Chapter 53 Population Ecology

Population Ecology A population Is a group of individuals of a single species living in the same general area Population ecology is the study of populations in relation to environment Including environmental influences on population density and distribution, age structure, and variations in population size Northern Fur Seals

Density and Dispersion Is the number of individuals per unit area or volume Dispersion Is the pattern of spacing among individuals within the boundaries of the population

Population Dynamics Density is the result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it Births and immigration add individuals to a population. Births Immigration PopuIation size Emigration Deaths Deaths and emigration remove individuals from a population. Ins Outs

Patterns of Dispersion Environmental and social factors Influence the spacing of individuals in a population

A clumped dispersion Is one in which individuals aggregate in patches May be influenced by resource availability and behavior (a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

A uniform dispersion Is one in which individuals are evenly distributed May be influenced by social interactions such as territoriality (b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

A random dispersion Is one in which the position of each individual is independent of other individuals (c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

Demography Demography is the study of the vital statistics of a population and how they change over time Death rates and birth rates are of particular interest to demographers

Life Tables A life table Is an age-specific summary of the survival pattern of a population

Life table of Belding’s ground squirrels

Survivorship Curves A survivorship curve Is a graphic way of representing the data in a life table Shows a pattern of survival

Survivorship curve for Belding’s ground squirrels 1000 100 10 1 Number of survivors (log scale) 2 4 6 8 Age (years) Males Females This survivorship curve shows a fairly constant death rate

Survivorship curves (types I, II, and III) 50 100 1 10 1,000 Percentage of maximum life span Number of survivors (log scale)

Reproductive Rates A reproductive table, or fertility schedule Is an age-specific summary of the reproductive rates in a population

Life History Life history traits Traits that affect an organism’s schedule of reproduction and survival They are Trade-Offs between survival and reproduction (parental care, # of offspring, etc.) They are very diverse They are evolutionary outcomes that are reflected in the development, physiology, and behavior of an organism

Life History Life history entails 3 basic variables Age at which reproduction begins Frequency of reproduction # of offspring produced per reproductive cycle

Semelparity, or “big-bang” reproduction Reproduce a single time and die Iteroparity, or repeated reproduction Produce offspring repeatedly over time Agave Rabbit

Life History trade-off Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.) EXPERIMENT The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. CONCLUSION 100 80 60 40 20 Reduced brood size Normal brood size Enlarged brood size Parents surviving the following winter (%) Male Female RESULTS

Some plants produce a large number of small seeds ensuring that at least some of them will grow and eventually reproduce (a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.

Some plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established (b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

Population Growth A Couple Questions for Population Ecologists. What environmental factors stop a population from growing? Why do some populations show radical fluctuations in size over time, while others remain stable?

Population Change and Population Density In density-independent populations Birth rate and death rate do not change with population density In density-dependent populations Birth rates fall and death rates rise with population density

Population-Limiting Factors Competition Territoriality Health Predation Waste accumulation Intrinsic factors (factors related to the biology/physiology of the particular species)

Competition for Resources In crowded populations, increasing population density intensifies intraspecific competition for resources 10 100 1,000 10,000 Average number of seeds per reproducing individual (log scale) Average clutch size Seeds planted per m2 Density of females 70 20 30 40 50 60 80 2.8 3.0 3.2 3.4 3.6 3.8 4.0 (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Territoriality In many vertebrates and some invertebrates territoriality may limit density Cheetahs are highly territorial Chemical communication can warn other cheetahs of their boundaries

Territoriality Territories can be small Gannets exhibit territoriality in nesting behavior

Health Population density can influence the health and survival of organisms In dense populations pathogens can spread more rapidly

Predation As a prey population builds up Predators may feed preferentially on that species

Toxic Wastes The accumulation of toxic wastes Can contribute to density-dependent regulation of population size The Wine Example Alcohol content of wine generally peaks out at about 13%. Beyond that level the yeast die off from ethanol toxicity

Intrinsic Factors For some populations Intrinsic (physiological) factors appear to regulate population size (Such as an increase in aggressive behavior in a crowded colony of animals leading to a decrease in reproduction rate) Stress has a negative impact on the reproductive system.

Population Dynamics Population dynamics Focuses on the interactions between biotic and abiotic factors that cause variation in population size

Commercial catch (kg) of male crabs (log scale) Extreme fluctuations in population size are typically more common in invertebrates than in large mammals 1950 1960 1970 1980 Year 1990 10,000 100,000 730,000 Commercial catch (kg) of male crabs (log scale)

Metapopulations and Immigration Groups of populations linked by immigration and emigration This butterfly exists in many small populations. Individuals migrate between patches (populations) and can also colonize unoccupied patches.

Population Cycles Year 1850 1875 1900 1925 40 80 120 160 3 6 9 Lynx population size (thousands) Hare population size (thousands) Lynx Snowshoe hare Many populations undergo regular boom-and-bust cycles. Cycles are influenced by interactions between biotic and abiotic factors

Human Populations Human population growth has slowed after centuries of exponential increase No population can grow indefinitely humans are no exception

The Global Human Population The human population increased relatively slowly until about 1650 and then began to grow exponentially 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 1000 A.D. The Plague Human population (billions) 2000 A.D. 1 2 3 4 5 6

The rate of growth began to slow approximately 40 years ago 1950 1975 2000 2025 2050 Year 2003 Percent increase 2.2 2 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 1.8 The rate of growth began to slow approximately 40 years ago

Regional Patterns of Population Change To maintain population stability (zero population growth) A regional human population can exist in one of two configurations Zero population growth = High birth rates – High death rates Zero population growth = Low birth rates – Low death rates

The demographic transition is the move from the first toward the second state. Is associated with various factors in developed and developing countries The transition in Sweden was slow. In Mexico the change was abrupt early and has since slowed. 50 40 20 30 10 1750 1800 1850 1900 1950 2000 2050 Birth rate Death rate Year Sweden Mexico Birth or death rate per 1,000 people

Age Structure The relative number of individuals at each age Affects present and future growth trends

Age structure represented in pyramids Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Age 8 6 4 2 Percent of population 80–84 85 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19

Global Carrying Capacity Just how many humans can the biosphere support? …no agreement in the scientific community on a number. But, estimates of global population in 2050 are between 8 and 11 billion (currently about 6 billion) Will technological advances continue to increase the Earth’s capacity for humans.

Ecological Footprint Concept The aggregate land and water area needed to sustain the people of a nation. What area is needed to produce the resources consumed and absorb wastes generated. Ecological capacity the actual resource base of a country Looking at the ecological footprint is one measure of how close we are to the carrying capacity of Earth

Ecological footprints for 13 countries Show that the countries vary greatly in their footprint size and their available ecological capacity 16 14 12 10 8 6 4 2 New Zealand Australia Canada Sweden World China India Available ecological capacity (ha per person) Spain UK Japan Germany Netherlands Norway USA Ecological footprint (ha per person) Red dots are countries that were in ecological deficit in 1997 At more than 6 billion people the world is already in ecological deficit http://www.myfootprint.org/en/