Why tropical forest lizards are vulnerable to climate warming

Abstract

Biological impacts of climate warming are predicted to increase with latitude, paralleling increases in warming. However, the magnitude of impacts depends not only on the degree of warming but also on the number of species at risk, their physiological sensitivity to warming and their options for behavioural and physiological compensation. Lizards are useful for evaluating risks of warming because their thermal biology is well studied. We conducted macrophysiological analyses of diurnal lizards from diverse latitudes plus focal species analyses of Puerto Rican Anolis and Sphaerodactyus. Although tropical lowland lizards live in environments that are warm all year, macrophysiological analyses indicate that some tropical lineages (thermoconformers that live in forests) are active at low body temperature and are intolerant of warm temperatures. Focal species analyses show that some tropical forest lizards were already experiencing stressful body temperatures in summer when studied several decades ago. Simulations suggest that warming will not only further depress their physiological performance in summer, but will also enable warm-adapted, open-habitat competitors and predators to invade forests. Forest lizards are key components of tropical ecosystems, but appear vulnerable to the cascading physiological and ecological effects of climate warming, even though rates of tropical warming may be relatively low.

Figure 1

Thermal sensitivity of diurnal lizards versus latitude. (a) Heat tolerance (CT_max), (b) optimal sprint temperature (T_o), (c) mean-field body temperature (T_b) and (d) cold tolerance (CT_min) versus latitude. In phylogenetic analyses, CT_max, T_o and T_b are independent of the latitude but are relatively low for certain taxa and for non-basking species, especially tropical ones (black circles; see text); by contrast, CT_min decreases with latitude. The four points for non-baskers and mid-latitude are cryptozoic species (see text). Points for non-baskers are offset and shifted by 0.5° latitude to increase spread (see data in table 1 in the electronic supplementary material).

Figure 2

Thermal safety margin (T_o−T_a,max) for diurnal lizards increases with latitude. Open circles, basking species; filled circles, non-basking species. In a phylogenetic analysis, thermal safety margin increases with latitude. The tropical species with a large safety margin (Sceloporus magister) is montane.

Figure 3

Impact of climate warming on T_b and performance of a tropical lizard. (a) Body temperature and (b) predicted relative sprint performance of A. cristatellus in a shaded forest at Punta Salinas, Puerto Rico before (grey boxes, 1973) and after (red boxes) predicted climate warming (air temperature increase of 3°C). (c) Body temperature and (d) predicted relative sprint performance of A. cristatellus in a forest at San German, Puerto Rico, before (1983–1984) and after predicted warming (T_a=+3°C). In (a, c), the shaded grey rectangle delimits the preferred temperature range of this species, and the red dashed line indicates the CT_max (Huey & Webster 1976). Box plots depict the median, interquartile range and range.

Figure 4

Predicted impact of recent climate warming on a tropical forest lizard. Observed climate warming between 1972 and 2008 (see figure 1 in the electronic supplementary material) is likely to make forest habitats (El Verde, Puerto Rico) less suitable in summer for resident forest species but more suitable for an open-habitat species. T_bs of the forest species (A. gundlachi) in July 1972 are shown in grey boxes, and most T_bs were close to the T_p range of this species (lower shaded grey rectangle). By 2008, observed warming of 2.1°C at El Verde should have elevated predicted T_b of lizards in the forest (red boxes). These predicted T_b are much higher than those preferred by A. gundlachi, but should now be close to the preferred temperatures (upper shaded red rectangle) of A. cristatellus, which was restricted to open habitats in 1972. If warming continues, A. cristatellus could displace A. gundlachi.