Xantus' Murrelet, Sythliboramphus hypoleucus II: Assessment of Listing Status.

Abstract We considered the status of Xantus' Murrelet (Synthliboramphus hypoleucus), a rare seabird of the western United States and Mexico, under the World Conservation Union quantitative criteria for listing as vulnerable, threatened and endangered. Based on Worm Conservation Union Criterion B, Xantus' Murrelet qualifies for listing as vulnerable, although our information base is incomplete for many sites. Known and/or suspected population declines at colonies in Baja California and the Channel Islands (USA) and threats from a variety of sources make listing of this species as threatened under the U.S. Endangered Species Act an appropriate course of action.

Introduction

As detailed by Roth and Sydeman (this volume) the distribution of the Xantus' Murrelet (Synthliboramphus hypoleucus) is restricted to islands off the coasts of California and Baja California (Drost and Lewis 1995). There are two subspecies: S. h. scrippsii breeds on the California Channel Islands south to the San Benitos Islands, and S. h. hypoleucus occurs on the San Benitos and Guadalupe islands. Herein, we summarize and evaluate the listing status of the species based on the best available information on population distribution and abundance and threats facing the species. We evaluate this information in relation to quantitative criteria established by the World Conservation Union (IUCN) for listing of vulnerable, threatened and endangered species (IUCN 1994).

The IUCN categories of threat to a species include a major division: species are either listed as "threatened" which includes "critically endangered," "endangered" and "vulnerable" sub-classifications, or "lower-risk," which includes sub-classifications of "least concern," "near threatened," and "conservation dependent." Currently, the IUCN lists Xantus' Murrelet as "near threatened" owing to its limited breeding range, concentration of sub-populations at few colony sites, and pervasive threats including predation by native (mice, owls and falcons) and introduced (rats, cats) species and the close proximity of human population centers to colonies (Collar et al. 1994). The IUCN is currently considering uplisting the species to "vulnerable." Therefore, our analysis focuses on the question of whether a classification of "vulnerable" under IUCN criteria is appropriate. We base this evaluation primarily on population surveys conducted by Carter et al. (1992, 1996) and Keitt (1999) and syntheses by Drost and Lewis (1995) and Sydeman et al. (1998).

Evaluation of Xantus' Murrelet status

According to the IUCN (IUCN 1994), a taxa is considered "vulnerable" when it is not considered "critically endangered" or "endangered," but is facing a high risk of extinction in the wild in the "medium-term" future, as defined by any of the following criteria:

Criterion (A)

Population reduction in the form of either of the following:

(1) an observed, estimated, inferred, or suspected reduction of at least 20% over the last 10 years or three generations, whichever is longer, or

(2) a reduction of at least 20%, projected or suspected to be met within the next 10 years or three generations, based upon direct observation, an index of abundance appropriate for the taxon, a decline in the area of occupancy, actual or potential levels of exploitation, and ecological effects which may reduce population size.

In relation to the best information available, Xantus' Murrelet does not qualify for listing as "vulnerable" based on the above criteria, mainly because the species is data deficient in regards to population trends. For most colony locations, population dynamics are unknown. At Santa Barbara Island (SBI), however, location of the largest Xantus' Murrelet population in the U.S. and representing about 25% of the global population, estimates from 1977 and 1991 indicate a population reduction of 30 to 50% over a 14-year period (Sydeman et al. 1998). The criterion above "... reduction of 20% over the last 10 years," implies an annual rate of change of -2.2% per year. However, three generations for a Xantus' Murrelet is at least 20 years (Sydeman et al. 1998). Hence the criterion should be interpreted as "... reduction of 20% over the last 20 years, which implies an annual rate of change of -1.1% per year. The population decline on Santa Barbara Island between 1977 and 1991 has exceeded this rate (-2.5% per year or greater [Sydeman et al. 1998]). Moreover, based on current and past levels of predation by native deer mice (Peromyscus maniculatus) on murrelet eggs and by barn owls (Tyto alba) on adults, we infer that the SBI population will continue to decline at [is greater than] 2% per year for the next 10 years (Sydeman et al. 1998). If the results observed at Santa Barbara apply across the whole species, this would qualify the species as "vulnerable" under Criterion (A).

