Post-establishment changes in habitat selection by an invasive species: beavers in the Patagonian steppe

Abstract

Changes in habitat use over the course of a biological invasion may influence the fraction of the landscape that is ultimately affected by the invader. However, this intermediate stage of invasion has been less studied than the initial or final stages. Here, we investigated the recent invasion by an ecosystem engineer, the American Beaver (Castor canadensis), in an area of the Patagonian steppe. We utilized repeated high resolution satellite images to identify beaver ponds, and used them to study changes in beaver abundance and habitat use over time. The number of beaver ponds increased 85 % between 2005 and 2014. During this period, beavers changed their habitat selection pattern, presumably as a response to increased density. Beavers established on small watercourses in canyons first, but as more canyons became occupied over time, beavers moved to less preferred watercourses in plains and U-shaped valleys. Potential new beaver colonies established close to existing beaver ponds, suggesting proximity to a beaver pond is an important determinant of beaver colonization. Identifying habitat preferred by beavers in the steppe, could help to increase early detection of the invader at the invasion front. Our work highlights the importance of the use of high resolution remote sensing technologies to better understand and control biological invasions.

Mapping the status of the North American beaver invasion in the Tierra del Fuego archipelago

“…In addition, beavers also prefer areas that are not too sloped (>15˚), and avoid semi-arid conditions (>600 mm.yr -1 EVTP). Pietrek and González-Roglich [36] also noted ecological thresholds in the steppe ecosystem, where beavers probably changed their habitat use patterns as a trade-off for the increased density. In the Northern Hemisphere, it was also noted that the habitat requirements largely determined the distribution and abundance of this species [37], where beavers were associated with forests and some open ecosystems [38].…”

“…Unfortunately, for this study, detailed information about the species composition (e.g., understory plants) or ecosystem types (e.g., shrubland or grassland types) was not available for the whole archipelago at a local level, which limited larger scale knowledge about beaver preferences. The shrub vegetation of the steppe and ecotone may provide enough material for dam construction to all beaver presence in steppe [36] or even in habitats such as tundra [29], which would suggest that vegetation areas without forest can provide substitute environments. In the steppe, Pietrek and González-Roglich [36] showed that beavers first settled in optimal environmental conditions (e.g., watercourses in canyons) and then use other habitats that were initially less preferred, as the numbers of beaver ponds in the study area increased.…”

“…The shrub vegetation of the steppe and ecotone may provide enough material for dam construction to all beaver presence in steppe [36] or even in habitats such as tundra [29], which would suggest that vegetation areas without forest can provide substitute environments. In the steppe, Pietrek and González-Roglich [36] showed that beavers first settled in optimal environmental conditions (e.g., watercourses in canyons) and then use other habitats that were initially less preferred, as the numbers of beaver ponds in the study area increased. The temperature affected the beaver dam density change, and its suitability at temperatures below <5˚C was less than that at >5˚C.…”

Direct and Indirect Interactions between Landscape Structure and Invasive or Overabundant Species

“…For example, landscape associations changed markedly over the course of the invasion of the Patagonian steppe by beaver (Castor canadensis) [10]. Small waterways in canyons initially facilitated their establishment, but as beaver populations grew, colonization was better predicted by the presence of plains, U-shaped valleys, and the proximity to existing beaver ponds [11]. Thus, the factors that facilitate species invasion may differ from those which predict establishment and potential distribution.…”

“…In some cases, these changes occur by virtue of the high densities of the offending species, but these impacts can also be disproportionate to species' biomass. For example, beaver are well known for their role as ecosystem engineers in North America, but their introduction to South America has hugely affected Patagonian steppe landscapes by causing an 85 % increase in ponds from 2005 to 2014 [11], and concomitant changes in landscape processes and species distribution [77,78]. Many ecosystem engineers are underrecognized for their roles.…”