Population Studies

1. Population Studies … Exploring how different species survive and interact in their environments

2. Population Basics • Population = the number of individuals of ONE species in an environment. • Community = all the populations of different species in an environment. • Species = a group of similar organisms that can reproduce and have fertile offspring. • Diversity = the number of different species in an environment (or how complex the community is). • Density = the number of individuals in a specific area (i.e., how crowded they are)

4. Population Growth • A population often increases in size when it enters a new environment • Eventually the population stabilizes when a limiting factor prevents it from growing larger

5. Calculating Population Growth I • Population growth is influenced both by the number of births and the number of deaths during a year • If Births > Deaths, then ____________ • If Births < Deaths, then ____________ • If Births = Deaths, then ____________

6. Calculating Population Growth II • The “Rate of Natural Increase” is symbolized by the letter r • r is calculated as a percentage of the current population size r = (Births minus Deaths) divided by the original Number of individuals r = b-d N

7. Population Growth Calculation Example N • 500 Moorish Idols lived on a coral reef. • They layed 10,000 eggs. • 2000 eggs hatched. • 100 larvae grew to adults. • 75 adults died during this time. • Find the growth rate and the new population after five years! b? d r = (b-d)/N

8. N = b = d = 500 100-75 500 = 25 500 = .05 = 5% r = 100 75 500 X .05 = 25 500 25 525 500 + 25 = 525 525 Now you fill in the rest!! Final Answer = 670 Moorish Idols after 5 years

9. Population Growth Calculation Example 2 N • 800 rattlesnakes lived in a desert. • They layed2000 eggs. • 1500 eggs hatched. • 200 “snakelings” grew to adults. • 175 adults died during this time. • Find the growth rate and the new population after five years! b? d r = (b-d)/N

10. Perhaps you’d enjoy a shortcut? • Try this: • F = I(1+r)t Where: • F= Final value • I = Initial value • r = Rate of growth • t = Time Compare your answer with this equation to your answer with the chart!

11. Calculating Population Growth III • A population that grows year after year grows faster every year, since there are more individuals to reproduce every year • This results in exponential growth, which appears as a J-shaped curve when graphed J 3 2 1 • Lag Phase: Population Grows Slowly • Exponential Growth Phase: Pop. Grows Rapidly • Stable Equilibrium Phase: Pop. Stops Growing 1+2= J 1+2+3= S

12. Biotic Potential I • Biotic Potential = the maximum possible growth rate (r) under ideal conditions • “Ideal conditions” varies between species, but it implies that the population has plenty of food, space, and mates, that there are few predators or parasites, that there is a good climate, etc. • The biotic potential of a population is seldom reached, since conditions are usually not ideal • Understanding the biotic potential is important when people want to influence the size of a population (such as increasing an endangered species or decreasing a pest species)

13. Biotic Potential II • The biotic potential of a population may be high or low depending on several factors related to how fast they can reproduce • The frequency of reproduction (how often they can reproduce) • The number of offspring per reproduction • The age required to begin reproducing • The likelihood of surviving to the reproductive age • The number of reproductions during a lifespan

14. Biotic Potential III • Example: the cui-ui fish of Pyramid Lake • Cui-ui are an endangered species found nowhere else in the world – • They are bottom-dwelling sucker fish that were a major food source for the Paiute people in the past

15. Biotic Potential IV • Cui-ui reproduce once a year (in spring) – but only if there is enough water flowing down the Truckee River for them to swim out of the lake and back after spawning • Each female lays 50,000 eggs at each spawning • Cui-ui must survive to between 10 and 25 years old before they are able to reproduce • Only one out of every 1000 to 10,000 eggs survive to one year old – and fewer survive to reproductive age • Cui-ui can live between 30 and 50 years

16. Biotic Potential V • Conclusions: the cui-ui fish of Pyramid Lake have a very low biotic potential • They seem to have adapted to variable amounts of spring runoff, reproducing extensively when there is plenty of water and then waiting for as much as a decade if there is a drought preventing them from spawning • The major limiting factor on their population growth seems to be the amount of water flowing down the river

17. Biotic Potential VI • The farmers in Fallon have been using about half of the Truckee’s water every year since 1905 • The Paiute tribe took the farmers to court to get more water for the cui-ui • The Endangered Species Act supported the tribe’s claim, and now the river flows are controlled to ensure that cui-ui can spawn • There is also a fish hatchery where the cui-ui are spawned in captivity, but it is only partially successful (the young cui-ui do not survive very long, so they have to be returned to the lake where predators can eat them before they grow very big) • A fish elevator was installed on a dam on the Truckee to help cui-ui get upstream to their spawning grounds

18. Biotic Potential VII

19. Carrying Capacity I • K is the symbol for the carrying capacity of the environment • K = the maximum population size that an environment can support • When the population reaches K, births = deaths, so r = 0 • The population stops growing at K, because they live in balance with their resources • Whichever resource runs out first is the major limiting factor for the population

20. Carrying Capacity II • Populations that reach K show logistic growth, making an S-shaped curve • The curve has three regions: • Rapid, exponential growth (r is large) – plenty of resources available • Slowing growth (r is small) – resources start limiting growth • Population remains constant at K (r is zero) – not enough resources for population to grow bigger, and population lives in balance with resources S 2 3 1 1+2+3= S

21. Carrying Capacity III • Populations usually don’t just stop growing at K – instead they grow past it • The resources can’t support the new population, so it drops (there are more deaths than births, and r is negative!) • Once the population gets low enough, it can expand again • The population goes through several cycles of overshooting K, then dropping below it, etc. • Each cycle is less drastic than the previous one • Eventually the population stabilizes at K when they live in balance with their resources 1 4 5 K 6 2 3

