Chapter 18 – Ecology of Organisms and Populations
Ecology – the scientific study of the interactions between organisms and their
environments. All organisms are intimately tied to all surroundings. When an organism
breathes, it exchanges gases with the atmosphere. When an organism eats, it is acquiring
second-hand energy that was originally harvested from the sun and atmosphere. Waste
products of an organism’s metabolism are returned to the surrounding environment where
they are recycled. Organisms absorb and radiated heat from and to the surrounding
environment. Organisms compete with other organisms for limited resources (food,
water, space). Organisms eat and are eaten by other organisms.
The environment in which organisms live can be divided into 2 basic categories (1)
abiotic (2) biotic.
(1) abiotic includes non-living chemical and physical factors (light, temperature,
pH, gravity, pressure, minerals , air).
(2) Biotic includes living factors (other organisms including bacteria, protozoans,
plants, fungi, animals).
The science of ecology is broad within itself and can be subdivided into many fields of
research. A few specialties include: population ecology, community ecology, ecosystem
ecology, landscape ecology, biosphere ecology, microbial ecology, biogeography,
molecular ecology, ecological modeling, individual-based ecological modeling, chemical
ecology, and physiological ecology. Here are a few definitions that are important for
general biology:
Organismal Ecology – evolutionary adaptations that enable individual organisms
to survive in their environment.
Population Ecology – studies processes of population growth, density, and how
members of a population coexists.
Community Ecology – studies how interactions between species (competition,
predation, and symbiosis) affect community structure and organization.
Ecosystem Ecology – broadens to include the interactions of communities with all
the abiotic factors taken into consideration. This branch of ecology often places
research emphasis upon energy flow and chemical cycling between living and
non-living components among and between communities.
Biosphere Ecology – The most complex level to study ecology. This is especially
a new field of study; it has only recently become practical due to great advances
in satellite imagery of Earth, global communication systems, and breakdown oof
national isolation which has allowed scientists from many countries to work
together and share results. This is definitely he most interdisciplinary Ecological
Science; it brings together scientific findings from many branches of science
including: climatology, oceanography, meteorology, soil science, geology,
physics, chemistry, and all the biological sciences.
It might help to re-examine the various levels of biological organization (levels of
complexity among living systems).
It is at the organism level that we really begin to see how living systems interact with
each other and their respective environments – to study these interactions is to practice
ecology. Ecology provides us with science-based knowledge of how the processes that
shapes life on this planet.
Ecology & Environmentalism
It is important draw a distinction between these 2 terms which are often portrayed as
being the same. We have already defined Ecology as the science of studying interactions
between organisms and their environments ay many levels. Environmentalism is the
movement that began in the 1960’s, grew strongly in the 70’s and is very apparent to us
all today. The environmental movement began as the scientific world started noticing
that when even small environmental conditions change, there are often drastic
consequences for living systems. Once a population, community, or ecosystem is altered,
large-scale changes result. WHY? Because these systems are usually very finely tuned
(through evolution) over millions of years (+ or -). That is, all organisms within a
population, community, ecosystem, biosphere are intimately dependent upon one another
for survival – take one organism or group of organisms out and things have a way of
falling apart.
For more, read about Rachael Carson & the chemical DDT which almost drove many
species of birds into extinction during the 1960’s and 1970’s. We have talked earlier in
the semester about the Post-WWII manufacturing and over-use of agricultural chemicals
(herbicides, pesticides, and fertilizers). Over-use, unregulated use, and misuse have all
added up to a serious threat to biodiversity alone. There is no denying the role agriculture
has played in deteriorating the environment.
The Evolutionary Adaptations of Organisms (Organismal Ecology)
The fields of ecology and evolutionary biology are tightly linked. Although the word
“ecology” had yet to be coined, Darwin noticed that organisms in various regions are
usually well adapted for living in specific conditions and he suggested that interactions
between organisms and their environments (abiotic and biotic) led to evolutionary change
through natural selection. By default, Darwin was an early ecologist.
Events that occur in “Ecological Time” lead to effects over “evolutionary time.”
Good examples of this are seen in predator/prey a relationship which leads to an
“evolutionary arms race” where changes in body armor or body coloration seem
to coincide with constant pressure from predators. This is seen in the fossil record
and in the living world.
