Chapter 44: Population Ecology
Chapter 44: Population Ecology
Habitat - The specific environment in which a population lives, as characterized by biotic and
Age Structure - Astatistical description or graph of the relative numbers of individual in each
age class in a population.
Generation Time - The average time between the birth of an organism and the birth of its
Sex Ratio - The relative proportions of males and females in a population.
Demography - The statistical study of the process that change a population's’size and density
Cohort -Agroup of individuals of similar age.
Life History - The lifetime pattern of growth, maturation, and reproduction that is characteristic
of a population or species.
Carrying Capacity - The maximum size of a population that an environment can support
Intraspecific Competition - The dependence of two or more individuals in a population on the
same limiting resource.
R-selected -Ashort-lived species adapted to function well in a rapidly changing environment.
K-selected - Long-lived, slow reproducing species that thrive in more stable environment.
44.1 The Science of Ecology
● In basic ecology, major research questions relate to the distribution and abundance of species
and how they interact with each other and the physical environment.
● In applied ecology, workers develop conservation plans and amelioration programs to limit,
repair, and mitigate ecological damage caused by human activities.
● Organismal ecology is the study of organisms to determine adaptations to the abiotic
environment, including morphological, physiological, biochemical, behavioural, and genetic
● Population ecologists document changes in size and other characteristics of populations of
species over space and time.
● Community ecologists study sympatric populations, the interactions among them, and how
these interactions affect the community’s growth. Interactions may include predation and
● Ecosystem ecologists study nutrient cycling and energy flow through the biotic and the abiotic
environment. ● Mathematical models express hypotheses about ecological relationships and different variables,
allowing researchers to manipulate the model and document resulting changes. In this way,
researchers can stimulate natural events before investing in lab work.
● Experimental and control treatments are necessary because they allow ecologists to separate
cause and effect.
44.2 Population Characteristics
● Seven characteristics can be described for any population:
a. Every population has a Geographic range
b. Every population occupies a habitat
c. Population size is the number of individuals making up the population at a specified time.
d. Population density is the number of individuals per unit area or per unit volume of habitat.
e. Populations can vary in their dispersion, the spatial distribution of individuals within the
f. All populations have an age structure, a statistical description of the relative numbers of
individuals in each age class.
g. Populations growth is also affected by its generation time, the average time between the birth of
an organism and the birth of its offspring.
h. Populations of sexually reproducing organisms also very in their sex ratio, the relative
proportions of male and females.
● Geographic range is the overall spatial boundary around a population. Individuals in the
populations often live in a specific habitat within a range.
● Alower population density means that individuals have greater access to resources such as
sunlight and water. The capture-mark-recapture technique assumes that:
a. a mark has no effect on an individual`s survival
b. marked and unmarked individuals mix randomly
c. there is no migration throughout the estimation period
d. marked and unmarked individuals are equally likely to be caught
● Three types of dispersion are clumped, uniform, and random.
● Clumped dispersion is the most common in nature because suitable conditions are usually
patchily disturbed and animals often live in social groups.Asexual reproduction patterns can also
lead to clumped aggregations.
● Uniform dispersion occurs when individuals repel one another because resources are in short
● Random dispersion occurs when environmental conditions do not vary much within a habitat,
and individuals are neither attracted to nor repelled by others of their species.
● Generation time increases with body size.
● The number of males in a population of mammals has little impact on population growth because
females bear the costs of reproduction (pregnancy and lactation), thus limiting population
growth. 44.3 Demography
● Alife table summarizes the demographic characteristics of a population, using a cohort to
● Age-specific mortality and age-specific survivorship deal with age intervals. In any one
interval, age-specific mortality is the proportion of individuals that died during the time.Age-
specific survivorship is the number surviving during the interval. The two values must sum 1.
● Age-specific fecundity is the average number of offspring produced by surviving females during
each age interval.
● In a type 1 curve, high survivorship at a young age decreases rapidly later in life. Type 1 curves
are common for large animals, including humans.
● In a type 2 curves, the relationship is linear because there is a constant rate of mortality across
the lifespan. Songbirds fit in this category.
● Atype 3 curve shows high mortality at a young age that stabilizes as individuals grow older and
larger, Insects fall into this category.
44.4 The Evolution of Life Histories
● Life history of an organism includes the lifetime patterns of growth, maturation, and
reproduction. There is a tradeoff in the allocation of resources to these three activities. Natural
selection adjusts the allocation of resources to maximize an individual's number of surviving
● Every organism has a finite energy budget, the total amount of energy it can accumulate and use
to fuel its activities.
