Allometric scale model reveals temperature effects on growth and reproduction in Daphnia magna

Abstract

Climate change amplifies temperature variability, thereby subjecting organisms to increased stress as they more frequently encounter temperatures outside their optimal range. Temperature influences resource distribution across fundamental processes in organisms, such as metabolism, reproduction and overall fitness, yet energy allocation strategies are primarily understood under stable temperature conditions. Predicting organisms' responses to fluctuating temperatures, however, remains challenging. To address this gap, we develop an allometric growth model to predict energy allocation between growth and reproduction under both constant and variable temperature conditions. The model predictions perform and align well with the observed growth patterns of Daphnia magna, a keystone species in aquatic ecosystems, exposed to various thermal scenarios. Results indicate that exposure to unpredictable temperatures elicits growth and reproduction responses similar to those observed under consistently high temperatures. However, individuals exposed to unpredictable temperatures incur a disproportionate energetic cost compared to those in constant average or low-temperature conditions, significantly reducing estimated fecundity over time and lifespan. These findings highlight the relative energetic impacts of increased unpredictability in temperature and underline its critical role in shaping life-history traits. Given the growing concern over modern climate change scenarios, the allometric growth model provides a straightforward yet essential approach for integrating energetic and subsequent ecological effects, enabling not only predictions of responses across keystone species but also an enhanced understanding of anthropogenic impacts on aquatic ecosystems.

Fig. 1

The estimated parameters with their confidence intervals against temperature scenarios.

Fig. 2

The body length curve and the estimated fecundity changes. Top: the calculated body length growth curve and its confidence envelope for each temperature treatment. Each curve is a numerical solution of model (7), given estimated parameters, and . The length of the trajectories differ as the longevity of D. magna varies; Bottom: the estimated fecundity investments over time, time-varying parameter .

Fig. 3

The scatter plots and the estimated growth curves.

Fig. 4

The calculated quantity equivalent to the number of neonates produced for the average lifetime. Each dashed line illustrates the average lifetime (days) : 81 (Constant Low), 72 (Constant Rearing), 48 (Constant High) and 52 (Unpredictable Variation). The corresponding number of neonates : 239 (Constant Low), 236 (Constant Rearing), 181 (Constant High) and 194 (Unpredictable Variation).

Fig. 5

The scatter plots show the number of neonates produced by individuals over the lifetime. The superposed solid line represents the theoretical number of neonates up to days t. The dashed line illustrates the average lifetime and the corresponding number of neonates .