Forecasting the cumulative effects of multiple stressors on breeding habitat for a steeply declining aerial insectivorous songbird, the olive-sided flycatcher (Contopus cooperi)

To halt ongoing loss in biodiversity, there is a need for landscape-level management recommendations that address cumulative impacts of anthropogenic and natural disturbances on wildlife habitat. We examined the cumulative effects of logging, roads, land-use change, fire, and bark beetle outbreaks on future habitat for olive-sided flycatcher (Contopus cooperi), a steeply declining aerial insectivorous songbird, in Canada's western boreal forest. To predict the occurrence of olive-sided flycatcher we developed a suite of habitat suitability models using point count surveys (1997–2011) spatially- and temporally-matched with forest inventory data. Flycatcher occurrence was positively associated with small (~10 ha) 10–20-year-old clearcuts, and with 10–100% tree mortality due to mountain pine beetle (Dendroctonus ponderosae) outbreaks, but we found no association with roads or distance to water. We used the parameter estimates from the best-fit habitat suitability models to inform spatially explicit state-and-transition simulation models to project change in habitat availability from 2020 to 2050 under six alternative scenarios (three management × two fire alternatives). The simulation models projected that the cumulative effects of land use conversion, forest harvesting, and fire will reduce the area of olive-sided flycatcher habitat by 16–18% under Business As Usual management scenarios and by 11–13% under scenarios that include protection of 30% of the land base. Scenarios limiting the size of all clearcuts to ≤ 10 ha resulted in a median habitat loss of 4–6%, but projections were highly variable. Under all three management alternatives, a 50% increase in fire frequency (expected due to climate change) exacerbated habitat loss. The projected losses of habitat in western boreal forest, even with an increase in protected areas, imply that reversing the ongoing population declines of olive-sided flycatcher and other migratory birds will require attention to forest management beyond protected areas. Further work should examine the effects of multiple stressors on the demographic mechanisms driving change in aerial insectivore populations, including stressors on the wintering grounds in South America, and should aim to adapt the design of protected areas and forest management policies to projected climate-driven increases in the size and frequency of wildfires.

为遏制当前的生物多样性丧失，亟需制定景观尺度的管理方案，以应对人为与自然干扰对野生动物栖息地的累积影响。本研究以加拿大西部北方针叶林内的橄榄色捕蝇鸟（olive-sided flycatcher, *Contopus cooperi*）——一种种群数量急剧下降的空中食虫鸣禽为研究对象，探究了伐木、道路建设、土地利用变化、火灾以及树皮甲虫暴发对其未来栖息地的累积效应。为预测橄榄色捕蝇鸟的出现概率，本研究基于1997至2011年的样点计数调查数据，结合时空匹配的森林清查数据，构建了一系列栖息地适生模型。研究结果显示，捕蝇鸟的出现概率与面积约10公顷、林龄10-20年的皆伐迹地，以及因山松甲虫（*Dendroctonus ponderosae*）暴发导致10%-100%树木死亡的区域呈显著正相关，但未发现其与道路距离或距水源的远近存在关联。本研究利用最优适生栖息地模型的参数估计值，驱动空间显式状态转移模拟模型，预测了2020年至2050年在6种不同情景（3种经营方案×2种火灾情景）下的栖息地面积变化。模拟结果表明，在常规经营情景下，土地利用转换、森林采伐与火灾的累积效应将使橄榄色捕蝇鸟的栖息地面积减少16%-18%；在保护30%陆地面积的情景下，栖息地面积减少幅度为11%-13%。若将所有皆伐迹地的面积限制在10公顷以内，生境损失中位数仅为4%-6%，但该情景的预测结果存在较高变异性。在全部3种经营方案中，火灾频率提升50%（预计由气候变化引发）均会加剧生境损失。即便通过扩大保护区面积，西部北方针叶林的栖息地仍面临预估的损失，这意味着扭转橄榄色捕蝇鸟及其他候鸟的种群数量下降趋势，需关注保护区以外的森林管理措施。未来研究应进一步探究多重胁迫因子对空中食虫鸣禽种群动态变化机制的影响，包括南美洲越冬地的胁迫因子，并致力于调整保护区设计与森林管理政策，以应对气候变化驱动下野火规模与发生频率的上升趋势。