Our discussion of Organismal Ecology here will be limited to 3 types of adaptations seen
in organisms; physiological, anatomical, and behavioral. These adaptive categories
mostly are in the context of how individual organism survives in their environment’s
abiotic conditions. If you look at an aerial photo of any landscape, you will notice that
there is patchiness where many different habitats exits side by side. If you examine the
habitats closer, you will find that there are different communities within habitat. In fact, if
you zoom into a specific habitat you will find many smaller scale micro-habitats with
their own inherent small-scale communities. For instance, if you fly over Comanche
County in an airplane, you will notice that there is a definite patchiness to the landscape;
you will see large tracts of residential land, agriculturized land, native prairie, forests, and
lakes. Essentially, you could refer to each as a major category of environments. Now you
decide to parachute down into one of these environments, lets say you end up in a forest.
So you are now in a major habitat type, clearly different from prairie, residential, and
agriculturalized. So if you begin looking, you will see that there are still different types of
habitats in the forested habitat – you have mature forest sections where big mature trees
dominate, you have some sections where there was a fire or tree-cutting and smaller
shrubs, grasses, and forbs dominate. What if you encounter a stream running through the
forest? That sections of the stream will be very biologically different that the stream
sections running through the prairie, residential, or agriculturized habitats and the stream
section of the forest is even different that the surrounding forest. That is, the stream in
the forest supports different life-forms (bacteria, algae, fungi, plants, and animals) than
the other portions of forest. When you examine the other portions of the forest (matureforest and un-mature forest) you will find the same phenomenon – each supports different
forms of life and it share in common a few other forms of life. You can even take it a step
further – start flipping over rocks and logs; you will undoubtedly find little communities
of living things under them and yes, those micro-habitats will most likely have different
life-forms. That is, you will find different species under the rock in the mature forest
compared to under the rock in the immature forest than compared to under a rock at the
edge of the stream in the forest. Just pick one single tree in the forest and compare the
life-forms you find at the bottom of the tree (0 – 1’) and compare those groups of
organisms to the ones you find between 1’ – 5’ and to the ones you find at 5’ – 10’ and to
10’ – 20’ and to 20’ – 30’ …………………………………………………… What you
will soon learn is that yes even at different heights above the ground you will find groups
of living organisms that are specialized to live at exactly that height. Now walk overt to
the stream and begin walking down it. What you will find is that the stream itself is a
very diverse system with its shallow riffles, deep pools, shaded areas, and areas receiving
sunlight. Yes, if you compare the living organisms among each of those microhabitats
you will find that they differ – there are species adapted specifically to each those very
narrow range of conditions. You will also find that there are a few species that seem to
do well in all range of conditions (generalists).
Why do habitats (and the communities within) differ even at very subtle scales?
Abiotic Factors
Sunlight - Solar energy powers nearly all ecosystems. Since sunlight is such an important
resource, many life-forms spend energy competing for it.
Water – Water is limited resource that life is very dependent upon. The amount of water
on the ground, in the air, in the soil, and at different times of year influence the types of
life-forms that are successful in different habitats. The ability to conserve water or get rid
of excess water are some major adaptations we see in organisms.
Temperature – Temperature is highly important because of its influential role on
metabolism. Most organisms operate at optimally at between 32º F and 122 º F. Above
and below those temperatures, most organisms don not function well, partially due to the
denaturing of enzymes and destruction of tissues and cells. However, some species are
well adapted to extreme temperatures, including many desert species of plants and
animals as well as some reptile and amphibian species that can withstand being
completely frozen for several months each year. The ability to maintain internal
temperatures is a big hurdle facing living organisms and likewise, there are many
adaptations (physiological, anatomical, & behavioral) that enable organism to survive in
different habitats on Earth.
Wind – Wind has many effects upon organisms. Some groups that live on nutrient poor
substrates depend on wind to blow in nutrients (i.e., bacteria, protozoans, & some
insects). Many plants depend on wind to disperse their pollen (sperm) and seeds. Wind
storms create patchiness in forest by blowing down trees. Wind also influences the rate of
water loss by plants. Wind also plays apart in evaporative cooling which directly affects
the internal temperature control of animals (mostly mammals). Wind chill can be a factor
for organism s in temperate and arctic regions where any from of shelter serves as an
important micro-habitat type.
