Chapter 9: Adaptation to Life in Varying Environments

1. Chapter 9: Adaptation to Life in Varying Environments Robert E. Ricklefs The Economy of Nature, Fifth Edition (c) 2001 by W.H. Freeman and Company

2. Background - Life in Varying Environments • The giant red velvet mite lives in one of the most forbidding desert environments on earth. • Its life history represents a series of adaptations that optimize survival, growth and reproduction while minimizing exposure to unsuitable conditions. (c) 2001 by W.H. Freeman and Company

3. Responses Right and Wrong • The appropriateness of a response is a function of: • the qualities of the environment • ecological circumstances • Consider the storage of fat by sparrows: • the “right” choice if this fat will be needed: • for migration • to carry the bird through bad weather • the “wrong” choice in the absence of such needs, because stored fat: • reduces speed and maneuverability • increases risk of predation (c) 2001 by W.H. Freeman and Company

4. Adaptation results from natural selection. • Genotype is the unique genetic constitution of an individual. • Evolution is any change in the genetic makeup of a population. • Natural selection results in evolutionary change when genetic factors cause differences in fecundity and survival among individuals. • Fitness is the reproductive success of an individual. (c) 2001 by W.H. Freeman and Company

5. Evolution - An Example • Evolution of cyanide resistance in citrus scale: • fumigation with cyanide gas was an effective control measure early in the 20th century • as time passed, fumigation became less effective as scale evolved genetically based resistance to cyanide • eventually scale regained its former pest status • Citrus scale had three main ingredients of evolution by natural selection: • variation among individuals • inheritance (genetic basis) of this variation • differences in fitness related to genetic variation (c) 2001 by W.H. Freeman and Company

6. Evolution guides diversification. • The diversification of living beings over the history of life has been guided primarily by natural selection. • Natural selection is not an external force. • Natural selection occurs because of differences in reproductive success among individuals endowed with different form or function within a particular environment. (c) 2001 by W.H. Freeman and Company

7. The phenotype is the expression of the genotype. • The phenotype is the outward expression of the genotype manifested in structure and function: • the genotype is a set of instructions • the phenotype is the expression of the genotype as modified by environmental conditions affecting growth and development (c) 2001 by W.H. Freeman and Company

8. Genes and alleles • Genes encode proteins: • used as part of an organism’s structure • may function as enzymes or hormones • Different forms of a particular gene are called alleles: • alleles may cause perceptible and measurable differences in the phenotype (e.g., eye color) • defective alleles may cause genetic disorders: • directly: sickle-cell anemia, albinism • indirectly: tendencies to develop certain cancers (c) 2001 by W.H. Freeman and Company

9. Allelic Diversity in Individuals • Each diploid individual has two copies of each allele, one inherited from its mother, the other from its father: • a heterozygous individual has two different alleles • a homozygous individual has identical alleles • alleles may be: • dominant (expressed in heterozygous individual) • recessive (masked by dominant allele) • codominant (result in intermediate phenotype in heterozygotes) • most deleterious alleles are recessive (c) 2001 by W.H. Freeman and Company

10. Phenotypic Plasticity • Environmentally induced variation in the phenotype is referred to as phenotypic plasticity. • The capacity to exhibit phenotypic plasticity may itself be an evolved trait. • It is important that we keep in mind the difference between: • plastic responses of individuals • evolutionary responses by populations (c) 2001 by W.H. Freeman and Company

11. Each type of organism has an activity space. • The organism functions best within a narrow range of environmental conditions which define its activity space. • activity is equivalent to performance • activity may be measured as any trait (swimming speed, photosynthesis, survival) that influences individual’s fitness (c) 2001 by W.H. Freeman and Company

12. Biological activity is related to environmental conditions. • For a given environmental factor, we can identify various ranges relative to activity: • optimal range (above minimum level required to maintain population) • suitable range (above minimum level required to maintain organism) • marginal range (above minimum level required to maintain critical functions) • unsuitable range (fatal for extended periods) (c) 2001 by W.H. Freeman and Company

