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Lecture 15

Lecture 15-16.docx

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University of Toronto St. George
John Stinchcombe

BIO220: Lecture 15-16 Humans harvesting of biodiversity  Can we harvest plants/animals in a sustainable way that prevents inadvertent negative results? o Results such as extinction, or by harvesting too much of it, driving its evolution in a way that makes it less useful for us dN= change in abundance (N) Simple Models of species growth and harvesting strategies dt= change in time dN/dt= change in abundance dN/dt= r* N * [(K-N)/K] over time As N approaches K, K goes to 0 so as a population approaches its carrying capacity, it approaches 0 r= intrinsic rate of pop growth If N really small, it’s going to be some number close to K ~1 of species K= carrying capacity Inflection point at K/2 Per capita rate of increase= Decline as pop gets bigger because getting closer to carrying capacity Ideal Harvesting strategy: Harvest K/2 individuals/year, so population is at max growth rate How harvesting affects demographics of harvested species Theoretically ideal scenario  harvest individuals of each species/year, so allow harvesting and self- sustaining populations Does this work?  Going to go through a best-case scenario where we don’t account for random chance, weather, predation, etc. Unstable: Stable: Left side of the line is Unstable and it is the Maximum Economic Yield (MEY) (little star) Right side of the line is Unstable and Maximum Sustained yield (MSY) (Big star) Harvest done to stock MSY so Harvest at MEY > Harvest at MSY There is always economic pressure to harvest so most fisheries tend to fish until they collapse. So the more you harvest, the more cost there is Stable: Little bits of harvesting are stable for the population because the population is being harvested down where the pop growth rate is at it’s maximum. Unstable: The unstable part which is where the economic benefits are the greatest, are going to be harvesting the population to such low levels that the population growth will be really small and won’t be able to replenish itself. The consequences of MEY> MSY  Constant financial pressure to harvest more  Populations are harvested to the point where they have low population growth rates  If harvest quotas are fixed in tonnage from year to year, you get the same amount removed each year and if the population starts to be harvested below K/2, it will experience slower pop growth which will lead to extinction  Improvements in technology make the cost curve flatter (reduces it) but does not alter the population dynamics Until the fishery collapses  almost every fish population that we try to harvest for we have fished into extinction. We can have tight regulation, enforcement and cautious catch quotas below the estimated MSY How harvesting affects their evolution How is harvesting determined?  By size thresholds (ex. if the fish is larger than x, it gets caught in the net, but if smaller, goes through)  By trophy ornaments (size of antler racks/horns/ etc. ) bigger fish competition  Continuous traits: Phenotypes that vary continuously quantitative traits (size, mass, length) o Influenced by lots of genes and have small effects o Also influenced by environment  Discrete traits: Mendelian traits (i.e. eye colour, tongue rolling, blood groups) o Can’t infer genotype from phenotype therefore we have to work with statistical descriptions Bell-shaped curve. It describes how many individuals there are in a given size. Individuals above a certain size threshold are targeted (ex. size of net so that bigger fish will only get caught and smaller fish will swim right through) So fish below threshold will live, fish above threshold will die. Truncation Selection: We’re imposing really strong selection for individuals to live or die based on whether or not they are above/below this threshold Example: Tuna  Fished on a size ratio (above/below a certain size can be caught/not caught)  Measure how strong this selection is by measuring the mean before and after selection o S=z*-z (s) Selection differential= Mean after selection – Mean before selection Ex. Mean is 10. Mean before selection is 10. So the mean after selection is going to be smaller because we preferentially removed the biggest individuals So by killing all of the biggest individuals, we’re imposing selecti
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