LAP Code:
LAP Subject Title: People and the Earth’s Ecosystem
No. of Hours: 3 hrs/meeting
LAP 05-WEEK 05
Population Ecology
A. Topic Outline
Unit
Chapter 5:
Population
Ecology
Content
Standard
LIFE
EXPE LIFE
EXPECTANCY
CTANCY
Objectives
Activities
To define ecology and population
ecology.
To identify types
of population factors.
To describe changes in population
growth in an ecosystem.
Assignment
1. Graph
analysis
2. Writing an
inference
B. Activity
Crude Birth Rate, Crude Death Rate and Population & Number of Live Births and Life Expentancy in the
Philippines: 2010-2015.
YEAR
POPULATION
NO. OF LIVE
BIRTHS
CRUDE
BIRTH
RATE
NO. OF
DEATHS
CRUDE
DEATH
RATE
LIFE
EXPECTANCY
2010
93, 135, 100
1, 782, 981
19.1
488, 265
5.2
68.32
2011
94, 823, 800
1, 746, 684
18.4
498, 486
5.3
68.43
2012
96, 510, 900
1, 790, 367
18.6
514, 745
5.3
68.55
2013
98, 196, 500
1, 761, 602
17.9
531, 280
5.4
68.68
2014
99, 880, 300
1, 748, 857
17.5
536, 999
5.4
68.81
2015
101, 562, 300
1, 744, 767
17.2
560, 605
5.5
68.95
Answer the following questions. Refer to the above table for your answer.
1. What year has the greatest number of populations?
2. What is the effect of the population to the economy of the country?
C. Abstraction
Population – is a group of individuals of the same species living in a given area at a given time.
Ecology – a science that deals with the relationship between groups of living things and their
environments. Ecology is the study of the relationships between living organisms, including
humans, and their physical environment; it seeks to understand the vital connections between
plants and animals and the world around them.
Population ecology - Population ecology is the study of these and other questions about what
factors affect population and how and why a population changes over time.
Characteristic of population
A population’s genes pool is the basis for characteristic ranges of morphological,
physiological, and behavioral traits. Ecologist considers its genetic make-up as well as reproductive
modes and overall behavior. They also consider demographics, or vital statistics of the
populations, which include population size, density, distribution and age structure.
Population density – is the number of individuals of a species per unit area or volume.
a. Indirect indicators – Indirect methods (relative estimates). These are many measures
which may correlate with population density, visual counts or counts of
animal products (grass, nests).
b. Mark recapture method – places traps within the boundaries of the population under
study and puts marks to it by using tags, collars, or dye.
Population distribution – is the pattern of dispersal of individuals across an area of interest.
Limiting factors – are those environmental aspects that particularly determine where an
organism lives.
Dispersion pattern – is the way individuals are spaced within the population’s geographic range.
A. Random dispersion – individuals in a population are spaced in a pattern less and
unpredictable way.
B. Uniform pattern of dispersion – often results from interactions among the individuals
of a population.
C. Clumped pattern – most common in nature in which individuals are aggregated in
patches and often results from unequal distribution of resources in
the environment.
The population growth rate
The population’s annual growth rate is dependent upon natality, the number of
individuals born each year, mortality, the number of individuals that dies each year, annual
immigration, the number of individuals of species moving into an existing population, and
emigration, the number of individuals of a species moving out of an existing population.
Abiotic potential – is the highest possible growth rate if a population and is achieved when
resources are unlimited.
Abiotic potential factors:
1. Usual number of offspring per reproduction
2. Chances of survival age of reproduction
3. How often each individual reproduces
4. Age at which reproductions begin
D. Analysis: Make an inference on the species dispersion patterns—or distribution patterns. Refer to
the diagram below on how the individuals in a population are distributed in a space at a given time.
E. Application
Demography: describing populations and how they change
In many cases, ecologists aren't studying people in towns and cities. Instead, they're studying
various kinds of plant, animal, fungal, and even bacterial populations. The statistical study of any
population, human or otherwise, is known as demography.
