EE Bio 100 (Ecology)

Ecology

The scientific study of our home/environment

Study of relationships between organisms and their environments

"study of our house"

History of Ecology

18th-19th century widespread exploration to discover and claim the natural world

Darwin and Wallace

Today human ecology -> how humans impacting the environment and what can we do to protect it

Global Living Planet Index

scientific analysis of the health of the planet that assesses the vitality of life systems given the burdens imposed by human activity

Measure of the world's biodiversity based on population trends of vertebrate species and has been adopted by convention of biological diversity (CBD) as indicator of progress

reduced number oof organisms over 50% loss of species

Fisheries Downfall

Not enough individuals in species to be harvested -> all fisheries collapsed by 2050

Flying insect abundance fallen

Arable land per capita is decreasing -> as the human population increases, available resources per capita decreases

Downward Curve

We do not see/feel our impact until we are at the bottom of the curve

Ecology problems and solutions

Save biodiversity -> save tropical rainforests, coral reefs, and natural ecosystems, stop climate change

How -> stop burning fossil fuels, reduce consumption and wast from animal based foods, lower human fecundity

Example policy

Policy and science not complementary -> science tells us dams may be worse than fossil fuels

Why society ignore ecologists?

Ecologists do not work in real world

nobody reads scientific publications

scientific knowledge is not disseminated widely

lack of core ecological education

it would mean major change to our lifestyles

nobody cares

economics

politics

greed, power, and money

racism

theoretical ecology

investigations of specific organisms and environments that lead to the development of theories about how the natural world works

examples: soil food webs, nutrient cycling, community assembly, animal behavior

ecology does not conservation

but ecology is scientific backbone of conservation

Applied Ecology

applying ecological science to solve real world systems

-> centered around conservation

examples:

sustainably manage resources

restore degraded ecosystems

conserve endangered species

save biodiversity on Earth

How can we perform real applied ecology?

Ecologists get involved outside of science - in policy, on-the-ground projects

conduct collaborative research with people who will directly apply findings

disseminate knowledge through non-academic circles -> pampas wolf story (did not share information)

be advocate for science

get out in the streets/get political

promote ecological education

Community based science

Count turtles through using K-12 kids

What is a population?

A. A group of people that live in the same place

B. A group of individuals of the same species that inhabit a given area

C. A group of individuals that are able to interbreed

D. B and C

D. B and C

Properties of Populations

Populations have size and structure -> (number of organisms/individuals)

- abundance

- density

- spacing

- age distribution

Populations are dynamic, changing over time

Why study Population Ecology?

Helps with conservation, because in conservation we need to know:

- which species are threatened

- population too small -> what is the viable population size for a species

- Cause of decline -> which part of the population is in decline, and why, is decline related to size, density, distribution, or range, problem of numbers or genetic diversity, habitat change, or something else

- are conservation efforts working -> see population overtime is increasing or decreasing, is a species population increasing, decreasing, or stable, change in distribution/density

Distribution of a population

- Distribution of a population is the AREA over which it occurs (where individuals are present) -> where find a population

Why do we care about species distribution?

- What are the habitat requirements

- are there threats to the habitat or threats in the habitat

- what other species share the habitat

- how to preserve the habitat -> where is it?, who owns it?, what is the population?

- climate effects/shifts?> -> affects species distribution because it affects habitat

geographic range

Encompasses all of the individuals of a species, usually many populations

individuals are found in suitable habitats within that geographic range

the range is limited by

- abiotic factors -> temperature, soil moisture, elevation

- biotic factors -> predation, competition, parasitism (amazon intact because malaria)

distribution of entire species -> preferred range and temperature

Red Maple

the range of the red maple is limited by temperature in the north and drier conditions midwest

Geographic barriers limit species range

Reduce/prevent individuals colonizing new areas -> bodies of water, mountains, deserts

geographic barriers limit species distribution

Endemic species (species found in limited spaces) often have specialized habitat requirements

Rare vs. Widespread Species (Waldron 2010)

Endemic species more likely to become extinct but study found opposite for primate species -> widespread went more extinct than endemic -> have major changes in environment and habitat become fragmented and these are species that depend on large areas of habitat whereas endemic only have little bit of habitat but widespread not adapt to such a small space

