Human systems depend on natural systems.

St. Croix’s unique and complex natural environment plays an important role in the island’s opportunities and challenges. Rich soils on the island advanced a long agricultural history under colonialism. But in the present day, the island’s hot and dry climate puts stress on island systems, while natural threats like sargassum and flooding jeopardize island infrastructure. While the rich biodiversity of the island is a major selling point, contributing to its resilience and beauty, it is also under threat. In the midst of a changing climate, the protection of St. Croix’s natural environment will inform how the island responds to shocks now and into the future.

The climate of St. Croix is defined by its geographic positioning, which places it in the sub-tropical zone, where the trade winds blow along the length of the island. The island is warm all year, with little variation in average temperature between months or seasons. Temperatures are expected to grow more extreme with the onset of climate change.36

Precipitation across the island is quite variable. The western part of the island receives substantially more precipitation than the east end. Annually, on average, it rains 30 inches in the east and 50 inches in the west and most rainfall occurs during the hurricane season.37 Precipitation varies substantially by season. The wettest month is September and the driest months are February and March. Severe seasonal droughts are a reoccurring problem on the island and are expected to worsen in the future.38

Geology

Geology and Groundwater

Terrestrial systems depend heavily on soil types. Soils contribute to the diversity of habitat types on the island and range from stony, to gravelly, sandy, silty, and loamy.39 Some soils are more suitable for agricultural practices than others, but these properties would not exist without the underlying geology of St. Croix. Much of St. Croix is well-suited for agriculture, and it has historically been known as the breadbasket of the Lesser Antilles.40

St. Croix is an elevated island with few regions existing below 70 feet;41 this topography influences how and where rainwater collects. When St. Croix formed, it was uplifted and created an extensive reef system composed of limestone.42 The Limestone Plain separates the Northside Range and the East End Range. However, limestone and alluvial deposits (small areas with a specific geologic features) exist throughout the island. During rain events, the alluvial deposits are responsible for recharging the aquifer systems on St. Croix.43 The aquifers on St. Croix are associated with dissolved solids contaminants, complicating access to fresh water.44 The threats of drought on the island are compounded by the lack of fresh groundwater or freshwater streams.

Ecology

St. Croix’s natural environment is rich in unique features and biotic communities, from the rugged terrain of the upland forests to the grassy plains down the south coast and the immense coral reef system.45 The island’s natural resources provide a variety of habitats that sustain diverse species, and this rich biodiversity contributes to the resilient systems that support human life.

The marine shelf, reef system, and mangroves all have a unique relationship with the hydrology of St. Croix. The increasing depth of the marine shelf, composed of scattered elkhorn, brain, fire, boulder, and staghorn corals, and its associated canyon help attenuate wave action.46 However, attenuating wave action is something mangroves and coral reefs do as well. Coral reefs can help stabilize shorelines and support a variety of marine life.

Likewise, mangroves stabilize shorelines and blossom with various wildlife species; additionally, mangroves sequester carbon and reduce flooding. Red, black, and white mangroves are located primarily along shorelines with red being the most salt and water-depth tolerant and white mangroves dominating inland regions and higher elevations.47 Mangroves are unique systems playing a specific role in marine and terrestrial ecosystems due to their unique tolerance to flooding and salinity. The underwater habitats created by submerged mangrove roots are important to the life cycles of some economically relevant fish species. Unfortunately, the USVI has lost 50% of mangrove habitats between 1980 and 2005. The largest mangrove forest on St. Croix was destroyed in the 1960s to build the island’s oil refinery. The loss of these special habitats threatens the biodiversity of the island as well as its resilience to storm forces.

Mangroves cover less than 1% of the island’s land and reefs cover 67% of its shelf.

St. Croix has diverse terrestrial habitats as well; these habitats include forests and shrublands. 72% of the island is categorized as subtropical dry habitat, with the remaining being categorized as subtropical moist forest.48 Variations in rainfall across the island create a variety of ecosystems. Most protected natural spaces on St. Croix include one of the above natural systems - highlighting the need to appreciate and steward these systems well. These spaces are not only protected for their beauty but, also, for their ecological services.

The existing conditions of some of St. Croix’s natural systems display how interconnected human systems are with natural systems. One native species, Sargassum—a type of floating seaweed, has led to several challenges for St. Croix’s infrastructure ultimately resulting in a declared emergency. Sargassum is naturally occurring; however, it is currently overabundant and creates thick mats. These thick mats clog the guts and the Seven Seas Desalination Plant intake pipes. Clogged guts can lead to slow drainage of water on land and clogged Plant intake pipes make it nearly impossible to treat and deliver potable water. The Sargassum influx may be linked to pollution that originates outside of the Caribbean.49

The threat of pollution is significant given the island’s industrial history, which has impacted air, soil, and water quality. Pollution is a major threat to coral reefs and aquatic ecosystems because these systems rely on delicate water chemistry to survive. Similarly, mangroves and other important terrestrial and shoreline systems are threatened by development, which can either decrease the functionality of or completely remove habitats altogether.