Olivier Dezerald
email: olivier.dezerald@gmail.com

Claudio Latorre

Julio L Betancourt

Gabriel A Brito Vera

Angélica L González
email: angelica.gonzalez@rutgers.edu

Olivier Dézerald

Gabriel A Brito Vera

Ecological fidelity and spatiotemporal resolution of arthropod death assemblages from rodent middens in the central Atacama Desert (northern Chile)

Keywords: arthropods, death assemblages, historical baseline, Holocene, taphonomy, time averaging INTRODUCTION

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

 [START_REF] Smith | Beyond the barn beetles: difficulties in using some Coleoptera as indicators of stored fodder[END_REF][START_REF] Kenward | The visibility of past trees and woodland: testing the value of insect remains[END_REF][START_REF] Smith | Can we characterise 'openness' in the Holocene palaeoenvironmental record? Modern analogue studies of insect faunas and pollen spectra from Dunham Massey deer park and Epping Foresty England[END_REF][START_REF] Smith | Can dung beetles from the palaeoecological and archaeological record indicate herd concentration and the identity of herbivores?[END_REF][START_REF] Forbes | The life and death of barn beetles: faunas from manure and stored hay inside farm buildings in northern Iceland[END_REF]. The main use of fossil arthropods in paleoecological reconstructions has been, instead, to infer past climatic conditions from the presence of indicator species (mostly Coleoptera and Diptera) in peatland, wetland, and floodplain deposits worldwide (https://www.bugscep.com/downloads/qbib.pdf; see also [START_REF] Ashworth | The paleoenvironment of the Olympia beds based on fossil beetles from Discovery Park[END_REF][START_REF] Coope | Coleoptera from the Cromerian Type Site at West Runton, Norfolk, England[END_REF][START_REF] Elias | Differential insect and mammalian response to Late Quaternary climate change in the Rocky Mountain region of North America[END_REF][START_REF] Kuzmina | Insect faunal response to environmental changes during the last interglacial in Western Beringia[END_REF] for recent examples).

The lack of modern calibration has limited our understanding of how taphonomic and other biases may affect past arthropod assemblages.

An understudied source of fossil arthropod remains are late Quaternary middens made by rodents (several families) and possible other animals (e.g. hyraxes: Procavia) and preserved in rock crevices, shelters, and caves throughout arid and semi-arid regions of the world [START_REF] Pearson | Understanding arid environments using fossil rodent middens[END_REF]. The first midden studies were pioneered in the western North America beginning in the late 1980's, where more than two-thousand middens made by packrats (also called woodrats; Neotoma: Cricetidae) have been radiocarbon dated and analyzed (e.g., [START_REF] Betancourt | Synthesis and prospectus[END_REF]. Analogous middens made by other small mammals have been studied from three other continents (e.g., [START_REF] Betancourt | Paleomadrigueras de roedores, un nuevo método para el estudio del Cuaternario en zonas aridas de Sudamérica[END_REF][START_REF] Chase | Rock hyrax middens: a palaeoenvironmental archive for southern African drylands[END_REF][START_REF] Mccorriston | Holocene paleoecology, and prehistory in highland southern Arabia[END_REF][START_REF] Pearson | Understanding arid environments using fossil rodent middens[END_REF]. In South America, where our particular study is focused, middens are made by rodents in four different families: Abrocoma (Abrocomidae), Phyllotis (Cricetidae), Lagidium (Chinchillidae), and possibly Octodontomys and Octomys (Octodontidae; [START_REF] Betancourt | Paleomadrigueras de roedores, un nuevo método para el estudio del Cuaternario en zonas aridas de Sudamérica[END_REF][START_REF] Holmgren | Holocene vegetation history from fossil rodent middens near Arequipa, Peru[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF].

These South American rodents are considered dietary generalists and feed almost exclusively on surrounding vegetation and infrequently on ground-dwelling invertebrates (<0.1% of diet; [START_REF] Cortés | Hábitos alimenticios de Lagidium viscacia y Abrocoma cinerea: roedores sintópicos en ambientes altoandinos del norte de Chile[END_REF][START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] López-Cortés | Dietas de Abrothrix andinus, Phyllotis xanthopygus (Rodentia) y Lepus europaeus (Lagomorpha) en un ambiente altoandino de Chile[END_REF][START_REF] Sobrero | Octomys mimax (Rodentia: Octodontidae)[END_REF].

