Understanding Population Dynamics: Key Characteristics and Changes Over Time

1. Ch. 50

2. How We Describe Populations • Population dynamics characteristics of a population • Vary between locations and time periods • Geographical range boundaries it lives in; habitat specific abiotic/biotic factors it lives in • Small (one tree) or large (an ocean) • Population Density # of individuals/area of habitat • Small bodied organisms typically have higher density than large bodied organisms • Greatly influenced by birth/death rate

3. How We Describe Populations 3) Dispersion pattern how are individuals spread across the habitat • Random unpredictable; spiders and rainforest trees • Clumped small groups are randomly placed; lions and mushrooms • Most common; mating, resources, and protection all attract small groups • Uniform evenly spaced apart; conifers and large topped trees • Prevent clumped pattern by making it hard for others to share space

4. How We Describe Populations 4) Age Structure what percent are either pre-reproductive, reproductive, or post-reproductive • High pre-reproductive means fast future growth (India) • High post-reproductive means fast future shrinking (Russia) Generation time how long it takes for the new born babies to grow up and have babies (Humans 25 years) 5) Sex Ratio # males vs. # females • Most populations # of females is the important one • In Life-long mating both numbers are important

5. Monitoring Populations • Demography statistical analysis of population trends • Birth/Death Rate • Immigration movement in • Emigration movement out • Life Table reference chart made from data • Age-specific mortality % that will die during each age interval • Age-specific survivorship % that survive each age interval • Age-specific fecundity number of offspring produced by the females in each age interval

6. Survivorship Curves • Survival rate over the lifespan of an individual • 3 Types of Populations • Type I high survival rate till later in life • animals with few offspring that protect them for most of their development • Type II change of survival drops consistently with age • Chance of predation, disease, or starvation always present in life • Small mammals and lizards • Type III low survival rate until later in life • Many offspring that are not raised by parents; plants, fish, insects, etc...

7. It All Comes Down To Energy • Life Histories how do the organisms grow, mature, and then reproduce • Controlled by energy; only so much energy in you to use and you only get so much energy from food (energy budget) • How will you use it? • Growth • Homeostasis • Reproduction • Passive parental care energy used before birth; eggs, seeds, fetus • Active parental care energy used after birth; raise them or fend for themselves • One vs. Many; Late vs. Early

8. Modeling Growth • 2 types of growth: • Exponential J-shaped; fast initial growth due to abundant resources • Logistical S-shaped; growth slows and oscillates around carrying capacity of environment • Both track: N/t = B-D • N change in population • t change in time • B # of births • D # of deaths • Better to track rates: dN/dt= (b-d)N • b B/N= per capita birth rate • d D/N= per capita death rate

9. Exponential Model • dN/dt= (b-d)N • Assumes no outside factors will effect growth • r= b-d = pre capita growth rate • dN/dt= rN • Growing populations r > 0 • Shrinking populations  r < 0 • r=0 ? • Zero Population growth (ZPG) • Intrinsic rate of Increase maximum rate a population can grow; dN/dt= rmaxN • Limited by generation time and # of possible offspring

10. Logistical Model • As populations increase resources become limited; energy budgets tighten • b drops and d raises • Carrying Capacity (K) maximum number an environment can hold at that time • Limited by intraspecific competition conflict for resources inside the population • (K-N)/K= available capacity • dN/dt= rmaxN(K-N)/K • Closer (K-N)/K gets to 1, the closer rmax is to true value • As N increases to K, rmaxdrops • What happens when (K-N)/K=0? • ZPG, no growth • Growth always fluctuates around K because of time lag time it takes limits to effect reproduction

11. Ever Changing Populations • Changes in population are regulated by two types of factors: • Density-Dependent factors that increase/decrease with the density of the population • Competition food, space, and mates all becomes more difficult to obtain • Crowding stresses population and spread disease faster; can induce females to reproduce less or make smaller offspring • Predation increase population means more to kill

12. Ever Changing Populations 2) Density-Independent factors that do not fluctuate with population size • Temperature, natural disasters, etc… • Do not regulate population size since there is no standard effect • Often both types are present: • High density can lead to weakened offspring which die in larger numbers during bad weather • Smaller population might be more likely to all survive a disaster if they find a safe place

13. Exchange Game • What density factor is ignored in most growth rate calculations? • Immigration and Emigration • Though ignored, most populations exchange members with each other (metapopulations) • Source population stable or increasing; members emigrate out when too big • Sink population unstable or shrinking; new members immigrate in from sources

14. 2 Ways to Live • r-selected life pattern: • Do well in fast changing environments • Small body size, short generation time, reproduce few times but in large numbers, do not raise their young • High rmax; rarely can hit K because so few offspring survive • normally environment limits numbers before resources run out • Which type survivorship is this? • Type III

15. 2 Ways to Live 2) K-selected life pattern: • Do best in stable environments • Large body size, long generation time, reproduce often but with few offspring, spend energy raising young • Low rmax; Normally reach K in time and fluctuate in density • Can adjust to environment to keep constant growth • What type of survivorship is this? • Type I and Type II

16. Human Growth • Human population hit 1 billion in 1804 and is projected to hit 9 billion before 2050 • This exponential growth is due to the removal or suppression of density-dependent factors • 3 Ways: • Expansion of geographical range allows greater source of resources • Agriculture allows more available food than nature would provide • Public health and medicine limits affects of disease

17. Stages of Growth • Human population experience 4 stages of growth (Demographic Transition Model) • Preindustrial low control on environment causes high birthrate to even out high death rate; population does not grow much • Transitional increased control lowers death rate; birth rate still high so population raises quickly • Industrial society limits birthrate; growth slows • Postindustrial birth and death rate about even; birth rate may drop more to cause negative growth

18. Homework • Suggested Homework: • Test Your Knowledge • Actual Homework: • Discuss the Concepts #1 • Interpret the Data • Due Wednesday