DOI: 10.5061/dryad.rxwdbrv4d
title: A multivariate approach reveals diversity of ontogenetic niche shifts across taxonomic and functional groups

description: Shifts in the fundamental and realize niche of individuals during their
 ontogeny are ubiquitous in nature, but we know little about what aspects
 of the niche change and how these changes vary across species within
 communities. Yet, this knowledge is essential to predict the dynamics of
 populations and communities and how they respond to environmental change.
 Here I introduce a range of metrics to describe different aspects of
 shifts in the realized trophic niche of individuals based on stable
 isotopes. Applying this multi-variate approach to 2,272 individuals from
 13 taxonomic and functional distinct species (Amphibia, Hemiptera,
 Coleoptera, Odonata) sampled in natural pond communities allowed me to:
 (1) describe and quantify the diversity of trophic niche shift patterns
 over ontogeny in multi-dimensional space, and (2) identify what aspects of
 ontogenetic shifts vary across taxa, and functional groups. Results
 revealed that species can differ substantially in which aspects of the
 trophic niche change and how they change over ontogeny. Interestingly,
 patterns of ontogenetic niche shifts grouped in distinct taxonomic
 clusters in multi-variate space, including two distinct groups of
 predators (Hemiptera vs. Odonata). Given the differences in traits
 (especially feeding mode) across groups, this suggests that differences in
 ontogenetic niche shifts across species could at least partially be
 explained by variation in traits and functional roles of species. These
 results emphasize the importance of a multivariate approach to capture the
 large diversity of trophic niche shifts patterns possible in natural
 communities, and suggest that differences in ontogenetic niche shifts
 follow general patterns. Shifts in the fundamental and realize niche of
 individuals during their ontogeny are ubiquitous in nature, but we know
 little about what aspects of the niche change and how these changes vary
 across species within communities. Yet, this knowledge is essential to
 predict the dynamics of populations and communities and how they respond
 to environmental change.
description: Study species: Full description can be found
 in corresponding manuscript in Freshwater Biology. I identified 13
 species, including three tadpole species (three genera, two families:
 <i>Hyla versicolor, Acris crepitans, </i>and <i>Rana
 </i>(<i>Lithobates</i>)
 <i>sphenocaphala</i>), four predatory hemipteran species (four
 genera, three families: <i>Notonecta indica</i>,
 <i>Buenoa</i> <i>scimitar,</i> <i>Belostoma
 sp.</i>, <i>Pelocoris sp.</i>), larvae from five
 dragonfly species (five genera, one family: <i>Erythemis</i>
 <i>simplicicollis</i>, <i>Libellula incesta, Plathemis
 lydia, Pachidiplax longipennis, Tramea carolina</i>), and one
 predatory beetle species (<i>Cybister fimbriolatus</i>). All
 species differ considerably in their biology, including life histories,
 habitat use, behavior, feeding mode, and body morphology across and within
 orders, allowing me to examine ontogenetic niche shifts across diverse set
 of species. Data includes dry mass for each species and range covers up to
 &gt;3 orders of magnitude from smallest to largest individual (0.017mg
 to 38mg). Note that all species have a terrestrial part of their life
 cycle, but here I only focus on the aquatic habitat where all stages have
 access to the same resources and are part of the same community.
 Sample collection: I collected 2,272
 individuals from 13 species from three locations (ponds) in South East
 Texas: “Nick’s Pond” in Steven F. Austin Experimental Forrest
 (31.509376,-94.761019), “DC108-1” in Davy Crocket National forest
 (31.20547196 , -94.99097256) and “New Pond” at CBFS of Sam Houston
 University (30.746377,-94.473880). All ponds are fish free, but only the
 last two are permanent (i.e. never dried out completely in  past 10 years)
 while “Nick’s Pond” is temporary and dries out every year. All individuals
 of a species at a given site were collected during a single day or within
 two week period depending on their respective abundances. Overall, I found
 no systematic influence of sample date on the relationships between body
 size and stable isotopes. Consequently I pooled dates within a given
 location. All but three species were collected from the same site (DC
 108-1), and three species were additionally collected from one or two
 other sites. Details for locations and collection dates are
 given. Sample preparation:
 Collected individuals were immediately frozen and
 stored at -33ºC until further processing. I first photographed each
 individual and used Image J to measure body length. Individuals were dried
 at 65°C for 72 hours, allowed to cool off in an airtight desiccation
 chamber before they were weighed to obtain dry mass. All samples were
 analyzed by the UC-Davis Stable Isotope Facility following their standard
 preparation guidelines. Individuals larger than 1.2 mg were homogenized
 with an amalgamator to obtain ~1mg sample for the isotope analysis. I used
 whole body samples, as it is infeasible to obtain sufficient mass of
 specific tissues (or body parts) for most size classes. Following previous
 studies (Sanders et al., 2015), we combine multiple individuals (2-10
 depending on species) of similar weight of the very smallest sizes to get
 the minimum total dry mass necessary for stable isotope analysis, and used
 average mass for all further analyses. This was done for 268 out of 1,603
 samples. Pooling multiple individuals will not change metrics based on
 stage means (besides increasing confidence due to larger sample size), but
 could reduce intra-stage variation and related metrics (e.g. SEAc).
 However, inspection of stage specific variance showed that variation in
 first stage that included pooled individuals was not consistently smaller
 than variation in any other staged (see Results). Furthermore, there was
 no correlation between <i>SEAc</i> and mean number of
 individuals pooled per sample (Kendall’s rank correlation tau = -0.015,
 P=0.9342), indicating that variance in isotopes was not reduced by pooling
 individuals. Isotope rates for C and N were obtained simultaneously and
 reported in δ units and used to calculate all metrics in Table 1. All
 procedures were in compliance with ethical guidelines for animal use and
 approved by the Institutional Animal Care and Use Committee (IACUC
 Protocol no. A09022601).
description: All information on data preparation and analyses are given in
 knitter R code file.