Does greater thermal plasticity facilitate range expansion of an invasive terrestrial anuran into higher latitudes?

Abstract

Temperature has pervasive effects on physiological processes and is critical in setting species distribution limits. Since invading Australia, cane toads have spread rapidly across low latitudes, but slowly into higher latitudes. Low temperature is the likely factor limiting high-latitude advancement. Several previous attempts have been made to predict future cane toad distributions in Australia, but understanding the potential contribution of phenotypic plasticity and adaptation to future range expansion remains challenging. Previous research demonstrates the considerable thermal metabolic plasticity of the cane toad, but suggests limited thermal plasticity of locomotor performance. Additionally, the oxygen-limited thermal tolerance hypothesis predicts that reduced aerobic scope sets thermal limits for ectotherm performance. Metabolic plasticity, locomotor performance and aerobic scope are therefore predicted targets of natural selection as cane toads invade colder regions. We measured these traits at temperatures of 10, 15, 22.5 and 30°C in low- and high-latitude toads acclimated to 15 and 30°C, to test the hypothesis that cane toads have adapted to cooler temperatures. High-latitude toads show increased metabolic plasticity and higher resting metabolic rates at lower temperatures. Burst locomotor performance was worse for high-latitude toads. Other traits showed no regional differences. We conclude that increased metabolic plasticity may facilitate invasion into higher latitudes by maintaining critical physiological functions at lower temperatures.

Keywords: Aerobic scope; Rhinella marina; cane toad; invasive species; metabolic rate; thermal plasticity.

Figure 1:

Resting oxygen consumption (V.o₂rest) of cane toads from low- and high-latitudes acclimated to 15 and 30°C. Filled squares indicate toads acclimated to 30°C; open circles indicate toads acclimated to 15°C. Continuous lines indicate northern populations; dashed lines indicatesouthern populations. (a) Low-latitude toads acclimated to 30°C and 15°C; (b) high-latitude toads acclimated to 30°C and 15°C; (c) low- and high-latitude toads acclimated to 15°C; (d) low- and high-latitude toads acclimated to 30°C. Analysis indicates a significant three-way interaction between region, test temperature and acclimation temperature (see Results section for details). For plotting, values of log V.o₂rest have been adjusted for the scaling effect of log mass. *Statistically significant (P < 0.05) pairwise differences at individual temperatures. n = 20 for each data point, and error bars represent SEM.

Figure 2:

Peak post-exercise rate of oxygen consumption (V.o₂max) for toads acclimated to 15 and 30°C. Filled squares indicate toads acclimated to 30°C; open circles indicate toads acclimated to 15°C. Data points indicate the combined response of toads from both low- and high-latitudes. Analysis indicates a significant two-way interaction of test temperature and acclimation temperature (see Results section for details). For plotting, values for V.o₂max have been adjusted for the scaling effect of log mass. *Statistically significant (P < 0.05) pairwise differences at individual temperatures. n = 40 for the group acclimated to 30°C and n = 39 for the group acclimated to 15°C. Error bars represent SEM.

Figure 3:

Absolute aerobic scope for toads acclimated to 15 and 30°C. Filled squares indicate toads acclimated to 30°C; open circles indicate toads acclimated to 15°C. Data points indicate combined response of toads from both low- and high-latitudes. For plotting, values were calculated from V.o₂max and resting V.o₂ that was adjusted for the scaling effect of body mass. Analysis indicates a significant two-way interaction of test temperature and acclimation temperature (see Results section for details). *Statistically significant (P < 0.05) pairwise differences at individual temperatures. n = 40 for the group acclimated to 30°C and n = 39 for the group acclimated to 15°C. Error bars represent SEM.