PART SIX Ecology
Ecosystems and Their Interconnectivity
The interconnectivity of ecosystems means that different populations will have an impact on each other.
As mentioned earlier, an ecosystem is a collection of organisms and nonliving things. The number of interactions that occur within an ecosystem is vast and varied. A critter, for example, must call some specific location its home, or habitat. A habitat includes both biotic and abiotic factors. A tardigrade, for example, may enjoy spending its days in a freshwater droplet held within a lush bed of moss.
The glorious tardigrade—aka “water bear,” aka “moss piglet”—is one of the most amazing animals on Earth. They are obviously adorable little critters, but don’t mistakenly think you can put it on a leash and take it for a walk—they don’t get any bigger than 1 mm. They have eight legs with tiny little hands and tiny little claws.
Their superpower lies in their ability to withstand ridiculous environmental conditions. Tardigrades can survive temperatures from minus 300°F to over 300°F. Vacuum of space? No problem. Heavy radiation? Sure. Their survival abilities are rooted in their powers of cryptobiosis—going into a death-like state that enables them to slow their metabolism to 0.01% levels of normal. They pull in their tiny little legs and turn into a teeny dehydrated ball called a tun. Just add water and … voila!—rehydrated tardigrade, ready to go and find a nice, new patch of moss to call home.
Authors: Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. https://commons.wikimedia.org/wiki/File:SEM_image_of_Milnesium_tardigradum_in_active_state_-_journal.pone.0045682.g001-2.png
Well, back to our tardigrade habitat scenario. The temperature is 76°F and there are plenty of plant cells for it to eat. This is the tardigrade’s habitat, or where it lives. Now, how the tardigrade lives is a bit more complex. This is referred to as a species’ niche, and along with all the abiotic and biotic factors of its habitat, other factors are considered. When does the tardigrade eat? What might decide to prey upon the tardigrade? Will the tardigrade decide to seek out a partner and reproduce sexually, or undergo parthenogenesis (a form of asexual reproduction via the development of an embryo from an unfertilized egg)?
A bunch of different organisms occupy the same habitat, but they have their own distinct niches. When so many different species live so closely together, there will undoubtedly be interspecies interactions. In fact, two or more species cannot occupy the same niche. The species who is a bit better at acquiring resources will have a higher survival rate and reproduce more rapidly. The inferior species will die out. This is called the competitive exclusion principle, and it is an excellent example of our must know concept: the interconnectivity of ecosystems means different populations can have an impact on each other.
There is a very well-known example of the competitive exclusion involving two different species of Paramecium participating in a tiny, microscopic cage fight … two species go in, but only one will survive. This specific investigation was conducted by the Soviet biologist G. F. Gause in 1934, using the two Paramecium species P. caudatum and P. aurelia. He introduced the two species into the same ecological niche and then measured their population densities over time. Imagine if you wanted to conduct a similar experiment with Paramecium A and Paramecium B. Your hypothesis might be something like this: due to the competitive exclusion principle, one species will win out over the other, and there will be a decline in the loser species’ numbers because the dominant species will use up all the resources. In order for you to really know if any die-off was due to competition, you need a control experiment against which you can compare your experimental data. Your controls are the growth curves of each species living independently.
Growth curve of Paramecium A
The first graph shows you the growth curve of Paramecium A population if it lives alone in its very own bacteria-filled flask (your paramecia love to eat bacteria). It follows a healthy S-shaped growth curve of a population enjoying an initially brisk boom, followed by a leveling off once it hits the habitat’s carrying capacity (at about 380 critters per milliliter).
Growth curve of Paramecium B
Paramecium B also grows well in its own flask, and though its initial growth spurt is faster than its cousin’s, it has a smaller carrying capacity (240 critters per milliliter).
In the actual experiment run by Dr. Gauss, Paramecium B was a larger species than Paramecium A. It makes sense, then, that the carrying capacity for the larger species was a smaller number—bigger paramecia need bigger bacterial meals!
Finally, you are ready to run the competitive exclusion experiment, and you happily combine both species A and B into a single food-filled flask.
Growth curve when species A and B are combined
For the first 2 days, everything seems normal. Paramecium B’s population increases faster than Paramecium A’s. But then around the fourth day, something happens … Paramecium B numbers start to decline! Meanwhile, the Paramecium A population is growing like crazy, and continue to do so as their cousins dwindle in numbers until they disappear. Your hypothesis was correct, and species A outcompeted species B.
