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Chapter 6 Population and Community Ecology NATURE EXISTS AT SEVERAL LEVELS OF COMPLEXITY FACTORS THAT REGULATE POPULATION ABUNDANCE AND DISTRIBUTION 1. 2. 3. 4. 5. Population size- the total number of individuals within a defined area at a given time. Population density- the number of individuals per unit area at a given time. Population distribution- how individuals are distributed with respect to one another. Population sex ratio- the ratio of males to females Population age structure- how many individuals fit into particular age categories. 1. POPULATION SIZE Population size is the number of individuals that a population contains. Determining the size can be tricky Simply counting (if population is small Estimating (if populations are too & stationary) abundant, too widespread or too mobile) Mark-Capture-Recapture (scientists use traps to capture the animals alive and mark them in some way. The animals are returned unharmed to their environment. Over a long time period, the animals from the population are continued to be trapped and data is taken on how many are captured with tags) 2. POPULATION DENSITY Population density measures how crowded a population is. Usually expressed as: # individuals / unit of area 30 people/km2 Macau (China) = 54,882 people/mi2 United States = 84 people/mi2 (182nd in the world) 3. POPULATION DISPERSION Population dispersion is the spatial distribution of individuals within a population. Most populations live as clumped individuals. Clumped helps protect against predators, provides individuals to mate with and helps with gathering resources. Random Uniform Clumped 4. POPULATION SEX RATIO 5. POPULATION AGE STRUCTURE Sex ratio = the ratio of males to females In monogamous species, 50:50 will increase population size. Age Structure = the relative numbers of individuals of each age within a population. The more individuals in childbearing age, will increase population size. POPULATION CONTROL Density-Dependent Population Control- the size of the population will influence an individual’s probability of survival. Large populations are at greater risk! Limiting Resource Carrying Capacity (K) A resource (water, food, etc) that a community cannot live without and those quantities are lower than what is needed to sustain a growing population. How many individuals an environment can sustain. Disease POPULATION CONTROL Density-Independent Population Control- This is • • • • • • when a population is controlled by natural events other than population density. fires floods earthquakes hurricanes volcanoes drought GRAPHING POPULATION FACTORS Density-Dependent Factors - mortality increases as population density increases. (food shortage) Inverse-Dependent Factors - mortality decreases as a population increases in size. (mating) Density-Independent Factors - mortality remains unchanged. (natural disaster) Last slide GROWTH MODELS & POPULATION CHANGES Exponential & Logistic Growth Models K-selected species, r-selected species Survivorship Curves & Metapopulations POPULATION SIZES CAN GROW Growth rate- the number of offspring an individual can produce in a given time period, minus the deaths of the individual or offspring during the same period. Growth Rate = Birth - Deaths Intrinsic growth rate (r)- is the rate at which a population would grow if it had unlimited resources POPULATION GROWTH MODELS EXPONENTIAL GROWTH CURVE: J-SHAPED CURVE Exponential Growth Model J-shaped curve When populations are not limited by resources, growth can be very rapid. POPULATION GROWTH MODELS LOGISTIC GROWTH CURVE: S-SHAPED CURVE Logistic Growth Model S- shaped Curve But as resources become limited, its growth rate slows and levels off. Environmental resistance: water, space, food, predators or disease P OPULATION G ROWTH M ODELS VARIATIONS OF THE LOGISTIC MODEL LOGISTIC GROWTH CURVE: S-SHAPED CURVE If resources become scarce, the population will experience an overshoot by becoming larger than the carrying capacity and will result in a die-off, or population crash. DIFFERENT REPRODUCTIVE STRATEGIES CAN AFFECT POPULATION SIZES species- the population of a species that grows slowly until it reaches the carrying capacity. K-selected Ex. elephants, whales, and humans. species- the population of a species that grows quickly and is often followed by overshoots and die-offs. r-selected Ex. mosquitoes and dandelions DIFFERENT LIFESPANS CAN AFFECT POPULATION SIZES Survivorship Curves show that populations of different species vary in how long individual members typically live. SURVIVORSHIP CURVES Type I long life span K-selected Species Type II medium life span Type III short life span r-selected Species GEOGRAPHICAL MOVEMENT CAN AFFECT POPULATION SIZES Metapopulations- a group of spatially distinct populations that are connected by occasional movements of individuals between them. SPECIES INTERACTIONS Competition, predation, mutualism, commensalism Keystone species THERE ARE 4 MAJOR TYPES OF SPECIES INTERACTION 1. 2. 3. 4. Competition Predator-Prey Mutualism Commensalism Symbiosis COMPETITION occurs when two or more organisms attempt to use the same limited resource. Competition A limited resource is any resource that may run out. Examples: Interspecific: Different Species Hyenas fight with lions over the same animals Intraspecific: Same Species Two cacti that are side by side compete for water COMPETITION – GAUSE’S LAW The competitive exclusion principle, sometimes referred to as Gause’s Law of Competitive Exclusion or just Gause's law, is a proposition that states that two species competing for the same resource cannot coexist at constant population values. One species must go. RESOURCE PARTITIONING IN COMPETITION PREDATION (PREDATOR – PREY) Predation- the use of one species as a resource by another species. True predators- kill their prey. Herbivores- consume plants as prey. Parasites- live on or in the organism they consume. Parasitoids- lay eggs inside other organisms. With Predation, Populations rise & fall with each other. SYMBIOSIS is the close association between two or more organisms of different species living together 2 types: Mutualism Commensalism Symbiosis MUTUALISM Mutualism is a cooperative partnership between two species in which both species benefit. An example is the bacteria in your intestines and you. Billions of bacteria live in your intestines. They help break down food you would