Chapter 9: Properties of Populations

What is „population‟ and how is it important to ecology?

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

- Suggests the potential (in sexually reproducing organisms) for interbreeding among member of the population. As such,

the population is a genetic unit. It defines the gene pool, the focus of evolution.

- The population is a spatial concept, requiring a defined spatial boundary – for example, the breeding of Chinook salmon

inhabiting the Copper River of Alaska.

o Organisms may be „unitary‟ or „modular‟

o The distribution of a population describes its spatial location, the area over which it occurs. Distribution is based on the

presence and absence of individuals, and is influenced by the occurrence of suitable environmental conditions. In other

words, distribution is a spatial boundary within which all individuals in the population reside.

o When the defined area (of distribution) encompasses all the individuals of a species (total area occupied by a population),

the distribution describes the population‟s geographic range. Within the geographic range of a population, individuals are

not distributed equally throughout the area, and this range can be limited by factors such as geographic barriers (i.e. ocean

between two islands) and interaction with other species (i.e. predation/competition).

Abundance: whereas distribution defines the spatial extent of a population, abundance defines its size – the number of

individuals in the population. Abundance is a function of

two factors

: 1) the population density, and 2) the area over which

the population is distributed.

Population density vs. ecological density:

o

Population density

is the number of individuals per unit are (also known as

crude density

), or per volume

o To account for patchiness, ecologists often refer to

ecological density

, the number of individuals per unit of available

living space.

Determining Abundance:

o Sampling:

Quadrant sampling:

population size =

mean density of units sampled x total area

Capture-recapture or mark-recapture method: (where

N

= Total population,

n

= number of individuals in the sample,

M

=

initially marked individuals,

R

= recaptured marked individuals)

Detection: observations by techniques such as vocalizations, counts of animal scant seen along a length of roads traveled,

counts of animal tracks (number of individuals crossing a certain track). These counts are called indices of abundance and

cannot function alone as estimates of actual density. However, it can show trends in abundance and comparison between

different habitats.

Population age classes or stages: prereproductive, reproductive, and postreproductive.

Chapter 10: Population Growth

Population growth: refers to how the number of individuals in a population increases or decreases with time.

Two methods of calculating population growth: (assuming the population dynamics are a function only of

demographic processes relating to birth and death à i.e. population is „closed‟ or immigration = emigration)

7. 1.

Exponential growth:

predicts the rate of population change through time. Exponential growth results in a

continuously accelerating rate of population increase (or decelerating rate of decrease) as a function of population

size.

o ; (where „r‟ = birth rate – death rate)

9. 2.

Geometric growth:

described over discrete time intervals, using finite growth multiplier (λ). Unlike

exponential growth, this estimate of the per capita growth rate does not assume that all individuals in the

population are identical. This estimate does, however, assume that the age-specific rates of birth and death for the

population are constant (they do not change over time).

o N(t) = N(0) λ^t ; where „λ‟ = finite multiplier rate

λ = e^r or r=lnλ

Chapter 14- Predation

Predation: the consumption of one living organism by another

Heterotrophs consume organic matter, predators are

diff from decomposers and scavengers

o They feed on living organisms, function as agents of mortality, regulate prey populations (prey populations reduce/regulate

growth rate of predator populations)

Classifications of predators (excludes decomposers and scavengers) includes heterotrophic organisms

o Carnivores (eat animal tissue)

o Herbivores (eat plant/algal tissue) – eat part of the individual plant, may harm the plant, doesn‟t result in mortality

o Omnivores (eat both plant and animal tissues

Parasites and their host organisms share an intimate relationship (not seen in predators and herbivores)

o Parasitoid attacks prey(host) by laying its eggs in host‟s body, egss hatch, larvae feed on host and kill it, don‟t cause

immediate death

Population growth equation FOR PREY POPULATION has two parts:

o 1) exponential model (dN/dt = RN)

o 2) mortality term: represent removal of pretty from the predator population

o Rate at which predators eat prey increases as # of prey increases

o cNprey [c= efficiency of predation]

o rate of predation = per capita rate of consumption * # of predators

o # of prey captured = cNprey * Npred (Npred/ cNpreyNpred)

dNprey/dt = rNprey - cNpreyNpred (represents rate of change in prey population)

Population growth rate for PREDATOR POPULATION has two parts

1) birth rate: amount of food consumed increases as with the rate at which prey is captured (cNpreyNpred).

