A metapopulation is formed when a number of local populations are linked.
Local populations occupy discrete habitat patches in a sea of unsuitable habitat.
Patches vary in size, quality, and isolation, influencing individual movement.
Extinct populations can be recolonized by immigrants from other populations.
The Glanville fritillary butterfly exemplifies metapopulation dynamics.
Found in about 500 meadows across the Åland Islands of Finland, while roughly 4,000 patches are suitable.
Populations appear and become extinct regularly, shifting colonized patches.
The species persists through a balance of local extinctions and recolonizations.
Glanville Fritillary Movement
An individual's ability to move depends on genetic makeup.
The Pgi gene, coding for phosphoglucoisomerase, affects movement.
This enzyme catalyses the second step of glycolysis.
the activity correlates with the rate of CO_2 production
Heterozygous individuals for a Pgi nucleotide polymorphism fly farther at lower temperatures.
Movements ranged from 10 m to 4 km in two-hour periods.
They exhibit a fitness advantage and are more likely to colonize new locations.
Significance of Metapopulations
The metapopulation concept underscores the importance of immigration and emigration.
It helps ecologists understand population dynamics and gene flow in patchy habitats.
It provides a framework for conserving species in fragmented habitats and reserves.
Human Population Growth
The human population has grown at an unprecedented rate in the last few centuries.
In 1650, about 500 million people inhabited Earth.
The population doubled to 1 billion within the next two centuries.
It doubled again to 2 billion by 1930 and to 4 billion by 1975.
The doubling time decreased from 200 years to 45 years.
The global population is now more than 7.2 billion and increases by about 78 million each year.
This is equivalent to adding a city the size of Amarillo, Texas, each day.
Ecologists predict 8.1-10.6 billion people on Earth by 2050.
Slowing Growth Rate
The rate of growth began to slow during the 1960s.
The annual rate of increase peaked at 2.2% in 1962 but was 1.1% in 2014.
Projected growth rate of 0.5% by 2050 would add 45 million people per year if the population climbs to a projected 9 billion.
The reduction in annual growth rate resulted from fundamental shifts in population dynamics.
This includes diseases like AIDS and voluntary population control.
Regional Patterns of Population Change
Population dynamics vary widely from region to region.
Stable regional populations have birth rates equal to death rates, disregarding immigration and migration.
A unique feature of human population growth is the ability to control family sizes.
Social change and rising educational and career aspirations encourage women to delay marriage and postpone reproduction.
Delayed reproduction decreases population growth rates and moves a society toward zero population growth.
Zero population growth occurs under conditions of low birth rates and low death rates.
Territoriality
Territoriality can limit population density when space becomes the resource for which individuals compete.
Cheetahs use chemical markers in urine to mark territorial boundaries.
The presence of nonbreeding individuals indicates that territoriality restricts population growth.
Intrinsic Factors
Intrinsic physiological factors can regulate population size.
Reproductive rates of white-footed mice can drop even with abundant food and shelter.
This drop in reproduction at high density is associated with aggressive interactions and hormonal changes.
These changes delay sexual maturation and depress the immune system.
Toxic Wastes
Toxic wastes can regulate population size in certain organisms.
Yeasts, such as Saccharomyces cerevisiae, convert carbohydrates to ethanol in winemaking.
The ethanol that accumulates is toxic to yeasts and contributes to density-dependent regulation.
The alcohol content of wine is usually less than 13% because that is the maximum concentration of ethanol that most wine-producing yeast cells can tolerate.
Population Cycles: Snowshoe Hares and Lynx
Some populations fluctuate at unpredictable intervals, while others undergo regular boom-and-bust cycles.
Small herbivorous mammals like voles and lemmings have 3- to 4-year cycles.
Some birds like ruffed grouse and ptarmigans have 9- to 11-year cycles.
Snowshoe hares and lynx in northern forests of Canada and Alaska exhibit roughly 10-year cycling.
Lynx are predators that feed predominantly on snowshoe hares.
Lynx numbers rise and fall with hare numbers.
Hypotheses for Hare Cycles
First Hypothesis: Cycles are caused by food shortage during winter.
Hares eat terminal twigs of small shrubs like willow and birch.
Second Hypothesis: Cycles are due to predator-prey interactions.
Many predators other than lynx eat hares and may overexploit prey.
Evidence Against Food Shortage
Researchers conducted experiments in the Yukon for 20 years, providing extra food to hare populations.
Hare populations in areas with extra food increased about threefold in density but continued to cycle as the unfed control populations demonstrating food supplies alone do not cause the hare cycles.
Evidence for Predation Effects
Ecologists used radio collars to track individual hares and determine cause of death.
Predators, including lynx, coyotes, hawks, and owls, killed 95% of hares.
None of the hares appeared to have died of starvation, data supporting second hypothesis
Electric fences excluding predators from certain areas nearly eliminated the collapse in survival during the decline phase of the cycle.
Overexploitation by predators seems to be an essential part of snowshoe hare cycles.
Without predators, hare populations would not cycle in northern Canada.