Degree of Threat: High
Comments: Significant threats include destruction and degradation of habitat as a result of agricultural practices, urbanization, forest invasion, invasion of non-native vegetation, fire suppression, and changes in ecological processes that create suitable horned lark habitat; disturbance by humans; nest destruction caused by mowing of habitat during the nesting season; nest predation; and small population size (see Pearson and Altman 2005 for further details).
The primary threat is the loss, fragmentation, and degradation of suitable breeding habitat. Habitat has been and continues to be lost through urbanization, agricultural intensification, fire suppression, and alien species invasion (Altman 2000, Beauchesne and Cooper 2002). At least 60% of the remaining Oregon population is not secure (E. Scheuering, pers. comm.). In Washington, however, the habitat of the key remaining occurrences is more or less secure. Because the populations breeding at most occurrences are so small, they are threatened with extirpation by a host of unpredictable environmental events and fluctuations.
In the Willamette Valley, it is estimated that less than 1 percent of the native grassland and savanna remains (Altman 2000). There, the prime causes of habitat loss and degradation are conversion of grassland to agriculture; homes and other human development; encroachment of woody vegetation because of fire suppression; and encroachment onto prairies by alien plant species such as broom (Cytisus scoparius) and grasses (e.g. Holcus sp. and Arrhenatherum elatius). In the south Puget Sound region, only 3 percent of the historic prairie is considered to be intact (Crawford and Hall 1997).
On the Columbia (and probably the Fraser River) floodplain, the building of dams and dikes has eliminated seasonally flooded prairies and allowed riparian forests to grow (Rogers 2000).
At sites in Washington State that are not noticeably threatened by habitat loss, the primary threat comes from recreational and military use of the sites (J. Fleckenstein, pers. comm.). On San Juan Island, the species has disappeared from habitat that has not noticeably changed; there, however, introduced predators such as red foxes and ferrets may have reduced the numbers of ground-nesting birds (Rogers 2000).
In Canada, limiting factors include current scarcity of suitable habitat, historical decline of suitable habitat, increased human disturbance in remaining habitats, increased predation pressure related to increased urbanization, invasion of exotic plants, increased pesticide use, and severe decline of a source population for immigration (COSEWIC 2003).