possum                 package:secr                 R Documentation

_B_r_u_s_h_t_a_i_l _P_o_s_s_u_m _T_r_a_p_p_i_n_g _D_a_t_a_s_e_t

_D_e_s_c_r_i_p_t_i_o_n:

     Data from a trapping study of brushtail possums at Waitarere,
     North Island, New Zealand.

_U_s_a_g_e:

     data(possum)

_D_e_t_a_i_l_s:

     Brushtail possums (_Trichosurus vulpecula_) are an unwanted
     invasive species in New Zealand.  Although most abundant in
     forests, where they occasionally exceed densities of 15 / ha,
     possums live wherever there are palatable food plants and shelter.

     Efford et al. (2005) reported a live-trapping study of possums in
     _Pinus radiata_ plantation on coastal sand dunes.  The 294-ha site
     at Waitarere in the North Island of New Zealand was a peninsula,
     bounded on one side by the sea and on two other sides by the
     Manawatu river.  Cage traps were set in groups of 36 at 20-m
     spacing around the perimeter of five squares, each 180 m on a
     side.  The squares ('hollow grids') were centred at random points
     within the 294-ha area.  Animals were tagged and released daily
     for 5 days in April 2002.  Subsequently, leg-hold trapping was
     conducted on a trapping web centred on each square (data not
     reported here), and strenuous efforts were made to remove all
     possums by cyanide poisoning and further leghold trapping across
     the entire area.  This yielded a density estimate of 2.26 possums
     / ha.

     Traps could catch at most one animal per day.  The live-trapped
     animals comprised 46 adult females, 33 adult males, 10 immature
     females and 11 immature males; sex and/or age were not recorded
     for 4 individuals (M. Coleman unpubl. data).  One female possum
     was twice captured at two sites on one day, having entered a
     second trap after being released; one record in each pair was
     selected arbitrarily and discarded.

     The data are provided as a single-session 'capthist' object
     'possumCH'.  'possummask' is a matching mask object - see
     Examples.  Two fitted models ('possum.model.1' &
     'possum.model.1b') are provided for illustration.

_S_o_u_r_c_e:

     Landcare Research, New Zealand.

_R_e_f_e_r_e_n_c_e_s:

     Borchers, D.L. and Efford, M.G. (2008) Spatially explicit maximum
     likelihood methods for capture-recapture studies. _Biometrics_
     *64*, 377-385.

     Efford, M. G., Dawson, D. K. and Robbins C. S. (2004) DENSITY:
     software for analysing capture-recapture data from passive
     detector arrays. _Animal Biodiversity and Conservation_ *27*,
     217-228. 

     Efford, M. G., Warburton, B., Coleman, M. C. and Barker, R. J.
     (2005) A field test of two methods for density estimation.
     _Wildlife Society Bulletin_ *33*, 731-738.

_S_e_e _A_l_s_o:

     'capthist'

_E_x_a_m_p_l_e_s:

     ## Not run: 

     setwd('d:\density communication\webtest\foxton\')
     possumtraps <- read.traps(file = 'foxtraps.txt', detector = 'single')
     temp <- read.captures('foxton.txt', colClasses=c('character',
         'character', 'numeric', 'character'))
     ## drop within-day duplicates of animal 5861
     temp <- temp[-c(184,186),]

     possumCH <- make.capthist(temp, possumtraps)
     possummask <- make.mask(possumtraps, buffer = 300, type='pdot', pdotmin = 0.001,
         detectpar = list(g0=0.2, sigma=60), spacing = 10)

     ## fit constant-density model
     possum.model.1 <- secr.fit(possumCH, mask = possummask)
     ## fit learned trap response model
     possum.model.1b <- secr.fit(possumCH, mask = possummask, model = list(g0~b))

     ## End(Not run)

     require (graphics)
     data(possum)
     plot(possummask)
     plot(possumCH, tracks = TRUE, add = TRUE)
     plot(traps(possumCH), add = TRUE)
     summary(possumCH)

     ## compare & average pre-fitted models
     AIC(possum.model.1, possum.model.1b)
     model.average(possum.model.1, possum.model.1b)

