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Population Ecology Topic 3 This trio of sea otters is part of the population that lives near Monterey, California. Sea otters often rest by wrapping themselves in kelp to keep from drifting away. Kelp Forest Ecosystem America’s Pacific Northwest is home to an underwater “forest” that is the basis for a very diverse ecosystem. • What does kelp provide? food & habitat • Sea Otters are an important member of the kelp forest. Otters eat a lot of food (sea urchins) to stay warm in the cold water • Sea urchins, in turn, feed on kelp • The relationships along this food chain set the stage for a classic tale of population growth and decline. Kelp forest Sea Otters Sea Otters are Keystone Species A keystone is the stone at the top of an arch that supports the other stones and keeps the whole arch from falling. **A Keystone species is a species on which the persistence of a large number of other species in the ecosystem depends. Kelp Forest Continued A century ago, otters were nearly eliminated by hunting. Sea urchin populations increased greatly, and kelp forests nearly disappeared. Why? Then, sea otters were declared an endangered species and were protected from hunting Otter populations recovered, sea urchin numbers dropped, and kelp grew back But now, some otter populations are shrinking again… Click to Watch How sea otters help save the planet The sea otter, a voracious urchineater and possessor of the densest fur in the animal kingdom. Photograph: Alamy Sea otters floating in kelp. They help kelp forests to flourish by keeping sea urchin numbers under control. Photograph: Frans Lanting/Getty Images Sea otters use rocks to crack open the spiny shells of sea urchins. Photograph: Noel Hendrickson/Getty Images Jaguar (keystone predator) Prairie Dog (keystone modifier) Hummingbird (keystone mutualist) Beaver (keystone modifier) Saguaro Cactus (keystone host) Snowshoe Hare (keystone prey) …back to our topic, Populations A group of organisms of the same species that live in the same place There are Four Important Characteristics of Populations: 1. Geographic Distribution (Range) 2. Density 3. Growth Rate 4. Age Structure 1. Geographic Distribution (Range) Describes the area inhabited by a population The range can vary in size from a few cm3 to millions of km2 Which has a larger range? 2. Population Density The number of individuals per unit area This number can vary tremendously depending on the species and its ecosystem The tall saguaro cactuses in this Arizona desert have a low population density compared to the smaller desert plants Density of Whitetail Deer in the United States Which states have the highest density? 3. Growth Rate How the number of individuals in a population changes over time Three factors can affect population size: Number of births Number of deaths Number of individuals that enter and leave the population What happens when the Immigration: the movement of individuals birth rate is greater than into an area the death rate? Emigration: the movement of individuals What happens when life expectancy improves? out of an area Feedback Loop in Populations A feedback loop is a process in which the system responds to a disturbance either: In the same direction (positive feedback) In the opposite direction (negative feedback) Example Positive Feedback Loop = increase in birth rate (reinforcing) Click to Watch Negative Feedback Loop = increase in death rate (diminishing) 4. Age Structure Population growth depends, in part, on how many individuals of different ages make up a population Age-structure diagrams—graph of the number of individuals in different age groups GROWTH CURVES REPRESENT THE GROWTH OR DECLINE OF A POPULATION GRAPHICALLY There are two models or types of growth curves: 1. Exponential (J-shaped)—occurs when the individuals in a population reproduce at a constant rate 2. Logistic (S-shaped)—occurs when a population’s growth slows or stops following a period of exponential growth EXPONENTIAL GROWTH In the presence of unlimited resources and in the absence of predation and disease (ideal conditions), a population will grow exponentially If this growth pattern continued for several days without slowing down bacteria would cover the planet! LOGISTIC GROWTH Since bacteria doesn’t cover the planet, this means that exponential growth does not continue in natural populations for very long As resources become less available, the growth of a population slows or stops Carrying Capacity In logistic growth, why does the growth slow or stop? The ecosystem reaches the carrying capacity, or the largest number of individuals of a population that a given environment can support Note that even when the population growth is said to stop, it still rises and falls, but averages out to a certain