Life History Characteristics and Population Growth in Marine Ecosystems

1 source A. Sharov Population Ecology web course

Population Ecology

Key questions of population ecology include:

A population is a group of individuals of the same species that occupy a specified region at a specified time.

What is the size of a population?

What is the potential for growth in the population?

What form will growth take?

Population Growth

• N(t+1) = N(t) + B(t) + I(t) – D(t)– E(t)

where, N=number of individuals, B=births, I=immigrations, D=deaths, and E=emigrations ------BIDE

Overview of Ecological TheoryOverview of Ecological Theory

• Population Growth and regulation

• biological populations have great potential for increase, however, populations never realize this potential. There appears to be some factors (e.g., food limitation, war) that keep population regulation within some definable limits.

• Population Growth and regulation

• biological populations have great potential for increase, however, populations never realize this potential. There appears to be some factors (e.g., food limitation, war) that keep population regulation within some definable limits.

Population growth models

geometric

the rate of geometric growth

=the ratio of the population

in one year to the population in the previous

year

Geometric Growth ModelGeometric Growth Model• Population growth incrementally

• Geometric Growth rate ()– Ratio of the population in one year to that in the

preceding year = N(1)/N(0) = er

Where r=b-d

• Population growth incrementally

• Geometric Growth rate ()– Ratio of the population in one year to that in the

preceding year = N(1)/N(0) = er

Where r=b-d

Assumptions of the Geometric Growth Model

• Population is growing under optimum conditions

• Population has discrete generations (live and die at the same time)

• Breeds only one time per year (semelparous-salmon)

• Focuses only on females (millions of sperm/egg)

Population growth models

exponential

the rate of change in population

size

= x

the contribution of each

individual to population

growth

the number of individuals in

population

Population growth models

exponential

dN/dt = rN

N(t) = N(o)ert

where, N=number

r = (birth – death)t=time

Intrinsic Growth RateIntrinsic Growth Rate

r = births - deathsr = births - deaths

Components of the environment that affect birth or death rate will also affect r. Therefore, each environment a population lives in might produce a different r. And, if r can vary, it can be subject to natural selection and selective pressures can shape the values of r in different situations.

Components of the environment that affect birth or death rate will also affect r. Therefore, each environment a population lives in might produce a different r. And, if r can vary, it can be subject to natural selection and selective pressures can shape the values of r in different situations.

Intrinsic Rates of Increase• On average, small organisms have higher rates of

per capita increase and more variable populations than large organisms.

• Pops. of pelagic tunicates (Thalia democratica) grow at exponential rates in response to phytoplankton blooms.

– Life cycle involves a mixture of sexual and asexual reproduction

– Increase pop. size dramatically due to extremely high reproductive rates.

Small marine invertebrate:

Large marine mammal:

• Female gray whales (E. robustus) give birth, on average, every other year

• Reilly et al. (1983) estimated 2.5% growth for California population in 1967-1980

Assumptions of the exponential growth model

• Reproduction is continuous (no seasonality)

• All organisms are identical (e.g., no age structure)

• Environment is constant in space and time (resources are unlimited)

Logistic Growth• Because of “Environmental Resistance”

population growth decreases as density reaches a “carrying capacity” or K

• Graph of individuals vs. time yields a sigmoid or S-curved growth curve

• Reproductive time lag causes population to overshoot K

• Population will not be unvarying due to resources (prey) and predator effects

Population Biology: Logistic growth model

An example with

barnacles (Connell 1961):

K is determined largely by the amount of space available on rocks for attachment

barnacle(Balanus balanoides)

Population growth models

logistic

dN/dt = rN (K-N/K)

N(t) = K/1 + ea-rt

r = birth – deathK=carrying capacity

a= integration constant

Environmental Resistance• Factors that reduce the ability of

populations to increase in size– Abiotic Contributing Factors:

• Unfavorable light

• Unfavorable Temperatures

• Unfavorable chemical environment - nutrients

– Biotic Contributing Factors:• Low reproductive rate

• Specialized niche

• Inability to migrate or disperse

• Inadequate defense mechanisms

  

 

