Chapter 8 Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? They are classified as a keystone species o Help keep sea urchins and other kelp-eating species from depleting highly productive kelp forests that provide habitats for a number of species in offshore coastal waters They are making a comeback o Almost went extinct in the 1900’s Population dynamics: is a study of how and why populations change in their distribution, numbers, age structure, and density in response to changes in environmental conditions Population Dynamics and Carry Capacity Population Distribution There are three general patterns of population distribution/dispersion in a habitat o Clumping The location and size of these clumps vary with availability Why? The resources a species needs vary greatly in availability from place to place Living in herds/flocks/schools can provide some animals with better protection from predators and population decline Living in packs gives some predator species such as wolves a better chance of getting a meal Some animal species form temporary groups for mating and caring for their young o Uniform dispersion Better access to scarce water resources o Random dispersion Species that do this are fairly rare Most individuals in the populations of most species live in clumps or groups Changes in Population Size: Entrances and Exits Populations increase through births and immigration and decrease through disease and emigration The four variables that govern changes in population size are: o Births o Deaths o Immigration—arrival of individuals from outside the populations o Emigration—departure of individuals from the population Population change= (Births + Immigration) – (Deaths + Emigration) Age Structure: Young Populations Can Grow Fast How fast a population grows or declines depends on its age structure Age Structure: the proportions of individuals at various ages o Can have an effect on how rapidly its size increases or decreases Prereproductive Ages: not mature enough to reproduce Reproductive Ages: those capable of reproduction Postreproductive Ages: those too old to reproduce The size of a population made up mostly of individuals in their reproductive ages or soon to reach these ages is likely to increase A population dominated by individuals past their reproductive age will tend to decrease The size of a population with a fairly even distribution among these three age groups tends to remain stable o Because reproduction by younger individuals is roughly balance by the deaths of older individuals Limits on Population Growth: Biotic Potential versus Environmental Resistance No population can increase its size indefinitely Biotic potential: populations vary in their capacity for growth Intrinsic rate of increase (r): the rate at which a population would grow if it had unlimited resources A rapidly growing population reaches some sixe limit imposed by a shortage of one or more limiting factors There are always limits to population growth in nature Environmental resistance: consists of all factors that act to limit the growth of a population o This is an example of negative and corrective feedback Biotic potential and Environmental resistance together determine: o Carrying Capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat The growth rate of a population decreases as its size nears the carrying capacity of its environment o Because resources such as food and water begin to dwindle Exponential and Logistic Population Growth: J-Curves and S-Curves With ample resources a population can grow rapidly, but as resources become limited, its growth rate slows and level off A population with few, if any, resource limitations grows exponentially at a fixed rate such as 1% or 2% a year Exponential/geometric growth starts slowly but then accelerates as the population is increasing Logistic Growth: involves rapid exponential population growth followed by a steady decrease in population growth with time until the population size levels off o This slowdown occurs as the population encounters environmental resistance and approaches the carrying capacity of its environment o After leveling off, a population with this type of growth typically fluctuates slightly above and below the carrying capacity Changes in the population sizes of keystone species can influence the composition of communities by causing decreases in populations of species dependent on such keystone species and increases in the populations of species that move in to occupy part or all of their ecological niches Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size When a population exceeds its resource supplies, many of its members will die unless they switch to new resources or move to an area with more resources Overshoot: when populations use up their resource supplies and temporarily or exceed the carrying capacity of their environment Reproductive Time Lag: the period needed for the birth rate to fall and the death rate to rise in response to resource overconsumption o Populations will suffer a dieback or a crash, unless the excess individuals can switch to new resources or move to an area with more resources The carrying capacity of an area or volume in not fixed o Sometimes when a population exceeds the carrying capacity of an area, it causes damage that reduces the area’s carrying capacity Over time species may increase their carrying capacity by developing adaptive traits through natural selection that reduce environmental resistance to their population growth Carrying capacity can increase or decrease seasonally and from year-to-year because of variations in weather, climate, and other factors Some species nearing their carrying capacity may be able to keep growing in size by migrating to other areas with more resources Technological, social, and other cultural changes have extended the earth’s carrying capacity for humans Can we continue to expand the earth's carrying capacity for humans? o a. No. Unless humans voluntarily control their population and conserve resources, nature will do it for us. o b. Yes. New technologies