 Types of Symbiosis Commensalism Commensalism: Using without Harming • Some species interact in a way that helps one species but has little or no effect on the other. Figure 7-10 Mutualism: Win-Win Relationship • Two species can interact in ways that benefit both of them. Figure 7-9 Mutualism Lichens are a combination of a fungus and an alga or cyanobacteria Parasites: Sponging Off of Others • Although parasites can harm their hosts, they can promote community biodiversity. – Some parasites live in host (micororganisms, tapeworms). – Some parasites live outside host (fleas, ticks, mistletoe plants, sea lampreys). – Some have little contact with host (dumpnesting birds like cowbirds, some duck species) Parasitism A tomato hornworm is covered with cocoons of pupating braconid wasps Adult heartworms in a dog’s heart Defensive Mechanisms • Defensive adaptations to protect species from predation – Examples • Noxious odors • Poisonous secretions • Mimicry – Batesian: species that are harmless will evolve colors, patterns, or body shapes that mimic noxious species – Mullerian: two species, both of which are noxious, have evolved to look alike • Camouflage Animal Camouflage ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION • New environmental conditions allow one group of species in a community to replace other groups. • Ecological succession: the gradual change in species composition of a given area – Primary succession: the gradual establishment of biotic communities in lifeless areas where there is no soil or sediment. – Secondary succession: series of communities develop in places containing soil or sediment. Primary Succession • Pioneer species – first ones on site • Ecological development – accumulation of organic debris allowing germination of seeds – orderly sequence of stages occurs as organisms modify the environment in ways that allows one species to replace another Primary Succession: Starting from Scratch • Primary succession begins with an essentially lifeless area where there is no soil in a terrestrial ecosystem Figure 7-11 Secondary Succession • Occurs due to disruption of community – Bare soil colonized by rapidly growing annual plants • Annual plants replace by perennial plants • Biomass accumulates and allows richer soil, better shelter, and allows a more complex community Climax woodland community Climax Forest on Stone Mountain Beech Forest (Fernbank Science Center) Damage done by Yellowstone Fire in 1988. Pictures taken in May, 2010 Slow succession due to destruction of beneficial soil bacteria and fungi This is primary succession on a recent lava flow in the Galapagos Islands, Ecuador. The vegetation near the center of the photo is mangrove, on the slopes there are cactus and other desert plants. Before After This is the northern lower flank of Mt. Saint Helens taken about 24 years after the eruption. This area was buried under hundreds of feet of ash, so all the plant life you can see has come in from wind-blown seed. The plants are things like willows, cottonwoods, grasses and forbes that have fine, easily wind-transported seeds. In the thin ash area, most of the new vegetation arises from perennial plants whose roots survived the blast underground. This is reforestation of the blast zone around Mt. Saint Helens. All of the trees were planted (by Weyerhaeuser) about 20 years ago. This photo shows plant succession following the retreat of the Emmons Glacier on Mt. Rainier. The glacier began retreating around the time of the Civil War and has moved up the valley several miles. The glacier itself is the dark mass (not the white area) that terminates in the lower left third of the picture. You can see plant succession marching up-valley toward the snout of the glacier. If you look closely you can see the White River emerging from the snout. This river flows all the way to Puget Sound at Tacoma. To the right is a beautiful lateral moraine. Plant succession is visible on the right (north) slope of the moraine but is being impeded by hot dry conditions on the south (left) slope of the moraine. Most of the pioneering vegetation is alder and willow, with a few conifers gradually becoming established. The seeds are coming down from the forests above. The dark old growth forest to the right is mainly mountain hemlock and subalpine fir. Climax Community • Culmination of successional process; is a stable, complex mature form. • Equilibrium communities – Never reach a stable climax because they are characterized by and adapted to periodic disruption. • Example: fire-climax communities (Long-leaf pine forest) Fire Ecosystem • The longleaf/wiregrass community is part of a fire ecosystem. • Fire is required to clear brush to allow pine seedlings to grow • Wire grass is most likely to bloom and set seed after a late spring or early summer burn. Jackson Pine Forest, Lake Tahoe, CA Resilience and Stability • Resilience or stability: Resistance to change. Three types: – Constancy: ability to keep populations within the limits of available resources – Inertia: ability to resist change due to perturbations – Resilience: ability to repair damage after disturbance ECOLOGICAL STABILITY AND SUSTAINABILITY • Having many different species appears to increase the sustainability of many communities. • Human activities are disrupting ecosystem services that support and sustain all life and all economies.