Must Know High School Biology - Kellie Ploeger Cox 2019

PART SIX Ecology


Imagine yourself outside, relaxing under a tree, reading your biology book. Without even making an effort, you are interacting with your ecological surroundings. Ecology is an important subject because it takes a large-scale view of biology. Instead of focusing on the systems contained within a single organism, ecology considers how the organism interacts with its surroundings. Nothing lives in true solitude! You are constantly sharing your environment with other life-forms, whether they’re obvious (other humans, swaying trees, that pesky mosquito that is buzzing in your ear) or not so obvious (mutualistic bacteria colonizing your digestive tract, or that annoying athlete’s foot infection).


Image An ecosystem’s populations are often changing in numbers and composition.

The living world is filled with different populations of organisms coexisting and interacting with one another. Furthermore, as our must know points out, these populations are not static and unchanging. Through either natural events or man-made circumstances, a population’s numbers may change drastically (or even disappear altogether).

When you refer to a species, you are talking about organisms that can reproduce together and create fertile offspring. A group of the same species living in the same area is called a population, and a bunch of different populations occupying the same area is referred to as a community. But if you look around you, there are certainly things in your environment beyond living organisms (referred to as biotic factors). As you can probably guess, abiotic factors refer to the nonliving properties of an environment, such as the weather, water, oxygen, temperature, and sunlight. All of these—the communities and abiotic factors with which they interact—create that inclusive term ecosystem.

For example, consider the meerkat. These little mongooses live in the Kalahari Desert of southern Africa. They form groups of between 20—50 individuals called gangs or mobs, usually consisting of extended family members. Meerkats are omnivores (eating both plants and animals), but prefer delicious insects over anything else. The species is well adapted to life in the desert and has evolved dark patches of fur around their eyes to reduce sun glare. This is important, because meerkats need to keep a sharp lookout for predators, such as cheetahs and eagles.



Author: George Hodan.

A meerkat’s ecosystem is in the Kalahari Desert.A population of meerkats live amongst other populations, such as cobras, palm trees, beetles, and cheetahs. This community (collection of different populations) shares the abiotic factors of the desert, including temperature and water availability. This sum of abiotic and biotic factors defines the ecosystem of the Kalahari Desert.


Meerkat image author: George Hodan.

Cobra image author: Kamalnv.

Palm tree image author: Karen Arnold.

Cheetah image publisher: U.S. Fish and Wildlife Service.

Beetle image author: Heiti Paves.

Desert image author: Luca Galuzzi.

When you consider the impact a population has on the ecosystem, or if you are worried about an endangered species’ fate, you must consider the size of the population. If there are too many individuals for a given area, it may have a negative impact on their habitat. Conversely, if there are too few young individuals in a population, it may be in danger of dying out completely. Changes in population size—our must know concept—are natural occurrences and are often determined by two factors: the birthrate and the death rate.


Suppose you have a population of warblers (a type of small, vocal, perching bird). There are some old warblers, some middle-aged warblers, and some young warblers. This distribution of age groups describes the age structure of the population and helps to determine the birthrate. Is the population of warblers having a lot of babies? That means it has a high birthrate! This is significant because a population with many individuals in their reproductive years will have current (and future) growth due to a strong birthrate.


Age structure of a growing warbler population

In the previous figure, the highest percentage of members are young individuals whose reproductive years stretch ahead of them. There is definite potential for future reproduction and growth.

If, however, a population is mainly composed of older individuals, the population at best will remain stable (at worse, it will decline). There are many sad scenarios where habitat loss has doomed an endangered species because the population cannot thrive and reproduce. One example is the kakapo of New Zealand:



Author: Mnolf.

The kakapo (also called “night parrot”) is a large, round, flightless, nocturnal parrot of New Zealand. This adorable bird is a heavy and rotund little guy who does not fly because it evolved on an island without predators. But when humans moved in and introduced predators (such as cats and ferrets), the kakapo population crashed. Even though conservation efforts began as early as the 1890s, nothing really helped until the 1980’s Kakapo Recovery Plan. The current surviving population is kept safe on three predator-free islands. Even so, the adorable kakapo is critically endangered. As of 2018, fewer than 150 adults exist.


Age structure of a declining kakapo population

These percentages are hypothetical and should not be considered actual data. Yet the dire situation depicted in this graph is unfortunately true.

As the graph shows, there are not many young individuals, and the older kakapos are no longer able to have babies. Who will create the next generation? This is an extreme (and sad) example of our must know concept: populations change, and the kakapo population may even be entirely removed from the island ecosystem.