Matter and Energy Flow in Ecosystem
It would be remiss not to address the issue of energy and matter flow within an ecosystem. You can learn about a specific ecosystem with its distinct critters and habitats, and it’s easy to think of it as a closed system that can exist on its own. Nothing could be further from the truth. There is a constant flow of energy into an ecosystem from the sun, and after the light energy is converted to a chemical form by autotrophs (photosynthetic organisms), it is then transferred throughout the ecosystem via a web of intricate and detailed food chains and complex food webs. Though this interconnectivity is not as obvious as different species competing with each other for niche domination, it is still a perfect example of our must know concept. Please recall the laws of thermodynamics we talked about in Chapter 1.
Don’t forget those two laws of thermodynamics:
• Energy cannot be created, nor can it be destroyed. It can, however, be converted from one form to another.
• Any time energy is converted from one form to another (meaning it is being used by the critters of Earth), some of the potential energy is lost as heat. It is a thermodynamic rule that when energy is transformed, the process is never ever 100% efficient (and most of the time, the lost energy escapes as heat).
A food chain is a model of how energy is transformed as it is moved from one organism to another (through predation). Every chain must begin with a critter that captures the sun’s energy and transforms it into a chemical form that is used by all subsequent members of the food chain. An autotroph (also called a producer) is a photosynthetic organism at the base of every chain. Most of the time, a plant comes to mind, but many bacteria and protists are able to photosynthesize, as well. Each step in a food chain is referred to as a trophic level.
If the first trophic level is a producer, then the second trophic level must be composed of critters that eat the producers, either an herbivore (that only eats plants), or an omnivore (that eats both plants and other animals). A plant-eating critter that is the second member of a food chain is referred to as a “primary consumer.” A food chain cannot have too many steps because energy is lost along the way (recall the second law of thermodynamics). Eventually, there is not enough energy left to support another trophic level.
In summary, a chain goes like the following:
Producer → primary consumer → secondary consumer → tertiary consumer
In an actual ecosystem, however, there are no distinct and isolated food chains (once again referring back to our must know concept); instead, many chains are interlinked into complex food webs.
Carl Sagan said, “We’re made of star stuff.” It’s beautiful and true. The atoms on Earth today are the same atoms from 4.6 billion years ago, constantly recycled into new shapes, new forms, and new purposes.
Every move from one trophic level to the next only transfers about 10% of the energy locked within the tissues of the prey. When a leafhopper eats a plant, it receives only 10% of the plant’s energy (the rest is lost as heat). When a bluebird eats the grasshopper, it gains only 10% of the energy locked up in the grasshopper’s tissues. A model that quantifies the amount of energy available at each trophic level is called an energy pyramid, and the pyramidal shape is derived from the significant loss of energy at each subsequent trophic level.
An energy pyramid
Notice that each trophic level receives only 10% of the energy from the level below it.
A biomass pyramid has the same shape because it compares the amount of biomass at different trophic levels within an ecosystem. Biomass is a term used to describe the total dried weight of a group of organisms in a particular ecosystem. It makes sense that if each trophic level only receives a fraction of the energy stored within the previous trophic levels (because of energy and heat loss), there are fewer organisms. Each level shows the mass needed to support all the organisms above it! Luckily, the sun keeps replenishing the base.
African biomass pyramid
Look at the secondary consumer in the above biomass pyramid … do you know what that is? A pangolin is the most adorable critter, and it looks sort of like a cross between an anteater and a pinecone.
Indian Pangolin (Manis crassicaudata) in Kandy, Sri Lanka
Author: Dushy Ranetunge. https://commons.wikimedia.org/wiki/File:Scaly_ant_eater_by_Dushy_Ranetunge_2.jpg
A pangolin in defensive posture, Horniman Museum, London
Author: Stephencdickson. https://en.wikipedia.org/wiki/File:A_pangolin_in_defensive_posture,_Horniman_Museum,_London.jpg.
