By Paula Ezcurra
Not all islands are created equal
The Gulf of California extends 1,070km along a northwest-southeast axis between the Baja California Peninsula and mainland Mexico. Dotting the waters are over 100 islands and smaller islets that can be divided into three broad categories: the northern islands, the central midriff islands, and the southern gulf islands. In November 2018, we visited 14 of the southern gulf islands from Isla Tortuga to Isla Cerralvo (Fig.1), documenting the flora and fauna of each island. As I began to compare the islands we visited, it became apparent how vastly different each one was from the other. Though superficially all islands have similar desert scrub vegetation type and associated fauna, these islands differ deeply among themselves due to three factors: geologic origin, connectedness to the continent, and age. These factors have a strong bearing on the biological and evolutionary history of an island.
The islands of the gulf can be primarily classified into either volcanic or uplift islands. Volcanic islands, such as Tortuga Island (Fig.2a), formed via the accumulation of mountains of cooled lava from eruptions of underwater volcanoes. Uplift islands, as the name suggests, formed from uplifting of the ocean floor during events of seafloor spreading from huge tectonic forces causing oceanic crust to rise and form emerged islands. These such islands can be formed by granite (Fig.2b) or by gypsum (Fig.2c). Additionally, some islands were also formed by faulting, breaking off from the continent when the present-day peninsula separated from mainland Mexico, leaving fragments of this faulting as islands.
Figure 2 – Examples of islands of different geologic origin. San Diego island (top) is composed of granite, an igneous intrusive rock. San Marcos island (middle) is partially volcanic and partially gypsum, a type of sedimentary rock made of calcium sulfate. Lastly, Tortuga island (bottom) is extrusive igneous rock formed by cool lava. Photo credit: Jon Rebman
The degree of connectedness an island has with the continent depends not only on its physical distance from the coast, but also on the depth of the channel that separates islands from the Baja California Peninsula. During Pleistocene Glaciation events, where a large proportion of Earth’s water was trapped in icecaps, there was a marked decrease in sea level of around 80-100m. Consequently, islands with shallower (less than 100m) channels became connected to the mainland. These islands are known as “landbridge” islands, in contrast with true oceanic islands that have much deeper channels and were never connected to the mainland.
The islands along the Gulf of California vary considerably in their age. Many of the volcanic islands, like Tortuga, formed during relatively recent eruptions less than 40 thousand years ago. Other islands, like San José, were formed when the Gulf was a shallow body of water over 3 million years ago. To put this difference in age in context: If we convert the age of the millennia-old islands into the time-scale of a single year, then the more recent volcanic islands formed on December 29th of our geological “year”!
What influence do these differences have?
The origin, connectedness, and age of an island in the Gulf has profound influences on the organisms that inhabit it. For example, in San Marcos Island, where a large portion of the substrate is gypsum, there is unique flora adapted to grow on this calcium sulfate sediment. Landbridge islands allowed for easier dispersal of plants and animals from the mainland, whereas migration to oceanic islands would be much more difficult, impacting the resulting biological make up of each island. Finally, the age of an island is a indicator of evolutionary time, and could potentially impact the level of endemism found on each island. For these reasons, an understanding of the original and age of islands in critical to the study the biogeography of a region.