CONCEPTS OF POPULATION
ARIZONA STATE UNIVERSITY
ABS 370 - ECOLOGY
SPRING 2024
A.
PROPERTIES OF GROUPS POPULATION
Population is a group of organisms of the same species that inhabit a place, having special
characteristics or properties of the population/group and not individual characteristics. These
characteristics include: density, natality (birth rate), mortality (death rate), age distribution,
biotic potential, dispersion, growth and development.
B.
POPULATION DENSITY AND RELATIVE NUMBER INDEX
Population Density is the size of a population in relation to a unit of space. It is generally
expressed as the number of individuals or population biomass per unit area or volume, e.g.
200 trees/Ha, 5 million diatoms/m3 . The calculation of numbers is too concerned with the
meaning of small organisms, while biomass is too concerned with the meaning of large
organisms, while the energy flow component provides a better index for comparing any
population in the ecosystem.
In practice it is often more important to know if a population is (changing/evolving) than
to know the number of population members population at any one time. In this case, the
relative abundancy index is useful in relation to time, such as the number of birds seen each
hour.
The difficulty in measuring population density is that organisms are not evenly
distributed, but are unevenly distributed or clustered, therefore in taking a sample of
population density research must be careful.
1.
Basic Concepts About Rates
Since populations are ever-changing entities, we are not only interested in the size and
composition at any one time, but also how the population is changing.
Some important characteristics associated with population change are rates. A rate is
obtained by dividing the change by the period of time over which the change takes place.
Number of births per year = birth rates. The term rates indicates the speed at which
something changes over time.
2.
Natalitas Birth Rate
Natality is the ability of a population to grow. Natality rate, birth rate in demography is
obtained by hatching births, or germination, and so on.
Ecological natality or true natality or simply called natality is the increase in population
under actual circumstances. It is not fixed depending on the environment.
3.
Mortality
Mortality is the death rate in a population. Minority rate is the death rate in demography
is the number of individuals who die at a unit time (= deaths per time).
Ecological mortality i.e. real morality, i.e. the number of individuals who die in actual
environmental conditions, the price is not fixed depending on the environmental conditions.
Minimum (theoretical) mortality is the loss of individuals from the population under ideal
environmental circumstances and the price is fixed.
Often the survival rate is more interesting than the death rate. If death rate = M, then
survival rate = 1- M.
Since we are often more interested in living organisms than dead organisms, it is often
more meaningful to express the mortality rate in its inverse, i.e. by expressing the survival
rate.
4.
Age Distribution of Population
Age Distribution Age is an important trait of a population as it can affect mortality and
natality. A comparison of the various age groups in the population can determine the
reproductive state that takes place in the population and can be used to estimate the future
state of the population.
Rapidly growing populations contain mostly young individuals, while stationary
populations are more evenly distributed in age and declining populations are mostly old
individuals. In a steady state population distribution, exceptional births/deaths will result in a
temporary change in the population which then returns to a steady state.
According to Bodenheimer (1939) in the population there are 3 ecological age groups,
namely:
1.
pre-reproductive
2.
reproductive
3.
post-reproductive
The relative length of this ecological lifespan compared to the length of the lifespan
varies greatly. In modern humans these three elements are approximately equal in length, in
primitive humans, post-reproductive is short. In some animals (insects) and plants pre-
reproductive is very long, reproductive is short and post-reproductive is absent. The
theoretical age pyramid shows the high percentage of low, medium and high population sizes.
Age levels are grouped into immature (prereproductive), fecound (reproductive), and
nonfecound (post-productive).
5.
Intrinsic Rate of Natural Increase
If the environment is unlimited (space, food, and organisms have no constraints) then the
specific growth rate (i.e. the growth rate per individual) becomes constant and maximum, and
is characteristic for the general structure of a given population and is a single index for the
strength of population growth expressed by r.
the growth rate of the overall population in an unbounded environment (r) depends on :
1.
age composition
2.
the specific growth rate that binds to the reproduction of the age group component.
6.
Population Fluctuation and Cyclic Swing (isolation)
When the population has completed its growth, the average N/t equals zero, the
population density tends to fluctuate above and below the upper level of the asymptote or
carrying capacity.
These fluctuations are the result of changes in the physical environment or interactions
within populations or both or between populations. Fluctuations can therefore occur even
when environmental conditions are fixed, for example in a laboratory.