● Maintenance, growth, and reproduction are the three main energy-consuming process.
● Passive care occurs in animals that simply lay eggs and leave them, or in mammals, as nutrients
cross the placenta from the mother to the developing baby.
● Active care involves nursing and other care provided after birth.
● Salmon have a short lifespan and devote a great deal of energy to reproduction. Deciduous trees
may reproduce more than once and use only some energy in any reproductive more than once
and use only some energy in any reproductive event, balancing reproduction and growth.
● Early reproduction is favoured if adult survival rates are low or if, when animal age, they do not
increase in size. In this case, fecundity does not increase in size.
44.5 Models of Population Growth.
● An exponential model is used when a population has unlimited growth.
● dN/dt = change in a population’s size during a given time period; b = per capita birth rate; d = per
capita death rate; N = number of individuals in the population; b-d = per capita growth rate = r
● When r>0, the birth rate exceeds the death rate, and the population is growing. When r<0, the
birth rate is less than the death rate, and the population is decreasing. When r=0, the birth rate is
equal to the death rate, and the population is neither growing nor shrinking. The intrinsic rate of
increase (r max) is the maximum per capita growth rate. This value usually varies inversely with
generation time, so a shorter generation time means a higher r max. ● Alogistic model has the following pattern: when the population growth is low, the population is
small. At intermediate population sizes, growth is more rapid because more individuals breed
and r is high. When population growth approaches K (carrying capacity), competition increases,
r decreases, and the growth of the population is reduced.
44.6 Population Regulation
● Density-dependent factors include intraspecific competition and predation.At high density,
fewer resources are available for individuals, which, in turn, use more energy in maintenance
needs and less in reproduction. Offspring produced at higher population densities are often
smaller in number or size and less likely to survive.At high population levels, adults may be
smaller and weaker.
● Density-independent factors, such as fire, earthquakes, storms, floods, and other natural
disturbances, reduce a population size regardless of density.
● r-selected species often have large numbers of small young, whereas k-selected species usually
have small numbers of larger young.
● Extrinsic control includes interactions between individuals in a population and their food and
predators. Once a food supply is exhausted, reproduction will decrease and mortality will
● Intrinsic control can be hormonal changes within dispersal, population that cause increased
aggressiveness, faster dispersal, and reduced reproduction.Aggression can cause weaker
individuals to be forced to disperse to reduce the population density.
44.7b Population Growth andAge Structure: NotAll Populations Are the Same
● For all long-lived species, differences in age structure are a major determinant of differences in
population growth rates.
● Countries with ZPG, there are approximately equal numbers of people of reproductive and pre
reproductive ages. The ZPG situation is exacerbated when reproductives have very few
offspring, meaning that pre reproductives may not even replace themselves in the population. ● In countries with negative growth (without immigration), post reproductives out-number
reproductives, and these populations will not experience a growth spurt when today’s children
reach reproductive age.
● Countries with rapid growth rate have a broad-based age structure, with many youngsters born
during the previous 15 years.
Ecologists study levels of organization ranging from individual organisms to the
Study of the interactions between organisms (biotic) and their environments (abiotic)
Ecologists use experimental or observational data
Field or laboratory studies used to test predictions by manipulating variables
Data collected about “natural experiments” do not require manipulations
Sometimes hypotheses framed in mathematical models
Allows computer models to simulate natural events and large-scale experiments
Science of Ecology
Focuses on undisturbed natural systems
Considers effects of human disturbance
Overarching goal of ecology is to understand the causes and consequences of changes of
What characters to study?
Geographic range / Habitat
Proportion of individuals in reproductive condition
Geographic Range: Overall spatial boundaries within which a population lives
Habitat: Specific environment in which a population lives, as characterized by its biotic
and abiotic features
Estimating Population Size
Often a difficult task
Too many? Cryptic?
Mark-Recapture methodology is one solution
Step 1: Capture a Random Sample of Individuals (n ) 1hat are marked
Step 2: Release marked individuals back into population
Step 3: Capture a second random sample of Individuals (n )2 some of whom are marked
Step 4: Calculate initial population size (x) using the formula1n /x =2m /2
Assumption of Mark-Recapture
Marking has no effect on survival or probability of being resampled later
Marked individuals have time to mix with unmarked individuals before second sample
Marks are not lost
Population is “closed”
No births, deaths, immigration or emmigration
Distribution of resources in environment/Landscape features
Habitat requirements of each individual
Social structure of species
Dispersal capacity/mode of reproduction of species
Presence/absence of other species
Distance between individuals is “random” with respect to each other’s location
Individuals neither attract nor repel one another.
