Chapter 3: Adaptation to Aquatic and Terrestrial Environments

1. Chapter 3: Adaptation to Aquatic and Terrestrial Environments Robert E. Ricklefs The Economy of Nature, Fifth Edition

2. Sperm whales: how do they dive to 500-2 km?

3. The Weddell Seal can dive to 500 meters depth and stay for up to 80 minutes. How?

4. Chapter Overview - Bottom Line • It is important for us to understand the mechanisms organisms use to interact with their environment. • This understanding may lead to insights: • why organisms are specialized • why organisms have specific geographic distributions • why certain adaptations are associated with certain environments

5. What’s next? • This chapter examines adaptations by considering various challenges facing organisms, for example: • how do plants acquire water and nutrients from soils and transport these? • how do plants carry out photosynthesis under varied environmental conditions? • how do plants and animals cope with extremes of temperature, water stress, and salinity?

6. Availability of Soil Water • Water molecules are attracted to: • each other (causes surface tension) • surfaces (causes capillary action) • When a soil is saturated and excess (gravitational) water drains: • remaining water exists as thin films around soil particles (mineral and organic) • the greater the area of such particles (as in clayey soils), the more water the soil retains

7. Which soil holds more water? Why?

8. All soil water molecules are not equal. • It’s all a matter of physical attraction... • the closer a water molecule is to a soil particle, the greater the force with which it is attracted (water potential) • this force is the matric potential of the soil, contributing to the overall water potential • matric potentials (units are MPa or atm) are considered increasingly negative as they represent greater attractive forces

9. It’s all a matter of potential... • Soil water potential is: • usually dominated by matric forces • determined as the force required to remove the most loosely bound water molecules • Typical “benchmark” values are: • -0.1 atm (field capacity) • -15 atm (wilting point/wilting coefficient) • -100 atm (exceedingly dry soil)

10. Plants obtain water from the soil. • How do water molecules move? • in the direction of more negative potential • across most biological membranes • Why does water move from the soil into plant roots? • water potential in cells of the root hairs is more negative than that in the soil • negative potential in root cells is generated mostly by solutes -- osmotic potential

11. Membranes are selectively leaky. • Can solutes exit root cells as readily as water enters? • no, internal and external concentrations would equilibrate and osmotic potential gradient would disappear • cell membranes are semipermeable; large molecular weight solutes (carbohydrates and proteins) cannot readily leave the cell • Active transport – needs use of energy

12. So why does water move into roots? • Internal (cellular) osmotic potential is more negative than external (soil) matric potential, up to a point: • root hair cells with 0.7 molar concentration of solutes maintain inward flux of water against a soil matric potential as low as -15 atm: • as soil becomes drier, water flux ceases and may reverse, leading to wilting and death • desert plants may obtain water to soil matric potentials as low as -60 atm (high solute conc.)

13. Moving Water from Roots to Leaves • Once water is in root cells, then what? • water moving to the top of any plant must overcome tremendous forces caused by gravity and friction in conducting elements (xylem): • opposing force is generated by evaporation of water from leaf cells to atmosphere (transpiration) • water potential of air is typically highly negative (potential of dry air at 20 oC is -1,332 atm) • force generated in leaves is transmitted to roots -- water is drawn to the top of the plant (tension-cohesion theory)

14. Water potential that moves water from the roots to the leaves of a plant is generated by transpiration

15. Adaptations to Arid Environments 1 • Most water exits the plant as water vapor through leaf openings called stomates: • plants of arid regions must conserve limited water while still acquiring CO2 from the atmosphere (also via stomates) - a dilemma! • potential gradient for CO2 entering plant is substantially less than that for water exiting the plant • heat increases the differential between internal and external water potentials, making matters worse

16. Adaptations to Arid Environments 2 • Numerous structural adaptations address challenges facing plants of arid regions by: • reducing heat loading: • increase surface area for convective heat dissipation • increase reflectivity and boundary layer effect with dense hairs and spines • reducing evaporative losses: • protect surfaces with thick, waxy cuticle • recess stomates in pits, sometimes also hair-filled

17. Spines and hairs help plants adapt to heat and drought. (a) cross section; (b) surface view of the leaf of the desert perennial herb