Rocks and Soil – The physical and chemical structure of rock and soil limit the
distributions of plants and the animals that feed on plants. The chemical content of soil
and rock impact the conditions of water sources (streams, rivers, and lakes).
Disturbances – Disturbances (i.e., fires, hurricanes, tornadoes, volcanoes, drought, flood,
grazing) can greatly impact biological communities and ecosystems. After a disturbance,
the habitat begins to reemerge and is slowly re-structured. This process of disturbance
and re-structuring is called succession and is a vital role in biological communities and
ecosystems.
Organism adaptations to changing or varying abiotic factors are what allows for them to
survive and reproduce in all the different habitats on Earth. Responses of organisms to
their local abiotic conditions include (1) physiological (2) anatomical (3) behavioral.
Examples of each are:
(1) Physiological - when temperatures drop, mammals involuntarily raise their
hair follicles by actions of tiny muscles at the base of each hair. This allows
for mammals to increase the amount of trapped air around the body – dead air
space is a natural insulator (what is the purpose of an attic in a house?).
Another example is when animals are cold, they involuntarily shiver – this
shivering creates heat between the muscle cells and thereby protects the
tissues from freezing. Another example is animals in higher elevations are
confronted with lower oxygen concentrations in the air. The body responds by
producing more red-blood cells to carry more oxygen. Some groups are
geographically limited by physiologies that don’t allow for them to acclimate
to extreme conditions – the inability of reptiles to effectively regulate their
internal temperature inhibits them from living in arctic and sub-arctic
environments.
This map shows the number of lizard species found along a North-South
gradient in N. America. As you know, the different shaded regions indicate a
gradient in severity of winter weather. Its no coincidence that cold weather
and the abundance and distribution of lizard species seem to be correlated –
they are! This is good example of biogeography as an important data set in
biology.
(2) Anatomical Responses – Many organisms respond to the environment by
changing some aspect of their anatomy. Examples include growing a heavier
coat of fur and hair. Plants are more finely tuned into this type of response.
Plants can shift their energy allocations am ong various parts of their anatomy
depending on changes in environmental conditions. Examples include
growing towards light, devoting more energy to growing deeper roots in cases
of drought, changing energy allocations between seed production and starch
storage in the roots.
(3) Behavioral Responses – In contrast to plants, animal can move to more
favorable conditions in response to changing environmental conditions. This
can be daily movements fomr shade to sunlight (reptiles). This can also
include great long-distance movements with change in seasons – migration
(mammals, birds, and fish). Behavioral responses also include building and
maintaining shelters in preparation for incelement conditions.
Population Ecology – Biologists define a population as a group of individuals of the
same species living in the same area at the same time. Population ecology is the scientific
study of how populations interact with the environment. There are many interesting
questions that can be asked concerning populations and many population ecology studies
have shed light on processes that contribute to population dynamics. This branch of
biology has been instrumental in conservation and understanding evolution (remember,
the population is the smallest unit on which evolution occurs). Population ecology has
also helped us figure out how diseases spread.
Here are a few things that population ecologists study:
(1) Population density – the number of individuals of a species per unit are or volume.
Determining how many individuals are in a given space helps to shed light on basic
biology of the species. For instance, you can determine how much land is needed for the
conservation of an endangered species until you do such basic research on population
density. Ecologist often use Mark-Recapture Analysis which requires catching,
marking, and re-capturing individuals of a population to determine an estimate of how
many individuals live in that given amount of space. Various versions of this method use
different formulas and sampling techniques but they all allow researchers to estimate
population density.
(2) Patterns of Dispersion – dispersion refers to the way in which individuals are spaced
in a population. There are 3 general types of dispersion patterns: clumped, uniform, and
random. A species will usually demonstrate 1 dispersion pattern and the dispersion
pattern says a lot about the biology, ecology, natural history, conservation needs, and
sometimes the evolution of a species.
(3) Population Growth Models – the rates at which populations grow is a reflections of
the reproduction biology of the respective species. Consider a single bacterium that can
reproduce by dividing into 2 parts every 20 minutes. After 20 minutes there would be 2
bacteria, 4 after 40 minutes, and so on. If this continued for only 36 hours, there would be
enough offspring of that original one bacterium to cover the entire earth 1 foot deep! At
the other extreme, a pair of elephants may produce only 6 young over a period of 100
years. Still it would only take 750 years for a single pair of elephants to give rise to an
eventual population of 19,000,000 elephants!