Threat tolerance is negatively associated with geographic range size

When climate change reduces habitat to small, isolated fragments, tolerance becomes more important to species persistence, and rare species may gain the persistence advantage (at least those that lie within distributional reach of an environmental refuge)

Metapopulations

Populations divided into subpopulations that live in suitable habitat patches surrounded by unsuitable habitat

the environment is heterogeneous

spatially separated but connected by the movement of individuals (gene flow) -> not separated socially

some species adapted in metapopulation -> not homogenous but have patches -> each patch too small for a population -> key to survival is work as larger population and travel to maintain diversity and genetic viability -> gene flow between populations/subpopulations

Human altered landscapes (habitat fragmentation) often force species to function as metapopulations

Mountain lions in LA -> forced to live in metapopulations due to habitat fragmentation

viable overtime -> increase gene flow

What is a metapopulation?

The following is MOST important for survival of a metapoluation:

A. One big population with a large range

B. A huge number of subpopulations

C. Gene flow between subpopulations

D. genetic variability between subpopulations

C. Gene flow between subpopulations

do not want genetic differences means lack of gene flow

3 kinds of distribution

1. Random

2. Uniform

3. Clumped

Random Distribution

The position of one individual is independent of another.- The scattering of plant seeds by the wind can lead to a random distribution of plants after the seeds germinate.

Uniform Distribution

Organisms are found at a regular distance from one another

Often the result of negative interactions among individuals such as competition

Equal allocation -> highly competitive and territorial -> avoid competition or predation with others in population -> ex. Nesting Shorebirds

Acacia: uniformly spaced because of competition for water and nutrients -> trees because of competition for resources

Clumped Distribution

Individuals are found in groups

This is the most common spatial distribution and results from a number of factors

- Suitable habitat or resources are found in patches

- Species form social groups (herds, flocks, schools)

- Ramets formed by asexual reproduction.

resources in certain areas and individuals interact in social/positive way

Spatial distributions of individuals may be described at multiple spatial scales

Forms clumped and uniform because availability of resources

How do we quantify populations?

Abundance: # of individuals in the population

Population density: # of individuals / area

Why care about abundance/density?

More dense more competition and detrimental to species

What is a viable population size?

Determine trends over time.

Crowding

- Competition, resource decline, disease, etc.

Too sparse

- Reproduction effects, social breakdown, decline, (Allee effect), etc.

Determining Abundance Requires Sampling

A complete count may be possible if both the abundance and area occupied are small, or if an area is very open so that all individuals can be seen.

Determining Abundance by Sampling of Density

If an organism is sessile (attached), like a plant or coral, sampling can be done using quadrats / sampling units

- Area is divided into subunits

- # of individuals counted in a random sample of subunits

- Mean density x Total area = Estimate of population size

works well for stationary/slow-moving species

Determining Density Requires Sampling

Depends largely on the spatial distribution of individuals in the population

- works well if individuals have a uniform distribution

- works less well with a random or clumped distribution

important to report a confidence interval or some estimate of variation

Randomly sampling - # of samples matters!

Number of samples really matter -> more samples the better -> when number is stabilized know correct number of samples -> used for organisms that move around

Determining Density Requires Sampling (mark and recapture)

Used for organisms that move around

Mark-recapture is the most commonly used technique to measure animal population size.

This method is based on

- capturing a number of individuals in a population

- marking them

- releasing of marked individuals (M) back into the population

- after an appropriate period of time, recapture a sample of the population

Mark and recapture equation

N = Total population (what you want to know)

m = # of initially captured and marked individuals

s = # of captured animals on the second visit

r = # of animals with marks on the 2nd visit

Ratio: N/m (Total pop/ # in initial capture) = s/r (# in 2nd capture/ # with marks in 2nd capture )

N/m=s/r

N = MS / r

Assumptions -How often do we meet these?

1. No effect of marking on probability of recapture -tags should not be obvious or slow the individual, or reduce fitness in any way. (How might fitness be reduced?) -> equally easy to capture individuals the second round does not work if injuring or negatively impacting the animals

2. Mixing of marked and unmarked - mix into the entire population (how much time between sampling events)

3. Captured individuals are representative of the whole population, not a certain age group or one sex vs another, e.g., only weak individuals -> method of capture should not favor only males or females or specific age (old) -> equal probability of any species -> turtle males always in the water

4. Marks are not lost -> not strip of paint

Methods of Marking

Tags

Leg bands

Pit tags

Paint

Chopping off toes

Etc. Do these affect fitness?

toes -> make sure benefit outweigh harm and minimize negative effect

You capture and mark 80 snails by putting a small spot of white paint on their shells. When you return five months later, you capture 45 snails and 5 of them have the mark. Based on these data, the population has ________________ individuals.

