AP Biology Energy and Ecosystems

Welcome to Educator.com.0000

In this final lecture on the topic of ecology, we are going to be discussing the flow of energy through ecosystems0002

as well as the cycles of chemicals that are essential to life: water, carbon, nitrogen and phosphorus.0009

So, I am going to start out with just talking about energy within an ecosystem.0016

Recall that an ecosystem is all the organisms and the non-living components in one area, so it includes both the biotic and the abiotic components.0022

As we discuss the flow of energy through ecosystems, you should recall the first law of thermodynamics that we talked about back in an early lecture.0041

And the first in the lecture on...actually, when we talked about energy at the cellular levels.0051

The first law of thermodynamics states that energy cannot be created or destroyed. It can only be changed from one form to another.0059

Virtually, all life on earth depends on the input of energy from the sun, so life depends on energy from the sun.0103

Everyday, about 10 to the 22nd Joules of energy from the sun reaches the earth.0121

Most of this solar energy is actually reflected or absorbed by the atmosphere, clouds and airborne dust.0129

However, a small portion of the light from the sun eventually reaches photosynthetic organisms.0136

But, because photosynthesis only utilizes certain wavelengths of the light,0143

only about 1% of the light energy reaching photosynthetic organisms is converted into chemical energy in the form of organic compounds.0147

So, when I say converted to chemical energy, I mean converted into organic compounds.0174

For the purposes of describing energy flow within an ecosystem or within the earth as a whole, energy can be expressed a couple of ways.0178

Energy can be expressed in using an actual energy measurement.0187

So, sometimes, we will talk about Joules when we talk about energy, or we can talk about the mass of organic matter produced.0191

Usually, energy is also discussed or expressed per unit time, so an example would be,0198

we might talk about Joules per second or kilograms per year of organic matter produced.0204

Energy can also be produced - or excuse me - discussed or expressed per unit area.0212

So, you may also see discussions within the ecosystem talking about Joules per second0218

per meter squared or kilograms of organic matter produced per year per kilometer squared.0225

Two important terms that you should be familiar with are gross primary productivity, which is known as GPP and net primary productivity.0233

Gross primary productivity is the total amount of light energy converted to chemical energy.0266

So, it is the total amount of light energy converted to chemical energy.0274

Net primary productivity is the GPP minus the energy required for respiration.0291

Since about half of the energy actually end up getting used for respiration,0315

what you will end up with is that NPP is approximately equal to half or 50% of GPP.0321

And the other half of that energy ends up getting used for respiration.0333

Now, ecosystems vary in their productivity. Tropical rainforests have the highest NPP per unit area of any ecosystem.0337

Tropical rainforests have the highest net primary productivity per unit area.0349

Oceans actually have a low NPP per unit area. However, they occupy a very large portion of the earth's surface.0359

So, they contribute more to the global NPP than any other biome because of the large area that they cover.0370

Now, it would follow that net primary productivity and, therefore, the amount of0377

biomass it has accumulated per year is dependent on the amount of sunlight available.0382

And this is true to a large extent, but there are other factors involved.0387

So, as we learned earlier, solar energy is greater near the equator than toward the poles.0391

And the productivity of terrestrial biomes generally follows that pattern: greater productivity near the equator, lower productivity near the poles.0397

However, there are many cases where productivity does not correlate with sunlight energy.0407

Examples would be the oceans near the equator also the Sahara desert, and this because there are other limiting factors.0413

Precipitation: water is a major one that is going to limit productivity in a desert biome.0422

Other nutrients such as nitrogen and phosphorus can also be limiting factors.0427

And we are going to discuss these factors right now as we talk about biogeochemical cycles.0431

Chemicals and other nutrients are constantly being transported through the environment by biological activity, weather and other natural processes.0440

Human activity can also alter some of these natural processes as we will talk about at the end of this lecture.0449

Now, recall that the law of conservation of mass dictates that mass cannot be created or destroyed.0456

It follows that on a long-term worldwide basis, the process by which chemicals and0463

nutrients are exchanged among the various parts of the ecosystem, are actually cycles.0470

