We asked: Is Lycium californicum an ecotone species and what factors restrict it from overlapping more broadly with dominant plants of the salt marsh (downslope) and the coastal sage scrub (upslope)? Field sampling at Tijuana Estuary revealed that Lycium grows within a 1.1-m elevation range intermediate between that of Eriogonum fasciculatum (a subshrub of the coastal sage scrub) and Salicornia subterminalis (an obligate wetland subshrub of tidal marshes). Hence, Lycium was confined to the wetland-upland ecotone. In greenhouse experiments salt water wetting and soil moisture influenced these three species as follows: Salicornia required saturated soil to establish, restricting it to the wetland. Eriogonum failed to tolerate salt water wetting of the soil, both as seedlings (100% mortality) and as adults (100% mortality), likely restricting it to the upland. Lycium adults tolerated seawater wetting of the soil (100% survival) but seedlings did not (100% mortality). Both Lycium and Eriogonum should be able to establish seedlings near the wetland when soil salinity is low, but only Lycium would survive subsequent tidal surges (seawater wetting). Lycium showed evidence of drought tolerance but no requirement for groundwater (which would restrict it to the lowland); hence, its absence from the coastal sage scrub community may be due to competition with less salt-tolerant species. The few remaining populations of Lycium are restricted by the narrow range of suitable habitat and continuing pressure for coastal development. Thus, the transition from salt marsh to coastal sage scrub should be protected and restored to support this rare ecotone species.