The preservational fidelity of evenness in molluscan death assemblages

Paleobiology ◽  
2007 ◽  
Vol 33 (1) ◽  
pp. 1-23 ◽  
Author(s):  
Thomas D. Olszewski ◽  
Susan M. Kidwell
Keyword(s):  
Environmental Gradient ◽  
Time Averaging ◽  
Data Sets ◽  
Sufficient Time ◽  
Soft Bottom ◽  
Rapid Loss ◽  
Fine Mesh ◽  
Death Assemblages ◽  
Species Abundances ◽  
Depositional Setting

The richness (number of species) and evenness (uniformity of species abundances) of death assemblages can differ from corresponding living communities due to processes such as between-habitat transport, environmental condensation, and differential taphonomic destruction. Analysis of 132 single-census live-dead comparisons of benthic molluscs from a variety of soft-bottom marine settings indicates that on average evenness does not differ greatly between live and dead assemblages, regardless of the particular depositional setting or grain size of associated sediment. However, individual death assemblages can deviate quite substantially from their corresponding living assemblages, especially if processed using a fine mesh. In addition, death assemblages collected using sieves with 2 mm mesh or coarser showed consistently and significantly greater evenness than corresponding living assemblages. These results are encouraging for broad-scale assessments of evenness in the fossil record based on the comparison of average values (rather than for individual assemblages) and where trends in evenness are the aim of the study.Our live-dead comparisons of richness sample-size corrected by rarefaction revealed that death assemblages were on average ~1.45 times richer than the corresponding living assemblages regardless of rarefied size. In 63.6% of death assemblages both dead richness and dead evenness were greater than live, suggesting sufficient time-averaging to catch significant random or directional changes in the living community and/or introduction of individuals from outside the sampled habitat. In 12.9% of collections both dead richness and dead evenness were less than live, suggesting either rapid loss of dead shells so that dead diversity is depressed below the local living community or selective loss of taphonomically vulnerable taxa. In 18.2% of data sets dead richness was elevated but dead evenness was depressed relative to live: these are interpreted to reflect the addition of low-evenness allochthonous material. The remaining 4.5% of data sets had elevated dead evenness but depressed dead richness, suggesting that live and dead in this case may not be closely related.In seven available time series, temporal volatility in living communities over 6–24 months was considerable but could not account for observed (mostly higher) evenness values in corresponding death assemblages, whose evenness and composition were quite stable in the few examined studies. A densely sampled spatial transect shows that changes in living-assemblage evenness along an environmental gradient were preserved in the corresponding death assemblages, although dead evenness at any location on the gradient was substantially higher than living evenness.

Testing the effects of durability (shell thickness) on species over-representation in death assemblages, a taphonomic baseline for conservation paleobiology

2021 ◽  
Author(s):  
Adam Tomašových ◽  
Susan M. Kidwell
Keyword(s):  
Southern California ◽  
Anthropogenic Impacts ◽  
Spatial Scales ◽  
Organic Content ◽  
Time Averaging ◽  
Shell Size ◽  
Interspecific Differences ◽  
Conservation Paleobiology ◽  
Death Assemblages ◽  
Species Abundances

<p>Differences in the taxonomic or functional composition of living and death assemblages is a key means of identifying the magnitude and drivers of past ecological changes in conservation paleobiology, especially when assessing the effects of anthropogenic impacts. However, such live-dead differences in species abundances can arise not only from ecological (stochastic or deterministic) changes in abundance over the duration of time averaging but also from interspecific differences in the postmortem durability of skeletal remains or from the lifespan of the individuals. Here, we attempt to directly incorporate the effects of durability on species abundances in death assemblages by modeling dead abundance as a function of species’ durability traits and using abundances in living assemblages as a prior. Species inferred to be negatively affected by anthropogenic impacts should be over-represented in death assemblages relative to their abundance in death assemblages predicted by the durability model (rather than just relative to their abundance in living assemblages). Using species-level durability trait data for bivalves (shell size, thickness, mineralogy, shell organic content, and life habit) from the southern California shelf, we find that, among these traits, valve thickness correlates consistently positively and at multiple spatial scales with the log of the dead:live ratio of species abundances, and accounts for ~20-30% of live-dead mismatch. Using this benchmark for the discordance that might be taphonomic in origin, we confirm that the over-representation of epifaunal suspension-feeders and siphonate deposit-feeders in death assemblages of the southern California shelf owes in fact to their ecological decline in recent centuries, even when accounting for their greater durability.</p>

