DOI: 10.5061/dryad.1225
title: Data from: Predicting the effects of increasing temporal scale on species composition, diversity, and rank-abundance distributions

description: Paleoecological analyses that test for spatial or temporal variation in
 diversity must consider not only sampling and preservation bias, but also
 the effects of temporal scale (i.e., time averaging). The species-time
 relationship (STR) describes how species diversity increases with the
 elapsed time of observation, but its consequences for assessing the
 effects of time averaging on diversity of fossil assemblages remain poorly
 explored. Here, we use a neutral, dispersal-limited model of metacommunity
 dynamics, with parameters estimated from living assemblages of 31
 molluscan datasets, to model the effects of within-habitat time averaging
 on the mean composition and multivariate dispersion of assemblages, on
 diversity at point (single station) and habitat scales (pooled multiple
 stations), and on beta diversity. We hold sample size constant in STRs to
 isolate the effects of time averaging from sampling effects. With
 increasing within-habitat time-averaging, stochastic switching in the
 identity of species in living (dispersal-limited) assemblages (1)
 decreases the proportional abundance of abundant species, reducing the
 steepness of the rank-abundance distribution, and (2) increases the
 proportional richness of rare, temporally-impersistent species that
 immigrate from the neutral metacommunity with many rare species. These two
 effects together (1) can shift the mean composition away from the
 non-averaged (dispersal-limited) assemblages towards averaged assemblages
 that are less limited by dispersal, resembling that of the metacommunity,
 (2) allow the point and habitat diversity to increase towards
 metacommunity diversity under a given sample size (i.e., the diversity in
 averaged assemblages is inflated relative to non-averaged assemblages),
 and (3) reduce beta diversity because species unique to individual
 stations become shared by other stations when limited by a larger but
 static species pool. Surprisingly, these changes occur at fixed sample
 sizes and can become significant after only a few decades or centuries of
 time averaging, and are accomplished without invoking ecological
 succession, environmental changes, or selective postmortem preservation.
 We find that time averaging results in less inflation of diversity at
 habitat than at point scales; paleoecological studies should thus analyze
 data at multiple spatial scales, including that of the habitat where
 multiple bulk samples have been pooled in order to minimize time averaging
 effects. The diversity of assemblages that have accumulated over 1000
 years at point and habitat scales is expected to be inflated by an average
 of 2.1 and 1.6. This degree of inflation is slightly higher than that
 observed in molluscan death assemblages at these same spatial scales (1.8
 and 1.3). Thus, neutral metacommunity models provide useful quantitative
 constraints on directional but predictable effects of time averaging. They
 provide minimal estimates for the rate of increase in diversity with time
 averaging because they assume no change in environmental conditions and in
 the composition of the metacommunity within the window of averaging.
description: abundances-S California
 1975.xlsProportional abundances
 of bivalve species in living and death assemblages sampled on the southern
 California shelf. Living assemblages are represented by unpublished data
 from the 1975 BLM-funded survey conducted by Alan Hancock Foundation,
 which we have digitized from rescued print-outs. Death assemblages - data
 generated by authors of this paper from unprocessed sieve residues of
 these same samples, archived by the Los Angeles County Museum of Natural
 History. Mesh size 2 mm. In this study, 2 habitats are represented by
 sandy muds and muddy sands ranging from 30 to 90 meters of water depth.
 All stations are outside the impact areas of publicly operated wastewater
 outfalls. See the excel file with environmental assignment and samples
 sizes for each station (sample size can be used to recompute proportional
 into absolute abundances).abundances-S California
 2003.xlsProportional abundances
 of bivalve species in living and death assemblages sampled on the southern
 California shelf. Living assemblages are represented by unpublished data
 from the 1975 BLM-funded survey conducted by Alan Hancock Foundation,
 which we have digitized from rescued print-outs. Death assemblages - data
 generated by authors of this paper from unprocessed sieve residues of
 these same samples, archived by the Los Angeles County Museum of Natural
 History. Mesh size 2 mm. In this study, 2 habitats are represented by
 sandy muds and muddy sands ranging from 30 to 90 meters of water depth.
 All stations are outside the impact areas of publicly operated wastewater
 outfalls. See the excel file with environmental assignment and samples
 sizes for each station (sample size can be used to recompute proportional
 into absolute abundances).Station
 information.xlsSample sizes of living
 and death assemblages and sedimentologic, geographic and bathymetric
 information about the position of individual stations sampled in 1975 and
 2003 in the southern California Bight. Stations with muddy sand and sandy
 mud were analyzed in this study (four habitats in
 total).R language source
 code.txtThe code in R language
 used in simulations