In nature it is necessary to distinguish :
a.
Seasonal changes in population size are largely influenced by life history adaptations
together with changes in environmental factors.
b.
Annual fluctuations.
There are two kinds of annual fluctuations (swings):
a.
Fluctuations influenced by changes in physical environmental factors that occur on an
annual basis or extrinsic factors (i.e. factors outside of interactions within the
population).
Fluctuations influenced by differences in physical environmental factors tend to be
irregular and are clearly related to variations in limiting physical factors such as
temperature, rainfall and so on.
b.
Fluctuations that are mainly influenced by population dynamics or intrinsic factors (i.e.
factors within the population). Fluctuations of this type often show regularity, so the term
"cycle" is adequate. Annual fluctuations will be great in relatively simple ecosystems
where the community consists of only a few populations such as polar populations,
artificial forests, and so on. It can be said that the older and more organized the
community, the lower the population fluctuations.
7.
Population Regulation and Control
In low- and medium-diversity ecosystems, physical stresses tend to depend on physical
components such as weather, currents, pollutants, and pollution. Whereas in ecosystems with
high diversity or no physical stress, the population tends to be controlled biologically.
In all ecosystems there is a strong tendency for populations to evolve through natural
selection and towards self-control.
Ecological factors that are either adverse limiting factors or non-limiting factors
(favorable factors) or negative factors or positive factors to the population can be classified as
factors:
a.
Density independent/density legislative or density independent if the influence or effect
does not depend on the size of the population. Examples: climatic factors, hurricanes,
drastic temperature drops, light factors, and so on.
b.
Density dependent is an ecological factor whose influence/effect on the population is a
function of population density. The influence of density dependent factors is like an
engine regulator because it can be the main tool to prevent overpopulation and is
responsible for achieving a state of balance (steady state).
Density dependent factors are biotic factors, such as competition, parasitism, pathogens,
natality, morality, and so on. Examples of density dependent are flies Weeds and their
parasites where the percentage of weed flies killed by parasites increases with population
size. Sometimes the steady state of the population is disrupted by weather changes such as a
drastic drop in temperature which can lead to a decrease in insect parasites and as a result the
insect population will rise rapidly and form a J-shape curve. If such a situation occurs, it can
lead to deforestation of Eucalyptus trees so that insects will lack food and the insect
population will decline again drastically. The decline in insect populations can also be caused
by the increase in predator populations as a result of many insects.
C.
POPULATION GROWTH PATTERNS AND THE CONCEPT OF CARRING
CAPACITY
Populations have a distinctive growth pattern, called the population growth form.
There are two basic patterns of growth based on the growth curve, namely :
1.
Growth curve of J shape.
2.
S-shape or sigmoid growth curve.
The population growth pattern can be J-shaped or S-shaped or a combination of both
according to the specificity of the growth of the population of organisms and their
environment.
In the J-shape growth pattern, density rises exponentially fast and then stops abruptly as
environmental barriers or limiting factors suddenly become effective.
In the sigmoid shape population growth pattern the population initially rises slowly
(positive acceleration phase) then becomes fast (logrithmatic phase) then slows down again
after environmental barriers begin to work (negative acceleration phase) and finally almost
balanced.
The upper limit where there is no more growth is the asymptote of the sigmoid curve
which is commonly called the carrying capacity. In the J-shaped growth pattern there is no
equilibrium level but the N limit is the upper limit determined by the environment.
D.
POPULATION SPREAD
Population dispersal is the movement of individuals or offspring (seeds, spores, larvae)
out of a population or population area. There are three patterns of population dispersal:
a)
Emigration: one-way outward movement
b)
Immigration: one-way inbound movement
c)
Migration: periodic movement in and out The effect of dispersal on a population
will be :
a.
Small, if there are few individuals entering/exiting the population or the population
is large.
b.
Large, if the spread that occurs is missal (very large in number) and occurs in a
short period of time.
Population dispersal is influenced by :
1.
Barriers, for example: rivers, mountains, valleys, and so on.
2.
Vigor or the ability to move organisms generally organisms with high vigor will
facilitate dispersal, for example birds, insects.
Dispersal is the means by which new and empty areas that were previously unoccupied
will become inhabited so that a new balance is formed, besides that dispersal is also
important for gene flow and the formation of new species.
