Chapter 8

Population Ecology

WHAT IS A KEYSTONE?

• THE TOP BLOCK IN AN ARCHWAY

• REMOVE KEYSTONE

• ARCHWAY COLLAPSES

KEYSTONE SPECIES

• PLAY CRITICAL ECOLOGICAL ROLE IN HELPING SUSTAIN A COMMMUNITY

• EX. POLLINATORS, TOP PREDATORS (WOLF, LEOPARD, LION, ALLIGATOR, SOME SHARKS) DUNG BEETLES, SEA STARS, SOUTHERN SEA OTTER

KEYSTONE SPECIES

• LOSS OF KEYSTONE SPECIES CAN LEAD TO POPULATION CRASHES AND EXTINCTION OF OTHER SPECIES IN COMMUNITY THAT DEPENDS ON THE K.SPECIES FOR CERTAIN SERVICES/ROLES;

POPULATION DYNAMICS AND CARRYING CAPACITY

• 3 Types of Population dispersal:• 1) Clumped• 2) Uniform 3) Random Figure 8-2Figure 8-2

THE WORLD IS MOSTLY CLUMPY

• HERDS, FLOCKS, SCHOOLS, PACKS• RESOURCES SPECIES NEEDS VARIES GREATLY

IN AVAILABILITY FROM PLACE TO PLACE;• CAN PROVIDE BETTER PROTECTION FROM

PREDATORS & POPULATION DECLINE• BETTER CHANCE FOR FOOD, MATING & CARING

FOR YOUNG

POPULATION DYNAMICS

• MOSTLY REPRODUCTIVE AGE – POPULATION INCREASE

• MOSTLY POSTREPRODUCTIVE AGE – POPULATION DECREASE

• EVEN DISTRIBUTION PRE & POST – POPULATION IS STABLE

• NO POPULATION CAN INCREASE ITS SIZE INDEFINITELY

Population Dynamics• Depends on: size, density, dispersion, age distribution• Biotic potential = capacity for growth • Intrinsic rate of increase (r) – rate population grows if resources

unlimited• High r 1) reproduce early, 2) short generation times, 3) long

reproductive life, & 4) have many offspring• Carrying capacity (k) – Biotic potential + environmental resistance

• Limiting Factors – light, water, space, nutrients, competition, predators,

disease (environmental resistance)

CARRYING CAPACITY (K)

• THE MAXIMUM POPULATION OF A GIVEN SPECIES THAT A PARTICULAR HABITAT CAN SUSTAIN INDEFINITELY WITHOUT DEGRADING THE HABITAT

• GROWTH RATE DECREASES AS POPULATION SIZE NEARS K

Changes in Population Size: Entrances and Exits

• Rule of 70: To calculate doubling time for a population in years, divide 70 by the percent growth rate

Age Structure: Young Populations

Can Grow Fast• 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.

Biotic Potential vs. Environmental Resistance

• No population can increase its size indefinitely.– Intrinsic rate of increase

(r) -rate at which a population would grow if it had unlimited resources (biotic potential).

– Carrying capacity (K): maximum population of a given species that a particular habitat can sustain

Exponential and Logistic Population Growth: J-Curves and

S-Curves– J-curve:

exponential growth

– S-curve: logistic growth

Figure 8-4Figure 8-4

POPULATION GROWTH

• NO OR FEW RESOURCE LIMITATIONS

• POPULATION WILL GROW AT A FIXED RATE OF

• 1% TO 2% EACH YEAR

EXPONENTIAL GROWTH

• GEOMETRIC

• STARTS SLOWLY

• THEN ACCELERATES AS POPULATION INCREASES DUE TO BASE SIZE OF POPULATION INCREASING

• FIGURE 8-3 BOTTOM HALF

LOGISTIC GROWTH

• S-CURVE: RAPID EXPONENTIAL GROWTH FOLLOWED BY STEADY DECREASE IN POPULATION GROWTH WITH TIME UNTIL POPULATION GROWTH LEVELS OFF

• FIGURE 8-3 WHOLE CURVE

Exceeding Carrying Capacity: • Density-independent

factors: ctrl size without consideration of population size – natural disasters

• Density-dependent factors: do depend on population size – competition, space, predation, disease, parasitism Figure 8-6Figure 8-6

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 in regular cyclic or

boom-and-bust cycles.– Irregular: erratic changes possibly due to chaos or

drastic change.

Types of Population Change Curves in Nature

• Predator-prey cycles – top-down vs. bottom-up control

Figure 8-7Figure 8-7

REPRODUCTIVE PATTERNS• Some species reproduce without having

sex (asexual).– Offspring are exact genetic copies (clones).

• Others reproduce by having sex (sexual).– Genetic material is mixture of two individuals.– Disadvantages: males do not give birth,

increase chance of genetic errors and defects, courtship and mating rituals can be costly.

– Major advantages: genetic diversity, offspring protection.

Reproductive Patterns:Opportunists and Competitors

• r-strategists - Large number of smaller offspring with little parental care

• K-strategists - fewer, larger offspring with higher invested parental care

Figure 8-9Figure 8-9

Reproductive Patterns

• r-selected species tend to be opportunists while K-selected species tend to be competitors.

Figure 8-10Figure 8-10

Survivorship Curves: Short to Long Lives

• Late loss: live to old age

• Constant loss: die at all ages (birds, lizards, small mammals)

• Early loss: die at young ages

Figure 8-11Figure 8-11