Population Dynamics

Population Dynamics

Population Dynamics Outline • Characteristics of a Population • Population Dynamics and Carrying Capacity • Reproductive Strategies

Characteristics of a Population • Population - individuals inhabiting the same area at the same time • Population Dynamics: Population change due to • Population Size - number of individuals • Population Density - population size in a certain space at a given time • Population Dispersion - spatial pattern in habitat • Age Structure - proportion of individuals in each age group in population

Population Size • Natality • Number of individuals added through reproduction • Crude Birth Rate - Births per 1000 • Total Fertility Rate – Average number of children born alive per woman in her lifetime • Mortality • Number of individuals removed through death • Crude Death Rate Deaths per 1000

Population Density • Population Density (or ecological population density) is the amount of individuals in a population per unit habitat area • Some species exist in high densities - Mice • Some species exist in low densities - Mountain lions • Density depends upon • social/population structure • mating relationships • time of year

Population Dispersion Population dispersion is the spatial pattern of distribution There are three main classifications Clumped: individuals are lumped into groups ex. Flocking birds or herbivore herds due to resources that are clumped or social interactions most common http://www.johndarm.clara.net/galleryphots/

Uniform: Individuals are regularly spaced in the environment - ex. Creosote bush due to antagonism between individuals, or do to regular spacing of resources rare because resources are rarely evenly spaced Random: Individuals are randomly dispersed in the environment ex. Dandelions due to random distribution of resources in the environment, and neither positive nor negative interaction between individuals rare because these conditions are rarely met www.agry.purdue.edu/turf/ tips/2002/clover611.htm Population Dispersion http://www.calflora.net/bloomingplants/creosotebush2.html

Age Structure • The age structure of a population is usually shown graphically • The population is usually divided up into prereproductives, reproductives and postreproductives • The age structure of a population dictates whether is will grow, shrink, or stay the same size

Age Structure Diagrams Positive Growth Zero Growth Negative Growth (ZPG) Pyramid Shape Vertical Edges Inverted Pyramid

Biotic Potential • factors allow a population to increase under ideal conditions, potentially leading to exponential growth • Environmental Resistance • affect the young more than the elderly in a population, thereby affecting recruitment (survival to reproductive age)

Biotic Potential • Ability of populations of a given species to increase in size • Abiotic Contributing Factors: • Favorable light • Favorable Temperatures • Favorable chemical environment - nutrients • Biotic Contributing Factors: • Reproductive rate • Generalized niche • Ability to migrate or disperse • Adequate defense mechanisms • Ability to cope with adverse conditions

Environmental Resistance • Ability of populations of a given species to increase in size • Abiotic Contributing Factors: • Unfavorable light • Unfavorable Temperatures • Unfavorable chemical environment - nutrients • Biotic Contributing Factors: • Low reproductive rate • Specialized niche • Inability to migrate or disperse • Inadequate defense mechanisms • Inability to cope with adverse conditions

Population Growth • Population growth depends upon • birth rates • death rates • immigration rates (into area) • emigration rates (exit area) Pop = Pop0 + (b + i) - (d + e) ZPG (b + i) = (d + e)

Population Growth • Populations show two types of growth • Exponential • J-shaped curve • Growth is independent of population density • Logistic • S-shaped curve • Growth is not independent of population density

Exponential Growth • As early as Darwin, scientists have realized that populations have the ability to grow exponentially • All populations have this ability, although not all populations realized this type of growth • Darwin pondered the question of exponential growth. He knew that all species had the potential to grow exponentially • He used elephants as an example because elephants are one of the slowest breeders on the planet

Exponential Growth One female will produce 6 young over her 100 year life span. In a population, this amounts to a growth rate of 2% Darwin wondered, how many elephants could result from one male and one female in 750 years? 19,000,000 elephants!!!

Population Dynamics and Carrying Capacity • Basic Concept: Over a long period of time, populations of species in an ecosystem are usually in a state of equilibrium (balance between births and deaths) • There is a dynamic balance between biotic potential and environmental resistance

Carrying Capacity (K) • Exponential curve is not realistic due to carrying capacity of area • Carrying capacity is maximum number of individuals a habitat can support over a given period of time due to environmental resistance (sustainability)

Exponential and Logistic Population Growth: J-Curves and S-Curves • As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4

Logistic Growth • Because of Environmental Resistance, population growth decreases as density reaches carrying capacity • Graph of individuals vs. time yields a sigmoid or S-curved growth curve • Reproductive time lag causes population overshoot • Population will not be steady curve due to resources (prey) and predators

Types of Population Change Curves 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.

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size • Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6

Types of Population Change Curves in Nature • Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7

Hare Lynx Population size (thousands) Year Fig. 8-7, p. 166

Reproductive Strategies • Goal of every species is to produce as many offspring as possible • Each individual has a limited amount of energy to put towards life and reproduction • This leads to a trade-off of long life or high reproductive rate • Natural Selection has lead to two strategies for species: r - strategists and K - strategists

r - Strategists • Spend most of their time in exponential growth • Maximize reproductive life • Minimum life K

R Strategists • Many small offspring • Little or no parental care and protection of offspring • Early reproductive age • Most offspring die before reaching reproductive age • Small adults • Adapted to unstable climate and environmental conditions • High population growth rate – (r) • Population size fluctuates wildly above and below carrying capacity – (K) • Generalist niche • Low ability to compete • Early successional species

K - Strategists • Maintain population at carrying capacity (K) • Maximize lifespan K

K- Strategist • Fewer, larger offspring • High parental care and protection of offspring • Later reproductive age • Most offspring survive to reproductive age • Larger adults • Adapted to stable climate and environmental conditions • Lower population growth rate (r) • Population size fairly stable and usually close to carrying capacity (K) • Specialist niche • High ability to compete • Late successional species

Reproductive Patterns:Opportunists and Competitors • Large number of smaller offspring with little parental care (r-selected species). • Fewer, larger offspring with higher invested parental care (K-selected species). Figure 8-9

Reproductive Patterns • r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10

Survivorship Curves • Late Loss: K-strategists that produce few young and care for them until they reach reproductive age thus reducing juvenile mortality • Constant Loss: typically intermediate reproductive strategies with fairly constant mortality throughout all age classes • Early Loss: r-strategists with many offspring, high infant mortality and high survivorship once a certain size and age

Survivorship Curves: Short to Long Lives • The populations of different species vary in how long individual members typically live. Figure 8-11

Carrying capacity K K species; experience K selection Number of individuals r species; experience r selection Time Fig. 8-9, p. 168

Late loss Constant loss Percentage surviving (log scale) Early loss Age Fig. 8-11, p. 169

Case Study: Exploding White-Tailed Deer Populations in the United States • Since the 1930s the white-tailed deer population has exploded in the United States. • Nearly extinct prior to their protection in 1920’s. • Today 25-30 million white-tailed deer in U.S. pose human interaction problems. • Deer-vehicle collisions (1.5 million per year). • Transmit disease (Lyme disease in deer ticks).

Population Dynamics