Transcript Ecology as a discipline can be subdivided into: 1)

Ecology as a discipline can be subdivided into:
1)
2)
3)
4)
5)
6)
Physiological Ecology - the adaptations of individual
organisms
Behavioural Ecology - the behaviour(s) of individuals
in an ecological setting
Population Ecology - the dynamics of groups of
individuals living in [potentially] reproductive
groups
Community Ecology - the dynamics of the groups of
species living together in a habitat
Ecosystem Ecology - the processes that occur within
a community as an integrated unit
Landscape Ecology - a new area that considers larger
scale processes among related ecosystems
We’ll consider each of these approaches to ecology
basically in that order through the semester.
Physiological Ecology
By example, consider the adaptations necessary for success:
1) in a fresh water fish (a rainbow trout)
Where did fish evolve? Under what physical conditions?
Are these the same conditions as those where rainbow trout
are found today?
2) a desert lizard species (a Chihuahuan whiptail)
What is the desert like (daytime and nighttime temperature,
water regime)?
What adaptations are necessary (activity time, place to
spend hot days, physiological adaptations to water limits)?
We can consider physiological ecology to be the study of
adaptations to the physical conditions of the environment.
A species has tolerance to some range of each abiotic factor,
i.e. temperature, water availability, salinity, nutrient availability. Here is a generalized curve of distribution:
In the range labeled as “OPTIMAL ENVIRONMENT”…
• survival, growth, and reproduction can occur
• more individuals are found
BUT …
In sub-optimal environments (zones of stress)
• individuals may survive, but growth rate will be lower
• reproduction is impossible or unlikely
• fewer individuals are found
The concept of a tolerance limit is embodied in Leibig’s
law of the minimum. It states:
Under stable conditions, the essential constituent most
closely approaching the minimum for survival (and/or
reproduction) tends to limit the occurrence of a species.
An alternative statement, stolen from a WWII movie:
“A chain is only as strong as its weakest link.” (Hayakawa
via Cavett)
When conditions are varying, this ‘law’ doesn’t work very
well. What was limiting at one time may be abundant only
a short time later.
The law of the minimum neglects the possibility that there is
too much of something – salt, calcium, temperature, water…
That led to the Shelford law of tolerance.
In a paraphrase:
A species will be found only where its needs are met and
its tolerances are not exceeded.
Temperature is one of the abiotic factors for which tolerance
limits are frequently apparent.
In the Rocky Mountains the distribution of tree species is
evidence of species specific differences in tolerance...
Another example (indicative of climate change during the
20th century: the change in the position of treeline along
the eastern shore of Hudson Bay. The water of the Bay
is colder than latitude would lead you to expect, since
water enters Hudson Bay from the high arctic to the west,
then circulates through the Bay to exit on the east.
Tree line has moved 12km closer to the water on the eastern
margin, indicating a general warming trend through the
century:
One last example: southern flying squirrels (Glaucomys
volans). They are small, nocturnal, and ‘fly’ by gliding.
They are particularly vulnerable to thermal stress. As they
approach their northern limit (~45N), animals huddle
together in their nest during colder months. Otherwise the
expenditure of energy to keep warm would be too great.
A key number is expenditure of 2.5x basal metabolic rate.
That also parallels the range limit for a number of birds.
The previous examples have dealt with low temperature…
What about high temperature? Some animals have
physiological tolerance to a wide temperature range, even
during the course of a single day…
The antelope ground squirrel of the Mohave and Sonoran
deserts forages during the day, but must frequently withdraw
to its burrow, where it lies on the cool, moist soil and
‘dumps’ heat, before going back to foraging. The squirrel
is a homeotherm, but look at the core body temperature
variation it tolerates...
In the deserts of the American southwest, 70 out of 70
studied vertebrates use burrows. Most are nocturnal,
avoiding the heat of the day. Many have physiological
adaptations to accompany the behavioural pattern.