Why is demography important? Populations can change in their numbers and structure—for
example age and sex distribution—for various reasons. These changes can affect how the population
interacts with its physical environment and with other species.
By tracking populations over time, ecologists can see how these populations have changed and
may be able to predict how they're likely to change in the future. Monitoring the size and structure of
populations can also help ecologists manage populations—for example, by showing whether conservation
efforts are helping an endangered species increase in numbers.
In this article, we'll begin our journey through demographics by looking at the concepts of population size,
density, and distribution. We'll also explore some methods ecologists use to determine these values for
populations in nature.
Population size and density
To study the demographics of a population, we'll want to start off with a few baseline measures.
One is simply the number of individuals in the population, or population size—NNN. Another is
the population density, the number of individuals per area or volume of habitat.
Size and density are both important in describing the current status of the population and, potentially, for
making predictions about how it could change in the future:
Larger populations may be more stable than smaller populations because they’re likely to have
greater genetic variability and thus more potential to adapt to changes in the environment through
natural selection.
A member of a low-density population—where organisms are sparsely spread out—might have
more trouble finding a mate to reproduce with than an individual in a high-density population.

Measuring population size
To find the size of a population, can’t we just count all the organisms in it? Ideally, yes! But in
many real-life cases, this isn’t possible. For instance, would you want to try and count every single grass
plant in your lawn? Or every salmon in, say, Lake Ontario, which is 393 cubic miles in volume? ^11start
superscript, 1, end superscript Counting all the organisms in a population may be too expensive in terms
of time and money, or it may simply not be possible.
For these reasons, scientists often estimate a population's size by taking one or more samples
from the population and using these samples to make inferences about the population as a whole. A
variety of methods can be used to sample populations to determine their size and density. Here, we’ll
look at two of the most important: the quadrat and mark-recapture methods.
I. Quadrat method
For immobile organisms such as plants—or for very small and slow-moving organisms—plots
called quadrats may be used to determine population size and density. Each quadrat marks off an area of
the same size—typically, a square area—within the habitat. A quadrat can be made by staking out an area
with sticks and string or by using a wood, plastic, or metal square placed on the ground, as shown in the
picture below.
After setting up quadrats, researchers count the number of individuals within the boundaries of
each one. Multiple quadrat samples are performed throughout the habitat at several random locations,
which ensures that the numbers recorded are representative for the habitat overall. In the end, the data
can be used to estimate the population size and population density within the entire habitat.
II. Mark-recapture method
For organisms that move around, such as mammals, birds, or fish, a technique called the markrecapture method is often used to determine population size. This method involves capturing a sample of
animals and marking them in some way—for instance, using tags, bands, paint, or other body markings,
as shown below. Then, the marked animals are released back into the environment and allowed to mix
with the rest of the population.
Later, a new sample is collected. This new sample will include some individuals that are marked—
recaptures—and some individuals that are unmarked. Using the ratio of marked to unmarked individuals,
scientists can estimate how many individuals are in the total population.
Example:
One thousand two hundred and seventy deer are living on an island that is eight hundred and thirty
square kilometers in size. What is the population density of the deer per square kilometer?
Population density = total population
total area
Population density = 1,270 deer = 1.53 deer/sq. km
830 sq. km
D. Output for Application: Solve the following problem. Show your solution. Use the above computation
as your pattern.
1. At the end of 2002, there were 1,284.53 million people living in China. China is the third
largest country in the world with an area of 9.6 million square kilometers. What is the population
density of China?
2. In 2000, there were 30,750,087 people living in Canada, which has a total area of 9,984,670
km 2. What was the population density of Canada?
LAP Code:
LAP Subject Title: People and the Earth’s Ecosystem
No. of Hours: 3 hrs/meeting
LAP 06-WEEK 06
Community Ecology
A. Topic Outline
Unit
Chapter 6:
Community
Ecology
Content Standard
Explain the nature of
interactions between
organisms in different
symbiotic relationships (i.e.,
mutualism, commensalism,
parasitism.
