BOTTOM-UP AND TOP-DOWN FACTORS CONTRIBUTE TO REVERSING A MOOSE POPULATION INCREASE IN NORTHEASTERN WASHINGTON
Keywords:Alces alces, body condition, bottom-up, Canis lupus, carotid worm, Elaeophora schneideri, fecundity, moose, survival, population trajectory, predators, top-down, Washington state, wolves
Newcomers to the state, moose increased in abundance and distribution throughout northeastern Washington from the 1970s through 2013 when we began a study of moose demography north of Spokane, Washington. The study was designed and analyzed with 2 adjacent but geographically distinct population subunits – a northern study area with wolf (Canis lupus) packs present and a southern study area without wolf packs. We followed the fates of 67 GPS-collared cow moose (41 and 26 in the northern and southern study areas, respectively), and monitored production and apparent survival of their (unmarked) calves using ground-based approaches during 2014–2018. We used the Cormack-Jolly-Seber estimator to account for imperfect detection of calves monitored via their mothers, and AICc to evaluate competing models of calf survival. We supplemented these analyses with indices of calf recruitment to mid-winter obtained from helicopter-based surveys over a larger survey area. The best supported calf survival model included neither study area nor temporal covariates; estimated annual calf survival in both study areas combined was 0.36 (SE = 0.05). Adult survival rates were similar in the 2 study areas (0.80 overall; 95% confidence interval 0.76–0.86) but causes of death differed. Estimated observed fecundity (calves/females in early summer) was 0.56 in the northern study area and 0.70 in the southern; pregnancy rates showed a similar trend (0.70 northern, 0.93 southern). Populations in both study areas were declining; λ was estimated as 0.87 (SE = 0.03) in the northern study area and 0.90 (SE = 0.03) in the southern. Body condition data indicated moose from both study areas entered winter with low energy reserves, increasing susceptibility to morbidity and mortality. We found multiple factors acting on the northern population including equal rates of wolf predation and winter tick mortality of adults and low marrow fat in many tick- and predation-related mortalities. We suggest the marked population decline measured during the study was related to multiple and often interacting factors including the combined and often interacting top-down effects of predation and bottom-up effects of nutrition.
How to Cite
- Authors retain copyright and grant the journal right of first publication with the work simultaneously licensed under a Creative Commons Attribution License that allows others to share the work with an acknowledgement of the work's authorship and initial publication in this journal.
- Authors are able to enter into separate, additional contractual arrangements for the non-exclusive distribution of the journal's published version of the work (e.g., post it to an institutional repository or publish it in a book), with an acknowledgement of its initial publication in this journal.