18. This drought-resistant plant reduces water loss by placing its stomates in hair-filled pits on the leaf’s undersurface

19. Plants obtain mineral nutrients from soil water. • Nutrients must move from the soil solution into cells of root hairs… • a nutrient element moves passively (via diffusion) into root when its concentration in soil water exceeds that of root cells • when nutrient concentration in soil water is lower than that in roots, active uptake (energy-demanding) is essential

20. Allocation of root or shoot growth (a) Supplies of soil nutrients or water are limited (b) Absence of those restrictions

21. Other Plant Strategies for Obtaining Nutrients • Enlist partners! • many plants have intimate associations (symbioses) with fungi -- fungal partners enhance mineral absorption • Regulate growth! • plants of nutrient-poor soils typically: • grow slowly, maintain leaves for multiple growing seasons (evergreenness), and store surplus • shift growth toward more root and less shoot

22. Photosynthesis varies with levels of light. • Photosynthetic rate is a function of light intensity (proportional to light intensity at low light levels, leveling off at high levels): • in dim light, plants fail to offset respiratory losses with photosynthetic gains • as light intensity increases, a break-even point (losses offset by gains) is reached, called compensation point • at saturation point, further increase in light level does not stimulate further photosynthesis

23. Photosynthesis balances respiration at compensation point

24. Plants modify photosynthesis in stressful environments. • Know the general aspect of C3 and C4 and CAM • Particularly the advantages • No equations will need to be memorized.

25. Balancing Salt and Water • Osmotic regulation is not just a problem for plants • Aquatic animals are rarely in equilibrium with their surroundings: • fresh-water fish are hyperosmotic (internal salt concentration higher than that of medium) • marine fish are hypo-osmotic (internal salt concentration lower than that of medium) • Hyper: more in than out • Hypo: less in than out

26. Ion retention is critical to freshwater organisms. • Freshwater fish must eliminate excess water and selectively retain dissolved ions: • they gain water by osmosis • they eliminate excess water in their urine • their kidneys selectively retain dissolved ions • active uptake of ions via gills is also important

27. Pathways of exchange of water and solutes differ between marine and freshwater fish

28. Water retention is critical to marine organisms. • Saltwater fish must retain water and excrete excess ions: • they tend to lose water to surrounding sea water and must drink to replace this • excess salt must be excreted from gills and kidneys • some fish (sharks and rays) raise osmotic potential of blood by retaining waste nitrogen as urea in their bloodstream -- their high internal osmotic potential matches that of seawater; thus no net movement of water across a shark’s surfaces

29. Osmotic potential of seawater

30. Start here

31. Chapter 3 (continued) • Reminder: • Exam 1 – April 8 • Exam 2 – May 22 • Make-up sessions • I will be absent on March 25 and March 27 • Make-up scheduled for Thursdays • March 19. 12.30-2.00 Khoury 132 • April 2. 12.30 – 2.00 Khoury 132

32. Your ? From Wed… • The Saunders Comprehensive Veterinary Dictionary sums it up pretty well: A period of rest during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended; a behavioral state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli. • Some fish and amphibians reduce their awareness but do not ever become unconscious like the higher vertebrates do. • Some fish are motionless in the water during the night, while other fish, like rockfish and grouper, don’t appear to sleep at all. • Funny: some fish don’t hide the fact that they take an occasional nap. • It’s probable that fish do sleep in some form, whether slowing down or coming to a complete stop, whether hiding or doing it right in the open.

33. Water and Salt Balance in Terrestrial Plants • Plants take up excessive salts along with water, especially in saline soils. • plants must actively pump salts back into soil • In coastal mudflats, mangroves must acquire water while excluding salts. They: • establish high root osmotic concentrations to maintain water movement into root • exclude salts at the roots and also excrete excessive salts from specialized leaf glands

34. Mangrove plants have adaptations for coping with a high salt load. (a) roots immersed in salt water at high tide; (b) specialized glands in the leaves excrete salt