Explain how cooperative (e.g.,
symbiosis) and competitive
(e.g., predator/prey)
relationships help maintain
balance within an ecosystem Explain why no two species
can occupy the same niche in
a community
Objectives
 To understand ecological
levels of organization.
 To describe the flow of
energy
through
an ecosystem.
 To describe and analyze the
components.
B. Activity: Study the picture and answer the following questions.
Activities
Assignment
Levels of organization in Ecology
1. Based on the picture, what are the levels of organization in ecology?
2. What is the highest level of organization in ecology?
C. Abstraction
Community Ecology
Community ecology is the study of the organization and functioning of communities of organisms.
As populations of species interact with one another, they form biological communities. A community of
organisms consists of all the interacting populations of the species living within a particular area or within
a particular habitat. Community ecology also studies the relationships of the members of a community to
their environment. Community ecology is usually subdivided according to habitat or biome. Typical
habitats include forest, grassland, desert, and stream or lake environments.
Community ecology is the division of ecology that deals with the study the interaction among
species and interaction with the abiotic environment. A community is composed of all of the population
of a given area. It is a group of populations interacting with one another within the same environment.
A rainforest community may include different species of trees, thick vines called lianas, and
epiphytes (orchids, cacti and ferns). Mammals such as monkeys, reptiles, and amphibians, and colorful
birds and insects are common inhabitants of this community.
Community ecology is the study and the theory of how population of organisms interact with each
other and react to their non-living surroundings. As a subset of the general study of ecology, this field of
specialization explores the organization and functioning of biological communities.
Community ecologists protect the environment and save species from extinction by assessing and
monitoring environmental conditions such as global warming.
One of the earliest formal definitions of community ecology was suggested by Cornell professor
Robert Whittaker characterized community ecology as an assemblage of living organisms that interact
and form a community with a unique structure and species composition. Knowing how a community
functions is vital to promoting and preserving biodiversity.
Assemblage of living organisms or assemble from plants, animals and others then interact with
each other, unique structure, in the sense that there is a form of structure or pyramid of food. What was
the easiest insect or anything to eat, for example a certain frog eats the insect, a frog eats by snake, a
snake eat by eagle and eagle eat by human. So there is a pyramid of food that happened, from insect to
frog, frog to snake, snake to eagle, and eagle to human. So this is what Robert Whittaker means about the
community ecology.
Community ecology examines how coexisting organisms interact and compete in a particular
niche or geographical location such as a woodland prairie or a lake. Community ecology encompasses all
populations of all species that live together in the same area.
Community ecology encompasses many types of ecological interactions that continue to change
over time. A forest community includes the plant community, all trees, birds, squirrels, deer, foxes, fungi,
fish in a forest stream, insects and all other species living there or migrating seasonally.
Similarly, a coral reef community includes a vast number of different species of corals, fish and
algae. Abundance and distribution are strong forces that shape the biological community.
Community ecology focuses on how interactions between different species affect health, growth,
dispersion and abundance of the ecological system. At the community level, species are often
interdependent. Several short food chains are common in most biological communities. Food chains often
overlap and form food webs of producers and consumers.
Community Ecology Theory: American, European and British scientists have long held many
differing theories on the definition of community ecology, which was first called plant sociology. In the
20th century, opinions differed as to whether ecological niches were self-organized organism communities
or random assemblages of species that thrived because of their particular traits. By the 21st century,
theories broadened to include such ideas as the metacommunity theory that focuses on community
structures and the evolutionary theory that incorporates principles of evolutionary biology into
community ecology.
Currently held community ecology theory is based on the supposition that ecological communities
are the result of different types of assembly processes. Assembly processes involve adaptation, speciation
in evolutionary biology, competition, colonization, altitude, climate, habitat disturbances and ecological
drift. The theory of community ecology expands upon niche theory, which has to do with an organism
having a specific place and role in an ecosystem.
