Population Dynamics

Populations are dynamic Size Density Age distribution Dispersion (spatial pattern) These changes (population dynamics) occur in response to environmental stress

Dispersion patterns within populations

Why clumping? Resources vary from place to place Moving in groups allow better chance finding resources Living in groups protects some animals from predators Hunting in packs give some predators an advantage of finding and capturing prey Some species form temporary groups for mating and caring for young Raft of sea otters

Four variables which govern population size Births Immigration Death Emigration Population Change = (Births + Immigration) – (Deaths + Emigration)

Age Structure How fast a population grows or declines depends on its age structure. Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce. Baby hippos stay with their mother for 4 years and reach their reproductive age at about 5 – 6 years

What would you say about this population of fish if they are capable of reproducing between the ages of 10 and 20 years? What would you say about this population if they did not reach reproductive age until the age of 26 years?

Factors affecting population size Intrinsic rate of increase Environmental Resistance Carrying Capacity Minimum Viable Population

Limits on population growth No population can increase its size indefinitely. The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Factors affecting carrying capacity Competition for resources Immigration and emigration of other species Natural and human-caused catastrophes Seasonal fluctuations Lake Lanier, GA in Spring 2008

What happens when a population exceeds its carrying capacity Dieback (crash) Easter Island Cause damage – reduce area’s carrying capacity Overgrazing by cattle Reduces grass cover in some areas Allows nonnative species to inhabit the area (opportunists) Sagebrush invaded after overgrazing occurred by cattle (the cattle do not eat the sagebrush)

Minimum viable population The smallest possible size at which a population can exist without extinction from natural disasters, genetic changes, human influence, or environmental factors

The intrinsic rate of increase falls and extinction is likely If a population falls below minimum viable population (MVP) needed to support a breeding population: Individuals may not be able to locate mates Genetically related individuals may interbreed Genetic diversity may be too low California condor The intrinsic rate of increase falls and extinction is likely

Genetic diversity can affect the size of a population Founder effect When only a few individuals in a population colonize a new habitat that is geographically isolated Demographic bottleneck When only a few individuals in a population survive a catastrophe (i.e. fire or hurricane) and this lack of genetic diversity may limit ability to rebuild population Genetic drift Random changes in gene frequencies where some individuals breed more than others and their genes may eventually dominate the gene pool Inbreeding Individuals in a small population mate with one another

Under some circumstances population density affects population size Density-independent population controls Their effect is not dependent on the density of the pop. Floods, hurricanes, severe drought, seasonable weather, fire, habitat destruction, and pesticide spraying Density-dependent population controls Competition for resources, predation, parasitism, and disease

Density-dependent population controls Higher density may help find mates, but can lead to increased competition for mates, food, living space, water, sunlight, and other resources

Density-dependent population controls Can help shield members from predators, but it can also make large groups such as schools of fish vulnerable to human harvesting methods

Density-dependent population controls Close contact among individuals in a dense population can increase the transmission of parasites and infectious disease April 2010: Connecticut deer population is 64 deer/ square mile – the deer population’s density is contributing to the spread of Lyme disease to humans (they would like to get the deer population down to 12 deer/ square mile

Several different types of population change occur in nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Cyclic fluctuations (boom-and-bust cycles)

Top-down population control Predators (high on trophic pyramid) regulate prey population size Example of top-down effects Increase in number of predators Reduces size of herbivore population Increases size of plant population Reduces nutrients in soil

Bottom-up population control Each trophic level derives its energy from the level below it Productivity of the species on the lower layer sets the bounds on the population sizes of the species on the higher layer Example of bottom-up effects Increase nutrients in soil More nutrients increases size of plant population More plants increases herbivore population More herbivores increases size of predator population

Lynx-Hare Population Cycle Is it top-down or bottom-up?

How does productivity influence top-down or bottom-up population control? With high primary productivity Higher primary productivity results in herbivore populations large enough to support prey populations Predators suppress the herbivore populations = top-down regulation With low primary productivity When primary productivity is low, there is not enough production of biomass to sustain large herbivore populations = bottom-up regulation

Reproductive Patterns

Survivorship Curves K-selected species r-selected species