let’s eat!!

U.S.G.S.

Let’s Eat!! Trophic levels

What do the First and Second Laws of Thermodynamics Tell us?

• First Law: Energy in = Energy Out• Until humans: • Energy =sit by fire, or in the sun• Sun: 30% reflectd, 50% converted to heat,

the rest goes to the water cycle, except <1% used by plants

• Second Law: No process is 100% efficient– Energy In = Work + Heat

What is Ecological Efficiency?• Plants absorb how much sunlight?

– 1-3%• Herbivores use how much of the plant energy?

– 10%• Where does the rest go?

– Heat and respiration• What is the efficiency of a carnivore?

– 10%• Example: Humans

– 0.02 x 0.1 x0.1 = 0.0002, 2% of the solar energy that passed through the plant, cow, human

Species interaction tactics

• Unique niches• Competition--competitive exclusion by

specialization vs. extinction• Specialization• Symbiosis--commensalism, mutualism,

parasitism• Predation• ==population ecology

Population EcologyPopulation Ecology

1. Density and 1. Density and DistributionDistribution

2. Growth2. Growtha. Exponentiala. Exponentialb. Logisticb. Logistic

3. Life Histories3. Life Histories4. Population 4. Population

Limiting FactorsLimiting Factors5. Human 5. Human

population growthpopulation growth

(Modified from a WWW site that I have lost the reference to!

Examples of applications

• Invasive species• Endangered species• Pest control (e.g., agriculture)• Human population growth

DensityDensity: the number of organisms in a unit area: the number of organisms in a unit areaDistributionDistribution: how the organisms are spaced in the area: how the organisms are spaced in the area

Fig. 52.2

Population. Population. Individuals of same species occupying same general Individuals of same species occupying same general area.area.

Changes in population sizeChanges in population size

Growing Fig. 52.9

Fig. 52.19

Fluctuating

Shrinking Fig. 52.16

Northern Pintail Duck

Questions

• Why do populations change in size?• What factors determine rates of population

growth or decline?• How do these differ among species?

2. Population Growth2. Population Growtha. exponential growtha. exponential growth

The change in population size (The change in population size (NN) in an interval of time is) in an interval of time is

number of births – number of deaths, ornumber of births – number of deaths, or

??????

If If bb (birth rate) is the number of offspring produced (birth rate) is the number of offspring produced over a period of time by an average individual, and over a period of time by an average individual, and dd (death rate) is the average number of deaths per (death rate) is the average number of deaths per individual, thenindividual, then

????????

Population GrowthPopulation Growth: exponential growth: exponential growth

The difference between the birth rate and the death The difference between the birth rate and the death rate is the per capita growth raterate is the per capita growth rate

??????

The growth equation can be rewritten asThe growth equation can be rewritten as ??????

Exponential growth occurs when resources are Exponential growth occurs when resources are unlimited and the population is small (doesn’t unlimited and the population is small (doesn’t happen often). The r is maximal (happen often). The r is maximal (rrmaxmax) and it is called ) and it is called the the intrinsic rate of increaseintrinsic rate of increase..

Population GrowthPopulation Growth: exponential growth: exponential growth

Fig. 52.8

Note that:1. r is constant, but N grows

faster as time goes on.2. What happens with

different r’s in terms of total numbers and time to reach those numbers?

r can also be negative (population r can also be negative (population decreasing)decreasing)

if r is zero, the population does not if r is zero, the population does not change in sizechange in size

thus, the rate of increase (or decrease) thus, the rate of increase (or decrease) of a population can change over time.of a population can change over time.

Exponential growth does not happen often:Exponential growth does not happen often:

Fig. 52.9 – Whooping crane

reindeer slide

Reindeer on the Pribalof Islands, Bering Sea

Or indefinitely:

Unlimited growth, carrying capacity, and limited growth

Models of population growth

• This is the simplest model of population growth for species with discrete breeding seasons.

• In this model, there is no competition, and population dynamics are governed solely by the net reproductive rate, R.

• If R > 1, the population increases indefinitely and exponentially.

Nt+1 = NtR

Or

Nt = N0Rt

• Graphically, we can see that the population increases exponentially when Nt is very low.

• But the rate of increase declines as population size rises.

• At carrying capacity, the growth rate is zero.

• Above carrying capacity, the population will decline.

• K is therefore a stable equilibrium.

Models of population growth: incorporating competition

St. Matthew Island, Alaska

Reindeer on St. Matthew Island, Alaska

• In 1944, 29 reindeer introduced to St. Matthew Island (300 km2)

• Approximate initial density 0.1/km2

• 24 females, 5 males, all 2 years old

• R. Rausch visited the island in 1954, and on the basis of counts, estimated the population size at 400-500.

