November 2021 issue of HAI Flier

The Corrales Bosque Comes Under Imminent Threat

By Trevor Fetz, Ph.D.

Hawks Aloft Research Director

In October, the Corrales Mayor and Village Council approved a clearing project that would remove all woody vegetation within 10 feet of the eastern levee toe for the entire 8+ mile length of the Corrales bosque. The only exceptions would be the retention of cottonwoods and the very sparsely occurring Goodding’s willow. In addition, all non-native vegetation out to 20 feet from levee (with the exception of “some healthy, mature Russian olives”) would be removed.

Although the width of the impacted area for this project may seem small, it will directly impact 2.4 acres of bosque per mile, with a total impact in excess of 19.2 acres. Unfortunately, much of the impacted area includes some of the best bird habitat within the entire middle Rio Grande bosque. This plan, if enacted, will be extremely detrimental to the avian community.

The approved version of this project is actually a revision that will be more detrimental to the avian community than the original version. In the original version, all non-native vegetation would have been removed, but all native vegetation would have been retained. The Corrales Bosque Advisory Commission (CBAC), which presumably oversees all activity in the Corrales bosque, was completely bypassed by the supporters of this project. CBAC asked the mayor and village council to reject both the original version and the second version that was ultimately approved.

Corrales Bosque Edge Transects

Based on Hawks Aloft’s extensive experience in the bosque, we would expect a minimum 25-30% reduction in avian use within 30 meters of the levee road if the proposed project is conducted. But, bird use could decrease by as much as 50% or more, as we’ve previously documented in areas with a thinned bosque understory. In some areas where extensive understory clearing has occurred, we have documented a nearly 300% reduction in avian density.

An additional 6.5 acres would be impacted by this project for maintenance in and around existing fuel breaks, but that work would have a much more limited impact on the avian community. Hawks Aloft does not oppose the maintenance of existing fire breaks. In general, we don’t oppose the thinning of non-native vegetation. However, we do believe healthy, berry-producing Russian olive should be retained due to its importance as a nesting substrate and year-round food source for birds. We also aren’t opposed to the removal of cottonwoods (or portions of cottonwoods) that are likely to fall and pose an imminent threat to the general public. We are, however, vehemently opposed to any other removal of native vegetation, especially New Mexico olive, which it can be argued is the single most important plant species for bosque birds.

To understand why this project would be so detrimental to the avian community, it is necessary to understand the unique aspects of this area. It is well-established that bird use is generally higher in edge habitat than interior habitat of the same type. Edge habitat is more dynamic and attracts more species. Data collected in the middle Rio Grande bosque reflects this. The landmark Middle Rio Grande Biological Survey (Hink and Ohmart 1984) compared avian use in like habitat types between interior areas and levee edge areas (areas within 15 meters of a levee road). During all times of year, they found that avian density in edge stands was significantly higher than density in interior stands of the same habitat type. They also found avian richness in edge stands to be substantially (but not significantly) higher than interior stands. The 18 years of data that Hawks Aloft has collected during the Middle Rio Grande Songbird Study (MRGSS) has shown the same trends for both avian density and richness between edge and interior habitat of the same type. This is important because the targeted vegetation in the proposed project area is edge vegetation and its removal would have a greater negative impact on bird use in the Corrales bosque than would a similar project in the interior of the bosque (although Hawks Aloft would oppose the extensive removal of native vegetation regardless of the location).

Looking specifically at cumulative bird numbers for the Corrales bosque, both summer and winter avian density are significantly higher on our six edge transects (873 and 612 birds/100 acres respectively) than our 10 interior transects (740 and 312 birds/100 acres respectively). Winter avian richness also is significantly higher on our edge transects (23.5 species/transect) than interior (18.8 species/transect), although summer richness is only slightly higher on edge transects (33.4 species/transect) than interior (32.1 species/transect).

In addition, for a vast majority of the length of the Corrales bosque the edge vegetation in the proposed project path is dominated by a New Mexico olive understory, and supports some of the highest bird numbers throughout the entire 79-mile-long MRGSS study area. This includes extensive areas with a dense, New Mexico olive-dominated understory with a mature cottonwood canopy (C/NMO 1) and the only substantial patch of habitat throughout the entire study area with a dense, New Mexico olive and silver buffaloberry-dominated understory (and lacking a cottonwood canopy; NMO-SB 5).

Four of the five C/NMO 1 transects in the MRGSS study area would be impacted by this project. The fifth is an interior transect located in the Albuquerque bosque in the only extensive stand of C/NMO 1 outside of Corrales within the entire study area. The two tables below show the value and rankings of the Corrales C/NMO 1 transects relative to all 81 transects of all habitat types, as well as the 63 transects that are entirely terrestrial (i.e., don’t incorporate any standing water).

Corrales Chart 1
Corrales Chart 2

Finally, it is important to understand that the negative impact would go beyond just the removal of vegetation in the project area, but also include adjacent habitat due to the fragmentation and increased disturbance that would occur. Additionally, through substantial portions of the proposed project area dense patches of New Mexico olive are quite narrow (sometimes less than 30 meters wide) and clumped relatively close to the levee. Thus, considerable portions of this habitat would be lost.

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