Two Schools of ThoughtTwo Schools of Thought

Density independent school - changes in physical environmental factors (generally climatic changes); which lead to dramatic shifts in populations. Density dependent school - work with larger organisms (vertebrates) or sessile organisms (barnacles); biotic interactions and their importance (competition, predation, or parasitism)

Density independent school - changes in physical environmental factors (generally climatic changes); which lead to dramatic shifts in populations. Density dependent school - work with larger organisms (vertebrates) or sessile organisms (barnacles); biotic interactions and their importance (competition, predation, or parasitism)

*large controversy over the relative importance of these factors *large controversy over the relative importance of these factors

Density-dependent Density-dependent responsesresponses

Pop

ula

tion

or

ind

ivid

ual

para

mete

r

Density

Density independent

Density dependent

r and K selectionr and K selection

r vs K (intrinsic growth rate) (carrying capacity)

r vs K (intrinsic growth rate) (carrying capacity)

Any organism has three categories that it needs to allocate energy to:Any organism has three categories that it needs to allocate energy to:

1. Growth2. Reproduction3. Maintenance (basal metabolic activity;

building of bone and supporting structures)

1. Growth2. Reproduction3. Maintenance (basal metabolic activity;

building of bone and supporting structures)

r selected traits - rapid development- small body- early reproduction- semelparity (single reproduction)

r selected traits - rapid development- small body- early reproduction- semelparity (single reproduction)

K selected traits

- slow development- large body- delayed reproduction- iteroparity (repeated reproduction)

K selected traits

- slow development- large body- delayed reproduction- iteroparity (repeated reproduction)

Thought to divide invertebrates from vertebrates; many exceptions; all are relative (barnacle to whales)

Overgeneralization, but it does seem to have some usefulness in organizing thinking

Thought to divide invertebrates from vertebrates; many exceptions; all are relative (barnacle to whales)

Overgeneralization, but it does seem to have some usefulness in organizing thinking

Hypothetical metapopulation dynamics. Closed circles represent habitat patches, dots represent individual plants or animals. Arrows indicate dispersal between patches. Over time the regional metapopulation changes less than each local population.

Demography

• Demography is the study of the vital statistics of a population

DemographyDemography

• Life Tables are the main tool for demographers, and they have 2 main components

• Survivorship schedule – average # of individuals that survive to any particular age

• Fertility schedule (fecundity) – average # of daughters produced by one female on each life stage

• Life Tables are the main tool for demographers, and they have 2 main components

• Survivorship schedule – average # of individuals that survive to any particular age

• Fertility schedule (fecundity) – average # of daughters produced by one female on each life stage

Population size = double the # of daughters born to each female (assumes the same # of sons as # of daughters)

Population size = double the # of daughters born to each female (assumes the same # of sons as # of daughters)

Types of Life TablesTypes of Life Tables

1. Static life table – calculated on a cross section of the population at a specific time. Estimate # of individuals from each age group and look at # of deaths for each age group

2. Cohort life table – follow a cohort through out their life and record the # of individuals surviving to each stage.

Both will give the same results if birth rate and death rate remain constant

1. Static life table – calculated on a cross section of the population at a specific time. Estimate # of individuals from each age group and look at # of deaths for each age group

2. Cohort life table – follow a cohort through out their life and record the # of individuals surviving to each stage.

Both will give the same results if birth rate and death rate remain constant

Notes on length measurements

1

2

3

4

5

Age Determination-Otoliths• Otoliths are composed primarily of aragonite, which is a

form of calcium carbonate • Readers count bands

Coral growth rings

• Each of the light/dark bands in this x-ray of a cross-section of a coral core formed during a year of growth

Seagrass Demography

Leaf Scars(plastochrone intervals)

Rhizomescars

Short- ShootStem

Leaves

Age Structure DiagramsAge Structure DiagramsPositive Growth Zero Growth Negative Growth (ZPG)Pyramid Shape Vertical Edges Inverted Pyramid

Survivorship table

Using these data, calculate the proportion of population surviving at the start of each period (lx).

