POPULATION ECOLOGY The branch of ecology environmental science

POPULATION ECOLOGY The branch of ecology & environmental science that deals with characterizing the makeup, growth, and impact of populations of organisms on the environment

POPULATION ⦿A group of the same species living in the same place at the same time �Ex. Earthworms living in a garden or maple trees that live in a particular forest ⦿ Species may consist of multiple populations that are geographically isolated from one another

3 METHODS FOR MEASURING POPULATION SIZE… 1. Direct count method (census) ⦿ Just as it says… �Count all individuals in population �Done every 10 years in U. S. �Not exact due to immigration, emigration, births & deaths

2. QUADRAT METHOD (Sampling) ⦿ Construct a grid of sample squares, each of known area, within the larger area where population lives ⦿ Individuals counted in a fraction of those sample squares & then total population is estimated ⦿ Works best for counting organisms that don’t move (trees)

3. CAPTURE-RECAPTURE ⦿ Used to estimate size of a mobile population ⦿ Count & mark a sample of individuals in an area on a given day & then return them to the habitat. ⦿ Next day, go back and mark another sample of individuals ⦿ Continue �Ex. over the course of several days Japanese beetles in a field.

HOW MANY INDIVIDUALS MAKE UP A POPULATION? ⦿Population density �The # of individuals to a given unit of area • Ex. Dandelion population in a field= the # of dandelion plants per square meter

POPULATION DENSITY PROBLEM ⦿ 1, 270 deer are living on an island that is 830 km 2 in size. What is the population density of the deer per square kilometer?

⦿ 1, 270 POPULATION DENSITY PROBLEM deer are living on an island that is 830 km 2 in size. What is the population density of the deer per square kilometer? ⦿ 1, 270/830 ⦿ 1. 53 deer/km 2

⦿ The ability to predict growth or decline is useful in monitoring and managing threatened or endangered species

POPULATION DENSITY ⦿ In general, larger organisms have lower population densities because they require more resources– and thus more area– to survive

POPULATION DENSITY Specialist species tend to be advantaged in habitats that remain constant Generalist species tend to be advantaged in habitats that are changing

HIGH VS LOW POPULATION DENSITY ⦿ High density � Easy to find mates � Conflict over space, food � Vulnerable to predators � Increase transmission of disease ⦿ Low density � Hard time finding mate � Less competition for space, food

CALCULATING BIRTH AND DEATH RATES ⦿To calculate the birth or death rate of a population, just take the number of births/deaths and then divide by the total population ⦿Ex. If a population has 29 births a year and the original population size is 1, 000, what is the birth rate for this population?

CALCULATING BIRTH AND DEATH RATES Ex. If a population has 29 births a year and the original population size is 1, 000, what is the birth rate for this population? ⦿ 29/1, 000=. 029= 2. 9%

HOW DO POPULATIONS GROW? ⦿ 1. Population with no immigration or emigration occurs r= (b-d)/N ⦿ N= population size ⦿ b= births ⦿ d= deaths ⦿ r= growth rate of the population

HOW DO POPULATIONS GROW? 1. If a small country of 90, 000 have 3, 000 births and 421 deaths annually, what is the growth rate for this country? 2. If the crude birth rate of a country is 36 per 1, 000 people and a crude death rate of 9 per 1, 000, the natural increase annual percent increase of its populations is. .

HOW DO POPULATIONS GROW? 1. 3, 000421/90, 000=0. 028 x 100= 2. 9% 2. 36 -9/1000=0. 027 x 100=2. 7%

POPULATIONS GROW WITH IMMIGRATION & EMIGRATION r= (b + i) –(d + e)/N ⦿ N= population size ⦿ b= births ⦿ d= deaths ⦿ i= immigration ⦿ E= emigration ⦿ r= growth rate of the population

POPULATION DOUBLING TIME ⦿ To calculate the time it takes for a population to double- we use the Rule of 70 ⦿ 70 r ⦿ How long would it take a population with a growth rate of 2% to double in size? 70 = 35 years 2

HOW DO POPULATIONS GROW? 1. A country has an estimated annual growth rate of 3%. At this rate of growth, approximately how many years will it take for the population of this country to double?

