Regulation of population size marking territory = competition Limiting factors density dependent competition: food, mates, nesting sites predators, parasites, pathogens density independent abiotic factors sunlight (energy) temperature rainfall Exponential growth cannot be sustained for long in any population The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size In density-independent populations, birth rate and death rate do not change with population density In density-dependent populations, birth rates fall and death rates rise with population density Density-dependent birth and death rates are an example of negative feedback that regulates population growth They are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate Population density can influence the health and survival of organisms In dense populations, pathogens can spread more rapidly As a prey population builds up, predators may feed preferentially on that species Accumulation of toxic wastes can contribute to density-dependent regulation of population size swarming locusts competition for nesting sites

Introduced species Non-native species kudzu transplanted populations grow exponentially in new area out-compete native species loss of natural controls lack of predators, parasites, competitors reduce diversity examples African honeybee gypsy moth zebra mussel purple loosestrife gypsy moth kudzu

Number of breeding male Carrying capacity Time (years) 1915 1925 1935 1945 10 8 6 4 2 Number of breeding male fur seals (thousands) Maximum population size that environment can support with no degradation of habitat varies with changes in resources 500 400 300 200 100 20 10 30 50 40 60 Time (days) Number of cladocerans (per 200 ml) Some populations fluctuate greatly and make it difficult to define K Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small The logistic model fits few real populations but is useful for estimating possible growth What’s going on with the plankton?

Changes in Carrying Capacity Population cycles predator – prey interactions At what population level is the carrying capacity? Some populations undergo regular boom-and-bust cycles Lynx populations follow the 10 year boom-and-bust cycle of hare populations Three hypotheses have been proposed to explain the hare’s 10-year interval Hypothesis: The hare’s population cycle follows a cycle of winter food supply If this hypothesis is correct, then the cycles should stop if the food supply is increased Additional food was provided experimentally to a hare population, and the whole population increased in size but continued to cycle No hares appeared to have died of starvation Hypothesis: The hare’s population cycle is driven by pressure from other predators In a study conducted by field ecologists, 90% of the hares were killed by predators These data support this second hypothesis Hypothesis: The hare’s population cycle is linked to sunspot cycles Sunspot activity affects light quality, which in turn affects the quality of the hares’ food There is good correlation between sunspot activity and hare population size The results of all these experiments suggest that both predation and sunspot activity regulate hare numbers and that food availability plays a less important role K K

Human population growth Population of… China: 1.3 billion India: 1.1 billion Human population growth adding 82 million/year ~ 200,000 per day! Doubling times 250m  500m = y () 500m  1b = y () 1b  2b = 80y (1850–1930) 2b  4b = 75y (1930–1975) 20056 billion Significant advances in medicine through science and technology What factors have contributed to this exponential growth pattern? Industrial Revolution No population can grow indefinitely, and humans are no exception The population doubled to 1 billion within the next two centuries, doubled again to 2 billion between 1850 and 1930, and doubled still again by 1975 to more than 4 billion. The global population now numbers over 6 billion people and is increasing by about 73 million each year. The population grows by approximately 201,000 people each day, the equivalent of adding a city the size of Amarillo, Texas, or Madison, Wisconsin. Every week the population increases by the size of San Antonio, Milwaukee, or Indianapolis. It takes only four years for world population growth to add the equivalent of another United States. Population ecologists predict a population of 7.3–8.4 billion people on Earth by the year 2025. To maintain population stability, a regional human population can exist in one of two configurations: Zero population growth = High birth rate – High death rate Zero population growth = Low birth rate – Low death rate The demographic transition is the move from the first state toward the second state The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women Most of the current global population growth is concentrated in developing countries The carrying capacity of Earth for humans is uncertain The average estimate is 10–15 billion Is the human population reaching carrying capacity? Bubonic plague "Black Death" 1650500 million

individual at standard of living of population Ecological Footprint 30.2 15.6 6.4 3.7 3.2 2.6 USA Germany Brazil Indonesia Nigeria India Amount of land required to support an individual at standard of living of population 2 4 6 8 12 10 14 16 18 20 22 24 26 28 30 32 34 Acres over-population or over-consumption? uneven distribution: wealthiest 20% of world: 86% consumption of resources 53% of CO2 emissions The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation It is one measure of how close we are to the carrying capacity of Earth Countries vary greatly in footprint size and available ecological capacity A more comprehensive approach to estimating the carrying capacity of Earth is to recognize that humans have multiple constraints: We need food, water, fuel, building materials, and other requisites, such as clothing and transportation. The ecological footprint concept summarizes the aggregate land and water area appropriated by each nation to produce all the resources it consumes and to absorb all the waste it generates. Six types of ecologically productive areas are distinguished in calculating the ecological footprint: arable land (land suitable for crops), pasture, forest, ocean, built–up land, and fossil energy land. (Fossil energy land is calculated on the basis of the land required for vegetation to absorb the CO2 produced by burning fossil fuels.) All measures are converted to land area as hectares (ha) per person (1 ha = 2.47 acres). Adding up all the ecologically productive land on the planet yields about 2 ha per person. Reserving some land for parks and conservation means reducing this allotment to 1.7 ha per person—the benchmark for comparing actual ecological footprints. The graph is the ecological footprints for 13 countries and for the whole world as of 1997. We can draw two key conclusions from the graph. First, countries vary greatly in their individual footprint size and in their available ecological capacity (the actual resource base of each country). The United States has an ecological footprint of 8.4 ha per person but has only 6.2 ha per person of available ecological capacity. In other words, the U.S. population is already above carrying capacity. By contrast, New Zealand has a larger ecological footprint of 9.8 ha per person but an available capacity of 14.3 ha per person, so it is below its carrying capacity. The second conclusion is that, in general, the world was already in ecological deficit when the study was conducted. The overall analysis suggests that the world is now at or slightly above its carrying capacity.

Evolutionary adaptations Coping with environmental variation regulators endotherms homeostasis (“warm-blooded”) conformers ectotherms (“cold-blooded”)