The Ecology of Tijuana Estuary, California: An Estuarine Profile

Each "island" of higher topography allows species diverse; the larger the mound, the longer the plant
of the higher marsh to extend farther into the species list. In addition, the mounds provide
wetland. The marsh periphery is thus patchy and habitat for several herbivorous mammals that in
turn influence mound and Intermound vegetation.
Cox attributes the mound formation to ground
squirrels and other burrowing mammals that utilize
the high ground. He suggests that over centuries,
they gradually transport so~ls toward a central
burrow opening until a pattern of mounds and
intermound areas is created. Whether the
concentration of squirrels on mounds is cause or
effect, however, is hard to demonstrate in shortterm
studies. What is clear is that the mounds of
high ground, which are surrounded by wetter
marsh, add small-scale hab~tat diversity to the
wetland. Where adjacent areas have been filled or
developed, these islands provide the only clue to
the higher-elevation communities that might have
occurred rn the transition zone. The invasion
process can be seen in wet winters, such as 1983,
when high spots are leached of their salts and
upland weeds (e.g., the wild radish, Raphanus
sativum) germinate and grow. Such salt-intolerant
species rarely persist beyond the next dry season.
Herpetofauna are l~kewise lim~ted to these areas
of high ground. The San Diego horned lizard
(Phrynosoma coronatum blainv~llei; Figure 21) is
declining because of frequent collection, but a few
remain in isolated areas. Snakes are not common,
and their rarity partly explalns the abundance of
rodents such as ground squirrels (Spermoph~lus
beechyid and rabbits (Lepus californicus and
Sylv~lagus audubonil sactidregil that populate the
upper marsh
Birds that nest in the high marsh include the
white-crowned sparrow (Zonotnchia leucophrys),
song sparrow (Melospira melodia), western
meadowlark (Sturnella neglecia), and killdeer
(Charadnus vociferus). Other birds use the area
extensively for foraging. Raptors, such as the
northern harrier (Circus cyaneus), Amertcan kestrel
1 (Falco sparverius), and golden eagle (Aquila
chrysaetos), explolt the populations of small
mammals; while Belding's Savannah sparrows,
horned larks (Eremophila alpestris), and
loggerhead shrikes (Lanius ludovic~anus) feed on
Expisnation of X axis
insects and other small prey (A. White, SDSU, pers.
Cla~r~ Elevation Quadrats Class Elevation Quadrats comm ,,
numbef (cm MSL) in crass number (cm MSL) in class
" -- - .
3 30-39 7 8 80-89 35 Gharacteristic plants of the higher marsh are
4 40-49 f 2 B 90-99 30 the perennial glasswort (Salrcornla subterm~nalts),
5 50-59 22 10 100-109 39 the shore grass (Mananthochloe Iittoral~s), alkali
6 60-69 113 11 110-130 8
7 70.79 95 heath, sea lavender, and Atr~plex watsoni~. Ail of
these specles are perenn~als that reproduce
Figure 1% The frequency of occurrence of abundant vegetatively, and seedlings are rare. Only dur~ng
Plant species in the salt marsh of Tljuana Estuary (from the long ralny season of 1983 were there
Zedter 1977).
conspicuous episodes of seedling establishment
2 0