1. Energy Flow & Nutrient Cycle

2. Food Chains
Artificial devices to illustrate energy flow from one trophic level to another
Trophic Levels: groups of organisms that obtain their energy in a similar manner
Food Chains
Although the term 'food chain' has entered into common usage, in most ecosystems food chains do not occur. The idea that energy flows along a chain of consecutive links made up of various consumers is unrealistic. As we will see shortly, trophic interactions are considerably more complex than a series of linear steps. Food chains are a useful beginning to illustrate the concept of trophic levels.
Trophic levels are a way of identifying what kinds of food an organism uses.
Primary producers obtain their energy from the sun or chemical sources and utilize inorganic compounds from the environment to make organic compounds.
Herbivores feed on primary producers that utilize the sun for energy
Carnivores feed on herbivores and other heterotrophic organisms.

3. Example Food Chain
This simplified food chain illustrates links in a food chain. The chain begins with diatoms which are consumed by herbivorous copepods. The copepods are consumed by carnivorous zooplankton (in this case, chaetognaths) and the chaetognaths are consumed by planktivorous fishes.
In a food chain, energy moves in a linear fashion from producers through consumers.

4. Food Chains
Total number of levels in a food chain depends upon locality and number of species
Highest trophic levels occupied by adult animals with no predators of their own
Secondary Production: total amount of biomass produced in all higher trophic levels
Food Chains
In food chains, the total number of trophic levels depends upon the location and number of different species.
In general, the highest trophic level is occupied by adult animals with no predators of their own. For example, killer whales would occupy the highest trophic level in an antarctic food chain.
Secondary production refers to the total amount of animal biomass produced in all trophic levels above the primary producers. That is, it reflects all heterotrophic production.

5. Nutrients
Inorganic nutrients incorporated into cells during photosynthesis
- e.g. N, P, C, S
Cyclic flow in food chains
Decomposers release inorganic forms that become available to autotrophs again
Nutrients
Inorganic nutrients are incorporated into cells during photosynthesis and chemosynthesis.
Examples of important nutrients are nitrogen, phosphorus, carbon and sulfur.
The flow of nutrients in a food chain is cyclical. A pool of nutrients resides in a trophic level until animals die or excrete it. Then decomposers can release it in a form that is utilizable by autotrophic organisms.

6. Energy Non-cyclic, unidirectional flow
Losses at each transfer from one trophic level to another
- Losses as heat from respiration
- Inefficiencies in processing
Total energy declines from one transfer to another
- Limits number of trophic levels
Energy
Unlike nutrients, the flow of energy is not cyclical but rather is unidirectional. Energy is captured by primary producers and transferred to higher trophic levels. At each transfer, only a fraction of the energy is passed on and much is lost.
These losses are in the form of heat and inefficiencies in processing and assimilating energy.
Thus, the total available energy declines as one moves up trophic levels in a food chain.
This places a limit on the number of trophic levels that can exist. At some point, there is too little energy available to sustain further transfers.

7. Energy Flow

8. Energy Flow through an Ecosystem
sun
Food Chain
Primary
Producer
Primary Consumer
Secondary Consumer
Tertiary Consumer
zooplankton
larval fish
phytoplankton
fish
heat
Example Food Chain
This simplified food chain illustrates links in a food chain. The chain begins with diatoms which are consumed by herbivorous copepods. The copepods are consumed by carnivorous zooplankton (in this case, chaetognaths) and the chaetognaths are consumed by planktivorous fishes.
In a food chain, energy moves in a linear fashion from producers through consumers.
heat
heat
water
Nutrients
Fungi & bacteria
Decomposer

9. Transfer Efficiencies
Efficiency of energy transfer called transfer efficiency
Units are energy or biomass
Pt = annual production at level t
Pt-1 = annual production at t-1
Et = Pt
Pt-1
Transfer Efficiencies
Only a portion of the energy in one trophic level makes its way to the next. This is called the transfer efficiency. The currency may be energy or biomass.