Indicators of Ecological Health: Species richness refers to the richness, or number of species
found. Species diversity looks at overall biodiversity. Species diversity measures species richness as well
as the relative number of species present. High species diversity characterizes stable ecological
communities. Sudden of significant changes in a community such as an influx of predators can disrupt the
predator-prey ecological balance and reduce species diversity.
When we say influx of predators it is the increasing of predators. If there is an increasing of
predators than prey and if it is not balance then the ecological balance may possibly affect. If the supply
of a certain community is lacking or if there is something lacking or doesn’t even exist, possibly the
ecological might become unbalance. For example if a tree might cut by the humans which birds are most
dependent to it, they may become affected and there are changes to their own biodiversity because they
are no longer comfortable the new environment which it may cause unbalance of ecological health. The
ecological health may become unbalance because of these reasons, one, the influx of predators, second,
change of biodiversity, and third one, is the climate change.
Community Ecological Structure:
Community ecologists study the interaction between structure and organism. Structure describes
characteristics of ecological niches, species richness and species composition. Species interact with each
other and with their environment in many different ways, such as competing the finite resources or
working together to trap game.
The energy pyramid shows how energy is made and transferred by organisms that comprise the
food chain. Heterotrophic producers of usable food energy from the sun from the broad base of the
pyramid. Primary consumers such as herbivores cannot make food to fuel their cells and must eat
producers to live. Secondary consumers are carnivores that eat primary consumers. Tertiary consumers
devour secondary consumers, but the apex predator at the top of the pyramid has no natural enemies. A
food chain represents the flow of food energy in a community
Types of Species in Community Ecology: The founding species, Keystone species, and Invasive species.
Founding species like coral in a coral reef community play a pivotal role in community ecology and shaping
structure. Coral reefs are commonly called “rainforests of the sea” because they provide food, shelter,
breeding areas and protection for up 25% of all marine life, according to Smithsonian Museum of Natural
History. Threats to coral reefs include climate change pollution, overfishing and invasive species. Keystone
species like wolves profoundly affect community structure relative to the abundance of the other species.
Invasive species are invaders that are not to the habitat and disrupt the community. Invasive species like
zebra mussel destroy native species.
Community Ecology Definition for Succession
Ecological succession is a series of changes over time to community structure that affect
community dynamics and encourage the assemble of plants and animals.
Primary succession starts with the introduction of organisms and species, usually on newly exposed rock.
It is where the role enters to the species or to introduce a certain species in a certain area. When
we say succession it is the changes of area because of the unbalance of community.
Secondary succession happens when orderly recolonization occurs in an area that was previously
inhabited before a disruption.
Example, in that area before it is just something like inhabited then they reinvent again or it will
be inhabited again. In secondary succession it mentions there the word recolonization which means to
reinvent and repair to become inhabited again.
Many types of interaction occur in an ecological community and influence population dynamics. These
are a few examples of interactions:
◙ Mutualism
◙ Protocooperation
◙ Commensalism
◙ Parasitism
◙ Competition
◙ Predation
◙ Amensalism
Type of Interaction
Mutualism
Species 1
+
Species 2
+
Protocooperation
+
+
Commensalism
+
o
Parasitism
+
-
Amensalism
-
o
Competition
-
-
Predation
+
-
Nature of Interactions
Both organisms are
benefited and obligatory
Both organism are
benefited and not
obligatory
One organism benefits
while the other is neither
benefited nor injured
One organism benefits
and the other is harmed
One organism is inhibited
while the other is not
affected
Both organisms are
affected
One organism benefits at
the expense of another
1) Mutualism is a given-and-take relationship. In mutualism, both organisms are benefited and
obligatory, thus a double positive (+,+) interaction. The term mutualism refers to a type of relationship
that mutually benefits two species sharing an environment. Mutualism in biology refers to symbiotic
species interactions that are mutually beneficial, or even essential, for survival. Mutualism is common
in all ecosystems, including the human body. For instance, Harvard Medical School estimates that
trillions of bacteria called “gut microbiota” live in the human intestine and aid in digestion and overall
health.