Preservation of spatial and environmental gradients by death assemblages

Paleobiology ◽  
2009 ◽  
Vol 35 (1) ◽  
pp. 119-145 ◽  
Author(s):  
Adam Tomašových ◽  
Susan M. Kidwell
Keyword(s):  
Community Structure ◽  
Community Composition ◽  
Environmental Gradients ◽  
Compositional Variation ◽  
Time Averaging ◽  
Data Sets ◽  
Square Root ◽  
Spatial Gradients ◽  
Death Assemblages ◽  
The Impact

Although only a few studies have explicitly evaluated live-dead agreement of species and community responses to environmental and spatial gradients, paleoecological analyses implicitly assume that death assemblages capture these gradients accurately. We use nine data sets from modern, relatively undisturbed coastal study areas to evaluate how the response of living molluscan assemblages to environmental gradients (water depth and seafloor type; “environmental component” of a gradient) and geographic separation (“spatial component”) is captured by their death assemblages. We find that:1. Living assemblages vary in composition either in response to environmental gradients alone (consistent with a species-sorting model) or in response to a combination of environmental and spatial gradients (mass-effect model). None of the living assemblages support the neutral model (or the patch-dynamic model), in which variation in species abundance is related to the spatial configuration of stations alone. These findings also support assumptions that mollusk species consistently differ in responses to environmental gradients, and suggest that in the absence of postmortem bias, environmental gradients might be accurately captured by variation in species composition among death assemblages. Death assemblages do in fact respond uniquely to environmental gradients, and show a stronger response when abundances are square-root transformed to downplay the impact of numerically abundant species and increase the effect of rare species.2. Species' niche positions (position of maximum abundance) along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species positions in living assemblages in seven of nine data sets (both square-root-transformed and presence-absence data).3. The proportion of compositional variation explained by environmental gradients in death assemblages is similar to that of counterpart living assemblages. Death assemblages thus show the same ability to capture environmental gradients as do living assemblages. In some instances compositional dissimilarities in death assemblages show higher rank correlation with spatial distances than with environmental gradients, but spatial structure in community composition is mainly driven by spatially structured environmental gradients.4. Death assemblages correctly identify the dominance of niche metacommunity models in mollusk communities, as revealed by counterpart living assemblages. This analysis of the environmental resolution of death assemblages thus supports fine-scale niche and paleoenvironmental analyses using molluscan fossil records. In spite of taphonomic processes and time-averaging effects that modify community composition, death assemblages largely capture the response of living communities to environmental gradients, partly because of redundancy in community structure that is inherently associated with multispecies assemblages. The molluscan data sets show some degree of redundancy as evidenced by the presence of at least two mutually exclusive subsets of species that replicate the community structure, and simple simulations show that between-sample relationships can be preserved and remain significant even when a large proportion of species is randomly removed from data sets.

scholarly journals Remembrance of things past: modelling the relationship between species' abundances in living communities and death assemblages

2011 ◽  
Vol 8 (1) ◽  
pp. 131-134 ◽  
Author(s):  
Thomas D. Olszewski
Keyword(s):  
Half Life ◽  
Species Differences ◽  
Species Abundance ◽  
Time Averaging ◽  
Ecological Communities ◽  
Post Mortem ◽  
Death Assemblages ◽  
Species Abundances ◽  
The Relationship ◽  
Range Of Variation

Accumulations of dead skeletal material are a valuable archive of past ecological conditions. However, such assemblages are not equivalent to living communities because they mix the remains of multiple generations and are altered by post-mortem processes. The abundance of a species in a death assemblage can be quantitatively modelled by successively integrating the product of an influx time series and a post-mortem loss function (a decay function with a constant half-life). In such a model, temporal mixing increases expected absolute dead abundance relative to average influx as a linear function of half-life and increases variation in absolute dead abundance values as a square-root function of half-life. Because typical abundance distributions of ecological communities are logarithmically distributed, species' differences in preservational half-life would have to be very large to substantially alter species' abundance ranks (i.e. make rare species common or vice-versa). In addition, expected dead abundances increase at a faster rate than their range of variation with increased time averaging, predicting greater consistency in the relative abundance structure of death assemblages than their parent living community.