X #barnacles at start (Prop. Surviving (Nx/NO)

0 12 100% (12/12) 1 6 50% (6/12) 2 3 25% (3/12) 3 1 8% (1/12)

Survivorship CurvesSurvivorship Curves

Type III Curve

• High juvenile mortality

• Little chance of surviving to adulthood

• Oysters, clams

Fertility Schedule

Provides the average number of daughters produced by one female at each particular age

Customarily, only females are tracked, since it is virtually impossible to measure the fecundity of males

It is assumed that the male population will grow the same way the female population grow

Net Reproductive Rate (Ro)

Is the average number of offspring produced by each female during her entire lifetime

Can be calculated by summing the products of the survivorship and fecundity schedules from birth to death

When Ro<1, the population declines; when Ro=1, the population is stable; and when Ro> 1, the population increases

A highly significant way of reducing fecundity is increase the age of first reproduction

Population 1 Population 2 X(age) Lx mx X(age) Lx mx

0 1.0 0 0 1.0 0

1 0.5 1.0 1 0.5 0

2 0.4 3.0 2 0.4 1

3 0.2 0 3 0.2 3

Ro=1.7 Ro=1.0

Life History Components

• The important components include:– the age and size at which reproduction occurs– the relative apportionment of energy to

reproduction, growth, survivorship, and predator avoidance

– production of many small or a few large offspring

– the age of first reproduction– age of death

Life History StrategiesLife History StrategiesAssumptions:

• Natural selection will produce a life history tactic (strategy) that will maximize the individual fitness of the organism under study by optimizing the allocation of energy between these function.

• Fixed amounts of energy (investing energy in one will take it from another)

Assumptions:

• Natural selection will produce a life history tactic (strategy) that will maximize the individual fitness of the organism under study by optimizing the allocation of energy between these function.

• Fixed amounts of energy (investing energy in one will take it from another)

The goal of life history strategy: to predict the characteristics of any organism that you might expect to find under any given set of conditions

The goal of life history strategy: to predict the characteristics of any organism that you might expect to find under any given set of conditions

Examples of life history considerations• If environmental conditions change, what effect might this that have

on growth or reproduction?

• Should most energy be allocated to growth in year 1 or should more energy be put into factors that protect you from predators or to reproduction?

• How many young should be produced?

• How many reproductive events would best take place during an organism’s life to maximize fitness?

• Should organisms allocate energy to care for young?

• If environmental conditions change, what effect might this that have on growth or reproduction?

• Should most energy be allocated to growth in year 1 or should more energy be put into factors that protect you from predators or to reproduction?

• How many young should be produced?

• How many reproductive events would best take place during an organism’s life to maximize fitness?

• Should organisms allocate energy to care for young?

In area where large amounts of density independent mortality occurs (catastrophic events that cause high mortality; weather or climatic)

In area where large amounts of density independent mortality occurs (catastrophic events that cause high mortality; weather or climatic) r selection dominates: organisms should survive here when they allocate a lot of energy to early reproduction, rapid growth, and dispersal to new habitats

r selection dominates: organisms should survive here when they allocate a lot of energy to early reproduction, rapid growth, and dispersal to new habitats

Stressful EnvironmentsStressful Environments

When K selection dominates: populations will increase until they approach K where competition will increase. In this situation good competitors will be selectively favored (density dependent mortality)

When K selection dominates: populations will increase until they approach K where competition will increase. In this situation good competitors will be selectively favored (density dependent mortality)

Stable EnvironmentsStable Environments

organisms should survive when they allocate energy to slow development, delayed reproduction, usually large and may have frequent reproductive events throughout life.

organisms should survive when they allocate energy to slow development, delayed reproduction, usually large and may have frequent reproductive events throughout life.

Bet hedgingBet hedging

Occurs where environmental conditions vary greatly and juveniles or adults are subject to high density independent mortality

Occurs where environmental conditions vary greatly and juveniles or adults are subject to high density independent mortality

If high juvenile mortality: smaller reproductive output, smaller litters at any given time, and longer lived organisms

If high juvenile mortality: smaller reproductive output, smaller litters at any given time, and longer lived organisms

If high adult mortality: increased reproductive effect, larger litters, and shorter lifespanIf high adult mortality: increased reproductive effect, larger litters, and shorter lifespan

Egg Size and Number in Fish• Fish show more variation in life-history than any other

group of animals.– clutch size (# of offspring per brood):

• ranges from 1-2 live births produced by mako shark (Isurus oxyrinchus)

• to 600,000,000 eggs produced by ocean sunfish (Mako mako)

Life History Variation Among Fish Species

Gunderson (1997) studied adult survival and reproductive effort of several fish spp.