Passive dispersal of small organisms generally follows an exponential pattern, meaning
that population density decreases by the same amount for equal multiples of distance from the
source. Active dispersal of large organisms deviates from this pattern.
The effect of dispersal on population depends on :
a.
The status of the population growth form, whether the population is near/far from
carrying capacity, growing or declining.
b.
The speed of spread and this is influenced by barriers and the vigor of the organism.
If the population is in balance with its environmental factors, then moderate migration
or emigration has little effect on the population, but if the population is above or below the
carrying capacity of the distribution, the effect will be more obvious, for example, emigration
will accelerate population growth or vice versa, emigration will accelerate extinction.
Migration often involves the mass movement of populations (birds, locusts). It is often
practiced by vertebrates and insects. Seasonal and diurnal (day and night) migrations are
important for :
a.
Occupy an empty area
b.
Allows organisms to maintain optimum density and high activity.
Populations that are unable to migrate are often subject to reduced population densities
or dormancy under unfavorable circumstances.
1.
Population Patterns: Internal Dispersion Pattern
Individuals in a population can be dispersed according to three patterns:
a.
Random
b.
Uniform (more regular than random)
c.
Clustering (disorganized, not random)
Random dispersal is rare in nature and can occur when the environment is very uniform
and there is no tendency to cluster.
Uniform dispersal occurs when competition between individuals is intense or there is
positive anatagonism that encourages equal sharing of space.
Clustering of varying degrees is the most common pattern in populations and is almost
the rule when viewed from an individual perspective. Note, however, that the distribution of
clusters is close to random.
From the three basic patterns of organism dispersal, 5 (five) types of dispersal can be
established:
a.
Uniform
b.
Random
c.
Random clumping/grouping
d.
Clustered/grouped uniform
e.
Group get together
Sampling the population for the latter three patterns should be done carefully as it can
give very different results. A small sample of a population with a clumped distribution may
give results with densities that are too high or too low.
The tendency of organisms to group for example when breeding, forming colonies
(ants, termites). Examples of random populations are rice bugs, mussels in the mud this
happens because the environment is very homogeneous.
E.
TYPE OF INTERACTION BETWEEN TWO SPECIES
1.
Negative Interaction: Interspecific Competition
Interspecific competition is any interaction between two or more populations of a
species that can affect the growth and life of the population.
The tendency to compete leads to ecological segregation of adjacent or similar species,
and this is known as the competitive exclusion principle. Interspecific competition can result
in:
1.
Adjustment of the balance between the two species;
2.
Resulting in replacement population species onewith populations of another species;
3.
Forced to move;
4.
Forcing the use of other types of food.
Table 2.1. Types of interspecies interactions
No.
Type of interaction
Species
General nature of interaction
I
II
1.
Neutralism
0
0
Second the population of not
each other
disturbing.
2.
Competition: type of
disruption
-
-
Direct barriers to both
directly
population.
3.
Competition: type
-
-
Indirect barriers, when the source
use of resources
power decreases.
4.
Amensalism
-
0
Population I is inhibited,
population II is not
inhibited.
5.
Parasitism
+
-
Population I parasites, generally
smaller
population II (host).
6.
Predation
+
-
I predator populations, generally
more
(host) is not disturbed.
7.
Commensalism
+
0
Population I commensal profit,
population II
(host) is not disturbed.
8.
Protokooperasi
+
+
Interaction benefits both,
but not interdependent.
Both populations benefit from each
other and
9.
Mutualism
+
+
both interdependent
dependent,
cooperation is an obligation.
In nature, closely related species or those with similar interests generally occupy
different geographical areas or have different daily activities to reduce the amount of
competition. Competitive interactions can lead to changes in morphology, bill shape, body
size (through natural selection) and this can accelerate ecological separation.
Islands are a good place to observe habitat selection trends that are more likely to be
Was if inter-competition is present. Specific (between populations) competition is reduced,
consequently intraspecific (within populations of a single species) competition will be
prominent and populations of species tend to disperse.
2.
Negative Interactions: Predation, Parasitism, and Antibiosis
Predation and parasitism are examples of interactions between two populations that
have a negative effect on the growth and life of one of the populations. The same result
occurs if one population produces a substance that is detrimental to the other. This interaction
is known as antibiosis.
