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Environmental Science: Toward a Sustainable Future Richard T. Wright. Chapter 4. Ecosystems: How They Change. Environmental Science: Toward a Sustainable Future Richard T. Wright. Chapter 4. Ecosystems: How They Change. Factors That Contribute to Ecosystem Change. Key Topics:
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Environmental Science: Toward a Sustainable Future Richard T. Wright Chapter 4 Ecosystems: How They Change
Environmental Science: Toward a Sustainable Future Richard T. Wright Chapter 4 Ecosystems: How They Change
Factors That Contribute to Ecosystem Change Key Topics: • Dynamics of natural populations • Mechanisms of population equilibrium • Mechanisms of species adaptation • Ecosystem response to disturbance • Lessons to learn
Chapter 4: Ecosystems: How They Change • Yellowstone National Park contains 2.2 million acres • In 1988, 10% of the park burned • May take 25 years for the diversity of plants and animals to completely recover • How can biodiversity be enhanced by this type of disturbance?
4:1 Dynamics of Natural Populations Key Topics: • Population growth curves • Biotic potential versus environmental resistance • Density dependence and critical number
4:1 Dynamics of Natural Populations Population Equilibrium Births A dynamic balance between births and deaths. Deaths Over time, the populations of most species in an ecosystem tend to remain more of less constant in size and geographic distribution.
4:1 Dynamics of Natural Populations Population Growth Curves • J-curve shows growth under optimal conditions with no limiting factors • S-curve shows a population at equilibrium/carrying capacity
4:1 Dynamics of Natural Populations Population Growth Curves • When first introduced, Gypsy moth caterpillars devastated oak trees in the Eastern US • Currently, the trees have recovered and moth populations remain low
4:1 Dynamics of Natural Populations Biotic Potential and Environmental Resistance • Reproductive strategies Biotic Potential • Slow growth/development • Large bodies • Long life spans • Few offspring with high • parental care • High recruitment/low • mortality of young • Predictable environment • Rapid growth/development • Small bodies • Short life spans • Many offspring with low parental care • Low recruitment/high mortality of young • Unpredictable environment S-shaped growth curve Low Biotic Potential J-shaped growth curve High Biotic Potential
4:1 Dynamics of Natural PopulationsBiotic Potential and Environmental Resistance • Environmental resistance:combination of biotic and abiotic factors that may limit population increase in natural ecosystems • Reduces recruitment • Predators, competitors, disease • Adverse weather, limited food/nutrients • If recruitment is at replacement level population remains constant/at carrying capacity
4:1 Dynamics of Natural Populations Biotic Potential and Environmental Resistance
4:1 Dynamics of Natural Populations Density Dependence and Critical Numbers • Factors of environmental resistance are either: • density-independent: effect does not vary with population density; e.g., adverse weather • density-dependent: effect varies with population density; e.g., infectious disease/stress/ competition
4:1 Dynamics of Natural Populations Density Dependence and Critical Numbers • Critical number: the lowest population level for survival and recovery • Threatened species: populations declining rapidly • Endangered species: population is at its critical number • Habitat destruction • Invasive species • hunting Fire salamanders are threatened Toucans are endangered
4:2 Mechanisms of Population Equilibrium Key Topics: • Predator–prey dynamics • Competition • Interspecific • Intraspecific • Introduced species
4:2 Mechanisms of Population EquilibriumPredator–Prey Balance: Wolves and Moose Predator–prey dynamics • Top-down regulation • Small number of wolves = low environmental resistance for moose moose population increases • Abundance of moose = low environmental for wolves wolf population increases • Large number of wolves = high environmental resistance for moose moose population decreases • Decline in moose population = high environmental resistance for wolf wolf population decreases Royal Isle, 45-mile long island in Lake Superior
4:2 Mechanisms of Population Equilibrium Lessons to Be Learned about Predator–Prey Balance • Absence of natural enemies allows a herbivore population to exceed carrying capacity, which results in overgrazing of the habitat. • The herbivore population subsequently crashes. • The size of the herbivore population is maintained so that overgrazing or other overuse does not occur.
4:2 Mechanisms of Population Equilibrium Plant–Herbivore Dynamics • No regulatory control (No predation) on herbivores • Went into exponential growth pattern • Overgrazed habitat • Massive die-off of herbivores Reindeer on St. Matthew Island 128-mile island in Bering Sea between Alaska & Russia 5 males & 24 Females introduced No predators
4:2 Mechanisms of Population EquilibriumPlant–Herbivore Dynamics • Compare the predator–prey with plant–herbivore methods of controlling the size of the herbivore population. • How would the herbivore population growth curve look if diseases or predators were used as the control mechanism?