– Reproductive effort measured as gonadosomatic index (GSI)

= (ovary weight / body weight) x (# of batches of offspring produced per year)

– Species with higher rates of mortality show higher relative reproductive effort

Modular growth

• Modular growth occurs when organisms reproduce asexually by increasing the # of modules they possess (e.g., corals, bryozoans, and cnidarians).

• r and K have no meaning for these organisms; colonial growth allows them to have incredibly long life spans.

• A common theme is that under stressful conditions organisms with modular growth turn on sexual reproduction for dispersal.

Frequency & Timing of Reproduction

Semelparous – Spawn only once and die, synchronized and massive gonad, usually seasonal

Iteroparous – Spawn multiple times a) Dailyb) Seasonalc) Opportunisticd) Variations of the above

• Semelparity

• Favored by stable, predictable environments

• less energy required for maintenance

•more energy devoted to reproduction

• produces cohorts of similar-aged young

Reproductive Strategies

•Iteroparity – offspring are produced multiple times during an organisms lifetime; found in most marine organisms

•Favored by unstable, non-predictable environments

- likened to “bet-hedging”

- survival of juveniles is low and unpredictable, thus selection favors repeated reproduction and long reproductive life

- tends to produce young of different ages

- much variation in # of clutches and size of clutch

• INSERT POPULATION DYNAMICS

Comparing models of population dynamics

Malthusian

• Unstructured

• per-capita instantaneous rate of increase r is independent of density

• N grows exponentially when r is positive.

• dN/dt increases linearly with N

• Populations growing in this manner are Not Regulated: they are predominantly affected by DI factors.

Pearl-Verhulst

• Unstructured

• per-capita instantaneous rate r of increase (ractual) is negatively density dependent

• N maxes out at K - sigmoid (S shaped) curve

• dN/dt maxes out at K/2 and tends towards 0 when N approaches 0 as well as K

• Population is regulated.

Logistic Model • Is it real?

– In the laboratory - sometimes

Paramecium

Daphnia

• Often overshoot carrying capacity

• Usually fluctuate around carrying capacity

• However, most organisms are limited by factors other than density.

Biotic PotentialBiotic Potential• Ability of populations of a given species to

increase in size– Abiotic Contributing Factors:

• Favorable light

• Favorable Temperatures

• Favorable chemical environment - nutrients

– Biotic Contributing Factors:• Reproductive rate

• Generalized niche

• Ability to migrate or disperse

• Adequate defense mechanisms

• Ability to cope with adverse conditions

?human activities

140biological

fixation

200denitrification

SOIL

ATMOSPHERE

OCEANS

15biological

fixation

140denitrification

1200internal cycling 8000

internal cycling

?burial

?river flow

<3fixation inlightening

groundwater

Nitrogen Cycle Without Microbes

All processes slow.Would life be possible?

Density Independent GrowthDensity Independent Growth• Rate of increase at any particular instant of time

Two principles

- exponential growth rate (r) expresses the population increase on a per individual basis

- rate of increase (dN/dt) varies in direct proportion to the size of the population (N)

• Rate of increase at any particular instant of time

Two principles

- exponential growth rate (r) expresses the population increase on a per individual basis

- rate of increase (dN/dt) varies in direct proportion to the size of the population (N)

r = intrinsic rate of population increase (birth rate – death rate)N = # of individuals in populationr = intrinsic rate of population increase (birth rate – death rate)N = # of individuals in population

dN

dtrN

N N et ort Exponential growth equationExponential growth equation

r vs. K SelectionPianka (1970, 1972):

• r and K are ends of a continuum. Most spp. fall somewhere in between.

• r- vs. K-selection correlated with env. and pop. attributes:– r-selection:

• Variable, unpredictable envs

• Type III survivorship

– K-selection: • Constant, predictable envs.

• Type I or II survivorship

•Uniform dispersion is when individuals are evenly spaced.

•Clumped dispersion is when individuals aggregate in patches.

•Random dispersion is the position of each individual is independent of the others.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

How many bacterialspecies are there?

