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

�A community is a group of interacting populations living in the same area

�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

HOW MANY INDIVIDUALS MAKE UP A POPULATION? � Actual size �The total # of individuals in the population Direct count or estimation � Population density �The # of individuals to a given unit of area Ex. Dandelion population in a field many be the # of dandelion plants per square meter

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 � 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.

� Population density= The # of individuals to a given unit of area � Equation: �number of individuals divided by given unit of area � 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 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

� Used to be most abundant bird in North America � Early 1800 s, an ornithologist watched 2 billion birds that took 5 hours to fly over him � What happened? �We started cutting down the forests & the birds became easy targets for hunters �The last passenger pigeon died in 1914 in the Cincinnati Zoo

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

� 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

� 1. Random Distribution �individuals are not grouped in any particular pattern across an area. � 2. Uniform Distribution �Individuals � 3. are equally spaced across a given area Clumped Distribution �individuals associate in small groups or clusters that are unequally spaced across a given area Most populations live in clumps although other patterns occur based on resource distribution

�Random Distribution �Most unusual �Occurs when resources are found throughout an area and other organisms do not strongly influence where members of a population settle Windblown seeds may settle and establish themselves in an area in this manner Some spider species

�Uniform Distribution �Can occur due to… � Territoriality � Limited resources Space or Water �Plants that exhibit allelopathy

�Clumped Distribution �Most common because resources are usually concentrated in specific areas Protection (zebra herd) Social species (bees/ants)

� 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? � 29/1, 000= . 029= 2. 9%

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

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

� 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

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”

� 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 This can vary, depending on changes in the environment

Ø Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8 -6

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

Ø Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8 -4

Ø As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8 -4

Ability to Have offspring The more offspring you produce the higher your biotic potential 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

Temperature changes in the environment Weather Habitat destruction Availability of water Sunlight Salinity Soil chemistry Suitable Breeding Sites Dissolved Oxygen Fire, flood, earthquake, etc.

Competition for food resources 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)

• r-strategists/r-selected species • Mortality due to density independent factors J shaped growth curve • Insects, fish, bacteria, algae, rodents, weeds • k- strategists/k-selected species • Mortality due to density dependent factors S shaped growth curve • 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

Ø Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. l Stable: exhibits dynamic equilibrium. l Irruptive: when populations explode and crash. l Cyclic: populations fluctuate up and down l Irregular: erratic changes.

Ø Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.

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

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