I notice when I work in my garden in the Dominican Republic that every shovelful of soil contains an abundance of earthworms – this, despite the somewhat clayey texture of the soil, the ubiquity of fire ants, and the presence of at least two species of invasive land planarians.
Conversely, I note that where I am currently staying in California, I cannot find any earthworms at all – perhaps the climate is too dry.
I have read of the problem of invasive earthworms in areas that were under ice during the last glaciation, and on oceanic islands. Neither of those situations apply to the Dominican Republic; the Greater Antilles, at least, are made from continental crust. The fossil record in Dominican amber has suggested the vicariance theory – the idea that the Caribbean Islands were once a “Central America,” that is, a land bridge connecting the two American continents, and separated into islands later on as tectonic plates moved around. If that is the case, it would be possible for earthworms to have colonized on their own during that time. The absence of those fossil South American taxa in the Caribbean today is explained by the idea that during the last glacial period, the Islands experienced a temperate climate, perhaps like the mid-Atlantic United States today, where tropical taxa could not survive. This might not have affected earthworms as much, though, since they live underground where temperatures are more uniform.
Unfortunately, when trying to find information about Caribbean earthworms, not only could I not find any identification materials, but I could also not even find any definite statement as to whether earthworms, as a group, are native or invasive in the Islands. Does anyone know of any research into this?
Trying to ID earthworms, I tried to find the reason for European worms being present on continents that are simply not reachable by subterranean animals. Google Scholar yielded several articles that made it clear for me that the puzzling worm observations are due to the gardeners of the past empires transporting soil for their colonial government lawns and mansions.
I would start to eliminate or confirm the possibility of the Hispaniola island getting its earthworm load from France and Spain, or their other colonies.
Check out Fragoso et al. 1999 (“A Survey of Tropical Earthworms: Taxonomy, Biogeography and Environmental Plasticity”) and Gonzalez et al. 2006 (“Earthworm invasions in the tropics”). Hispanola apparently has a very large number of endemic earthworms, with Fragoso et al. 1999 predicting ~130 native species on the island. The Greater Antilles have a number of earthworms because of their history as continental islands, whereas the Lesser Antilles apparently have no native species.
Of course, there are also the typical invasives like Lumbricus in the Greater Antilles in addition to the endemics. Some of these species appear to have been brought in by Europeans (some of the species like Gordiodrilus peguanus and Eudrilus eugenia are only native to Africa and appear to have been brought over as a result of the slave trade (they don’t occur in regions like Mexico and Peru where few slaves were imported; Fragoso et al. 1999), whereas other genera (Pontoscolex, Onychochaeta and Eukerri) appear to have been introduced to the island by pre-Columbian civilizations migrating to the islands from South America.
Are you in Southern California, in a particularly rocky area, or on a steep slope with little soil?
In my experience, earthworm abundance varies enormously across California by soil type. You can have one area with many earthworms, and a few meters away find none.