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Lecture 2: Population Ecology – Ch. 8

Why study Population Ecology?

In order to conserve species we need to know:

 Which species are threatened?  In decline?

 Vulnerable to habitat change?

 Cause of decline?  Which part of the population is in decline, and

why?

 Is decline related to density, distribution, or range?

 Problem of numbers or genetic diversity?

 Are conservation efforts working?  Is a species’ population increasing, declining, or

stable?

 Change in distribution?

Properties of Populations

 A population is a group of individuals of the

same species that inhabit a given area and are

able to interbreed

 Populations have structure

 density

 spacing

 age distribution

 Populations are dynamic, changing over time

Organisms May Be Unitary or Modular

 Unitary organisms exist as individuals

 After fertilization, the zygote grows into a genetically

unique organism through a series of predictable

stages

 Most animals are unitary organisms

 Modular organism

 produces more, similar modules

 Most plants are modular

 develop by branching, producing repeated structural

units

 The fundamental unit above-ground is the leaf, with

its axillary bud and internode

 Roots also show modular growth

Organisms May Be Unitary or Modular

Figure 8.1 Step 4 Slide 4

Axillary bud

Stem

Leaf

Node

Basic vegetative unit

Internode

Vegetative units

repeat along the stem Vegetative branches

form from axillary buds,

and are made of

additional vegetative

units

Branch

Shoots

Roots

Whole plant in a collective of

units making up both the shoot

and root systems

 Suckers – new stems that sprout

from surface roots and may appear

to be individuals

 Genet – plant produced by sexual

reproduction, a genetic individual

 Ramet – module produced

asexually by a genet (a clone)

Reproduction in Plants

Stolon

Clones Parent

In woody plants (shrubs and trees), such as an aspen tree (Populus tremuloides),

ramets develop from root suckers. They appear to be individuals!

http://www.9news.com/story/life/2014/07/02/aspens-colorado/12051273/

Pando -105-acre clonal colony of Quaking Aspen in Utah, connected by a single root

system. At least 80,000 years ago. Some estimates as old as 1 million years.

At 6,615 tons, Pando is the heaviest living organism on earth.

 To study populations of modular organisms, both

individual (genet) and module (ramet) must be

recognized

 This can be challenging

 molecular studies can distinguish

 Ramets are often counted as individuals, and

they often function this way, but diversity must be

considered in conservation applications.

 The distribution of a

population is the area over

which it occurs.

 Where individuals are present

The Distribution of a Population

Geographic Range

 Encompasses all of the individuals of a species

 Individuals are found in suitable habitats within that

geographic range

 This range is limited by

 Abiotic factors, e.g., temperature, soil moisture,

elevation

 Biotic factors, e.g., predation, competition, parasitism

Giant Amazon River Turtle (Podocnemis

expansa)

 The range of the red maple is limited by temperature

in the north and drier conditions in the Midwest

Endemic Species

 Ubiquitous species - geographically widespread

 Endemic species - geographically restricted

 many have specialized habitat requirements

Geographic Barriers

 Reduce/prevent individuals colonizing new areas

 bodies of water, including rivers; mountains; large

areas of unsuitable habitat such as deserts

Genetic differences across ranges

 It’s important to understand the

genetic/physiological variability across a range

 E.g. Habitat restoration

• 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

Metapopulations

Worldwide

Cluster

Pine barrens

400

4000

10,000 8500

100

Locality Colony Clump

Swampy

Stream

20'

Region

Ocean

River

Continental

Physiographic

area

Coniferous

stump

clay bank

Distribution of

moss (Tetraphis pellucida)

that requires

microclimates

Human altered landscapes function as

metapopulations…

 Abundance: # of

individuals in the

pop

 Population

density: # of

individual/area

 Density of cell 1 =

5 ind/m2

 3 kinds of

distribution…

 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

Random

 Organisms are found at a regular distance

from one another

 Often the result of negative interactions

among individuals such as competition

Uniform Distribution

Uniform

Nesting Shorebirds

Acacia: uniformly spaced because of competition for water and nutrients

 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

Clumped Distributions

Clumped

Figure 8.11

(b)

(a)

Acacia tortilis

Euclea divinorum

20 m

Spatial distributions of

individuals may be

described at multiple

spatial scales…

How to determine population size?

Population size (abundance)

 population density  the area occupied

Determining Density 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

 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 a random sample of subunits

 Mean density X total Area = Estimate of population size

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

Determining Density Requires Sampling

10 Randomly Placed Quadrats 20 Randomly Placed Quadrats

Randomly sampling - # of samples matters!

 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

Determining Density Requires Sampling

N -Total population

m - initially captured and marked individuals

s - captured animals on the second visit

r - the # of ind. marked on the 2nd visit

Ratio: N/M (Total pop/ Initial capture) =

s/r (2nd capture/ marked recapture)

N/m= s/r

Assumptions

1. No effect of marking on probability of recapture - tags

should not be obvious or slow the individual, or reduce

fitness in any way

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,

only weak

4. Marks are not lost

Methods of Marking

 Tags

 Leg bands

 Pit tags

 Paint

 Chopping off legs

 Etc.

Do these affect fitness?

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.

A. 80

B. 128

C. 720

D. 2000

 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

Determining Density Without Direct Counts

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.

 Individuals migrate great distances throughout

the year.

What we did…

 We did mark recapture – didn’t work.

 We counted ALL the nests on a sample of

nesting beaches – this worked!

Measuring turtle tracks…

Population Structure - Age, Developmental

Stage, and Size

 Abundance doesn’t provide information on the

population characteristics…

 Why would you want to know the age structure

of a population?

 A population with non-overlapping generations does

not have an age structure

 annual plants and some insects

 A population with overlapping generations has an

age structure

 reproduction is restricted to certain age classes

 mortality is more common in certain age classes

 Populations can be divided into three ecologically important age classes

 pre-reproductive

 reproductive

 post-reproductive

 How long an individual is in each age class depends on the organism’s life history

 Mice, have a very short span of time between generations

 Elephants, have a very long span of time between generations

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

 Valid for dominant canopy trees, but less so for

understory trees

6 4 2 0 2 4 6

Percentage of population

6 4 2 0 2 4 6

Percentage of population

6 4 2 0 2 4 6

Percentage of population

8 8

Age

80+

75–79

70–74

65–69

60–64

55–59

50–54

45–49

40–44

35–39

30–34

25–29

20–24

15–19

10–14

Under 5

5–9

Egypt- growing United States- stable Japan- aging

Male Female Male Female Male Female

Population Structure – Age Pyramids

 Plant populations, the distribution of age classes

may be highly skewed

 In a forest, the tall tress can inhibit the survival of

seedlings and the growth and survival of young

trees

 only when the older trees die can trees in younger

age classes access the light, water, and nutrients

they need to grow and develop

Population Structure

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

Passive vs. Active Dispersal

 Passive dispersal (plants & animals) may include:

 wind

 water

 gravity

 animals

 Active dispersal (animals)

 often the young or subadults

 Factors that affect dispersal

 crowding, food availability/quality, temperature change

 Animals can be important dispersers of plant

seeds

 Dispersed when an animal eats fruits and the seeds

they contain

 Spines or hooks that attach to animal fur or bird

feathers

Individuals Move within a Population

 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

The gray whale

summers in the

Arctic and Bering

seas; it winters in

the Gulf of California

and the waters off

Baja California

 The populations of many species of trees have

shifted north after the glaciers retreated and the

climate warmed

Population Distribution and Density Change

in Both Time and Space