35. Water and Salt Balance in Terrestrial Animals • Terrestrial animals must eliminate excess salts acquired in diet: • copious amounts of water can serve to flush excess salts in more humid climates • where water is scarce, other options exist: • desert mammals produce highly concentrated urine • birds and reptiles eliminate excess salts via salt glands

36. Animals excrete excess nitrogen. • Carnivorous animals acquire excess nitrogen from their high-protein diet: • excess nitrogen must be eliminated: • aquatic animals eliminate nitrogen as ammonia • terrestrial animals cannot afford copious amounts of water necessary for elimination of ammonia • mammals excrete urea • birds and reptiles excrete uric acid, which can be eliminated with very little water

37. Conserving Water in Hot Environments 1 • Animals of deserts may experience environmental temperatures in excess of body temperature: • evaporative cooling is an option, but water is scarce • animals may also avoid high temperatures by: • reducing activity • seeking cool microclimates • migrating seasonally to cooler climates

38. Conserving Water in Hot Environments 2 • Desert plants reduce heat loading in several ways already discussed. • Plants may, in addition: • orient leaves to minimize solar gain • shed leaves and become inactive during stressful periods

39. The Kangaroo Rat - a Desert Specialist These small desert rodents perform well in a nearly waterless and extremely hot setting. • kangaroo rats conserve water by: • producing concentrated urine • producing nearly dry feces • minimizing evaporative losses from lungs • kangaroo rats avoid desert heat by: • venturing above ground only at night • remaining in cool, humid burrow by day

40. Organisms maintain a constant internal environment. • An organism’s ability to maintain constant internal conditions in the face of a varying environment is called homeostasis: • homeostatic systems consist of sensors, effectors, and a condition maintained constant • all homeostatic systems employ negative feedback -- when the system deviates from set point, various responses are activated to return system to set point

41. Negative feedback system

42. The ice man

43. Temperature Regulation: an Example of Homeostasis • Principal classes of regulation: • homeotherms (warm-blooded animals) - maintain relatively constant internal temperatures • poikilotherms (cold-blooded animals) - tend to conform to external temperatures • some poikilotherms can regulate internal temperatures behaviorally, and are thus considered ectotherms, while homeotherms are endotherms

44. Homeostasis is costly. • As the difference between internal and external conditions increases, the cost of maintaining constant internal conditions increases dramatically: • in homeotherms, the metabolic rate required to maintain temperature is directly proportional to the difference between ambient and internal temperatures

45. Limits to Homeothermy • Homeotherms are limited in the extent to which they can maintain conditions different from those in their surroundings: • beyond some level of difference between ambient and internal, organism’s capacity to return internal conditions to norm is exceeded • available energy may also be limiting, because regulation requires substantial energy output

46. Partial Homeostasis • Some animals (and plants!) may only be homeothermic at certain times or in certain tissues… • pythons maintain high temperatures when incubating eggs • large fish may warm muscles or brain • some moths and bees undergo pre-flight warm-up • hummingbirds may reduce body temperature at night (torpor)

47. Hummingbirds maintain a constant low body temp when in torpor

48. Delivering Oxygen to Tissues • Oxidative metabolism releases energy. • Low O2 may thus limit metabolic activity: • animals have arrived at various means of delivering O2 to tissues: • tiny aquatic organisms (<2 mm) may rely on diffusive transport of O2 • insects use tracheae to deliver O2 • other animals have blood circulatory systems that employ proteins (e.g., hemoglobin) to bind oxygen

49. Countercurrent Circulation • Opposing fluxes of fluids can lead to efficient transfer of heat and substances: • countercurrent circulation offsets tendency for equilibration (and stagnation) • some examples: • in gills of fish, fluxes of blood and water are opposed, ensuring large O2 gradient and thus rapid flux of O2 into blood across entire gill structure • similar arrangement of air and blood flow in the lungs of birds supports high rate of O2 delivery

50. Conservation and Countercurrents • Countercurrent fluxes can also assist in conservation of heat; here are two examples: • birds of cold regions conserve heat through countercurrent circulation of blood in legs • warm arterial blood moves toward feet • cooler venous blood returns to body core • heat from arterial blood transferred to venous blood returns to core instead of being lost to environment • kangaroo rats use countercurrent process to reduce loss of moisture in exhaled air