The poaching of rhino for their horns has reached unprecedented levels, and the world can expect to witness their extinction in the wild by 2035 if a breakthrough is not made. The links between poaching, global instability and possibly terrorism have led to substantial investment from the developed world into conservation security development in Africa. Such an investment requires a quantitative monitoring approach that allows for the effectiveness of the expenditure to be determined. By combining criminological deterrence theory and spatially explicit field-ranger patrol monitoring, we develop a framework to measure the presence of field-rangers in the landscape. We test this framework empirically by comparing the presence of field-rangers in the landscape against the presence around 40 rhino poaching incidents. We empirically demonstrate that the analysed field ranger human resources and their deployment in a well-staffed protected area in Africa did not deter rhino poachers.