Petra K. Wallem

Pontifical Catholic University of Chile, CiudadSantiago, Santiago, Chile

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Publications (7)12.98 Total impact

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    ABSTRACT: Aim Ecological theory predicts that invasive ecosystem engineers like the American beaver (Castor canadensis) in Tierra del Fuego (TDF) affect landscape-level biodiversity and ecosystem function (BEF) when engineered habitats are novel or extensive. We tested these hypotheses on freshwater BEF, sampling benthic habitat and macroinvertebrates in natural lotic (forest and grassland streams) and natural lentic habitats (bogs, lakes) and beaver-modified lentic ecosystems (active and abandoned ponds). Location Tierra del Fuego Archipelago (Chile and Argentina). Methods To determine effects on patch-scale BEF, we assessed two drivers: substrate diversity (H′) and benthic organic matter standing crop (BOM, g m−2). Extent of impact was estimated as relative stream length (%) for each patch type in four 1000 ha images. Results The freshwater landscape was 56% free-flowing streams (natural lotic), 13% bogs and lakes (natural lentic) and 31% active and abandoned beaver ponds (beaver lentic). While engineering significantly modified lotic habitats (converting them to ponds), the beaver ponds were largely similar to natural lentic systems, but engineered lentic patches retained more BOM. While benthic biodiversity in beaver ponds was less than streams, the assemblage contained no habitat-specific taxa and was a subset of the natural lentic community. Main conclusions Invasive beavers engineer habitats whose biodiversity is similar to the landscape's natural lentic habitats, but by increasing the surface area and unit area retention of BOM via its impoundments, this invasion augments carbon standing stock approximately 72% in watersheds. While this invasion is considered the largest alteration to TDF's forested biome in the Holocene, here we discover that its impact is to ecosystem function, rather than biodiversity in the aquatic landscape.
    Diversity and Distributions 11/2013; · 6.12 Impact Factor
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    ABSTRACT: Background/Question/Methods South America’s sub-Antarctic ecoregion not only hosts the world’s southernmost forests, but also the largest extent of temperate wetlands, forests and ice fields south of the equator. Additionally, its remoteness, with no latitudinal replica in the Southern Hemisphere, has kept it isolated from most modern environmental threats including nutrient deposition and habitat fragmentation, leading it to be declared one of the world’s last wilderness areas. Nonetheless, the globalized phenomenon of introduced species does affect this region, and today at the extreme southern tip – in the Fuegian Archipelago – the terrestrial mammalian assemblage is dominated by exotics 2:1. Among the introduced fauna, the North American beaver is believed to cause the largest impact to this landscape in the Holocene. To assess this assertion, we compared beaver’s effects on stream benthic taxa richness, assemblage and production in natural lotic (forested and grassland streams) and lentic (bogs, lakes) habitats and compared them to disturbed habitats (streams affected by forestry management, beaver ponds and beaver meadows). We evaluated this invasion at the habitat and landscape-scales by a) calculating the extent of invasion (% impacted stream length) and b) modeling natural and engineered landscapes with regards to diversity and ecosystem function. Results/Conclusions Beavers have invaded 30-50% of the archipelago’s streams, causing the retention of organic material (+40-115%). At the patch-scale, impacts of exotic beavers were predictable based on studies in their native range with localized decreases in richness and increases in productivity. Forest harvesting with a riparian buffer had no detectable impact on benthic communities, while beaver engineering (ponds and meadows) significantly altered assemblages, compared to streams, but were not significantly different than those in natural lentic habitats. Therefore, influence on benthic assemblages at the landscape-scale was much less, but in contrast, the total effect on stream function significantly changed carbon dynamics. In conclusion, since engineered patches were similar to natural habitats (bogs and lakes), beavers did not significantly alter landscape-level community patterns, but significantly affected ecosystem function. At the same time, beaver introduction did constitute the largest modern impact to sub-Antarctic forests, but this affect is attenuated by the natural lentic features found in the Fuegian landscape. Understanding the underlying ecological mechanisms operating differentially at patch versus landscape scales and between community and ecosystem levels of organization allowed us to predict the effects of this species introduction, which demonstrates the power of ecology for management and conservation decision-making as well.
