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Serengeti Wildebeest Conservation, management, models and data

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Introduction to ecosystem World Heritage Site #1 Home to the largest migratory mammal populations in the world, the highest mammalian biodiversity, and equal diversity among birds The regions - rainfall

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Patterns of rainfall The short rains – Nov – Dec The long rains – Feb – May The dry season – Sept – November Mean dry season rainfall 100 mm in SE – 300 mm in NW On the plains soils have hardpan that prevents tree and bushes from growing

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Wildebeest life history

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Types of data Census Rainfall Mortality Grass production

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What do do with limited data 1977 situation –Wildebeest –Wildebeest and predators

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Population and rainfall data

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History of wildebeest Rinderpest Rains 1977 situation

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1977 Questions What will happen if rainfall returns to 150 mm dry season instead of 250 mm If population crashes is there a danger of predation becoming important and perhaps a predator pit collapse?

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Rinderpest antibodies in wildebeest 195886% 195986% 196079% 196167% 196251% 19630 19640 19650 19660 19690

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The 1977 model (the old way) Put together an understanding of the biology from existing literature using functional knowledge Do not integrate any estimation of parameters with the model – all estimation is done outside the model

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Key elements in 1977 model Grass production related to rainfall by simple regression Calf survival related to kg/grass per individual Adult mortality key to population trends

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Adult dry season mortality Wildebeest eat 4.2 kg/day grass when plentiful A ruminant can lose 30% of body weight and survive, but become vulnerable to disease after losing 20% Green grass has 8% crude protein, dry grass has 2% Studies of cattle show weight loss related to % protein in diet

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Basic arithmetic 150 mm rain produces 100 kg/ha/mo 1 million animals is 1 per ha So 150 mm rain produces 3.3 kg/animal/day with 1 million animals So no problem But 2 million animals is 1.7 kg/animal/day, and we expect mortality to go way up

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The population will track the current rainfall up and down. One should note that the equilibria are large, even with the low dry-season rainfall observed in the 1960s (150 mm) the wildebeest population will be about the same as 1977 (1.4 million). Thus a return to the 1960s rainfall levels would possibly not lead to a catastrophic decline at 1977 levels.

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Population and rainfall data

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Simple model Food/ha (F) = -200 + 2*rainfall Food/animal (FPA)= F / density of wildebeest (W) Calf Survival (CS) = (.5 FPA)/(75+FPA) Calves surviving (C) =.5 W CS Wildebeest eaten /predator (WEP) = (317 W)/(1 +.05*317*w +.08*100*A) Wildebeest eaten (WE)= Predators (P) * WEP Adult survival (AS) = graph shown previously W(t+1) = W(t)*AS +C(t) – WE(t)

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Key lessons from 1977 Some basic biological knowledge can provide insight in what would otherwise be a data poor situation Simple 3 trophic level model is straightforward No formal integration of data doesnt allow us to discuss uncertainty

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Jump ahead to 1991 We now integrate data fitting to modeling and prediction We have more long term data The population has leveled off Poaching has increased dramatically targeting wildebeest, but there is by-catch of predators and rare ungulates

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1991 Questions Can harvesting wildebeest be legalized in a way that reduces or eliminates by catch How large is the illegal harvest

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Data sources census of total wildebeest population with standard errors estimates of yearling/adult ratio estimates of dry season adult mortality rate pregnancy rates rainfall and dry season grass production relationship dry season rainfalls

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The model(s)

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The parameters The parameters of the survival vs food functional relationships, a,b, e, and f And how harvest is calculated –Assume constant after 1977

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The likelihoods Census numbers: normal with specified s.d. or lognormal with cv=.2 Yearling adult: lognormal cv=.2 Adult mortality lognormal cv=.2

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Alternative hypotheses survival parameters harvest constant after 1977 harvest proportional to human population as above but change in enforcement three harvest periods pre 77, 78-87, 88- present

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Wildebeest numbers 0 500 1000 1500 2000 195019601970198019902000 Wildebeest numbers (x1000)

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Yearling:adult ratio

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Percent adult monthly mortality

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Figure 1: fit with no harvest

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Figure 2: without harvest

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Post 1977 harvest

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Alternative hypotheses

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Summary re modeling Modern likelihood theory provides a powerful framework for analysis of complex data sources Include all your observations Integrate data fitting with evaluation of alternative policies

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Summary re Serengeti Model performed very well in predicting the impacts of the 1993 drought Estimates of illegal harvest are much lower than methods estimated by interviews No legalization program has been implemented

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Publications Hilborn, R. and A.R.E. Sinclair. 1979. A simulation of the wildebeest population, other ungulates and their predators. pps 287-309 In: Serengeti: Dynamics of an Ecosystem. A.R.E. Sinclair and M. Norton-Griffiths, eds. University of Chicago Press. Mduma, S.A.R., Hilborn, R. & Sinclair, A.R.E. 1998. Limits to exploitation of Serengeti wildebeest and implications for its management. Dynamics of tropical communities, the 37th Symposium of the British Ecological Society (eds D.M. Newbery, H.B., H.H.T. Prins & N.D. Brown) pp. 243-265. Blackwell Science, Oxford.

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