Fidelity of variation in species composition and diversity partitioning by death assemblages: time-averaging transfers diversity from beta to alpha levels

Paleobiology ◽  
2009 ◽  
Vol 35 (1) ◽  
pp. 94-118 ◽  
Author(s):  
Adam Tomašových ◽  
Susan M. Kidwell
Keyword(s):  
Species Composition ◽  
Temporal Variation ◽  
Temporal Resolution ◽  
Beta Diversity ◽  
Fossil Record ◽  
Time Averaging ◽  
Data Sets ◽  
Diversity Partitioning ◽  
Preservation Potential ◽  
Death Assemblages

Despite extensive paleoecological analyses of spatial and temporal turnover in species composition, the fidelity with which time-averaged death assemblages capture variation in species composition and diversity partitioning of living communities remains unexplored. Do death assemblages vary in composition between sites to a lesser degree than do living assemblages, as would be predicted from time-averaging? And is the higher number of species observed in death relative to living assemblages reduced with increasing spatial scale? We quantify the preservation of spatial and temporal variation in species composition using 11 regional data sets based on samples of living molluscan communities and their co-occurring time-averaged death assemblages. (1) Compositional dissimilarities among living assemblages (LA) within data sets are significantly positively rank-correlated to dissimilarities among counterpart pairs of death assemblages (DA), demonstrating that pairwise dissimilarity within a study area has a good preservation potential in the fossil record. Dissimilarity indices that downplay the abundance of dominant species return the highest live-dead agreement of variation in species composition. (2) The average variation in species composition (average dissimilarity) is consistently smaller in DAs than in LAs (9 of 11 data sets). This damping of variation might arise from DAs generally having a larger sample size, but the reduction by ∼10–20% mostly persists even in size-standardized analyses (4 to 7 of 11 data sets, depending on metric). Beta diversity expressed by the number of compositionally distinct communities is also significantly reduced in death assemblages in size-standardized analyses (by ∼25%). This damping of variation and reduction in beta diversity is in accord with the loss of temporal resolution expected from time-averaging, without invoking taphonomic bias (from differential preservation or postmortem transportation) or sample-size effects. The loss of temporal resolution should directly reduce temporal variation, and assuming time-for-space substitution owing to random walk within one habitat and/or temporal habitat shifting, it also decreases spatial variation in species composition. (3) DAs are more diverse than LAs at the alpha scale, but the difference is reduced at gamma scales because partitioning of alpha and beta components differs significantly between LAs and DAs. This indicates that the effects of time-averaging are reduced with increasing spatial scale. Thus, overall, time-averaged molluscan DAs do capture variation among samples of the living assemblage, but they tend to damp the magnitude of variation, making them a conservative means of inferring change over time or variation among regions in species composition and diversity. Rates of temporal and spatial species turnover documented in the fossil record are thus expected to be depressed relative to the turnover rates that are predicted by models of community dynamics, which assume higher temporal resolution. Finally, the capture by DAs of underlying variation in the LA implies little variation in the net preservation potential of death assemblages across environments, despite the different taphonomic pathways suggested by taphofacies studies.

TAPHONOMY AND TIME AVERAGING OF MOLLUSCAN DEATH ASSEMBLAGES IN FLORIDA SPRINGS AND RIVERS

2018 ◽  
Author(s):  
Kristopher M. Kusnerik ◽  
◽  
Harley Means ◽  
Roger W. Portell ◽  
Michal Kowalewski
Keyword(s):  
Time Averaging ◽  
Death Assemblages ◽  
Florida Springs

Some distributional models for fossil animals

Paleobiology ◽  
1980 ◽  
Vol 6 (1) ◽  
pp. 77-95 ◽  
Author(s):  
John C. Tipper
Keyword(s):  
Environmental Gradient ◽  
Additive Model ◽  
Data Sets ◽  
Substrate Type ◽  
Benthic Species ◽  
Marine Animals ◽  
Mode Of Life ◽  
Geological Work ◽  
The Individual ◽  
Fossil Data