Death Rate

The death rate is obviously working against the birthrate. If there’s a high death rate, many members of a population are dying and the overall numbers are dropping (unless the birthrate is high enough to offset the deaths). And the death rate can vary for different populations, depending on when the majority of deaths occur. For example, a fish may lay hundreds of eggs, but only a small percentage of these individuals reach adulthood (there is a high death rate early in the lives of the fish). Large mammals, however, have relatively few offspring, but most survive and reach old age (most of the deaths occur late in life). A survivorship curve is a diagram comparing numbers of survivors to the percentage of their maximum life span.


Type I, II, and III survivorship curves

Type I survivorship shows a low level of infant mortality, probably because the parents care for their young; this curve is for the large mammals, including primates. Type III survivorship has a rapid drop-off early on, with relatively few survivors. Species showing this survivorship curve (plants, invertebrates, and fish) need to have a large number of offspring to compensate for this loss, and animals that suffer a high predation rate also tend to produce large numbers of offspring. The Type II survivorship curve is common in birds, some reptiles, and smaller mammals, because they tend to have an equal chance of dying throughout their lifetimes.

Limiting Population Growth

Clearly, there are many environmental factors that keep down a population’s size. The steep initial drop-off of a Type III survivorship curve may be due to predation. A population’s growth eventually slows due to limited resources such as food and space. Anything that limits the population size is called a limiting factor, and can be categorized as either density-dependent or density-independent. A density-dependent limiting factor gets worse as the population increases and crowding gets worse. Because this type of limitation is due to interactions with living organisms, most density-dependent limits are biotic.

Disease, for example, is caused when a pathogenic organism (bacteria or parasitic worms, for example) is passed from host to host. A disease can more easily spread and wreak havoc if the members of the population are crammed in together without much space. Competition is worse if there are a huge number of individuals fighting for the same resources. If there are a ton of prey milling about, predation is going to be significant. In contrast, a density-independent limiting factor will impact a population regardless of its density. Weather events such as floods and droughts can severely impact an ecosystem’s resources, and natural disasters such as earthquakes, fires, and tsunamis will kill a great number of organisms in the area, regardless of the population density. And, unfortunately, human activity has significant effects on populations. If we destroy entire ecosystems by clearing forests or draining wetlands, there is little chance of the native population’s survival.

Limiting Factors


Even though fluctuations occur, the number of individuals in a population (number of individuals/area) is defined as the population density. The limiting factors we just learned about can have a significant impact on a population’s density. For example, a change in population density can be due to resource availability. If there are unlimited resources, then a population will grow very, very fast. This isn’t a realistic scenario, however, because an environment will eventually run out of nutrients, space will become limited, and there will be an increase in predation. This is why exponential growth doesn’t often happen (probably a good thing … could you imagine if the chipmunk population grew exponentially? You’d find them in your sock drawer.).

The one time you would see exponential growth is when a species moves into an uninhabited area. No predators! Plenty of food and space! Rampant reproduction! It is way more common, however, to see a logistic growth pattern. This is when a population increases slowly at the beginning, enjoys a short burst of exponential growth, and then eventually levels off at a stable (and sustainable) population size. This maintainable population density is referred to as an ecosystem’s carrying capacity.


Exponential and logistic growth (including carrying capacity)

Interspecies Interactions

Predation and competition are two harsh examples of how species can interact. Consider the tardigrade, a tiny animal who enjoys sucking the juice from moss and algae (more on this little guy in the next chapter). The tardigrade is the predator, and it is feeding upon another organism, in this case, an algal blob. Now, if another tardigrade decides to take up residence in the same moss clump, there will now be competition for the same limited resources (delicious algal blobs). Competition can occur between members of the same species, or entirely different species. If a resource is limited, there’s gonna be a battle.


You may accidentally use the term symbiosis to mean a relationship between individuals that is beneficial to both parties. This is not necessarily true! A symbiotic relationship can be really, really bad for one participant. You probably mean “mutualism.”

When individuals of different species live in direct contact with one another, this is referred to as a symbiotic relationship. There are three types of symbiosis: mutualism, parasitism, and commensalism. Each of these refer to an ecological relationship between different species.

Symbiotic Relationship: Parasitism

In a parasitic relationship, one organism benefits while the other is harmed. This is unlike predation because the parasite reaps the most benefit by keeping its host alive as long as possible (unlike a predator, who immediately kills and eats its prey). Some parasites live inside the host (endoparasites), whereas other parasites live and feed on the outside of the unsuspecting host (like ticks and lice and … shudder … leeches). Not only are parasites creepy, but they can have a significant impact on a host population. Parasitic interactions are, in fact, perfect examples of a density-dependent limiting factor. Usually, the parasites that come to mind are the obvious worms and other creepy-crawlies, but plants and fungi can be parasites, too. There are so many awesome examples of cool/horrific parasitic relationships it’s hard to choose. Here are two:

Cool Parasite 1

Mistletoe brings to mind wintery seasons and happy holiday decor. But a far more sinister truth lies beneath that beautiful sprig of glossy green leaves and beautiful white berries … mistletoe is a plant parasite that sucks the life-juice out of its photosynthetic victims with nutrient-absorbing projections called haustoria. A growing mistletoe will wrap itself around a host plant and use its haustoria to puncture through to the host-tree’s circulatory system (xylem and phloem) to steal its water and minerals.