Fossil middens are amalgamated deposits of plant (macrofossils and pollen), arthropod, and vertebrate remains, including copious fecal pellets of the midden agent, all cemented into a hard mass by crystallized urine ("amberat"). Most of the midden materials, except for some of the pollen from anemophilous species, are presumed to originate from living organisms within the animal's foraging range. In arid lands, and when protected from the elements in a cave or rock shelter, middens can commonly be thousands of years old. The excellent preservation of plant and animal remains allows for a broad suite of morphological, geochemical, and even genetic analyses (see [START_REF] Kuch | Molecular analysis of a 11 700-year-old rodent midden from the Atacama Desert, Chile[END_REF][START_REF] Murray | Highthroughput sequencing of ancient plant and mammal DNA preserved in herbivore middens[END_REF]. Fossil rodent middens have provided unparalleled taxonomic and spatial resolution in reconstructing past vegetation change in the Americas during the late Quaternary [START_REF] Betancourt | A 22,000-year record of monsoonal precipitation from Northern Chile's Atacama Desert[END_REF][START_REF] Betancourt | Synthesis and prospectus[END_REF][START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF][START_REF] Mujica | Late Quaternary climate change, relict populations and present-day refugia in the northern Atacama Desert: a case study from Quebrada La Higuera (18 degrees S)[END_REF].

Despite their many advantages, middens also have some limitations. The depositional episode that created a particular midden can be brief (months) or prolonged (decades to centuries), and usually indeterminable from stratigraphy or 14 C dating. Even the thickest of deposits (meters deep) tend to be stratigraphically discontinuous. To create a local midden series requires collating and radiocarbon dating a number of individual and separate middens (from different locations in the same rock shelter or different rock shelters in the same area) and their assemblages, which often results in long temporal gaps [START_REF] Betancourt | A 22,000-year record of monsoonal precipitation from Northern Chile's Atacama Desert[END_REF][START_REF] Betancourt | Synthesis and prospectus[END_REF]. Also, there have been few attempts to test and calibrate the reliability of plant macrofossil and pollen assemblages in rodent middens to reflect local plant communities [START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF][START_REF] Lesser | Reliability of macrofossils in woodrat (Neotoma) middens for detecting low-density tree populations[END_REF][START_REF] Lyford | Validating the use of woodrat (Neotoma) middens for documenting natural invasions[END_REF][START_REF] Nowak | Probability that a fossil absent from a sample is also absent from the paleolandscape[END_REF].

Midden research in the Americas has focused primarily on the rich plant macrofossil and pollen assemblages, and less so on animal remains, including the arthropod assemblages that are the focus of this study. Arthropod remains are common and well preserved enough in packrat (Neotoma) middens from western North America to attract the attention of entomologists [START_REF] Ashworth | Climate of the Chilean channels between 11[END_REF][START_REF] Elias | Paleoenvironmental significance of late Quaternary insect fossils from packrat middens in South-Central New Mexico[END_REF][START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] Elias | Fossil insect evidence for late Quaternary climatic change in the Big Bend region, Chihuahuan desert, Texas[END_REF][START_REF] Hall | Late Quaternary and modern arthropods from the Ajo mountains of Southwestern Arizona[END_REF][START_REF] Hall | Late quaternary arthropod remains from Sonoran Desert packrat middens, southwestern Arizona and northwestern Sonora[END_REF][START_REF] Hall | Arthropod history of the Puerto Blanco mountains, Organ Pipe Cactus National Monument, Southwestern Arizona[END_REF][START_REF] Van Devender | Fossil arthropods from the Sierra Bacha, Sonora, Mexico[END_REF][START_REF] Van Devender | Holocene arthropods from the Sierra Bacha, Sonora, Mexico, with emphasis on beetles (Coleoptera)[END_REF]. [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF] surmised that most of the sclerotized exoskeletal remains in packrat middens were not consumed by packrats, but instead derive from facultative arthropods (particularly ground beetles and darkling beetles) that spend the winter in temperature-buffered packrat dens, are packrat parasites, or were drug in on plant material or by other animals. A taphonomic study of arthropods in these middens showed that herbivores are under-represented, whereas scavengers are over-represented [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF]. Similar arthropod studies have, however, lagged for South American middens.