• C.J. Rhode visited the island in 1955, and estimated the population size at 700-900.


• David Klein visited the island in 1957, and made a total count of 1,350 animals.

• This implies an average annual growth rate of 34 percent.

• Klein assumed that the population growth rate earlier in the explosion must have been near the theoretical maximum for the species.


• Population growth during this period looks like unlimited growth.

• Klein recognized the potential importance of this study during his 1957 visit.


Natural mortality was assessed from skeletons

Physical condition was assessed from animals shot during fieldwork

At a density of 4.5 inds./km2, the animals were in excellent condition

• Noticeable, extensive fat deposition, especially on large males

• Weights of all reindeer collected exceeded the average weight range for other Alaskan reindeer

• No external parasites noted• Very large and uniform antler growth on

males and females

What contributed to the unlimited growth and excellent condition of reindeer on St.

Matthew Island?• Abundant winter and summer forage• No competitors• No large predators• No large herbivores had been there

previously• But Klein sensed there was trouble on the

horizon

Signs that limits to population growth were imminent in 1957

• Lichen beds were showing signs of fracturing due to overgrazing and trampling (winter range)

• Prostrate willows were also showing signs of heavy browsing (summer range)

• Calf percentage of 26% was “below the indicated level of previous years”

• Klein concluded/warned that “the population decline may be rapid after the peak is reached”.

What happened next:• Klein revisited the island in 1963 and surveyed it with 2 Coast

Guard helicopters.

• “As their boots hit the shore, they saw reindeer tracks, reindeer droppings, bent-over willows, and reindeer after reindeer.” – Ned Rozell, Alaska Science Forum

• The survey revealed the population had increased to 6000• Calf percentage was lower than in 1957• Recruitment was down from 29% in 1957 to 17% in 1963

• “There was ample evidence of overpopulation, and the stage was apparently set for wholesale die-off.”

What happened next:

• May 1964: an aerial survey of the island located no reindeer. “We were unaware, of course, that a die-off had already taken place.”

The introduction, increase, and crash of reindeer on St. Matthew Island

Klein, D.R. 1968. J. Wildl. Manage. 32:350-367.• Upon returning in 1966,

Klein found only 42 reindeer;

• Of these, 1 male; the rest were females 2yrs old and older

• No calves or yearlings, indicating the crash die-off probably occurred in late winter 1964.

What caused the crash die-off?

• Extremely high density (20/km2)• Unusually harsh winter in 1963-64 (exceptionally

cold, with unusually deep snow)• Long bones of examined skeletons contained no

marrow fat, indicating starvation• Many skeletal remains were found in groups,

suggesting the animals died over a very short period.

• By the mid 1980s, there were 0 reindeer on the island.

Sex and age composition of the die-off

Compare natural mortality (1957) with crash die-off (1966)

• Physical characteristics of the animals in 1957 and 1963:

• Avg body weight declined by 38% for adult females and by 43% for adult males

• Not only were they smaller just before the crash, regressions between body weight and skeletal parameters indicated growth rates were lower in 1963

• Lichens had been completely eliminated as a significant component of the winter diet

Carrying capacity

• Klein (1968) suggested that forage quantity primarily governs population size, while quality determines the size of the individual.

• The winter component governs the upper limit of the population, and the summer component determines the stature of the individual.

Klein (1968) attributed the large-scale die-off to the following factors:

• Overgrazing of lichens, with no possibility of the reindeer expanding into alternative range;

• Excessive density of reindeer competing for a very restricted winter resource;

• Relatively poor condition of reindeer going into the winter of 1963, resulting from intense competition;

• Extreme weather conditions, primarily deep snow, during the winter of 1963-64.

Intraspecific competition and carrying capacity

• Competition may be defined as (Begon et al. 1984):

An interaction between individuals, brought about by a shared requirement for a resource in limited supply, and leading to a reduction in the survivorship, growth, and/or reproduction of the competing individuals.

Effects of competition on individuals

• Increased energy expenditure (searching for the unexploited resource), increased risk of mortality, and decreased rate of food intake may all decrease individual’s chances of survival

• Increased energy expenditure and decreased food intake may leave less energy available for development and less available for reproduction.

• Increases in density will therefore decrease the contribution made by each individual to the next generation.

Effects of competition on individuals

Common features of intraspecific competition

• The ultimate effect of competition is a decreased contribution to the next generation;– Intraspecific competition leads to decreased rates of

resource intake per individual, decreased rates of individual growth or development, or to decreases in the amounts of stored reserves;

– These may lead to decreases in survival and/or fecundity.

– Evidence from St. Matthew Island?

