Chapter 7 Notes: Energy and Nutrient Relations
For Monday: 1. Read Molles 149-151 and 155-162 (sec 7.3) and answer Concept Review 7.3
2. Case Study: Mystery in Alaska – Why Have All the Sea Lions Gone?; complete Pts 1 and 2
Monday: Due: Vodopich 2, 15
Due: Case Study: Mathematics in Conservation
1. ID Quiz: Large Mammals
2. Complete Sea Lions, Pt 3
3. Discussion: Size and Habitat Use
For Friday: 1. Read Molles 162-170 and answer Concept Reviews 7.4, and 7.5
Friday: Due: SP Opportunity: Case Study: Mathematics in Conservation Q 12a-d
Due: Case Study: Sea Lions
Meet in lab
1. Vodopich 4.2, Q 3 on p 43
Life is essentially about converting energy and nutrients into descendants.
Ecology is essentially about studying how organisms solve the problems associated with
balancing their need for food with the dangers of living in a dynamic system.
We can group organisms by trophic biology (what they eat):
Autotrophs can be:
1. Photosynthetic – use light energy to convert carbon dioxide to sugars
2. Chemosynthetic – use chemical energy to convert CO2 to sugars
Heterotrophs:
3. Obtain organic molecules from the living tissues of other organisms
Prokaryotes can be any of 1, 2, 3
Protists are 1 or 3
Plants are mostly 1
Animals are mostly 3
Fungi are all 3
SKIP SECTIONS 7.1 AND 7.2
Section 7.3: Heterotrophs
Ecological stoichiometry – balancing multiple chemical elements in interactions between
individuals or populations.
Heterotrophs depend on chemicals produced by autotrophs, but a wide variety in
obtainment methods exist.
Three major categories –
I. Herbivores
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Elser Q asked: how ubiquitous is the imbalance between the nutrient content in herbivores
vs. their food? Measured by counting C:N and C:P ratios of plants and herbivorous insects.
*This method is valid because herbivores eat plants, which are mostly fiber/cellulose, i.e.,
lots of carbon, (and HO) and relatively lower NPS.
A: insects eat 5x to 10x as much C as they need to get enough NP.
Q: What happens to all the extra carbon they must eat?
Obvious deterrents to herbivores include thorns, abrasives (mostly silica), lignin (hard to
chew, fibrous, but also high C:NP ratio). Cellulose and lignin cannot be digested by most
animals without help.
Non-obvious deterrents
1. toxins – chemicals that kill, impair, or repel
2. digestion inhibitors – phenolic compounds (tannins) which reduce digestion by enzymes
Tropical plants produce more toxic alkaloids than temperate plants.
A larger number of herbivores in the tropics, so leaf removal is 2x to 7x higher in the tropics
than in temperate zones, suggesting that natural selection for chemical defenses by
herbivores is more intense in the tropics.
Bolser, Hay
Q asked: do tropical seaweeds have more chemical defenses than temperate?
Fed both temperate and tropical urchins with tropical and temperate seaweeds
A: both native urchins chose temperate seaweeds
Exam question: How did Bolser and Hay standardize their synthetic algae samples before
feeding them to the urchins?
Conclusion: tropics produce stronger arms race in chemical defenses
Some specialized herbivores have mechanisms for avoiding, altering, or using the chemicals
produced for defense.
II. Detritivores
Most tissues only become nutrient poor after death. 2x.
III. Carnivores
Stoichiometrically similar to their food source. This allows for quick processing time, shorter
guts, can eat less mass per body weight. BUT, food is harder to get and grazing is not
possible, thus more energy and time must be spent on finding food. Prey capture rates are
low – sometimes less than 1%.
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Prey must defend themselves.
1. morphological/physiological – camouflage, spines, shells, repellents, poisons
2. behavioral – flight, burrowing, grouping, playing dead, fighting, and showiness
Aposematic: defense that clearly visibly stands out as warning.
Müllerian mimicry: Comimicry among noxious animals
Batesian mimicry: nonharmful animal appearing as harmful. Must have a model and a
mimic (fig 7.15a/b)
Predators are most responsible for selection of prey defenses. Predators eliminate poorly
camouflaged/defended individuals, leaving well-defended, increasing and reinforcing the
defense over time.
Exam question: Explain how predators are responsible for selection of defense behaviors in
prey.
Even where prey choices differ or prey differ naturally as a consequence of geography, the
total stoichiometry remains balanced.
Q: how aware are carnivores of their own stoichiometry?
Size-selection predation: predators must select prey of a manageable size. Prey size tends
to increase with predator size.
CR7.3.1. Why do pumas face fewer stoichiometry challenges than deer?