In the present study, we aimed to establish the ecological fidelity and spatiotemporal resolution of arthropod death assemblages found in rodent middens from the central Atacama Desert (22-24° S latitude) in northern Chile. To this end, we used the live-dead approach to compare four sources of soil arthropod data: two contemporary surveys of live communities ("local live data" and "regional live data"), and two sources of death assemblages ("modern dead data" and "fossil data" in modern and fossil middens, respectively). Specifically, we ask: (1) do arthropod assemblages found in rodent middens reflect living arthropod communities (live-dead agreement)? (2) What is the spatial resolution of those death assemblages? And (3) what is the magnitude and direction of the effect of time and time-averaging on community structure and composition? We hypothesized that: (1) the death assemblages may be biased towards highly sclerotized and ground-dwelling arthropods such as Coleoptera [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF]; (2) the live communities found in the close vicinity of fossil middens may represent a subset of the regional species pool [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF], and thus be more similar to death assemblages than the regional fauna; and (3) because fossil middens represent temporal "snapshots" of past floristic assemblages (i.e., limited effect of time averaging; de [START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF], the effect of time averaging on the structure and composition of arthropod assemblages also may be limited.

MATERIALS & METHODS

Study area and rodent middens

This study was located in the southern part of the Salar de Atacama (Chile), ~10 km south of Tilomonte (Latitude: -23.84°; Longitude: -68.18°; altitude: ~2,600 m), near the lower, dry limits of vascular plants in the central Atacama Desert and at the well-studied site of Vegas de Tilocalar/Lomas de Tilocalar, where 46 middens were previously collected, dated, and analyzed spanning the last ~40,000 cal yr BP [START_REF] Betancourt | A 22,000-year record of monsoonal precipitation from Northern Chile's Atacama Desert[END_REF][START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF]. Midden ages reported here were calibrated using the SHCal13 calibration curve (see Table A). In this area, modern and fossil middens are produced by two main families of rock-dwelling rodents, chinchilla rats (Abrocoma, mostly A. cinerea: Abrocomidae) and leaf-eared mice (Phyllotis spp.: Cricetidae). The agent of deposition (i.e., rodent species) for each of the fossil middens was identified by the size and shape of the fecal pellets in the middens [START_REF] Diaz | Rodent middens reveal episodic, long-distance plant colonizations across the hyperarid Atacama Desert over the last 34,000 years[END_REF]. The occurrence of any vegetation, today and in the past, in this hyperarid desert is controlled by the amount and seasonality of precipitation, which vary sharply with latitude and elevation [START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF]. The Tilomonte study site is dominated by sparse (<5% cover) shrubs and succulent annuals and perennial halophytes (e.g., Cistanthe salsoloides:

Portulacaceae; Atriplex imbricata: Amaranthaceae; and Tessaria absinthioides: Asteraceae).

Live arthropod communities found in vicinity of rodent middens: "local live data"

Several sampling methods exist to provide an accurate assessment of the local ground-dwelling arthropod community, yet their capture efficiency depends upon the species characteristics (e.g., surface activity, population density), the trap-related features (e.g., size, shape, bait/preservative type, and material of constructions), as well as the sampled environments (e.g., [START_REF] Zou | A Comparison of Terrestrial Arthropod Sampling Methods[END_REF]. Among all sampling methods, pitfall traps are used extensively for sampling ground-dwelling communities [START_REF] Pekár | Differential effects of formaldehyde concentration and detergent on the catching efficiency of surface active arthropods by pitfall traps[END_REF][START_REF] Phillips | Effects of Habitat Structure and Lid Transparency on Pitfall Catches[END_REF], especially in hyperarid environments [START_REF] Cheli | Efficient sampling of ground-dwelling arthropods using pitfall traps in arid steppes[END_REF]. Sticky traps were, however, the only realistic alternative in our study sites, as the rocky shelters of rodents preclude the proper burial of pitfall traps. Sticky traps consist of a piece of cardboard with a sticky surface that can catch both crawling and flying species [START_REF] Atakan | Evaluation of yellow sticky traps at various heights for monitoring cotton insect pests[END_REF]. Within the close proximity of rock shelters (both within and < 15 m outside the caves), we set ten squared plots each containing 16 sticky traps (Nb. total = 160). Within a given plot, the sticky traps (catching surface: 20 cm x 15 cm) were located from two to seven meters away from each other depending on the configuration of the rock shelters. All plots were ~60 m apart. Finally, each sticky trap was checked every day in the morning over five consecutive days. Arthropods were kept in 70% alcohol and later identified in the laboratory using a dissecting microscope.