Discussion: This relationship has double positive interaction and both are obligatory. For example of
this are the bee and the flower. If the bee pollinate to the flower it may cause vitamins or energy that help
it to form or make a honey, in some ways if the bee might pollinate into another flower, the flower also
bloom because of the pollination of the bee. Therefore it is an example of mutualism because they are
both benefited and obligatory. They are obligatory in the sense that bee is dependent to the flower for
them to have a honey and the flower also dependent to the bee because only the bee and the pollination
of this can result a flower to bloom and they are obligatory to each other.
2)Protocooperation is a relationship which both organisms are benefited but the relations are not
obligatory, thus a double positive (+,+) interaction. Protocooperation is a type of interaction between
two organisms that harbors a parasite, typically providing food and protection.
Discussion:
relationship has a
This
double
positive
interaction but not
obligatory both sides. For examples are the heron and the rhino, if the heron go to the body of the rhino
there is a possibility that he/she can get the food to eat and because of this the rhino is benefited because
the insects from his/her body are remove and he/she become safe from any harm of the insect. On the
other side also the heron is benefited because he/she get the food from the rhino, therefore they are both
benefited but not obligatory. Not obligatory in the sense that they are not obligatory to just get the food
from the body of the rhino by heron and also not only the heron can remove the insects from the body of
the rhino but there are other animals which can remove also. So therefore this is not obligatory relationship
of organisms.
3)Commensalism is a relationship in which one organism benefits while the other is neither
benefited nor injured, thus a positive-neutral (+,-) interaction. The one benefited is known as
“commensal”. Commensalism is defined as a unilateral relationship between two species
without consequence to the other. Most of the interactions occurring in the natural world
affect both organisms in some way.
Discussion: This relationship in which one is benefited and one is neutral. They are one but only one is
benefited and the other is not. For example are the frog and the leaves. Frog uses leaves for their protection
without harming the plants to be destroyed. Another example is the cattle egrets and carabao, cattle
egrets benefited from the carabao that they eat insects without harming the carabao.
4) Parasitism is a relationship in which one organism benefits and the other is harmed, and thus a
positive-negative (+, _-) interaction. The one benefited is called “parasite”, while the organism that is
harmed is the host. Parasitism is an interaction that harms the host species: in case like the strangler
fig, the parasitic species can even parasites that carry pathogenic bacteria that infect its host. The
parasite are usually smaller than their hosts and are classified into ectoparasite and endoparasite. Most
tapeworms and roundworms are endoparasites, while fleas, ticks, and mites are ectoparasites.
Discussion: This relationship in which one organism is benefited while one is not. For example are the
mosquito and the human. If mosquito bite the human we are the negative of this it’s because mosquito
benefited us by our blood and we human doesn’t have any benefit from the mosquito. So mosquito is the
positive here. Another example is the insect in our head. They are also the positive here because they are
the parasite that benefits from our head while we are the negative it’s because we are the affected one.
5)Amensalism is a relationship in which one organism is inhibited or destroyed and the other is not
affected. Amensalism is a unilateral interaction like commensalism. However, one organism causes
harm to another without being helped or harmed in the process. Amensalism is any relationship
between organisms of different species in which one organism is inhibited or destroyed while the other
organism remains unaffected.
Discussion: This relationship in which one is neutral and the other is negative. One is affected and one is
neutral. For example is black walnut. Black walnut secretes a chemical from its roots that harm
neighbouring plants, which was an example of amensalism. Black walnut here is neutral while the other
plants are negative because they are the affected one which cause the chemical in the black walnut.
6)Competition is a relationship in which organisms struggle with each other to obtain limited resources.
The resources such as food, water, sunlight, territory, shelter, nesting sites, and so are the things the
organism needs to survive. Due to this, competition occurs. Thus, competition is defined as a double
negative (-,-) interaction. Competition can occur within a species or between different species.
