Trophic ecology of adult male Odonata. I. Dietary niche metrics by foraging guild, species, body size, and location

Abstract

Abstract. 1. Information on the dietary niches of adult odonates is sparse, as they are highly mobile and evasive animals, which makes them difficult to observe in their natural habitat. Moreover, there is a lack of knowledge on how varying behavioural traits of odonates relate to phenomena like niche partitioning.2. This study investigated niche partitioning amongst odonate species, foraging guilds and size classes in a riverine system in the Eastern Cape province of South Africa. A combination of stable isotope and fatty acid-based niches was used to infer odonate feeding.3. Both fatty acid and stable isotope-based niches showed that there was niche separation amongst odonates that forage in flight (fliers) and those that forage from a perch (perchers), amongst odonates of different size classes (damselflies, medium-and large-sized dragonflies), and amongst species, although varying levels of niche overlap were observed in each case.4. Niche sizes of odonates varied between an upstream and a downstream site. Generally greater niche overlap was recorded at the narrow upstream site (associated with low insect emergence rates) than the wider downstream site (associated with high insect emergence rates), indicating that a greater degree of resource sharing occurred at the upstream site where aquatic food was less abundant.5. The findings of this study suggest that dietary niches of odonates can be influenced by foraging guild, body size, and/or environmental conditions, and additional study in a variety of regions is recommended to determine the greater applicability of these findings.

Trophic ecology of adult male Odonata. II. Dietary contributions of aquatic food sources

“…We assigned trophic fractionation values of 3.4‰ (SD = ±1) for 15 N and 0.4‰ (SD = ±1.3) for 13 C per trophic level (Post, 2002), so fractionation from the producers to the odonates was estimated by multiplying these values by the calculated values of trophic position for each odonate species (see Chari et al, 2017). When applied to entire food webs with multiple trophic pathways and many species, a mean 15 N trophic fractionation of 3.4‰ per trophic level is a robust and widely applicable assumption (Post, 2002).…”

“…The Kowie River is approximately 90 km long and drains into the Indian Ocean on the south-east coast of South Africa. Descriptions of climate, rainfall, and annual run-off are provided in Chari et al (2017). The study sites consisted of a narrow section of the river (stream width = 1.5 ± 0.4 m) that had typical upstream properties (shallow and heavily shaded), and a wide downstream section (stream width = 11.3 ± 4.9 m) that was deeper with less vegetative cover.…”

“…Samples were covered in chloroform containing 0.01% butylated hydroxytoluene and nitrogen gas, and stored at −20 ∘ C until further processing. The procedure for extraction of fatty acids was as described in Chari et al (2017).…”

“…Samples (in tin capsules) were sent for stable nitrogen and carbon analysis at IsoEnvironmental cc, Grahamstown, South Africa, and were combusted in a Europa Scientific 20-20 isotope-ratio mass spectrometer linked to an ANCA SL Prep Unit (Europa Scientific Limited, Nantwich, England). The calibration of internal standards was as described in Chari et al (2017).…”

“…Odonates in the Kowie River (South Africa) have distinct isotopic and fatty acid niches based on hunting mode and body size (Chari, 2016;Chari et al, 2017). Odonates that spend most of their time flying, foraging, and mating in flight are called 'fliers', and those that capture their prey and return to feed from their perches are called 'perchers' (Corbet & May, 2008).…”