Since mass cannot be created or destroyed, what is happening is it is just being cycled in different forms.0475

These cycles are known as biogeochemical cycles, and these cycles or parts of these cycles, occur with varying speeds and distributions.0483

If you look at water transport through the atmosphere, it is a relatively quick process, and it is also a global process.0495

Whereas, transport of phosphates in a terrestrial ecosystem remains relatively local because phosphates are solids.0502

And so, they are not easily transported over long distances.0511

Now, as I said, sunlight is obviously very important in determining productivity, but what is important as well is the availability of nutrients.0516

So, availability of nutrients is often a limiting factor in biological productivity.0525

Each chemical that we are going to talk about, or each nutrient, is located in varying proportions among the atmosphere,0551

the water, soil, ocean sediments, rock and biomass, and these are known as reservoirs.0559

So, reservoirs for different substances or nutrients like nitrogen or carbon include the atmosphere, water, soil, ocean sediments, rock and in biomass.0566

There are four biogeochemical cycles that we are going to focus on because they are the most significant to living organisms.0590

Then, we are going to go through each of these in detail. The first is the water cycle.0601

As you know and as we discussed in the lecture on properties of water, water is critical to life.0609

It is required for the growth and metabolism of all living things.0614

Carbon: carbon is the building block for all of organic matter.0619

Nitrogen: nitrogen is a very important nutrient, and it is required for the formation of proteins and other macromolecules.0625

Phosphorus is also found in various macromolecules, as well, notably nucleic acids and ATP.0634

So, in the next four slides, I am going to go over each of these cycles including the reservoirs,0641

how the nutrient is transported among the reservoirs, and the importance of that chemical or nutrient to living organisms.0647

We are going to start out with the water cycle.0655

So, as I said, water is critical to all of life, and in the lecture on the properties of water,0657

we talked about the many unique properties of water that make it such an important substance to living organisms.0663

The availability of water is particularly important to primary producers.0671

Now, water is used by living organisms primarily in its liquid form, and I am going to start out with some of the reservoirs for water.0677

About 97% of all water in the biosphere is in the oceans, so talking about reservoirs of the water, 97% of the water reservoir is oceans.0687

There is about 2% found in glaciers, so 2% is in the form of ice, in glaciers and in the polar ice caps.0702

That leaves only about 1% water of the total water reservoir in freshwater lakes, rivers, streams, wetlands and ground water.0718

Now, the atmosphere is critical to the water cycle as we are going to see, but it holds only a negligible amount of the earth's water.0741

Looking at this illustration, we are going to focus on how water moves between the various reservoirs, and we will start out with precipitation.0749

With precipitation, condensed water vapor falls to the earth's surface.0757

Then, because this is a cycle, the water is going to be returned to the atmosphere, and that occurs through several processes.0763

First, evaporation: about 90% of all evaporation occurs in the oceans.0770

Sublimation also helps to return water into the atmosphere, and remember, sublimation is when water goes from its solid form directly into water vapor.0788

So, when snow or ice changes directly into water, that is sublimation.0800

Water is returned to the atmosphere when it is in a terrestrial environment largely through transpiration.0805

Recall in the lecture on plants, we talked about transpiration being absorption of water by plants from the soil.0813

For most plants, it is the roots that take up the water.0822

And then, it will leave or evaporate into the atmosphere out via the leaf, mostly via the stoma in the leaves.0825

The three major ways that water is returned to the atmosphere are evaporation, sublimation and transpiration.0837

You should be familiar with the term evapotranspiration, and this refers to the amount of water transpired plus the amount that evaporates from land,0845

so here, focusing just on what is happening on land plus amount of water that evaporates from the land.0865

Canopy interception - I will put it up here by these trees - refers to when precipitation0879

lands on plants and then, ends up evaporating before it ever hits the ground.0890

Water can also move across the land, and what you see here is run off.0897

So, run off is when the water moves across the land, and it can either move across the ground or move via streams and rivers.0902

Snowmelt is run off from the melting snow, so we are talking about now water moving along the land.0917