13. Organisms can select microhabitats. • Within habitats, there are finer-scale variations referred to as microhabitats or microenvironments: • these represent distinct differences in temperature, moisture, salinity, and other factors within a particular habitat • In desert habitats, for example: • shaded ground under shrubs is cooler and moister than surrounding areas exposed to direct sunlight • such differences may vary diurnally or seasonally (c) 2001 by W.H. Freeman and Company

14. Behavioral Cycles in Lizards • Lizards can regulate body temperature by diurnal behavioral cycles: • lizards do not regulate temperature by generating metabolic heat • by moving about, they select various microhabitats: • lizards take advantage of differences in solar radiation and temperatures of various surfaces to maintain body temperatures within a suitable range during a day (c) 2001 by W.H. Freeman and Company

15. Cactus wrens select microhabitats to optimize energy budgets. • The desert habitat offers varied microhabitats, ranging from exposed ground in full sun to deep shade of trees. • Cactus wrens of our southwestern deserts take advantage of various microhabitats: • in early morning, they forage widely • as the day becomes progressively warmer, they restrict activity to cooler microhabitats • nests are positioned to shelter from (spring) or face (summer) prevailing winds (c) 2001 by W.H. Freeman and Company

16. Acclimation is a reversible change in structure. • Acclimation is a shift in the range of physiological tolerances of the individual. • Some examples: • growing thicker fur in winter • producing smaller leaves in the dry season • increasing the number of red blood cells at higher elevations • producing enzymes with different temperature optima • producing lipids that remain fluid at different temperatures (c) 2001 by W.H. Freeman and Company

17. Thermal Acclimation in Goldfish • Goldfish swim most rapidly when: • acclimated at 25oC • placed in water between 25oC and 30oC • When acclimated at 5oC, goldfish: • swim most rapidly at 15oC • sacrifice ability to swim fast at 25oC • Increased tolerance of one extreme often brings reduced tolerance at another. (c) 2001 by W.H. Freeman and Company

18. Variation in Potential for Acclimation • The ability to acclimate reflects the typical range of conditions experienced: • creosote bush (Larrea) experiences a wide range of temperatures and its photosynthetic ability acclimates well to both cool and warm temperatures • Atriplex grows under cool conditions and does not acclimate well to high temperatures • Tidestromia grows under hot conditions and does not acclimate well to low temperatures (c) 2001 by W.H. Freeman and Company

19. Developmental responses are irreversible changes. • Consider the developmental responses of loblolly pines grown under different light regimes: • shade-grown seedlings allocate more energy to stem and needles • sun-grown seedlings allocate more energy to root systems • greater proportion of needles in shade-grown plants enhances photosynthetic rate per unit plant mass, especially under low-light conditions (c) 2001 by W.H. Freeman and Company

20. Developmental Responses in Grasshoppers • The grasshopper Gastrimargus africanus can match its color to that of its surroundings: • helps avoid detection by predators • epidermal pigments laid down at each molt respond to hormones produced in the brain in response to quality and intensity of light: • animals are green in rainy season • as dry season comes on, animals are brown • following fires, animals are black (c) 2001 by W.H. Freeman and Company

21. Where do we find developmental responses? • As a rule, plants and animals in habitats with persistent variation exhibit such responses. • For plants, spatial heterogeneity creates the kind of persistent variation favoring developmental responses. (c) 2001 by W.H. Freeman and Company

22. Migration, Storage, and Dormancy • When extremes of environment are so adverse as to prevent normal activities, organisms: • cannot adapt to such extreme conditions • or they can adapt, but such adaptations would be too costly • Alternative strategies include migration, storage, and dormancy. (c) 2001 by W.H. Freeman and Company

