Biome: Tropical Scrub

On every continent except Antarctica, there are areas with dry or desert climates that receive enough rainfall to support shrubs as a dominant plant growth form. The land cover of these places consists of various types of shrublands or woodlands, which can be collectively known as “scrub.” Tropical scrub refers both to this climate zone as found in tropical and subtropical latitudes and also to the typical scrub vegetation type of those latitudes, characterized by dominance of multi-stemmed woody plants that are 1 to 5 m (meter) tall. It may also be known as brush, bush, or thicket, depending on local usage. Slightly higher woody plants with thorns, form the closely associated “thorn woodland.”

Globally, tropical scrub and its variants can be found in large areas of Africa (including Madagascar), Southern Asia (India, Thailand), Australia, Mexico and Central America, the Andes Mountains, Eastern Brazil, and enclaves in Venezuela and elsewhere. It may be localized in pockets of suitable climate in mountainous areas and is also positioned on the dry sides of tropical and subtropical islands. It is found where mean annual precipitation is on the order of 20 to 50 (70) centimeters/year, with mean annual temperatures from as low as 12 °C and as high as 30 °C.

One way to conceptualize the biogeography and distribution of tropical scrub is in relationship to its relative position on a theoretical environmental gradient associated with annual precipitation. Such a gradient would extend from vegetation formations such as tropical rain forests and tropical deciduous forests to tropical and subtropical deserts, following a gradient of wet to dry climates. Scrub would be situated in climate zones somewhat moister than the deserts, while drier than the conditions found in tropical deciduous forests. However, its position along this idealized moisture gradient is also shared with tropical savanna, a vegetation formation wherein dominance is typically shared between both grasses and scattered trees. Climate on its own is not enough to predetermine the dominant vegetation in a particular area. Instead, the influence of disturbances such as fire and the complicating effect of edaphic or soil-related factors also must be taken into account.

The shrubs and short-statured trees that dominate tropical scrub are often deciduous, losing their leaves as an adaption to extended dry seasons when little or no rain may fall for 4 to 8 months a year. The evergreen species often have small leaves to minimize water loss. In addition, many species have thorns, providing protection from herbivores. These plant species also have varying degrees of adaptation to fires. They may have some combination of a fire-resistant bark, the ability to resprout following limb loss, or seeds cued to grow on recently burned substrate. By definition, tropical scrub does not have a continuous graminoid (grass or sedge) layer, so it is less prone to annual or semiannual fires than in many tropical savannas because fuel loads are less. Some tropical scrub may include cacti or other succulent plants that are well adapted to survival in seasonally dry environments.

Tropical scrub offers an excellent opportunity to examine evolutionary convergence globally. This phenomenon occurs when unrelated species develop similar appearances and adaptations to the prevailing climatic conditions found in a certain biome type in different parts of the world. Because tropical scrub is found worldwide, there are lineages of plants that have independently evolved suitable adaptations and so are generally similar in size, morphology, and physiology, despite not being close relatives. The same would be true for animals specialized in using scrub habitats, including a variety of bird, lizard, and insect species.

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