4.2 Mechanisms of Population Equilibrium Keystone Species Keystone Species • A single species that maintains biotic structure of the ecosystem • Pisaster ochraceus: a starfish that feeds on mussels, keeping them from blanketing the rocks • Allows barnacles, limpets, anemones, and whelks to colonize inter-tidal habitat http://www.marine.gov/
4.2 Mechanisms of Population Equilibrium Type of Competition Competition: • No species exists in isolation • If niches overlap may compete for scarce resources • 2 kinds of competition: • Intraspecific- occurs when members of the same species compete for resources • “Bottom-up” regulation- when resource is in limited supply • Interspecific-occurs when members of different species compete • “Top-down” regulation- occurs by predation
4.2 Mechanisms of Population Equilibrium Competition: Intraspecific Territoriality: defense of a resource against individuals of the same species • Examples: wolves and songbirds • Results in priority access and use of resources • Nesting area • Adequate food • Breeding is restricted to only individuals capable of claiming and defending the territory • Density-dependent Wolves on Isle Royale
4.2 Mechanisms of Population Equilibrium Competition: Intraspecific Territoriality • Usually the younger, weaker members of the population are unable to claim territory • Leads to Natural Selection • Weaker members must choose to disperse or die • Leads to stabilization of the population • Leads to Self-Thinning-where the “strongest” survive and reproduce, sending the “stronger” genes to the next generation
4.2 Mechanisms of Population Equilibrium Competition: Interspecific (In Plants) • Consider all the different species of green plants competing for nutrients, water, and sunlight • The adaptation of a certain species to a specific microclimate reduces competition • A single species cannot utilize all the resources in an area • Left-over resources may be claimed by another species • Example: Grasslands contain plants with both fibrous roots and taproots • Coexist by accessing resources from different soil levels
4.2 Mechanisms of Population Equilibrium Competition: Interspecific (In Plants) • Spring wildflowers (temperate deciduous forest) show another adaptation to reduce competition • Sprout from perennial roots or bulbs in the early part of the season • Take advantage of the light that reaches the forest floor before the trees grow leaves
4.2 Mechanisms of Population Equilibrium Competition: Interspecific (In Plants) • Bottom Line No one plant species is able to outcompete all others • Different species have becomes specialized in different ways in their use of resources
4.2 Mechanisms of Population Equilibrium Competition: Interspecific (In Animals) • 2 species of Barnacles(Cape Cod) • Competetive Expulsion- one species (Semibalanus balanoides) will outcompete and eliminate the other (Chthamalus fragilis) • But in warmer waters, Chthamalus fragilishas the advantage Chthamalus fragilis Semibalanus balanoides
4.2 Mechanisms of Population Equilibrium Competition: Interspecific (In Animals) • Bottom Line Competition helps drive natural selection bringing about greater specialization of ecological niches and allowing resources to be divided up among species Here, an anemone, Epiactis prolifera, competing with a red coralline alga (pink) and a sponge colony (orange).
4.2 Mechanisms of Population Equilibrium Introduced Species • Introduced species can become: • Naturalized- joins the native fauna or flora and does no measurable harm • Invasive- finds conditions hospitable/population explodes/ drives out native species by outcompetiting • Estimate of economic losses due to introduced species in the US excess of $138 billion per year • Chestnut blight • Japanese beetles, fire ants, gypsy moths • Water hyacinth, kudzu, spotted knapweed, purple loosestrife
4.2 Mechanisms of Population Equilibrium Introduced Species Chestnut blight: • Fungal disease introduced to US when Chinese chestnut trees carrying the disease were planted in NY • Disease spread through forest killing neatly every American chestnut tree by 1950’s • Have crossbred hybrid between Chinese and American chestnut which is resistant to the fungus Surviving 85-foot American chestnut tree in Atkinson, Maine.