Such negative effects tend to be reduced in populations that have interacted for a long
time and in stable ecosystems where one species does not exterminate the other.
Destruction can occur in new and unstable ecosystems, such as sudden changes due to
human actions. This can lead to what is known as the principle of instant pathogen, which
explains why human actions often lead to epidemic problems.
Predators and parasites do suppress population growth rates, but would populations be
better off without them. Predators and populations play a role in keeping exploding
populations, such as populations of insects, birds, and so on, from becoming overpopulated.
Population explosions can occur if a species is introduced into a new area, where there
are unexploited resources and no negative interactions (e.g. predators).
3.
Positive Interaction: Commensalism, Cooperation, Mutualism
Positive interactions between two populations of species are common and are as
important as competence and parasitism in determining population and community traits.
Evolutionarily, positive interactions start with commensalism (+0) which then evolves into
mutualism (++) where both species are interdependent.
Generally interaction positive less received attention than negative interactions although
both are equally important.
Commensalism is a simple type of positive interaction and may be the first step towards
a mutually beneficial relationship (epiphytes, anemones on shellfish).
In older ecosystems, mutualism will regulate parasitism and this is important when
some aspects of the environment are in a state of limitation, in which case cooperation is
beneficial.
Obligate symbiosis between cellulose-digesting microorganisms and animals, important
for the detritus food chain. In the symbiosis between termites and flagellates (order
hypermastigina), termites will die without the cooperation of flagellates, because termites
cannot digest cellulose and will eventually starve. Coordination between termites and
flagellates (Spirotrichonimpha bispira) are very good, for example, flagellates are responsive
to the host molting hormone, i.e. when termites molt (hormone influence), flagellates will
form cysts, thus ensuring transmission and reinfection after molting.
Community Concepts:
Community is an important concept because in nature different types of organisms live
together in an orderly way and are not just spread out and what is experienced by the
community will also be experienced by the organism. So to destroy an organism we can do so
by changing its community. For example, mosquitoes can be controlled efficiently and
cheaply by changing the aquatic community by raising and lowering water levels and
currents. The control of weeds on the roadside is not by clearing the road with
plowing/hoeing, but by developing a steady vegetation where weeds are outcompeted.
A.
COMMUNITY AND DOMINANT CONCEPTS ECOLOGY
In ecology there is a collection of populations called a community, where the
Community is a collection of various populations living at a certain time and area that
interact and influence each other. Communities have a more complex degree of integration
when compared to individuals and populations.
The name of the community should be able to provide information about the
characteristics of the community. The simplest way to name it is to use words that indicate
what the community looks like, such as grassland, desert, teak forest.
There are several principles at play in community-level organization. A biotic
community is a collection of populations that occupy a habitat and are organized in such a
way that they exhibit additional characteristics of individual and population characteristics as
a whole, such as food web structure and energy flow.
Major communities are large communities that do not depend on other communities
nearby. Minor communities are communities that still depend on other communities nearby.
The best way to name a community is to take some clear and definite characteristics,
whether living or not. Naming a community can be based on :
1.
Main forms or structures such as dominant species, living forms or other indicators such
as pine forests, agathis forests, teak forests, or Dipterocarphaceae forests, can also be
based on dominant plant traits such as sclerophyll forests.
2.
Based on the physical habitat of the community, such as mudflat communities, sand
beach communities, ocean communities, etc.
3.
Based on functional traits or signs such as the metabolic type of the community. Based
on the nature of the natural environment such as climate, for example found in tropical
areas with rainfall that is evenly distributed throughout the year, it is called a tropical
rainforest.
In nature, there are various communities that can be broadly divided into two parts:
1.
Aquatic community.
These communities are for example those found in the sea, in lakes, in rivers, in ditches
or in ponds.
2.
Terrestrial communities.
It is a group of organisms found in the yard, in the forest, in the meadow, in the field.
desert, etc.
Not all organisms in a community are equally important in determining the natural state
and function of the entire community. Of the hundreds/thousands of organisms in a
community, only a few species play an important role in controlling the community based on
their number, size, production, or activity.
The relative role of populations in the community is not indicated by the taxonomic
relationships of species because the controlling or dominating organisms often vary widely in
taxon.