Competition in community ecology sustains life and strengthens the gene pool. Better competitors are
more likely to survive and pass on their advantageous genetic traits to offspring. Whether a
characteristic is favorable or unfavorable depends on environmental conditions.
Discussion: This relationship in which both are affected and negative because they compete each other in
order to survive and get some foods to eat. For example are the tigers and the lions. They compete with
each other it’s because they are both predators who just depend on prey for them to eat. They are more
likely survival of the fittest and competition of basic needs is always within their perspective. They compete
each other for them to survive. Competition happens mostly in times of lacking of supply.
7) Predation occurs when one organism benefits at the expense of another. This is a positive-negative
(+,-) interaction in that the predator species benefits, while the prey species is harmed. Predation is a
specific type of symbiotic relationship because the predator and prey relationship is a long term and
close one within an ecosystem.
Discussion: In this relationship of organism, one is benefited while the other one is not. The predator is the
one who is benefited while the prey is not. In short predator is much competitor than the prey. For example
of this are cats and dogs in which cat is inferior and dog is superior. There are a lot of examples in predator
and prey and some of this is shown below.
Intraspecies and Interspecies Competition
Competition- is a negative type of interaction between individuals, brought about by a shared
requirement for a resource, and leading to a reduction in the survivorship, growth and/or reproduction
of at least some of the competing individuals concerned. Competition in this strict sense occurs when a
number of animals (of the same or of different species) utilize common resources, whose supply is short.
Examples:
Ecological significance- separation of two closely related or similar organisms by the process of dispersal
or adaptions.
TYPES OF COMPETITION
1. On the basis of taxonomic relationship:
(a) Intraspecific competition (within species)
It is an interaction in population ecology, whereby members of the same species compete for limited
resources. This leads to a reduction in fitness for both individuals.
So, example for this is the Bracon hebetor or the minute wasp this insect is has an internal parasitoid to
larval stage of Indian meal moth.
Individuals of the same species compete for the exact same thing in the environment most especially
when it comes to food. And the intraspecific competition is the strongest type of competition.
There are characteristics of intraspecific competition that affects the community ecology:
● The ultimate effect: decreased contribution of individuals to the next generations.
● The resource must be in limited supply.
● Competing individuals are all essentially equivalent.
● The effect on any individual increases with increasing number of competitors.
(b) Interspecific competition (between species)
● Competitions between different species compete for the same resources in an ecosystem.
● It can be violent, if the competing species are similar; but it is never as strong as intraspecific
completion.
The example for this is the interaction between entomo-pathogenic nematodes for lepidopterans larva.
INTERACTIONS IN INTERSPECIFIC COMPETITION
● CONSUMPTION COMPETITION- occurs when individuals of one species inhibit another species by
consuming a shared resource.
● PREEMPTIVE COMPETITION- occurs primary among sessile organisms where the competition by one
individual precludes establishment (occupation) by others.
● OVERGROWTH COMPETITON- occurs when one organism literally grows another (with or without
physical contact), inhibiting access to some essential resource.
● CHEMICAL COMPETITION- chemical growth inhibitors or toxins released by an individual inhibit or kill
other species. Ex: Sunflower (Allelopathic).
● TERRITORIAL COMPETITION- results from the behavioral exclusion of others from a specific space that
is defended as a territory.
● ENCOUNTER COMPETITION- when non-territorial meetings between individuals negatively affect one
or both of the participant species.
On the basis of mechanism
● Exploitation:
In many cases, competing individuals do not interact with one another directly. Instead, individuals,
respond to the level of a resource, which has been depressed by the presence and activity of other
individuals. It also referred to as resource competition. Thus, it is an indirect competition.
Example: Aphid species competing over the sap in plant phloem. Each aphid species that feeds on host
plant sap uses some of the resource, leaving less for competing species.