    96th ESA Annual Convention 2011; 08/2011
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    ABSTRACT: The North American (or Canadian) beaver (Castor canadensis) is the largest rodent native to North America (Figure 29.1), being found in an extensive range from northern Canada to northern Mexico. It has only one other extant congener, C. fiber, distributed originally throughout western and northern Eurasia. Both species affect extensive areas by 'engineering' stream and riparian habitat through their habits of dam building and cutting streamside vegetation. They were also both prized for their pelts, leading to near extinction in their native ranges. Castor canadensis was hunted heavily and locally extirpated throughout North America by the late 1800s; subsequent conservation and restoration efforts succeeded in reintroducing the species in much of its native range by the mid-to late 20th century (Naiman et al, 1988). Simultaneously, in the 1940s and 1950s, efforts in various countries sought to introduce North American beavers outside of their native range for the perceived commercial value of its fur, including countries in Europe (e.g. Finland, Poland, Austria and Russia), where the native C. fiber had been decimated, and Chile and Argentina, where no native species occupied a similar niche (Anderson and Valenzuela, 2011). Due to the unique nature of the introduction of beavers to southern South America, this chapter will focus on the socioecological role of this invasive exotic species in sub-Antarctic forests and the policy-research response of local and international managers and scientists. The sub-Antarctic archipelago, shared between Chile and Argentina (Figure 29.2), presents a paradox. On one hand, portions of this ecoregion are some of the least disturbed ecosystems on the planet (i.e. low human population density, highly intact native vegetation cover and over 50 per cent of its territory falling within the system of state protected areas; Mittermeier et al, 2003), while at the same time, it experiences pressing global environmental threats such as invasive alien species, climate change, the ozone hole and rapid commercial development, including tourism and salmon farming (Anderson et al, 2006a; Rozzi et al, 2006). The North American beaver has large impacts across the Tierra del Fuego and Cape Horn Archipelagos. The species was introduced in a single release of 25 pairs in 1946 by the Argentine government to Isla 29
    A handbook of global freshwater invasive species, Earthscan Press edited by R. Francis, 01/2011: chapter Building alliances between research and management to better control and mitigate the impacts of an invasive ecosystem engineer: the pioneering example of the North American beaver in the Fuegian Archipelago of Chile and Argentina.: pages 347-359; Earthscan Press., ISBN: 978-1-849-71228-6
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    ABSTRACT: The present study measures the resilience of riparian herbaceous communities to beaver invasion in subantarctic forests of southern Chile and Argentina. Divergence in community composition and spatial structure was measured comparing beaver-disturbed and undisturbed vegetation assemblages along a sequence of beaver meadow ages; the former by performing a Principal Component Analysis and the later by estimating a co-occurrence index (C-score). Community composition and spatial structure of vegetation showed an increasingly divergent trend from undisturbed sites to older beaver meadows. These results indicated that understory vegetation in deciduous subantarctic forests was not resilient to beaver invasion. Using “assembly rules” as a conceptual framework, we propose a resilience index of host communities to disturbances caused by herbivore invaders that also can be used for subsequent restoration programs to monitor the effectiveness of intervention and mitigation efforts.
    Biological Invasions 01/2010; 12(2):325-335. · 2.51 Impact Factor
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    ABSTRACT: Twenty-five pairs of North American beavers Castor canadensis Kuhl were introduced to Tierra del Fuego Island in 1946. The population has expanded across the archipelago, arriving at the Chilean mainland by the mid-1990s. Densities range principally between 0.5-2.05 colonies/km. They have an impact on between 30-50% of stream length and occupy 2-15% of landscape area with impoundments and meadows. Beaver impacts constitute the largest landscape-level alteration in subantarctic forests since the last ice age. 2. The colonization pattern, colony densities and impacted area indicate that habitat in the austral archipelago is optimal for beaver invasion, due to low predator pressure and suitable food resources. Nothofagus pumilio forests are particularly appropriate habitat, but a more recent invasion is occurring in adjacent steppe ecosystems. Nonetheless, Nothofagus repro- ductive strategies are not well adapted to sustain high beaver population levels. 3. Our assessment shows that at the patch-scale in stream and riparian ecosystems, the direction and magnitude of exotic beaver impacts are predictable from expectations derived from North American studies, relating ecosystem engineering with underlying ecological mechanisms such as the relationships of habitat heterogeneity and productivity on species richness and ecosystem function. 4. Based on data from the species' native and exotic range, our ability to predict the effects of beavers is based on: (i) understanding the ecological relationships of its engineering effects on habitat, trophic dynamics and disturbance regimes, and (ii) having an adequate compre- hension of the landscape context and natural history of the ecosystem being engineered. 5. We conclude that beaver eradication strategies and subsequent ecosystem restoration efforts, currently being considered in southern Chile and Argentina, should focus on the ecology of native ecosystems rather than the biology of this invasive species per se. Further- more, given the nature of the subantarctic landscape, streams will probably respond to restoration efforts more quickly than riparian ecosystems.
