Mammals, Reptiles and Amphibians
of Lake Merced

Elizabeth J. Proctor, Graduate Student, Geography San Francisco State University

California red-legged frog (Rana aurora draytonii)   Photo: Frank E. (Ed) Ely

See Sidebar: Lake Merced’s Potential Special Status Animal Species for more information 

Introduction

Lake Merced is like an "island" of wildlife habitat in an ocean of urban environment. Houses built close together, wide roads, and constant traffic surround it on all sides. In light of this, we might be tempted to look at the number and type of species inhabiting Lake Merced in terms of the theory of island biogeography. Using this model, we could examine the Lake Merced area in terms of 1) its isolation from other natural areas that might provide new species as colonists, and 2) its size, indicating its potential for supporting sustainable populations (MacArthur & Wilson 1967). The model of island biogeography would suggest that because Lake Merced is relatively small and isolated from other natural areas, the number of species found there would be low.

Which, in fact, is true. At a glance, Lake Merced’s vegetation appears to be diverse and plentiful enough to support a diversity of mammal, reptile and amphibian species. Lake Merced has areas of coniferous and hardwood forest habitat, shrub-dominated habitat, marsh, and grassland, each having the potential to support wildlife. But recent site visits have shown that few of the expected species actually inhabit the area.

Could the reason be Lake Merced’s island-like size and isolation? More likely, the paucity of mammals, reptiles and amphibians is the result of a combination of complex factors (C. Reading, personal conversation), which merit further study. Likely factors include the stocking of non-native fish species in the lake, water pollution issues, and the close proximity of human disturbances such as the shooting range and the golf course.

Lake Merced’s urban setting also influences the kind of species we might find there. Human-dominated settings tend to support the most tolerant species, which share certain common characteristics. In general, species that live in disturbed ecosystems are those that have the capability to reproduce quickly and disperse widely. They tend to be generalists – able to tolerate a broad range of habitat conditions and food sources; they also tend to be cosmopolitan species – found over a wide range of habitat locations (Wilson 1992). Taken to the extreme, author David Quammen paints a picture of the future in which most natural areas are small islands of habitat in an ocean of urban environments. As described by Quammen, species that thrive in human-dominated terrain share certain human traits – aggressiveness, versatility, prolific, and opportunistic. The future "Planet of Weeds" that he describes will be rich with such "weedy" species (Quammen 1998). Though not nearly as bleak a scenario as this, it is true that some of the animals found in urban areas like Lake Merced will be familiar to city dwellers – raccoons, opossums, rats, and mice.

Predicting Species Distribution

In addition to the species we expect to inhabit natural areas in urban environments, we can look also look at the evidence of Lake Merced’s relict fauna. To further predict what species might live at Lake Merced, we can also focus on the vegetation found there. Using wildlife/habitat relationship models, we can predict which species are likely to be found within vegetation in which they are known to inhabit, within the range of their overall distributions. In this way, we can look Lake Merced’s vegetation types, and make a prediction of what mammal, reptile and amphibian species could potentially inhabit that vegetation.

Broadly speaking, Lake Merced consists of four general habitat types: coniferous and hardwood forest, shrub-dominated, marsh, and grassland habitats. (For more details on Lake Merced’s vegetation, see the flora section) The SF Public Utilities Commission’s 1998 Baseline Natural Resources Inventory compiled species lists based on these vegetation types and their associated species, using the California Natural Diversity Data Base (CNDDB).

Following is that list of Lake Merced’s vegetation types and associated species, illustrated with photos from the CalPhotos website maintained by the University of California, Berkeley’s Digital Library Project (see References and Resources for more information). It should be noted then, that this is a list of Lake Merced’s potential species based on habitat. In some cases, like the muskrat and the frog species, these animals have not actually been reported at Lake Merced. The gray fox, for example, is not known to inhabit Lake Merced, but the PUC’s 1998 Baseline Natural Resources Inventory contains an anecdotal report of one found as roadkill in the area. Similarly, coyote are not generally sighted at Lake Merced, though they have been sighted around Skyline Boulevard to the south, and because of their wide-ranging distribution and tolerance for urban habitats, the potential for coyote exists. See Mammal, Reptile and Amphibian Studies below for more information on the results of actual species surveys.