720

What if you conduct mark recapture, and when you go to recapture, zero of the animals you capture have a mark? What size is your estimated population?

What if you conduct mark recapture, and when you go to recapture, 100% of the animals you capture have a mark? What size is your estimated population size?

Are these reliable estimates? Why/not?

- too small sample size -> think about method, maybe mark came off or negatively affected survival of animals or animals figured out how to not be marked again

Individuals did not mix in the population once marked

Determining Density Without Direct Counts

Signs of the presence of animals include:

- counts of vocalizations, such as bird song

- counts of animal scat seen along a length of trail

- counts of animal tracks, such as footprints in snow

environment DNA or other marks animals leave

How would you sample populations of the giant Amazon river turtle?

Situation:

Individuals stay submerged until the nesting season

Individuals weigh a LOT, and are very difficult to capture from the water

Individuals migrate great distances throughout the year.

mark and recapture -> only reproductive females as indicator reproduction viability

counted all the nests on a sample of nesting beached

also measured and counted turtle tracks

Population Structure - Age, Developmental Stage, Size

Abundance doesn't provide information on a population's characteristics

Why would you want to know the age structure of a population?

- Is the population in growth or decline? (e.g. turtles)

E.g., Overhunted populations will have younger/smaller populations.

- Reproductive effects

A population with non-overlapping generations does not have an age structure

e.g., annual plants and some insects -> same age such as plants and insects -> different ages have different survival rates

A population with overlapping generations has an age structure

pre-reproductive, reproductive & post-reproductive

mortality is more common in certain age classes

Population Structure - Age Pyramids

Egypt - Growing -> pyramid

US -> stable -> rectangle

Japan -> aging -> inverted pyramid

Measures of Population Structure Include Age, Developmental Stage, and Size

The most accurate method is to mark young individuals in a population and follow their survival through time• Dendrochronology -counting annual growth rings to determine the age of a tree

- Diameter can give an imperfect estimate!

Individuals Move within the Population

Dispersal is the movement of individuals in space

- emigration -when individuals leave a subpopulation

- immigration -when individuals enter a subpopulation

Movement of individuals is an important part of meta-population dynamics

- maintains gene flow between the subpopulations

Organisms do not stay in the same place -> dispersal

Why should we care about movement within a population?

How is movement related to estimates of abundance? How is movement necessary for survival?

Has movement been affected e.g., by habitat loss or change?

What habitats are required by the population?

Migration

Migration is movement of organisms that is round-trip

- zooplankton move in the water column; lower depths during the day and the surface at night

- bats leave caves at dusk, move to feeding areas, then return at dawn

- earthworms move deep into the soil for winter to avoid freezing, then move back up in the spring

- gray whales feed in the Arctic during the summer, winter off the California coast where calves are born

Caribou migration (largest land migration): Under threat things happen along migration route

What is Life History?

Theory of biological evolution that seeks to explain how aspects of organisms' anatomy, behavior, reproductive development, and life span have been shaped by natural selection -> way we are (attributes we have) are that way for a reason -> traits of different species -> how functions and conservation solutions

An organism's lifetime pattern of growth, development, and reproduction

- At what age/size do organisms mature?

- How do organisms reproduce?

- How long do they live?

- How many offspring do they produce?

- At what age do they stop reproducing?

Why study species life histories?

How to conserve / sustainably harvest a fish?

How to conserve / sustainably harvest a turtle?

How to conserve / sustainably harvest a mammal?

From what we know about fish and turtles (Congdon et al.)

What are good resource management strategies for each?

Example fish:

10 year lifespan

Age at maturation ~ 5 yrs

Senescence

High juvenile mortality; lower mortality in mid-life; accelerating mortality with age

No post-reproductive phase: larger females have more successful reproduction

Example turtle:

45+ year lifespan

Age at maturation ~ 15 yrs

Negligible senescence?