Live communities present in the region: "regional live data"

To evaluate the specificity of arthropod communities living in rodent middens (compared to both local data and death assemblages), we sampled live arthropod communities in distant sites ("regional live data"). These distant sites thus provide information on the composition of the regional fauna. To this end, we set 15 squared plots at ca. 0.5 to 3 km away from our closest fossil or modern midden sites. Each plot contained 16 traps (sticky and/or pitfall traps), which were spaced from each other by seven meters. All plots were separated by ~60 m. Pitfall traps consisted in plastic cups (radius: 7.5 cm x height: 10.5 cm) and were set with antifreeze (ethylene glycol) or no preservative was added [START_REF] Cheli | Efficient sampling of ground-dwelling arthropods using pitfall traps in arid steppes[END_REF]. Each trap was checked every day in the morning over four consecutive days. Arthropods were kept in 70% alcohol and later identified under a dissecting microscope.

Pitfall traps and sticky traps are two passive sampling methods that can catch both crawling and, to a lesser extent, flying arthropods [START_REF] Zou | A Comparison of Terrestrial Arthropod Sampling Methods[END_REF]). Yet, the proportion of each trap type-pitfalls with (N=100) or without (N=100) preservatives and sticky traps (N=200)may influence the probability of catching a given species. We therefore estimated the trap-type standardized catch of each species for all habitats combined (local and regional data; see Terry, 2010b for more details). Our results indicate high consistency between estimated and observed abundances of each taxon except for the abundant Entomobryomorpha (Collembola), which was mainly caught in pitfalls set with ethylene glycol (taxa code = 1; Table B; Figure 1).

Modern death assemblages: "modern dead data"

Replicated samples of time-averaged surficial death assemblages were collected close to fossil and modern middens (N tot =60). Modern middens were identified by the lack of amberat (crystallized urine) and occasional presence of green leaves and twigs. Each sample consisted in cylindrical volumes (radius: 22 cm x height: 2 cm) of soil debris, including recent rodent fecal pellets, plant material, dead arthropod remains, and rocks of various sizes. The exact magnitude of time averaging represented by these "modern" samples is unknown but most likely to be within the last few decades.

Arthropod remains were identified to the lowest taxonomic level when possible using our reference database of modern arthropods, the collection of the Museo Nacional de Historia Natural in Santiago (Chile), and by reference to specialists. The minimum number of individuals (MNI; a proxy of species abundance) of each species was calculated by counting the maximum number of analogous arthropod remains belonging to a given species (Terry, 2010a). For instance, if we identified one thorax, three head capsules, and nine elytra (four left-sided and five right-sided elytra) that were found apart from each other but belonging to the same species, we reported a MNI of five individuals (four complete pairs of elytra plus one). We did not use the number of leg remains due to the difficulty to identify and attribute those remains to any given specimens.

Fossil death assemblages: "fossil data"

In this study, we included a total of 46 samples of fossil middens, covering a time span of >40,000 cal yr BP from Vegas de Tilocalar/Lomas de Tilocalar (Table A). Middens were extracted by using a hammer and chisel, cleaned in the field for surface contaminants, and split along clear stratigraphic units when recognizable. A "sample" can thus correspond to a small fossil midden or a distinct stratigraphic unit from a large fossil midden. Hereafter, those samples are referred to "fossil middens". [START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF] describe the original extraction, location and processing of these middens. Of possible consideration to taphonomy is the potential fragmentation and mechanical damage done to fragile arthropod remains during midden processing. Middens are typically soaked for 2-3 weeks (enough to dissolve the crystallized urine) followed by wet screening of dissolved midden materials with water faucet. They are then dried on cardboard plates at 50-60°C in a rack oven for an average of two to three days. Dried midden samples were then sieved into size classes using USGS Standard Soil Sieves (mainly #10 and #18), and hand sorted for three hours each under a microscope for arthropod remains. This standardized methodology was used for the sake of consistency with previous studies on paleomiddens [START_REF] Betancourt | A 22,000-year record of monsoonal precipitation from Northern Chile's Atacama Desert[END_REF][START_REF] Betancourt | Paleomadrigueras de roedores, un nuevo método para el estudio del Cuaternario en zonas aridas de Sudamérica[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF][START_REF] Latorre | A vegetation history from the arid prepuna of northern Chile (22-23 degrees S) over the last 13,500 years[END_REF][START_REF] Diaz | Rodent middens reveal episodic, long-distance plant colonizations across the hyperarid Atacama Desert over the last 34,000 years[END_REF][START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF], and because the sizes and weights of fossil middens can vary across locations. The identity of fossilized arthropod species and MNI were then established using the same procedure as used for "modern dead data" described above (see Table C for a list of taxa identities).