Potential Wildlife in Lake Merced Habitats

Coniferous and hardwood forest and Shrub-dominated habitats

Reptiles and amphibian

Gopher snake (Pituophis melanoleucus)
Western fence lizard (Scleroporus occidentalis)

Mammals

California ground squirrel (Citellus beecheyi)
Botta’s pocket gopher (Thomomys bottae)
Virginia opossum (Didelphus virginiana)
Raccoon (Procyon lotor)
Striped skunk (Mephitis mephitis)
Rabbit (Syvilagus sp.) (no photo)
Rat (Rattus sp.) (no photo)
Mouse (Peromyscus sp.)

Gopher snake (Pituophis melanoleucus) Photo: Jens V. Vindum	Western fence lizard (Scleroporus occidentalis) Photo: J. Taylor

California ground squirrel (Citellus beecheyi)   Photo: Jules Strauss

Botta’s pocket gopher (Thomomys bottae)  Photo: Dr. Lloyd Glenn Ingles

Opossum (Didelphis virginiana)  Photo: Alden M. Johnson

Raccoon (Procyon lotor)  Photo: Gerald and Buff Corsi

Striped skunk (Mephitis mephitis)  Photo: Dr. Lloyd Glenn Ingles

Marsh habitat

Reptiles and amphibians

Pacific treefrog (Hyla regilla)
Bullfrog (Rana catesbiana)
Red-eared slider (no photo)
Garter snake (Thamnophis sp.)

Mammals

Muskrat (Ondatra zibethica)
Raccoon (Procyon lotor)
Virginia opossum (Didelphis virginiana)
Striped skunk (Mephitis mephitis)

Pacific treefrog (Hyla regilla) Photo: Jens V. Vindum

Bullfrog (Rana catesbieana) Photo: Jens V. Vindum

Muskrat (Ondatra zibethica) Photo: Dr. Lloyd Glenn Ingles

Grassland habitat

Reptiles and amphibians Gopher snake (Pituophis melanoleucus) Western fence lizard (Scleroporus occidentalis)
Mammals Black-tailed hare (no photo) Striped skunk (Mephitis mephitis) California ground squirrel (Citellus beecheyi) Botta’s pocket gopher (Thomomys bottae) Coyote (Canis latrans) Western harvest mouse (Reithrodontomys megalotis) Gray fox (Urocyon cinereoargenteus)
	Western harvest mouse (Reithrodontomys megalotis)  Photo: Dr. Lloyd Glenn Ingles
Coyote (Canis latrans)  Photo: Dr. Lloyd Glenn Ingles	Gray fox (Urocyon cinereoargenteus)  Photo: Alden M. Johnson

Sidebar: Lake Merced’s Potential Special Status Animal Species

Lake Merced has the potential to support several "special status" animal species, including the San Francisco garter snake (Thamnophis sirtalis tetrataenia), the Western pond turtle (Clemmys marmorata), and the Californian red-legged frog (Rana aurora draytonii). These were determined based on literature searches, the CA Natural Diversity Data Base (CNDDB), site-specific habitat assessment, and geographic ranges of sensitive species, as reported in the PUC’s 1998 Baseline Natural Resources Inventory.

"Special status" is a term referring to taxa that are being tracked by the CNDDB, and includes species listed as state or federal endangered or threatened, as well as species associated with a California habitat that is in serious decline. (For more information on the CNDDB and California’s endangered species, see the References and Resources section).

A recent report by EIP Associates indicates that two of the three species have been recorded recently, noting that the San Francisco garter snake my not be present because of lack of food supply and predatory species present (EIP Associates 2000).

California red-legged frog (Rana aurora draytonii)  Photo: Dong Lin, 1995

• California species of special concern; federally threatened
• Subspecies only occurs in the coastal portions of its historic range, apparently extinct in the valley and foothills and in most of southern California
• Confined to aquatic habitats – ponds, lakes, streams, or deep, still, or slow-moving water, with pools 1m deep
• Adjacent habitat typically includes dense emergent or riparian vegetation including arroyo willow, cattails and bulrushes
• In Lake Merced, as elsewhere, the main reason for decline is attributed to exotic aquatic predators – in this case, bass and bullfrogs that feed on tadpoles and frogs
• Most recently observed March 2000 (EIP Associates 2000).

