Northwest Forest Plan—the first 25 years (1994–2018): status and trends of northern spotted owl habitats

Abstract

This is the fourth in a series of periodic monitoring reports on status and trends of forests used by northern spotted owls (Strix occidentalis caurina; NSO) for nesting, roosting, and dispersal on federally administered lands within the Northwest Forest Plan (NWFP) area (NSO range in the United States) since its implementation in 1994. The objective of this monitoring is to determine if federal forest lands are providing sufficient conservation of forests that are important elements of NSO habitat, and thus populations. Here we present models on the amount, distribution, and spatial arrangement of nesting, roosting, and dispersal forest types across the NWFP area, and losses and gains resulting from disturbance and forest succession, respectively. Forests suitable for nesting and roosting are one of the most critical components of NSO habitat, defined as the area with the full range of environmental conditions necessary to support occupancy, survival, and reproduction. Given the importance of habitat to support population recovery, this is the first monitoring report to model habitat using distribution and amount of suitable nesting/roosting forest at multiple spatial scales, and in combination with abiotic environmental covariates. We used estimates of territory occupancy in conjunction with available habitat to estimate changes in population size and distribution of territorial NSOs on federal lands at the start and end of this monitoring cycle (1993 and 2017). We found a 3 percent net increase (from 8,890,500 to 9,155,700 ac) of nesting/roosting forest on federal lands between 1993 and 2017. This net gain occurred despite gross losses from wildfire of 7.9 percent (703,700 ac), 2.9 percent from timber harvest (257,700 ac), and 0.9 percent from insects or other causes (83,700 ac), indicating that processes of forest succession more than compensated for the losses resulting from disturbance during the first 25 years of the NWFP. Dispersal forest on federal lands increased by 1 percent, but dispersal-capable landscapes decreased by 9 percent because of forest losses on surrounding nonfederal lands and large wildfires on federal lands. The forest landscape that allowed for owl movement between one reserved area to another became more confined and fragmented. Despite net increases in NSO forests on federal lands during the monitoring period, the population of territorial owls on federal lands decreased by an estimated 61.8 percent. A primary cause for population declines on federal lands was displacement from native habitat by the invasive barred owl (S. varia), which highlights the increasing threat to NSO persistence.