22. Carrying Capacity IV • The environment stops populations from growing through “environmental resistance” • Two types of environmental resistance: • Density-dependent effects: Environmental factors that stop growth faster as the population gets more crowded, such as lack of food, spread of disease, competition for resources, & predation • Density-independent effects: Environmental factors that stop growth regardless of how crowded it is, such as climate change, natural disasters, & storms

23. Carrying Capacity V • The carrying capacity can change for several reasons: • K might change with the seasons, leading to a cycle of population increases and decreases • K might change permanently due to overexploitation of the environment, causing a loss of resources and an abrupt drop in population size • K might change if a new predator or competitor is introduced to the environment (an “exotic species”) • K might change if there is a long-lasting change in climate (either natural or human-caused) • K might change due to habitat destruction by people or nature

24. Dynamic Equilibrium • Populations and communities at their carrying capacity are in dynamic equilibrium • Dynamic equilibrium means “changing balance:” the overall population sizes do not change, but the individuals in the population change due to births and deaths • The equilibrium point depends on the limiting factors, and this determines K • Changing the equilibrium point changes K, and the population will either grow or shrink to match the new equilibrium

25. Mortality & Survivorship • Mortality = the death rate (percent of individuals that die each year) • Survivorship = the opposite of the death rate (percent of individuals still alive at the end of each year) • Humans have dramatically lowered their mortality and increased their survivorship twice in our history: • 5,000 to 10,000 years ago we developed agriculture • 100 or 200 years ago we developed modern medicine • Both of these technologies raised our carrying capacity and allowed our population to undergo exponential growth

26. Survivorship Curves Survivorship Curves • Survivorship curves show the percent of individuals still alive at a particular age • Type I survivorship (aka, late loss) has most individuals living to old age – there is low infant mortality, and the majority of deaths occur in older organisms • Type II survivorship (aka, constant loss) has an equal mortality rate among all ages – the chances of dying stay the same throughout the lifespan • Type III survivorship (aka, early loss) has very few organisms surviving to adulthood (high infant mortality), but those that survive usually live out their full life span Humans Mice Salmon

27. Strategies for Survival I • Organisms must reproduce faster than they die in order to avoid extinction • Two main strategies for survival: • r-Strategists survive by maximizing their growth rate • K-Strategists survive by maintaining their population at the carrying capacity

28. Strategies for Survival II • r-Strategists: Maximize their growth rate by having many young • Often have Type III survivorship curves • Often have a lifestyle of rapidly colonizing available habitats • “Opportunists” that make a living by taking over quickly • Growth is eventually slowed by density-independent effects • Examples: grasses, mice, locusts

29. Strategies for Survival III • K-Strategists: Balance their population at K and try to stay there • Often have Type I survivorship curves • Often have a lifestyle of dominating the environment and keeping their population in equilibrium • “Specialists” that make a living by maintaining a specific niche and competing effectively against other species • Often have few young and long-term parental care • Environmental changes can be disastrous, since their populations grow slowly • Examples: Pines, chimpanzees, preying mantis

30. Population Interactions • Population sizes are influenced by both abiotic and biotic factors • Biotic influences on populations can occur by either intraspecific interactions (within a single species) or interspecific interactions (between different species) • Main types of population interactions: competition, symbiosis, predator/prey

31. Predator – Prey Interactions I • One organism feeding on another obviously influences the populations of both predators and prey • Predators and their prey are considered to be in an “evolutionary arms race:” the predators have adaptations to help them capture prey, and prey have adaptations to help them avoid capture – they both continue to improve their adaptations, resulting in a constant race (example: as the cheetah gets faster, so does the gazelle… or when the cougar gets better at hiding, the deer get better at finding it)

32. Predator – Prey Interactions II • Antipredator defenses include: • Camouflage • Scaring or startling a predator • Finding & escaping the predator before being caught • Staying in large groups that may confuse or distract the predator • Possessing protective body parts or noxious chemicals that stop the predator (poisonous animals often have “warning coloration” as a signal to the predator that they are poisonous) • Mimicry of another animal’s antipredator defense also occurs, even if the mimic doesn’t actually have that defense itself

33. Predator – Prey Interactions III • Predators may regulate prey populations at a constant level (and vice versa), since they are each a part of the biotic environment of the other, which means they influence the carrying capacity of each other • Predator and prey populations may also oscillate in a pattern like this: • Predator numbers increase when prey is plentiful • The increase in predators decreases the prey population • The lack of prey makes the predator population drop • The low number of predators allows the prey population to increase again, returning the cycle to #1 • These oscillations mainly occur with predators that specialize in one type of prey, since a predator that eats many types of prey (a generalist) will just switch to another food source after step 2

34. Predator–Prey Interactions IV • Records of the number of pelts sold were maintained for several decades by the Hudson’s Bay fur company • Does the predator control the prey population or vice versa? • One other factor was not included in the original data: the plant populations that the hares “preyed” upon! • The arctic willow releases a toxin when it’s growth is stunted by herbivores – so when the hares multiplied beyond a certain point, the willow fought back to reduce the hare population – and this impacted the lynx in turn! Canadian Lynx & Snowshoe Hare Populations

35. Integrated Pest Management • Controlling pest populations using a wide variety of methods. • Trying to keep the populations low – might use some pesticides, but also studies pest life cycle in order to find ways to slow their growth. • Possibilities include releasing sterilized pests to mate with wild pests, releasing predators, making pheromone traps that attract pests, etc.

36. Maximum Sustainable Yields • Harvesting a living resource (lumber, fish, etc.) at the fastest rate that can be maintained for a long period. • Usually requires keeping the population on the rising portion of the J-curve, where the population grows rapidly.

37. Biodiversity • Population studies also focus on maintining biodiversity – having healthy populations in an area ensures that the ecosystem is stable.