Impacts of development on boreal birds

Some of the most notorious photos of oil and gas development in the boreal forests of Alberta include the surface mines for oil sand deposits. However, the cumulative area occupied by these mines is minor proportion of oil and gas development’s footprint in northern Alberta. In most of northern Alberta where oil and gas exploration and development occurs, the landscape is crisscrossed by thousands of seismic lines hundreds of kilometers long. Areas where seismic exploration pinpoints promising oil or gas deposits may be further developed when pipelines and wells are constructed, or when seismic lines are widened and converted to roads. Even when seismic lines are not developed further, they may continue to be used by ATV riders.

In contrast to surface mining, relatively little boreal forest is destroyed or converted for individual seismic lines, pipelines, wells, and access roads. For example, older seismic lines (up to 50 years old) may be as little as 6-10 m wide and newer techniques permit the construction of seismic lines that are just 2-3 m wide. Individual gas wells, including the surrounding cleared land are approximately 1 hectare in extent. Much more habitat may be lost as the density of seismic lines, pipelines and wells increases. However, separate from habitat loss, a bigger environmental issue with the construction of thousands of km of seismic lines, pipelines, and roads is the enormous increase in the amount of habitat edges, which are sharp transitions between one type of vegetation and another type of vegetation.

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photo by Hedwig Lankau

(photos by Hedwig Lankau)

Traditionally, creation of habitat edges was seen as beneficial for wildlife and was encouraged by wildlife managers throughout most of the 20th Century. In the 1970’s, however, some ornithologists noted that while many forest birds were attracted to forest-farmland edges in eastern North America, possibly because of increases in resources like dense understory vegetation or insect prey, nest predators and brood parasites like Brown-headed Cowbirds were also drawn to the same edges. The result was what is described as an ecological trap, in which organisms (here, forest songbirds) were drawn into a habitat that is actually bad for them, resulting in lower reproductive success (Gates and Gysel 1978). With further study and growing concern about habitat destruction and fragmentation in the 1980’s and 1990’s, more negative edge effects were documented or hypothesized as biologists became more aware of edge-sensitive or area-sensitive species that needed large undisturbed core areas of forest away from edges to breed successfully.

A palm warbler watches field researchers in Northest Territories. photo by Hedwig Lankau

A palm warbler watches field researchers in Northest Territories.
(photo by Hedwig Lankau)

While concerns about habitat fragmentation and area-sensitive species remain, the overall impact of the amount of edges on species has swung back from negative to a mixture of positive and negative effects. The strength and direction of edge effects on species abundance and reproductive success has been found to depend strongly on landscape, the dominant vegetation type in the landscape, and the types of vegetation comprising the edge. For example, the edge effect of increased nest predation and lower nesting success along forest edges is thought to be stronger in forest-farmland landscapes, but may be relatively small in boreal forest landscapes fragmented by logging (Bayne and Hobson 1997) or energy sector developments (Ball et al. 2009).

The Bayne and Boutin research groups at the University of Alberta have been studying the impacts of oil and gas development on wildlife, including effects of habitat edge on boreal forest birds and mammals for many years.

Yellow rumped warbler photo by Hedwig Lankau

Yellow rumped warbler
(photo by Hedwig Lankau)

Impacts of edges associated with pipelines, roads, and seismic lines have been well-studied for predators like wolves and herbivores like moose, white-tailed deer, and woodland caribou. Increases in edge habitats along pipelines, roads and seismic lines are associated with more moose and white-tailed deer (prey for wolves), more wolves (predators of caribou, as well as moose and deer), and fewer woodland caribou. As a result, most woodland caribou populations have declined and are at risk of extinction in areas with heavy concentrations of seismic lines and roads.