CR7.3.2 Compare the challenges of being a detritivore with those of being a herbivore (fig
7.14)
CR7.3.3 Explain how a Batesian mimic might evolve from a nonaposematic ancestor.
7.4 Energy Limitations
Feeding rates increase as food availability does, but levels off when constraints are reached.
Energy intake is limited, either by environmental or internal constraints.
In Plants
Plants intake photons, which fluctuate with photoperiod, shade, and weather conditions.
Photon flux density/irradiance: quantity of light available for energization of water.
Pmax = maximum rate of photosynthesis
Isat = irradiance required for Pmax
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These responses are what separate “sun” and “shade” plants.
Shady habitats select for plants have higher photosynthetic rates than sun plants.
Net photosynthesis: total gross CO2 uptake minus CO2 produced by plant’s own
respiration.
Pmax and Isat shown by Adantium decorum, a lowlight forest fern (maidenhair), are much lower
than those in sunny environments
A high Pmax and Isat are helpful where water is low. When water does appear, these plants
ccan use all of it very quickly and efficiently.
All curves eventually level off.
Internal constraints are more applicable at high levels of food availability; environmental at
low levels.
In Animals
Functional response curves – response is a function of another stable or dynamic variable.
Type I: constant feeders with unlimited unchanging food supply. Think filter feeders, like
sponges. Curve is linear; feeding rate increases with food supply, until internal constraints are
reached.
Type II: rate is limited by searching time at low density; at medium density by searching and
handling time; and at high density entirely by handling. Intake levels off as animal has
maximized intake per prey/food item.
Type III: S-shaped curve. Response is slower at low density because of increase search time.
Q: Which categories of organisms (from section 7.3) would you expect to follow which
functional response curve?
Q: Why is Gross’ measured difference a valid tool for calculating intake rates?
Curve II is most common both in controlled studies and in the wild, across all levels and
categories of organisms.
CR7.4.1. In a Type III curve, what mechanisms may be responsible for low rates of food
intake compared to I and II at low densities?
CR7.4.2. Why are plants such as mosses living in the understory of a dense forest, which
show higher rates of photosynthesis at low irradiance, unable to live in environments where
they are exposed to full sun for long periods of time?
CR7.4.3. What conclusion can we draw from the parallel between photosynthetic response
curves in plants and functional response curves in animals?
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7.5 Optimal Foraging Theory
OFT models feeding behavior as an optimizing process/cost-benefit analysis
IF access to energy is limited, individuals that are more effective at acquisition will be
selected for.
Further, organisms cannot simultaneously maximally support all of lifes functions. Consider
energy availabity and balancing between competing demands as a sort of energy budget.
Principle of allocation: inevitable conflict between energy demands and allocation (p 103104). As a population adapts to a particular set of environmental conditions, its fitness in
other environments is reduced.
OFT scientists attempt to predict what consumers eat, and when and where they forage.
The predictions made using OFT can be tested by observing behavior or by economic
modeling. Theories themselves cannot be tested in whole, but in parts by testing specific
individual predictions made using the theory.
Abudndance of a food item is most important when evaluating foraging behavior.
Hihg prey abundance correlates with high energy return . A more abundant prey itme yields
larger return than uncommon do.
OFT can be used to predict composition of diets
Prey adbundance = number of prey encountered = Ne
Amount of energy (cost) expended by searching = Cs
Time spent processing/handling = H
Q asked: Given search and handling capabilities and a certain array of available prey, do
animals select a diet in a way that yields max rate of intake?
Q asked: What mix of prey will max intake under a particular set of circumstances?
We can model accurate answers to these questions, by setting E/T as intake rate. That is,
intake rate can be measured as E = energy available in a prey time taken in per unit time T.
For a single type of prey,
Ne1 = # of type 1 prey encountered (must be standardized per unit time if/when comparing
prey types)
E1 = net energy gained by eating one individual of type 1 prey (energetic return)
CS = energetic cost of searching
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H1 = handling time required for one individual of type 1 prey
To measure effectiveness, we arrange the variables as a ratio of energy over time.
The numerator must give total NET energy, so we have to multiply the energy available per
each prey item, and then subtract the total energetic cost of searching behavior.
In the denominator, handling time must be multiplied per prey item encountered (per time,
if applicable). We add a 1 to the denominator to ensure not dividing by zero. Because this
equation is only useful when comparing prey types, the 1 is both negligible and irrelevant.
When the predator has a choice between two prey types, will it choose a diet that still
maximizes efficiency?
We can add:
Encounter (rates) for prey 2 = Ne2
Energetic return for prey 2 = E2
Handling time for prey 2 = H2
Searching costs are assumed to not increase because the predator is still foraging in the same
area at the same time, and energy spent searching does not increase for available prey, but
still must be counted on a per prey basis
Clearly, this rule can be extended to as many prey items as are present:
OFT asks whether organisms feed in a way that maximizes E/T.