10. Transfer Efficiency Example
Net primary production = 150 g C/m2/yr
Herbivorous copepod production = 25 g C/m2/yr
= Pcopepods
Et = Pt
Pt-1
= 25 = 0.17
Pphytoplankton
150
Transfer Efficiency Example
Let's assume that we wish to calculate the transfer efficiency between primary producers and herbivorous copepods.
Our currency will be grams of carbon.
The annual production of primary producers is 150gC per square meter per year.
The annual production of copepods is 25 gC per square meter per year.
The transfer efficiency is then 25/150 or about 17%.
Typical transfer efficiencies from primary producers to herbivores are about 20% while efficiencies between higher levels are about 10%.
Typical transfer efficiency ranges
*Level 1-2 ~20%
*Levels 2-3, …: ~10%

11. Energy and Biomass Pyramids
Kaneohe Bay
10 J
Tertiary consumers
100 J
Secondary consumers
1000 J
Primary consumers
10,000 J Limu
Primary producers
1,000,000 J of sunlight

12. Energy Use By An Herbivore
Algae eaten by Uhu
Cellular Respiration
Feces
Growth

13. Food Webs Food chains don’t exist in real ecosystems
Almost all organisms are eaten by more than one predator
Food webs reflect these multiple and shifting interactions
Food Webs
Remember that food chains are an artificiality that don't really exist. In reality, the trophic linkages between organisms are much more complicated. Most organisms have more than one predator and the diets of animals shift as they develop.
Food webs reflect the complexity of trophic interactions.

14. Antarctic Food Web

15. Some Feeding Types Many species don’t fit into convenient categories
Algal Grazers and Browsers
Suspension Feeding
Filter Feeding
Deposit Feeding
Benthic Animal Predators
Plankton Pickers
Corallivores
Piscivores
Omnivores
Detritivores
Scavengers
Parasites
Cannibals
Ontogenetic dietary shifts
Food Webs ...
There are many trophic categories that are too complicated to fit into the simple concept of a food chain.
Many animals are omnivorous. That means that they consume a wide variety of prey. An omnivore might consume diatoms and crustacean larvae. Thus, it's feeding at trophic levels one and two.
Detritivores feed on dead organic matter that can be derived from a wide range of sources at varying trophic levels.
During development (ontogeny) animals often shift their diet as they grow larger. Consider a tuna which may begin by feeding on copepods and zooplankton but which progresses to large fish at adulthood.
Parasites complicate the picture because they may have a number of different hosts of different trophic status.

16. Food Webs…
Competitive relationships in food webs can reduce productivity at top levels
Phytoplankton
(100 units)
Phytoplankton
(100 units)
Herbivorous
Zooplankton
(20 units)
Herbivorous
Zooplankton
(20 units)
Food Webs ...
The presence of two competitors feeding on the same prey items may alter the availability of energy to higher trophic levels. Consider the example on the left where carnivorous zooplankton of species A feed on herbivorous zooplankton and are themselves consumed by fishes.
Let's introduce a second species of carnivorous zooplankton. Species B isn't consumed by fish but shares the supply of herbivorous zooplankton with species A.
The result is that the availability of energy to fishes is diminished.
Food webs contain many of these sorts of competitive relationships and prey preference.
Carnivorous
Zooplankton A
(2 units)
Carnivorous
Zooplankton A
(1 units)
Carnivorous
Zooplankton B
(1 units)
Fish (0.2 units)
Fish (0.1 units)

17. Recycling: The Microbial Loop
All organisms leak and excrete dissolved organic carbon (DOC)
Bacteria can utilize DOC
Bacteria abundant in the euphotic zone (~5 million/ml)
Numbers controlled by grazing due to nanoplankton
Increases food web efficiency
The Microbial Loop
All organisms leak organic carbon compounds into the water. This organic carbon (DOC) is an important food source that would be a net loss to each trophic level.
Bacteria are abundant in seawater and many bacteria are capable of utilizing this DOC.
Bacterial numbers are controlled via grazing by nanoplankton (ciliates and flagellates). These small zooplanktors are then consumed by larger zooplankton. In this way, the lost DOC is recycled and returns to the food web.