We talked about water moving from the atmosphere to the land and the oceans0928

and water being returned to the atmosphere through transpiration, evaporation, sublimation.0933

OK, so, we have run off. We also have subsurface flow.0939

In subsurface flow, this is the flow of water underground, and what can eventually happen is that this water seeps into the ocean.0942

And so, this is movement of water along the land via run off, snowmelt, infiltration. I did not mention that yet.0965

And this is water moving from the surface underground, so what can happen is there could be snowmelt run off from that.0975

It enters the ground, or precipitation that is hitting the ground can end up infiltrating, moving along via subsurface flow and eventually, entering the ocean.0991

And then, it could evaporate out from there, so you see how this is definitely a cycle.1000

Water can also move through the atmosphere, and the term for that is advection.1004

Finally, one more process that is not mentioned right on here but that you should know is that of condensation.1019

And this is when water vapor is transformed into liquid droplets, for example, fog, or if it is transformed on the ground, then, we call it dew.1025

Here, this gives you an overview of how water is recycled through the system from precipitation, water landing on the ground,1045

evaporating, sublimating, transpiring back up into the atmosphere, moving along the land via run off,1056

infiltration and subsurface flow, as well as moving through the atmosphere in the process of advection.1064

The second cycle that we are going to talk about is the carbon cycle.1073

And the carbon cycle is the exchange of carbon among the biosphere, atmosphere, rocks, soil and water.1078

Recall that carbon in the form of carbon dioxide is used during photosynthesis by primary producers to generate organic compounds.1086

So, CO₂ is used by plants, and it is used to produce organic compounds.1098

Herbivores will, then, consume plants, utilize those organic compounds. Carnivores can eat the herbivores and get organic compounds that way.1106

And the plants release oxygen into the atmosphere during the process of photosynthesis.1120

Living organisms, as you know, are made up of carbon-containing organic compounds.1127

Now, starting out with some of the reservoirs for the carbon cycle, the atmosphere is approximately 400 parts per million CO₂.1133

So, it holds about 750 gigatons of carbon, and you do not need to memorize these numbers or anything.1144

But, I just want to give you a relative idea of the size of the reservoirs.1152

Plant and animal biomass or just biomass in general, about half of the dry weight of living organisms is carbon.1157

If we put together all the living biomass, plants and animals together, we will get about 600 gigatons of carbon.1173

Soils contain about 1500 gigatons of carbon. Some of this is organic carbon, and some is inorganic forms such as limestones,1184

so organic plus inorganic, so as I said, limestone, which is calcium carbonate, so example, calcium carbonate.1202

The oceans hold about 36,000 gigatons of carbon, but this mostly in the form of inorganic; so it is mostly inorganic carbon.1215

Rocks and the interior of the earth also hold large quantities of carbon.1232

However, with the exception of volcanoes that will add some CO₂ to the atmosphere,1236

the carbon found in rocks and within the earth is largely unavailable to the biosphere.1241

Now, there are three main pathways that carbon moves among living things.1246

The first, I already mentioned photosynthesis.1252

During photosynthesis, photosynthetic organisms such as plants use CO₂ to generate organic compounds.1255

Consumers, so animals, utilize carbon. They eat the producers, and they utilize the carbon.1270

During respiration, these organisms release CO₂.1284

So, here, CO₂ is being consumed by the producers. The producers are being eaten by the consumers.1289

The consumers are respiring and releasing CO₂ back into the atmosphere.1296

All living things also contribute carbon to the soils, the sediments and the oceans, as well as lakes via detritus.1301

So, detritus consist of dead organisms and waste, so fecal material, and the dead organisms and the waste products are broken down via decomposers.1314

And the result is that this carbon is returned to the soil and the oceans and lakes.1331

I also want to talk about fossil fuels. Combustion of fossil fuels by humans is adding CO₂ to the atmosphere.1338

And I am not going to go into detail on that now because at the end of this lecture,1347

I am going to do a slide talking about human impact in the environment, increased CO₂ levels and the Greenhouse Effect.1351

The third cycle is one that I introduced earlier in the lessons, and that was the nitrogen cycle.1362

