2 An Introduction to Ecology
Ecology is the study of the interactions of living organisms with their environment. One core goal of ecology is to understand the distribution and abundance of living things in the physical environment. Attainment of this goal requires the integration of scientific disciplines inside and outside of biology, such as biochemistry, physiology, evolution, biodiversity, molecular biology, geology, and climatology. Some ecological research also applies aspects of chemistry and physics, and it frequently uses mathematical models.
Levels of Ecological Study
When a discipline such as biology is studied, it is often helpful to subdivide it into smaller, related areas. For instance, cell biologists interested in cell signaling need to understand the chemistry of the signal molecules (which are usually proteins) as well as the result of cell signaling. Ecologists interested in the factors that influence the survival of an endangered species might use mathematical models to predict how current conservation efforts affect endangered organisms. To produce a sound set of management options, a conservation biologist needs to collect accurate data, including current population size, factors affecting reproduction (like physiology and behavior), habitat requirements (such as plants and soils), and potential human influences on the endangered population and its habitat (which might be derived through studies in sociology and urban ecology). Within the discipline of ecology, researchers work at four specific levels, sometimes discretely and sometimes with overlap: organism, population, community, and ecosystem (Figure 2.1).
Figure 2.1: Ecologists study within several biological levels of organization (credit “organisms”: modification of work by “Crystl”/Flickr; credit “ecosystems”: modification of work by Tom Carlisle, US Fish and Wildlife Service Headquarters; credit “biosphere”: NASA).
Organismal Ecology
Researchers studying ecology at the organismal level are interested in the adaptations that enable individuals to live in specific habitats. These adaptations can be morphological, physiological, and behavioral. For instance, the Karner blue butterfly Lycaeides melissa samuelis (Figure 2.2) is considered a specialist because the females preferentially oviposit (that is, lay eggs) on wild lupine. This preferential adaptation means that the Karner blue butterfly is highly dependent on the presence of wild lupine plants for its continued survival.
Figure 2.2: The Karner blue butterfly Lycaeides melissa samuelis is a rare butterfly that lives only in open areas with few trees or shrubs, such as pine barrens and oak savannas. It can only lay its eggs on lupine plants (credit: modification of work by J & K Hollingsworth, USFWS).
After hatching, the larval caterpillars emerge and spend four to six weeks feeding solely on wild lupine (Figure 2.3). The caterpillars pupate (undergo metamorphosis) and emerge as butterflies after about four weeks. The adult butterflies feed on the nectar of flowers of wild lupine and other plant species. A researcher interested in studying Karner blue butterflies at the organismal level might, in addition to asking questions about egg laying, ask questions about the butterflies’ preferred temperature (a physiological question) or the behavior of the caterpillars when they are at different larval stages (a behavioral question).
Figure 2.3: The wild lupine Lupinus perennis is the host plant for the Karner blue butterfly.
Population Ecology
A population is a group of interbreeding organisms that are members of the same species living in the same area at the same time. (Organisms that are all members of the same species are called conspecifics.) A population is identified, in part, by where it lives, and its area of population may have natural or artificial boundaries: natural boundaries might be rivers, mountains, or deserts, while examples of artificial boundaries include mowed grass, manmade structures, or roads. The study of population ecology focuses on the number of individuals in an area and how and why population size changes over time. Population ecologists are particularly interested in counting the Karner blue butterfly, for example, because it is classified as federally endangered. However, the distribution and density of this species is highly influenced by the distribution and abundance of wild lupine. Researchers might ask questions about the factors leading to the decline of wild lupine and how these affect Karner blue butterflies. For example, ecologists know that wild lupine thrives in open areas where trees and shrubs are largely absent. In natural settings, intermittent wildfires regularly remove trees and shrubs, helping to maintain the open areas that wild lupine requires. Mathematical models can be used to understand how wildfire suppression by humans has led to the decline of this important plant for the Karner blue butterfly.
Community Ecology
A biological community consists of the different species within an area, typically a three-dimensional space, and the interactions within and among these species. Community ecologists are interested in the processes driving these interactions and their consequences. Questions about conspecific interactions often focus on competition among members of the same species for a limited resource. Ecologists also study interactions among various species; members of different species are called heterospecifics. Examples of heterospecific interactions include predation, parasitism, herbivory, competition, and pollination. These interactions can have regulating effects on population sizes and can impact ecological and evolutionary processes affecting diversity.
