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A multivariate approach reveals diversity of ontogenetic niche shifts across taxonomic and functional groups

Citation

Rudolf, Volker (2019), A multivariate approach reveals diversity of ontogenetic niche shifts across taxonomic and functional groups , Dryad, Dataset, https://doi.org/10.5061/dryad.rxwdbrv4d

Abstract

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.

Methods

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: Hyla versicolor, Acris crepitans, and Rana (Lithobates) sphenocaphala), four predatory hemipteran species (four genera, three families: Notonecta indica, Buenoa scimitar, Belostoma sp., Pelocoris sp.), larvae from five dragonfly species (five genera, one family: Erythemis simplicicollis, Libellula incesta, Plathemis lydia, Pachidiplax longipennis, Tramea carolina), and one predatory beetle species (Cybister fimbriolatus). 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 >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 SEAc 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).

Usage Notes

All information on data preparation and analyses are given in knitter R code file.

Funding

National Science Foundation, Award: DEB-1256860

National Science Foundation, Award: DEB-1655626