Question of the day All of the following are examples of abiotic factors except temperature pH wind salinity vegetation

Population Dynamics & Carrying Capacity NOTES Chapter 9-1 Population Dynamics & Carrying Capacity

Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? Sea otters are an important keystone species for sea urchins and other kelp-eating organisms. They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback. Figure 8-1

Key Terms & Concepts (1 of 2) Population- the number of individuals of a species that inhabit a particular area. Population structure- the density and spacing of individuals within a landscape. Spacing (or dispersion) refers to the dispersion pattern of individuals in a population. Geographic distribution – a species’ range; the range can change with changing population dynamics. Population dynamics- population changes over time. Reasons for caring: Aesthetics Keystone species Ethical

Population Spacing or Dispersal Spacing is dependent on the availability of resources Clumped Spacing - individuals are clustered in groups, most common. Uniform spacing – regular spacing or dispersal Random spacing - no spacing pattern is apparent

Key Terms & Concepts (2 of 2) Natality Number of individuals added through reproduction Crude Birth Rate - Births per 1000 Total Fertility Rate – Average number of children born alive per woman in her lifetime Mortality Number of individuals removed through death Crude Death Rate Deaths per 1000 Age Structure - proportion of individuals in each age group in population

Population Growth Population growth depends upon birth rates (+) death rates (-) immigration rates (+ into area) emigration rates (- exit area) Population = Pop0 + (b + i) - (d + e) Zero Population Growth (ZPG) (b + i) = (d + e)

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. What factors influence the growth of a population?

Age Structure Diagrams Positive Growth Zero Growth Negative Growth (ZPG) Pyramid Shape Vertical Edges Inverted Pyramid

Limits on Population Growth Biotic potential is a population’s capacity for growth. Biotic potential is limited by environmental factors. Thus no population can increase its size indefinitely. Environmental resistance: all the factors that act to limit the growth of a population. Biotic factors: reproductive rate, niche, food supply, disease resistance, etc. Abiotic factors: light, temperature, chemical environment (i.e. pH)

Limits on Population Growth The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat. Determined by the interaction of biotic potential and environmental resistance. Growth rate (r) decreases as its size nears the K. Why?

Population Growth Populations show two types of growth Exponential J-shaped curve Growth is independent of population density Logistic S-shaped curve Growth is not independent of population density

Exponential and Logistic Population Growth: J-Curves and S-Curves Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8.3 Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers. Figure 8-4

Logistic Growth Because of Environmental resistance, population growth decreases as density reaches carrying capacity Graph of individuals vs. time yields a sigmoid or S-curved growth curve Reproductive time lag causes population overshoot Population will not be steady curve due to resources (prey) and predators

Population Dynamics and Carrying Capacity (K) Over time, populations of species in an ecosystem are usually in a state of equilibrium (balance between births and deaths) There is a dynamic balance between biotic potential and environmental resistance Exponential curve is not realistic due to carrying capacity of area

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8.6 Exponential growth, overshoot, and population crash of reindeer introduced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, with the herd size plummeting to only 8 reindeer by 1950. Figure 8-6

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Over time species may increase their carrying capacity by developing adaptations. Some species maintain their carrying capacity by migrating to other areas. So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

Population Density and Population Change: Effects of Crowding Population density: the number of individuals in a population found in a particular area or volume. Density depends upon social/population structure, mating relationships, time of year A population’s density can affect how rapidly it can grow or decline. e.g. biotic factors like disease Some population control factors are not affected by population density. e.g. abiotic factors like weather

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 and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Types of Population Change Curves in Nature Figure 8.7 Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich) Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7

Question of the day When a population’s growth rate changes to match local conditions, it is known as Exponential growth Density-independent growth natality Logistic growth Pioneer growth

NOTES Section 9-2 Reproductive Patterns & Survival

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 Large number of smaller offspring with little parental care (r-selected species). Fewer, larger offspring with higher invested parental care (K-selected species). Figure 8-9

r - Strategists Most of their time is in exponential growth. High growth rate (r) Max. reproductive life Minimum life span Pop. size fluctuates wildly above & below carrying capacity – (K) Generalist niche K

K - Strategists Maintain population at carrying capacity (K) Maximize lifespan Lower population growth rate (r) Specialist niche K

Reproductive Patterns r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10

Survivorship Curves Survivorship curves represent the age structure of a population. Type 1: Late loss population live to an old age. Usually K-strategists Type 2: Constant loss population die at all ages. Usually intermediate reproductive strategies Type 3: Most members of early loss population, die at young ages. Usually r-strategists

Survivorship Curves The populations of different species vary in how long individual members typically live. Figure 8-11

NOTES Section 9-3 Effects of Genetic Variations on Population Size

EFFECTS OF GENETIC VARIATION Small, isolated populations are most vulnerable Founder Effect – a few individuals colonize a geographically isolated habitat. Ex. Limited genetic diversity may threaten the survival of the population. Demographic Bottleneck: a few individuals survive a catastrophe. Lack of genetic diversity limits the ability to rebuild the population Genetic Drift: random changes in the gene frequencies lead to unequal reproductive success.