18. Microbial Loop Solar Energy CO2 nutrients DOC Phytoplankton Herbivores
Planktivores
DOC
Piscivores
Bacteria
Nanoplankton
(protozoans)

19. An Ecological Mystery An Ecological Mystery
Let's take a look at a food web in the north Pacific ocean that has changed substantially in the past decade.

20. Keystone Species
Kelp Forests

21. An Ecological Mystery
Long-term study of sea otter populations along the Aleutians and Western Alaska
1970s: sea otter populations healthy and expanding
1990s: some populations of sea otters were declining
Possibly due to migration rather than mortality
1993: 800km area in Aleutians surveyed
- Sea otter population reduced by 50%
An Ecological Mystery
Sea otters are marine mammals that live in kelp beds along the western coast of North America from Baja Mexico to Alaska. Once hunted to near extinction, their protection has been one of the success stories of conservation.
In the 1970's, sea otter populations were healthy and expanding throughout their range.
Scientists noted that by the 1990's some populations of sea otters were declining.
One possibility was that the animals had moved rather than died.
In 1993, an 800 km long section of the Aleutian Islands was surveyed and the results were alarming. The sea otter population had declined by 50%.

22. Vanishing Sea Otters 1997: surveys repeated
Sea otter populations had declines by 90%
: ~53,000 sea otters in survey area
: ~6,000 sea otters
Why?
- Reproductive failure?
- Starvation, pollution disease?
Vanishing Sea Otters
In 1997 the Aleutian survey was repeated and the results were worse. Sea otter populations had declined by 90%. In 1970, some 53, 000 sea otters lived in the study area. By 1997, that population was down to about 6,000 animals.
A number of possible causes were considered. These included reproductive failure, starvation, pollution and disease. The problem with these hypotheses was that there was no evidence of dead otters that might support the idea of some epidemic or source of mortality that would kill many over a wide range.

23. Cause of the Decline
1991: one researcher observed an orca eating a sea otter
Sea lions and seals are normal prey for orcas
Clam Lagoon inaccessible to orcas- no decline
Decline in usual prey led to a switch to sea otters
As few as 4 orcas feeding on otters could account on the impact
- Single orca could consume 1,825 otters/year
Cause of the Decline
In 1991, one scientist noticed an orca (killer whale) eating a sea otter. This was unusual because sea lions and seals are the normal prey for orcas and a small animal such as a sea otter wouldn't provide much nutrition.
At one site called Clam Lagoon, populations of otters remained healthy. Interestingly, that site was inaccessible to orcas.
It turned out that orcas had indeed been responsible for the decline in otters. A decline in the abundance of their usual prey forced them to switch to otters.
Not all the orcas needed to switch to generate the mortality observed along the Aleutians. As few as 4 orcas feeding solely on otters could have produced an impact of the magnitude observed. A single orca could consume about 1,825 otters per year.

25. This diagram illustrates the cascade that swept through the food web.
Declines in oceanic fish due to overfishing and climatic changes led to a reduction in food for sea lions and seals. This forced the orcas to enter into the coastal waters where they consumed sea otters.
Sea otters normally feed on sea urchins. Without this control, the urchins increased in abundance.
Urchins graze on kelp, particularly on the holdfast and large numbers of urchins damaged kelp forests. The decline in the kelp forests has had an impact on many others species ranging from sea ducks to sea stars.