Live-dead agreements of ecological information

The degree to which ecological information obtained from death assemblages reflect that of modern communities was evaluated using various complementary measures of community structure and composition: species richness, evenness, composition, and abundance (Terry, 2010a, b). Species richness was compared using individual-based rarefaction curves (rareNMtests-package in R; [START_REF] Chao | Rarefaction and extrapolation with Hill numbers: a framework for sampling and estimation in species diversity studies[END_REF]. Species evenness was evaluated using the Probability of Interspecific Encounter (PIE) as H/log(S), where H correspond to the Shannon index, and S the number of species. PIE is the probability that two randomly picked individuals belong to two different species and correspond to the steepest slope of a rarefaction curve.

Species composition was estimated by calculating the percentages of "live species" from modern communities found in death assemblages and "dead species" found in modern communities [START_REF] Kidwell | Taphonomy and time-averaging of marine shelly faunas, Taphonomy: releasing the data locked in the fossil record[END_REF]. This procedure was extended to include all type of data (local live, regional live, modern dead, and fossil data). Similarity in species composition (presence/absence) among death assemblages and live communities was assessed using the Simpson's turnover index (0 = dissimilar; 1 = similar; vegan-package; [START_REF] Koleff | Measuring beta diversity for presence-absence data[END_REF]. Both Simpson's turnover and Raup-Crick indices can handle variations in sample sizes when evaluating similarities in species richness. The Raup-Crick index was not chosen, however, because it is too sensitive to the composition of the regional species pool, which was unknown for death assemblages [START_REF] Chase | Disentangling the importance of ecological niches from stochastic processes across scales[END_REF]. Similarity in species rank abundances among death assemblages and modern communities was evaluated with the Spearman rank correlation test (Terry, 2010a, b). Similarity in species relative abundances among death assemblages and live communities was estimated with the Horn-Morisita index (0 = dissimilar; 1 = similar; vegan-package). All of these measures of community structure and composition (rarefied species richness, Simpson's turnover, Spearman Rho, Horn-Morisita) were selected for their robustness to variations in sample sizes among data types.

Effect of time and time averaging

We used two procedures to estimate the direction and magnitude of time averaging on community structure and composition. First, we successively pooled the fossil middens through time to simulate time averaging and compared the resulting assemblages with local live communities (pooled local live data) and modern death assemblages (pooled modern dead data) using the Simpson's turnover index, Spearman rank correlation, and Horn-Morisita index. To evaluate the effect of single (and pooled) midden ages on those agreements, we regressed these measures of community composition against time and assessed the significance of relationships using simple Spearman rank correlations. We expect a significant effect of midden ages if, for example, a lower agreement is consistently recovered from very old than from more recent fossil middens. Second, we repeated the procedure previously described using species richness, Shannon and Simpson indices on individual and successively pooled fossil middens. All analyses were performed in R version 3.3.0 (R Core [START_REF] Core | R: A language and environment for statistical computing[END_REF].

RESULTS

Live-dead agreements of ecological information

The agreement among all four sources of soil arthropod data was highly variable among samples and across the various measures of community structure and composition used in this study.

Rarefied species richness was not statistically different between all four sources of data when samples were pooled by data type (Figure 2A; Table 1). Individual samples treated separately showed, however, large variability in species richness within each source of data (Figure 2B).

Regional live communities ("regional data") and modern death assemblages ("modern dead data") displayed the lowest and highest evenness, respectively (0.74±0.05, 0.92±0.01; Table1). In contrast, local live communities ("local live data") and fossil death assemblages ("fossil data") exhibited intermediate values of evenness (0.83±0.02, 0.86±0.02; Table 1).

