Ecology as a discipline can be subdivided into:

Ecology as a discipline can be subdivided into: 1) Physiological Ecology - the adaptations of individual organisms 2) Behavioural Ecology - the behaviour(s) of individuals in an ecological setting 3) Population Ecology - the dynamics of groups of individuals living in [potentially] reproductive groups 4) Community Ecology - the dynamics of the groups of species living together in a habitat 5) Ecosystem Ecology - the processes that occur within a community as an integrated unit 6) 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 OrganismHabitatWaste form Birds Terrestrial uric acid Snakes & lizards Terrestrial uric acid Gastropods Terrestrial uric acid Mammals Terrestrial urea Amphibians Aquatic ammonia Teleost fishes Aquatic 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 marine salmon shark Na+ 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 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.

Ecology as a discipline can be subdivided into: