Transcript POPULATIONS
POPULATIONS AND HOW THEY GROW • Biosphere • Ecosystems ECOSYSTEM LEVEL Eucalyptus forest COMMUNITY LEVEL All organisms in eucalyptus forest • Community • Population POPULATION LEVEL Group of flying foxes ORGANISM LEVEL Flying fox Brain ORGAN SYSTEM LEVEL Nervous system ORGAN LEVEL Brain Spinal cord Nerve TISSUE LEVEL Nervous tissue CELLULAR LEVEL Nerve cell MOLECULAR LEVEL Molecule of DNA Figure 1.1 POPULATIONS Population-all of the individuals of a species that live together in one place at one time. Demography-the statistical study of populations. It is used to predict how the size of a population will change. COUNTING POPULATIONS direct observation-count indirect observation-estimate sampling-count small number in a small area and multiply to get the number in the large area mark and recapture studies-capture, mark, release mark-recapture method depends on likelihood of recapturing the same individual Figure 35.2A KEY FEATURES OF POPULATIONS 1. Population size is the number of individuals in a population. had an important effect on the ability of the population to survive. A small population is more likely to become extinct: in the case of random events or natural disaster due to inbreeding where the population is more genetically alike. with reduced variability it is harder to adapt to changes. KEY FEATURES OF POPULATIONS, CON’T 2. Population density – the number of individuals in a given area. – if they are too far apart they may only rarely encounter one another resulting in little reproduction. KEY FEATURES OF POPULATIONS, CON’T POPULATION SIZE IS LIMITED BY: density-independent factors density-dependent factors Disease Competition Predators Parasites Food Crowding The greater the population, the greater effect these factors have. Ex. Black plague in the Middle Ages – more deaths in cities Volcanic eruptions Temperature Storms Floods Drought Chemical pesticides Major habitat disruption (as in the New Orleans flooding) Most are abiotic factors KEY FEATURES OF POPULATIONS, CON’T 3. Dispersion – the way in which the individuals are arranged. Most common POPULATION GROWTH Population Growth Three factors can affect population size: number of births (natality) the number of deaths (mortality) the number of individuals that enter or leave the population (immigration and emigration). * Simply put, a population will increase or decrease in size depending on how many individuals are added to it or removed from it IMMIGRATION & EMMIGRATION Immigration the movement of individuals into an area, is another factor that can cause a population to grow. Emigration the movement of individuals out of an area, can cause a population to decrease in size. PREDICTING POPULATION GROWTH, CON’T Nearly all populations will tend to grow exponentially as long as there are resources available. Two of the most basic factors that affect the rate of population growth are the birth rate, and the death rate. r(rate of growth)=birth rate – death rate PREDICTING POPULATION GROWTH, CON’T Exponential growth curve: population growth plotted against time. As a population gets larger, it also grows at a faster rate. Maximum population growth under ideal circumstances. Includes plenty of room, unlimited resources (food, water) and no predators. FACT: No population exhibits this type of growth for long. PREDICTING POPULATION GROWTH, CON’T Logistic model: This model accounts for the declining resources available to populations as they grow. It assumes the birth and death rates are not constant. As the population grows, births decline and death rises. Eventually birth=death so the population stops growing. Carrying capacity (K): The number of organisms of one species that an environment can support indefinitely. PREDICTING POPULATION GROWTH, CON’T TWO MODES OF POPULATION GROWTH. The Exponential curve (also known as a Jcurve) occurs when there is no limit to population size. The Logistic curve (also known as an S-curve) shows the effect of a limiting factor (in this case the carrying capacity of the environment). POPULATION GROWTH STRATEGIES There are 2 ways a population can prosper: 1. Depends on the rate of growth (r) 2. Influenced by the carrying capacity (K) POPULATION GROWTH STRATEGIES, CON’T r-strategists: “boom and crash” characterized by exponential growth, which results in temporarily large populations, followed by sudden crashes in population size. Ex. Insects, bacteria, some plants live in unpredictable and rapidly changing environments Reproduce quickly when conditions are favorable Many offspring: small, mature rapidly, no parental care r = rate of growth POPULATION GROWTH STRATEGIES, CON’T K-strategists: characterized by a high degree of specialization. Ex. Trees, whales, tigers, etc. Live in stable and predictable environments Can compete effectively Reproduce late in life Few offspring: large, mature slowly, often much parental care K = carrying capacity POPULATION GROWTH STRATEGIES, CON’T Human Populations: K- strategist characteristics But, recently, humans learned to expand carrying capacity of environment by increasing food supply, combating pests and curing diseases. Can Earth support this increase? THE HUMAN POPULATION doubled three times in the last three centuries about 7 billion and may reach 9.3 billion by the year 2050 improved health and technology have lowered death rates The history of human population growth Figure 35.8A Figure 2.10x