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.

9-1 Population Dynamics and Carrying Capacity OBJ 9.1 Population dynamics study of how populations change in size, density, and age distribution populations respond to their environment change according to distribution

Factors Governing Changes in Population Size OBJ 9.2 Four variable births, deaths, immigration and emigration Population Change = (births + immigration) – (deaths + emigration)

Age Structure Stages PREREPRODUCTIVE AGE - Not mature enough to reproduce REPRODUCTIVE AGE - Capable of reproducing POSTREPRODUCTIVE AGE - too old to reproduce

LIMITING FACTOR OBJ 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 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) Fig. 9-3 p. 166

Exponential and Logistic Growth OBJ 9.4 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 9.5

Population Density Effects OBJ 9.6 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

Natural Population Curves OBJ 9.7 Fig. 9-7 p. 168

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 OBJ 9.8 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

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 OBJ 9.10 r-selected species vs. K-selected species Fig. 9-10 p. 170

Survivorship Curves OBJ 9.11 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. Fig. 9-11 p. 171

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