Rarest type of distribution
Found in homogeneous environments
Maximal distance between any two individuals.
Found in many plants due to competition for resources
Also can result from territorial behavior Clumped Distribution
Most common form of distribution (Why?)
Poor dispersal capacity
Benefits to group living
Foraging, defence, parental care
Further Population Characteristics
Population age structure
Relative numbers of individuals of different ages
Relative proportion of males and females
Particularly relevant to conservation of any species in which individuals are rare
or widely dispersed in habitat
Average time between individual’s birth and birth of its offspring
Apopulations’characteristics can have dramatic effects on capacity for future growth
(strong target of selection)
Statistical study of processes that change a population’s size and density through time
Population growth factors
Births and immigration
Population decline factors
Deaths and emigration
Summarize demographics of a population
Group of individuals of similar age
Type I : High survivorship until late in life
Type II : Constant rate of mortality in all age classes
Type III : High juvenile mortality, followed by low mortality after critical age and size Life Histories
The schedule, frequency and duration of events in an organisms’lifetime: patterns of
growth, maturation, and reproduction
Allocations of resources influence evolution of maintenance, growth, and
Usually adjusted to maximize an individual’s number of surviving offspring
(shaped by selection)
Evolution of Life Histories
Influenced by organism’s physical and ecological environment:
age of sexual maturity / first reproduction
number of offspring / frequency of breeding
level of parental investment
senescence and death
Fecundity vs. Parental Care
Fixed energy budget may result in trade-offs between fecundity and parental care
Passive parental care before offspring born:
10 eggs at 100 units energy each = 1,000 total
1,000 eggs at 1 unit energy each = 1,000 total
Active parental care after offspring born:
many young = little care
few young = more care
How Often to Breed?
One reproductive episode
Devotes all stored energy
Maximum fecundity Death after reproduction
Multiple reproductive episodes
Only some energy devoted in each event
Age at First Reproduction
Early reproduction favoured
Adult survival rates low
Animals do not grow more fecund with age
Later reproduction favoured
If sexually mature adults likely to become older
If organisms grow older with age
If larger organisms have higher fecundity
Life History Differences
Most organisms fall between two extremes:
r-selected species and K-selected species
• organisms often have the capcity to produce more offspring than are needed to replace
themselves in the next generation
• study of population growth is central to ecology
o change in N/change in T = B-D
o N = change in population size
o T = time period
o B = births
o D = deaths
Exponential Growth Model By expressing per capita birth and death rates, equation describes unlimited population
dN/dt = rMAXN
rMAX = Intrinsic rate of increase; i.e. maximum per capita growth rate (a constant)
• When there are no limits on population growth, follows exponential pattern
Life History Evolution
Trade-offs reflect differences in strategies to maximize fitness
LH Character 1: Male displays
Probability of being predated increases brightness/number of spots
Attractiveness vs survival trade-off
Trade-offs reflect differences in strategies to maximize fitness
LH Character 2: Body size
Probability of being predated decreases with body size
Effect on time of first reproduction
LH Character 3: Parental Care/Fecundity trade-off
Many small offspring, produced more frequently vs. fewer, better provisioned
offspring produced more occasionally
Bacterial Population Growth
• Bacteria reproduce by binary fission; their populations double in size each generation
o Short generation time
o Small body size
o Large r MAX
Life History Traits and Populations
The specifics of a species’life history characteristics can have important consequences
Nature of interactions with other species (including in context of community
Logistic Growth Model
Most population growth is NOT exponential – limited in some fashion
Carrying capacity (K): Maximum population size that environment can sustain*
Per capita population growth rate (r) decreases as N approaches K
dN/dt = rmaxN[K-N/K]
• Modify exponential rate of growth relative to carrying capacity
• Time lags in responses to increased density may cause N to oscillate around K
• Crowding decreases individual growth rates, adult size, and survivorship
Density-Dependant • Fecundity is density-dependant
• Competition within populations or between species
• Predator–prey interactions
• Spread of infectious diseases
Reduce population growth regardless of population size (abiotic environmental factors)
May interact with density-dependent factors
Food shortage caused by high population density (a density-dependent factor)
may lead to malnourishment and…
Malnourished individuals may be more likely to succumb to stress of extreme
weather (a density-independent factor)
Some animal populations exhibit cyclic fluctuations in size