Relics of beavers past: time and population density drive scale‐dependent patterns of ecosystem engineering

Abstract

Like many ecological processes, natural disturbances exhibit scale-dependent dynamics that are largely a function of the magnitude, frequency and scale at which they are assessed. Ecosystem engineers create patch-scale disturbances that affect ecological processes, yet we know little about how these effects scale across space or vary through time.Here, we investigate how patch disturbances by beavers Castor canadensis, ecosystem engineers renowned for their pond-creation behavior, affect ecological processes across space and time. We evaluated how beaver population recovery influenced surface water dynamics in relation to population density over 70 years across multiple spatial scales (pond, watershed and regional) in northern Minnesota. Surface water area was positively related to population density at the watershed scale; however, despite variation in beaver densities (and therefore surface water area) at the watershed scale, regional-scale surface water area was stable through time. This stability appears to have been driven by asynchronous beaver density fluctuations among watersheds, combined with the increasing importance of abandoned ponds. Beavers initially created and occupied larger ponds with greater surface water area, but through time shifted towards occupying smaller ponds. As ponds accumulated on the landscape proportionally more surface water was stored within abandoned ponds, which offset the smaller size of occupied ponds. Beaver engineering -driven by density-dependent mechanisms and the legacy effects from abandoned ponds -not only follows general patterns of patch disturbance dynamics by creating a spatial mosaic of patches, but the organism-created mosaic also appears to generate ecological stability at greater spatial scales. We suggest restoring beavers to landscapes is a viable method for increasing surface water storage and will ultimately help advance numerous conservation and rewilding objectives. Our study demonstrates that ecosystem engineering effects can be scale-dependent, indicating researchers should evaluate the ecological impact of engineers across diverse spatiotemporal scales to fully understand their functional roles in ecosystems.

Foxes engineer hotspots of wildlife activity on the nutrient-limited Arctic tundra

“…The lush vegetation that grows on Arctic fox dens (Bruun et al 2005, Gharajehdaghipour et al 2016, Fafard et al 2020) continues to indirectly affect the spatial distribution of caribou even if no foxes are present at the dens, demonstrating the long-term effects that fox ecosystem engineering has on other wildlife. Our findings that fox ecosystem engineering affects wildlife differently through these short- and long-term pathways adds to other studies that have demonstrated ecosystem engineering effects may be scale-dependent (Ferry et al 2020, Johnson-Bice et al 2022).…”

A Biodiversity Boost From the Eurasian Beaver (Castor fiber) in Germany’s Oldest National Park

“…At the landscape scale, the presence of different successional stages of beaver patches can result in increased species diversity (Bush et al, 2019;Nummi et al, 2021). Particular long-term beaver presence can add a further level of complexity to the landscape, as the strength of its engineering effects seems to be time and density dependent (Johnson-Bice et al, 2022).…”

“…For these reasons, beavers have been suggested as a "restoration tool" to promote the recovery of riparian environments and to increase landscape heterogeneity through creation of new ponds (Johnston and Naiman, 1990;Law et al, 2017). Moreover, a mosaic of old, new and abandoned ponds creates a dynamic mosaic of patches with different ecological features at the landscape scale (Kivinen et al, 2020;Johnson-Bice et al, 2022).…”

Exploring the dynamics of flow attenuation at a beaver dam sequence

“…As beaver populations expand, family groups will abandon territories, leaving dams unmaintained. Where these dams have previously persisted for some time, they are often stabilized by vegetation (Johnson‐Bice et al, 2022 ; Pollock et al, 2014 ). Many of these dams can therefore remain in the landscape and continue to exert strong controls over surface water storage/flow routing at catchment and regional scales after the abandonment of territories (Johnson‐Bice et al, 2022 ).…”