For example, Karner blue butterfly larvae form mutualistic relationships with ants. Mutualism is a form of a long-term relationship that has coevolved between two species and from which each species benefits. For mutualism to exist between individual organisms, each species must receive some benefit from the other as a consequence of the relationship. Researchers have shown that there is an increase in the probability of survival when Karner blue butterfly larvae (caterpillars) are tended by ants. This might be because the larvae spend less time in each life stage when tended by ants, which provides an advantage for the larvae. Meanwhile, the Karner blue butterfly larvae secrete a carbohydrate-rich substance that is an important energy source for the ants. Both the Karner blue larvae and the ants benefit from their interaction.
Ecosystem Ecology
Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem is composed of all the biotic components (living things) in an area along with the abiotic components (non-living things) of that area. Some of the abiotic components include air, water, and soil. Ecosystem biologists ask questions about how nutrients and energy are stored and how they move among organisms and the surrounding atmosphere, soil, and water.
The Karner blue butterflies and the wild lupine live in an oak-pine barren habitat. This habitat is characterized by natural disturbance and nutrient-poor soils that are low in nitrogen. The availability of nutrients is an important factor in the distribution of the plants that live in this habitat. Researchers interested in ecosystem ecology could ask questions about the importance of limited resources and the movement of resources, such as nutrients, through the biotic and abiotic portions of the ecosystem.
Trophic Dynamics
All ecosystems, save those centered on chemosynthesis around geothermal vents in the ocean, rely on a continual input of energy from the sun. Indeed, all energy in our ecosystems is born from colliding hydrogen atoms, fusing to form helium, in the sun’s core. A process known as nuclear fusion. This fusion releases massive amounts of energy, in the form of waves which flow away from the sun, some of which make their way to our planet. Only some species can use this energy to create food for themselves. We call these autotrophs, with “auto” meaning self, and “troph” meaning food or nourishment. This self-nourishment (autotrophy) occurs via photosynthesis in plants, algae, and many bacteria. Essentially, these organisms convert the suns “light” energy into chemical energy. For example, a plant, the quintessential photosynthesizer, uses sunlight to synthesize food from CO2 and H2O. The “food” is in the form of a carbohydrate, glucose, while the byproduct, oxygen (O2), is released into the atmosphere. That glucose now harbors the energy originally created by nuclear fusion in the sun, but now residing as potential within carbon-carbon bonds and carbon-hydrogen bonds. Autotrophs use this energy for their own growth and reproduction, often producing many different forms of carbohydrates in the process (sugars, starches, and fibers).
All other living things (ourselves included) are dependent on the autotrophs that capture the sun’s energy. They are called heterotrophs, with “hetero” meaning other, as they rely on taking energy from other organisms, and are not able to produce it via photosynthesis. Some heterotrophs feed directly on autotrophs, e.g., herbivores, while others feed on other heterotrophs, e.g., carnivores, and of course some do both, e.g., omnivores. In this fashion the energy from the sun, captured as potential in those glucose molecules, eventually makes its way through, and fuels all life in an ecosystem. But life is messy, and physicists have a rule about conversion, meaning every time energy gets transferred from one organism to another, e.g., from the grass to the elk, or the elk to the wolf, much of it is lost.
We use the term trophic level to describe how many steps the sun’s energy has been through to reach a given organism. For instance, a plant, as an autotroph, is the first step in the sun’s energy entering an ecosystem. As such we call plants, and the trophic level the occupy the “producers”. When an animal eats a plant, it is now one step away from the origin of the energy, and rather than producing energy, it is instead consuming it. We call these organisms the “primary consumers”, i.e., the first consumers of the producers. If another animal then eats a primary consumer we are one further step away from the original energy input from the sun. We call these “secondary consumers”, i.e., they are eating something that ate a plant, thus are getting the plants energy after it has already been through another organism. This terminology then continues to tertiary (3rd), quaternary (4th), and quinary (5th) consumers.
Trophic dynamics describes this flow of energy through an ecosystem and how nutrients are cycled during this process. In the simple example above, we can consider how much of the sun’s energy is captured by the producers and retained as chemical energy, and then what amount of that chemical energy makes its way into each subsequent trophic level, e.g., primary consumers to quinary consumers.
Energy is “lost” along its trophic level journey in three primary ways:
1. Not all that consumed can be digested and is excreted as waste.
2. During cellular respiration (when cells break down glucose to form ATP) physics comes into play and a substantial amount of energy is lost as heat.