So why doesn’t this happen? The answer is limited resources, disease, and so on
Ecologists use several measures to calculate changes in population size. The growth of a
population is equal to the number of births minus the number of deaths. Growth Rate is
the change of in population size per time interval. There are 2 population Growth Models
that we will look at here.
(1) Exponential Growth Model – the rate of expansion of a population under ideal
conditions in which the population multiplies by a constant factor during constant time
intervals (generation time). The constant factor for bacteria is 2, because each parent cell
divides to produce 2 daughter cells. The generation time for the bacteria in the graph
below is 20 minutes. With each passing generation, the population size increases by an
exponent of the number 2 – hence the name Exponential Growth Model. This Growth
model occurs under ideal conditions and foe only short bursts of time. Eventually, limited
resource become exhausted and/or disease spread more and more rapidly. In nature, we
usually only see this type of growth when organisms are confronted with a new or underexploited environment.
(2) Logistic Growth Model – In nature, a population may grow exponentially for a short
while but eventually, one or more environmental factors will limit its growth – at which
the population will stops increasing or crashes. Environmental factors that restrict growth
are referred to as population-limiting factors. Logistic Growth is characterized by
carrying capacity, the highest number of individuals which an environment can support;
remember resources are limited.
Notice that with Logistic Growth, a population tends to grow rapidly when resources are
abundant but growth starts to rapidly slow and even come to a halt as resources become
depleted. Resources can include food, water, light, oxygen, mates, shelter, breeding sites,
nesting space, and so on.
Regulation of Population Growth – There are 2 major categories of limiting factors (1)
Density-Dependent (2) Density-Independent.
(1) Density-Dependent – these are factors that change in intensity as a function of
population density (i.e. the more individuals crammed into a finite space with finite
resources, the more severe the density-Dependent Limiting Factors.
Includes Intra-specific Competition – members of the same species and same
population compete for limited resources. This is a major factor in population dynamics.
As you can imagine, this type of struggle for resource does in fact usually lead to survival
of the fittest. Those individuals more capable of acquiring resources will usually survive
better and leave more offspring.
(2) Density-Independent – these are limiting factors that are more or less random in that
they don’t act as a function of population density. However, these limiting factors can
have extreme effects upon population growth. These factors include: floods, drought,
lighting, fires, volcanic eruptions, …………………
Human Population Growth – The graph below says it all. Yes, we have escaped (for
the time being) the bounds of nature; that is, most limiting factors. With medical
advancements and abundant food supplies, the worldwide human population has
increased dramatically. Since the industrial revolution, the human population has become
less and less dependent on the ability to acquire resources directly from nature – we have
become a consumer species with many luxuries. As you will soon learn, over-population
of humans is straining all of Earth’s resources.
Remember, population growth is characterized by birth rate minus death rate. So, lower
birth rates doesn’t simply lead to lower population growth – if the death rates are also
dropping then the population might be growing.
Age Structure and Population Growth – You can determine a lot by simply looking at
the age structure of a population. Knowing the average life expectancy and how many
individuals are in each generation (seniors, middle age, and young) gives you some
predictive power of what the future holds. Think about it, why do we hear so much about
the “Baby Boomers?” Because very soon a lot of people are going be retiring and
applying for social security benefits and other entitlement programs.
Look at the diagram below – it shows the age structure of 3 different countries and the
projected population growth based merely on age structure. If you were to look at
impoverished nations versus industrialized nations you see some more interesting trends.
It turns out that the poorest of countries are experiencing the most rapid population
growth of all – you may ask why? There are lots of factors that play a part in this.
Accelerated human population growth will cause many things to occur in our own
lifetime – increased war, famine, disease, religious mania, environmental degradation.
The Rule of 72 – How long does it take for a population to double? If you take the
average % population growth of a population and divide that into the number 72, you will
get the number of years it will take for that population to double.
Example: In some poor regions the annual population growth is around 3.5%; that
doesn’t sound like much does it? So how many years will it take for that region to double
in humans?
72 ÷ 3.5 = 20.5 years. So, in about 21 years the population will double. If we are
talking about a poor country where the people are already having a tough time, in 21
years they will be having double the problems. This leads to things like famine and
genocide – sound familiar? It should, because these are the scenarios that many regions
are facing today (Darfur, Zimbabwe, ect).