High juvenile mortality; negligible adult mortality

No post-reproductive phase: larger females have more successful reproduction

Fish -> when reproduction happening -> do not harvest reproductive individuals

Turtles -> make general limits -> adults also contribute to the population

Post-reproductive age -> own species very important-> remove have less impact than one still in the reproductive phase

What about sex?

Maintain naturally evolved sex ratio.

Conserve reproductive stages of BOTH sexes.

Females may be especially important -> only target female turtles because can only get to them

Evolution of Life Histories Involves Trade-offs

NOTHING IN LIFE IS FREE! - IF IT SOUNDS TOO GOOD TO BE TRUE, IT IS!

Individuals have a limited amount of resources that can be allocated to specific tasks -> no species has every trait maximized

An allocation to one aspect reduces the resources available for other aspects

- E.g., Allocating resources to reproduction reduces the resources available for growth.

can't grow and reproduce at the same time -> delay maturity to allocate to growth

Trade Offs

These trade offs include:

1. mode of reproduction

2. age at first reproduction

3. allocation to reproduction

4. number and size of eggs, young, or seeds

5. timing of reproduction

These trade-offs are imposed by genetic, physiological, energetic, and environmental constraints (biotic & abiotic)

Depending on the environment, some life history traits will increase fitness, others will lower fitness

Trade-off: Mode of Reproduction Sexual vs. Asexual Reproduction

Benefits of asexual reproduction

- Offspring are genetically identical to parent, so well adapted to the local environment

- All individuals can reproduce, potential for high population growth

Benefits of sexual reproduction

- Genetic recombination - more variation among offspring -> Better ability to respond to changes in environmental conditions

- Removes bad genes from the population

Costs of sexual reproduction

- Requires specialized reproductive organs

- Production of gametes and mating energetically expensive

Trade Off: Age at First Reproduction /Delayed Reproduction

A number of species show a positive correlation between body size and fecundity

Allocation to reproduction often reduces allocation to growth

Individuals that postpone reproduction for growth will (often) have more offspring per reproductive period

Correlation between body size and fecundity (number of offspring)

Graph of annual tree rings and mean number of cones per tree

More cones made -> less energy for growth

Douglas fir trees exhibit an inverse relationship between the allocation to reproduction (number of cones produced) and annual growth (as measured by radial growth)

When to Reproduce?

Intro to Life Tables:

- Ix shows the probability of an individual surviving to age x (from birth)

- sx shows the probability of an individual at age x surviving to age x+1

E.g., probability you will make it from age 1 to 2

- bx is average number of female offspring produced by an individual at age x

Natural selection will favor those individuals whose age at maturity results in the greatest number of offspring produced over the lifetime of an individual

X -> age

Ix -> probability survive to that age

Sx -> probability survive to next age

bx -> fecundity (number of average offspring at that age)

R0 -> replacement -> 1 is replacement

R0

R0 = average number of offspring produced over lifetime

R0 = 1 = replacement/stable

R0 > 1 increasing

R0 < 1 in decline/decreasing

Net reproductive rate: R0 = sum of all lx x bx (R0 = 1 is replacement)

R0: Average number of offspring produced by an individual in its lifetime

Do you want to harvest all the individuals in one life stage or all post reproductive individuals?

No, only take out a small percent

Which female has higher TOTAL fecundity? What is that dependent on?

Female 1 - starts to reproduce at age 3 and has 10 offspring each year until death

Female 2 - delays reproduction until 5 and has more body mass therefore can have 15 offspring each year until death

All depends on survival

Die young first option is better, but die later means 2nd option better

maturity -> Females that delay maturity by two years will have a greater total number of offspring if they live past age 8

Age at Maturity Is Influenced by Patterns of Age-Specific Mortality

Need to survive to that age:

Natural selection favors:

- earlier maturation when adult survival is low compared to juvenile survival

- delayed maturation when juvenile survival is low compared to adult survival

Look at costs and benefits.;

The benefit of delaying maturity is a larger body size at age of first reproduction -> increase in fecundity

The cost of delaying maturity is the increased risk of death before reproducing

Example of natural selection at work

Predators target specific size class of prey, affecting age specific mortality -> species interaction affects reproduction

Trade Off: Energy Allocation to Reproduction

The amount of energy invested in reproduction varies widely across organisms (as % of annual energy budget)

- female Allegheny Mountain salamander - 48%

- some grain crops (corn and barley) - 35 to 40% -> larger amount of energy to reproduction than wild plants because of human/artificial selection

- wild annual plants - 15 to 30%

- herbaceous perennials - 15 to 20%

- common lizard - 7 to 9%

Reproductive Effort Is Governed by Trade-offs between Fecundity and Survival

Reproductive effort - the total energetic cost of reproduction per unit of time

The energetic costs of reproduction include:

gonad development

movement to breeding area

competition for mates

production of gametes

nutrient demands

nesting

parental care

These vary by species!

