Chapter 53: Population Ecology

Chapter 53 Population Ecology 53.1 Biotic and Abiotic Factors affects population density, dispersion, demographics Population: A group of individuals of a single species in the same general area. Usually defined by boundaries and the number of individuals in the boundaries. Population Density: The number of individuals in a population per unit of area or volume Population Dispersion: the spacing pattern among individuals within the boundaries To estimate a population’s size, or how many individuals are in the population, many methods are used such as the mark-recapture method, using population density to generalize the size, counting nests or burrows. Each has its pros and cons, but a combination of these methods is often used. Population density and size in an area can fluctuate because of factors such as births in the population, deaths in the population, immigration into the population, and emigration out of a population. *Immigration vs Emigration : Think of immigration as I stand for coming in and emigration as exiting * Types of dispersion Demography: The study of the key characteristics of populations and how these change over time Often in studying the demography of a population, life tables are needed. These tables summarize the survival and reproduction of individuals at specific ages which follows cohorts from birth until all of them are dead. The cohorts are all individuals of a similar specific age. The results of the life tables can be used to make survivorship curves which show the proportion of the cohort that are still alive at each age. Survivorship curves can be represented by three different types of curves:   Type I: In the type I curve there are a low number of deaths in the early and middle stages of life but are higher among the older stages. A common example of this curve is either humans or animals such as elephants Type II: In the type II curve, the chance of death is constant over a lifespan. A common example of this curve is birds  Type III: In the type III curve, there is a high death rate in the early and middle parts of life and then slowly flattens as an organism gets older. 53.2 The Exponential model describes population growth in an ideal, unlimited environment Change in a population over time can be represented with the following equation: ΔN/Δt= births-deaths (we ignore immigration and emigration) Population ecologists are more interested in R which is the intrinsic rate of increase of the growth of a population per capita per year. Can lead to the following equation if we make the substitution: R=B-D because ΔN/Δt=R Through derivation and a couple of substitutions, to calculate a population at a given point of time we use the following equation: dN/dt= RN In which R= the per capita rate of change N is the population at a given point of time dN/dt= can be used to express the exponential population growth When graphing the following equation, it shows a J shaped graph that start small and then increases as the slope becomes steeper and steeper. 53.3 The logistical model describes how a population grows more slowly as it nears it carrying capacity. The exponential model is used to show population growth in ideal conditions where resources and space are unlimited and that is often not realistic. The logistical model shows a more realistic picture of population growth which shows a population’s growth rate as fast when it is smaller but as it nears the carrying capacity or K, it decreases. Carrying capacity: Symbolized by K, it is the maximum population size that a particular environment can sustain. The carrying capacity can vary on the environment, resources available, organism, and point in time. Things that can influence carrying capacity are:      Energy Shelter from predators Nutrients Water Shelter or space (this is the biggest thing that limits the population) If there are not enough resources the per capita growth rate tappers off and then stabilizes. The new equation is: dN/dt= rN* (N-K)/K The idea is that most growth happens in the intermediate states of the population. As a minor disclaimer, the perfect S curves that are shown don’t really exist in nature and some time may need to elapse past K to see the effects in the population. 53.4 Life histories are products of natural selection. Life histories: the traits that affect an organism’s schedule of reproduction and survival. The types of questions that are asked:    When does reproduction begin? When does it reproduce? How much and how often does reproduction occur? Two main types of reproduction: Semelparity: Reproduction of all offspring in an organism occurs in one event. This is also called bigbang reproduction. Iteroparity: Reproduction of offspring happens many times over the organism’s life. These strategies have trade-offs between reproduction and survival. Organisms cannot do both reproduction and survival with offspring because of the limited resources. Selective pressures also influence the trade-offs between reproduction and survival. Two types of selective pressures K-Selection: Selection for traits that are advantageous in high population densities. R- Selection: Maximizes reproductive success in low population densities. Both the terms come from the logistical equation for population growth. 53.5 Density-dependent factors regulate population growth. When a population’s birth or death rate are influenced by factors that are not based on the population’s size, the factor is said to be a density- independent factor ex: Temperature, Precipitation, Natural Disasters When the population’s birth rates, and death rates are influence by factors that are based on the population’s size, the factor is said to be a density- dependent factor ex: Competition for resources, disease, territoriality, intrinsic factors, toxic waste products. The density-dependent factors create feedback loops that regulate the birth and death rates. Population dynamics: When the population fluctuates in size year to year or place to place and focuses on the complex factors, abiotic and biotic, in these boom-and-bust cycles. Emigration and competition increase as a population increases. Metapopulations: A group of spatially separated population of one species that interacts through immigration and emigration. 53.6 The human population is no longer growing exponentially, but it is still increasing rapidly. The human population has exploded in the last four centuries and has at some points gone beyond exponential growth. Some have speculated that by 2050, there will be 9.7 billion people even though the rate of growth has decreased in recent years. The change from high birth and death rates to low birth and death rates is called a demographic transition and the human population in many developed nations are going through this because of a greater control over family size through contraception and increased medical access. Age structure also affects population: relative number of people in each age group can be graphed as pyramids. Infant mortality and life expectancy also reflect the condition of various countries. Currently there is not a consensus among scientists what the global carrying capacity is for humanity. Scientists are currently concerned about the ecological footprint of people: the summarization of the aggregate land and water area required for each person, city, or nation to produce all resources needed for consumption and absorb all the waste it generates. Ultimately, many don’t know what the limiting factor on the human population will be: food water or space but there needs to be changes to determine a more sustainable development in reaching net zero growth.