Criterion (B)

Extent of occurrence estimated to be less than 20,000 [km.sup.2] or area of occupancy estimated to be less than 2000 [km.sup.2], and estimates indicating any two of the following:

(1) severely fragmented or known to exist at no more than 10 locations, or

(2) continuing decline, inferred, observed or projected, in any of the following:

(a) extent of occurrence,

(b) area of occupancy,

(c) area, extent, and/or quality of habitat,

(d) number of locations or sub-populations, or

(e) number of mature individuals, or

(3) extreme fluctuations in any of the following: extent of occurrence, area of occupancy, number of locations or sub-populations, or number of mature individuals.

In relation to Criterion (B), Xantus' Murrelet qualifies for listing as "vulnerable" due to its small area of occupancy on offshore islands ([is less than] 2000 [km.sup.2]) and its fragmented distribution (B1), coupled with extirpation/declines observed or inferred at some locations (B2d), and continuing declines in the number of mature individuals (B2e).

In the U.S., Xantus' Murrelets are known to breed at five locations (in this case we define a location as an island plus closely adjacent islets and sea-stacks): San Miguel Island and associated Prince Islet, Santa Cruz Island, Anacapa Island, Santa Barbara Island and associated Sutil Islet, and San Clemente Island (Carter et al. 1992; Drost and Lewis 1995). Breeding on Santa Catalina at present is suspected but unconfirmed (Drost and Lewis 1995).

In Baja California, Mexico, murrelets are known to breed at four locations: Islas Los Coronados, Isla San Geronimo, Islas San Benitos and Isla Guadalupe (Drost and Lewis 1995; Carter et al. 1996; Keitt 1999). Breeding populations on Islas Los Coronados, San Benitos, and Guadalupe are relatively large. Drost and Lewis (1995) indicated that the populations on Todos Santos, San Geronimo, and San Martin were extirpated, but Keitt (1999) found active nesting on San Geronimo. Keitt (1999) also suspected a small number of breeding individuals on Todos Santos and San Martin, but this was not confirmed.

Thus, the known breeding population is restricted to nine locations. Of these locations only four, Santa Barbara Island (c. 1,500 individuals), Islas Los Coronados (c. 1,500 individuals), Islas San Benitos (c. 500 individuals) and Isla Guadalupe (c. 2,500 individuals), support relatively large populations (defined as at least 500 breeding birds; see Sydeman et al. 1998 for justification of this criterion).

Recent Xantus' Murrelet population declines have been documented on Santa Barbara Island (see discussion above) and there is ample reason to suspect that this decrease has continued.

Based on Criterion (B) alone, Xantus' Murrelets appear to qualify for a listing of "vulnerable" under IUCN rules. In our view, this listing would be analogous to listing as "threatened" under the U.S. Endangered Species Act.

Criterion (C)

Population estimated to number less than 10,000 mature individuals and either:

(1) an estimated or continuing decline of at least 10% within 10 years or three generations, whichever is longer, or

(2) a continuing decline, observed, projected, or inferred, in numbers of mature individuals and population structure in the form of either:

(a) severely fragmented (i.e., no subpopulation estimated to contain more than 1,000 mature individuals, or

(b) all individuals are in a single subpopulation.

Based on Criterion (C) alone, Xantus' Murrelet does not qualify as "vulnerable." While the global population of mature individuals at large colonies is likely to be in the range of about 6,500 birds (assuming c. 2,500 at Guadalupe, [Jehl and Everett 1985], 525 at Benitos [Keitt 1999], 1,500 at Coronados [Carter et al. 1996; Keitt 1999], and 1,500 at Santa Barbara [Carter et al. 1992]) and an additional 500 birds may be in elsewhere in Baja California (Keitt 1999) and the U.S (Drost and Lewis 1995), we lack information on overall population trends for the species. Also, three locations likely contain more than 1,000 mature individuals (Guadalupe, Coronados and Santa Barbara). However, if populations at Guadalupe and Coronados are also declining, then, coupled with the decline in population level of 30 to 50% over 14 years observed at Santa Barbara, it is likely that the global population is declining at a rate of at least 10% per 20 year period. In that case, Xantus' Murrelet would qualify as "vulnerable" under criterion C.

Criterion (D)

Population very small or restricted in the form of either of the following:

(1) population estimated to number less than 1,000 mature individuals

(2) populations is characterized by an acute restriction in its area of occupancy (typically less than 100 [km.sup.2]) or in the number of locations (typically less than 5). Such a taxon would thus be prone to the effects of human activities (or stochastic events whose impact is increased by human activities) within a very short period of time in an unforeseeable future, and is thus capable of becoming critically endangered or even extinct in a very short period.