and strategies will allow us to further delay exceeding the earth's carrying capacity. Population Density and Population Change: Effects of Crowding A population’s density can affect how rapidly it can grow or decline, but some population control factors are not affected by population density Population Density: the number of individuals in a population found in a particular area or volume Higher population densities may help sexually reproducing individuals find mates but can also lead to increased competition for mates, food, living space, water, sunlight and other resources High densities can help shield come members of predators but can also make large groups vulnerable to predators such as humans Close contact among individuals in dense populations can increase the transmission of infectious disease Density-dependent factors tend to regulate a population at a fairly constant size, often near the carrying capacity of its environment o Includes diseases o Mostly abiotic, can kill members of a population are density independent Meaning, their effect is not dependent on the density of the population Can include floods, hurricanes, fire, pollution, and habitat destruction Types of Population Change Curves in Nature Population sizes may stay about the same, suddenly increase and then decrease, vary in regular cycles, or change erratically Four general patterns of variation in population size o Stable Fluctuates slightly above and below carrying capacity o Irruptive Populations explode and then crash to a more stable level or in some cases to a very low level o Cyclic Populations fluctuate and regular cyclic or boom-and-bust cycles Sine curve (going high up and very low from the K-line) o Irregular Erratic changes possibly due to chaos or drastic change Ecologists distinguish between top-down population regulation by predation and bottomup regulation by the scarcity of one or more resources Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources Case Study: Exploding White-Tailed Deer Populations in the United States Since the 1930s the white-tailed deer population in the U.S. has exploded Today 25-30 million white-tailed deer in U.S. pose human interaction problems. o Deer-vehicle collisions (1.5 million per year). o Transmit disease (Lyme disease in deer ticks). Reproductive Patterns Ways to Reproduce: Sexual Partners Not Always Needed Some species reproduce without having sex and others reproduce by having sex Two types of sexual reproduction o Asexual Reproduction: in which offspring are generally exact genetic copies (clones) of a single parent Common is single-celled bacteria Each cell can divide to produce two identical cells that are replicas of the original Many plants and animals such as coral reefs reproduce this way o Sexual Reproduction: which mixes the genetic material of two individuals and produce offspring with combinations of genetic traits from each parent Three disadvantages Males do not give birth: females have to produce twice as many offspring as an asexually reproducing organism does to maintain the same number of young in the next generation There is an increased chance of genetic errors and defects during the splitting and recombination of chromosome Courtship and mating rituals consume time and energy: can transmit diseases, and can inflict injury on males of some species as they compete for sexual patterns Two important advantages Provides greater genetic diversity in offspring o A population with many different genetic possibilities has a greater chance of reproducing when environmental conditions change than does a population of genetically identical clones Males of some species can gather food for the females and the young and protect and help train the young Reproductive Patterns: Opportunities and Competitors While some species have a large number of small offspring and give them little parental care, other species have fewer offspring and take care of them until they can reproduce r-selected species: species with a capacity for a high rate of population increase o The species have many, usually small, offspring and give them little or no parental care or protection o Overcome the loss of so many offspring by producing so many of them that a few will survive o Such species tend to be opportunists The reproduce and disperse rapidly when conditions are favorable or when a disturbance opens up a new habitat or niche for invasion However, once established, their populations may crash because of unfavorable changes in environmental conditions or invasion by more competitive species K-selected species: tend to reproduce later in life and have small number of offspring with fairly long life spans o Typically the offspring if such species develop inside their mother (where they are safe), are born fairly large, mature slowly, and are cared for and protected by one or both parents until they reach reproductive age o This reproductive pattern results in a few big and strong individuals that can compete for resources and reproduce a few young to begin their life cycle again o They tend to do well in competitive conditions when their population size is near the carrying capacity (K) of their environment o Their populations typically follow logistic growth o They are prone to extinction The availability of a suitable habitat for individuals of a population in a particular area determines its ultimate population size r-selected species tend to be opportunists while K-selected species tend to be competitors Survivorship Curves: Short to Log Lives The population of different species vary in how long individual members typically live Individuals of species with different reproductive strategies tend to have different life expectancies Survivorship curve: the way to represent the age structure of a population o Three types Late loss population: live to an old age Constant loss population: die at all ages Early loss population: most members die at young ages Life table: show the projected life expectancy and probability of death for individuals at each age in a survivorship curve