Forest Investment Account (FIA) - Forest Science Program
FIA Project Y103119

    Determining thresholds of habitat quality for breeding birds in rangeland ecosystems in the Cariboo Region.
Project lead: David Green (Simon Fraser University)
Contributing Authors: Green, David; Mahony, Nancy
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Maintaining the ecological resilience of BCs rangelands is of vital importance to the ranching industry and to conserving rangeland biodiversity. BCs rangelands are a dynamic ecosystem with change driven by grazing, fire and changing climatic conditions. The purpose of this project is to determine if there are thresholds of habitat quality for rangeland birds in the Southern Interior. We will do this by defining response curves for the relationship between habitat structure and composition across a gradient of rangeland (dry forest and grassland) vegetation communities associated with varying levels of livestock grazing and the diversity, abundance and productivity of rangeland birds. In this project we will address the following research questions:

1) What are the threshold levels of key structural and compositional habitat variables related to bird community indices in grassland and forest rangelands

2) What are the threshold levels of key structural and compositional habitat variables related to productivity of selected rangeland bird species?

3) At the micro-habitat scale, what habitat attributes are rangeland birds selecting for nesting habitat and is that selection related to nest success?

Defining key habitat attributes for biodiversity indictors is a powerful approach allowing managers to tie indicator responses to anthropogenic habitat alteration. North American rangeland birds have shown more persistent and widespread declines than other avian groups, with almost all grassland species showing significant 30 year declines (Vickery et al. 1999, Sauer et al. 2005, Brennan and Kuvlesky 2005). Livestock grazing adversely affects some bird species but favours others, therefore, bird species assemblages have been used as indicators of rangeland quality (Bock et al. 1993, Saab et al. 1995, Bradford et al. 1998, Walk and Warner 2000). Horned Larks prefer reduced cover and thrive under moderate levels of grazing, while Western Meadowlarks require more cover and may be at risk under similar grazing regimes (Bock et al. 1984, Davis 2005). Grazing can have negative impacts by reducing of nesting cover, altering vegetation composition and therefore important food resources and by trampling of nests.

The issue addressed by this project is the sustainable resource use of rangelands, an essential ecosystem for avian diversity in the southern interior. The results will be applied to the development of Best Management Practices for land managers from government, industry and NGOs. To support the widest range of bird species, a variety of grazing conditions and intensities is needed. Rangelands in the Southern Interior are composed of a mosaic of grasslands and forests, habitats that support a diverse suite of birds and are widely grazed by cattle. Cattle grazing can alter natural vegetation assemblages in forests and grasslands by reducing vegetative cover and changing plant communities. Grazing affects habitat attributes of the forest-grassland edge such as age class, stem density and ecotone breadth (Belsky and Blumenthal 1997). Grazing allows the invasion of non-native weeds and shifts the grass community from late seral stage dominated by decreasers, (plant species that decrease with grazing), to early seral stage dominated by increasers, (plant species that increase with grazing; Gayton 2003).
Most North American literature on rangeland bird responses to grazing comes from the prairies. In BC, where vegetation composition and structure are different, Hooper and Pitt (1993) showed Vesper Sparrows need sites with complex vegetation structure and were common in the absence of spring or summer grazing and Savannah Sparrows were more common on wetter sites with less grass cover. However, no studies in the Cariboo region have assessed how productivity is related to habitat characteristics; information that is critical in managing for viable bird populations. There is an almost total lack of information on the responses of birds of dry forests to grazing. However one recent study found a 75% reduction of nest success of ground-nesting Dark-eyed Juncos in grazed versus ungrazed dry forests as a result of a reduction in vegetative cover over nests (Walsberg 2005). To manage for healthy populations of native breeding birds in BC rangelands, we need more information about their ecological needs and the effects of rangeland management on their abundance and productivity.

In years 1-2 of this project, we systematically surveyed bird abundance and distribution across a gradient of grazing intensities, including sites that have been ungrazed for at least 10 years and currently grazed ones, across the forest-grassland ecotone in the Cariboo Region. We worked at three sites; a working ranch, the OK Ranch (owner/operator: Mr. Lawrence Joiner) and at two provincial protected areas, the Churn Creek Protected Area, where there is active cattle grazing and the Junction Sheep Range Provincial Park where there is only native ungulate grazing. A fourth site, the Talking Mountain Ranch (owned by the Land Conservancy of BC) proved to be inaccessible due to high water levels at he Fraser River ferry crossing in both years. Plots within each site were selected based on the current vegetation community, a result of the potential natural community and the sites grazing history. We selected grassland sites that are predominantly late seral stage, dominated by bluebunch wheatgrass, a decreaser, and those that are earlier seral stage with more needle-and-thread grass and other known increasers. This will allow us to assess bird responses to a gradient of plant communities that result from cattle grazing. Grazed and ungrazed forested plots were also selected. We measured vegetation structure and composition at survey plots to determine the habitat features important to bird communities at a site-specific scale relevant to range management. In the coming year (2009-10), we will continue with a detailed study of the effect of vegetation structure and composition on nest-site selection and productivity of Vesper Sparrows in the grassland plots and Chipping Sparrows in the forest plots to identify nesting habitat requirements vital to bird productivity.

We envision this as a long-term project. Initial pilot work in 2006 at the OK Ranch allowed us to test and learn appropriate methodologies for sampling bird and vegetation data. In 2007 and 2008 and we collected data to assess the relationships between vegetation characteristics of grazed and ungrazed plots and the breeding bird community and plot and site level differences in breeding bird productivity. We completed the data collection on bird abundance and distribution in relation to site and plot characteristics in 2008. In 2009 we will focus our efforts on acquiring robust estimates of seasonal fecundity of Vesper Sparrows and nest success of Chipping Sparrows, analysis of the relationship between vegetation structure and composition on avian community structure, abundance and productivity, the development of Best Management Practices and the dissemination of results. We will refine, promote and test the Best Management Practices we develop as a result of this research over the following five years in an iterative, adaptive management approach.
Related projects:  FSP_Y081119FSP_Y092119
Contact: David Green, (778) 782-3981,


Executive summary (25Kb)

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Updated August 16, 2010 

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