Wilson 1988

Total number species: ~ 1.4 millionBacteria: ~3,500

Hammond 1995

Total number species: ~ 11 millionBacteria: ~10 million

100human activities

140biological

fixation

200denitrification

SOIL

ATMOSPHERE

OCEANS

15biological

fixation

140denitrification

1200internal cycling 8000

internal cycling

10burial

36river flow

<3fixation inlightening

groundwater

Example: The Nitrogen Cycle

IV. Logistic growth

Biologists beginning with Thomas Malthus & Charles Darwin understood that populations cannot grow exponentially for very long

Increasing population size gives rise to …

… shortages in food and other limiting resources

… greater intraspecific aggression

… increased attention from predators

… greater risk of disease outbreaks

These factors can act to lower birth rates and elevate death rates

Antarctic sponges and asteroids

Work by Paul Dayton at McMurdo Sound in 1960s showed strong interannual variability in recruitment of a dominant hexactinellid sponge, linked to predation by a sea-star and oceanographic variability (El Niño)

Acanthaster (crown-of-thorns starfish) and coral recruitment

Acanthaster outbreaks kill coral but allow for new recruitment

Population Growth and regulation

• Darwin incorporated many of Malthus’ idea’s and proposed that diversification of species was the direct result of intraspecific competition driven by food/resource limitation

66CS Fig. 4.7

Competition for resources causesevolution

Population growth models

geometric -- used when there is a discrete breeding season

exponential -- used when populations are growing continuously

A highly significant way of reducing fecundity is increase the age of first reproduction

Population 1 Population 2 X(age) Lx bx X(age) Lx bx

0 1.0 0 0 1.0 0

1 0.5 1.0 1 0.5 0

2 0.4 3.0 2 0.4 1

3 0.2 0 3 0.2 3

Ro=1.7 Ro=1.0

Survivorship Schedule

Provides the average number of individuals surviving to each particular age

The fundamental processes of sexual reproduction

Adult

Embryo/LarvaZygote

Juvenile

Fertilisation

Development

Metamorphosis

Growth & Maturation

Life Cycle

Definitions: Life Tables

• Cohort is a group of individuals within a population that are born at the same time. A population is a collection of cohorts

Life Table Construction Steps

• Determine the age distribution and # of intervals you want

• Standardize lx to 1,000 (some do it to 1.0 or 100%)

• Calculate dx

• Qx= dx/ lx

• Lx= (lx+lx+1)/2

• Tx = Sum Lx

• ex = Tx/lx

Life Table TerminologyTerm Definition

x Age interval

nx No. of survivors at the start of age interval x

lx Proportion of organisms surviving at the start of age interval x

dx No. dying during age interval x to x+1

qx Rate of mortality during age interval x to x+1

ex Mean expectation of life for organisms alive at the start of age x

Lx Avg. length of time lived by all individuals entering an age category (1/2(nx+ nx+1)

Tx The total amount of time remaining for all individuals in an age group.

How to calculate a life table: an example

Age nx lx dx Qx Lx Tx ex

0-1

1-2

2-3

3-4

4-5

5-6

6-7

93

74

32

14

5

1

0

1000

796

344

151

54

11

0

204

452

193

97

43

11

0

.204

.568

.561

.642

.796

1.0

0

898

570

248

103

33

6

0

1858

960

390

142

39

6

0

1.858

1.21

1.134

.94

.72

.55

Survivorship Table (lx) and Fertility Table (bx) for Women in the United States, 1989

Age Up

Midpoint or Pivotal Age x

Proportion Surviving to

Pivotal Age lx

No. Female Offspring per Female Aged x per 5-Year

Time Unit (bx) Product of lx

and bx

0-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49

and above

5.0

12.5 17.5 22.5 27.5 32.5 37.5 42.5 47.5 ----

0.9895 0.9879 0.9861 0.9834 0.9802 0.9765 0.9712 0.9643 0.9528

----

0.0

0.0020 0.1233 0.2638 0.2772 0.1807 0.0650 0.0125 0.0005

0.0

0.0

0.0020 0.1216 0.2594 0.2717 0.1765 0.0631 0.0121 0.0005

0.0

R0 = lx bx = 0.9069