Population Dynamics Chapter 10
Outline • Estimating Patterns of Survival • Survivorship Curves • Age Distribution • Rates of Population Change • Overlapping Generations • Dispersal • In Response to Climate Change • In Response to Changing Food Supply • In Rivers and Streams
Estimating Patterns of Survival • Three main methods of estimation: • Cohort life table • Identify individuals born at same time and keep records from birth. • Static life table • Record age at death of individuals. • Age distribution • Calculate difference in proportion of individuals in each age class. • Assumes differences from mortality.
High Survival Among the Young • Murie collected Dall Sheep skulls, Ovis dalli. • Major Assumption: Proportion of skulls in each age class represented typical proportion of individuals dying at that age. • Reasonable given sample size of 608. • Constructed survivorship curve. • Discovered bi-modal mortality. • <1 yr. • 9-13 yrs.
Survivorship Curves • Type I: Majority of mortality occurs among older individuals. • Dall Sheep • Type II: Constant rate of survival throughout lifetime. • American Robins • Type III: High mortality among young, followed by high survivorship. • Sea Turtles
Age Distribution • Age distribution of a population reflects its history of survival, reproduction, and growth potential. • Miller published data on age distribution of white oak (Quercus alba). • Determined relationship between age and trunk diameter. • Age distribution biased towards young trees. • Sufficient reproduction for replacement. • Stable population
Age Distribution • Rio Grande Cottonwood populations (Populus deltoides wislizenii) are declining. • Old trees not being replaced. • Reproduction depends on seasonal floods. • Prepare seed bed. • Keep nursery areas moist. • Because floods are absent, there are now fewer germination areas.
Dynamic Population in a Variable Climate • Grant and Grant studied Galapagos Finches. • Drought in 1977 resulted in no recruitment. • Gap in age distribution. • Additional droughts in 1984 and 1985. • Reproductive output driven by exceptional year in 1983. • Responsiveness of population age structure to environmental variation.
Rates of Population Change • Birth Rate: Number of young born per female. • Fecundity Schedule: Tabulation of birth rates for females of different ages.
Estimating Rates for an Annual Plant • P. drummondii • Ro = Net reproductive rate; Average number of seeds produced by an individual in a population during its lifetime. • Ro=lxmx • X= Age interval in days. • lx = % pop. surviving to each age (x). • mx= Average number seeds produced by each individual in each age category.
Estimating Rates for an Annual Plant • Because P. drummondii has non-overlapping generations, can estimate growth rate. • Geometric Rate of Increase (): • =N t+1 / Nt • N t+1 = Size of population at future time. • Nt = Size of population at some earlier time.
Estimating Rates when Generations Overlap • Common Mud Turtle (K. subrubrum) • About half turtles nest each year. • Average generation time: T = xlxmx / Ro • X= Age in years • Per Capita Rate of Increase: r = ln Ro / T • ln = Base natural logarithms
Dispersal • Africanized Honeybees • Honeybees (Apis melifera) evolved in Africa and Europe and have since differentiated into many locally adapted subspecies. • Africanized honeybees disperse much faster than European honeybees. • Within 30 years they occupied most of South America, Mexico, and all of Central America.
Collared Doves • Collared Doves, Streptopelia decaocto, spread from Turkey into Europe after 1900. • Dispersal began suddenly. • Not influenced by humans. • Took place in small jumps. • 45 km/yr
Rapid Changes in Response to Climate Change • Organisms began to spread northward about 16,000 years ago following retreat of glaciers and warming climate. • Evidence found in preserved pollen in lake sediments. • Movement rate 100 - 400 m/yr.
Dispersal in Response to Changing Food Supply • Holling observed numerical responses to increased prey availability. • Increased prey density led to increased density of predators. • Individuals move into new areas in response to higher prey densities.
Dispersal in Rivers and Streams • Stream dwellers have mechanisms to allow them to maintain their stream position. • Streamlined bodies • Bottom-dwelling • Adhesion to surfaces • Tend to get washed downstream in spates. • Muller hypothesized populations maintained via dynamic interplay between downstream and upstream dispersal. • Colonization cycle
Review • Estimating Patterns of Survival • Survivorship Curves • Age Distribution • Rates of Population Change • Overlapping Generations • Dispersal • In Response to Climate Change • In Response to Changing Food Supply • In Rivers and Streams