POPULATION DYNAMICS

Population Dynamics and the Sea Otter The population dynamics of the sea otter have helped us to better understand the ecological importance of this keystone species. Sea otter – almost extinct in 1900’s, caused a population explosion of sea urchins because nothing to keep pop. In check Kelp beds lost, decrease in diversity When sea otters made a comeback, deforested kelp areas recovered

Population Dynamics and the Sea Otter

Population Dynamics Population dynamics is the change in population structure due to environmental stress and changes in environmental conditions.

Population Dynamics Four ways in which population structure changes: –Size (# of individuals) –Density (# of individuals in a certain space) –Dispersion (spatial patterns) –Age distribution

Dispersion – Spatial Patterns

Population Size Population change = (births + immigration) – (deaths + emigration) –Immigration – organisms moving into a population –Emigration – organisms leaving pop.

Population Size To determine percent growth rate: =[(CBR-CDR) + (IR-ER)] *100 CBR crude birth rate = #births/1000 CDR crude death rate = #deaths/1000 IR Immigration rate = #immigrating/1000 ER emigration rate = #emigrating/1000

Population Size Biotic Potential – a population’s growth potential Intrinsic Rate of Increase (r) – rate at which a population would increase with unlimited resources –Ex. One single female housefly could give rise to 5.6 trillion flies in 13 months with no controls on the pop.

Population Size ….but, as we know, in nature there are always limits to population growth. –Environmental resistance are all the factors that limit population growth

Population Size Together the biotic potential and environmental resistance determine carrying capacity (K)

Population Size Carrying capacity (K) -- # of individuals of a given species that can be sustained indefinitely in a given space (area or volume)

Population Size Exponential Growth – a population that has few resource limitations; growth starts out slowly, but gets faster and faster (j-shaped growth curve)

Exponential Growth

Logistic Growth Involves exponential growth, with a steady decrease in population growth as it encounters environmental resistance, approaching carrying capacity and leveling off Sigmoid growth curve

Logistic Growth

In reality, populations fluctuate slightly above and below the carrying capacity

Population Size – Doubling Time How long it takes for the population to double = 70/ % growth rate –Ex. In 2002, world pop. Grew by 1.28%…..so 70/1.28=54.7 (so world pop. Should double in approx. 55 yrs.

What if carrying capacity is exceeded? This happens when a pop. Uses up its resource base and temporarily overshoots carrying capacity Occurs because of reproductive time lag: period needed for birth rate to fall and death rate to rise Pop. Would then suffer a crash or dieback

Example of Overshoot 26 Reindeer were introduced to an island off of Alaska in – pop.= 2,000 (no predators and plentiful resources) By 1950, the pop. Crashed with only 8 reindeer remaining

Factors Affecting Carrying Capacity Competition within and between species Immigration and emigration Natural and human caused catastrophes Seasonal changes in resource availability

What About Human Pop. Carrying Capacity? Currently growing at an exponential rate Humans can be affected by overshooting carrying capacity –Ex. Potato famine in 1845 (Ireland) 1 million people died and 3 million emigrated

Population Density Density-independent population controls:affect pop. Size regardless of density Examples: floods, fires, hurricanes, unseasonable weather, habitat destruction, pesticide spraying

Population Density Density-dependent population controls:have a greater affect as population increases Examples:competition, predation, parasitism, disease

Population Density Human Example: Bubonic Plague (bacterium usually found in rodents) spread like wildfire through cities of Europe in 14 th century killing 25 million people

Revisiting Predator-Prey Relationship

Top-down control hypothesis: the predator population keeps the prey population in check …but is this really true?

Revisiting Predator-Prey Relationship Research shows that the snowshoe hare pop. Has a similar cycle even when lynx aren’t present Bottom-up control hypothesis: the hare population overshoots its carrying capacity, and then crashes, so in reality the hare pop. Controls the lynx pop.

Revisiting Predator-Prey Relationship It has been found that both of these hypotheses are not mutually exclusive, they exist in different ecosystems

Reproductive Patterns Asexual Reproduction: all offspring are clones or identical copies

Reproductive Patterns Sexual Reproduction: half of genetic material coming from each parent; 97% of known organisms Risks:females only ones producing offspring, chance of genetic errors, mating may spread disease, injury

Reproductive Patterns If sex is so risky, why do so many organisms reproduce this way? –Greater genetic diversity, so more likely to survive environmental change –Males can help provide for offspring, increasing chances for survival

Reproductive Patterns Two different patterns: r-selected species K-selected species

Reproductive Patterns r-selected species or opportunists: reproduce early and put most of their energy into reproducing Called opportunists because can rapidly colonize a new habitat or colonize after a disturbance; usually boom and bust cycles

Characteristics of r- selected species Little or no parental care Early reproductive age Many offspring at once Short lived

Characteristics of r- selected species examples

K-selected species or competitors Tend to do well in competitive conditions when population size is near carrying capacity (K) Thrive best when environmental conditions are stable

K-selected species or competitors Characteristics: –Develop inside mothers –Reproduce late in life –Mature slowly –Parental care –Fewer offspring –*prone to extinction due to these char.

K-selected species or competitors

Survivorship Curves Shows the # of survivors of each age group

Survivorship Curves Type I:late loss curves (humans, typical K-selected species) –High survivorship (parental care) until a certain age, then a high mortality

Survivorship Curves Type II – constant loss curve (songbirds, lizards) –Fairly constant mortality rate in all age classes

Survivorship Curves Type III:early loss curves (r- selected species, fish, insects) –High juvenile mortality rate

Conservation Biology A multidisciplinary science to take action to preserve species and ecosystems 3 principles: –Biodiversity is necessary to life on earth –Humans should not cause or hasten ecological damage including extinction –Best way to preserve biodiversity is to protect intact ecosystems

Conservation Biology

How humans have altered natural ecosystems: –Fragmenting and degrading habitat

How humans have altered natural ecosystems: –Simplifying natural ecosystems (monocultures)

Conservation Biology –Using, wasting, or destroying a percentage of earth’s primary productivity

Conservation Biology Genetic resistance of some pest and bacteria pops. Due to overuse of pesticides and antibiotics

Conservation Biology Eliminating some predators

Conservation Biology Deliberately or accidentally introducing nonnative species

Conservation Biology Overharvesting renewable resources

Conservation Biology Interfering with the normal chemical cycling and energy flows in ecosystems

Conservation Biology So… what can we do? –Learn about processes and adaptations by which nature sustains itself –Mimic lessons from nature