Fluxes =Area X Gradient X Conductance Surface area to volume….. r2 X r3… volume

increases faster than surface area. In a circle its 2pi-r vs pi-r2.

More examples follow ….

Animals also modify components of flux equation to obtain materials--e.g., countercurrent exchange mechanisms Countercurrent circulation in fish allows

concentration of O2 from water into blood stream Blood flows across gill lamellae (of gill filaments) in

vessels that flow opposite to direction of water flowing across gills

This countercurrent maintains a concentration gradient for absorption of O2 throughout gills

According to physical laws O2 diffuses from areas of higher concentration to lower

Anatomy of countercurrent circulation in fish

Theory of counter-current flow mechanism

Concurrent flow, by contrast, would not allow concentration of O2 in blood of fish

Some birds use countercurrent mechanism to cool extremities, so as to minimize gradient (and thus minimize heat loss) to cold environment; heat flows from artery to vein along length of leg, to conserve heat proximal to body

Figure 3.17

Minimize water-loss gradient by nocturnal activity-- illustrates importance of behavior

Large surface area of nasal passages conserves H2O Inhalation of hot, dry air evaporates H2O, cools surfaces Exhalation of moist, warm air condenses on cooled

surfaces, retains water Large & small intestines resorb water efficiently

Kangaroo rats of SW deserts illustrate variety of mechanisms to minimize water loss

Conclusions: Physiological adaptations covers a huge topic--we’ve

just skimmed surface with a few examples Re-emphasizes the constraints imposed by physical

environment Every specialization comes with costs

Jack of all trades is master of none Corollaries: “A master of one is master of no others”

and “there’s no free lunch” Adaptations can be observed at many levels of

organization--e.g., biochemistry, cell and tissue anatomy, whole-organism anatomy and behavior

Most organisms have many, diverse adaptations to physical environment

Introduction to life-histories All organisms are similar in that they tend to reproduce

enough over their lifetimes to replace themselves, on average: replacement = 1 offspring per adult

However, organisms differ spectacularly in how they grow and replace themselves Maximum age (pattern of survival) Age at reproduction Number of reproductive efforts per lifetime Number of offspring per reproductive effort (e.g., clutch

size in birds) Care of offspring Senescence Number of mates per reproductive effort

Sockeye salmon, for example, swim as far as 6,000 km from Pacific Ocean feeding grounds to spawning streams, lay thousands of eggs, then die from the exertion.

African elephants produce one offspring at a time, once every few years over a long lifetime, and protect each offspring intensively (much like humans)

By contrast, many plants and arthropods, such as insects, reproduce once (annually), producing vast numbers of seeds/eggs that are poorly

protected, if at all

Desert annuals

A goal of ecologists is to explain how this awesome variety of life histories arose Life-histories, just like physiology and

behavior, are adapted to environments, shaped by natural selection E.g., David Lack

-clutch size in birds is shaped by individual level natural selection,

-maximizes number of chicks that parents can feed Prediction

-larger and smaller clutches will produce fewer offspring than found in natural populations.

Hogstedt tested Lack’s hypothesis experimentally

-added, and subtracted eggs to modal clutch size of 7 eggs.

-manipulated clutches produced fewer offspring than modal clutch size in unmanipulated populations

European magpie, a relative of jays

a few life-history characteristics, and relevant ecological conditions

Number of reproductive efforts per lifetime

Mating systems Correlation of life-history

characteristics adapted for disturbed, stable, or stressful (extreme) environments

First trait, number of lifetime reproductive efforts

semelparity, is to grow many years, then reproduce once before dying (e.g., salmon, bamboo, some yuccas, agaves, and Lobelia telekii)

annual habit (e.g., many herbaceous plants, many insects), involving one reproductive effort, typically after a year of growth

iteroparity, reproductive effort is spread out over many episodes over a long lifetime (e.g., most mammals, birds, and most long-lived plants)

Environmental uncertainty may favor iteroparous species

Unpredictable environments (e.g., in terms of rainfall, which changes with season, and across years according to patterns like El Niño/La Niña) favor repeated reproductive attempts and iteroparity “bet hedging”, (i.e., living long enough to hit the

jackpot reproductively during an environmentally favorable period)

In stable climates annual plants/insects often prevail Organisms put all their energy into reproduction,

rather than expensive maintenance; dormancy (seeds) is a strategy of desert annuals to await rain

These ideas are not well tested empirically

What about semelparity… Semelparity may make sense as a bet-hedging

strategy in desert plants (e.g., Agave, Yucca) Growth over many years increases likelihood that

plant can “choose” to flower in a year with relatively plentiful rainfall (favorable conditions predictable)

Such plants appear to need abundant resources to be able to produce flowering stalks from shallowly rooted leaf rosettes

Pollination success may depend on size of inflorescence…i.e., plants put all their energy into one “big bang” to make the most of rare good conditions

Semelparity seems advantageous with low adult survival probability, long intervals between good years

Agave parryi of Arizona…a kind of semelparous “century plant”

Note large inflorescence for size of plant

Another example comes from two closely related species of Lobelia, from mountains of East Africa

Lobelia telekii is semelparous (see table, next slide) This species lives on dry, rocky slopes. Much like

environments where Agave grows in N. America) Produces single, shallow-rooted leaf rosettes (like Agave) Number of seed pods produced strongly related to size of

inflorescence (i.e., to reproductive effort) Lobelia keniensis, living in nearby environments, is iteroparous

Production of seed pods not related to inflorescence size Grows in moister valley bottoms, with shorter intervals

between good years The greater predictability of rainfall in L. keniensis environment

may favor iteroparity over semelparity

Lobelia telekii (foreground) and L. keniensis (stalked rosettes in background)

What about Salmon, bamboo (also semelparous)?