3. Some plants and animals die, but are not eaten, so their biomass is not passed on to the next trophic level.
Ecologist have reached the 10% “rule of thumb”, meaning a practical guideline lacking precision, to describe the amount of energy making its way from one trophic level to the next. Meaning that of all of the energy captured by plants via photosynthesis only 10% of that amount will be present in the primary consumers, and only 10% of that 10% will be present in the secondary consumers and so on, with the quaternary consumers only having 0.01% of the energy that is present in the producers (Figure 2.4). This rapidly dwindling availability of energy is the driving force behind the structure of our plant and animal communities in an ecosystem. Look out your window, if you are lucky enough to have one nearby, and you will likely see many producers, e.g., plants, but fewer primary consumers, and very few if any of the upper trophic level species. With less energy in each successive trophic level there are subsequently fewer organisms!
Figure 2.4: An energy pyramid is a presentation of the trophic levels in an ecosystem. Energy from the sun is transferred through the ecosystem by passing through various trophic levels. Roughly 10% of the energy is transferred from one trophic level to the next, thus preventing large amounts of trophic levels. There must be higher amounts of biomass at the bottom of the pyramid to support the energy and biomass requirements of the higher trophic levels (credit: a modification of work by Swiggity.Swag.YOLO.Bro from Wikipedia page Food Chains).
Of course, food webs and the organisms in them are much more complicated than the simple chain we have been using so far to discuss the flow of energy. Instead, an ecosystem might have thousands of species of producers, fed on by a complex assemblage of herbivores and omnivores. Layered above can be a myriad of predators that feed at various trophic levels, getting some of their energy by eating primary consumers, but also by eating other organisms that might be secondary, tertiary, or even quaternary consumers. Not only does this make calculating the amount of energy in any given trophic level more complex, it also creates a system of interdependencies in which removing one species may have a cascading effect on an entire system, or, with trophic redundancy, may go nearly unnoticed.
Dealing with a changing environment
There are lots of amazing and sometimes bizarre adaptations out there in the world. For example, some species of frogs (e.g., wood frogs) that live in temperate climates can tolerate the freezing of their blood and other tissues. These frogs allow about 65% of their bodies to freeze solid, stop breathing, and stop their heart when temperatures drop below freezing. Come spring, as temperatures rise, the frog’s body thaws and basic motor functions restart, allowing these frogs to survive incredibly harsh winter conditions. Other examples of interesting adaptations include carnivorous plants that obtain their nutrients from insects (e.g., pitcher plants), or rodents (kangaroo rat) that obtain enough water from metabolism that they do not need to drink water at all.
Figure 2.4: Frog (photo by Patti Black on Unsplash), pitcher plant (photo by Adrian Pingstone released to the public domain), and kangaroo rat (US Fish & Wildlife).
While each of these examples are fascinating in their own right, perhaps a better place to start when thinking about adaptation are the basic, or broad strategies that organisms have adapted to survive in the environment. Specifically, if we think about the fact that the environment that an organism lives in can vary considerably. The environment can vary temporally, on both short and long-term time scales, and spatially in terms of both abiotic and biotic factors. For example, the environment an organism experiences can change in temperature, precipitation, amount of sunlight, water availability, oxygen concentration, salinity, atmospheric pressure, etc. This can create problems for living things because most cellular functions (think enzymes, or neurotransmitters) require specific conditions for proper function. Biotic components of the environment can also change with time or space, including things like the availability of prey, the abundance of predators or competitors, or access to potential mates. While different groups of plants and animals may have solved different components of dealing with this variability in different ways, more broadly we can think of two basic solutions or strategies for dealing with environmental variation: conform or regulate.
Conforming is when an organism allows their internal environment to fluctuate with the external environment; we might call an organism that conforms a “conformer” for that particular environmental variable. An example of a conformer to external temperature is a frog that allows its body temperature to fluctuate with the environment (Figure 2.5 A). As the external temperature increases or decreases, the internal temperature of the frog increases or decreases along with the external environment.
If we’re thinking just about temperature, we often describe organisms using conforming strategies using the terms ectotherm (an animal that relies on the external environment to regulate its internal body temperature), or poikilotherm (an animal that varies its internal body temperature within a wide range of temperatures).
Figure 2.5: Basic strategies for dealing with fluctuations in the environment: conform or regulate.
Regulating is when an organism attempts to regulate or maintain a constant internal environment despite any environmental fluctuations; we might call an organism that regulates a “regulator” for that particular environmental variable. An example of a regulator for external temperature is a dog that attempts to maintain its internal body temperature within a relatively narrow range despite fluctuations in the external environment (Figure 2.5 B). As the external temperature increases or decreases, the internal temperature of the dog remains nearly the same with some limitations at extreme temperatures.
