Does energy availability predict gastropod reproductive strategies?

Functional space expansion driven by transitions between energetically advantageous traits in the deep sea

Climate change is shifting community structure and biodiversity on a global scale, in part due to alterations of chemical and thermal energy availability. These changes may impact ecosystem functioning through their influence on functional diversity. We investigate patterns of functional diversity, functional niches, and functional traits in bivalve communities across the energetic gradient of the deep Atlantic Ocean. We use the functional traits feeding type, tiering, and motility level to define the axes of functional space and the unique combinations of these traits as functional niches. We find that increased energy affords new species, added into functional space through niche expansion rather than niche packing. Underlying this pattern are complex dynamics of gains and losses of individual functional niches, with few adapted to the low- and high-energy extremes, and most occurring at intermediate energy. Adaptive qualities of specific traits are evidenced by those functional niches occurring at energetic extremes. Tradeoffs between these traits within the intermediate energy zone underlie an increased coexistence of functional niches, which in turn drives a unimodal pattern of functional niches and expansion of used functional space. This work suggests that energy-limited communities may be especially vulnerable to continued shifts in food availability through the Anthropocene.

Energetic increases lead to niche packing in deep-sea wood falls

Mechanisms leading to variation in diversity over energetic gradients continue to challenge ecologists. Changes in diversity may reflect the environmental capacity to support species' coexistence through increased niche packing or niche space expansion. Current ecological theory predicts that increases in energy may lead to both scenarios but not their relative strengths. We use experimental deep-sea, wood-fall communities, where energy supply can be controlled, to test for the importance of niche expansion and packing in functional space over an energetic gradient. Invertebrate communities were identified and counted from 16 Acacia sp. logs ranging in size from 0.6 to 20.6 kg in mass (corresponding to energy availability) deployed at 3203 m in the Pacific Ocean for 5 years. We use four fundamental energetic species-level functional traits-food source, trophic category, motility and tiering-to characterize species niches. Increases in energy on wood falls lead to increases in species richness. This higher species richness resulted from a substantial increase in mean niche overlap, suggesting that increases in energy may afford reduced competition.

Increased energy differentially increases richness and abundance of optimal body sizes in deep-sea wood falls

Theoretical and empirical studies suggest that the total energy available in natural communities influences body size as well as patterns of abundance and diversity. But the precise mechanisms underlying these relationships or how these three ecological properties relate remain elusive. We identify five hypotheses relating energy availability, body size distributions, abundance, and species richness within communities, and we use experimental deep-sea wood fall communities to test their predicted effects both on descriptors describing the species-richness-body-size distribution, and on trends in species richness within size classes over an energy gradient (size-class-richness relationships). Invertebrate communities were taxonomically identified, weighed, and counted from 32 Acacia sp. logs ranging in size from 0.6 to 20.6 kg (corresponding to different levels of energy available), which were deployed at 3,203 m in the Northeast Pacific Ocean for 5 and 7 yr. Trends in both the species-richness-body-size distribution and the size-class-richness distribution with increasing wood fall size provide support for the Increased Packing hypothesis: species richness increases with increasing wood fall size but only in the modal size class. Furthermore, species richness of body size classes reflected the abundance of individuals in that size class. Thus, increases in richness in the modal size class with increasing energy were concordant with increases in abundance within that size class. The results suggest that increases in species richness occurring as energy availability increases may be isolated to specific niches, e.g., the body size classes, especially in communities developing on discrete and energetically isolated resources such as deep sea wood falls.