Birth rate (b) = efficiency with which food is converted into population growth (reproduction) * rate of predation

(cNpreyNpred) or b(cNpreyNpred)

2) mortality rate: represented as dNpred [d= probability of mortality]

dNpred/dt = b(cNpreyNpred) - dNpred (represents rate of change in predator population)

These two lotka-volterra equations for predator and prey population growth function as density-dependent regulator on

each other

o Predators: regulate growth of prey population, function as source of density-dependent mortality

o Prey: function as source of density-dependent regulation on birthrate of predator population

o low predator population= prey population grows exponentially (dNprey/dt = rNprey)

o high predator population = prey mortality will increase until mortality rate (due to predation (cNpreyNpred)) is equal to

growth rate of prey population (rNprey) and net population rate growth for prey species = 0 (dNpred/dt = 0)

o Npred = r/c [growth rate of population prey is zero when # of predators = per capita growth rate of prey

population/efficiency of predation]. If predator population exceeds this value, prey population declines. Mortality due to

predation cNpreyNpred > prey population growth rate rNprey

Influence of prey population size on predator population‟s growth rate:

o Growth of predator will be 0 (dNpred/dt = 0) when the rate of predator increase is equal to the rate of mortality

(Nprey = d/bc) = growth of predator population is zero when size of prey population equals mortality rate of predator

population divided by product of efficiency of predation and conversion efficiency of captured prey into new predators

Growth of predator and prey populations are linked by cNpreyNpred

o For prey: cNpreyNpred regulates population growth through mortality

o For predator: cNpreyNpred regulates population growth through reproduction

Regulation of predator population growth is a result of 2 responses by predator to changes in prey population

1) Predator population growth depends on # of prey captured (cNpreyNpred)(more prey=more population)

o Functional response: relationship between rate of prey consumption & # of prey

2 ) Numerical response: increased consumption of prey = increase in predator reproduction [b(cNpreyNpred)]

If we define the rate of predation as # of prey eaten by a single predator during a period of search time (Ts) as Ne, the

per capita rate of predation is :

Ne = (cNprey)Ts

Ne = per capita rate of predation as # of prey eaten by a single predator during a period of search time (Ts)

Nprey = density of prey

C= efficiency of predation

Ts = period of search time

*fig 14.3 on page 286

Model of functional response (developed by Holling)

Type I functional response: # of prey taken per predator increases linearly as prey density increases. Expressed as a

proportion of prey density, rate of predation is constant- independent of prey density - all the time for feeding is spent

searching, search time is shorter than total time associated with consuming Ne prey, assumes no handling time below the

max rate of ingestion, occurs in passive predators (spiders)

Type II functional response: predation rates rise at a decreasing rate to the max level, expressed as a proportion of

prey density, rate of predation declines as prey population grows – most commonly reported for predators. Why approaches

an asymptote? Time is divided into searching time and handling time (T = Ts + NeTh), as # of prey captured increases,

handling time increases, further reaching time decreases, this causes a decline in mortality rate of prey with increasing prey

density

Type III functional response: rate of predation is low at first and then increases in a sigmoid (S) fashion, approaching

an asymptote. Plotted as a proportion of prey density, rate of predation is low at low prey density, rising to a maximum

before declining as the rate of predation reaches its maximum – high prey density = prey mortality increases in a density-

dependent fashion, low density = mortality rate is negligible.

Factors that result in type III functional response:

## Document Summary

A population is a group of individuals of the same species that inhabit a given area. Suggests the potential (in sexually reproducing organisms) for interbreeding among member of the population. As such, the population is a genetic unit. It defines the gene pool, the focus of evolution. Distribution is based on the presence and absence of individuals, and is influenced by the occurrence of suitable environmental conditions. Within the geographic range of a population, individuals are not distributed equally throughout the area, and this range can be limited by factors such as geographic barriers (i. e. ocean between two islands) and interaction with other species (i. e. predation/competition). Abundance: whereas distribution defines the spatial extent of a population, abundance defines its size the number of individuals in the population. Abundance is a function of two factors: 1) the population density, and 2) the area over which the population is distributed.