Compute Schoener's ratio

Description

schoener computes the Schoener's ratio on radio-tracking data.
schoener.rtest performs a randomization test of the equality of the Schoener's ratio to 2

Usage

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schoener(tr, keep, byburst = TRUE)
schoener.rtest(tr, keep, byburst = TRUE, nrep = 500)

Arguments

tr

an object of class traj

keep

a vector of length 2, giving the lower and the upper bound of the time interval for which a pair of relocations is considered in the computation of t\^2 (see details). These values are given in seconds.

byburst

logical. If TRUE, the Schoener's ratio is computed by burst. If FALSE, the ratio is computed by animal.

nrep

the number of randomisations of the test.

Details

The Schoener's ratio is a measure of time-autocorrelation in the data. This ratio is computed as the squared mean distance between "neighbour" relocations (t\^2) divided by the squared mean distance between the relocations and their barycenter (r\^2). The theoretical value of this ratio under the hypothesis of independance of the relocations is 2.

Swihart and Slade (1985) consider as neighbour two successive relocations. However, the Schoener's ratio computed in this way makes sense biologically only if the relocations are equally spaced in time. However, as indicated by these authors, "such a data set probably is the exception rather than the rule because many problems may arise in taking a locational reading at a specified time".

In this function, we define as "neighbour" two relocations (not necessarily successive relocations) separated by a time interval comprised within the bounds specified in the vector keep (in seconds). For example, if keep = c(60, 300), all relocations separated by a time interval comprised between 1 and 5 minutes are considered in the computation. Thus, the total number of pairs of relocations m taken into account in the computation may be larger than the number of relocations n (m can be at most equal to n*(n-1)/2).

Value

returns an object of class schoener.

Author(s)

Clement Calenge clement.calenge@oncfs.gouv.fr

References

Schoener, T.W. (1981) An empirically based estimate of home range. Theoretical Population Biology, 20, 281–325.

Swihart, R.K. and Slade, N.A. (1985) Testing for independence of observations in animal movements. Ecology, 66, 1176–1184.

Solow, A.R. (1989) A randomization test for independence of animal locations. Ecology, 70, 1546–1549.

Examples

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data(puechcirc)
puechcirc <- ltraj2traj(puechcirc)
puechcirc$date[1:10]

## Relocations are taken every 10 minutes
## For example we consider relocations as
## neighbour when they are separated by a time
## interval comprised between 5 and 15 minutes
schoener(puechcirc, keep = c(5*60, 15*60))
## Not run: 
schoener.rtest(puechcirc, keep = c(5*60, 15*60))

## End(Not run)

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