Brood enlargement study of European Kestrels (Dijkstra et al.):

Study showed sweet spot to reproduction -> parents can only take care of certain amount of offspring

Both parents hunted more

Adult survival declined

Food intake per chick declined

Nestling growth rate was reduced

Nestling mortality increased.

Law of Diminishing Returns

Sweet point in spending energy on reproduction

more siblings in your house decreases your chance of survival -> parents have more offspring increase biological fitness to a point then starts to decrease

As the number of offspring produced per reproductive effort increases, there is a corresponding decline in their probability of survival

The current reproductive success (number of offspring produced times their probability of survival) reaches a maximum value at intermediate values of reproductive effort

Natural selection maximizes parental fitness -> parental fitness is often at its maximum at intermediate values of reproductive allocation -> need to find intermediate-level -> more energy you put in less energy you have later which means as you put energy now reproductive success in the future decreases

Which is true?

A. Organisms produce few large offspring

B. Organisms produce many small offspring

C. Organisms produce few small offspring

D. A & B

D. A & B

Many small and fewer large

Many small

Each individual receives a smaller investment

- for plants - little energy stored in seeds

- for animals - little energy in parental care

These species often live in unpredictable environments or areas where parental care is difficult, so the probability of survival is lower

Large numbers increase the chance that a few will survive and reproduce.

Fewer large

fewer large -> makes sense in organism that has higher rates of parental care, safe and stable environment

Each individual receives a greater investment

- for plants - more energy stored in seeds

- for animals - more energy in parental care

Trade Off: Timing of Reproduction

Iteroparous organisms reproduce more than once -> multiple iterations of reproductive events

What are some examples of iteroparous species

- include most vertebrate animals, shrubs, trees, and perennial plants.

Species Differ in the Timing of Reproduction

Semelparous organisms reproduce only once -> one reproductive event in entire lifetime -> die after one reproductive event -> put all investment in one event

- initial energy investment to growth, development, and energy storage

- one large reproductive effort

- then the organism dies, sacrificing all future reproduction

Examples of organisms?

- This is seen in most insects, annual (short lived plants) and biennial plants, some fish (salmon)

Pacific Salmon:

Reproduction timing cons and pros

Semelparity would be favored if the external environment leads to high adult mortality relative to juvenile mortality

- little chance of future reproduction

Iteroparity would be favored if juvenile mortality is high relative to adult mortality

- good chance of future reproduction -> when adult survival is high

Patterns of Life History Characteristics

A species with high adult mortality has

- a shorter life span

- faster rate of development

- early age of maturity

- higher fecundity (per unit time)

A species with low adult mortality has

- a longer life span

- slower rate of development

- late age of maturity

- lower fecundity (per unit time)

Survivorship Curves

Type I survivorship -> juvenile survival is high and most mortality occurs among older individuals

Type II survivorship -> die at equal rates regardless of age -> equal levels of survival throughout life

Type III survivorship -> die at high rate as juveniles and then at much lower rates later in life

r-selected species

population size:

- more sporadic growth rate

- limited by reproductive rate (r)

- density independent

- relatively unstable

Organisms:

- smaller, short-lived

- produce many offspring

- provide no care for offspring

K-selected species

Population size:

- limited by carrying capacity (k)

- density dependent

- relatively stable

Organisms:

- larger, long lived

- produce fewer offspring

- provide greater care for offspring

Which of the following traits does not seem to fit into the following species' life history?

A.Long-lived

B.Delayed maturity

C.Small body size

D.High adult survival

C. Small body size

Which of the following traits does not seem to fit into the following species' life history?