If we’re thinking just about temperature, we often describe organisms using regulating strategies using the terms endotherm (an animal that regulates its own internal body temperature through metabolic processes), or homeotherm (maintains a constant internal body temperature, usually within a narrow range of temperatures).
Some organisms can differ in their strategy for different regulatory processes. For example, salmon are thermoconformers, but osmoregulators when they move between marine (saline) and freshwater environments..
There are costs and benefits to each of these basic strategies. For example, conformers will invest less energy into maintaining their internal environment, but can experience compromised cellular functions. On the other hand, regulators can live in a wider range of environments without experiencing reduced cellular functions, but they expend a great deal of energy to maintain their internal environment (Figure 2.6).
Figure 2.6: Theoretical costs and benefits of an organism regulating their internal environment or conforming to the external environment.
In addition to being a conformer or regulator, organisms may also be avoiders that will escape changes in the environment by moving locally or migrating long distances (read more about this in the behavioral ecology chapter). Other types of the “avoiding” strategy could include organisms that undergo some type of dormancy, which is when an organism decreases their metabolic activity under extended unfavorable conditions in order to conserve energy.
Examples of dormancy in animals include hibernation, a mechanism used by many mammals to reduce energy expenditure and survive food shortages over the winter. During hibernation, the animal undergoes many physiological changes, including decreased heart rate (by as much as 95%) and decreased body temperature. Another type of dormancy in animals, most commonly seen in insects, is diapause, when the organism completely suspends development between autumn and spring.
In plants, dormancy is a period of arrested growth, and is a survival strategy exhibited by many plant species that allows them to survive in climates where part of the year is unsuitable for growth, such as winter or dry seasons. A classic example of dormancy in plants is seed dormancy, where seeds are prevented from germinating during unsuitable ecological conditions. Many plant species that exhibit dormancy have a biological clock that tells them when to slow activity and to prepare soft tissues for a period of freezing temperatures or water shortage. On the other hand, dormancy can be triggered after a normal growing season by decreasing temperatures, shortened day length, and/or a reduction in rainfall.
Many bacteria can survive adverse conditions such as temperature, desiccation, and antibiotics by forming endospores, cysts, or states of reduced metabolic activity lacking specialized cellular structures.
For organisms that regulate components of their internal environment to big changes in the external environment, one key question we might have is: how do they do this? Mechanistically, the process of adjusting the internal environment in response to an external change is described as acclimation.
Acclimation
Acclimation is the process in which an individual organism adjusts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH), allowing it to maintain fitness across a range of environmental conditions. Acclimation occurs in a short period of time (hours to weeks), and within the organism’s lifetime (compared to adaptation, which is evolution, taking place over many generations). This may be a discrete occurrence (for example, when mountaineers acclimate to high altitude over hours or days) or may instead represent part of a periodic cycle, such as a mammal shedding heavy winter fur in favor of a lighter summer coat. Organisms can adjust their morphological, behavioral, physical, and/or biochemical traits in response to changes in their environment (The Unabridged Hutchinson Encyclopedia, 2009). While the capacity to acclimate to novel environments has been well documented in thousands of species, researchers still know very little about how and why organisms acclimate the way that they do.
Methods of acclimation
Biochemical
In order to maintain performance across a range of environmental conditions, there are several strategies organisms use to acclimate. In response to changes in temperature, organisms can change the biochemistry of cell membranes making them more fluid in cold temperatures and less fluid in warm temperatures by increasing the number of membrane proteins (Los & Murata, 2004). In response to certain stressors, some organisms express so-called heat shock proteins that act as molecular chaperones and reduce denaturation by guiding the folding and refolding of proteins. It has been shown that organisms which are acclimated to high or low temperatures display relatively high resting levels of heat shock proteins so that when they are exposed to even more extreme temperatures the proteins are readily available. Expression of heat shock proteins and regulation of membrane fluidity are just two of many biochemical methods organisms use to acclimate to novel environments.
Morphological
Organisms are able to change several characteristics relating to their morphology in order to maintain performance in novel environments. For example, birds often increase their organ size to increase their metabolism. This can take the form of an increase in the mass of nutritional organs or heat-producing organs, like the pectorals (with the latter being more consistent across species) (Liknes & Swanson, 2011; McKechnie, 2008).
Contributors and Attributions
Modified from the following sources:
Ecology for All! By LibreTexts, Chapter 1.2 and Chapter 4.2 licensed CC BY-NC-SA
Additional references and citations from the above sources can be found in References in the backmatter.
2. An Introduction to Ecology is shared under a CC BY-NC-SA license
