1. Overexploitation III Ch. 8

2. Effects of exploitation on non-target species & ecosystems ▪ Tropical terrestrial ecosystems ▪ Logging & forest flammability: selective logging, even for a single species, increases the number of canopy gaps, which creates hotter drier microclimates in the understory; this can affect plant regeneration and susceptibility to fire disturbance ▪ Hunting & loss of seed dispersal services: Successful seedling recruitment in many flowering plants depends on seed dispersal services provided by large frugivores (e.g., monkeys, apes, lemurs, agoutis); seeds that simply fall to the ground often do not germinate. Hunting can reduce frugivores to the extent that recruitment is severely altered

3. Agouti ( Rodentia )

4. Effects of exploitation on non-target species & ecosystems ▪ Temperate ecosystems ▪ Aquatic: Beavers are ecosystem engineers – their dams affect the biogeochemistry of wetlands, the dynamics of shifting successional mosaics of aquatic patches, and the population dynamics of other wetlands species including waterfowl ▪ Terrestrial: The loss of top predators or keystone species has caused trophic cascades in many ecosystems; for example, the loss of wolves and bears in Yellowstone NP caused increases in elk and moose, altering riparian vegetation and increasing erosion, ultimately affecting other biodiversity (birds in willows) and even stream structure (see movie in lecture 4)

5. Effects of exploitation on non-target species & ecosystems ▪ Aquatic ecosystems ▪ Marine ecosystems: by-catch makes up 1/4 to 1/3 of total take worldwide; most are from trawl fisheries, drift nets & gill nets. In Australia, prawn trawlers discard 70,000 individual organisms per night ▪ By-catch is threatening some species with extinction. For example, 15 of the 22 spp. of Albatross are threatened with extinction, mostly due to longline fishing.

7. Effects of exploitation on non-target species & ecosystems ▪ Other marine issues: ▪ Sea Turtles: By-catch by longlines and trawlers; for example, >20,000 are killed annually in the Mediterranean in longlines set for swordfish. Turtle excluder devices are required in many fisheries; these provide an exit flap for turtles to escape while retaining the fish

9. Effects of exploitation on non-target species & ecosystems ▪ Freshwater ecosystems ▪ The critically-endangered Mekong Catfish of the Mekong River Basin in Thailand, Laos, and Cambodia, reaches 600 lbs and 9 feet long, making it prized by fishermen. They are migratory and must swim a gauntlet of nets and lines to reach their only spawning grounds – whirlpools and rapids near the Thai-Laos border. The spawning area is being dynamited to enhance river navigation. Biologists have set up a scheme whereby they purchase individual caught by fishermen at market price, and tag and release them (short-term measure while a longer-term solution is reached).

12. Biological Theory of Sustainable Exploitation ▪ Biological populations are renewable; the key is the ability of birth or death rates to compensate for the removal of individuals ▪ Some reduction of populations via exploitation can lead to benefits: reduced competition for food, territories or shelter, and lower disease transmission rates. We call this density dependence (limitations on fitness at high densities)

14. At what population size should we exploit the population to allow compensation?

15. Problem ▪ Difficult to take the exact number planned (people don’t do as they are told, there is always uncertainty in conditions affecting the take, such as weather conditions) ▪ So, we have a new question: What happens when the population is exploited at rate that differs from the one that theory suggests should be maximally sustainable? ▪ We can use constant quotas….

17. Proportional quotas: a better way? ▪ It would be more sensible to tie quotas to populations size; that is, if population size is low, reduce the quota, if its high, raise it ▪ As long as the exploitation rate (quota) is below the intrinsic rate of natural increase (r), then all equilibria are stable

18. Proportional vs. constant quotas

20. Open access sharing & tragedy of the commons ▪ Imagine you have sole access to trout in a lake on your property. You will probably manage them carefully, because if you take too many you will be the one who suffers in the future ▪ Now imagine you sharing the pond with a neighbor. Each of you should take half. But will you be so prudent? Perhaps if you are close friends or have a strong belief that reinforces this, but otherwise you might be tempted to exploit your neighbor’s self-restraint ▪ Now scale this scenario up to the North Sea, bordered by many countries of thousands of fishers all competing for the same fish. The sea is a common fishing ground, and massive overfishing of the past century has been tragic for both fishers and their prey

21. Open access – how much effort should we exert?

22. Open access overexploitation ▪ Experience and data show that people will join the exploitation until their individual costs become equal to their profits; exploitation will proceed to this risky break-even point because people are competing with one another ▪ This is bad for both the resource and the consumer ▪ Governments often respond by providing subsidies. This leads to lower apparent costs, but encourages more overexploitation. People find it difficult to leave the business during tough times, leading to resistance against restrictions on exploitation rates

23. Discounting ▪ Even when people have exclusive rights to a resource, they may still be tempted to exploit it heavily now, rather than conserve it for the future ▪ This is because we place a higher value on current vs. future worth ▪ Which would you rather receive, $100 now, or $100 in a year’s time? What about $100 now vs. $1,000 in a year’s time? Now, if we keep moving the one-year payment closer to the amount of the immediate payment, there is a point where you will “hesitate with indifference”. That is called discounting (the value of some future sum of $ in today’s terms) ▪ Conclusion: it may be economically rational to exploit populations to extinction rather than leaving them to provide future returns

24. Methods for calculating sustainable yields ▪ Putting the previous theory into conservation practice ▪ Surplus production models: if we know how yields have responded to different levels of exploitation effort over time, then we can estimate the dome-shaped curves for yield ▪ Yield per recruit model: searches for the level of mortality that maximizes the yield under the tradeoff between numbers and value (older trees provide more wood but if we wait too long natural mortality will mean there are less individuals available to us) ▪ Full demography models: when there is enough economic or conservation interest, full-blown population models are produced using data from births, juvenile survival, age at maturity & adult survival, etc.

25. Compliance

26. Next week: Invasive Species