And recall that we talked about the nitrogen cycle when we discussed plants, but I am going to review it again here.1369

So, the nitrogen cycle is the conversion of nitrogen between its various forms in the biosphere.1374

The atmosphere is about 78% nitrogen.1383

However, recall that atmospheric nitrogen, which is in the form of dinitrogen, N₂, is unavailable for biologic organisms to use.1389

It needs to be converted into another form.1399

Now, nitrogen is essential to life because it is needed to produce proteins, nucleic acids,1402

amino acids, which are the building blocks of proteins and other biological molecules.1409

The productivity of primary producers is frequently limited by the availability of nitrogen, so nitrogen can be a limiting factor for primary producers.1416

I am just going to give you an overview of some of the reservoirs of nitrogen before we go into the cycle.1435

Nitrogen is found in...we talked about the atmosphere. That is not immediately utilizable.1442

But, reservoirs include the soil, sediments of the oceans, lakes and rivers.1449

Dissolved nitrogen that is found in surface water, in ground water, biomass, is also a reservoir for nitrogen.1462

So, as I mentioned, the atmosphere is 78% nitrogen, but it is dinitrogen that cannot be utilized by biologic organisms.1483

So, from the plant lecture, remember that this dinitrogen gas can be converted to ammonia by nitrogen-fixing bacteria.1491

These nitrogen-fixing bacteria take the nitrogen and convert it to ammonia, NH₃.1504

In the soil, the hydrogen ion that is present combines with the ammonia to form ammonium ions, and we are going to show this here, as well.1516

So, you will end up with NH₄⁺, which is ammonia.1530

Now, plants can use ammonium directly, but, it is toxic if it is in large concentrations in the soil.1534

So, instead of just having a bunch of toxic ammonia floating around, instead,1543

what happens is nitrifying bacteria oxidize the ammonium to form nitrates - excuse me - nitrite.1548

Nitrate is the first step. Then, nitrifying bacteria further oxidize the nitrites to nitrates.1559

Before we go on, I just want to point something out here. There are a couple of places where nitrogen-fixing bacteria exist.1579

There can be free-living...there is free-living nitrogen-fixing bacteria in the soil. That is what is shown here.1587

Recall that there is a group of plants known as legumes that have a mutualistic relationship with rhizobium.1597

And these plants have what is called root nodules, and these bacteria live in vesicles inside the cells in the root.1604

The bacteria within the nodules fix the nitrogen, so the plant receives nitrogen; and the bacteria receive protection and nutrition from the bacteria.1615

So, recall that this is a mutualistic relationship. So, some plants have nitrogen bacteria living right there inside their roots.1627

Whereas other plants rely on free-living nitrogen bacteria in the soil to take the nitrogen from the N₂ form to the form of ammonia.1634

And then, here, we have nitrifying bacteria going on and further converting the ammonia to nitrites and nitrates.1644

Lightning also fixes nitrogen, so in the atmosphere, some of the nitrogen actually ends up getting converted from N₂ to nitrate just naturally by lightning.1660

But, this is a very small amount.1679

Decomposers also have an important role in really most cycles, so decomposers break down dead plants and animals, as I mentioned before,1684

so the waste products, as well as the waste products from these organisms.1694

And in the nitrogen cycle, this breakdown will release nitrogen in the form of ammonia into the soil.1697

So, this process of converting organic nitrogen to ammonia in this step with the decomposers is called ammonification.1704

And here is just shown decomposition from animals. However, decomposition also occurs with plants.1721

So, just to sum up, although there is significant amount of nitrogen in the atmosphere,1729

it is in the form of dinitrogen, so it is not utilizable by most living organisms.1735

Instead, these organisms rely on nitrogen-fixing bacteria to put it in a form that can be utilized.1741

Nitrifying bacteria, then, oxidize the ammonium to nitrites and then, nitrates.1748

I also want to point out that denitrifying bacteria actually can end up taking this nitrate and converting it back into dinitrogen/N₂.1757

And then, that is just lost to the atmosphere and cannot be utilized by the plants.1769