OFT, then, predicts that a predator will choose a single prey type (when choices are
available) for which energy intake is greater than when feeding on multiple types.
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This is to say that feeding on one species has a higher E/T than feeding on two. If feeding
on two is more efficient than feeding on one, the sign would reverse.
Predators will only include a second prey type IF the reverse is true – feeding on both
species simultaneously gives a higher E/T than one alone.
Optimization: predators will continue to add new prey species until E/T reaches max.
Q asked: do animals select food in a way to maximize E/T, as predicted by OFT?
Q asked: Obviously, animals do not consciously do this math when selected food items.
How then, is it useful?
Werner/Mittelbach Bluegill sunfish choose prey by size, which is convenient behavior for measuring the
composition of available prey.
They knew or could directly measure CS, H, Ne (for all prey), and E (for all prey), so they
could easily solve for E/T.
Fish selected prey that were larger than average and much less common than the most
abundant type.
Exam question: Does the finding of Werner/Mittelbach support or reject OFT? Explain.
In Plants
Plants “forage” by growth toward resources. In two (at least) directions at once, plants can
allocate energy to growth. The two environments (up/down) can be starkly different in
terms of available resources. For example, plants in light poor environments invest more in
above ground tissues while plants in nutrient poor invest below.
*Plants allocate energy for growth to those structures that gather the resources that limit
growth in a particular environment.*
Root:shoot ratio: biomass of above ground tissues per biomass of below ground tissues.
Tilman/Cowan
Four grasses, four herbaceous flowers, on seven levels of nitrogen concentration in 500 pots
in high density to low density (of plants per pot)
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Low root:shoot ratios were found in more nitrogenous soils, i.e., plants reduced root
biomass and increased shoot biomass in high availability of nitrogen. (fig 7.27)
CR7.5.1. According to OFT, under what conditions should a predator add a new prey
species to its diet?
CR7.5.2. Do patterns of feeding by bluegills (fig 7.25) include evidence that the consumers
ignore certain potential prey?
CR7.5.3. Why did Tillman and Cowan plant several plots of each species in each of their
growing conditions?
SKIP Applications section
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Chapter 7 Review Questions
Skip 1, 2, 3, 6, 10
4. What are the relative advantages and disadvantages of being a herbivore, a detritivore, or a
carnivore? What kinds of organisms were left out of our discussions? Where do parasites fit?
Where does Homo sapiens fit?
5. What advantage does advertising give to noxious prey? How would convergence in
aposematic coloration among several species of Müllerian mimics contribute to the fitness of
individuals in each species? In the case of Batesian mimicry, what are the costs and benefits
of mimicry to the model and to the mimic?
7. What kinds of animals would you expect to have type 1, 2, or 3 functional responses?
How should natural selection for better prey defense affect the height of functional response
curves? How should natural selection for more effective predators affect the height of the
curves? What net effect should natural selection on predator and prey populations have on
the height of the curves?
8. The rivers of central Portugal have been invaded and densely populated by the Louisiana
crayfish Procambarus clarki, which looks like a freshwater lobster about 12 to 14 cm long. The
otters of these rivers, which were studied by Graça and Ferrand de Almeida, can easily catch
and subdue these crayfish. Using the diet breadth model for prey choice, explain why the
diets of the otters would shift from the highly diverse menu (fig 7.17), which included fish,
frogs, water snakes, birds, and insects, to a diet dominated by crayfish. For the crayfish,
assume low handling time, very high encounter rates, and high energy content.
9. The data of Iriarte and colleagues suggest that prey size may favor a particular body size
among pumas (fig 7.19). However, this variation in body size also correlates with latitude;
larger pumas live at high latitudes. Consequently, this variation in body size has been
interpreted as the results of selection for efficient temperature regulation. Homeothermic
animals are often larger at high latitudes, a pattern called Bergmann’s rule. Larger animals,
with lower surface area relative to the mass, would be theoretically better at conserving heat.
Smaller animals, with higher surface area relative to mass would theoretically be better at
keeping cool. So what determines predator size? Is predator size determined by climate,
predator-prey interactions, or both? Design a study of the influence of the environment on
the size of homeothermic predators.
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Vocabulary
Archaea
Müllerian mimicry
Autotroph
Net photosynthesis
Batesian mimicry
Optimal foraging theory
carnivore
Optimization
chemosynthetic autotroph
Photon flux density
detritivore
Photosynthetic autotroph
ecological stoichiometry
Pmax
functional response
Principle of allocation
herbivore
Prokaryote
heterotroph
Size-selection predation
irradiance
Trophic (feeding) biology
Isat
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