In relation to Criterion (D), Xantus' Murrelet does not qualify for a status of "vulnerable." The population is certainly larger than 1,000 mature individuals, and while its distribution is localized, there are nine locations with confirmed breeding populations. Then again, only four of these locations contain populations that are likely to exceed 500 mature individuals.

Criterion (E)

Quantitative analysis showing that the probability of extinction in the wild is at least 10% within 100 years.

In relation to Criterion (E), Xantus' Murrelet cannot be considered "vulnerable." There have been no attempts at quantitative analyses for the entire Xantus' Murrelet population. Using RAMAS/ METAPOP to model the population, Sydeman et al. (1998) investigated the probability of "quasi-extinction" (defined as fewer than 500 breeding birds) for the Santa Barbara Island sub-population. They found that this sub-population had a 30 to 80% chance of reaching quasi-extinction within 20 years. The likelihood of extirpation of this colony is essentially zero for 20 years (PRBO unpublished data), but analyses were not extended to a timeline of 100 years.

Conclusion

The Xantus' Murrelet is deserving of listing as "vulnerable" under IUCN criteria due to its small population, limited range, and continuing threats to its long-term viability. We view this recommendation as equivalent to a listing of threatened under the U.S. Endangered Species Act. A declining trend in abundance, evident on Santa Barbara Island, may apply to other colonies. Better information on the existence (or lack of existence) of breeding colonies on San Clemente, Santa Catalina, Todos Santos, and San Martin is required. Just as important is information on population trends at colonies other than on Santa Barbara Island. Finally, in order to fully evaluate the species' status and management options, more data are needed regarding population size and dynamics, demographic processes, and factors affecting mortality throughout its range in California and Baja California.

Literature cited

Carter, H.R., G.J. McChesney, D.L. Jaques, C.S. Strong, M.W. Parker, J.E. Takekawa, D.L. Jory, and D.L. Whitworth. 1992. Breeding populations of seabirds in California, 1989-1991. Unpublished Report, U.S. Fish and Wildlife Service, Dixon, California.

Carter, H.R, D.L. Whitworth, W.R. McIvor, J.B. Bulger, and G.J. McChesney. 1996. Survey of Xantus' Murrelets (Synthliboramphus hypoleucus) and other marine birds at Islas Los Coronados, Baja California Norte, Mexico, on 23-25 April 1995. Unpublished Report, U.S. Geological Survey, Biological Resources Division, Dixon, California.

Collar, N, M.J. Crosby, and A.J. Stattersfield. 1994. Birds to Watch 2: The World List of Threatened Birds. Birdlife International, Cambridge.

Drost, C.A. and D.B. Lewis. 1995. Xantus' Murrelet (Synthliboramphus hypoleucus). In A. Poole and F. Gill, eds. The Birds of North America, No. 305. The Academy of Natural Sciences, Philadephia, PA, and The American Ornithologists Union, Washington, D.C.

Jehl, J.R. Jr., and W. T. Everett. 1985. History and status of the avifauna of Guadalupe Island, Mexico. Trans. San Diego Soc. Nat. Hist. 20:313-336.

Keitt, B.S. 1999. Status of the Xantus' Murrelet (Synthliboramphus hypoleucus) on the islands of Baja California, Mexico. Unpublished Report, Island Conservation and Ecology Group, Davenport, California.

Sydeman, W.J., N. Nur, and P. Martin. 1998. Population viability analyses for endemic seabirds of the California marine ecosystem: The Ashy Storm-Petrel (Oceanodroma homochroa) and Xantus' Murrelet (Synthliboramphus hypoleucus). Unpublished Report, Point Reyes Bird Observatory, Stinson Beach, California.

World Conservation Union (IUCN). 1994. IUCN Red List Categories, Final Version, 18 pp. IUCN, Gland, Switzerland.

William J. Sydeman Point Reyes Bird Observatory, Marine Science Division, 4990 Shoreline Hwy., Stinson Beach, CA 94970; 415-868-1221, x19; [e-mail]

Nadav Nur Point Reyes Bird Observatory, Population Biology, 4990 Shoreline Hwy., Stinson Beach, CA 94970; 415-868-1221, x15; [e-mail]