Adrián Schiavini

The regulation of wild populations influenced by density (where changes in abundance induce competitive interactions that affect vital rates) is typically assessed by collecting time series of population counts. In the case of mammals, whether terrestrial or marine, density‐dependent changes in vital rates can occur in an apparent theoretical sequence, affecting population persistence. However, evaluations based on changes in vital rates of invasive populations completely removed (complementing population regulation studies) are less common. Understanding how these changes occur in biological invasion contexts can improve management decisions for invasive species populations. The North American beaver ( Castor canadensis ) is a territorial semiaquatic rodent. Harvested populations have lower densities and higher beaver kit proportions. Introduced to Tierra del Fuego, beavers invaded all watercourses. To assess whether differences in population density and/or vital rates induced by hunting align theoretically with density‐dependent population regulation, we analysed using a descriptive‐comparative approach the effects of various population management histories on 1121 invasive beavers ( C. canadensis ) eradicated between 2016 and 2018 from seven areas in Tierra del Fuego (Argentina) on their density, survivorship, reproductive value, generation time, breeding onset and age distributions. The areas with historical management (with sustained hunting) and one area that was reinvaded after being previously cleared in 2014 (reinvaded post‐eradication) exhibited the highest densities and vital rates consistent with a relaxation of density dependence: age distribution with a high proportion of beaver kits, shorter generation times, high reproductive value and lower survivorship compared to areas with null management (without previous hunting). Areas with null management presented vital rates reflecting density dependence: higher survivorship of young, delayed breeding onset, lower offspring production, higher survivorship of adults and low proportion of beaver kits. Practical implication . At the local level, the density dependence on vital rates of invasive beavers could be driven, at least in part, primarily by the effect of management history rather than by population density. Continuous but irregular management over time will increase density and exacerbate spatial damage caused by beavers, while leaving previously unmanaged areas alone may result in relatively low densities but eventually achieve density‐dependent vital rates, thereby compromising environmental restoration efforts.

The population regulation of animals depends on fertility and survivorship. Hunting can affect population growth through changes in reproduction and survivorship. Understanding these changes in vital rates is crucial for the control of invasive mammals. With the objective to assess the influence of different population removal pressures on the finite population growth rate (𝝀) of an invasive mammal, this research used demographic data estimated from 922 invasive beavers (Castor canadensis) eradicated from seven areas from Isla Grande de Tierra del Fuego, Argentina. Areas were classified into four removal pressures: higher, high‐intermediate, low‐intermediate, and lower removal pressure. The hypothesis states that the higher removal pressure produces higher population growth and resilience. Leslie matrix models were used to estimate 𝝀, generation time, damping ratio, stable age distribution, reproductive value, and the sensitivity–elasticity of survivorship and fertility of the females for each removal pressure. The 𝝀 and damping ratio increased with the removal pressure, as expected; the inverse occurred with the generation time. The survivorship of the younger age classes was the most sensitive and elastic vital rate regardless of removal pressure, followed by the breeding onset. The fertility was less relevant on 𝝀. The beaver population would depend more on the survival of kits, 1‐year‐olds, and subadults, than the number of offspring produced. A management strategy is proposed focusing on affecting the survivorship of younger age classes, eliminating first the adult animals and then the younger beavers.

Conflicts between people and wildlife have become widespread as people move to areas previously home to wildlife and as wild populations recover. In Patagonia, one of the main threats to guanacoLama guanicoeconservation is the animosity of sheep ranchers towards the species. As key stakeholders in guanaco conservation we assessed ranchers’ perceptions regarding guanaco abundance in Isla Grande de Tierra del Fuego, Argentina. We contrasted these perceptions with estimated guanaco abundance and explored the socio-ecological factors influencing perceptions and how perceptions of overabundance are rooted in contextual factors rather than actual abundance. We performed semi-structured interviews with ranchers from Isla Grande and estimated guanaco abundance independently using density surface modelling. Ranchers were divided into three categories depending on their perception of guanaco abundance: ‘too many’, ‘many’ and ‘normal’. Those in the ‘many’ and ‘normal’ categories perceived guanaco abundance as being similar to actual abundance, whereas those in the category ‘too many' overestimated guanaco abundance. The perceived issues affecting livestock production varied between categories, although feral dogs emerged as the main problem. Negative perceptions of the guanaco stemmed from ranchers' beliefs that the species reduces forage availability for livestock, and from their disappointment about the government's handling of concerns regarding livestock production. Greater understanding and integration of the human dimension in conservation are needed to design more inclusive and resilient management plans.

The North American beaver (Castor canadensis) has been an invasive species in the Tierra del Fuego archipelago since its intentional introduction in 1946. Its activities have led to such severe impacts on various environments that eradication has been tested as a management strategy. We assessed how vital rates (i.e., survivorship and reproduction) of the beavers were affected by time since invasion and population management history (such as lethal control). We constructed vertical life tables and compared survivorship, age‐specific survivorship, generation time, and life expectancy for different landscapes and management histories. As part of an eradication pilot program conducted between 2016–2018, 922 beavers were removed in seven pilot areas of the Isla Grande de Tierra del Fuego, Argentina. Areas in which beavers have resided longest since invasion, areas without hunting pressure, and areas previously cleared of beavers had higher survivorship and longer generation time and life expectancy. Areas where beaver management was historical (trapping performed by the provincial government) or focused (trapping by landowners) had shorter life expectancy and generation time and advanced age at first reproduction. Beavers showed demographic plasticity in relation to time since invasion (increasing survivorship, generation time, life expectancy, and cohort life expectancy consistent with populations close to carrying capacity in areas with low or null management). They also exhibited plasticity in response to management histories (advanced breeding onset, reduced survivorship, and life expectancy in managed areas—similar to managed populations in their native range). We present management implications from demographic perspective for a future eradication or control program of beavers. We also highlight the importance of landowners in shaping beaver demography for the future management of the species. Beaver demography reacts dynamically and rapidly to local removal, creating challenges for management based on the removal of beavers.

