Earth 125 1st Edition Lecture 23 Outline of Last Lecture I. Impactsa. Other impacts/extinctionsII. Biological causes (Examples)a. Great American InterchangeOutline of Current Lecture II. Predation as Cause of extinctiona. Extinction of megafauna in past 100,000 yearsCurrent LectureI. Case 3: Megafaunal extinctions1. Pleistocene – last 100ky2. Predationi. Why predation alone is generally not effective in causing extinction?1. Predation is a density dependent processa. Prey switching: as prey A becomes rare, predators switch to different prey B, allowing prey A to recoverb. Also affects predators:i. Predator survival rate is related to number of prey available1. more prey: most predators births survive2. fewer prey: less food, higher mortality of juvenilesc. Prey death relates are related to number of predatorsi. more predators: higher death rate of preyii. fewer predators: lower death ratesd. Mites laboratory experiment with spotted mite and predatory mites:i. Show dependent process – population size fluctuates depending on number of prey and predatorsii. Doesn’t necessarily lead to ultimate extinction, instead leads to fluctuations and cyclical process that continues ii. Why is predation generally not an effective extinction mechanism?1. When predators and prey coexist, prey will have time to evolveescape strategies rather than being driven into extinction, and this, in turn, may lead predators to either switch to other prey or to evolve new predatory strategies2. Escalation: Evolutionary race, “tit-for-tat”3. Megafaunal extinction: large animal extinctioni.ii. Major events: 1. dramatic climatic changes (glaciations)a. 120,000 years ago, the glaciers had retreated (looked similar to today)i. And then a new glacial epoch startedii. From 100,000 to 10,000 years ago, glaciers were presentiii. Last 10,000 years, time of deglaciationiv. Today, we are in an interglacial state (few glaciers, relatively warm temperatures)b. Last 40,000 yearsi. rapid drop in 10 degrees lower temperatures2. Dispersal of modern humans a. Modern humans (100 ka);i. Sophisticated way of using tools: well-developed hunting skillsb. Large megafauna – over 100lbsi. mastodons, mammothsii. predatory organisms: sabre-tooth cats, smilodoniii. These mammals became extinct around 20,000 to 10,000 years agoII. What is the cause of the extinction of the mega fauna?1. Climate change and Human hunting2. How can these be evaluated? What are the predictions?3. Timingi. Earliest sites of humans in the Americas: Clovis people 1. 11,000 to 15,000 years agoa. Had tools used for not only scavenging, but killing preyii. Youngest of large animals dated at 8,000 years1. Peak of humans – 15,000 years ago4. Human Predationi. Overkill Hypothesis1. Archaeological evidence of hunting tools2. Arrow points etc. embedded in fossilized bonesii. Blitzkrieg1. Naïve prey or ineffective defenses2. Even though we said predation would be inefficient, the humans that invaded were never seen by animals beforea. Animals had no exposure to the skilled predators (humans)III. What about places outside of North America?1.2. Climate change is global – would affect places universally as opposed to locally3. Human arrival seems to coincide with many megafaunal extinctionsIV. Thus, what is the cause of the Late Pleistocene extinction of the Megafauna?1. Human huntingi. Timing and evidence of hunting both match up with extinctionsii. Climate change could have been part of the cause of the extinction, but it was probably a combination of the two 1. Extinctions are better explained through appearance of