Ecological consequences of body size reduction under warming

Abstract

Body size reduction is a universal response to warming, but its ecological consequences across biological levels, from individuals to ecosystems, remain poorly understood. Most biological processes scale with body size, and warming-induced changes in body size can therefore have important ecological consequences. To understand these consequences, we propose a unifying, hierarchical framework for the ecological impacts of intraspecific body size reductions due to thermal plasticity that explicitly builds on three key pathways: morphological constraints, bioenergetic constraints and surface-to-volume ratio. Using this framework, we synthesize key consequences of warming-induced body size reductions at multiple levels of biological organization. We outline how this trait-based framework can improve our understanding, detection and generalization of the ecological impacts of warming.

Keywords: bioenergetics; body size shift; ectotherms; predictive ecology; surface-to-volume ratio; temperature-size rule.

Figure 1.

A framework for integrating the ecological consequences of warming-induced BSR. Warming generally leads to reduced body size of ectotherms, which can have ecological consequences through three mechanistic pathways: (P1) morphological constraints imposed by smaller body size per se, (P2) bioenergetic constraints, as smaller body size is associated with lower energy reserves and their faster depletion, which, together with an even greater increase in mass-specific ingestion rate, leads to increased energetic efficiency (i.e. ratio of energy gain owing to ingestion and assimilation to energy loss owing to metabolism), and (P3) an increase in the surface-to-volume ratio, leading to more passive exchange of molecules with the environment. These three size-related pathways can each have specific ecological consequences on (C1) life history and dispersal, (C2) intra- and interspecific interactions, and (C3) stressor sensitivity, ultimately cascading to changes in (C4) population dynamics, (C5) (meta)community dynamics, and (C6) ecosystem functions and services. We visualize only the most obvious linkages in the framework. For example, warming-induced smaller fish will have a lower per capita excretion rate (pathway 2), which may directly affect nutrient cycling (consequence 6).

Figure B1.

Influence of body mass on (a) energetic efficiency along a thermal gradient, and (b) the depletion of the relative fat content (the percentage of lipids relative to total body mass) of animals starving at 8°C (solid lines) and 18°C (dotted lines). The horizontal line in (a) indicates where the energetic efficiency equals one, hence where energy gains equal energy losses.

Figure B2.

Ecological consequences of warming-induced body size reductions (red arrows) are expected to differ among aquatic and terrestrial ecosystems because (a) the temperature-size rule is stronger for aquatic organisms, (b) aquatic ecosystems are more size-structured than terrestrial ones, and (c) aquatic and terrestrial systems differ in the type of stressors to which sensitivity will increase most.