4.2 Mechanisms of Population Equilibrium Introduced Species Zebra Mussels: • 1980’s- Introduced into the Great Lakes with discharge of ballast water from European ships • Now spreading through Mississippi River basin • Displacing native mussels and clogging water intake pipes
4.2 Mechanisms of Population Equilibrium Introduced Species Ctenophores (jellyfish): • 1982- Transported from East Coast of US to the Black Sea • Have cost Black Sea fisheries estimated $250 million • Totally shut down fisheries in the Sea of Azov (Ukraine) • Kill larval fish directly and deprive larger fish of food
4.2 Mechanisms of Population Equilibrium Introduced Species Water Hyacinth: • 1884- Introduced into Florida from South and Central America as ornamental flower • Escaped into waterways • No competition proliferated so navigation is impossible on some waters • Is under “maintenance control” in Florida
4.2 Mechanisms of Population Equilibrium Introduced Species Kudzu: • Introduced from Japan in 1876 • Planted on farms in Southern US as cattle fodder and erosion control • Now occupies over 7 million acres of the deep South
4.2 Mechanisms of Population Equilibrium Introduced Species Spotted knapweed: • Introduced in 1920’s with alfalfa seed imported from Europe • Bitter tasting and basically inedible- making rangeland worthless for grazing cattle & elk • Displacing native plants and grasses • No herbivore enemies • Secretes chemicals that inhibit growth of neighboring plants
4.2 Mechanisms of Population Equilibrium Introduced Species Purple loosestrife: • Introduced into wetlands in US & Canada in 1800’s as ornamental and medicine plant • Inedible • Outcompeting native wetland vegetation • Threatening waterfowl
4.2 Mechanisms of Population Equilibrium Rabbits Overgrazing in Australia Rabbits: • 1859- Introduced into Australia from England to be used for sport shooting • No carnivore or other natural enemies capable of controlling them • Rabbit population exploded • Devastated vast area of rangeland by overgrazing • Extremely damaging to native kangaroos and rancher’s sheep • Eventually, brought under control by release of a disease-causing virus in rabbits
4.2 Mechanisms of Population Equilibrium Rabbits Overgrazing in Australia On one side of a rabbitproof fence, there is lush pasture; on the other side, the land is barren.
4.3 Mechanisms of Species AdaptationChange through Natural Selection Natural Selection: • Leads to modification of a species’ gene pool toward features that enhance survival and reproduction • Examples include predators, parasites, drought…
ENVIRONMENT GENES + ADAPTATIONS NATURALSELECTION: For? or Against? 4.3 Mechanisms of Species Adaptation Recipe for Change
4.3 Mechanisms of Species Adaptation Adaptations to the Environment All characteristics of organisms can be grouped as: Coping with abiotic factors Obtaining food (energy) and water Escaping predation Attracting mates; pollination Migration; Seed dispersal
4.3 Mechanisms of Species Adaptation The Limits of Change When facing a new, powerful selective pressure, species have 3 alternatives: • Adapt to the new condition by natural selection • Move (migrate) to an area where conditions are suitable • Die (extinction)
4.3 Mechanisms of Species Adaptation Vulnerability of different organisms to environmental changes 4 Keys to Survival: 1. Geographic Distribution 2. Specialization to a habitat or food supply 3. Genetic Variation 4. Reproductive Rate
4.3 Mechanisms of Species Adaptation Vulnerability of different organisms to environmental changes
4.3 Mechanisms of Species Adaptation Prerequisites for Speciation Prerequisites for Speciation Formation of a new species: • Original population must separate into smaller populations that do not interbreed with one another. • Geographic Isolation • Reproductive Isolation • Separated populations must be exposed to different selective pressures. • Example: arctic and gray fox (next slide)
4.3 Mechanisms of Species Adaptation Speciation: Galápagos Finches Darwin’s Finches: Possible Speciation • S. American finches may have been blown to the islands during storms • Initial populations grew • Intraspecific competition occurred • Some birds dispersed to nearby islands • Separated subpopulation encountered different selectivepressures • Became specialized for feeding on different foods
4.4 Ecosystem Responses to Disturbance Equilibrium Theory • Ecosystems are stable environments in which the biotic interactions among species determinethe structure of the communities present. • Species interact constantly in well balanced predator-prey and competitive relationships
4.4 Ecosystem Responses to Disturbance How do ecosystems respond to disturbances? • Ecological succession • Disturbance and resilience • Evolving ecosystems
4.4 Ecosystem Responses to Disturbance Succession and Disturbance Ecological succession: transition between biotic communities • Occurs because the physical environment is modified by the growth of the biotic community itself, the area becomes more favorable to another group of species and less favorable to the current species • Primary succession: no previous biotic community • Secondary succession: previously occupied by a community • Aquatic succession: transition from pond or lake to terrestrial community
4.4 Ecosystem Responses to Disturbance Primary Succession Primary Succession: • Bare rock can be colonized by certain mosses and lichens • Spores lodge and germinate in tiny cracks in rock • Catch soil particles as they are eroded soil gradually forms • Mat of moss and soil provides suitable place for seeds of larger plants to lodge (continued)