Successful reproduction in species like salmon, which migrate long distances with high risk of mortality, may depend on putting all remaining effort into reproduction (since chance of survival to reproduce a second time may be very low)

Bamboo may have evolved a semelparous reproductive strategy for quite another reason: to increase seedling survival, by flooding the environment with huge numbers of seeds, too many for predators to consume Bamboo flower synchronously over large areas Periodical cicadas (13-, 17-year) may have evolved

semelparous reproductive strategy for similar reason

Second trait, mating systems Monogamy

Example--some birds such as geese, and many tropical birds

Note: many migratory birds are socially monogamous, but genetically polygamous--e.g., Least Flycatcher)

favored in species in which both mates contribute substantively to survival of offspring (e.g., chicks in birds can be fed by males, not just females)

Other mating systems... Polygamy Polygyny

Mammals tend to be polygynous because females carry, birth, then nurse young, leaving males free to abandon and mate with other females

Resource-defense polygyny

-, e.g., Elk defending area of grassland on which harem of females feed; elephant seal defending island space where females give birth

Male (not female!) can increase repro. success with more mates--if his care of family does not affect offspring survival

Yet other mating systems... Polyandry--

E.g., Some arctic sandpipers and tropical pond lilly-trotting Jacanas become egg-laying machines, needed to replace clutches lost to predators (males do best by providing parental care, and mating to female that can quickly replace lost clutch)

Promiscuity--no pair bond, i.e., individuals come together just to mate, e.g., leks (breeding arenas) E.g., shorebirds like ruffs, woodcock form leks, in which

females select mates based on courtship display Leks occur in organisms in which females can care for

young alone (e.g., fruit-eating birds; birds in very productive habitats)

Males forced to compete because lone males ignored

What about humans...

What is our own species’ social system, and why? Make an assertion and back it up as best you can with evidence...

Life-history parameters are often correlated in distribution (I.e., not independent) One of the most general explanations of such correlated traits

derives from consideration of disturbed versus stable environments Idea was conceived by Robert MacArthur and Edward O. Wilson:

“r- vs. K-selected strategies” These traits represent a continuum Derivation of the terminology comes from population models (see

future lecture): “r” is population growth rate; r-selected species have traits that

increase r “K” is population carrying capacity; K-selected species have traits that

increase carrying capacity and competitive ability when populations fill environment

Examples of r- and K-selected organisms?

r-selected organisms--e.g., dandelion, with rapid population growth rate, early maturity, production of many small seeds that can colonize disturbed areas)

K-selected organisms --e.g., oak tree with long life, production of few, large seeds that can grow readily in shaded environments

Summary of life-history traits in r-selected versus K-selected strategies

“strategy”: r-strategist K-strategist environment variable constant, predictable mortality dens.-indep. dens.-dep. population size variable, below K constant, at K Competition? variable, lax keen Favored traits rapid development (opposites) high “r max” early reproduction small body size semelparity short generation time good dispersal, colonizing ability high allocation to reproduction (small offspring/seed sizes; many offspring)

Habitat characteristic Short Season Long Season (N. Dakota) (Texas) Climatic variability 3.05 1.56 (variance:mean ratio frost-free days) Competition index (above ground biomass) 404 1336 Annual recolonization (winter rhizome mortality %) 74 5 PLANT TRAIT T. angustifolia T. domingensis Days to flower 44 70 Mean genet weight (g) 12.6 14.3 Mean no. fruits/genet 41 8 Mean wt. frts. (g) 11.8 21.4 mean total wt. frts (g) 483 171

Example of two species of Typha (cattail): -r-selected species living in climatically variable environment (N. Dakota); -K-selected species in more predictable environment (Texas)

How good is the r- vs. K-selection model of life histories?

In one review of life-histories (Stearns, 1977), only 18 of 35 studies fit predictions of r- vs. K-selected traits

Thus, the theory is useful, but lots of exceptions exist

Why the exceptions?

Alternatives to r- vs. K-selected life-history traits

Probably the most important alternative scheme for correlated life-history traits is that of Grime, emphasizing plants Third axis (besides competitors, weeds) is stress-

tolerance Stress tolerators adapted to tolerate extremes of

physical environment (e.g., hot, dry deserts; cold arctic barrens, hot-springs)

Ability to tolerate stress probably traded for competitive, ruderal abilities

Similarly animals may be adapted to extreme physical environments (e.g., desert, arctic animals), and are thus poor colonizers, poor competitors

Life-history traits involve tradeoffs, based on allocation of energy to competing activities

For example, organisms that have high fecundity, typically survive poorly; alternatively high survival has evolved to offset low reproductive rates in some environments This is a phenomenon observed between species Another kind of tradeoff, occurring within individuals

is the “cost of reproduction”--e.g., sparrow hawks

Bird species (dots) illustrate the tradeoff between “fast” organisms (high fecundity, high mortality) versus “slow” ones (long life, low annual fecundity)

Sparrow hawks (kestrels) illustrate “cost of reproduction” to parents--modal clutch is 5 eggs

Production of offspring was enhanced in this species by experimentally increased clutch size, but parents survived poorly to subsequent breeding season

Conclusions: Ecologists have made great progress explaining

many (but by no means all) life history traits, using arguments based on individual level natural selection

Life-history traits are often correlated in their distribution because of the effect of habitats on multiple traits

Tradeoffs among different traits are also very common, indicating the inability to evolve one phenotype that is perfect in all situations: Organisms have been selected to allocate resources differentially in different environments

Acknowledgements: Illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.