The final cycle that we are going to talk about is the phosphorus cycle.1775

And the phosphorus cycle involves the transfer of phosphorus among living organisms, the soil and the water.1778

The atmosphere does not play a role in the phosphorus cycle.1787

Phosphorus and phosphorus-containing compounds are solids, so this cycle compared to the other cycles, is much more localized.1792

Phosphorus is required for the formation of nucleic acids, so it is found in nucleic acids. As you know, it is found in ATP.1802

It is a component of shells, bones, teeth, so it is very important to living organisms.1811

Phosphorus availability is frequently a limiting factor in the productivity of aquatic organisms.1819

And we talked about productivity and that sunlight can be a limiting factor. Other limiting factors can be nutrients.1842

In an earlier lecture, I talked about nitrogen and phosphorus both being limiting factors in biological productivity in some systems particularly in the oceans.1854

The primary form that phosphorus is used and found in living organisms is phosphate.1864

Reservoirs of phosphorus: the largest reservoir is marine sediments and sedimentary rock.1875

It is also found in dissolved form in the oceans, so phosphorus is found in dissolved form in the oceans. Phosphorus is present in soil and in biomass.1900

Looking at this cycle, this is actually one of the slowest cycles, and as I said, it is a more localized process.1915

Starting here with the primary producers, they take up phosphates from the soil.1922

They are, then, consumed by consumers that obtain the phosphates from the primary producers.1930

Decomposers break down the detritus and excrement from animals and then, detritus from plants and then, return phosphates to the soil in that way.1939

So, we have uptake by plants of phosphates, animals consuming the plants and, thereby, receiving phosphates and then,1953

breakdown of animal wastes and dead animals as well as dead plants or plant parts.1962

These are broken down by the decomposers, thus, returning the phosphates to the soil.1969

In addition, weathering of rocks releases additional phosphates that are added to the soil.1974

There is also some loss of phosphate from the soil because it can be dissolved by rainwater and then, carried off.1982

So, there ends up with run off from the soil into the ocean, so there is actually a loss of phosphorus in that way.1993

Now, as we discussed earlier, the atmosphere is about 400 parts per million CO₂.2002

To be a little more exact, about 390 parts per million CO₂ in the atmosphere.2008

The CO₂ from the atmosphere, as you know, is used in photosynthesis by plants and other photosynthetic organisms.2017

And then, CO₂ is released to the atmosphere by cellular respiration.2025

So, if we look at, let's just say, plants releasing CO₂, and then, we have other organisms that are...actually, change that.2030

We have the atmosphere, and we have plants taking up the CO₂; and then, we have respiration from other organisms releasing CO₂.2047

And these two processes are in approximate equilibrium.2061

Therefore, the quantities of CO₂ taken up are approximately equal to the quantities of CO₂ released into the atmosphere.2066

This means that the CO₂ in the atmosphere should be pretty stable.2076

The level of CO₂ is not being changed by these biological processes.2081

However, since about 1850, humans have been burning relatively large and increasing quantities of fossil fuels, so increased burning of fossil fuels.2086

What are fossil fuels? Well, these include coal, oil and natural gas.2102

And what all of these fossil fuels are is actually the fossilized remains of biomass from millions of years ago.2112

By burning fossil fuels, what humans have done is they have removed carbon from the reservoir.2120

So, burning of fossil fuels results in removing carbon from the reservoir where it was and adding CO₂,2126

relocating the carbon to the atmosphere in the form of CO₂.2135

We are adding CO₂ to the atmosphere, removing it from where it was sitting in a reservoir of fossil fuels.2138

And if you look at the CO₂ concentration in the atmosphere, it has increased from recent historical levels of about 280 parts per million.2145

So, back here, we are at about 280 parts per million CO₂ in the atmosphere until about the year 1800.2157

Then, around 1850, the amount of fossil fuels being burned increased, and they have continued to increase.2166

At the present time, the concentration of CO₂ in the atmosphere is increasing at the rate of about 2 parts per million per year.2174

