High densities of large herbivores rapidly disrupt ecosystem integrity

Data and code repository of the manuscript (See README file): High densities of large herbivores rapidly disrupt ecosystem integrity Human-induced high densities of large wild herbivores may pose a threat to natural ecosystems, modifying biodiversity and ecosystem functioning. Although the negative impacts on individual taxa and processes are well documented, a comprehensive understanding of how ecosystem integrity responds to high herbivore densities remains limited. This study addresses this gap through an extensive herbivore-density manipulation experiment conducted in two Mediterranean woodlands. We established two scenarios of high red deer (Cervus elaphus) density, representative of some protected areas (~30 individuals·km²) and intensively managed hunting estates (~90 individuals·km²). We monitored 38 metrics encompassing a broad spectrum of biodiversity and functional responses to synthesize the short-term impacts of high herbivore densities. We revealed an overall 13% decline in ecosystem integrity after one year of exposure to high herbivore densities, with particularly widespread impacts on functional responses and detrimental effects on biodiversity components. We detected rapid declines in plant diversity and a positive response of epigeic invertebrate orders. Conversely, ants (analyzed separately), birds, and small mammals remained mostly unaffected. Functional responses were consistently impaired, including declines in plant regeneration and physiological performance, simplified pollination networks, degraded soil structure, and increased disease vectors. These findings show that human-induced high herbivore densities rapidly create conditions that disrupt ecosystem integrity, reveal early signs of ecological degradation, and underscore the urgent need for population regulation. Jorge Isla1, Emmanuel Serrano2, Juan A. Calleja3, Carlos Hernández-Castellano4, Rafael Alcalá-Herrera1, Elena Albanell5, Carlos Alonso6, Elena Baraza7, Matthew Brolly8, Niall Burnside9, Ana Burón-Ugarte10, Miquel Capó11, Jesús Cardells12, João Carvalho13, Macarena Cuerdo3, David G. del Olmo3, Xavier Espadaler14, Iolanda Filella15, Laura Fuentes-Moyano2, Daniel Gambra1, Alba Gómez1, Jennifer A. Krumins16, Santiago Lavín-González2, Victor Lizana12, Aida López-Sánchez1,17, María Martínez-Jauregui18, Helena Martínez-Torres2, Elena Parra-Quesada2, Marta Peláez1, Josep Peñuelas15, Carmen Rello1, Jesús Renedo6, Carlos Rodríguez-Vigal20, Mariana Rossa13, Alfonso San Miguel1, Irene Torres-Blas2, Daycy Valladares-Pérez2, José C. Wenzel-Vergas1, Ramón Perea1. 1- Plant and Animal Ecology Lab (PAELLA), Centro para la Conservación de la Biodiversidad y el Desarrollo Sostenible (CBDS), Universidad Politécnica de Madrid, 28040, Madrid, Spain. 2- Wildlife Ecology & Health Group (WE&H) and Servei d’Ecopatologia de Fauna Salvatge (SEFaS), Universitat Autònoma de Barcelona (UAB), Barcelona, Spain. 3- Departamento de Biología (Botánica); Centro de Investigación en Biodiversidad y Cambio Global (CIBC-UAM), Universidad Autónoma de Madrid, 28049, Madrid, Spain. 4- Department of Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Prague, Czech Republic. 5- Group of Research in Ruminants (G2R), Department of Animal and Food Sciences, Universitat Autònoma de Barcelona, 08193, Cataluña, Spain. 6- Grupo de Investigación Hidrobiología, Centro para la Conservación de la Biodiversidad y el Desarrollo Sostenible (CBDS), Universidad Politécnica de Madrid, Madrid, Spain. 7- Research Group of Botany (INAGEA), Universitat de les Illes Balears, 07122, Illes Balears, Spain. 8- Centre for Environment and Society, School of Applied Sciences, University of Brighton, United Kingdom 9- Scientific Robotics Academy (SAMS), Oban, United Kingdom 10- Research Line Terrestrial Ecosystems, Basque Centre for Climate Change (BC3), País Vasco, Spain 11- Research Group of Botany on Mediterranean Islands, Universitat de les Illes Balears, 07122, Illes Balears, Spain 12- Servicio de Análisis, Investigación y Gestión de Animales Silvestres (SAIGAS), Universidad Cardenal Herrera-CEU, Valencia, Spain 13- Centre for Environmental and Marine Studies (CESAM); Department of Biology, University of Aveiro, 3810-193, Aveiro, Portugal 14- CREAF, Universitat Autònoma de Barcelona, Cataluña, Spain 15- Global Ecology Unit (CREAF-CSIC-UAB), 08193, Cataluña, Spain 16- Department of Biology, Montclair State University; Pinelands Research Institute, New Jersey, USA 17- Grupo TEMSUS, Universidad Católica de Ávila, Castilla y León, Spain 18- Instituto de Ciencias Forestales (ICIFOR, INIA-CSIC), 28040, Madrid, Spain; Instituto Universitario de Gestión Forestal Sostenible (iuFOR), Universidad de Valladolid, 34004, Castilla y León, Spain Centro Quintos de Mora, Organismo Autónomo Parques Nacionales, Ministerio para la Transición Ecológica y el Reto Demográfico, Madrid, Spain