1  Dynamics of Population Growth  Factors that Increase or Decrease Populations  Factors that Regulate Population Growth  Conservation Biology Chapter 6 Outline

2  Biotic potential refers to unrestrained biological reproduction. Biological organisms can produce enormous numbers of offspring if their reproduction is unrestrained.  Constraints include:  Scarcity of resources  Competition  Predation  Disease Biotic Potential

3  Population - all the members of a single species living in a specific area at the same time  Exponential Growth - growth at a constant rate of increase per unit time (geometric) ; has no limit dN/dt = rN The change in the number of individuals (dN) per change in time (dt) equals the rate of growth (r) times the number of individuals in the population (N). r is often called the intrinsic capacity for increase. Dynamics of Population Growth

4  Number of individuals added to a population at the beginning of exponential growth is relatively small. But numbers increase quickly because a % increase leads to a much larger increase as the population grows.  J curve when the equation is graphed  Exponential growth is a simple, idealized model. In the real world there are limits to growth. Exponential Growth

5  Carrying capacity - limit of sustainability that an environment has in relation to the size of a species population  Overshoot - population exceeds the carrying capacity of the environment and death rates rise as resources become scarce  Population crash - growth becomes negative and the population decreases suddenly  Boom and bust - population undergoes repeated cycles of overshooting followed by crashing Carrying Capacity

6

7  Logistic Growth - growth rates regulated by internal and external factors until coming into equilibrium with environmental resources  dN/dt = r N (1 - N/K)  Terms have the same definitions as previous slide, with K added to indicate carrying capacity.  Growth rate slows as population approaches carrying capacity.  S curve when the equation is graphed Growth to a Stable Population

8 Logistic Growth Curve or S Curve

9  External factors include habitat quality, food availability and interaction with other organisms.  Internal factors include physiological stress due to overcrowding, maturity, body size, and hormonal status.  These factors are density-dependent, meaning as population size increases the effect intensifies.  Density independent effects (drought, an early frost, flooding, landslides, etc.) also may decrease population size. Factors Affecting Population Growth

10  r selected species rely upon a high reproductive rate to overcome the high mortality of offspring with little or no parental care. Example: A clam releases a million eggs in a lifetime.  K selected species have few offspring but more parental care. Example: An elephant reproduces every 4 or 5 years. r and K Selected Species

11 Reproductive Strategies

12  Natality - production of new individuals  Fecundity - physical ability to reproduce  Fertility - measure of actual number of offspring produced  Immigration - organisms introduced into new ecosystems  Dispersal of organisms by wind or water currents over long distances. Sometimes carried by animals or on rafts of drifting vegetation. Factors that Increase Population

13  Mortality - death rate  Survivorship - percentage of cohort surviving to a certain age  Life expectancy - probable number of years of survival for an individual of a given age  Increases as humans age. By older age, most individuals destined to die early have already done so.  Has risen in nations/areas with good nutrition, sanitation and medical care  Women live longer than men. Factors that Decrease Population

14  Life span - longest period of life reached by a given type of organism  Bristlecone pine lives 4,600 years.  Human maximum lifespan is 120 years.  Microbes may live a few hours.  Differences in relative longevity among species are shown as survivorship curves. Life Span

15  Four general patterns: a.Full physiological life span if organism survives childhood Example: Humans in the U.S. b.Probability of death unrelated to age Example: Sea gull c.Mortality peaks both early and late in life. Example: Deer d.Mortality peaks early in life. Example: Tree Survivorship Curves

16 Survivorship Curves

17  The last factor in our list of factors that decrease population is emigration, the movement of members out of a population.  Many organisms have specific mechanisms to facilitate migration into new areas. Factors that Decrease Population

18  Intrinsic factors - operate within or between individual organisms in the same species  Extrinsic factors - imposed from outside the population  Biotic factors - Caused by living organisms. Tend to be density dependent.  Abiotic factors - Caused by non-living environmental components. Tend to be density independent, and do not really regulate population although they may be important in increasing or decreasing numbers. Example: Rainfall, storms Factors that Regulate Population Growth

19 Density Dependent Factors  Reduce population size by decreasing natality or increasing mortality.  Interspecific Interactions (between species)  Predator-Prey oscillations

20  Intraspecific Interactions - competition for resources by individuals within a population  As population density approaches the carrying capacity, one or more resources becomes limiting.  Control of access to resources by territoriality; owners of territory defend it and its resources against rivals.  Stress-related diseases occur in some species when conditions become overcrowded. Density Dependent Factors Continued

21  Critical question in conservation biology is the minimum population size of a species required for long term viability.  Special case of islands  Island biogeography - small islands far from a mainland have fewer terrestrial species than larger, closer islands  MacArthur and Wilson proposed that species diversity is a balance between colonization and extinction rates. Conservation Biology

22  In a large population, genetic diversity tends to be preserved. A loss/gain of a few individuals has little effect on the total gene pool.  However, in small populations small events can have large effects on the gene pool.  Genetic Drift  Change in gene frequency due to a random event  Founder Effect  Few individuals start a new population. Conservation Genetics

23  Demographic bottleneck - just a few members of a species survive a catastrophic event such as a natural disaster  Founder effects and demographic bottlenecks reduce genetic diversity. There also may be inbreeding due to small population size. Inbreeding may lead to the expression of recessive genes that have a negative effect on the population. Conservation Genetics

24 Genetic Drift

25  Minimum Viable Population is the minimum population size required for long-term survival of a species.  The number of grizzly bears in North America dropped from 100,000 in 1800 to 1,200 now. The animal’s range is just 1% of what is once was and the population is fragmented into 6 separate groups.  Biologists need to know how small the bear groups can be and still be viable in order to save the grizzly. Population Viability Analysis

26  Metapopulation - a collection of populations that have regular or intermittent gene flow between geographically separate units  Source habitat - Birth rates are higher than death rates. Surplus individuals can migrate to new locations.  Sink habitat - Birth rates are less than death rates and the species would disappear if not replenished from a source. Metapopulations

27 Metapopulation