Hapter 18 Ecology of Organisms and Populations


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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 (mature-forest 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.


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