Microclimates: Thermal Shields Against Global Warming for Small Herps

Thermal microhabitats are often uncoupled from above-ground air temperatures. A study focused on small frogs and lizards from the Philippines demonstrates that the structural complexity of tropical forests hosts a diversity of microhabitats that can reduce the exposure of many cold-blooded animals to anthropogenic climate warming.

Reproductive pair of the Luzon forest frogs Platymantis luzonensis (upper left), a IUCN near-threatened species restricted to < 5000 km² of habitat. Lower left: the yellow-stripped slender tree lizard Lipinia pulchella, a IUCN least-concerned species. Both species have body lengths < 6 cm, and are native to the tropical forests of the Philippines. Right panels, top to bottom: four microhabitats monitored by Scheffers et al. (2), namely ground vegetation, bird’s nest ferns, phytotelmata, and fallen leaves above ground level. Photos courtesy of Becca Brunner (Platymantis), Gernot Kunz (Lipinia), Stephen Zozaya (ground vegetation) and Brett Scheffers (remaining habitats).

If you have ever entered a cave or an old church, you will be familiar with its coolness even in the dog days of summer. At much finer scales, from centimetres to millimetres, this ‘cooling effect’ occurs in complex ecosystems such as those embodied by tropical forests. The fact is that the life cycle of many plant and animal species depends on the network of microhabitats (e.g., small crevices, burrows, holes) interwoven by vegetation structures, such as the leaves and roots of an orchid epiphyte hanging from a tree branch or the umbrella of leaves and branches of a thick bush.

Much modern biogeographical research addressing the effects of climate change on biodiversity is based on macroclimatic data of temperature and precipitation. Such approaches mostly ignore that microhabitats can warm up or cool down in a fashion different from that of local or regional climates, and so determine how species, particularly ectotherms, thermoregulate (1). To illustrate this phenomenon, Brett Scheffers et al. (2) measured the upper thermal limits (typically known as ‘critical thermal maxima’ or CTmax) of 15 species of frogs and lizards native to the tropical forest of Mount Banahaw, an active volcano on Luzon (The Philippines). The > 7000 islands of this archipelago harbor > 300 species of amphibians and reptiles (see video here), with > 100 occurring in Luzon (3).

CTmax is typically estimated in experimental settings as the body temperature at which an ectotherm, exposed to gradually increasing temperatures, loses motor coordination prior to enzyme degradation and, ultimately, death (4). Scheffers et al. (2) quantified how often the temperature of the microhabitat and surrounding habitat exceeded the CTmax of the study species. They used thermal sensors to record environmental temperatures every 20 minutes above and below ground, within and outside phytotelmata (small hollows in plants that fill with water), in bird’s nest ferns, on ground vegetation, and in tree canopies – from May to September 2011. They found that frog and lizard CTmax varied between 33 and 37 °C, far above the average temperature recorded in their microhabitats (~20-23 °C). All seems nice up to here. But what is ecologically relevant is not the average temperature, but the temperature extremes.

Those days and hours of the day when temperatures peaked, Scheffers et al. showed that temperatures above ground, outside phytotelmata, and in tree canopies exceeded CTmax 10 to 30 times more frequently than below ground or within phytotelmata, bird’s nest ferns, or ground vegetation (2). Additionally, for a 1 °C increase in local temperatures, the target microhabitats heated up by only 0.1 (soil), 0.3 (ferns), 0.5 (ground vegetation), and 0.7 °C (phytotelmata).

We can confidently state that rain forest microhabitats buffer local thermal anomalies. Moreover, modelling of change in local temperatures, given the IPCC’s [Intergovernmental Panel on Climate Change] worst-scenario projections of anthropogenic climate warming by 6 °C by the end of this century (5), further indicated that the native small frogs and lizards of Mount Banahaw might only be able to survive future heat waves by sheltering mainly below ground or within phytotelmata, likely forcing them to change their thermoregulatory behavior.

Landscape heterogeneity is a composite of variation in vegetation structure and topography (e.g., altitude, shading, exposure), and generates a spatial mosaic of thermal microhabitats. Within the same landscape, it is not rare to find spatial differences in air temperature at the same time of the day of up to 20 °C! (6, 7). In the face of the ongoing and predicted increase in the frequency of heat waves as a result of anthropogenic climate change (8, 9, 10, 11), small animals already rely, and will do so more strongly in the next few decades, on the availability of benign microhabitats and on their capacity to move from one microhabitat to another.

Not only that, air temperature and water availability are two faces of the same coin when it comes to understanding biodiversity responses to climate change. Thus, extreme temperatures come together with rain and air-humidity declines (12), which can also impact microhabitat quality. So a hole in a fallen tree can be thermally cosy for a frog, but inhospitable if waterless or too dry.

Overall, half of the tropical forests of the Philippines have been logged in the 20^th Century (13), and South East Asia can claim the fame for the largest planetary rate of human-made forest destruction (14) – recently named ‘Navjot’s nightmare’, giving credit to the legacy of late conservation biologist Navjot Sodhi (15). In particular, large-scale logging and fires^* in this region are destroying rainforests (and peatlands) at an unprecedented pace, adding toxic hazes to the problem of ecosystem destruction (16).

Because canopy temperatures in native forests are hotter than at ground levels (17), native forests, including their tallest and shortest trees and bushes, and the associated network of microhabitats, constitute a thermal shield against ongoing climate warming. But logging and fire, and the growing replacement of primary forests by secondary forests and agricultural lands, is wiping out that thermal shield for multiple species; in other words, there is a pronounced synergy in the ecological impacts driven by anthropogenic climate change and habitat loss (18, 19, 20).

One reason (one more!) for a pledge to conserve native forests everywhere.

^*See here and here two NASA’s satellite images showing an everyday scene of multiple fires (red dots) and smoke clouds in Southeast Asia.

—

by Salvador Herrando-Pérez & David R. Vieites

(with support of the British Ecological Society and the Spanish Ministry of Economy, Industry and Competitiveness) 

References

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