Ecology – Biology 3500
Dr. Adams
INDIVIDUAL ECOLOGY UNIT
Chapter 7: Energy and Nutrient Relations
Trophic levels (we'll discuss this in much greater
detail later): see page 150
Autotrophs: organisms that can
do either of the following 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 surrounding water) 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.
Includes some prokaryotes, a few plants, and all animals and fungi.
Photosynthesis: 6 CO2 + 12 H2O (with light in
presence of chlorophyll) º
C6H12O6
+ 6 O2 + 6 H2O. Involves the light dependent
reactions (to capture light
energy in molecules (ATP & NADPH + H+)), and then the carbon fixation
reactions (the
Calvin Cycle), with
the enzyme Rubisco, to grab CO2 from the air, and, using the energy
from the light dependent
reactions, make glucose. First molecule formed in the Calvin
Cycle is a 3 carbon molecule. PS involving just these sets of reactions is
C3 PS.
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 effici-
ently fixes only CO2
and is not light activated,
is thus the perfect "fixit", though it requires more
energy.
Heterotrophy -- Herbivory, carnivory, detritivory
Chemical requirements for organisms: (see page 156,
157)
≥93 % of the body of
all organisms consists of C, O, H, N, P (and S)
Plants most variable, but contain low N and P; leaves are about 2% N and 0.3% P.
Inverts,
bacteria, and fungi usually 5-10% N and 1% P. Verts even higher for both
as well as Ca.
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 lignin, and possibly many different secondary
plant compounds -- compounds for
defense (toxins) and digestion-reduction (eg. tannins). Also,
must deal with the N and P poor quality of plant food; do so by
typically eating parts of the plant
that are richer in proteins (LIKE?),
and consuming large amounts. 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
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?
Tropical seaweeds are similar in that they also have more chemical defense than
temperate ones.
Giraffes
tongue
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 . . .
Food density and functional response by animals
(section 7.4)
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
organism.
Optimal foraging (section 7.5)
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, and more
(see equations, page 166; note complexity of equations;
you will NOT have to know them)
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;
this typically makes the female the CHOOSY sex, and the male the COMPETITIVE sex.
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).
Examples:
I. Guppies
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
population ecology).