When the seeds go floating in: a salt marsh invasion.
Biological invasions are one of the most important causes of global biodiversity loss. The human-mediated movement of species has increased significantly with globalization and the expansion of international trade. Seaports have thus become the entry points for a variety of organisms transported with cargo, ballast water or as biofouling, and, therefore, coastal and marine habitats around the world have become especially vulnerable to this problem. Salsola soda L. (Amaranthaceae) is an annual halophytic plant, native to the Old World. Its presence beyond its native range was recorded for the first time in the mid-twentieth century in two estuarial habitats, on the Pacific coast in the United States and on the Atlantic coast of Argentina, becoming invasive at both sites. It grows as dense, practically monotypic populations, just above the high tide line. When S. soda colonizes the elevated zones where some colonial coastal birds nest, it causes them to move to lower adjacent sites, increasing their exposure to the effect of tides and storms that can result in the loss of nests. To contribute to the understanding of the dynamics of the invasion and its projection, we assessed the production, viability and germination capacity of S. soda seed, analyzed the effects of different salt concentrations on its germination and measured its dispersal capacity by hydrochory. The species produces large quantities of fruit, exceeding 16,000 in larger plants. The percentage of germination is very high for young seed (almost 100% during the first five months after their release), decreasing to less than 10% a year later. The effects of salinity are almost negligible in NaCl solutions of 0-300 mM, with a slight decrease in the germination rate at the higher salinities. Fruits can remain floating in seawater for up to a week, retaining a high germination capacity, so marine currents can be highly effective vectors for their dispersal. The biological features of Salsola soda make it a serious threat to the study site, in particular the production of large quantities of seed easily transportable by water and wind and capable of becoming established under typical marine coastal conditions, anticipating a high potential for expansion in this environment and in others nearby, and invading other similar coastal areas worldwide. On the other hand, its annual life cycle and the short survival time of its seeds could be key for the development of control and eradication actions in invaded sites.