• The resource for which individuals compete must be in limited supply– Competing individuals might or might not interact

directly;– Exploitation competition occurs when individuals

remove an item needed by others;– Interference competition occurs when individuals

interact directly and prevent others from occupying a portion of habitat and exploiting its resources;

– Which type presumably occurred on St. Matthew Island?


• The competing individuals are in essence equivalent, but in practice they are not– “One-sided reciprocity” or “Asymmetric competition”;

– The effects of competition are not the same on all individuals in the population;

– Evidence of asymmetry on St. Matthew Island?


• The likely effect of competition on any individual is greater the more competitors there are.

– The effects of intraspecific competition are thus said to be density dependent.


Logistic Growth EquationLogistic Growth Equation: incorporates changes in : incorporates changes in growth rate as population size approaches carrying growth rate as population size approaches carrying capacity.capacity.

dNdN = r = rmaxmaxNN dtdt

(K - N)(K - N) KK

Fig. 52.10

At what point is the “effective” r the highest?At what point are the most individuals added to the population?Are these the same?

Fits some populations well, but for many there is not stable Fits some populations well, but for many there is not stable carrying capacity and populations fluctuate around some carrying capacity and populations fluctuate around some long-tem average density.long-tem average density.

Logistic Model

Fig. 52.12

3. Life Histories

• How do we figure out r for different populations?

• What accounts for different patterns or rates of population growth among different species?– For example, different rmax

a. Life History Tablesa. Life History Tables : follow a cohort from : follow a cohort from birth until all are dead.birth until all are dead.

life history table

How do we figure out r?

Reproduction TablesReproduction Tables : follow a cohort from : follow a cohort from birth until all are dead.birth until all are dead.

b. Life history strategies

Life histories are determined by traits that Life histories are determined by traits that determine when and how much an determine when and how much an organism reproduces and how well it organism reproduces and how well it survives.survives.

““big-bang” reproductionbig-bang” reproduction Vs. reproduction for Vs. reproduction for consecutive yearsconsecutive years

very high reproductive rates very high reproductive rates per eventper event

fewer young produced per fewer young produced per event but often more event but often more parental careparental care

b. Life history strategies i. reproduction

Survivorship curvesSurvivorship curves

b. Life history strategies ii. mortality

Fig. 52.3

There are often There are often trade-offs between reproduction and survivaltrade-offs between reproduction and survival. .

Fig. 52.6 - European kestrelFig. 52.6 - European kestrel

Reproduction has a cost when energy is limiting. Reproduction has a cost when energy is limiting.

Fig. 52.5 – Red deer in ScotlandFig. 52.5 – Red deer in Scotland

Near carrying capacity natural selection will favor traits that Near carrying capacity natural selection will favor traits that maximize reproductive success with few resources (high densities).maximize reproductive success with few resources (high densities).

Density-dependent selection.Density-dependent selection.

K-selectionK-selection3.b. Life history strategies3.b. Life history strategies

iii. r- and K-selectioniii. r- and K-selection

Below carrying capacity natural selection will favor traits that Below carrying capacity natural selection will favor traits that maximize reproductive success in uncrowded environments (low maximize reproductive success in uncrowded environments (low densities).densities).

Density-independent selection.Density-independent selection.

r-selectionr-selection

Any characteristic that varies according to a Any characteristic that varies according to a change in population density.change in population density.

Density-dependentDensity-dependent

Any characteristic that does not vary as Any characteristic that does not vary as population density changes.population density changes.

Density-independentDensity-independent

food availability, territories, water, nutrients, food availability, territories, water, nutrients, predators/parasites/disease, waste accumulationpredators/parasites/disease, waste accumulation

weather events, salinity, temperature weather events, salinity, temperature

Space-limitedSpace-limited Food-limitedFood-limited

Density dependent: decreased fecundityDensity dependent: decreased fecundity

Fig. 52.14

Density dependent: decreased survivorshipDensity dependent: decreased survivorship

Fig. 52.15

Density-dependent changes Density-dependent changes in birth and death rates slow in birth and death rates slow population increase.population increase.

They represent an example They represent an example of negative feedback.of negative feedback.

They can stabilize a They can stabilize a population near carrying population near carrying capacity.capacity.

Fig. 52.13

• Density dependent birth and death rates (as we just discussed). Many of these reflect – competition for resource (food/energy,

nutrients, space/territories).– predation, parasites, disease– waste accumulation (e.g., ethanol)

4. Factors that limit population growth4. Factors that limit population growth

• Density independent survivorship or mortality– Extreme weather events– Fluctuations in wind and water currents

4. Factors that limit population growth4. Factors that limit population growth

Interactions among population-limiting factorsInteractions among population-limiting factorsThe dynamics of a population result from the interaction The dynamics of a population result from the interaction between biotic and abiotic factors, making natural populations between biotic and abiotic factors, making natural populations unstable.unstable.