Overall, the percentages of "live species" and "dead species" found in each data type were low and varied from 17.0% to 40.0% (Table 2). The highest percentages of "live species" were found between local and regional data (38.78%) and between dead and fossil data (40.0%). The lowest values were found between regional and dead data (22.22%) and between regional and fossil data (20.45%). The percentages between local data and death assemblages were intermediate (29-30%). These patterns were quantitatively similar for the percentages of "dead species" found in each data type (Table 2).

The averaged similarity in species presence/absence between samples within a given data type was not significantly different across data types (all pairwise comparisons between live-live=LL, regional-regional=RR, modern dead-modern dead=DD, and fossil-fossil=FF; Simpson's turnover; Appendix A; Figure 3A). Similar patterns were observed when evaluating rank abundances (Spearman rank test; except significant differences between RR and DD; Appendix B; Figure 3B) and species composition and abundances (Horn-Morisita; Appendix C; Figure 3C).

On average, 50 ± 0.1% of Spearman rank correlations were significant (see Figure A for the distribution of p-values grouped by data type). Species richness, rank abundances and species composition were significantly different when comparing local live communities and death assemblages (LL-LD and LL-LF comparisons; Appendix A-C; Figure 3). In addition, the agreements between regional data and death assemblages were significantly different and on average lower than comparisons between local live data and death assemblages (all RR-RD and RR-RF comparisons; Appendix A-C; Figure 3). Finally, when repeating those analyses with arthropods identified at the family levels, agreements between local live communities and death assemblages become not significantly different regardless of the community measure (Simpson's turnover, Spearman, and Horn-Morisita; Figure B; Appendix D-F). Agreements between regional data and death assemblages remained, however, significantly different with respect to rank abundances and family composition (Spearman, Horn-Morisita; Figure B; Appendix D-F).

All measures of community structure and composition (i.e., evenness, percentages of "live-dead species", Simpson's turnover, Spearman rank, and Horn-Morisita index), but the rarefied species richness, agree that the regional fauna is relatively unique compared to local live, modern dead, and fossil data. For instance, in terms of relative abundances, local live communities and modern dead and fossil assemblages are dominated by Coleoptera, whereas the regional fauna is dominated by Entomobryomorpha (Order; Figure 4). In addition, Hemiptera were relatively more abundant in the regional communities than in any other data type. Finally, two important differences were observed between live communities and death assemblages. First, Ixodida (ticks) and Siphonaptera (fleas) only were found in death assemblages (both dead and fossil data). Ticks and fleas are likely present in modern live communities but were not caught with our sampling design (i.e., sampling biases). Second, Entomobryomorpha, Acari, Diptera and Zygentoma were found in live communities, but were not preserved in death assemblages, probably due to taphonomic processes (Figure 4).

Effect of time and time averaging

In subsequent analyses, we excluded the regional data due to their relatively unique community structure and composition compared with local live, modern dead, and fossil data. In addition, to isolate the effect of time averaging on death assemblages, we also restricted our datasets to the orders present in all three data types (i.e., excluding taphonomic processes and sampling biases; Figure 4). The effect of time averaging on species richness was significant, but only noticeably at millennial scales (all pooled fossil middens) compared with local live and modern dead data (Figure 5A). The species richness within each fossil midden is variable, and not consistently higher than that in local live communities or in modern death assemblages, suggesting a limited effect of time averaging on death assemblages of single fossil middens (Figure 5B).

As data from fossil middens are successively pooled though time, the mean local-fossil Simpson's turnover decreases and then increases, whereas the mean Spearman agreement remains relatively constant, and the Horn-Morisita similarity increases (green points; Figure 6). In addition, most Spearman rank correlations are not significant (Figure C). Pooling successive middens through time does not necessarily improve the local live-fossil agreement in terms of species presence/absence or rank abundances (Figure 6A, andC). The mean local live-fossil agreement in species composition, however, improved from 0.39 (local live-fossil agreement with individual middens) to 0.59, which corresponds to a plateau reached when six successively occurring middens are pooled (Figure 6E). Similar patterns occur when successively pooled middens are compared to pooled modern dead data (Figure 6B, D, andF). The modern deadfossil agreements (Simpson's Turnover, Spearman Rho, Horn-Morisita) were, however, higher than the local live-fossil agreements.

