WSP03252

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/16/1996
• Title: Biological Opinion Summary
• Water Supply Pro - Doc Type: Biological Opinion

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Status and Distribution

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Reasons for listinll: The southwestern willow flycatcher was listed as endangered in response
to documented declines in population size and extent of historic range occupied as a result of
loss, modification, and fragmentation of riparian habitat and parasitism by the brown-headed
cowbird (U.S. Fish and Wildlife Service 1993. 1995b). Critical habitat was proposed to provide
additional protection for areas (occupied and unoccupied) necessary for the survival and recovery
of this species.

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Ranllewide trend: Southwestern willow flycatcher populations are extremely small and unstable;
77 percent of extant flycatcher locations are occupied by 5 or fewer territorial males.
Rangewide monitoring continues to document declines in some locations. Some populations
apparently have been stabilized as a result of cowbird trapping programs.

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New threats: Additional habitat losses will likely include both small- and large-scale losses and
be of the same types as known to date (i.e. habitat loss, fragmentation, and modification). The
Service expects incidences of cowbird parasitism will vary spatially and temporally as a function
of local cowbird population dynamics and local changes in the extent of riparian habitats.

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Sensitivity to impacts: The soUthwestern willow flycatcher's sensitivity to changes in habitat is
high as a result of the small sizes of nesting groups, the small sizes of riparian habitats occupied,
and the highly fragmented distribution of habitats. The extent of riparian habitat, its
distribution, continuity. and species composition have been substantially altered in the Southwest
(Phillips et aI. 1964. Carothers et al. 1974, Rea 1983, Johnson and Haight 1984, Katibah 1984.
Johnson et at. 1987. Franzreb 1987, Unitt 1987, General Accounting Office 1988, Szaro 1989,
Dah1199O, State of Arizona 1990). Changes in the extent and composition of riparian habitat
decreases suitability and carrying capacity, thereby depressing numbers of flycatchers that can
occupy an area. These effects have resulted in a contraction of the range occupied by the
southwestern willow flycatcher, a reduction in the number of flycatcher populations rangewide.
and in isolation of flycatcher populations, potentially changing historical emigration/immigration
patterns and severing genetic exchange among Populations.

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Resilience: The resilience of the southwestern willow flycatcher and its habitat are both relevant
aspects of a species' survival. The southwestern willow flycatcher has declined in extent of
range occupied and population size as a result of habitat loss, modification, and fragmentation.
Riparian habitats by nature are dynamic, with their distribution in time and space governed
mostly by flood events and stream-flow patterns. Current conditions along southwestern rivers
and streams are such that normal flow patterns have been greatly modified, catastrophic flood
events occur with greater frequency as a result of degraded watershed conditions. stream
channels are highly degraded, floodplains and riparian communities are reduced in extent, and
the species composition of riparian communities modified with exotic species dominant. These
conditions have significantly diminished the potential for southwestern rivers and streams to
develop suitable habitat for the southwestern willow flycatcher. These factors, combined with
the small size of flycatcher populations, indicate that this species' resilience to disturbance is
low.

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Biological and Conference Opinions Glen Canyon BeachlHabilat-Building Flows 2116196

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