Ecology as a discipline can be subdivided into:

1) Physiological Ecology - the adaptations of individualorganisms

2) Behavioural Ecology - the behaviour(s) of individualsin an ecological setting

3) Population Ecology - the dynamics of groups of individuals living in [potentially] reproductivegroups

4) Community Ecology - the dynamics of the groups ofspecies living together in a habitat

5) Ecosystem Ecology - the processes that occur withina community as an integrated unit

6) Landscape Ecology - a new area that considers largerscale 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 troutare 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 tospend hot days, physiological adaptations to water limits)?

We can consider physiological ecology to be the study ofadaptations 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 avail-ability. 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 tolerancelimits are frequently apparent.

In the Rocky Mountains the distribution of tree species isevidence of species specific differences in tolerance...

Another example (indicative of climate change during the20th century: the change in the position of treeline alongthe eastern shore of Hudson Bay. The water of the Bayis 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 easternmargin, indicating a general warming trend through thecentury:

One last example: southern flying squirrels (Glaucomysvolans). They are small, nocturnal, and ‘fly’ by gliding.They are particularly vulnerable to thermal stress. As theyapproach their northern limit (~45N), animals huddle together in their nest during colder months. Otherwise theexpenditure 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 Sonorandeserts forages during the day, but must frequently withdrawto its burrow, where it lies on the cool, moist soil and ‘dumps’ heat, before going back to foraging. The squirrelis a homeotherm, but look at the core body temperaturevariation it tolerates...

In the deserts of the American southwest, 70 out of 70studied vertebrates use burrows. Most are nocturnal, avoiding the heat of the day. Many have physiologicaladaptations 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 commonphysical factor for which tolerance determines distribution…

On prairie slopes (e.g. eskers in Iowa and Nebraska) swalesare more mesic, and upper slopes more xeric. Differentgrasses are found in different portions of the slope.

Adaptations to the problem of water loss are necessary tolive on land…

• amphibians mostly remain in moist environments• some animals have evolved relatively impermeable skin -

keratinized skin of reptileschitinous 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 beexcreted in dilute solution, costing water. Uric acid is lesstoxic and can be excreted in concentrated form.

Nitrogenous waste excretion

Organism Habitat Waste form

Birds Terrestrial uric acidSnakes & lizards Terrestrial uric acidGastropods Terrestrial uric acid Mammals Terrestrial ureaAmphibians Aquatic ammoniaTeleost fishes Aquatic ammonia &

urea

Where does the water come from to support the needs ofdesert 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 isproduced, for starch, 0.56g, and for fat an average of 1.02g

Kangaroo rats can subsist on metabolic water and the smallfree water content in ‘dry’ seeds when the relative humidityis >10%.

So, we have most of the answers to understand how thewhiptail lizard survives desert conditions.

1. use of metabolic water2. excretion of a concentrated urine of uric acid3. adaptation in the time of activity4. tolerance of variation in body temperature5. drinking dew6. keratinized skin7. 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 organismsgenerally have salt concentrations similar to sea water.

However, their environments may have radically differentsalt 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 coastlineswhere fresh water enters the oceans, salt concentrationcan vary widely. Some exposed organisms are osmo-conformers, e.g. starfish and oysters. Others are osmo-regulators, e.g. crabs.

In fresh water, osmoregulation is necessary. Animalsin fresh water are hypertonic compared to their environment.

Osmosis tends to move water into their tissues. They haveto 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 activelytransport those salts from the water into the fish’s body.

So, there is the answer to how the trout has adapted to afreshwater 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 itto the osmotic potential of fish and sharks…

Seawater marine salmon sharkNa+ 45% 20% 28%K 10% 2% 4%other 45% 18% 27%urea _0_ _0_ 41% 100% 40% 100%

The shark brings its osmotic potential equal to seawaterwith 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 tolose water, but need to replace it.

They drink seawater to replace it, but the salt that comeswith it must be excreted. Excretion occurs across gills andkidneys at high metabolic cost.

So, the marine fish isn’t better off, it just has differentproblems.

What about plants?

You’ve already seen grasses and other prairie plantsdistributed along a water gradient.

Where water is scarce, there are three approaches to permitplant 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 …

Calvin-Benson or C3

C4 photosynthesis

The ‘binding’ process:

The third pathway is called Crassulacean acid metabolism.A number of desert plants use it. In CAM photosynthesisCO2 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 arelimited to ‘open’ environments. Forest species are allC3.

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 betweenthese 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 nutrientavailability: the nutrient recovery hypothesis…

Lemmings (like other microtine rodents) undergo dramaticcycles in population number with a cyclic period of about 4years. For lemmings, the cycling may well be related tothe nutritional quality of the plants they eat. There is astrong 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.