Negative relationships between population density and metabolic rates are not general

Abstract

Population density has recently been suggested to be an important factor influencing metabolic rates and to represent an important 'third axis' explaining variation beyond that explained by body mass and temperature. In situations where population density influences food consumption, the immediate effect on metabolism acting through specific dynamic action (SDA), and downregulation due to fasting over longer periods, is well understood. However, according to a recent review, previous studies suggest a more general effect of population density per se, even in the absence of such effects. It has been hypothesized that this results from animals performing anticipatory responses (i.e. reduced activity) to expected declines in food availability. Here, we test the generality of this finding by measuring density effects on metabolic rates in 10 clones from two different species of the zooplankton Daphnia (Daphnia pulex Leydig and D. magna Straus). Using fluorescence-based respirometry, we obtain high-precision measures of metabolism. We also identify additional studies on this topic that were not included in the previous review, compare the results and evaluate the potential for measurement bias in all previous studies. We demonstrate significant variation in mass-specific metabolism among clones within both species. However, we find no evidence for a negative relationship between population density and mass-specific metabolism. The previously reported pattern also disappeared when we extended the set of studies analysed. We discuss potential reasons for the discrepancy among studies, including two main sources of potential bias (microbial respiration and declining oxygen consumption due to reduced oxygen availability). Only one of the previous studies gives sufficient information to conclude the absence of such biases, and consistent with our results, no effect of density on metabolism was found. We conclude that population density per se does not have a general effect on mass-specific metabolic rate.

An empirical test for a zone of canalization in thermal reaction norms

“…The gut lengths (GL, mm, measured from the top of midgut to the bottom of hindgut when the animal is relaxed) of each individual as neonate and mature were measured using ImageJ v1.48 (National Institutes of Health, Bethesda, MD). These length measurements were then transformed to dry mass (DM, mg) using the following relationship between dry mass (DM) and gut length (GL): DM = 0.00679GL 2.75 (modified from Yashchenko et al ., ; see Appendix S1). Using dry mass of neonates (DM start ), dry mass at maturation (DM end ) and the number of days between the two measurements (duration), the somatic growth rate (SGR) was calculated as follows:…”

Is there plasticity in developmental instability? The effect of daily thermal fluctuations in an ectotherm

“…The lengths of the animals (GL; gut length, measured from the apex of the foregut to the base of the hindgut) were measured to the nearest 0.01 mm using photographs from a stereo microscope (Leica Microsystems GmbH, Germany) and the software ImageJ (Rasband, ‐2016). Body masses (BM, dry weight, mg) were estimated by linear regression, using previously measured data on gut length (GL) and BM (Yashchenko, Fossen, Kielland, & Einum, ): BM = 0.00681 × GL 2.75 ( df = 30, r 2 = 0.99, p < .001). Details regarding the respirometric and the BM‐GL regression procedure are given by Yashchenko et al.…”

“…Details regarding the respirometric and the BM‐GL regression procedure are given by Yashchenko et al. ().…”

Effect of Temperature Rising on the Stygobitic Crustacean Species Diacyclops belgicus: Does Global Warming Affect Groundwater Populations?

“…The microplate was integrated with a 24-channel fluorescence-based respirometry system (SDR Sensor Dish Reader) (PreSens Precision Sensing GmbH, Regensburg, Germany) that allows simultaneous measurement of 20 replicates and 4 controls (wells without animals). This respirometer is known for its simplicity, high throughput, and high temporal resolution and sensitivity [28,29]. The reader was placed inside the thermostatic cabinet at the appropriate testing temperatures (either 14 or 17 • C; accuracy: ±0.1 • C) 18 h before the beginning of the trials to bring the equipment and the standard water to the same temperature prior to measurements [20,29].…”

“…This respirometer is known for its simplicity, high throughput, and high temporal resolution and sensitivity [28,29]. The reader was placed inside the thermostatic cabinet at the appropriate testing temperatures (either 14 or 17 • C; accuracy: ±0.1 • C) 18 h before the beginning of the trials to bring the equipment and the standard water to the same temperature prior to measurements [20,29]. Twenty copepodids of D. belgicus were then individually loaded in the microwells that were in turn inspected for air bubbles under a stereomicroscope (Leica M80, Leica Microsystems Srl, Wetzlar, Germany) at 20× magnification.…”