26. What does whale poo, iron and climate change have in common?
Sperm whale waste isn't much to look at -- a diarrhea-like substance with a few squid beaks floating around -- but new research has found it removes carbon from the atmosphere, helping to offset greenhouse gases that have been tied to global warming.
Sperm whales in the Southern Ocean release 220,462 tons of carbon when they exhale carbon dioxide at the water's surface, but their poo stimulates the drawdown of 440,925 tons of carbon, according to the research, published in the latest Proceedings of the Royal Society B.
WATCH VIDEO: Can jellyfish predict climate change?
These ocean giants and certain other marine mammals may therefore be among the most environmentally beneficial animals on the planet.
"If Southern Ocean sperm whales were at their historic levels, meaning their population size before whaling, we would have an extra 2 million tonnes (2,204,623 tons) of carbon being removed from our atmosphere each and every year," lead author Trisha Lavery Told Discovery News.
Lavery, a marine biologist at Flinders University of South Australia, and her colleagues explained how the cleaning process works.
It begins with sperm whales feeding on squid and fish, their favorite prey, deep in the ocean. The whales then return to the water's surface to relieve themselves.
"They do this because they shut down their non-crucial biological functions when they dive," Lavery said. "So it's only when they come to the surface to rest that they defecate."
Their waste comes out as a giant liquid plume (save for the undigested squid beaks) that showers over minute aquatic plant "seed stocks," which she said are "just floating around waiting for nutrients so they can use them to grow and reproduce." The whale poo provides these nutrients, functioning as a natural fertilizer.
The plants -- phytoplankton -- take up carbon from the ocean as they grow. Through the entire life and death cycle of these plants, the carbon then stays "trapped" for centuries to millennium.
Published estimates suggest that 12,000 sperm whales currently inhabit the Southern Ocean. Lavery and her team estimated the amount of prey consumed by each whale, along with the iron content of that prey. Iron is a critical phytoplankton fertilizer component.
Assuming that 75 percent of defecated iron persists in the photic -- or light receiving zone -- of the ocean, Southern Ocean sperm whales contribute 40 tons of iron to this region each year.
Humans driving cars, burning coal and engaging in other activities pump enormous amounts of carbon into the atmosphere, something that whales could never entirely offset.
"However, most whales are currently at 1 to 10 percent of their historical population sizes, so in the past, whales may have made a substantial contribution to carbon drawdown," Lavery said, adding that other marine mammals probably beneficially redistribute carbon just as whales do. These may include seals, sea lions and other types of whales, such as fin whales.
Unfortunately, some of these species wind up on sushi plates in restaurants here and abroad. A recent covert operation conducted by Scott Baker, associate director of Oregon State University's Marine Mammal Institute, determined that sashimi purchased at prominent sushi restaurants consisted of fin whale flesh, along with that of Antarctic minke whales, sei whales and a Risso's dolphin.
Lavery hopes all of the new research will help fuel efforts to conserve whales and other marine mammals.
"It is sometimes thought that conservationists try to 'save the whales' only because they are cute, however my work and the research of others is increasingly showing that whales play a crucial role in marine ecosystems," she said. "We must protect whales in order to have healthy, well-functioning marine ecosystems."
Could whale poop save the planet? Not entirely, but when the ocean giants known as sperm whales relieve themselves after a fish dinner, massive amounts of carbon are removed from the atmosphere. And this helps offset the harmful greenhouse gases that are linked to global warming, according to the Discovery Channel.
“If Southern Ocean sperm whales were at their historic levels, meaning their population size before whaling, we would have an extra 2 million tons of carbon being removed from our atmosphere each and every year,” Trisha Lavery, lead author of a study published in the Proceedings of the Royal Society B, told the Discovery Channel.
The 12,000 sperm whales estimated to inhabit the Southern Ocean (the waters encircling Antarctica) release carbon when they exhale carbon dioxide at the surface of the water, too. Yet these whales – and some other marine mammals – are among the animals that are most environmentally beneficial since their waste provides the nutrients that act as a natural fertilizer for aquatic plants that take up carbon from the ocean as they grow. This carbon stays trapped for hundreds to thousands of years.
While this is all good news, the bad news is that the whale population isn’t as large as it once was. “Most whales are currently at 1 to 10 percent of their historical population sizes, so in the past, whales may have made a substantial contribution to carbon drawdown,” Lavery told the Discovery Channel.
She said she’s hopeful that the new research will aid efforts to conserve ocean giants like sperm whales. Sometimes, Lavery says, the perception is that conservationists should save the whales because they are cute. But, she added, “We must protect whales in order to have healthy, well-functioning marine ecosystems.”
Read more:

27. Inquiry Define keystone species.
What is the relationship between sea urchins and sea otters?
Why doesn’t a food chain illustrate what really happens in ‘who-eats-who’ relationships?
Why are decomposers important?
Why do animals that eat lower on the food chain gain more energy than a top carnivore?
Homework assignment: pick a ecosystem and draw a food web. (E.g. coral reef, arctic, salt marsh, mangrove, estuary, deep sea…).