So that you don’t think
only of temperature
tolerance, here’s the
distribution of prairie
plants that characteristically
grow on badger mounds,
separated along a soil
moisture gradient:
Water abundance and availability is another common
physical factor for which tolerance determines distribution…
On prairie slopes (e.g. eskers in Iowa and Nebraska) swales
are more mesic, and upper slopes more xeric. Different
grasses are found in different portions of the slope.
Adaptations to the problem of water loss are necessary to
live on land…
• amphibians mostly remain in moist environments
• some animals have evolved relatively impermeable skin keratinized skin of reptiles
chitinous exoskeleton of invertebrates like insects
• behavioural adaptations like a fossorial strategy
• countercurrent exchange - warm air breathed in evaporates
water from passages, cooling them. The cool passages then
condense water from air being exhaled.
• desert animals have long digestive tracts that absorb as
much water as possible before feces are excreted.
• how nitrogenous waste is excreted in urine...
Urea and ammonia are relatively more toxic, and must be
excreted in dilute solution, costing water. Uric acid is less
toxic and can be excreted in concentrated form.
Nitrogenous waste excretion
Organism
Birds
Snakes & lizards
Gastropods
Mammals
Amphibians
Teleost fishes
Habitat
Terrestrial
Terrestrial
Terrestrial
Terrestrial
Aquatic
Aquatic
Waste form
uric acid
uric acid
uric acid
urea
ammonia
ammonia &
urea
Where does the water come from to support the needs of
desert animals?
• drinking dew
• reduction of water excretion
• use of metabolic water
Oxidative metabolism has water as a waste product.
C6H12O6 + 6O2  6CO2 + 6H2O
For each gram of glucose metabolized, .6g of water is
produced, for starch, 0.56g, and for fat an average of 1.02g
Kangaroo rats can subsist on metabolic water and the small
free water content in ‘dry’ seeds when the relative humidity
is >10%.
So, we have most of the answers to understand how the
whiptail lizard survives desert conditions.
1. use of metabolic water
2. excretion of a concentrated urine of uric acid
3. adaptation in the time of activity
4. tolerance of variation in body temperature
5. drinking dew
6. keratinized skin
7. long digestive tract to resorb water
….
What about the freshwater fish?
Necessary adaptations are related to salinity...
Life evolved in the sea. Cells and tissues in living organisms
generally have salt concentrations similar to sea water.
However, their environments may have radically different
salt concentrations.
Organisms have two approaches to deal with this problem…
• they can be euryhaline - tolerate variation in salt
concentration; internally they are osmoconformers.
• they can be stenohaline - require a narrow range of salt
concentration; internally they regulate salt concentration in
response to environmental variation, they are
osmoregulators.
Even in the marine system, at least near the coastlines
where fresh water enters the oceans, salt concentration
can vary widely. Some exposed organisms are osmoconformers, e.g. starfish and oysters. Others are osmoregulators, e.g. crabs.
In fresh water, osmoregulation is necessary. Animals
in fresh water are hypertonic compared to
their environment.
Osmosis tends to move water into their tissues. They have
to get rid of excess water. They excrete dilute urine
(teleost fishes urinate 1/3 of their body weight per day).
In the process, they lose critical salts. The gills actively
transport those salts from the water into the fish’s body.
So, there is the answer to how the trout has adapted to a
freshwater environment:
1. They are osmoregulators.
2. They achieve regulation by excreting a copious,
dilute urine.
3. They collect salts needed in their tissues by active
transport of needed ions.
There are many ways achieve a goal.
Marine water is generally more saline than marine fish.
Set seawater at 100% of osmotic potential, and compare it
to the osmotic potential of fish and sharks…
Seawater
Na+
45%
K
10%
other
45%
urea
_0_

100%
marine salmon
20%
2%
18%
_0_
40%
shark
28%
4%
27%
41%
100%
The shark brings its osmotic potential equal to seawater
with urea. There is no net movement of water for the shark.
For the marine salmon, osmotic potential is a problem.