23. Migration • Migration is moving to another region where conditions are more favorable: • arctic terns make annual migrations of 30,000 km, moving from summer in Arctic to summer in Antarctic • monarch butterflies migrate seasonally from Mexico and southern US into southern Canada • African ungulates follow geographic patterns of rainfall and fresh vegetation • migration in locusts represents a developmental response at high population densities (c) 2001 by W.H. Freeman and Company

24. Storage • Storage is the reliance on resources accumulated under more favorable conditions: • desert cacti store water during rainy periods • plants of infertile habitats store nutrients during periods of temporary abundance • animals of temperate and polar regions store fat for periods of severe weather during winter • some mammals and birds cache food supplies (c) 2001 by W.H. Freeman and Company

25. Dormancy • Dormancy is becoming inactive: • tropical and subtropical trees shed leaves during seasonal droughts • mammals undergo hibernation • some insects enter winter diapause, reducing their freezing point and metabolic rate • other insects enter summer diapause, tolerating dessication • plant seeds and spores of bacteria and fungi exhibit effective dormancy mechanisms (c) 2001 by W.H. Freeman and Company

26. Stimuli for Change • How do organisms sense impending environmental severity? • proximate factors are cues (such as day length) used to assess environmental factors but which do not directly affect well-being • ultimate factors are features of the environment (such as food supply) which directly affect well-being • Different populations of the same species may respond in dramatically different ways to the same cues. (c) 2001 by W.H. Freeman and Company

27. Animals forage optimally • Theories of optimal foraging seek explanations for decisions that animals make while foraging: • where to forage • how long to remain in a particular patch • which types of food to eat • Optimal foraging theories examine costs and benefits to animals of various decisions: • expectation is that animals will select the behaviors that yield the greatest benefits (c) 2001 by W.H. Freeman and Company

28. Central Place Foraging • When animals are tied to a particular place (e.g., a nest with offspring in it) they experience tradeoffs associated with distance they forage: • increasing foraging range results in: • greater potential for finding food • greater time, energy costs, risks of travel • foraging range should maximize amount of food returned per unit time (c) 2001 by W.H. Freeman and Company

29. Are starlings optimal foragers? • Research with starlings shows that they can maximize return rates by selecting intermediate foraging times and returning with less than the maximum possible amount of food. • Experimental studies show that starlings do forage optimally, adjusting upward their load size as round-trip travel time increases, as predicted by foraging theory. (c) 2001 by W.H. Freeman and Company

30. Risk-Sensitive Foraging • The value of a feeding area is reduced by the presence of risks, particularly predation: • predation has been incorporated into foraging theory in studies of risk-sensitive foraging • do animals incorporate risk of predation into decision-making? • experimental studies, in which availability of food and predator density were both varied, showed that minnows incorporated predation risks into their foraging decisions (c) 2001 by W.H. Freeman and Company

31. Prey Choice • Foraging decisions include choices concerning prey items: • each food item has intrinsic value based on: • nutrient and energy content • difficulty of handling • potential danger from toxins • poor-quality foods may require more handling time or take more time to digest, reducing overall rate of food intake (c) 2001 by W.H. Freeman and Company

32. Diet Mixing • Why do foragers consume a mixed diet? • different foods may be complementary, each providing essential nutrients missing in the other: • humans can subsist on rice and beans, but not on either of these alone (complementary amino acids) • grasshoppers foraging on mixed diets grow faster than those fed a single food • birds selectively consume fruits that differ from the more abundant background (c) 2001 by W.H. Freeman and Company

33. Summary 1 • Responses of organisms to their environments have evolved in response to selective pressures in these environments. • Organisms have characteristic activity spaces and can select appropriate microhabitats. • Acclimation and developmental responses permit organisms to respond to varying environments. (c) 2001 by W.H. Freeman and Company

34. Summary 2 • When environmental conditions exceed tolerances, organisms may migrate, rely on stored materials, or become dormant. • Animals adjust their foraging activities to optimize the net capture of resources per unit time. • Foragers also account for risks, and balance nutritional needs. (c) 2001 by W.H. Freeman and Company