Edge effects due to energy sector development on most boreal forest bird species in Alberta are not as clear. One species that has been well studied by the Bayne research group has been the Ovenbird (Seiurus aurocapillus), a small migratory songbird in the wood-warbler family (Parulidae). In eastern North America, Ovenbirds appear to be negatively influenced by increasing amounts of forest edge, in that their preferred foods (leaf-litter invertebrates) were less abundant at forest edges than in forest interior (Burke and Nol 1998). In some boreal forest studies, Ovenbirds also decrease in abundance closer to edges. Leaf-litter invertebrate abundance for Ovenbirds may also be lower along boreal forest edge than deeper within boreal forests (Van Wilgenburg et al. 2001). Pairing success of Ovenbird males, nesting success, and return rates of individual Ovenbirds were lower in boreal forests fragmented by logging or agriculture than for Ovenbirds in contiguous boreal forests, but pairing success rates in forest fragments were higher than for Ovenbirds in forest fragments in eastern North America (Bayne and Hobson 2001, 2002).

Ovenbird photo by Hedwig Lankau

Ovenbird
(photo by Hedwig Lankau)

With respect to boreal forest edges associated with energy sector development, changes in Ovenbird abundance near edges are not quite as clearcut. Some studies suggest that Ovenbirds avoid placing territories near newly constructed seismic lines (Machtans 2006), while other studies suggest that Ovenbirds do not decline strongly near older seismic lines (Lankau et al. 2013) or that Ovenbirds will use seismic lines as territory boundaries (Bayne et al. 2005). Ovenbirds may appear to decline along forest edge point counts relative to forest-interior point counts early in the morning, when point counts are conducted and Ovenbirds are detected by their singing; however, radiotracking has revealed that Ovenbirds may frequent forest edges for non-singing activities like foraging later in the day (Mazerolle and Hobson 2003). Therefore, declines in Ovenbirds along forest edges during point counts don’t necessarily prove that Ovenbirds will decline as forest edge increases in larger landscapes due to energy sector development (Bayne et al. 2005).

by Lionel Leston

Citations

Ball, J. R., Bayne, E. M., & Machtans, C. S. (2009). Energy sector edge effects on songbird nest fate and nest productivity in the boreal forest of western Canada: a preliminary analysis. Pages 161-170 In Proceedings of the Fourth International Partners In Flight Conference: Tundra to Tropics. McAllen, Texas, February, 2009.

Bayne, E.M. and K.A. Hobson. 1997. Comparing the effects of landscape fragmentation by forestry and agriculture on predation of artificial nests. Conservation Biology 11: 1418-1429.

Bayne, E.M. and K.A. Hobson. 2001. Effects of habitat fragmentation on pairing success of Ovenbirds: importance of male age and floater behavior. Auk 118: 380-388.

Bayne, E.M. and K.A. Hobson. 2002. Apparent survival of male Ovenbirds in fragmented and forested boreal landscapes. Ecology 83: 1307-1316.

Bayne, E.M., S.L. Van Wilgenburg, S. Boutin, and K.A. Hobson. 2005. Modeling and field-testing of Ovenbird (Seiurus aurocapillus) responses to boreal forest dissection by energy sector development at multiple spatial scales. Landscape Ecology 20: 203-216.

Burke, D.M. and E. Nol. 1998. Influence of food abundance, nest-site habitat, and forest fragmentation on breeding Ovenbirds. Auk 115: 96-104.

Gates, J.E. and L.W. Gysel. 1978. Avian nest dispersion and fledgling success in field-forest ecotones. Ecology 59: 871-883.

Lankau, Hedwig Erika Gertraude. 2014. Songbird responses to regenerating seismic lines in the boreal forest. Masters thesis, University of Alberta, Edmonton, Alberta.

Machtans, Craig S. 2006. Songbird response to seismic lines in the western boreal forest: a manipulative experiment. Canadian Journal of Zoology 84: 1421-1430.

Mazerolle, D.F. and K.A. Hobson. 2003. Do ovenbirds (Seiurus aurocapillus) avoid boreal forest edges? A spatiotemporal analysis in an agricultural landscape. Auk 120: 152-162.

Van Wilgenburg, S.L., D.F. Mazerolle, and K.A. Hobson. 2001. Patterns of arthropod abundance, vegetation, and microclimate at boreal forest edge and interior in two landscapes: implications for forest birds. Ecoscience 8: 454-461.

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