In 2011, we are at about 390 parts per million.2192

Also note that the records since 1958 are from monitors located in Hawaii.2201

And what you can see here is that there is an annual cycle due to increased photosynthesis in the Northern Hemisphere in the spring.2207

So, the carbon levels are different seasonally depending on how much photosynthesis is going on and how much CO₂ is being utilized.2215

And that is what is you are seeing right here in the background.2222

But, overall, if you just look at the trend, it has been upward and then, sharply upward in the 20th and 21st centuries.2224

So, what is the impact of this?2235

Well, CO₂ and the water vapor in the atmosphere absorb and reflect heat that is radiated by the surface of the earth.2237

And this prevents the heat from escaping into space. This is known as the Greenhouse effect.2250

So, in the Greenhouse effect, heat is essentially trapped by the CO₂ and the water vapor in the atmosphere.2257

Now, some of this is necessary and good because without the Greenhouse effect,2263

the earth would not be warm enough to be hospitable to life and for us to have moderate temperatures and for the earth to support life.2268

However, the increased CO₂ concentration since about 1800 and2276

especially 1850 has contributed to an increase in the average global temperatures.2282

In particular, temperatures in the higher latitudes especially in the Northern Hemisphere have increased significantly.2288

And this has led to dramatic decreases in Arctic sea ice and in the resilient glaciers.2297

The result of this increased CO₂ concentration is increased productivity by plants.2307

So, more CO₂ means more uptake of CO₂ by plants.2315

But, this is not enough to offset the increase in CO₂ in the atmosphere due to fossil fuel combustion.2321

The likely effect is that there will be changes in some ecosystems as plants whose2331

productivity is more sensitive to CO₂ concentration become more dominant.2336

So, certain plants that can really thrive due to this increased CO₂ concentration can take over, so it could affect the balance in ecosystems.2341

Projections show that there is going to be likely a continued increase in CO₂ through the end of the 21st century, and the estimates vary.2351

But, one estimate is that global average temperature could increase several degrees Celsius.2364

And this could be the highest global average temperature in about 100,000 years.2371

Another effect of human activity on ecosystems is that of eutrophication, and I have mentioned this elsewhere in the course.2379

But, I want to really focus in on it now.2389

When we talked about the nitrogen and phosphorus cycles, I mentioned that these nutrients can be a limiting factor in ecosystem productivity.2393

So, nitrogen and phosphorus can be limiting and frequently are limiting factors - I will say frequently limiting factors - in the productivity of an ecosystem.2402

Humans have affected the availability of these nutrients as well as their distribution.2429

Looking first at nitrogen, nitrogen is found in fertilizers, so what happens is when crops are harvested, then, the nitrogen is removed.2435

So, nitrogen is lost from the soil. It is used by the plants.2448

The crops are harvested. They want to plant another crop, so they need to provide nitrogen in the soil.2451

What is done is fertilizers containing nitrogen are added, so fertilizers contain nitrogen, and some fertilizers are carried by run off into lakes and streams.2458

So, human activity can end up increasing the nitrogen levels in bodies of water.2482

Now, let's talk about phosphorus. There is also phosphorus in fertilizers, and phosphorus is used by households and industries.2490

The use by households and industries subsequently causes a discharge of...so then, that phosphorus is used.2510

The wastewater goes to treatment facilities and then, ends up released into rivers, lakes and oceans.2519

So, eventually, the phosphorus in fertilizers ends up in bodies of water via run off.2529

Household and industrial use of phosphorus causes additional phosphorus to end up in rivers, lakes and oceans through discharge of waste water.2535

Now, improvements have been made in controlling these sources of pollution. For example, phosphates have largely been eliminated from detergents.2556

And improved farming techniques have reduced the amount of nitrogen that is required to be added to the soil.2569

However, what can happen when nitrogen and phosphorus end up in lakes, rivers, streams, body of water, is this process of eutrophication.2574

So, let's go through what happens. First, you will end up with increased phosphorus and/or nitrogen in a body of water, let's say a lake.2587

The result is that since these nutrients were the limiting factors in productivity, now that this limiting factor has been increased, we have more of it.2608

The primary production is going to be increased, so increased primary production.2619