Models for the areal distribution of marine animals may be formulated in such a way that their validity can be tested statistically. Five such models are described here: in each case the distributions of the individual taxa are made functions of depth of water, oceanicity, and substrate type.The simplest model proposes that benthic species are substrate-dependent: it can be verified by a series of χ2 tests. Benthic species can also be seriated along an environmental gradient: this second model can be tested by using a Monte Carlo simulation procedure. The third model proposes that planktic species are to some degree stratified in the water column, and hence that their remains are distributed additively in the underlying sediments: tests on modern and sub-fossil data sets indicate that realistic versions of this model are probably valueless for most geological work. The final models are extensions of the additive model: they are used here to demonstrate the importance of selecting an appropriate model and, in particular, of knowing the mode of life of each species before the model is selected.

Live, dead, and fossil mollusks in Florida freshwater springs and spring-fed rivers: Taphonomic pathways and the formation of multisourced, time-averaged death assemblages

Paleobiology ◽  
2020 ◽  
Vol 46 (3) ◽  
pp. 356-378
Author(s):  
Kristopher M. Kusnerik ◽  
Guy H. Means ◽  
Roger W. Portell ◽  
Mark Brenner ◽  
Quan Hua ◽  
...  
Keyword(s):  
Complex Mixture ◽  
Freshwater Ecosystems ◽  
Time Averaging ◽  
Multiple Sources ◽  
Near Surface ◽  
Radiocarbon Dates ◽  
Freshwater Gastropods ◽  
Melanoides Tuberculata ◽  
Death Assemblages

AbstractTaphonomic processes are informative about the magnitude and timing of paleoecological changes but remain poorly understood with respect to freshwater invertebrates in spring-fed rivers and streams. We compared taphonomic alteration among freshwater gastropods in live, dead (surficial shell accumulations), and fossil (late Pleistocene–early Holocene in situ sediments) assemblages from two Florida spring-fed systems, the Wakulla and Silver/Ocklawaha Rivers. We assessed taphonomy of two gastropod species: the native Elimia floridensis (n = 2504) and introduced Melanoides tuberculata (n = 168). We quantified seven taphonomic attributes (aperture condition, color, fragmentation, abrasion, juvenile spire condition, dissolution, and exterior luster) and combined those attributes into a total taphonomic score (TT). Fossil E. floridensis specimens exhibited the greatest degradation (highest TT scores), whereas live specimens of both species were least degraded. Specimens of E. floridensis from death assemblages were less altered than fossil specimens of the same species. Within death assemblages, specimens of M. tuberculata were significantly less altered than specimens of E. floridensis, but highly degraded specimens dominated in both species. Radiocarbon dates on fossils clustered between 9792 and 7087 cal BP, whereas death assemblage ages ranged from 10,692 to 1173 cal BP. Possible explanations for the observed taphonomic patterns include: (1) rapid taphonomic shell alteration, (2) prolonged near-surface exposure to moderate alteration rates, and/or (3) introduction of reworked fossil shells into surficial assemblages. Combined radiocarbon dates and taphonomic analyses suggest that all these processes may have played a role in death assemblage formation. In these fluvial settings, shell accumulations develop as a complex mixture of specimens derived from multiple sources and characterized by multimillennial time-averaging. These findings suggest that, when available, fossil assemblages may be more appropriate than death assemblages for assessing preindustrial faunal associations and recent anthropogenic changes in freshwater ecosystems.

The living, the dead, and the expected dead: variation in life span yields little bias of proportional abundances in bivalve death assemblages

Paleobiology ◽  
2010 ◽  
Vol 36 (4) ◽  
pp. 615-640 ◽  
Author(s):  
Susan M. Kidwell ◽  
Thomas A. Rothfus
Keyword(s):  
Mortality Rate ◽  
Life Span ◽  
Data Sets ◽  
Population Turnover ◽  
Marine Bivalves ◽  
Death Assemblages ◽  
Living Species ◽  
Biological Relationship ◽  
The Difference ◽  
Positive Correlations