otherwise not be able to digest. They also produce beneficial substances, such as Vitamin K for you. In return, you give them a warm, dark, food-rich environment. COMMENSALISM is a relationship in which one species benefits from another species and the other is neither harmed nor helped This is the rarest and strangest type of species interaction Commensalism Example: Remoras and Sharks The remoras attach to the sharks and feed on scraps left over from the sharks meals. The shark is neither harmed nor helped by this relationship Relationship Commensalism Mutualism Parasitism Predation Competition Organism #1 Organism #2 + + + + - 0 + - KEYSTONE SPECIES HAVE LARGE EFFECTS ON COMMUNITIES Keystone species- a species that plays a role in its community that is far more important than its relative abundance might suggest. Example: Sea Otters in California Kelp Beds Beavers build dams which provides many species with a home. Dams are also vital in restoring wetlands. Gray wolf is an effective predator in Yellowstone National Park. Bees help to pollinate flowers providing food for the ecosystem. Pine trees are keystone species in the Sierra Nevada Mountains. They regulate snowmelt runoff and soil erosion. Sharks prey on the sick and weak species in their ecosystem. COMMUNITIES CHANGE OVER TIME Ecological Disturbances Primary succession Secondary succession Aquatic succession ECOLOGICAL DISTURBANCE A disturbance is an event that affects environmental conditions rapidly and drastically, resulting in changes to the community and ecosystem. o A disturbance can be as localized as a tree falling in a forest, creating a gap in the canopy that lets in light and alters conditions for plants and animals in the gap. o A disturbance can be as large and severe as a hurricane, tornado or volcanic eruption. ECOLOGICAL DISTURBANCE A disturbance is an event that affects environmental conditions rapidly and drastically, resulting in changes to the community and ecosystem. o Some disturbances are sudden, such as landslides. o Some disturbances are gradual, such as climate change. o Some disturbances regularly reoccur and are considered normal aspects of a system. • Periodic fire • Seasonal storms • Cyclic insect outbreaks ECOLOGICAL DISTURBANCE Communities are dynamic systems and may respond to disturbance in several ways. Resistance – a community that resists change and remains stable despite disturbances is resistance. 2) Resilience – a community that changes in response to disturbance but later returns to is original state. 1) ECOLOGICAL DISTURBANCE Communities are dynamic systems and may respond to disturbance in several ways. Once a community is disturbed and changes are set in motion, there is no guarantee that the community will ever return to its original state. 3)Permanent change – the characteristics of the community fundamentally change. This can occur when… • If some crucial climatic threshold is passed • A keystone species is lost • A invasive species enters ECOLOGICAL SUCCESSION What happens to an ecosystem after a disturbance? The gradual, sequential growth of a community is called ecological succession This can occur in areas where no life has been before (primary succession) This can also occur when there is a disaster that completely wipes out a community (secondary succession) ECOLOGICAL SUCCESSION In both primary and secondary succession there is a specific sequence of growth The first species to grow in succession is called the pioneer species Pioneer species are typically small, grow quickly, good at growing under harsh conditions, and good at dispersing offspring PRIMARY SUCCESSION No true soil is present before Primary Succession Because of this it is slower than secondary Occurs on Newly exposed land under melting ice caps Newly created land formed by volcanoes PRIMARY SUCCESSION are usually the pioneer species They break down the rocks into dirt and when they die, they leave behind organic material, which will be broken down by decomposers This creates soil which is basically dirt, minerals, and organic material This process takes thousands of years Lichens PRIMARY SUCCESSION Primary succession- occurs on surfaces that are initially devoid of soil. Lichens are the first to appear SECONDARY SUCCESSION This happens after an existing community is disrupted by a disturbance like a fire Soil is already present Grasses and weeds tend to be the pioneer species After many years bigger plants will begin to grow This process takes about 100 years SECONDARY SUCCESSION Secondary succession- occurs in areas that have been disturbed but have not lost their soil. AQUATIC SUCCESSION Aquatic Succession turns a freshwater lake into a terrestrial habitat. CLIMAX COMMUNITY When does succession end? When it reaches a climax community This is simply a stable end point At this point, the community remains relatively stable assuming there are no more disturbances Some scientists don’t support the concept of a climax community because environmental disturbances are always occurring. RESTORATION ECOLOGY We can help! Restoration ecology emerged as a separate field in ecology in the 1980s. It is the scientific study supporting the practice of ecological restoration, which is the practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats in the environment by active human intervention and action. COMMUNITY SUSTAINABILITY Species Richness Theory of Island Biography COMMUNITY SUSTAINABILITY Species-Rich Communities tend to be more productive and sustainable. The more diverse an ecosystem is, the more productive it will be. That is, with a greater variety of producer species, that community will produce more plant biomass, which in turn will support a greater variety of consumer species. The more diverse an ecosystem is, the more sustainable it will be. That is, with a greater variety the greater ability to withstand environmental disturbances such as drought or insect infestations. FACTORS THAT DETERMINE SPECIES RICHNESS Latitudinal Gradient Time As we move away from the equator, species richness declines Why?... milder climate at the equator. The older the communities have more species richness Why?… more time for speciation. Habitat size Larger habitats have more species Why?… wider environmental conditions and resources. THEORY OF ISLAND BIOGEOGRAPHY Theory of Island Biogeography- the theory that explains that both habitat size and distance determine species richness.