What that means is that there is going to be an increase in the biomass of phytoplankton. This causes algal blooms, which you may have heard of.2626

Some of the algae are toxic. Some are toxic, and these can, then, in turn, harm consumers,2646

so consumers farther...even, not necessarily direct consumers of the algae, although, those can be harmed,2657

but also the tertiary and the quaternary consumers can be harmed as well, as this toxic algae enters the food chain.2663

Due to this increase in biomass, what can occur is an increase in turbidity of the water. Also, the level of dissolved oxygen is reduced.2676

This reduction in dissolved oxygen causes fish kills, and it decreases the diversity of fish, shellfish and the organisms that depend on them for nutrients.2698

So, you can see the multitude of effects adding a limiting factor like nitrogen or phosphorus to an ecosystem can have.2717

Alright, let's go ahead and do some examples to discuss what we have covered today.2730

Consider two ecosystems.2734

The first one is the Florida peninsula, which is between latitudes 25 North and 28 North, primarily subtropical forests and wetland biomes.2737

The second ecosystem: the Baja California peninsula. It is at the same latitude as Florida.2748

So, note that it is at the same latitude, and it is an arid desert biome.2754

So, here, we have subtropical forest and wetland biome, and here, we have a desert biome; but they are at the same latitude.2762

Which system has a higher gross primary productivity? Well, Florida, the Florida ecosystem would.2770

And that is because areas with a larger plant biomass like subtropical rainforest and wetlands2781

have higher productivity because of the larger quantity of photosynthesis that is occurring.2790

Florida, because I would expect it to have a larger plant biomass with the result that there is going to be larger quantity of photosynthesis.2797

Which ecosystem receives more sunlight?2818

Well, note that these are at the same latitude, so they actually receive the same amount of sunlight.2821

What is the possible explanation for why these two do not have the same growth primary productivity or the similar GPP?2833

So, we said that at the same latitude, they are receiving the same sunlight. Why do they have different primary productivity?2845

Well, sunlight is not the only limiting factor. In a desert biome, a major limiting factor is water, so in a desert biome, water is a limiting factor.2853

Without sufficient water, plants cannot develop enough biomass to utilize all this available sunlight.2869

In addition, in biomes where there is a lack of water like in deserts, plants end up with physical structures that conserve water.2877

So, looking in at a cactus with just spines or needles versus a broad leaf that is going to maximize exposure to the sun,2885

that plant may not be as effective as photosynthesis as a plant in a wetter biome because the plant in the desert biome has to conserve water.2894

So, plants in dry areas may have structures with decreased surface area and, therefore, not as efficient to have photosynthesis.2905

Example two: which of the following could be a contributing cause to increased atmospheric CO₂ over the last 200 years?2926

What could result in increased carbon dioxide?2935

A. decrease in the size of fish populations due to overfishing.2938

B. conversion of large quantities of forest to farmland.2944

C. migration of humans to cities from rural areas.2949

or D. increased volcanic activity.2955

So, what would increase atmospheric CO₂?2957

Well, fish, when they respire, are going to release CO₂. Therefore, if anything, less fish, would mean less CO₂ release.2963

So, less fish is not going to likely increase the atmospheric CO₂, so let's get rid of that.2980

Conversion of large quantities of forest to farmland, well, recall that forests have high productivity, so forests are going to consume a lot of CO₂.2986

If you decrease the forest, you do not have as much CO₂ consumption, and CO₂ levels will increase.3003

So, this could definitely be a contributing factor to increasing CO₂ levels.3012

Let's go ahead and check out the other two answers.3017

Migration of humans from cities to rural areas- this alone should not impact carbon dioxide levels.3020

And if anything, cities are more CO₂-efficient. There is less carbon dioxide produced per capita.3030

So, I would not expect the movement, the migration of humans from rural areas to the city to increase CO₂ levels over the past 200 years.3038

Increased volcanic activity- now, volcanoes do emit CO₂.3048

But, they are not a significant source compared with biological respiration releasing CO₂ and the combustion of fossil fuels.3054

