The subtropical climate and its diversity

If there is anything that defines the islands located in the subtropical band of the planet, it is their ability to concentrate biodiversity, unique beauty, and singular ecosystems in just a few hundred or thousand square kilometers. From Hawaii and Cuba to the Canary Islands and Madeira, subtropical islands surprise with their rich mosaic of lives and landscapes. This is largely due to the meteorological conditions of the climate that envelops them.

Much more than a 'pleasant temperature all year round', the subtropical climate leaves a deep mark on everything that occurs in these territories. The relative mildness of winter and the warm but rarely extreme heat of summer, the alternation—in some cases—of wet and dry seasons, the role of the sea, and the influence of the trade winds, shape the ecological rules of the game. None of this is static: each archipelago, even each island, experiences the subtropical in a particular way, closely linked to its geographical situation, its orography, and the characteristics of each season.

A globally decisive climatic visibility

Despite occupying only about 5% of the land territory, subtropical islands gather a wealth of flora and birds that represents nearly 17% of global biodiversity. This data, obtained from reports by UNESCO and IPCC, illustrates the impact of a climate of relative thermal stability and modulated humidity on biological diversity.

Only in the Canary Islands, nearly 45% of the birds are found only in these islands, and in Cuba almost half of the flora is exclusive. The reason is twofold: on the one hand, the temperate but never extreme climate offers stable refuge to species that, in other places, succumbed to glaciations, fires, or droughts; on the other hand, isolation fosters each species to evolve at its own pace, seeking its own ecological niche.

These characteristics have allowed the proliferation of species and ecosystems that often border on the unbelievable. The case of the Hawaiian honeycreepers, those birds that diversified into dozens of species with astonishing beaks from a single colonizing ancestor, is paradigm. In the Canary Islands, the same happened with the lauraceae of the fog forest and the Canary pine at drier altitudes.

Climatic characterization of subtropical islands

• Approximate latitude: between 20° and 35°.
• Average temperatures: winters rarely below 5–6 °C; warm summers between 27–32 °C.
• Precipitation:
  • Mediterranean style (e.g. Canary Islands, Balearic Islands): 350–900 mm/year, rainy winters and dry summers.
  • Monsoonal style (e.g. southern Japan, southeastern USA): up to 2,000 mm/year, concentrated summer rains.
• Relative humidity: high (70–80%), especially in wet seasons, due to marine influence.
• Frost risk and extremes: low, except in summits or particularly exposed areas.
• Wind patterns: predominance of trade winds or other systems that bring orographic clouds.

The sea, the wind, and the mountain: a climate shaped by reliefs

The ocean surrounding all subtropical islands acts as a gigantic thermal buffer. Minimum and maximum temperatures rarely reach extremes, and frost cycles are limited to the highest altitudes. A December 30th in Santa Cruz de Tenerife you can walk in short sleeves, while in the interior of the Iberian Peninsula frost falls.

The role of the trade winds is fundamental. These winds, upon hitting the coast and rising over the mountains, generate the famous 'sea of clouds'. On the windward slopes (usually the north and northeast), humidity is trapped, forming dense laurisilva forests or cloud forests, relict ecosystems from the Tertiary Europe that disappeared from the continent millions of years ago. On the leeward slopes, the rain shadow is so pronounced that the landscape turns dry, shaping pine forests and perfectly adapted xerophytic scrubs.

Miniature climatic diversity

Latitude, altitude, and wind exposure create microclimates, especially marked in mountainous islands. Thus, in Tenerife you can travel from arid beaches to cloud forests and summit meadows just 30 km apart. Madeira presents a similar gradient in just a few kilometers, and Hawaii, on its volcanic peak, observes occasional snowfall despite being in the tropics.

Biological adaptation: astonishing solutions to known challenges

Thermal stability and the absence of severe frost in subtropical climates favor communities dominated by shrubs and evergreen trees, with leaves that do not fall all at once. The ecological consequence is a green forest all year round, capable of maintaining constant primary production. However, the existence of highly marked dry or wet seasons also demands innovative evolutionary recipes.

Some species reduce their activity during the less favorable period; others synchronize reproduction with the onset of the rains. There are cases of true rarity, such as some reptiles or small mammals that only reproduce after the first major storm of the year, multiplying their population in a matter of days.

For its part, the fauna in subtropical islands shows unique responses to the lack of large predators and frost-free climates: flightless birds, giant insects, dwarf rodents, or viviparous reptiles. These adaptations, along with those inherent to the phenological cycle, increase the risk in the face of unusual climatic events or invasive species.

Examples of island adaptations

• Gigantism: insects, reptiles, or birds evolve to much larger sizes than their continental relatives (allowing them to exploit resources or defend themselves in the absence of predators).
• Dwarfism: in contrast, certain island mammals drastically reduce their size to adapt to limited resources.
• Loss of flight: birds, due to the absence of predators, stop flying and become more vulnerable with the arrival of invaders.
• Reproductive synchrony: species such as sea urchins, reptiles, or amphibians only reproduce in the climatically suitable season (after rains or in the midst of the humid summer).

Iconic ecosystems shaped by the climate

Just like the Canary laurisilva or the Macaronesian pine forests, unique biological formations arise in other subtropical regions. Mediterranean scrubs thrive thanks to the dry summer and rainy winter; in Caribbean and eastern Pacific islands, mangroves and xerophytic forests alternate based on water availability.

In the following list, some of the main ecosystems that depend on the subtropical climate in islands:

• Laurisilva forest: common in the Azores, Madeira, and the Canary Islands, it is the largest reservoir of Macaronesian endemic plants.
• Thermophile pines and junipers: dominate the dry areas of mid-elevation and intermediate altitudes.
• Mediterranean sclerophyllous shrubland: based on hard and aromatic shrubs, such as mastic trees and heathers.
• Mangroves, seagrass meadows, and coral reefs: in islands adjacent to warm seas.
• Summit ecosystems: unique in high-altitude islands, refuge for extreme endemisms.

Functions and ecological relationships

In these systems, the mutual dependence between species is absolute. A single actor – for example, a fruit bat in Samoa – can be responsible for all the seed dispersal of dozens of plants. If it disappears, the ecological chain breaks.

Similarly, the maintenance of ambient moisture by the laurisilva supports the existence of endemic invertebrates and birds; atmospheric alterations due to climate change could abruptly impoverish the entire biological spectrum.

Threats and vulnerability: climate as a double-edged sword

The subtropical climate, although conducive to diversity, has a dangerous flip side, as can be seen in the weather forecasts that frequently predict extreme eventualities. Island species, exquisitely adapted to stable conditions, are especially fragile to any alteration. Climatic eventualities such as extreme storms, hurricanes, prolonged droughts, or unusual heatwaves cause damages that take a long time to repair.

Some figures make it clear:

• A single cyclone can wipe out 30% of an endemic forest in just hours.
• 23% of island birds are threatened (compared to 11% globally).
• More than a third of threatened plants are endemic to islands.

As if that weren't enough, the introduction of exotic species and human pressure increase the exposure of these communities to new diseases, predators, and ecological disturbances.

Experience has shown, however, that the instrumentalization of biological reserves and the eradication of invaders can restore balances and allow for the recovery of species on the brink of extinction. Protecting this heritage, a result of millions of years of climatic and biological evolution, is an exciting challenge for science and environmental management in the 21st century.