    Mammal Review 10/2008; 39(1):33 - 52. · 3.42 Impact Factor
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    ABSTRACT: RESUMEN Castor canadensis es considerado un ingeniero ecosistémico arquetípico, ya que altera el estado de factores bióticos y abióticos, mediante interacciones no tróficas, modificando sustancialmente los ecosistemas que habita. Esta especie fue introducida en la Isla Grande de Tierra del Fuego en 1946 por el gobierno argentino, con el fin de estimular la industria peletera. Actualmente se encuentra presente en las principales islas del archipiélago, incluyendo Tierra del Fuego, Navarino, Dawson y Hoste y ocupando hábitat tan contrastantes como los bosques subpolares magallánicos, el matorral y la estepa patagónica. Su éxito en estos hábitat sugiere que su expansión solo se verá limitada por la disponibilidad de recursos hídricos. De acuerdo a lo observado en Tierra del Fuego, en ausencia de un programa de control poblacional, los castores amenazan con establecerse en todos los bosques templados nativos del continente (entre los 35 y 55º S) y ocupar los hábitat de matorral y pastizal circundantes a los mismos. El presente artículo propone tres hipótesis referentes a los mecanismos subyacentes al éxito de esta invasión: el escape de enemigos naturales, la oportunidad de recursos, y la autofacilitación mediante interacciones no tróficas. Los antecedentes disponibles develan patrones conductuales (e.g., el establecimiento de colonias en forma independiente de la cercanía de recursos maderables, un aparente crecimiento poblacional irruptivo, así como una explotación centrada en Nothofagus pumilio, especie dominante en los bosques magallánicos) que sugieren la operación de los mecanismos de escape de enemigos naturales y oportunidad de recursos. También se observa una mayor densidad de colonias activas en hábitat que presentan mayor extensión de modificación ambiental, sugiriendo el accionar del mecanismo de autofacilitación. Si bien la información reunida no permite poner a prueba las hipótesis propuestas, sí entrega un marco conceptual sobre el cual se pueden desarrollar futuras investigaciones referentes a esta invasión. ABSTRACT
    Revista Chilena De Historia Natural - REV CHIL HIST NAT. 01/2007; 80(3).
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    ABSTRACT: Castor canadensis is considered an archetypical ecosystem engineer, which modifies the state of biotic and abiotic factors through non-trophic interactions. This species was introduced by the Argentinean government into Tierra del Fuego island in 1946, and subsequently colonized autonomously the neighboring islands of Navarino, Dawson and Hoste. Currently this invader occupies contrasting ecosystems such as the Magellan subpolar beech forest and Patagonian scrub and steppe. This ability to colonize contrasting habitats suggests that beaver expansion will be limited mainly by hydrological resources, threatening to colonize the complete extent of temperate beech forests on the mainland (from 35 to 55° S). The present review proposes three hypotheses regarding the underlying mechanisms to this successful invasion: natural enemy escape, resource opportunities, and self-facilitation through non-trophic interactions. Current knowledge regarding beaver colonization and foraging behavior (e.g., habitat selection independently of forest availability, irruptive population growth, and apparent selective exploitation of Nothofagus pumilio, dominant species in the Magellan forest) suggests that enemy escape and resource opportunity are the main mechanisms underlying this invasion. The observation of higher densities of active colonies, where the extent of beaver habitat modification is larger, suggests that self-facilitation may be relevant to the success of this invasion. Current information does not allow testing these hypotheses, but it provides a framework to develop future investigations regarding this invasion in Tierra del Fuego.
    Revista chilena de historia natural 01/2007; 80(3):309-325. · 0.93 Impact Factor