● Interference:
Here individuals interact directly with each other, and one individual will actually prevent another from
exploiting the resources within a portion of the habitat. These individuals interfere with foraging, survival,
reproduction of others, or by directly preventing their physical establishment in a portion of the habitat.
It is called direct competition.
Example: Different species of Burying Beetles (Nicrophorus sp.) compete to other species of same
genera for reproductive success.
Succession in Communities
ECOLOGICAL SUCCESSION is a series of progressive changes in the species that make up a community over
time. Ecologists usually identify two types of succession, which differ in their starting points:
In primary succession, newly exposed or newly formed rock is colonized by living things for the first time.
In secondary succession, an area that was previously occupied by living things is disturbed, and then recolonized following the disturbance.
Succession often involves a progression from communities with lower species diversity which may be less
stable to communities with higher species diversity which may be more stable though this is not a
universal rule.
Primary Succession and Pioneer Species
Primary succession occurs when a new land is formed or bare rock is exposed, providing a habitat
that can be colonized for the first time. For example, primary succession may take place following the
eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new rock is
formed. On the Big Island, approximately 32 acres of land are added each year.
First, weathering and other natural forces break down the substrate, rock, enough for the
establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species,
see image below. These species help to further break down the mineral-rich lava into soil where other,
less hardy species can grow and eventually replace the pioneer species. In addition, as these early species
grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil
formation.
During primary succession on lava in Maui, Hawaii, succulent plants are pioneer species.
This process repeats multiple times during succession. At each stage, new species move into an
area, often due to changes to the environment made by the preceding species, and may replace their
predecessors. At some point, the community may reach a relatively stable state and stop changing in
composition. However, it's unclear if there is always—or even usually—a stable endpoint to succession,
as we'll discuss later in the article.
Secondary succession
In secondary succession, a previously occupied area is re-colonized following a disturbance that
kills much or all of its community.
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire.
Wildfires will burn most vegetation and kill animals unable to flee the area. Their nutrients, however, are
returned to the ground in the form of ash. Since a disturbed area already has nutrient-rich soil, it can be
recolonized much more quickly than the bare rock of primary succession.
Before a fire, the vegetation of an oak and hickory forest would have been dominated by tall trees.
Their height would have helped them acquire solar energy, while also shading the ground and other lowlying species. After the fire, however, these trees do not spring right back up. Instead, the first plants to
grow back are usually annual plants—plants that live a single year—followed within a few years by quickly
growing and spreading grasses. The early colonizers can be classified as pioneer species, as they are in
primary succession.
Due at least in part to changes in the environment caused by the growth of grasses and other species,
shrubs will emerge, followed by small pine, oak, and hickory trees. Eventually, barring further
disturbances, the oak and hickory trees will become dominant and form a dense canopy, returning the
community to its original state, its pre-fire composition. This process of succession takes about 150 years.
The path and endpoint of succession
The early ecologists who first studied succession thought of it as a predictable process in which a
community always went through the same series of stages. They also thought that the end result of
succession was a stable, unchanging final state called a climax community, largely determined by an
area's climate. For instance, in the example above, the mature oak and hickory forest would be the climax
community.
Today, the ideas of a set path for succession and a stable climax community have been called into
question. Rather than taking a predetermined path, it appears that succession can follow different routes
depending on the specifics of the situation. Also, although stable climax communities can form in some
cases, this may be uncommon in many environments. Ecosystems may experience frequent disturbances
that prevent a community from reaching an equilibrium state—or knock it quickly out of this state if it
manages to get there.
D. Analysis
Using what you have learned about ecological interactions, think of an example of each interaction
in which humans are involved:
a. Competition: ___________________________________________________________________
b. Parasitism: _____________________________________________________________________
c. Mutualism: _____________________________________________________________________
d. Commensalism: _________________________________________________________________
E. Application
Do you agree with this statement? Why or why not?
“All populations living together within a community interact with one another and with their
environment in order to survive and maintain a balanced ecosystem.”