Ecology – Biology 3500
INDIVIDUAL ECOLOGY UNIT
Chapter 7: Energy and Nutrient Relations
Trophic levels (we'll discuss this in much greater detail later):
Autotrophs: organisms that can do either of these in order to capture energy and
produce organic molecules -- producers.
Photosynthesis: Utilizes CO2 (from air) and H2O (from ground) to make organics
using light energy
Chemosynthesis: Utilizes CO2 (from H2O) and H2S (from vents as source of energy,
along with heat) to make organics; some bacteria in soil can use ammonium as an
energy source (which also gets them nitrogen) (see page 155)
Prokaryotes both photosynthetic and chemosynthetic; plants photosynthetic.
Heterotrophs: must consume organics produced by autotrophs in order to get not
only the necessary molecules for growth but energy as well -- consumers
Photosynthesis: 6 CO2 + 12 H2O (with light in
presence of chlorophyll) º
C6H12O6 + 6 O2 + 6 H2O
Utilizes light in the human visible spectrum (400 - 700 nm), mostly in the blue and red ends
this represents the PAR (photosynthetically active radiation)
The intensity and quality of the light available for photosynthesis changes with latitude,
seasons, weather, time of day, and depth within a biome (think of the floor of a tropical
rain forest; for aquatic systems, light changes with depth)
Alternatives to regular C3 photosynthesis: see page 152 and handout
C4 and CAM photosynthesis: both use an extra enzyme (PEPC) and extra steps to shuttle
CO2 to the "normal" Calvin Cycle reactions of the C3 pathway, which is used by ALL
plants to make glucose. The only difference is that the C4 pathway fixes CO2 during the
day and the malic acid (C4 acid) formed is immediately shuttled to the bundle sheath cells
around the veins where CO2 is refixed by Rubisco (the enzyme starting the Calvin Cycle)
and glucose is made. CAM plants open stomates at night and fix CO2 into malic acid,
which has to be stored until morning when the Rubisco enzyme (which is light activated)
can then work. This has to be done because Rubisco happens to be an oxygenase as well
as a carboxylase. In a nutshell, Rubisco "wastes time" fixing O2 in hotter, drier environments
(photorespiration; no organic molecule production), the very environment in which PS needs
to be MORE efficient to reduce water loss through open stomates. PEPC, which very
efficiently fixes only CO2, is thus the perfect "fixit", though it requires more energy.
Heterotrophy -- Herbivory, carnivory, detritivory
Chemical requirements for organisms: (see page 156)
≥93 % of the body of all organisms consists of C, O, H, N, P (and S)
High C:N ratios in plants (@25:1) indicate high carbohydrate (cellulose)/lower protein content
than other organisms (heterotrophs) with a lower C:N ratio, between 5:1 to 10:1 for fungi,
bacteria, and animals. The C:N ratio is particularly high, up to 300:1 for woody plants (WHY?)
Other minerals are essential nutrients, however, for both plants and animals (some are
unique to plants [B] and some to certain animals [I]). Plants obtain minerals with water from
ground, animals obtain minerals with their food and drink.
Herbivores: must deal with indigestible cellulose, and potentially many different secondary plant
compounds -- compounds for defense (toxins) and digestion-reduction (eg. tannins). Also,
must deal with the nitrogen poor quality of plant food; do so by typically eating parts of the
plant that are richer in proteins (LIKE?). Many plants may have evolved physical protection
(thorns, hairs, sticky compounds) as well. Usually, however, there are at least a few herbivores
that have overcome, and in the case of chemical protection, taken advantage of, the protective
mechanisms of plants. There are higher levels of secondary compounds in tropical plants, yet
herbivores remove more leaf mass (by %) than in temperate forests. What does this suggest?
Sequestration and utilization of chemicals: leading to Mullerian and Batesian mimicry
complexes (see below)
Detritivores: feed on decaying plant (and fungal/animal) material
Play a vital role in (re)cycling of nutrients; since feed largely are dead plant material, are
faced with similar problems as herbivores. Indeed, nitrogen content of food is about half
of what is found in living leaves. Fungi in the soil may help with sequestering nitrogen
(we'll talk about this more later).
Carnivores: feed on nutritionally rich prey, with complete proteins. Since prey are such
desirable pieces of food, most have defensive mechanisms for avoiding predation, much
like the plants have evolved defenses against herbivores. Understand that the predators
themselves have been the agents of selection for defense in prey.
Crypsis (eg. salt and pepper moths), chemical repellants, aposematism, Batesian and
Mullerian mimicry, physical defenses (goo, stingers, spines, hairs, shells, etc.), and behaviors
(fast flight, noise, playing dead, grouping together, etc.)
