Ecological Archives E096191A6
G. P. Hempson, A. W. Illius, H. H. Hendricks, W. J. Bond, and S. Vetter. 2015. Herbivore population regulation and resource heterogeneity in a stochastic environment. Ecology 96:2170–2180. http://dx.doi.org/10.1890/141501.1
Appendix F. Estimation of the FPAR zone threshold value.
The time that herds spent in the dry season range (DSR; i.e., the riparian zone) in each year was estimated by identifying a threshold FPAR value at which herds typically migrate to the wet season range (WSR; i.e., the plains). Herd position and FPAR data for the period from January 2007 to September 2009 were analyzed. Herd position was classified as being in either the DSR or WSR, with this boundary placed first at 5 km from the Orange River, and then at 10 km from the river. We then fitted a binomial GLM for each distance classification, with herd position (DSR or WSR) as the response variable and FPAR as the explanatory variable.
glm(Herd.position_5km ~ FPAR, family = binomial)
glm(Herd.position_10km ~ FPAR, family = binomial)
Whether herd position was located in the DSR or WSR during the period from 2007 to 2009 was reliably predicted by FPAR for both the 5 km (z = 11.93, p < 0.001) and 10 km (z = 12.92, p < 0.001) distance classifications. Based on these models, 50 % of herds would be expected to be > 5 km from the river at an FPAR threshold of 17.2, and > 10 km when FPAR reaches 20.9. Accordingly, an FPAR threshold value of 20 was selected (73.6 % of herds > 5 km, or 42.4 % > 10 km from the river) to estimate the time herds spent in the DSR in each year from 2000 to 2007.
Table F1. Parameter estimates for herd position models for the period January 2007 to September 2009. Statistical support for parameter estimates is indicated as follows: *** p < 0.001; ** p < 0.01; * p < 0.05; p < 0.01.

DSR: 5 km from river 

DSR: 10 km from river 







Constant 
6.253 (0.495) 
*** 

7.261 (0.509) 
*** 






FPAR 





Mean FPAR in FPAR zone 
0.364 (0.030) 
*** 

0.348 (0.027) 
*** 