EE32 Flashcards
why manage populations? (3 examples)
1) stop invasive species leading others to extinction ie Goat–> tortoise extinction
2) help endangered species recover e.g Gold lion tamarin
3) balance economics with exploitation ie wwf suggested Saiga antelope could substitute rhino horns of Chinese medicine market
what is recruitment?
the number of new individuals
what growth does net recruitment curve and productivity curve have? what equation?
LOGISTIC GROWTH
dN/dt = rN(1-N/K)
what does dN/dt=rN(1-N/K) show?
basically population growth rate and shows how much you can harvest per unit time with decreasing the population further
Problems with net recruitment curve? (3)
- it is an approximation and real life recruitment curves are often not log model shape
- why does density dependence affect growth when pop is 50% of carrying capacity
- maximum growth at pop size of 50% of K is mathematically convenient but doesn’t correspond to reality (e.g Fin whale stocks where enviro effects e.g salinity affects population size)
what do net recruitment curves show?
How many new individuals are added per unit time at each level of the population
How much you can harvest per unit time at each level of the population without decreasing the population further
Describe the net recruitment curve at deifferent population size as % of K.
0?
50?
100?
0%- small pop so growth limited
50%-growth rate and pop size are middling so max new individuals added
100%-growth limited as reaching K
what is an example of an organism with non-logistic recruitment curve?
North sea herring
recruitment declines at a lower population density than 50% of K
what is MSY?
maximum sustainable yield
how to you calculate MSY from net recruitment curve? what effects can this have on pop size?
- max offtake possible
- use peak of net recruitment curve (not necessarily 50% of K)
- MSY supreses pop depending on net recruitment curve shape
what enables inference of what the population will do wrt recruitment curves?
plotting the harvesting strategy and recruitment curve together
what is a harvesting strategy?
an amount that you plan to take depending on the population level
what are 2 harvesting strategies/ explain them?
fixed quota system- take percentage of K regardless of pop size
fixed effort-proportional off take (good for mammals)
what 2 critical points are there on net recruitment curve with harvesting strategy added horizontally?
points where harvesting strategy crosses net recruitment curve
1= STABLE EQUILIBRIUM where the recruitment curve is lower than the harvesting strategy to the right and higher to the left
2=BREAKPOINT where the recruitment curve is higher than the harvesting strategy to the right, lower to the left
if harvest= recruitment?
population size stays the same
what is unstable equilibria?
when the harvesting strategy JUST touches the recruitment curve as it means either side slightly from where it is touching will lead to the harvest being above recruitment and population falling
Dangers of MSY?
- even if you know MSY exactly the population is only going to an unstable equilibrium and random variation will cause eventual extinction
- if overestimate MSY and have a fixed quota harvesting strategy higher =automatic extinction
- ignores individual variation in the population
- harvesting is predominatly on certain individuals ie older larger bigger male fish
- ignores environment
Is fixed quota harvesting below MSY sustainable?
tehoretically should stay at equilibrium but if bad year goes below breakpoint for other reasons –> EXTINCTION
what is an example of fixed quota system failing?
Peruvian anchovy fishery
what happened in the pruvian anchovy fishery case study?
experts estimated MSY was 10 million tonnes/yr and catch was fixed at this–> stocks plunged–> population crash
why fixed quota? 1)20,000 livelihoods depended on it 2) politcal pressure
what are examples of tipping point behaviours?
breakpoints
What happens if a system crosses the tipping point?
it goes into a new stable state and then it can be very hard to bring back to the original state e.g peruvian anchovies
how do el nino events affect anchovian fisheries?
Every few years the pattern of air circulation of the equatorial Pacific changes in a way that affects oceanic upwelling. This weather condition is known as El Niño. During El Nino, upwelling brings up warm water with few nutrients. A serious economic consequences of El Niño is its devastating effect on the Peruvian anchoveta fisheries. Populations of fish and seabirds vanish and anchovy catches dwindle during El Niño.
how widescale was the use of MSY in fisheries? give figures
before 1980 all 39 national agencies responsible for managing marine fisheries used MSY