I do not expect that to have that much of an effect especially compared with B, so the correct answer is B.3066

Conversion of large quantities of forest to farm land over the past 200 years has likely contributed to the increase in the atmospheric CO₂.3073

Example three: step through one sequence by which nitrogen and animal waste is returned to plants.3083

Alright, so, we have the animal waste, which is going to contain nitrogen.3092

But, what we need, then, is for the decomposers to generate ammonium through this process of ammonification.3096

Now, plants do not directly usually use ammonia because it is toxic in large quantities in the soil.3114

So, what happens is nitrifying bacteria will convert the ammonium to nitrites and then, continue on and convert the nitrites to nitrates.3124

These nitrates are, then, taken up by the roots of the plant.3148

And then, inside the plant, they are reduced back to ammonium and then, used in organic compounds, OK?3158

So, animal waste broken down by decomposers, the ammonium is utilized by nitrifying bacteria.3170

They release nitrites. It is further oxidized to nitrates.3180

Nitrates are taken up by the roots of the plant, reduced back into ammonium.3183

And that ammonium can be used as a component of organic compounds in the plants. You could see how nitrogen is cycled through the ecosystem.3188

Example four: which of the following would not be a likely outcome after a forest near a lake is converted to a farmland?3201

So, what would be not be a likely outcome when we have a forest near a lake converted to farmland?3211

The water becomes cloudy. The health of animals living near the lake is negatively impacted.3221

Dissolved oxygen increases from 1 milligram per liter to 5 milligrams per liter.3228

The number of fish species in the lake reduces from 5 to 2, or a non-native fish becomes the dominant species.3239

Now, this is looking at this, making an assumption that this farmland is going to have a negative impact,3247

that the farming methods that are being used include use of fertilizers, and there are some run off.3253

But, given that assumption that there is a negative impact on a forest by the nearby farmland, let's see what would happen.3259

Often, fertilizers are used that contain nitrogen and phosphorus, and the result can be eutrophication.3270

It has nitrogen and phosphorus that are added to a lake.3278

And that limiting factor for the primary producers is, then, removed because now, they have enough nitrogen and phosphorus, or at least they have more.3283

One result of eutrophication is that the water becomes cloudy, and this increase in turbidity is due to increased algae and phytoplankton in the body of water.3290

So, what would not be a likely outcome?3302

Water becomes cloudy- that is a likely outcome of eutrophication. Really, the topic here is eutrophication.3304

B. the health of animals living near the lake is negatively impacted-3311

this is likely if eutrophication occurs because the toxic algae in the lake during an algal bloom can make their way up the food chain as the consumers.3316

And then, the secondary consumers and the tertiary consumers end up ingesting this toxic algae directly or indirectly, so this is a likely outcome.3327

Dissolved oxygen increases- so, is the dissolved oxygen going to increase in eutrophication? No, no.3336

So, this is not a likely outcome because the dissolved oxygen typically goes down.3344

And the reason that the dissolved oxygen goes down - I am not sure if I highlighted this before, so I want to now - is that the decomposers in3351

eutrophication increase because now, there is all this detritus by the increased biomass from phytoplankton and zooplankton and algae and all this.3359

So, the decomposers increased from consuming the detritus, and they consume oxygen, so the decomposers increase as they consume the detritus.3373

And the decomposers, then, in turn, consume oxygen, so there will be decreased oxygen concentration in this lake- not increased.3399

The number of fish species in the lake reduces from 5 to 2- this is likely.3409

This dissolved oxygen reduction is going to reduce the number of fish species that can live in the lake, so this is likely.3414

A non-native fish becomes the dominant species- this is possible because the native species might not3423

be able to survive these new reduced oxygen conditions, and then, an invasive species can take over.3428

It is an invasive species that might be tolerant to reduced oxygen.3436

As you can see that the impact of nitrogen and phosphorus being added to the lake can be vast as eutrophication occurs.3440

And it changes the whole balance of the community within the lake, so the only thing I would not expect is for dissolved oxygen to increase.3448

That concludes this section on ecology here at Educator.com.3457

Thank you for visiting.3461