Population Dynamics, Carrying Capacity, and Conservation Biology Chapter 9 Population Dynamics, Carrying Capacity, and Conservation Biology

Key Questions How do populations change in size, density, and makeup? What is the role of predators in controlling population size? What is conservation biology? How can we live more sustainably?

Major Characteristics of a Population Populations can change in… Size (number of individuals) Density (number of individuals in a certain area) Dispersion (spatial pattern) Age distribution

Dispersion Clumped (elephants) Uniform (creosote bush) Random (dandelions)

These changes are called… Population Dynamics! -occur in response to environmental stress OR changes in environmental conditions

What Limits Population Growth? Births Deaths Immigration (to a new area) Emigration (out of an area) Population change = (births + immigration) – (deaths + emigration)

Populations Populations vary in biotic potential (growth) Intrinsic rate of increase (r)-rate at which a population would grow if it had unlimited resources

High intrinsic rate of increase populations… Reproduce early in life Short generation times Reproduce many times Many offspring each time they reproduce

Example… FLIES!-high intrinsic rate of increase/biotic potential Without control, there would be 5.6 trillion flies within 13 months Within a few years, flies could cover the surface of the earth!

Of course… This is not realistic because no population can grow indefinitely There are always limiting factors!

Environmental Resistance all limiting factors together the biotic potential and environmental resistance determine the carrying capacity

(environmental resistance) POPULATION SIZE Growth factors (biotic potential) Favorable light Favorable temperature Favorable chemical environment (optimal level of critical nutrients) Abiotic Biotic High reproductive rate Generalized niche Adequate food supply Suitable habitat Ability to compete for resources Ability to hide from or defend against predators Ability to resist diseases and parasites Ability to migrate and live in other habitats Ability to adapt to environmental change Decrease factors (environmental resistance) Too much or too little light Temperature too high or too low Unfavorable chemical environment (too much or too little of critical nutrients) Low reproductive rate Specialized niche Inadequate food supply Unsuitable or destroyed habitat Too many competitors Insufficient ability to hide from or defend Inability to resist diseases and parasites Inability to migrate and live in other Inability to adapt to environmental © 2004 Brooks/Cole – Thomson Learning Population Size

A Few More Terms Carrying Capacity (K)-number of individuals that can be sustained in a given space Minimum viable population (MVP)-minimum population size needed to support a breeding population (below MVP, extinction is likely)

Exponential and Logistic Growth A population has exponential growth when it has few/no resource limitations (J-shaped curve) Logistic growth- exponential population growth that approaches carrying capacity and levels off (S-shaped curve)

© 2004 Brooks/Cole – Thomson Learning Population size (N) Population size (N) Time (t) Time (t) Exponential Growth Logistic Growth

What Happens If Population Size Exceeds Carrying Capacity? Overshoot-exceeds carrying capacity Dieback-happens unless individuals change resources or move to another area Humans are not exempt from this! Potato fungus in Ireland; 1 million died/3 million emigrated

Number of sheep (millions) 2.0 Overshoot 1.5 Number of sheep (millions) 1.0 .5 1800 1825 1850 1875 1900 1925 Year

How Does Density Affect Population Growth? Density-independent population controls (do NOT depend on size of population): floods, fires, hurricanes, habitat destruction, pesticides Density-dependent population controls (depend on size of population): competition, predation, disease, parasitism

Population Curves in Nature Stable: fluctuates slightly above and below carrying capacity (undisturbed areas) Irruptive: fairly stable with occasional explosions Irregular: chaotic, no recurring pattern Cyclic: regular cycles

Natural Population Curves

Do Predators Control Population Size? YES! Lynx-Hare Cycle: Shortage of hares reduces lynx population Hare population builds up because there are fewer predators Lynx population increases because there are more hares Cycle begins again

Two Ideas About Lynx-Hare Cycle Top-down control hypothesis: lynx controls hare population Bottom-up control hypothesis: hare controls lynx population

Population size (thousands) 160 140 Hare 120 100 Lynx Population size (thousands) 80 60 40 20 Year

Reproductive Patterns and Survival Asexual reproduction: all offspring are exact copies (clones) of a single parent Sexual reproduction: combination of gametes (97% of species) r-selected species K-selected species

Reproductive Patterns r-selected: opportunist species; high intrinsic rate of increase; reproduce early; algae, bacteria, rodents, insects Many offspring each time they reproduce Reproduce at young age Short generation times Little or no parental care Short life-spans Irregular and unstable changes in population size

Little or no parental care and protection of offspring r-Selected Species cockroach dandelion 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

Reproductive Patterns, continued K-selected: competitor species; mammals, long-lived plants, birds of prey Reproduce late in life Few offspring Long generation times Nurture and protect their young Logistic growth curve

Fewer, larger offspring High parental care and protection of offspring K-Selected Species elephant saguaro 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

Figure 9-9 Page 196 Time Number of individuals Carrying capacity K K species; experience K selection Number of individuals r species; experience r selection Time

Survivorship Curves Shows the number of survivors of each age group for a species 3 Types: 1. Late Loss Curves: high survivorship to a certain age, then high death rate (elephants, humans) 2. Constant Loss Curves: constant death rate at all ages (songbirds) 3. Early Loss Curves: survivorship is low early in life (r-selected species)

Percentage surviving (log scale) 100 10 1 Age

Conservation Biology Started in the 1970s Uses science to take action to preserve species and ecosystems 3 principles Biodiversity is necessary to all life on earth and should not be reduced by humans Humans should not disrupt vital ecological processes The best way to preserve earth’s biodiversity is to protect ecosystems