Movement of prey items means that predators must expend more energy, and will be less
successful than herbivores at finding prey, and, when found, even less successful at
capturing the prey (1% successful capture rate by bald-faced hornets, for example.)
Typically, most predators use, not surprisingly, size-selective predation.
This leads us to the concept of . . .
Optimal foraging Theory: (section 7.4 - 7.5)
Food density and functional response by animals
Three types of responses (all which can reach a maximum possible feeding rate):
Type 1: feeding increases in direct proportion to increasing food density (only occurs if
food requires little to virtually no processing time)
Type 2: feeding rate increases quickly, then slows as more and more food available; typical
in animals which require some time to find and then handle (process) food. This curve
is easily the most representative of a large number of animals.
Type 3: uncommon; might expect this as predator is forming a search image for "rare" food
Since energy expenditure in capture must in turn be balanced by energy gained from food, very
careful decisions must be made when selecting and chasing potential prey. After all, energy
(and nutrients) once obtained, must be allocated to parts of the body in which the energy/
nutrients are needed. Optimal foraging theory attempts to predict what consumers will eat,
based on their needs to maximize (assimilation) or minimize (water loss) some aspect of the
If a heterotroph is trying to optimize energy intake within a given amount of time, then the
consumer should technically take into account the following:
food availability (number of prey items), food nutritional quality, handling time, encounter
rates, abundance and nutritional value of different food types available
(SEE EQUATIONS, page 166; note complexity of the equations)
Bluegill Sunfish example
Chapter 8: Social Relations
Behavioral Ecology -- Sociobiology
Mate Choice: The one fundamental social interaction that all sexually reproducing organisms must
exhibit. Realize that each mate's goals are not necessarily the same (though the overall goal,
increasing fitness, is the same). Female's typically produce fewer, larger gametes, while males
produce many, smaller gametes (sperm are cheap!). So, female success is generally considered
to be limited by access to resources*, while male success is generally limited by access to females.
Sexual Selection -- we tend to see, due to the dichotomy indicated above, males attempting to
"convince" females, while females being highly selective in choosing a mate.
As such, we see the development of secondary sexual characteristics in one/both sexes,
leading to sexual dimorphism in many species.
Two distinctive selective processes at work:
1. intrasexual selection -- typified by competition within one sex for mates
2. intersexual selection -- typified by one sex selecting mates based on certain characteristics of
individuals of the opposite sex. This is mate choice, and can lead to the development of
remarkable structures (such as the male Peacock's tail).
1. Intersexual selection in Guppies (Endler)-- in no to low predatory environments, over time
colorful males increase in the population, indicating that color is beneficial in mating, but in high
predatory environments colorful males reduce in number (why?). In addition, Endler carried
out a transplant experiment in a natural situation where the end results again were as in the pond
experiments indicated above. Shows that the same trait can have conflicting affects on fitness
depending on what factors are present in the ecosystem.
2. Intrasexual selection in Guppies (Kodric-Brown) -- females clearly choose the males with the
showy, colorful spots, but when put together with two males (one showy, one not) females do
NOT always mate with the showy individual. Males are aggressive towards one another, and
colorful individuals are NOT always DOMINANT individuals. Dominant individuals sired
more offspring, whether attractive or not.
II. Scorpionflies (see Fig. 8.9 - 8.11) -- Resource provisioning*
Male Scorpionflies offer nuptial food gifts to potential mates; typically they find their "prey" dead
(though some species are active predators). Some species even risk death by stealing food
from spider webs. Males will get "gift", call female with pheromone, and wait for arrival of
female. Males with largest gifts have remarkable mating success, whereas those with smaller
gifts, and particularly no gifts have (much) reduced mating success. Females choosing males
with larger gifts get both the benefit of not having to find as much food themselves as well as a
significant nutrient head start in laying a larger numbers of eggs. Who can get the largest "gifts"?
No surprise. Largest scorpionflies out compete smaller males for food gifts.
So, are both intrasexual and intersexual selection going on here?
III. Wild Radish
Pollinators include bees, syrphid flies, and butterflies. Pollinators arrive at a flower carrying
pollen from several different males and, on average, have seven different mates. Flowers are
dioecious, but are self-incompatible (pollen from own stamen cannot grow pollen tube on
own pistil). So, must outcross. Is there non-random mating, and therefore uneven mating
success in plants?
Marshall clearly showed that all pollen donors are not "created equal". Some sired more
seeds total, some sired seeds of greater weight, etc. Is sexual selection going on? Certainly
SOME kind of selection is potentially going on, where certain fathers (pollen) clearly have
an advantage over others.
In SOCIAL organisms, chances for mating are often reduced even further than by direct one on one
competition. In such organisms exhibiting social structure, it is often just a few individuals that get
to mate in a given time period. (We will save a larger discussion of sociality until we discuss