A plant sinking its haustoria into its prey plant

Author: Chrissicc.

Cool Parasite 2

Toxoplasma gondii is a unicellular, parasitic protist that can cause a disease called toxoplasmosis. Cats are a common host for T. gondii, which is why pregnant women shouldn’t clean out cat litter boxes (the parasite can be passed through cat feces). Cats themselves usually aren’t affected by the infection, but their prey—rats—are another matter. When a rat is infected by T. gondii, it causes unusual changes in neural activity and the rat loses its fear of cats. Even worse, the rat becomes attracted to the smell of cat urine! This may help the parasite spread farther, because there’s a good chance that bold rat is going to end up as some cat’s dinner.

Furthermore, recent research suggests that T. gondii could contribute to mental disorders in humans. A study in mice showed that infection with T. gondii may cause brain cells to release a higher-than-normal level of the neurotransmitter dopamine; altered dopamine levels are linked to some mood disorders. Other studies have discovered a potential link between infection by T. gondii and schizophrenia. There is no definitive answer to the causation between infection and mood disorders, but the research suggests a significant link. It’s also notable that infection by T. gondii is so widespread, up to half of all humans on Earth are infected. Luckily, most of the time there are no symptoms and you don’t even know you have it. If you’re curious, your doctor can perform a simple blood test to screen for antibodies against the protist to see if you have ever had the infection!

Symbiotic Relationship: Mutualism

A mutualistic relationship is a peaceful and happy symbiosis where both parties benefit—everyone’s a winner! This sort of relationship is often the result of coevolution, with changes in one species affecting the survival of the other species (and vice versa); it’s like a friendly arms race. For some fascinating examples, read further, friend.

Cool Mutualistic Relationship 1

If you have ever gone out for a walk and looked closely at the trunks of some trees, you may have noticed a dry, flat, greenish growth on the bark. Or maybe when flipping over large rocks to see what scurries out from underneath, saw patches of green flakey stuff that looked too dry and dead to be a thriving life-form. If so, you have just noticed some lichen.


Author’s photos of lichen

Lichen is not a single organism. Instead, it is a permanent mutualistic relationship between a non-photosynthetic fungus and a photosynthetic algae or cyanobacteria.


Diagram of the mutualistic relationship that makes up lichen

Filaments of fungus (also called hyphae) surround and cradle their photosynthetic partners. And “partners” is a perfect description. Fungi, by definition, are unable to photosynthesize. Instead, they reap the benefit of their partner alga (or cyanobacteria) producing too much glucose—excess sugars and vitamins are passed from the alga onto the fungus. The alga is happy because it is protected by the fungal hyphae, reducing the chances of it drying out in the hot sun. This allows lichen to colonize many different habitats and they are remarkably resistant to drought.

Cool Mutualistic Relationship 2

There is this clever little leafhopper called a glassy-winged sharpshooter that feeds on the sap of woody plants (including grapevines, to the chagrin of vineyard owners in California).


A leafhopper

Image source:

Sap, however, is not a very healthy diet … it’s lacking essential amino acids and vitamins that are needed in the insect’s diet. Cleverly enough, the insect has mutualistic relationships with not one but TWO different microbes. These microbes produce the nutrients not supplied by the leafhopper’s primary diet: one species of bacterium uses sap-derived carbon to make amino acids (but cannot make any of the required vitamins). The other species of bacteria uses the carbon to make vitamins (but can’t make any of the needed amino acids). The insect is supplying the two bacterial species with a steady source of carbon, and the bacteria are, in turn, providing either the amino acids or the vitamins needed by the leafhopper. Furthermore, the next-door bacterial species are supplying each other with chemicals needed to make their own nutrients.


Multiple mutualistic relationships within the glassy-winged sharpshooter

These mutualistic relationships are so important to all three participants that the insect evolved to have specific housing for its two resident microbes. These houses are called bacteriomes and they reside on each side of the insect’s abdomen.

