While there were no studies that 

 quantified the effect of estuary mouth closure 

 on shorebirds, Jorgensen (pers. comm.) 

 estimates that shorebird abundance was 

 reduced 70% during this period, an effect 

 attributed to the absence of tidal submergence 

 and exposure of bottom sediments. Since the 

 sandy shore habitat was not affected by the 

 obstruction of the estuary mouth, some 

 species may have shifted to this habitat for 

 feeding or may have migrated to other 

 intertidal flats in the region. The return of 

 shorebirds to Tijuana Estuary was much more 

 rapid than that of the former dominant 

 bivalves, and a diverse community utilized the 

 mudflat and sandflat habitats during the fall 

 1985 migration. The potential for recovery 

 appears to be high for migratory species; if 

 suitable habitat is available, birds will use it. 

 The regional impact on birds of temporary 

 habitat losses may never be known. 



3.8 DUNES AND BEACH 



The shoreline is a dynamic habitat, 

 subject to a variety of environmental 

 influences such as wind and wave action, salt 

 spray, high temperature, and moisture stress. 

 A number of plants and animals have become 

 adapted to these factors and many are found 

 only on dunes. Because of this and because 

 most of the habitat in southern California has 

 been destroyed (Powell 1981), dune species 

 are particularly vulnerable to extinction on a 

 local scale. Thus, this small portion of the 

 estuary is of high value to wildlife. 



Dunes that are not stabilized by vegetation 

 can be blown or washed away. The original 

 dune vegetation at Tijuana Estuary has been 

 disturbed by construction and recreation 

 activities that resulted in the denudation of the 

 system. In the winter of 1983, storms 

 washed the dunes into the main channel of the 

 estuary. 



Before housing development in the late 

 1940's and early 1950's, the vegetation of 

 the Tijuana Estuary dune system was similar 

 to that of the Silver Strand, approximately 3 

 km to the north (Table 3.8). Plant species 

 such as dune primrose (C am isson ia 



cheiranthifolia), sand verbena (Abronia 

 maritima), and dune ragweed (Ambrosia 

 chamissonis) were present (Figure 3.27). 

 The dunes were stable and did not change 

 position significantly. Following human 

 impacts, some native species, most notably 

 the shrub, lemonadeberry (Rhus integri- 

 folia), were eliminated, while several 

 exotics, such as hottentot-fig (Carpobrotus 

 edulis), sea rocket (Cakile maritima), and 

 Atriplex species invaded. 



Brian Fink set out to determine what the 

 natural distribution of native dune plants may 

 have been prior to disturbance, so that 

 replanting could be accomplished in 

 microhabitats that were suitable for each 

 species. He carried out stress experiments 

 with three native species at PERL, subjecting 

 potted plants to salt and sand burial (Fink 

 1987). In addition, he planted each species on 

 the dune at Tijuana Estuary, to observe 

 growth and survival. Camissonia 

 cheiranthifolia ssp. suffruticosa proved to be 

 intolerant of seawater inundation and sand 

 burial and hence was not recommended for 

 planting along the strandline (Fink and Zedler 

 1990). Sand verbena (Abronia maritima) 

 and Ambrosia chamissonis were more tolerant 

 of maritime stresses. An unexpected 

 washover event in December 1986 further 

 tested these species' tolerances to seawater 

 inundation and sand burial. Camissonia did not 

 survive (Fink and Zedler 1990). 



The exotic dune plants have adverse effects 

 on native plants and animals. The interaction 

 between the exotic sea rocket and the native 

 sand verbena was investigated experimentally 

 by Wood (1987), who found the exotic to have 

 competitive superiority. The hottentot-fig 

 displaces native dune plants (Williams and 

 Williams 1984). It is an extremely 

 aggressive colonizing species, which is why it 

 is sometimes planted to reduce dune erosion. 

 However, where it dominates large areas of 

 coastal sand, it may alter more than just the 

 native vegetation. 



Hottentot-fig provides little food or 

 habitat for native insects (Nagano, pers. 

 comm.). Recent research at Tijuana Estuary 

 (S. Snover, in prog.) indicates that native 

 burrowing insects are at a disadvantage under 



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