It waddles along on its hind legs as if its center of gravity is a bit too far back, and when it’s scared, it rolls up into a scaly sphere and lies still until the threat goes away. It does no one any harm (well, except to those delicious ants it ladles into its mouth with its long, sticky tongue). It breaks my heart that the pangolin may disappear from Earth because humans poach them for their scales, which are believed to have medicinal properties. Luckily, there are programs like the African Wildlife Foundation that is working to save these beautiful creatures. For more information, please check this out: https://www.awf.org/wildlife-conservation/pangolin.
All of these trophic levels are showing organisms being eaten by other organisms, and they themselves being devoured … and so on and so forth. The interconnectivity is stunning; we are all linked. Bacteria, belugas, brown bears, or bamboo, we are all tethered to one another through cycled energy and matter.
Earth is an open system in regards to energy, but a closed system in regards to matter. A closed system is like a vacuumed-sealed environment that doesn’t have anything new coming in (nor is anything leaving). Imagine one of those cool glass sphere ecosystems filled with water and plants and a tiny invertebrate or two. It is filled with water and, unless you drop it on the floor, there won’t be any water lost from it through evaporation. Also, you cannot add water to it. The water is an example of a closed system.
Inside, there are plants and animals composed largely of carbon, and any of this organic matter contained within the sphere is certainly going to stay in there (another closed system). Inside of the closed sphere there are food chains and energy transformations as the plants are eaten by the tiny herbivores, and once these critters die, decomposers unlock the carbon stored within their tissues. The carbon will be taken up by the plants through photosynthesis, renewing the bottom trophic level and providing food for the next generation of herbivores.
But recall that pesky second law of thermodynamics—every energy transformation has an inherent loss of energy, heat that escapes this tiny ecosystem and is lost forever. How, therefore, can these cycles keep going, if so much energy is lost at every trophic level step along the way? The answer is because this living sphere may be a closed system in regards to matter, but it is an open system in regards to energy. Sunlight is constantly streaming down on this sphere, renewing the energy cycling throughout the ecosystem. The producers grab the sun energy and transform it into glucose. Consumers eat the plants and convert the stored glucose into energy through the process of cellular respiration. The waste produce of carbon dioxide is freed into the water and captured by the plants, continuing the cycle. Meanwhile, a constant stream of sunlight balances the constant loss of energy (heat). If you place your glass ecosystem in the dark, it will eventually die after all the enclosed energy is spent, without any source to renew it. Open systems require a constant input (in this case, of sunlight).
This glass sphere is a model of Earth. Our planet has a closed system for the cycling of matter (carbon cycle and the water cycle, for example), but an open system for energy, thanks to the sun. Next up, we will talk about how carbon and nitrogen are cycled within our closed Earth system.
Carbon is arguably the most important element for life on Earth, and the study of organic chemistry is based on the study of carbon-based chemicals. Every one of the organic compounds critical to life contain carbon. Clearly, the cycling of carbon is really important to an ecosystem’s health.
The carbon cycle
Carbon is found in the atmosphere as the gas carbon dioxide (CO2). Autotrophs (i.e., producers) capture this gas in the process of photosynthesis, fixing it into the organic compound glucose, thus making it accessible to all heterotrophs. The carbon moves along the food chain as each trophic level devours the tissues of its previous trophic level, ingesting the carbon locked in its meal. Along the way, carbon is released back into the atmosphere as each form of life converts the glucose back into carbon dioxide through the process of cellular respiration. Furthermore, when an organism dies, the decomposers that consume the tissues also unlock the carbon through cellular respiration. Finally, large amounts of carbon dioxide are also exhausted back into the atmosphere through the burning of woods and fossil fuels.
It’s important to notice the number of sources adding CO2 to the atmosphere, versus the number of sources removing CO2 from the atmosphere. Even though there are many pathways releasing carbon dioxide into the air, the only metabolic pathway removing it is photosynthesis. An unfortunate unbalance. Along these lines, why is it really bad when we clear-cut and burn huge expanses of forest and grasslands? By doing so, not only do we remove a CO2 sink (the plants), we release a ton of CO2 into the atmosphere by burning all the plant matter.
We can never underestimate the impact of plants’ removal of atmospheric carbon dioxide on the global climate. When the huge fern forests of the Carboniferous era (359—299 million years ago) first formed, they removed so much carbon dioxide from the atmosphere it caused global cooling and glacier formation! It’s unfortunate that this is now occurring in reverse. Increasing levels of atmospheric CO2 are increasing global temperatures, and the blame lies with us (humans) instead of it being a natural event.