Variations in local live-fossil and modern dead-fossil agreements were not significantly related to midden ages for all measures of community structure and composition. Those relationships become significant when pooling up to six (plateau) successive middens through time (Figure 7; Table D). Note that those combinations of three and six pooled middens encompass an average temporal window of 1056 ± 276.84 and 2165 ± 397.53 cal yr BP, respectively, when all middens, except the oldest ones (> 40,000 cal yr BP; N = 3), are included (Table 3). These examples from individual middens up to three and six pooled middens thus represent a range of combinations that increase agreements in species composition and abundances (Horn-Morisita) but also decrease the temporal resolution of resulting assemblages (from centennial to millennial scales; Table 3).

We found similar results when above analyses were repeated using several indices of diversity within each fossil midden (i.e., Shannon and Simpson indices). On average, single fossil middens harbor 6.48 ± 0.58 species (±SE; from 1 to 18). Species richness increases linearly as successive middens are pooled through time until all middens are pooled together (Figure 8A). The Shannon's diversity reaches a plateau (~2.47 ± 0.03) after pooling ten successive middens (Figure 8B) whereas three successive middens are needed for the Simpson index to reach its plateau (~0.78±0.02; Figure 8C). Finally, none of the diversity indices are significantly affected by the midden ages, and pooling successively occurring middens does not significantly change these patterns (Figure 9A-C) with one exception; a significant (and positive) relationship appears between species richness and the increasing amount of time from present when six successive middens are pooled (purple points, Figure 9A; Table D).

DISCUSSION

Altogether, these results demonstrate that individual fossil middens capture a highly resolved spatiotemporal picture of the structure and composition of common arthropod species and, to a lesser extent, rare species compared to raw species counts. The agreements between local live, modern dead, and fossil data in species composition and abundances are significantly different, but are statistically indistinguishable from one another when evaluated at the family level. Yet, these agreements present substantial variability. The main biasing factors that could influence the ecological fidelity of arthropod death assemblages to modern live communities include: taphonomic processes, spatial resolution, midden age and temporal resolution, sampling biases of modern and fossil surveys, and finally the true signal of past climate conditions, each of which is discussed below. Because the effect of taxonomic resolution on live-dead agreements was evaluated previously, it will not be discussed hereafter [START_REF] Albano | Taxonomic sufficiency in a live-dead agreement study in a tropical setting[END_REF].

Taphonomic processes

In agreement with our prediction and that of other Quaternary arthropod death assemblage studies, hard-bodied taxa are better preserved than soft-bodied taxa. Amongst the well-preserved arthropods, Coleoptera dominate fossil midden assemblages, a pattern also observed in fossil middens from the Sonoran and Chihuahuan Deserts in North America [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] Hall | Late Quaternary and modern arthropods from the Ajo mountains of Southwestern Arizona[END_REF][START_REF] Hall | Late quaternary arthropod remains from Sonoran Desert packrat middens, southwestern Arizona and northwestern Sonora[END_REF][START_REF] Van Devender | Fossil arthropods from the Sierra Bacha, Sonora, Mexico[END_REF]. The dominance of Coleoptera in both local live communities and death assemblages thus reflect a true feature of communities living in arid environments (past and present), rather than the sole effect of taphonomic processes that enhance the presence of those highly-sclerotized taxa [START_REF] Pizarro-Araya | Diversidad taxonómica de los artrópodos epígeos de la Región de Atacama (Chile): Estado del conocimiento[END_REF].

In contrast, Entomobryomorpha, Acari, Diptera, and Zygentoma are the taxa most impacted by taphonomic processes; they are completely absent from death assemblages (Figure 4). Although those taxa are absent from death assemblages sampled at out study site, they could potentially be recovered as in other paleontological or archaeological studies [START_REF] Langdon | Ecological influences on larval chironomid communities in shallow lakes: implications for palaeolimnological interpretations[END_REF][START_REF] Braga | Identification of Megaselia scalaris (Loew, 1866) (Diptera: Phoridae) in mummified human body from Itacambira (MG), Brazil, using scanning electron microscopy and cuticular hydrocarbons[END_REF]. In fact, we have found two dipteran larvae (Brachycera spp.) from paleomiddens sampled elsewhere in the central Atacama (unpublished data) suggesting that the effect of taphonomic processes on fossil arthropod remains may not be uniform across locations.