population size. Limiting Factors Concept Map Let’s Review… Population Growth can be Logistic growth Exponential growth characterized by No limits on growth Unlimited resources represented by Constant growth rate J-shaped curve characterized by Limits on growth which cause a Falling growth rate represented by S-shaped curve Many factors may cause a decline in a population over time. Remember…predators such as killer whales are now feeding on populations of sea otters Another Example: primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient – limiting nutrient This is an example of a larger concept – a limiting factor. Limiting Factors Cause population growth to decrease Ex. Competition, Predation, Parasitism, Disease, Drought, Human disturbances Think of it like a dam holding water back. Density-Dependent Limiting Factors Depends on population size—only becomes limiting when the density (number of organisms per unit area) reaches a certain level This includes factors such as competition, predation, parasitism, & disease Ex. Where is your chance of getting the flu the highest? Predator-prey relationships The regulation of a population by predation is one of the best-known mechanisms of population control How many moose does a wolf need to eat to survive a year? In order to survive a 50 kg wolf needs to eat about 2700 kg of moose/yr The average mass of a moose is 385 kg How many “average” moose does a wolf need to eat each year? ~7 If there are 8 wolves in a pack? 2700 x 8 =21,600/385 = ~56 Density-Independent Limiting Factors Affect all populations in similar ways, regardless of the population size This includes factors such as weather, natural disasters, seasonal cycles, & human activity In response to such factors, many species show a characteristic crash in population size What could be the potential cause for this crash? Human Population Growth Like other organisms, the size of the human pop. tends to increase over time. Industrial Revolution begins Agriculture begins Plowing and irrigation Bubonic plague • To begin: Life was often difficult…many limiting factors • Until fairly recently only ½ of children survived to adulthood • When the food supply became more reliable, technology, sanitation, medicine, & healthcare reduced death rate. Demography – the scientific study of human populations Thomas Malthus predicted that human populations would not grow indefinitely due to war, famine, disease providing limits How large is the human population? http://www.census.gov/population/international/ NPR population video Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly. The Demographic Transition C. A. A. All countries begin here B. As countries modernize, they have better nutrition, sanitation, and medicine which lowers the death rate C. As modernization continues, families have fewer children which lowers the birth rate D. The transition is complete when population growth stops D. B. Know how changes to birth & death rates affect the population. Age-Structure Diagrams: used for predicting future growth Section 5-3 U.S. Population Males Females Rwandan Population Males Females Click to Watch Population growth depends on how many people of different ages make up a population. Animal Societies Some of the most extreme examples of relatedness, and the most complex animal societies (apart from humans) are found among social insects! An animal society is a group of animals of the same species that interact closely and often cooperate Members of a society are often related to one another An Ant Society Click to Watch Figure 1: Social insects have well protected or defended nests, including termites (a), wasps (b), and bees (c). Click to Watch Animal Societies Figure 1: Social groups which have formed to improve the probability of survival and reproduction of individual members: pack of wolves (a) and school of fish (b) Group Behavior Figure 1: Group-living is widespread in the animal kingdom. Although many species form temporary associations, such as (a) flamingo (Phoenicopterus minor) colonies and (b) zebra (Equus quagga) herds, some species such as (c) African elephants (Loxodonta africana), (d) snapping shrimps (Synalpheus brooksi), and (e) superb starlings (Lamprotornis superbus) form more permanent social groups and live together year-round. Great Migrations Click to Watch Tens of millions of bison once rumbled across the Great Plains on a quest for grazing. By the late 1800s nearly all had been slaughtered. Today most of the half million remaining bison are in captivity, like these on the Triple U ranch in South Dakota. The Genius of Swarms Click to Watch A single ant or bee isn't smart, but their colonies are. The study of swarm intelligence is providing insights that can help humans manage complex systems, from truck routing to military robots.