Water temperature,Water temperature,Competition,Competition,Cannibalism.Cannibalism.

Fig. 52.18

Population-Limiting FactorsPopulation-Limiting FactorsSome populations have regular boom-and-bust cycles.Some populations have regular boom-and-bust cycles.

PredationPredationFood shortage in Food shortage in winterwinter

Prey availabilityPrey availability

Fig. 52.19

Population. Population. Individuals same species occupying same general area.Individuals same species occupying same general area.Have geographic boundaries and population size.Have geographic boundaries and population size.

Key characteristicsKey characteristics Density.Density. Individuals per unit of area or volume. Individuals per unit of area or volume. Distribution:Distribution: uniform, clumped, random. uniform, clumped, random.

Additions (+) : Births and Immigration.Additions (+) : Births and Immigration.Subtractions (-) : Deaths and emigration.Subtractions (-) : Deaths and emigration.

SUMMARYSUMMARY

Demography. Demography. Studies changes in population size.Studies changes in population size.

Life histories. Life histories. Affect reproductive output and survival rate and Affect reproductive output and survival rate and thus population growth.thus population growth. Life history strategies are trade-offs between survival and Life history strategies are trade-offs between survival and reproduction.reproduction.

Population GrowthPopulation Growth Exponential. J-shaped. Idealized, occurs in certain conditions.Exponential. J-shaped. Idealized, occurs in certain conditions. Logistic. S-shaped. A little more realistic. Carrying capacity.Logistic. S-shaped. A little more realistic. Carrying capacity.

K-selection. Density-dependent selection.K-selection. Density-dependent selection. r-selection. Density independent selection.r-selection. Density independent selection.

Population growth is slowed by changes in birth and death Population growth is slowed by changes in birth and death rates with density.rates with density.

Interaction of biotic and abiotic factors often results in unstable Interaction of biotic and abiotic factors often results in unstable population sizes. In some populations they result in regular population sizes. In some populations they result in regular cycles.cycles.

6,417,531,489 people (as of 9:30, Feb. 8, 2005)

5. Human population growth

Questions1. 1. Human growth

For example,- What factors are correlated with changes in human population

growth rate?– How long has Earth’s population been similar to what it is now?– Over what time period has the human population shown the

greatest change in numbers?2. How do the patterns compare with what we have just

studied about natural patterns of population growth?3. What new questions does this raise for you?

Human Population= 6,339,110,260 (this morning)Human Population= 6,339,110,260 (this morning)Exponential growth since Industrial Revolution: better nutrition, Exponential growth since Industrial Revolution: better nutrition, medical care and sanitation.medical care and sanitation. Growth rates ( r )Growth rates ( r )1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%

Growth will slow Growth will slow down either due to down either due to decreased births or decreased births or increased deaths.increased deaths.

Likely both as Likely both as suggested by age-suggested by age-structure pyramids: structure pyramids: relative number of relative number of individuals in each individuals in each age-class.age-class.

http://www.ibiblio.org/lunarbin/worldpop

Fig. 52.20

Age-structure pyramidsAge-structure pyramids

Fig. 52.22

BELLINGHAMBELLINGHAM

CensusScope

When and how will human population growth stop?

• This question is likely to be answered one way or another in your lifetime.

• What is Earth’s carrying capacity for human’s?

• Have we already exceeded K?• What are consequences of human

population growth for other species on this planet?

Human impact

• Depends on – Total human population– Consumption by each individual– Ecological impact of each unit of consumption

• I = PAT (Ehrlich and Ehrlich)– P = population– A = affluence– T = technology

Unknown what Unknown what the carrying the carrying capacity of Earth capacity of Earth for humans is. A for humans is. A useful concept is useful concept is the ecological the ecological footprint: land footprint: land needed to needed to produce produce resources and resources and absorb wastes absorb wastes for a given for a given country.country.

World Wildlife Fund for NatureWorld Wildlife Fund for Nature

Fig. 52.23 – Ecological footprints for various countries and the world

Human population has been growing exponentially for a Human population has been growing exponentially for a long time.long time.

A reduction is expected either through lower birth rates or A reduction is expected either through lower birth rates or higher death rates. The age-structure suggest different higher death rates. The age-structure suggest different scenarios for individual countries.scenarios for individual countries.

Humans appear to be above Earth’s carrying capacity.Humans appear to be above Earth’s carrying capacity.

SUMMARYSUMMARY

let’s eat!!

Documents

population growtha

population sizegrowingfig

population dynamics

rates of population

growth rate

population size n

unlimited growth

growth equation