With lower osmotic potential than seawater, they tend to
lose water, but need to replace it.
They drink seawater to replace it, but the salt that comes
with it must be excreted. Excretion occurs across gills and
kidneys at high metabolic cost.
So, the marine fish isn’t better off, it just has different
problems.
What about plants?
You’ve already seen grasses and other prairie plants
distributed along a water gradient.
Where water is scarce, there are three approaches to permit
plant survival and growth:
• deep roots – Adropogon gerardii, big bluestem, growing
on the Ojibway prairie, can have roots 12’ deep, prairie
roses can have roots >20’ deep.
• Prairie plants also tend to have very thick cuticles to
minimize evaporative loss from leaf surfaces.
Here’s a basic comparison, not just for prairie plants, but
for differences on a larger scale:
How do roots ‘pull’ water from the soil into the plant?
Answer: osmotic pressure
The osmotic potential of the root tissues is ‘higher’ (it’s
actually a large negative number), and water moves from the
soil into the roots.
Because the cell membranes are semi-permeable, water can
enter, but may solutes (larger ions) cannot diffuse out. Root
cells may also spend energy to actively transport the samller
ions that can get through the membrane.
The osmotic potential of the roots of some desert plants can
reach -60 atmospheres (at significant metabolic cost).
Plants conduct water from roots to above ground tissues and
leaves through the xylem. How?
The water (osmotic) potential of the leaves must exceed that
of the roots. The difference must be sufficient to work
against both gravity and the resistance of the xylem
elements.
That potential is generated by transpiration. Dry air has a
water potential of -1,332 atmospheres. Add humidity and that
pressure drops, but is still much more than enough to dry
water up from roots into leaves to replace water lost to
transpiration. The theory underlying this is called the
tension-cohesion theory.
Here’s the diagram from your text:
• Reduced (or no) leaf surfaces - cactus in the New World &
Euphorbiaceae in the Old World can survive and grow
using green stems, but no leaves
• different photosynthetic systems - the most common type
of photosynthesis is Calvin-Benson (or C3) cycle. The
forward reaction binding CO2 requires a high concentration
of CO2 to proceed.
Alternate photosynthetic pathways, C4 and CAM, have
much higher binding affinities for CO2 and, as a result, can
proceed even with leaf stomates closed, so that evaporative
water loss is greatly reduced.
Here are what Calvin-Benson and C4 pathways (the binding
steps) look like …
The ‘binding’ process:
Calvin-Benson or C3
C4 photosynthesis
The third pathway is called Crassulacean acid metabolism.
A number of desert plants use it. In CAM photosynthesis
CO2 is assimilated at night when water loss is minimized.
The carbon is stored in the form of malate, a 4-carbon
molecule. The rest of photosynthesis occurs during the day
with stomates closed.
Both CAM and C4 require higher light levels, and are
limited to ‘open’ environments. Forest species are all
C3.
Corn (Zea mays) is an example of a C4 plant; the Kentucky
bluegrass on the lawn outside is a C3.
There is also an important anatomical difference between
these pathways. It has significant effect on herbivores...
C3
C4 or Krantz anatomy
Note the difference in the spongy mesophyll and bundle sheath!
A short digression on biological adaptation to nutrient
availability: the nutrient recovery hypothesis…
Lemmings (like other microtine rodents) undergo dramatic
cycles in population number with a cyclic period of about 4
years. For lemmings, the cycling may well be related to
the nutritional quality of the plants they eat. There is a
strong correlation to phosphorus content of the plants...
Lemming number
What is the “take-home lesson” from these various examples
of physiological adaptation?
The distributions of species indicate regions they can reach
and that have suitable conditions for sustenance/growth/
reproduction. What limits those distributions is the existence
of some limiting factor, whether insufficiently present or
overabundant. To achieve the distribution we observe,
species have evolved adaptations that permit survival…
under conditions that are not optimal.
Today we have looked at adaptations in the physiology of
organisms. We will later consider other types of adaptations.