I love the nitrogen cycle because bacteria play such an important role in ensuring this essential element reaches all corners of an ecosystem. Nitrogen is important because it is a building block of proteins and nucleic acids. The main reservoir (storage area) for nitrogen is in the atmosphere, and it is in the form of nitrogen gas (N2). Since the gaseous form is literally floating around the air, it needs to first be brought down to Earth and “stuck” in a form that plants can use. This process is called nitrogen fixation—converting gaseous nitrogen into forms that cells can use—and it is how nitrogen enters an ecosystem.
The nitrogen cycle
Nitrogen fixation can happen when lightning strikes! The enormous energy in a bolt of lightning shatters the N2 molecule, allowing it to combine with atmospheric oxygen to form nitrate (NO3-) which then dissolves in rainwater and falls to Earth. More commonly, we rely on bacteria to grab this nitrogen gas and turn it into a form that cells can work with (cells can’t grab nitrogen gas). There are many different forms of nitrogen-containing compounds, and they can be used by different life-forms.
In case you’re wondering why animals aren’t listed above, it’s because we get our nitrogen by eating plants and animals, and their tissues contain nitrogen as it moves through the food chain. I am only going to describe how nitrogen is initially fixed and converted into different forms through terrestrial cycling.
Bacteria that are able to grab gaseous nitrogen from the atmosphere and turn it into a usable form are called (unsurprisingly) nitrogen-fixing bacteria. Probably one of the most well-studied N2 fixers is a bacterium of the genus Rhizobium. Along with being a perfect example in the nitrogen cycle, it is also an excellent example of mutualism!
Rhizobium forms a close relationship with plants of the legume family, such as clover, alfalfa, peas, and beans. The plant gives the bacteria a protected, nice place to live and a steady source of nutrients. The bacteria, in turn, fix atmospheric nitrogen and produce ammonia for the plant. How this beneficial relationship grows (literally) in the first place is pretty cool. The bacteria hang out in the soil during the winter and infect a plant when it germinates in the spring. The bacteria are attracted to certain chemicals produced by the plant, and they slowly make their way over to the roots. The bacterial cells move into the root tissue and the plant cells in the invaded region are stimulated to grow and divide, forming a small spherical “house” (a nodule) in which the Rhizobium reside. Each nodule is chock full of Rhizobium bacteria that are now nitrogen-fixing machines, creating a slew of ammonia for the plant to use.
Root nodules along the roots of a soybean plant
Author: United States Department of Agriculture. https://commons.wikimedia.org/wiki/File:Soybean-root-nodules.jpg
Once nitrogen gas is turned into ammonia (NH4+) and deposited in the soil, it is available for plants to use. Another type of bacteria (nitrifying bacteria) also find this soil ammonia and convert it into another chemical form called nitrate (NO3-), which plants also love to use. Plants take in the nitrogen—an essential nutrient—and incorporate it into their tissues. When critters eat these plants, their own tissues take up the nitrogen … and when they are eaten by other critters, the nitrogen is then used in their own tissues, and so on and so forth. Once an animal dies and decomposers help to liberate the stored organic molecules, nitrogen is once again set free into the cycle. Considering that bacteria are driving this cycle, there’s one final group of microbes — the denitrifiers—that grabs on to the nitrate in the soil and converts it back into nitrogen gas, removing it from the ecosystem and releasing it back into that huge reservoir in the sky.
1. What is the difference between an organism’s habitat and its niche?
2. Explain the significance of the following graph. Both species are occupying the same hypothetical niche.
3. Why are food chains limited in steps (trophic levels)? Relate your answer back to the laws of thermodynamics.
4. What process removes carbon dioxide from the atmosphere? What processes release carbon dioxide back into the atmosphere?
5. Fill in the blanks: The competitive exclusion principle states that two different __________________ cannot occupy the same ecological __________________. The survivor is the one who was able to __________________ the other.
6. The first trophic level in a food chain must be a __________________. Explain your answer.
7. Choose the correct term from each pair: Earth is a(n) open/closed system in regard to matter, but a(n) open/closed system in regards to energy.
8. The first step of the nitrogen cycle is __________________, when nitrogen gas is fixed into organic molecules. The two means of nitrogen fixation are __________________ and _________________.