Logistic Paradigm. Logistic Paradigm. Paradigms. How should we consider them?

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1 Harvesting Paradigms Sustained Yield Harvesting Paradigms Net-Annual Increment Paradigm The Myth of MSY How should we consider them? The logistic paradigm obviously arises from logistic model. Says that we should keep the population at 1/2 K in order to maximize the sustainable yield. Sustainable yield is the amount of annual removal that a population can support and still maintain its current population size 1

2 Maximum sustainable yield is the highest sustainable yield that exists for a given population. In the logistic model the intermediate size has the greatest net production and this production falls off both above and below this point. But in reality we have much random variation or even oscillation within a population on an annual basis. Does this mean that the logistic growth model is correct and variations from it are due to just random variation? 2

3 But despite this the logistic paradigm is still in wide use. Due to some 'bottleneck There are some social or limiting habitat factors that determine the maximum number of animals that can survive the winter. According to this paradigm, we should harvest the surplus in the fall before natural mortality will 'kill them anyway'. 3

4 We need just enough animals to make it through the bottleneck and the lower the surplus the better the survivorship. This paradigm may be correct for restricted set of populations, perhaps upland game birds (grouse, pheasants), microtenes, other birds, but it has been applied beyond these types of populations In conservation biology, lends itself to the concept that habitat is limiting if you build it they will come Is the bottleneck paradigm based on intrinsic or extrinsic factors? Is the bottleneck paradigm based on intrinsic or extrinsic factors? Intrinsic: from the population itself Extrinsic: acting from outside the population. Has led to "man can never over harvest wildlife populations". Do you believe this? and "if you provide the habitat, the animals will care for themselves". 4

5 Do you believe this? Fails with big game and slow reproducers as well as many fish species. But we can still see the effects of this type of approach in waterfowl management. Efforts are towards habitat, difficult to restrict harvest. But it turns out that winter habitat is limiting! The Myth of MSY Which is the more stable situation? The Myth of MSY As a manager, what should your target population be? 5

6 The Myth of MSY The Myth of MSY What happens if you over harvest the following populations? Harvest quotas To fisheries may mean tons/foot of boat length To big game harvest may mean limited entry. with big game harvest, opportunity to manage selectively for harvest by age and sex class. Most populations contain more males than are needed to fertilize all the females capable of reproduction. Progressive reduction of the proportion of males in the population has little effect on the fecundity of females until a critical threshold is reached. 6

7 It may be very low, particularly when mating is promiscuous or when a few dominant males monopolize the receptive females. But does this mean that we should have just male-only harvests? But does this mean that we should have just male-only harvests? Or, if we have high population levels, should we reduce the population by just removing males? Age-specific harvests Unless the fecundity of older age classes decrease steeply, then generally there is no advantage to harvesting different age classes differently. Age-specific harvests An obvious corollary: when a population is harvested efficiently the imposition of a minimum size limit will have little or no effect on the MSY. Age-specific harvests unless we have a situation where large numbers of immature animals are competing successfully for resources with reproductive animals. 7

8 Sustainable Harvest Should this be our goal in management? Sustainable Harvest Should this be our goal in management? What data do you need? Approach with limited data? Do numbers matter? Sustainable Harvest Should this be our goal in management? Alternate models / approaches? British Columbia Approaches Limited Entry Hunting Quotas Settings Process ( British Columbia Approaches General Seasons Process ( How would you set a seasons? Range of public involvement? Resident hunters Guide Outfitters? Conservation groups? What are the inputs? Who makes the decision? 8

9 How would you set a seasons? Suppose, moose population could be down 50% Redo some of the survey areas: somewhat higher than last year s more extensive survey but data don t contradict a decline Potential causes? Potential actions Predators Habitat Bull, Cow, Calf seasons? 9