DOI: 10.5061/dryad.tqjq2bvx5
title: Seasonality and interspecific competition shape individual niche variation in co-occurring tetra fish in Neotropical streams

description: The drivers of intraspecific niche variation and its effects on species
 interactions are still unclear, especially in species-rich Neotropical
 environments. Here, we investigated how ecological opportunity and
 interspecific competition affect the degree of individual trophic
 specialization and the population niche breadth in tetra fish. We studied
 the four ecologically similar species (Psalidodon aff. gymnodontus, P.
 aff. paranae, P. bifasciatus, and Bryconamericus ikaa) in subtropical
 headwater streams (three sites with two co-occurring species and three
 sites with only one species). We sampled fish in two contrasting seasons
 (winter/dry and summer/wet), and quantified their trophic niches using gut
 content analysis. Psalidodon bifasciatus was the only species distributed
 over all the sampled streams. We observed seasonal differences in
 population trophic niche breadth of P. bifasciatus just when this species
 co-occurred with P. aff. gymnodontus. These findings confirm the complex
 nature of the effects of interspecific competition, depending, for
 instance, on the identity of the competitor. The degree of individual
 specialization of P. bifasciatus was higher in the winter, and it was not
 influenced by the presence of another species. Conversely, the other two
 Psalidodon species studied presented greater individual specialization in
 the summer, when fish consumed a higher proportion of allochthonous items
 (terrestrial insects and seeds), and there were no effects only for B.
 ikaa. Herein, our results suggest that seasonality in food-resource
 availability is a major driver of niche variation and it has the potential
 to play an important role in how these similar tetra species interact and
 coexist.
description: We sampled six headwater streams (2<sup>nd</sup> and
 3<sup>th</sup> orders – Figure S1, Table S1) and collected all
 small characids species found: <i>Psalidodon</i> aff.
 <i>paranae </i>(Eigenmann, 1914), <i>Psalidodon
 bifasciatus </i>(Garavello & Sampaio, 2010),
 <i>Psalidodon</i> aff. <i>gymnodontus</i>
 (Eigenmann, 1911) and <i>Bryconamericus ikaa</i> Casciotta,
 Almirón & Azpelicueta, 2004. We sampled fish in July (winter) and
 December (summer) 2017. According to the historical climate series of the
 region, 2017 represents a typical year with cold and dry winter, and hot
 and rainy summer . In each season, we realized one sampling event per
 stream. We conducted the fish sampling in reaches of 50 m using
 electrofishing with three passes of 40 min. After capture, specimens were anaesthetized, fixed
 in 10% formalin, and preserved in 70% alcohol. We collected
 fish with authorization from the Instituto Chico Mendes de Conservação da
 Biodiversidade (ICMBio) (license number 25039-1) and approved by the
 Ethics Committee on Animal Use of the Universidade Federal do Rio Grande
 do Sul (CEUA – 32734) in accordance with protocols in their ethical and
 methodological aspects for the use of fish. We
 identified specimens in the laboratory according to specific
 identification keys (Baumgartner et al., 2012; Ota, Deprá, Graça &
 Pavanelli, 2018). To avoid bias caused by ontogenetic variations, we
 selected 403 adult individuals (Table S2). Voucher specimens were
 deposited in the fish collection of the Departamento de Zoologia,
 Universidade Federal do Rio Grande do Sul. We analyzed the stomach contents under
 stereoscopic and optical microscope and feeding items were
 identified to the lowest possible taxonomic level, using
 specific literature for the algae (Bicudo & Bicudo, 1970) and
 invertebrates (Mugnai, Nessimian & Baptista, 2010). We quantified
 the proportion of feeding items in each individual’s diet following the
 volumetric method (Hyslop, 1980) using graduated test tubes and a glass
 counting plate (Hellawell & Abel, 1971).