Symbiotic Relationship: Commensalism

Commensalism is a symbiotic relationship between two organisms where one benefits and the other one doesn’t really care either way (neither harmed nor helped). I personally think this is the most difficult one to identify because so often it turns out to be a mutualistic relationship instead. For example, a common example of commensalism is when a species of bird is “hitchhiking” on the back of a buffalo. When the buffalo walks through the grass it stirs up bugs, which the bird then eats. If this was pure commensalism, the bird would benefit (tasty bug meal), and the buffalo would neither benefit nor be harmed. But this could be a mutualistic example, since the buffalo is benefiting from a reduction of annoying bugs in its personal space. I will give you some more clear-cut examples of commensalism.

Cool Commensal Relationship 1

The soil community is a lively and crowded one, populated by myriad bacterial and fungal species. There can be one species of bacteria that releases a chemical that is happily gobbled up by another species of bacteria. The bacteria that produces the chemical really doesn’t care what happens to the by-product, but the second species is thrilled to receive an essential nutrient (discarded from its microbial neighbor).

Cool Commensal Relationship 2

A species of coral reef jellyfish will have a juvenile fish swimming along within the protective safety of its tentacles. The jellyfish is “meh” about this ride-along, but the young fish is enjoying an umbrella of protection from predators. It’s important to note that the jellyfish does not dine on its ride-along friend (otherwise, this would quickly turn into an example of predation).

Symbiotic Relationship: It’s Complicated

It’s Complicated 1

Cymothoa exigua is called the “tongue-eating louse,” and for good reason. It is a crustacean that lives in the Gulf of California (among other places) and parasitizes at least eight different species of marine fish. The male Cymothoa exigua takes up residence by attaching to the fish’s gills. The females, however, are a bit more creative in where they live in the unsuspecting fish. She causes degeneration of the fish’s tongue until there’s nothing but a stub left. She then attaches to the stub with seven pairs of hooked legs and replaces the fish’s tongue! The parasite fits so perfectly it serves the same mechanical function as the tongue when the fish feeds, and the fish can still eat normally. Because of this, some people consider this an example of a commensal relationship, instead of the more obvious parasitic comparison.


Cymothoa exigua, or the tongue-eating louse

Author: Marco Vinci.

It’s Complicated 2

Wolbachia is a common and widespread group of bacteria that hang out in the ovaries and testes of many different insects, as well as some spiders and nematodes. This bacterial infection is actually passed down generation to generation (also called “vertical transmission”), because it’s inherited in the cytoplasm of the egg. Since the bacteria don’t hang out in mature sperm, only infected females (and their Wolbachia-laden eggs) pass the infection on to their offspring.

A phenomenon called cytoplasmic incompatibility helps Wolbachia spread like wildfire through the host population. Males that are infected with Wolbachia are unable to mate with uninfected females. The uninfected egg “phenotype” is selected against, because uninfected eggs will not become fertilized. In order to further help the spread, Wolbachia causes infected males to turn into infected females (feminization). In some species of insects, Wolbachia-infected females can even reproduce without the help of males (a phenomenon called parthenogenesis)! It must be mentioned, however, that Wolbachia isn’t a clear-cut parasite. Some infections appear to be a bit mutualistic. Elimination of the infection from some nematodes, for example, leads to the worms’ sterility or even death. Infection of Wolbachia in fruit flies helps them to better resist viral infections. But overall, any infection that messes around with a species’ ability to reproduce is considered parasitic.



What if we could harness Wolbachia and use it to control an insect population that spreads more death and misery than any other insect in the world? Yes, the evil mosquito. Studies have suggested that intentionally infecting a mosquito population with Wolbachia may reduce the mosquitos’ ability to spread horrible viruses such as Zika and dengue. Let’s go, science!


1. Match the following term with the correct example:


2. Which type of survivorship curve necessitates having a large number of offspring? Give an example of an organism with this type of curve.

3. Of the two growth curves, the S-shaped logistic model is more realistic. Explain why, and draw/label an example curve.

4. The glassy-winged sharpshooter houses two species of bacteria that, in turn, provide essential nutrients to the leafhopper. What kind of symbiosis is this an example of?

5. Choose the correct word from each pair: A population with a high/low death rate and a high/low birthrate would experience an overall increase in the population numbers. This population would most likely have an age structure with mostly young/old individuals.

6. Draw the three types of survivorship curves, and explain which one best describes large mammals.

7. For each of the following examples, indicate whether it is a density-dependent liming factor (DD) or a density-independent limiting factor (DI):

a. A viral disease

b. Competition for mates

c. Volcanic eruption

d. Predation

e. Wildfire

f. Oil tanker spill in ocean

g. Clearcutting forest

8. The female “tongue-eating louse” removes and replaces a fish’s tongue. In order for this to be an example of commensalism, what characteristic of the relationship must be true?