Climate impacts across latitude – from physiology to food security

Background/Question/Methods

Climate change poses both direct and indirect threats to global food security.  Over 800 million people are malnourished today, 95% of these people are in tropical or subtropical areas, and estimates suggest this number will grow substantially with climate change.   Estimating the effects of climate change on global food production is thus a critical challenge.  Two approaches are commonly used: crop models, which include direct effects of CO2, precipitation and temperature changes, and field studies, which use correlations between field yield and climate. Neither of these approaches explicitly models changes in agricultural pest pressure, most of which comes from ectotherms.  Here we focus on three crops: Rice, Wheat, and Maize, which together provide 50% of the calories consumed by the food insecure and suffer yield losses of 8 to 15% due to ectotherm pest pressure,  and we calculate changes in insect pest pressure using two semi-mechanistic compound relationships:  1) the relationship between insect food consumption, insect metabolism and ambient temperature, and 2) the relationship between climate variation, insect population growth, and ambient temperature. 

Results/Conclusions

 The relationship between metabolism and temperature predicts large proportionate increases in food consumption in cooler temperate areas (> 50% increases throughout much of the temperate and polar regions) and much smaller increases in tropical regions, and direct estimates of insect consumption rates as a function of temperature predict even steeper gradients from the equator, where changes may be minimal, to cooler regions, where consumption could increase precipitously.   Projected insect population growth shows a similar gradient, with the potential for 20+% increases in population grown rates in temperate and polar regions, and declines of 10 to 15% in tropical regions. Combining these impacts predicts much stronger growth in insect pest pressure in north temperate regions (doubling of pest pressure in many areas), and moderate to zero growth throughout much of the humid tropics.  These projections present substantial problems for the production of all four of our focal crops, with particular problems for Rice and Maize, as areas with the highest yield for both these species are also areas where insect pest damage is projected to peak. These projected increases in insect pest damage do not account for changes in higher trophic levels (predators and parasitoids of herbivores), which could constrain growth in herbivore populations, but they point to the need for rigorous field studies focused on the links between temperature and insect pest physiology and consumption.