High temperature intensifies negative density dependence of fitness in red flour beetles

Abstract

Competition for food, space, or other depletable resources has strong impacts on the fitness of organisms and can lead to a pattern known as negative density dependence, where fitness decreases as population density increases. Yet, many resources that have strong impacts on fitness are nondepletable (e.g., moisture or temperature). How do these nondepletable resources interact with depletable resources to modify negative density dependence? We tested the hypothesis that negative density dependence is modulated by temperature in red flour beetles and tested the prediction that the strength of negative density dependence should decrease as temperature decreases. We measured the number of eggs laid, offspring development time, and the number of offspring that reached maturity at three temperatures and two food treatment combinations as we simultaneously manipulated adult population density. We demonstrated that low temperatures weaken negative density dependence in the number of eggs laid; this pattern was most evident when food was abundant. Density had no effect on development time, but low temperatures increased development time. The percent of eggs that emerged as adults decreased with both density and temperature and increased with food. Temperature, an abiotic driver, can thus modulate density-dependent processes in ectotherms. Therefore, models of population growth for ectotherms should incorporate the effects of temperature.

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

“…We assume that neither R Ti nor U Ti are linear; they should follow the general shape of the thermal reaction norm for fitness in the species: a gradual increase as temperature approaches T o followed by a sharp decrease as temperature increases above T o (Angilletta, ; Bulté & Blouin‐Demers, ; Halliday & Blouin‐Demers, ). Thus, as temperature deviates from T o , fitness decreases and negative density dependence of fitness weakens (Halliday et al ., ).…”

“…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.…”