Therefore, intracommunity classification is not the same as the taxonomic system of
flora and fauna, but the taxonomy is based on the role of organism species in the community.
The logical classification system in this view is based on the trophic level or functional
level. Communities are (mostly) composed of producers, macroconsumers and
microconsumers. The group of species that controls most of the energy flow and has a major
influence on the environment and other species is said to have ecological dominance.
(ecological dominant)
The degree of concentration of dominance in one or more species is expressed in the
dominance index, which indicates the role of the organism species in relation to the
community as a whole.
The conclusions from the observations are as follows: the dominant producer is
sampang grass, while the dominant consumer is cattle. So the community is a grazing area. A
more complete picture will be obtained if we ask about the seasonal use of the area.
If a dominant species is removed from a community, it will have a major effect on the
biotic and abiotic (microclimate) community. If non-dominant species are removed, the effect
will not be as great as the dominant species. Generally, dominant species are species with
high productivity. For small organisms, biomass can be used as an indicator of dominance.
On land, spermatophyta are dominant not only among otrophytes, but also in
communities because they provide protection against other organisms and densely modify
physical factors in many ways.
High-diversity communities such as rainforests will be more resilient to
climatic/environmental disturbances.
Diversity tends to be higher in older communities and lower in newly established
communities. Productivity is influenced by species diversity but the relationship is not linear.
Communities with high productivity can have high diversity (rocky areas) or low species
diversity (estuarine areas, temperate climates). Stability is more related to diversity than
productivity.
It turns out that species diversity is strongly influenced by food levels. For example, the
number of herbivores or predators greatly affects the grasses or communities that are preyed
upon.
Moderate predation often reduces the density of dominant organisms and thus reduces
competition between species, giving other species a better chance to gain space and food,
thus increasing diversity. But conversely, heavy predation will be stressful and reduce the
number of species. This is true as long as competition for space is high.
Diversity indices are the best way to recognize and assess pollution.
4
3
2
1
0 10 20 30 40 50 60 70
Figure 3.1. Changes in Shannon diversity index of Benthos in the stream during pollution
(domestic and factory pollution).
B.
PATTERN IN THE COMMUNITY
What is meant by pattern (Huchinson, 1953) is the structure that results from the
distribution of organisms in their environmental field and their interaction with the
environment.
Several kinds of diversity patterns in communities:
a.
Stratification pattern (upright/vertical layer)
b.
Zoning pattern (horizontal separation)
c.
Activation pattern (periodicity)
d.
Food web pattern
e.
Reproduction pattern
f.
Social patterns (groups and herds)
g.
Co-active patterns (results of competition, antibiosis, etc.) Some examples of patterns
in communities:
-
Stratification pattern
There are two basic layers in the forest: autotrophic and heterotrophic layers. Vegetation
layers: shrub layer, lower canopy trees, upper canopy. Animal layer: insect and bird
distribution. Water body layers: epilimnion, thermokline, and hypolimnion as well as
Pollution starts
stratification in fish and benthos.
-
Activity pattern (periodicity)
Most populations in communities exhibit periodicity that corresponds to changes that occur
over a 24-hour period (changes in light, temperature, and so on).
Diel periodicity is an event that recurs at intervals of 24 hours or less.
Circadian rhythm (circadian = approximately one day) is a fixed perodicity, which is
regulated by a biological clock related to the pattern of the day-night cycle.
Diurnal organisms are active during the day, nocturnal organisms are active at night,
and crepuscular organisms are active in dim light.
An example of daily periodicity in aquatic habitats is the vertical migration of
zooplankton, which generally moves upward during weak light and moves downward during
strong light (daytime).
In addition to the above, there is also seasonal periodicity that cycles over the course of
a year. In temperate climates, temperature in combination with day length can affect
flowering time and bird migration.
Conventionally we think of four seasons as spring, summer, fall, and winter, but
ecologists divide the seasons into six:
1.
Hibernal (winter or hienal)
2.
Prevernal (beginning of spring)
3.
Vernal (late spring)
4.
Aestival (beginning of summer)
5.
Serotinal (late summer)
6.
Autumnal
C.
ECOTONES AND THE CONCEPT OF EDGE EFFECTS
Ecotones are the transition of two or more distinct communities (forest-grassland, sea-
land, salt-fresh).
