Abstract
Dietary niche width and trophic position are key functional traits describing a consumer’s trophic ecology and the role it plays in a community. However, our understanding of the environmental and biological drivers of both traits is predominantly derived from theory or geographically restricted studies and lacks a broad empirical evaluation. We calculated the dietary niche width and trophic position of 2,938 marine fishes and examined the relationship of both traits with species’ maximum length and geographic range, in addition to species richness, productivity, seasonality and water temperature within their geographic range. We used Generalized Additive Models to assess these relationships across seven distinct marine habitat types. Fishes in reef associated habitats typically had a smaller dietary niche width and foraged at a lower trophic position than those in pelagic or demersal regions. Species richness was negatively related to dietary niche width in each habitat. Species range and maximum length both displayed positive associations with dietary niche width. Trophic position was primarily related to species maximum length but also displayed a non-linear relationship with dietary niche width, whereby species of an intermediate trophic position (3–4) had a higher dietary niche width than obligate herbivores or piscivores. Our results indicate that trophic ecology of fishes is driven by several interlinked factors. Although size is a strong predictor of trophic position and the diversity of preys a species can consume, dietary niche width of fishes is also related to prey and competitor richness suggesting that, at a local level, consumer trophic ecology is determined by a trade-off between environmental drivers and biological traits.
Highlights
The concept of a strict ecological niche to which a species is confined has been challenged on multiple occasions but is currently enjoying a revival as ecologists use species’ functional traits to describe their role within ecological networks, their response to environmental change and their contribution to ecosystem function[1,2]
We tested four hypotheses related to the environmental and biological traits which underpin a consumer’s trophic ecology: (1) dietary niche width (DNW) and TP are negatively related to prey and competitor richness[7]; (2) DNW and TP are positively related to species maximum length as larger species can feed on a greater variety of prey and may occupy a higher trophic position[11]; (3) DNW and TP are positively related to species range size as species with a broader geographic distribution are likely to be exposed to a greater diversity of prey[12]; (4) Species with an intermediate trophic position (i.e. 3–3.5) have a higher DNW than either specialist herbivores or piscivores[10]
Generalized Additive Models (GAM)’s indicated a strong positive relationship between a species’ DNW and the number of occurrences of that species in the Global Biodiversity Information Facility (GBIF) database, indicating that the DNW metric is influenced by the amount of data available for a specific species (Table 1, Fig. 3)
Summary
The concept of a strict ecological niche to which a species is confined has been challenged on multiple occasions but is currently enjoying a revival as ecologists use species’ functional traits to describe their role within ecological networks, their response to environmental change and their contribution to ecosystem function[1,2]. Characterizing the dietary niche width and trophic position of consumers across global geographic and richness scales may bridge the gap between the trophic ecology of consumers and their role in ecosystem function Assessing these relationships at a global scale using observational rather than theoretical approaches presents numerous challenges, and until recently was infeasible. We tested four hypotheses related to the environmental and biological traits which underpin a consumer’s trophic ecology: (1) DNW and TP are negatively related to prey and competitor richness[7]; (2) DNW and TP are positively related to species maximum length as larger species can feed on a greater variety of prey (i.e., small and large prey items) and may occupy a higher trophic position[11]; (3) DNW and TP are positively related to species range size as species with a broader geographic distribution are likely to be exposed to a greater diversity of prey[12]; (4) Species with an intermediate trophic position (i.e. 3–3.5) have a higher DNW than either specialist herbivores or piscivores[10]
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