Population Ecology

Key Concepts

Factors affecting population size

Species reproductive patterns

Species survivorship patterns

Conservation biology and human impacts on ecosystems

CASE STUDY: CANE TOADS• Cane toads (Bufo marinus, Bufonidae) naturally occur in the southern USA and the tropics of

South America.

Cane toads were deliberately introduced into Australia in an unsuccessful attempt to control pest beetles of sugar cane. About 3000 were first released near Cairns, northern Queensland, in July 1935.

• There are no specific predators of cane toads in Australia.• The ability of cane toads to rapidly increase in number and expand into new areas and eat a

large volume and variety of prey means they could displace many native species.• Toads prey on native animals especially insects and other invertebrates.• Toads out-compete native fauna such as small skinks and frogs for food.• Cane toads are poisonous at all stages of their life cycle.• Toads poison pets, humans, and native animals.

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9-1 Population Dynamics and Carrying Capacity

Population dynamics- study of how populations change in size, density, and age

distribution- populations respond to their environment- change according to distribution

OBJ 9.1

Factors Governing Changes in Population Size

• Four variable– births, deaths,

immigration and emigration

• Population Change = (births + immigration) – (deaths + emigration)

OBJ 9.2

Age Structure Stages• PREREPRODUCTIVE AGE

- Not mature enough to reproduce• REPRODUCTIVE AGE

- Capable of reproducing• POSTREPRODUCTIVE AGE

- too old to reproduce

LIMITING FACTOROBJ 9.3

DEFINITION: anything that tends to make it

more difficult for a species to live and grow, or

reproduce in its environment

ABIOTIC - temperature - water - climate/weather - soils (mineral component)

BIOTIC - competition:  interspecific and intraspecific - predation/parasitism - amensalism - mutualism

LIMITS TO POPULATION GROWTH: Resources & Competition

Fig. 9-3 p. 166

Biotic potential: capacity for growth

Intrinsic rate of increase (r): rate at which a population would grow if it had unlimited resources

Environmental resistance: all factors that act to limit the growth of a population

Carrying Capacity (K): maximum # of individuals of a given species that can be sustained indefinitely in a given space (area or volume)

Exponential and Logistic Growth

LOGISTIC GROWTH

- Rapid exp. growth followed by steady dec. in pop. Growth w/time until pop. Size levels off

EXPONENTIAL GROWTH- Population w/few resource

limitations; grows at a fixed rate

OBJ 9.4

OBJ 9.5

Population Density Effects Density-independent controls

- floods, hurricanes, unseasonable weather, fire, habitat destruction, pesticide spraying, pollution- EX: Severe freeze in spring can kill plant pop. regardless of density

Density-dependent controls- competition for resources, predation, parasitism, infectious diseases- EX: Bubonic plague swept through European cities in 14th century

OBJ 9.6

Natural Population Curves

Fig. 9-7 p. 168

OBJ 9.7

• STABLE– pop. Size fluctuates above or below its carrying

capacity– Stable population size– EX: undisturbed tropical rain forests

• IRRUPTIVE– pop. Growth occasionally explodes to a high peak then

crashes to stable low level– EX: Algae, insects

• CYCLIC– Fluctuations occur in cycles over a regular time period– EX: Lynx & snowshoe hare

• IRREGULAR– No recurring pattern in changes of population size

The Role of Predation in Controlling Population Size

Top-down control- lynx preying on hares periodically reduce the hare pop.

Bottom-up control- the hare pop. may cause changes in lynx pop.

Fig. 9-8 p. 168

OBJ 9.8

How do Species Reproduce• ASEXUAL REPRODUCTION

– all offspring are exact genetic copies of a single parent– Common in single celled species (bacteria)– Each cell divides to produce 2 identical cells

• SEXUAL REPRODUCTION– Organisms produce offspring by combining sex cells or

gametes from both parents– Produces offspring with combination of genetic traits

from each parent– Provides greater genetic diversity in offspring

• DISADVANTAGES– Males do not give birth– Increased chance of genetic errors and defects– Courtship & mating rituals consume time &

energy and transmit diseases

Reproductive Patterns and Survival

r-selected species vs. K-selected species

Fig. 9-10 p. 170

OBJ 9.10

Survivorship Curves

Fig. 9-11 p. 171

• Shows the % of members in a pop. Surviving at different ages

LATE LOSS- High survivorship to certain age; then

high mortality- EX: elephants, rhinos, humans

CONSTANT LOSS- Fairly constant death rate at all ages- EX: songbirds

EARLY LOSS- Survivorship is low early in life- EX: annual plants, bony fish sp.

OBJ 9.11

Human Impacts on Ecosystems

Habitat degradation and fragmentation Ecosystem simplification Genetic resistance Predator elimination Introduction of non-native species Overharvesting renewable resources Interference with ecological systems