A.Short-lived

B.Early maturity

C.Small body size

D.High levels of parental care

D. High levels of parental care

Exponential Growth Model

The exponential population growth model is based on several assumptions about the environment

dN/dt = rN = (b − d)N

- dN/dt = change in population size ("growth rate")

- r = intrinsic rate of increase -> growth rate that can change in population overtime

- b = birth rate

- d = death rate

- N = population size

Assumptions -> does not account for resources -> false assumption population can grow infinitely -> no population can grow infinitely

The Environment Functions to Limit Population Growth

The exponential model assumes:

- unending (constant) growth

- constant birth/death rates

- unlimited essential resources

These are not possible in natural populations.

- Resources are limited

- As the density of a population increases, demand for resources increases

What happens to the population when resource consumption is greater than replenishment?

The resource base shrinks, and there is increased competition for those resources

Intraspecific Population Regulation

Limitations are imposed on population growth by other members of the same species.

- What limitations (interactions)? -> conflict, availability/competition for resources

- How can these interactions regulate the population's size?

Mortality (d) increases, fecundity (b) decreases, or both - This means that b (birth) and d (death) rates are not constant

- No population is able to grow indefinitely!

Simple growth model

d0 = minimum death rate

b0 = maximum birth rate under ideal conditions

c is the slope of the death rate

a is the slope of the birth rate

growth rate: r = b - d

k = where birth rate equals death rate and stable

The Environment Functions to Limit Population Growth Equation with limitation on growth

dN/dt = [(b0 − aN) − (d0 + cN)]*N

As N increases, the birthrate declines and death rate increases; this results in a slowing of population growth

What happens when b = d?

N (K) = (b0 − d0)/(a + c)

Note: b0, d0, a, c are all constants, therefore this value of N is a constant, and we define it to be K

Carrying Capacity

The population size at which birth rate is equal to death rate is the maximum sustainable size in the current environment.

K = b0 - d0 / (a + c) -> all constants based on attributes of the environment -> maximum sustainable population size

K is the carrying capacity: max sustainable population size for a prevailing environment

Can K change?

A. No, K for a species is always the same

B. Yes, K for a species can increase

C. Yes, K for a species can decrease

D. B & C

D. B & C

Population Equation with Carrying Capacity

ON TEST

dN/dt = rN(1 − N/K)

This is the logistic model of population growth

This model has two components:

- rN = exponential growth -> r is defined as a constant = b0 - d0

- (1 − N/K) = slowing of population growth

(1-N/K)

The reduction of population growth as the population approaches carrying capacity

What happens to this term when N is much smaller than K?

- When N << K , then this term →1

x. (1-.01)= 0.99 -> very small population that is growing exponentially

What happens when N = K?

As N approaches K, then this term →0

ex. (1-K/K)= 1-1=0 -> stable

What happens when N is much larger than K?

If N >> K population growth then the term will be negative

ex. (1-100)= -99

Logistic Growth in a Population

Every biological population on earth follows this natural law because there are limits to growth on a finite planet!

As N approaches K...There is increased intraspecific competition for limited resources

Fastest growth of population

Transition from exponential to sigmodial -> 1/2 carrying capacity

Inflection point

K/2 -> point at which growth is fastest and just starting to slow down

Initially the population grows exponentially; at K/2 the rate of population growth decreases, eventually reaching zero as the population size approaches the carrying capacity (K).

As a population approaches its carrying capacity (but hasn't yet reached it):

A. Population and growth rate both stop

B. The population continues to increase, but growth rate slows

C. Population and growth rate begin to decline

D. Population and growth rate stay the same.

B. The population continues to increase, but growth rate slows

Is the human population above or below K?

A. Above K

B. Below K

C. At K

D. No way to determine

A. Above K

maximum size of population without resource distribution -> not living in sustainable fashion

What is human carrying capacity (k)?

Paul Ehrlich calculates at 1.5 billion based on current human consumption

What does this mean for humans?

- As a human society, what makes the most sense - to control our population through our birth rates or our death rates? -> birth rate down and death rate up

Density-Independent Factors

Affects r-selected species

limit population not related to K or population size

Examples:

Temperature

Flood

Drought

These can occur randomly!

Density-Dependent Factors

K selected species -> close to K or overshoot from K

Competition for resources

Disease

Predation

Territoriality

Intrinsic factors (physiological)

Parasitism

Population Regulation Involves Density Dependence

Density-dependent mortality

- As population density increases, the rate of mortality increases

Density-dependent fecundity

- As population density increases, the rate of fecundity decreases

Both can occur simultaneously in a population, or only one can occur.