title: Disease introduction is associated with a phase transition in bighorn sheep demographics.
abstract: Ecological theory suggests that pathogens are capable of regulating or limiting host population dynamics, and this relationship has been empirically established in several settings. However, although studies of childhood diseases were integral to the development of disease ecology, few studies show population limitation by a disease affecting juveniles. Here, we present empirical evidence that disease in lambs constrains population growth in bighorn sheep (Ovis canadensis) based on 45 years of population-level and 18 years of individual-level monitoring across 12 populations. While populations generally increased (λ = 1.11) prior to disease introduction, most of these same populations experienced an abrupt change in trajectory at the time of disease invasion, usually followed by stagnant-to-declining growth rates (λ = 0.98) over the next 20 years. Disease-induced juvenile mortality imposed strong constraints on population growth that were not observed prior to disease introduction, even as adult survival returned to pre-invasion levels. Simulations suggested that models including persistent disease-induced mortality in juveniles qualitatively matched observed population trajectories, whereas models that only incorporated all-age disease events did not. We use these results to argue that pathogen persistence may pose a lasting, but under-recognized, threat to host populations, particularly in cases where clinical disease manifests primarily in juveniles.
authors.affiliation: Center for Infectious Disease Dynamics, Pennsylvania State University, 208 Mueller Lab, University Park, Pennsylvania, 16802, USA.
authors.forename: Kezia
authors.identifier: http://orcid.org/0000-0002-7200-5236
authors.initials: K
authors.lastname: Manlove
authors.affiliation: Idaho Department of Fish and Game, 3316 16th Street, Lewiston, Idaho, 83501, USA.
authors.forename: E Frances
authors.initials: EF
authors.lastname: Cassirer
authors.affiliation: U.S. Geological Society Northern Rocky Mountain Science Center, 2327 University Way, Suite 2, Bozeman, Montana, 59715, USA.
authors.forename: Paul C
authors.initials: PC
authors.lastname: Cross
authors.affiliation: Department of Microbiology and Immunology, Montana State University, 109 Lewis Hall, Bozeman, Montana, 59717, USA.
authors.forename: Raina K
authors.initials: RK
authors.lastname: Plowright
authors.affiliation: Center for Infectious Disease Dynamics, Pennsylvania State University, 208 Mueller Lab, University Park, Pennsylvania, 16802, USA.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Hudson
mesh_terms: D000818:Animals; D000835:Animals, Wild; D011157:Population Dynamics; D011158:Population Growth; D012756:Sheep; D012757:Sheep Diseases; D034581:Sheep, Bighorn
keywords: bighorn sheep; childhood disease; demographic trends; disease-induced mortality; integrated population model; pathogen persistence; population projection matrix; vital rates; wildlife disease