All else being equal, species with short life spans are expected to be overrepresented in time-averaged death assemblages relative to their standing abundance in the living community, but the magnitude of the distortion of proportional abundance and assemblage evenness has received little attention. Here, information from 30 data sets on the living and dead abundances of marine bivalves in local habitats is combined with a global compilation of bivalve life spans to determine whether bias from mortality rate can explain observed differences in species proportional abundances. Although bivalve maximum life spans range from one to 75 years in these data sets, indicating annual mortality rates of 0.97 to 0.09, the “life span bias” (LB) of a species–the difference between its proportional abundance expected dead and that observed alive–is consistently small in magnitude (average change <2%, maximum about 20%) and random in sign relative to observed discordance (OD = difference between that species' proportional abundance observed dead and that observed alive). The aggregate result for 413 living species occurrences is a significantly positive but weak correlation of OD to LB, with only 10% of variation in OD explained. The model performs better among longer-lived species than among shorter-lived species, probably because longer-lived species conform better to the model assumption that species maintain a constant proportional abundance in the living assemblage over time. Among individual data sets, only seven exhibit significant positive correlations between OD and LB. The model also under-predicts the cases where a death assemblage is dominated by a species that is shorter lived than the dominant species in the living assemblage, indicating that some factor(s) other than or in addition to mortality rate is responsible for OD. We can find no evidence of preservational bias linked to life span, for example through body size. This negative outcome reflects a weak biological relationship between life span and living abundance among bivalves in local habitats, contrary to the terrestrial paradigm, and points toward a simpler model of time-averaged death assemblage formation where higher abundances reflect (under-sampled) past populations. Contrary to long-held expectations, variation in population turnover among species is not a major source of taphonomic bias in time-averaged death assemblages among bivalves and perhaps among other marine groups: bias must arise largely from other factors.

Community structure of Quaternary coral reefs compared with Recent life and death assemblages

Paleobiology ◽  
2001 ◽  
Vol 27 (4) ◽  
pp. 669-694 ◽  
Author(s):  
Evan N. Edinger ◽  
John M. Pandolfi ◽  
Russell A. Kelley
Keyword(s):  
Community Structure ◽  
Coral Reef ◽  
Reef Coral ◽  
Historical Record ◽  
Taxonomic Composition ◽  
Time Averaging ◽  
Rare Taxa ◽  
Life And Death ◽  
Death Assemblages

This paper assesses the reliability with which fossil reefs record the diversity and community structure of adjacent Recent reefs. The diversity and taxonomic composition of Holocene raised fossil reefs was compared with those of modern reef coral life and death assemblages in adjacent moderate and low-energy shallow reef habitats of Madang Lagoon, Papua New Guinea. Species richness per sample area and Shannon-Wiener diversity (H′) were highest in the fossil reefs, intermediate in the life assemblages, and lowest in the death assemblages. The taxonomic composition of the fossil reefs was most similar to the combination of the life and death assemblages from the modern reefs adjacent to the two fossil reefs. Depth zonation was recorded accurately in the fossil reefs. The Madang fossil reefs represent time-averaged composites of the combined life and death assemblages as they existed at the time the reef was uplifted.Because fossil reefs include overlapping cohorts from the life and death assemblages, lagoonal facies of fossil reefs are dominated by the dominant sediment-producing taxa, which are not necessarily the most abundant in the life assemblage. Rare or slow-growing taxa accumulate more slowly than the encasing sediments and are underrepresented in fossil reef lagoons. Time-averaging dilutes the contribution of rare taxa, rather than concentrating their contribution. Consequently, fidelity indices developed for mollusks in sediments yield low values in coral reef death and fossil assemblages. Branching corals dominate lagoonal facies of fossil reefs because they are abundant, they grow and produce sediment rapidly, and most of the sediment they produce is not exported.Fossil reefs distinguished kilometer-scale variations in community structure more clearly than did the modern life assemblages. This difference implies that fossil reefs may provide a better long-term record of community structure than modern reefs. This difference also suggests that modern kilometer-scale variation in coral reef community structure may have been reduced by anthropogenic degradation, even in the relatively unimpacted reefs of Madang Lagoon. Holocene and Pleistocene fossil reefs provide a time-integrated historical record of community composition and may be used as long-term benchmarks for comparison with modern, degraded, nearshore reefs. Comparisons between fossil reefs and degraded modern reefs display gross changes in community structure more effectively than they demonstrate local extinction of rare taxa.

Sign in / Sign up

Export Citation Format

Share Document