Factors affecting the recovery of Mexican wolves in the Southwest United States

Recovering and maintaining large carnivore populations is a global conservation challenge that requires better knowledge of the factors affecting their populations, particularly in shared landscapes (i.e., non-protected areas where people occupy and or utilize the land). The Mexican wolf (Canis lupus baileyi) is an endangered wolf subspecies being recovered on shared landscapes in the Southwest United States and Mexico. We used data from the U.S. program to model population growth, evaluate the impact of management removal and illegal killing relative to other demographic factors, and test hypotheses about factors influencing rates of management removal and illegal killing. From 1998–2019, the population growth averaged 12% per year. Rates of natural reproduction, illegal killing, and other mortality remained consistent over the 22 years, while releases, translocations, and management removals varied markedly between two time periods, phase 1: 1998–2007, and phase 2: 2008–2019. The number of wolves removed for conflict management was higher during phase 1 (average ~13 per year, rate = 24.8%) than phase 2 (average of ~5 per year, rate = 5.2%). This decrease in management removal resulted in the wolf population resuming growth after a period of population stagnation. Two factors influenced this decrease: a change in policy regarding the removal of wolves (stronger modeling support) and a decrease in the number of captive-reared adult wolves released into the wild (weaker modeling support). Illegal mortality was relatively constant across both phases, but after the decrease in management removal, illegal mortality became the most important factor (relative importance shifted from 28.2% to 50.1%). Illegal mortality was positively correlated with rates of reintroduction and translocation of wolves and negatively correlated with the rate of management removal. Using management removal to reduce human-carnivore conflict can have negative population impacts if not used judiciously. Recovering and maintaining carnivore populations in shared landscapes may require greater tolerance of conflict and more emphasis on effective conflict prevention strategies and compensation programs for affected stakeholders.

Methods
Within the United States, Mexican wolves are being recovered in south-central Arizona and New Mexico; specifics of the area can be found in (U.S. Fish and Wildlife Service 2017). Mexican wolves have been monitored intensively since the beginning of the reintroduction effort in 1998. To aid monitoring, a high percentage of wolves are radio-collared each year (range 38% to 100%, weighted average based on end-of-year population count and collars was 52%). Utilizing radio collars and other methods the Interagency Field Team (i.e., employees from Arizona Game and Fish Department, New Mexico Department of Game and Fish, USDA APHIS-Wildlife Services, US Forest Services, US Fish and Wildlife Service, and White Mountain Apache Tribe) then conducts annual population counts and pup counts and monitors continually for mortality events. Initially (1998–2004), the Interagency Field Team determined population estimates and pup counts via howling surveys (Harrington and Mech 1982, Fuller and Sampson 1988), tracks, and visual observations during aerial (fixed wing) and ground radio-telemetry efforts (White and Garrott 1990). Ground observations were collected opportunistically through the least intrusive methods possible, and the Interagency Field Team avoided any disturbance of den areas. In later years (2005–2019), they incorporated helicopter counts in January to verify and collect additional information from ground counts and incorporated the increased use of remote cameras, observations at den sites, and trapping for younger pups (2009–2019). Currently, the Interagency Field Team utilizes data collected from November 1 through mid-February to develop an end-of-the-year observed minimum population count. 

The only processing of the data that we have done was to combine different sources of non-management and non-illegal killing into “other mortality”. We combined natural mortality, mortality from vehicles, and other legal mortality into other mortality for our analysis.