Predation risk theory for optimising control of wild deer. A.D.M. Latham, M.C. Latham & D. Herries

Transcription

1 Predation risk theory for optimising control of wild deer A.D.M. Latham, M.C. Latham & D. Herries

2 Introduction Introduced game species are a resource for some stakeholders They can have negative impacts to ecological and productive assets To manage for both, the primary strategic option is sustained control conducted in perpetuity Wild deer introduced into Australia and New Zealand fall into this category

3 Introduced deer in NZ Seven species well-established Red deer (Cervus elaphus) Sika deer (C. nippon) Fallow deer (Dama dama) Sambar deer (Rusa unicolor) White-tailed deer (Odocoileus virginianus) Wapiti (C. canadensis) Javan rusa (R. timorensis)

4 Introduced deer in NZ Important game species, but also have unwanted impacts Controlled by recreational hunters and professional groundbased or helicopter-based hunters

5 Introduced deer in NZ In areas where recreational hunters cannot reduce deer numbers, helicopter-based hunters may be needed Expensive and optimising kills-per-unit-effort is important when budgets are limited

6 Predation risk theory The hunter recreational or professional can be viewed as a predator Predation risk for deer the risk of being killed per unit time can be estimated by decomposing two fundamental components: 1. the probability of encounter (α) 2. the conditional probability of kill given an encounter (d) Important because risk is spatiotemporally variable Revealing those environmental attributes that render deer more or less susceptible to being encountered or killed may increase control efficacy

7 Sika deer Main herd centred in Kaweka Forest Park, Central NI Helicopters target females

8 Questions Spatially and temporally decomposed sika deer predation risk from aerial culling 1. What landscape features are important for encountering and killing female deer? 2. Do landscape features associated with where female deer are encountered and killed differ from those they select? 3. Are female deer most likely to be encountered and killed in habitats where search effort is highest? 4. Can this improve deer management?

9 Sika deer GPS data from 16 female sika Helicopter flight paths Locations of: non-encounters (GPS collared deer within visual range but not seen) encounters kills

10 Approach 1. Assessed four stages of the predation process: Probability of occurrence (i.e., deer habitat selection) Probability of encounter Probability of kill given an encounter Helicopter search effort (using flight path tortuosity)

11 Approach 2. Created identical models for each stage of predation (including six environmental variables) Assessed each model seasonally Winter, spring and summer No culling during autumn (rut)

12 Results Total probability of encounter given presence = 0.19 Of 942 female sika encountered, 704 were killed Total probability of kill given encounter = 0.75 Seasonal probability of kill given encounter: - Winter = Spring = Summer = 0.83

13 Estimated beta coefficient P(occurrence) P(encounter) P(kill) Search Winter model Dist stream Dist hollow Slope Elevation Forest Open areas Environmental variables

14 Winter model

15 Summary Can these types of analyses improve current deer management? Decomposing predation risk can: 1. Identify habitats where the probability of kill given an encounter is low, e.g., forest 2. Identify habitats where the probability of kill is high but yield is low, e.g., river beds 3. Identify habitats where the probability of encounter and kill are high relative to search effort, e.g., tussock and forest gaps 4. Estimate whether operational search effort is being optimised compared to random

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18 Summary Can these types of analyses improve current deer management? Theoretically, yes Need to compare the number of deer killed (per unit effort) on optimised flight paths versus operational flight paths

19 Acknowledgements Game & Forest Foundation Guns & Tackle & Hamills hunting specialists NZ Outdoor & NZ Hunter magazines NZ Deer Stalkers Assoc. Hastings & Tutira Forest & Bird Collars Sirtrack & Kiwi Track Lakeland Aviation & East Kaweka Helicopters NZ Dept. of Conservation Investigation 4536 Landcare Research Animal Movement Modelling Group