Ecotone communities usually contain some members of both communities and overlap
with the addition of some species restricted to the ecotone. Generally, the species variety and
population density of the ecotone is greater than that of the adjacent community. The
tendency for diversity and density to increase in adjacent communities is called the edge
effect. Organisms that are abundant or mostly found in the ecotone are called edge species
(edge effect).
D.
COMMUNITY STRUCTURE
Community Structure Community character
1.
Qualitative, such as composition, life form, phenology and vitality. Vitality describes the
growth and reproductive capacity of an organism.
2.
Quantitative, such as frequency, density and relative density. Frequency of presence is a
value that expresses the number of times a species is present in a habitat. Density is
expressed as the amount or biomass per unit of sample, or unit area/volume, or unit of
capture.
3.
Synthesis is a process of change in a community that proceeds in a slow, orderly,
purposeful and predictable direction. Successions occur as a result of modifications in
the physical environment of the community and take time. This process ends with a
community or ecosystem called a climax. At this stage the community is homoestotic.
According to the latest concept, succession is the replacement of pioneer species by more
stable species that are well suited to their environment.
E.
INTERACTION WITHIN THE COMMUNITY
In a community, all organisms are part of the community and its components are
interconnected through a diversity of interactions. Interactions between ecological
components can be between organisms, between populations, and between communities.
1.
Interactions between organisms:
All living things are always dependent on other living things. Each individual will
always be in contact with other individuals of the same or different types, either individuals
within their own population or individuals from other populations. We see such interactions
all around us.
Interactions between organisms in the community are very close and some are less
close. Interactions between organisms can be categorized as The following. Neutral is a
relationship that does not interfere with each other¬ between organisms in the same habitat
that is not beneficial and does not harm both parties, called neutral. For example: between
dragonflies and cows.
Predation is the relationship between prey and predator. This relationship is very close
because without prey, predators cannot live. Conversely, predators also control the prey
population. Example: Lions with their prey, namely deer, gazelles, and owls with mice.
Parasitism is a relationship between organisms of different species, when one
organism lives on another organism and takes food from its host so that it is detrimental to its
host. Example: Plasmodium with humans, Taenia saginata with cows, and parasites with host
trees.
Commensalism is a relationship between two organisms of different species living
together to share a food source; one species benefits and the other species is not harmed. An
example is an orchid and the tree it lives on.
Mutualism is a relationship between two organisms¬ of different species that
mutually benefits both parties. For example, Rhizobium bacteria that live in the root nodules
of legumes.
2.
Interpopulation Interaction:
There is always direct or indirect interaction between one population and another in
its community. Examples of interpopulation interactions are as follows.
Allelopathy is an interaction between populations, where one population produces a
substance that prevents another population from growing. For example, around walnut trees
(juglans) other plants rarely grow because these plants produce substances that are toxic. In
microorganisms the term allelopathy is known as anabiosa. For example, the fungus
Penicillium sp. Can produce antibiotics that can inhibit the growth of certain bacteria.
Competition is an interaction between populations, when there are similar interests
between populations, resulting in competition for what is needed. For example, competition
between a population of goats and a population of cows in a pasture.
3.
Interaction between Communities:
A community is a collection of different populations in the same area that interact with
each other. Examples of communities include rice field and river communities. A rice field
community is made up of various organisms, such as rice, grasshoppers, birds, snakes and
weeds. The river community consists of fish, algae, zooplankton, phytoplankton and
decomposers. There is interaction between the river and paddy field communities in the form
of nutrient circulation from river water to paddy fields and circulation of living organisms
from both communities.
Interactions between communities are complex because they involve not only
organisms, but also energy and food flows. We can observe interactions between
communities, for example, in the carbon cycle. The carbon cycle involves different
ecosystems such as marine and terrestrial.
4.
Interaction between Biotic and Abiotic Components:
Interactions between biotic and abiotic components form ecosystems. The relationship
between organisms and The environment causes the flow of energy in the system. In addition
to the flow of energy, there are also trophic structures or levels, biotic diversity, and material
cycles in ecosystems. With these interactions, an ecosystem can maintain its balance. The
arrangement to ensure this balance is the hallmark of an ecosystem. If this balance is not
obtained, it will encourage the dynamics of ecosystem change to achieve a new balance.
