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Let’s Eat!!. Trophic levels. U.S.G.S. 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

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Trophic levels

U.S.G.S.

### 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 Ecology

• 1. Density and Distribution

• 2. Growth

• a. Exponential

• b. Logistic

• 3. Life Histories

• 4. Population Limiting Factors

• 5. Human population 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

Population. Individuals of same species occupying same general area.

Density: the number of organisms in a unit area

Distribution: how the organisms are spaced in the area

Fig. 52.2

Changes in population size

Northern Pintail Duck

Growing

Fig. 52.9

Shrinking

Fig. 52.16

Fluctuating

Fig. 52.19

### Questions

• Why do populations change in size?

• What factors determine rates of population growth or decline?

• How do these differ among species?

2. Population Growth

a. exponential growth

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

number of births – number of deaths, or

???

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

????

Population Growth: exponential growth

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

???

The growth equation can be rewritten as

???

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

Population Growth: exponential growth

• Note that:

• r is constant, but N grows faster as time goes on.

• What happens with different r’s in terms of total numbers and time to reach those numbers?

Fig. 52.8

r can also be negative (population decreasing)

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

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

Exponential growth does not happen often:

Fig. 52.9 – Whooping crane

Or indefinitely:

Reindeer on the Pribalof Islands, Bering Sea

reindeer slide

## 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

### Models of population growth: incorporating competition

• 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.

### 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

### Reindeer on St. Matthew Island, Alaska

• 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.

### Reindeer on St. Matthew Island, Alaska

• 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.

### Reindeer on St. Matthew Island, Alaska

• Population growth during this period looks like unlimited growth.

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

### 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.

### 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

### Effects of competition on individuals

• 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.

### 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?

### Common features of intraspecific competition

• 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?

### Common features of intraspecific competition

• 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?

### Common features of intraspecific competition

• 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 Equation: incorporates changes in growth rate as population size approaches carrying capacity.

dN = rmaxN

dt

(K - N)

K

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?

Logistic Model

Fig. 52.12

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

### 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

How do we figure out r?

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

life history table

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

b. Life history strategies

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

b. Life history strategies

i. reproduction

“big-bang” reproduction

Vs. reproduction for consecutive years

fewer young produced per event but often more parental care

very high reproductive rates per event

b. Life history strategies

ii. mortality

Survivorship curves

Fig. 52.3

There are often trade-offs between reproduction and survival.

Fig. 52.6 - European kestrel

Reproduction has a cost when energy is limiting.

Fig. 52.5 – Red deer in Scotland

3.b. Life history strategies

iii. r- and K-selection

K-selection

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

Density-dependent selection.

r-selection

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

Density-independent selection.

Density-dependent

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

Density-independent

Any characteristic that does not vary as population density changes.

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

weather events, salinity, temperature

Density dependent: decreased fecundity

Food-limited

Space-limited

Fig. 52.14

Density dependent: decreased survivorship

Fig. 52.15

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

They represent an example of negative feedback.

They can stabilize a population near carrying capacity.

Fig. 52.13

### 4. Factors that limit population growth

• 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 growth

• Density independent survivorship or mortality

• Extreme weather events

• Fluctuations in wind and water currents

Interactions among population-limiting factors

The dynamics of a population result from the interaction between biotic and abiotic factors, making natural populations unstable.

Water temperature,

Competition,

Cannibalism.

Fig. 52.18

Population-Limiting Factors

Some populations have regular boom-and-bust cycles.

Predation

Food shortage in winter

Prey availability

Fig. 52.19

SUMMARY

Population. Individuals same species occupying same general area.

Have geographic boundaries and population size.

Key characteristics

Density. Individuals per unit of area or volume.

Distribution: uniform, clumped, random.

Demography. Studies changes in population size.

Additions (+) : Births and Immigration.

Subtractions (-) : Deaths and emigration.

Life histories. Affect reproductive output and survival rate and thus population growth.

Life history strategies are trade-offs between survival and reproduction.

Population Growth

Exponential. J-shaped. Idealized, occurs in certain conditions.

Logistic. S-shaped. A little more realistic. Carrying capacity.

K-selection. Density-dependent selection.

r-selection. Density independent selection.

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

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

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

5. Human population growth

### Questions

• 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)

Exponential growth since Industrial Revolution: better nutrition, medical care and sanitation.

Growth rates ( r )

1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%

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

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

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

Fig. 52.20

Age-structure pyramids

Fig. 52.22

BELLINGHAM

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

World Wildlife Fund for Nature

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

SUMMARY

Human population has been growing exponentially for a long time.

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

Humans appear to be above Earth’s carrying capacity.