HOW DO POPULATIONS GROW? 1. If a country has an estimated annual growth rate of 3%. At this rate of growth, approximately how many years will it take for the population of this country to double? 70/3%= 23 years

HOW DO POPULATIONS GROW? 2. A population takes about 10 years to double in size. What would the country’s annual percentage growth rate be?

HOW DO POPULATIONS GROW? 2. A population takes about 10 years to double in size. What would the country’s annual percentage growth rate be? 70/r= 10

EXPONENTIAL GROWTH Exponential Growth Curve of a Hypothetical, Ideal Population This usually occurs with small populations & when a lot of resources are available Population Size (N) J curve Time (t) “Malthusian Growth”

HOWEVER, POPULATIONS DO NOT GROW INDEFINITELY… ⦿ Every population is constrained by limiting factors �Physical, chemical & biological characteristics of the environment ⦿ Populations grow until they reach the Carrying capacity (K) �Carrying capacity= the maximum population size of a species that a given environment can sustain based on available resources • This can vary, depending on changes in the environment

Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.

Logistical Growth Curve of a Hypothetical, Real Population Carrying Capacity (K) Population Size (N) S- curve Time (t)

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 -4

EXPONENTIAL AND LOGISTIC POPULATION GROWTH: J-CURVES AND S-CURVES ➢ As a population levels off, it often fluctuates above and below the carrying capacity. Figure 8 -4

EXCEEDING CARRYING CAPACITY Figure 8 -6

BIOTIC POTENTIAL AND ENVIRONMENTAL RESISTANCE AFFECT POPULATION SIZE biotic potential= the maximum reproductive rate of a population in ideal conditions Limiting factors

ENVIRONMENTAL RESISTANCE: LIMITING FACTORS ⦿ Density independent factors �Factors NOT influenced by the population density • They will affect the population the same, regardless of its size ⦿ Density dependent factors �Factors • influenced by population density They increase as population increases

DENSITY INDEPENDENT FACTORS Temperature changes in the environment Habitat destruction Precipitation Sunlight Salinity Soil chemistry Suitable Breeding Sites Fire, flood, earthquake, etc.

DENSITY DEPENDENT FACTORS Competition for food resources Dissolved Oxygen Predation Disease Territoriality Changes in reproductive capacity (some slow down when populations become overcrowded like mice/rats) Behavioral changes (some species resort to cannibalism and killing their young when they become overcrowded)

BIOTIC POTENTIAL: ORGANISMS FALL INTO ONE OF THE FOLLOWING TWO GROUPS • r-strategists/r-selected species • Mortality due to density independent factors • Minimally affected by invasive species; • Tend to be invasive species • Insects, fish, bacteria, algae, rodents, weeds k- strategists/k-selected species • Mortality due to density dependent factors • Adversely affected by invasive species • Sharks, birds, some reptiles, most mammals

Cockroach r-Selected Species Dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Fig. 8 -10 a, p. 168

K-Selected Species Elephant Saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Fig. 8 -10 b, p. 168

TYPES OF POPULATION CHANGE CURVES IN NATURE ➢ Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.

SURVIVORSHIP CURVES Proportion of individuals in a population that survive to a particular age

SURVIVORSHIP CURVES Type I: 1. 2. 3. 4. Long life span Young survive, death rates high among elderly K-strategists Humans Type II: 1. Constant death rate at all ages 2. K-strategists 3. Birds, rodents, flowering plants Type III: 1. High death rates among young; if live, likely to live entire life span 2. r-strategists 3. Fish, insects, amphibians, sea turtles, oysters, redwood trees, etc. (r-strategists)