Spatially explicit modelling allows to estimate population abundance and predict species’ distribution in relation to environmental factors. Abiotic factors are the main determinants of a herbivore´s response to environmental heterogeneity on large spatiotemporal scales. We assessed the influence of elevation, geographic location and distance to the coast on the seasonal abundance and distribution of guanaco (Lama guanicoe) in central Tierra del Fuego, by means of spatially explicit modelling. The estimated abundance was 23,690 individuals for the non-breeding season and 33,928 individuals for the breeding season. The factors influencing distribution and abundance revealed to be the elevation for the non-breeding season, and the distance to the coast and geographic location for the breeding season. The southwest of the study area presented seasonal abundance variation and the southeast and northeast presented high abundance during both seasons. The elevation would be the driving factor of guanaco distribution, as individuals move to lower areas during the non-breeding season and ascend to high areas during the breeding season. Our results confirm that part of the guanaco population performs seasonal migratory movements and that the main valleys present important wintering habitats for guanacos as well as up-hill zones during summer. This type of study would help to avoid problems of scale mismatch and achieve better results in management actions and is an example of how to assess important seasonal habitats from evaluations of abundance and distribution patterns.

Understanding the demography of an invasive species is crucial to better guide managers seeking to slow the spread of the invader. Habitat differences can affect demographic rates, which may in turn impact the speed of the invasion, but this has been rarely addressed. We studied the demography of invasive North American beavers (Castor canadensis) in 2 contrasting habitat types of the island, forest and steppe, on Tierra del Fuego in southern Patagonia. We used repeated observations, mark-resight methods, telemetry, and camera traps to estimate colony size and demographic rates of beavers in the 2 habitats. Colony size and the number of offspring (“kits”) produced per colony per year were higher in the steppe, contrary to the belief that forest is better habitat. This may be the result of the longer time since invasion in the forests of Tierra del Fuego and that the forest subpopulation is showing density-dependent regulation. Survival of beavers was high in all age classes and was higher than survival rates recorded in North America. Our work shows that plasticity of habitat use and predator release have likely facilitated beaver invasion in Patagonia. The higher productivity and detectability of beavers in the steppe call for active management in a habitat previously assumed to be subprime. Entender la demografia de las especies invasoras es crucial para guiar a los gestores que intentan frenar la expansión de los invasores. Las diferencias en el tipo de hábitat pueden afectar las tasas demográficas, lo que a su vez puede impactar en la velocidad de la invasión, pero esto ha sido raramente investigado. Estudiamos la demografía de castores norteamericanos invasores (Castor canadensis) en dos hábitats contrastantes, bosque y estepa, en la isla de Tierra del Fuego en el sur de Patagonia. Utilizamos observaciones repetidas, métodos de marcado-reavistaje, telemetría y cámaras trampa para estimar el tamaño de colonia y las tasas demográficas de castores en los dos tipos de hábitat. El tamaño de colonia y el número de crías producido por colonia por año fue mayor en la estepa, contrario a la creencia de que el bosque es un mejor hábitat. Creemos que este puede ser el resultado de un mayor tiempo de invasión en los bosques de Tierra del Fuego y que la subpoblación del bosque está mostrando regulación denso-dependiente. La supervivencia de castores fue alta en todas las clases de edad y fue más alta que las tasas de supervivencia registradas en Norteamérica. Nuestro trabajo muestra que la plasticidad en el uso de hábitat y la ausencia de depredadores han probablemente facilitado la invasion de castores en Patagonia. La alta productividad y detectabilidad de castores en la estepa llaman a un control activo en un hábitat que antes había sido considerado subóptimo.

Commerson’s dolphins (Cephalorhynchus c. commersonii) and Peale’s dolphins (Lagenorhynchus australis) are two of the most common species of cetaceans in the coastal waters of southwest South Atlantic Ocean. Both species are listed as Data Deficient by the IUCN, mainly due to the lack of information about population sizes and trends. The goal of this study was to build spatially explicit models for the abundance of both species in relation to environmental variables using data collected during eight scientific cruises along the Patagonian shelf. Spatial models were constructed using generalized additive models. In total, 88 schools (212 individuals) of Commerson’s dolphin and 134 schools (465 individuals) of Peale’s dolphin were recorded in 8,535 km surveyed. Commerson’s dolphin was found less than 60 km from shore; whereas Peale’s dolphins occurred over a wider range of distances from the coast, the number of animals sighted usually being larger near or far from the coast. Fitted models indicate overall abundances of approximately 22,000 Commerson’s dolphins and 20,000 Peale’s dolphins in the total area studied. This work provides the first large-scale abundance estimate for Peale’s dolphin in the Atlantic Ocean and an update of population size for Commerson’s dolphin. Additionally, our results contribute to baseline data on suitable habitat conditions for both species in southern Patagonia, which is essential for the implementation of adequate conservation measures.