San Francisco garter snake (Thamnophis sirtalis tetrataenia)   Photo: Ted Brown

• state and federally-listed endangered species
• Subspecies of the common garter snake
• Its historic range was the San Francisco county line south through Santa Cruz mountains to Point Ano Nuevo
• Has potential to inhabit Lake Merced, but its range is slightly south, and the Lake lacks the upland habitat required
• There are no known records of it occurring at Lake Merced
• Bullfrogs and bass are known predators

Western pond turtle (Clemmys marmorata)  Photo: Gerald and Buff Corsi

• California species of special concern
• Prefers permanent, still or slow-moving water, with partially exposed logs or ledges for basking, plus a terrestrial component, for oviposition, overwintering, occasional seasonal use, and overland dispersal
• Recently observed in East Lake (EIP Associates 2000)

Reports on mammal, reptile and amphibian species were included in two recent planning documents on Lake Merced: the SF Water Department Lake Merced Water Resource Planning Study (1993), and the SF Public Utilities Commission 1998 Baseline Natural Resources Inventory (1999). These studies consisted of site visits rather than extensive field surveys, but both reported very few reptile and amphibian species sightings and no mammal sightings, with the exception of feral cats.

The more recent report, complied by EIP Associates in the Fall of 2000, included surveying of both small mammal species as well as reptile and amphibian species.  In the small mammal trapping efforts, performed by Reading and Paquin (2000), only two species where collected, the California vole (Microtus californicus) and house mice (Mus musculus) (EIP Associates 2000).  They do point out that the surveys were most likely affected by recent restoration efforts, meaning long-term biodiversity may be higher than trapping results indicate.  Further surveying work must be done to provide conclusive results for mammal species present in the Lake Merced area.

The reptile and amphibian surveying was performed by EIP Associates and were conducted according to USFWS protocol for California red-legged frog (Rana aurora draytonii) surveying, either by boat or foot depending upon location.  During these efforts no red-legged frogs were observed (EIP Associates 2000).  A large tadpole, possibly a California red-legged frog, was seen during survey efforts in Impound Lake on May 2, 2000.  Also, a biologist from San Francisco State University, reported sighting a California red-legged frog on March 25, 2000 on the eastern shore of Impound Lake.  The report concludes that it seems likely that a population of California red-legged frogs does exist in Impound Lake (EIP Associates 2000).  Perhaps more notably then the lack of reg-legged frogs, these survey efforts did provide a partial species list in the Lake Merced area.  

One species of concern that was found during the efforts was the Western pond turtle (Clemmys marmorata) in the East Lake.  It is important to note that the non-native red-eared sliders (Trachemys scripta) and soft-shelled turtles [Apalone spp (a.k.a. Trionyx)] were also observed.  These two species are aggressive predators, which could pose a threat to specific species of concern, such as the Western pond turtle and the California red-legged frog.  Other species noted were the western skink (Eumeces skiltonianus), the northern alligator lizard (Elgaria coerulea), bullfrogs (Rana catesbeiana), and the California Slender salamander (Batrachoseps attenuatus) (EIP Associates 2000).  One final note, is the complete lack of Pacific chorus frogs (Pseudacris regilla) during the surveys.  EIP (2000) notes that they were heard calling during surveys, but were never seen, possibly indicating the overall suitability of the Lake Merced area for amphibian reproduction.

EIP Associates 2000. Significant Natural Resource Areas Management Plan. Prepared for Natural Areas Program, San Francisco Recreation and Parks Department.  

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, NJ.

Quammen, D. 1998. "Planet of Weeds."  Harper’s Magazine October: 57-69.

Reading, Craig. Personal Conversation.

Reading, C.R. and M. Paquin.  2000. Herptofuana and small mammal survey of Lake Merced, San Francisco Country, CA.

San Francisco Water Department. 1993. Lake Merced Water Resource Planning Study. Geo/Resource Consultants, Inc.

San Francisco Public Utilities Commission. 1999. Lake Merced 1998 Baseline Natural Resources Inventory. Camp Dresser & McKee Inc., Trihey & Associates, Inc.

Wilson, E.O. 1992. The Diversity of Life. W.W. Norton & Company, Inc. New York, NY.

Photo's, Links, and Sources

Browse the CalPhotos website for additional photos of California plants and animals at http://elib.cs.berkeley.edu/photos/

Find out about California’s special status species at the Department of Fish & Game’s Natural Diversity Data Base website at http://www.dfg.ca.gov/whdab/cnddb.htm

Search the California Academy of Science’s database of reptiles and amphibians at the Department of Herpetology website at http://www.calacademy.org/research/herpetology/

Learn about amphibian conservation efforts at the University of California, Berkeley’s Museum of Vertebrate Zoology website at http://www.mip.berkeley.edu/mvz/conservation.htm