Population Dynamics

1. Population Dynamics Chapter 8

2. Sea Otter – the other, other white meat • Why are sea otters considered keystone species? • They control urchin populations which feed on kelp, hence they keep the kelp forests healthy • Why did their populations decline? • Originally due to hunting, now chemical pollution is suspected

3. Characteristics of a population • Size – number of organisms • Density – number /space • Dispersion – spatial distribution • Age distribution – pre-breeding, breeding, or post breeding age • Population dynamics – how these factors change due to environmental stresses

4. Population growth • Population change (growth) = (births + immigration) – (deaths + emigration) • ZPG – zero population growth is when incoming equals outgoing • Biotic potential – max growth for that particular population • Intrinsic rate of increase – rate of growth with unlimited resources

5. High intrinsic growth rates • Reproduce early in life • Have short time between generations • Reproduce many times • Have many offspring each time • Roaches, mice, fish, flies

6. Environmental Resistance • These are the vast assortment of environmental factors which help keep populations from growing out of control • This is a way a population finds an equilibrium point

7. POPULATION SIZE Growth factors (biotic potential) Decrease factors (environmental resistance) Abiotic Abiotic Too much or too little light Temperature too high or too low Unfavorable chemical environment (too much or too little of critical nutrients) Favorable light Favorable temperature Favorable chemical environment (optimal level of critical nutrients) Biotic Biotic High reproductive rate Generalized niche Adequate food supply Suitable habitat Ability to compete for resources Ability to hide from or defend against predators Ability to resist diseases and parasites Ability to migrate and live in other habitats Ability to adapt to environmental change Low reproductive rate Specialized niche Inadequate food supply Unsuitable or destroyed habitat Too many competitors Insufficient ability to hide from or defend against predators Inability to resist diseases and parasites Inability to migrate and live in other habitats Inability to adapt to environmental change Fig. 9.3, p. 200

8. Carrying capacity • Biotic potential and environmental resistance will determine the population a given area can hold and sustain indefinitely • A population must not drop below the minimum viable population or lowest number needed to keep population from disappearing due to environmental resistance

9. 2.0 1.5 Number of sheep (millions) 1.0 .5 1800 1825 1850 1875 1900 1925 Fig. 9.5, p. 201 Year

10. Logistic growth • Exponential growth (J curve) is not possible forever because resources and space eventually run out. When a population reaches a certain point, environmental resistance increases causing the population size to stabilize. This is known as logistic growth (s curve) and this generally happens to all populations

11. K Population size (N) Population size (N) Time (t) Time (t) Exponential Growth Logistic Growth Fig. 9.4, p. 201

12. Can you overshoot your carrying capacity? • Absolutely, it happens all the time • When you have too many individuals for the area to support you will have a population crash • If the overshoot was not too drastic, and the crash was small the population re-stabilizes

13. Types of population curves • Stable – nearly flat line • Irregular – widely fluctuating pattern with no periodicity • Cyclic – regular growth and crash at set intervals, usually seasonal • Irruptive – normally stable, but with a random spike or crash

14. Irregular Stable Number of individuals Cyclic Irruptive Time Fig. 9.7, p. 202

15. Top-down or bottom-up? • Evidence seem to show both happening • Top-down – predators hunt and kill prey keeping their population stable • Bottom-up – prey are the food source that allow predators to keep the populations up

16. Types of reproduction • Asexual – cloning, single parent donates both parts of DNA (bacteria) • Sexual – two parents donate DNA • Females have to give birth more (males do not as in asexual) • More genetic errors from combining • Mating is more damaging, and energy intensive • Does provide more genetic diversity, hence a stronger species

17. R-selected species • Also known as r-strategists and fill generalist niche • Have many offspring • Reach reproductive age early • Short time between generations • Little to no parental care and adapted to unstable climate (low survivorship) • Short life span (usually under a year) • Algae, rodents, bacteria, annual plants and insects

18. r-Selected Species cockroach dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Fig. 9.10a, p. 205

19. K-selected species • K-strategists or competitors, specialist niche • Fewer, larger offspring (usually develop inside) • Mature slowly (often protected while vulnerable) • Lower population growth rate • Long lived with stable population near carrying capacity • Depend heavily upon suitable habitat • Large mammals, birds of prey, long lived plants such as oaks, redwoods, some cacti

20. K-Selected Species elephant saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Fig. 9.10b, p. 205

21. Survivorship curve • Late loss - typical for k-strategists • Early loss – typical for r-strategists • Constant loss – for species in the gray area in-between k and r strategists with intermediate reproductive patterns • Song birds, lizards, and small mammals

22. 100 10 Percentage surviving (log scale) 1 0 Age Fig. 9.11, p. 206

23. Conservation biology • Sensible use of natural resources • Originated in 1970’s – uses current science • Investigate human impact on the biodiversity • Develop practical approaches to maintain biodiversity • Maintain – endangered species, wildlife reserves, ecological restoration, ecological economics, environmental ethics

24. Assumptions of conservation bio • Biodiversity is necessary • Humans should not affect extinction or vital environmental processes • Protecting ecosystems is the best way to protect • Based on Aldo Leopold’s ethical principle, that if we maintain the earth’s life-support system it is appropriate

25. Human impact on ecosystems • Fragmentation – breaking up large tracts with roads, fences, towns, etc. • Habitat loss/degradation – pollution, lumber, mining, etc. • Simplifying ecosystems – lower biodiversity through habitat change (monocultures) • Strengthening species – pesticide use, antibiotics

26. Human impact continued • Predator elimination – wolves, coyotes, bear, etc. • Introduce alien species • Overharvest potentially renewable resources – trees, soil, other biomass (grasses, nuts, etc) • Interfere with natural chemical cycling – clear cutting, monocultures, pesticides (we kill and simplify a system)

27. Way to go humans!! You’re the best! • Goals for the future (if we want to be a part of it) • Maintain balance between human impacted simple ecosystems and natural rich ecosystems • Slow down rates at which we alter nature for our own purpose • Realize that we never do merely one thing, everything is interdependent and unpredictable

28. How can you help • Use consumer power – buy products that are friendly to the environment • Use voting power – elect officials that will strive to protect the environment • Educate – most people have no idea about the consequences of their actions • Identify “mother culture” that says spend, buy, consume and learn to tune it out • Exploit nature for its aesthetics and renewable resources

29. That’s all folks Have a nice day