Red flour beetles balance thermoregulation and food acquisition via density‐dependent habitat selection

Abstract

Theories of habitat selection assume that habitat selection patterns are based on the fitness consequences of selecting a particular habitat, and predict that individuals should be distributed between habitats so that each individual obtains the same fitness. The predictions are relatively simple when habitat suitability is based upon the quantity of depletable resources, such as food, in a habitat: individuals should be distributed between habitats in proportion to the depletable resources in those habitats. Yet, non‐depletable resources can also be important in habitat selection. For example, ectotherms must obtain heat from the environment, which causes them to select habitats based, at least partly, upon thermal quality. Non‐depletable resources can cause habitat selection that is independent of density and may modify the value of depletable resources. We used red flour beetles Tribolium castaneum to test the hypothesis that habitat selection by ectotherms depends upon both food abundance and temperature. We determined the thermal preference of red flour beetles. We then conducted habitat selection experiments with beetles when habitats were set at their preferred temperature and 10°C below their preferred temperature. We simultaneously manipulated food abundance in both habitats, and varied population density. We also examined the fitness effects of habitat selection by measuring oviposition rates of beetles. Beetles selected the habitat within their preferred temperature when food was equal between habitats and when food was higher in that habitat across all population densities. Beetles showed equal preference for high‐ and low‐temperature habitats when food was higher in the low‐temperature habitat across all population densities. Fecundity was always higher at the preferred temperature of beetles, regardless of food abundance or population density. Temperature is clearly an important factor in habitat selection of ectotherms and should be considered whenever thermal differences exist between habitats.

A stringent test of the thermal coadaptation hypothesis in flour beetles

“…T set for T. castaneum in the present study was similar to that in a previous study (30°C ;Halliday and Blouin-Demers, 2014), and so was the B 80 of the thermal reaction norm for egg laying (between 30 and 35°C; Halliday et al, 2015). A study over 60 years ago (Park and Frank, 1948) examined reproduction by T. castaneum and T. confusum at three temperatures (24, 29, 34°C) and showed similar patterns to those uncovered in the present study: highest reproductive output and quickest development time at 34°C for both species, although the differences between these variables at 29 and 34°C were small.…”

Can temperature modify the strength of density‐dependent habitat selection in ectotherms? A test with red flour beetles

“…Environmental temperature is a major component of habitat suitability for ectotherms due to the powerful effects temperature exerts on all aspects of their biology (Huey, ; Blouin‐Demers & Weatherhead, ; Huey & Berrigan, ; Deutsch et al ., ; Lelièvre et al ., ; Amarasekare & Savage, ). For example in ectotherms several important processes are maximized within a narrow range of body temperatures (termed the optimal temperatures), including locomotion (Stevenson, Peterson & Tsuji, ; Blouin‐Demers & Weatherhead, ; Halliday & Blouin‐Demers, ), growth rate (Angilletta, Steury & Sears, ), energy acquisition (Bergman, ), energy assimilation (Stevenson et al ., ; Angilletta, ) and reproductive output (Berger, Walters & Gotthard, ; Halliday & Blouin‐Demers, ; Halliday & Blouin‐Demers, ; Halliday, Thomas & Blouin‐Demers, ). Moreover, temperature can modulate the density dependence of fitness, where negative density dependence is strongest at the optimal temperature and weakens as temperature deviates from the optimal temperature (Halliday & Blouin‐Demers, ; Halliday et al ., ).…”

“…For example in ectotherms several important processes are maximized within a narrow range of body temperatures (termed the optimal temperatures), including locomotion (Stevenson, Peterson & Tsuji, ; Blouin‐Demers & Weatherhead, ; Halliday & Blouin‐Demers, ), growth rate (Angilletta, Steury & Sears, ), energy acquisition (Bergman, ), energy assimilation (Stevenson et al ., ; Angilletta, ) and reproductive output (Berger, Walters & Gotthard, ; Halliday & Blouin‐Demers, ; Halliday & Blouin‐Demers, ; Halliday, Thomas & Blouin‐Demers, ). Moreover, temperature can modulate the density dependence of fitness, where negative density dependence is strongest at the optimal temperature and weakens as temperature deviates from the optimal temperature (Halliday & Blouin‐Demers, ; Halliday et al ., ). Since the negative density dependence of fitness is a crucial aspect of density‐dependent habitat selection (Fretwell & Lucas, ; Rosenzweig, ; Morris, ; Morris, ), changes in temperature should modify the density dependence of habitat selection in ectotherms.…”

“…We then test these predictions using a controlled habitat selection experiment with red flour beetles ( Tribolium castaneum ). We use our previous results on the strength of fitness‐density functions (Halliday & Blouin‐Demers, ; Halliday et al ., ) to make specific predictions about how red flour beetles should select between habitats differing in the quantity of food as temperature changes. For the first time, this allows us to examine specifically how temperature affects the strength of density‐dependent habitat selection in an ectotherm.…”

“…Briefly, fitness was the per capita number of eggs laid over 4 days by red flour beetles at five density treatments (10, 20, 30, 40 and 50 beetles), two food treatments (0.625 and 2.5 mL of wheat flour) and three temperature treatments (20, 25 and 30°C). In red flour beetles, T o = 30°C according to multiple measures of fitness (Halliday & Blouin‐Demers, ; Halliday & Blouin‐Demers, ; Halliday et al ., ). We opted to use egg laying rate as our metric of fitness because it is easy to measure and is correlated with the number of adult offspring produced, which is a more ultimate measure of fitness (rho = 0.42 and 0.49 at 25 and 30°C, respectively, between the number of eggs laid and the number of adult offspring produced from data in Halliday et al ., ).…”