Yet, those two observations represent a very small fraction of all fossil remains we identified, thus highlighting the strong influence of taphonomic processes acting upon Diptera. Because dipterans could be abundant in living communities (third most abundant taxa in local live communities; Figure 4) and may play a crucial role in nutrient cycling (feeding upon rodent fecal pellets), the low preservation of dipterans in paleomiddens is particularly detrimental to a full understanding of the functioning of past midden assemblages.

The absence of Entomobryomorpha, Acari, Diptera, and Zygentoma from death assemblages may partly explain the significant difference in agreements in species richness, rank abundances, and, to a lower extent, species composition between live communities and death assemblages (Figure 3). At the family level, both modern and fossil death assemblages reflect, however, the composition of local live communities without significant differences (Figure B; Appendix D-F). These results suggest that taphonomic processes related to the consolidation cycles of middens may have a greater negative effect on species richness than species composition (i.e., species identities and abundances). Furthermore, we argue that modern death assemblages can be used reliably as modern analogs for fossil-live comparisons of community structure and composition. This result has important implications for midden studies because modern references of live communities are almost never collected, and they are more difficult to obtain than modern death assemblages.

Spatial resolution

Another key finding is the relative specificity of the regional fauna compared to local live communities and death assemblages (modern and fossil). This pattern was highlighted by all measures of community structure and composition (PIE's evenness, Simpson's turnover, Spearman rho and Horn-Morisita similarity) but the rarefied richness was not significantly different among all data types. Contrary to our expectations, however, local live communities do not represent a subset of the regional pool but rather a relatively distinct set of species. Likewise, most arthropods found in North American Neotoma middens are considered facultative inquiline taxa that spend, at least, part of the year in packrat dens [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF]. In the absence of arthropod remains from past regional pools, observations from modern communities do not allow us to ascertain, however, that the specificity of the midden-associated fauna was also true in the past. Yet, our results suggest that rodent-associated death assemblages record a reliable signal for arthropod communities at local scales. This high spatial resolution of midden-associated arthropod communities (fossil and modern) may contribute to the high variability in live-dead agreements (Figure 3). In the study area, local live communities, modern dead, and fossil assemblages were collected at elevations ranging from ca. 2,400 to 2,800 masl. Although the altitudinal effect on midden-associated arthropod communities is unknown, it may induce some variability among samples within and among data types (local live, modern dead, and fossil data). The heterogeneous deposition of middens across the landscape is an intrinsic feature of this system that needs to be considered explicitly when dealing with higher spatial scales than in the present study. When sufficient middens are available, pooling middens within a given spatial bin (2-D surface and/or altitudinal grid) may help reduce inter-midden variations in community structure and composition.

Midden age and temporal resolution

The exact depositional rate of rodent middens, be they modern death or fossil, is unknown, making it difficult to assess the effect of time averaging on midden-associated arthropod assemblages. Yet, by combining local live communities, modern death assemblages, and fossil arthropod assemblages (local, dead, and fossil data, respectively), we evaluated a time-averaging effect that potentially span from a few decades to millennial time scales. As expected, time averaging was significant only when fossil data were pooled over millennial scales (Figure 5), confirming our hypothesis that any fossil midden represents only a snapshot of past faunal assemblages (potentially less than a century; Figure 5; [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] Van Devender | Fossil arthropods from the Sierra Bacha, Sonora, Mexico[END_REF] akin to past floristic ones [START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF]. AMS (accelerator mass spectrometry) 14 C dating of individual arthropod and plant remains, though costly, may be the only means of accurately quantifying time averaging.

Midden age did not have a significant influence on local live-fossil and dead-fossil agreements in species richness, rank abundances, and composition (Figure 7; Table D). Likewise, no significant correlation was found between the midden age and any measures of diversity (species richness, and Shannon and Simpson's indices; Figure 9), suggesting that the effects of taphonomic processes are relatively constant over time. In our series of 46 middens ranging from 410 to >44,420 cal yr BP, 40 were recovered within the last 15,000 cal yr BP (Table A).

Therefore, even though old middens harbor a similar arthropod diversity as in most recent ones, they are recovered to a lesser extent with increasing age. Finally, we did not see major differences in the relationships between midden age and live-dead agreements or diversity indices as middens were successively pooled through time.

Rather, pooling up to three successively-occurring middens: (i) improved the live-dead agreements in species composition; (ii) reduced the inter-midden variability in species richness and composition; while (iii) keeping a relatively well-resolved temporal window (1056±276.84 cal yr BP; Table 3, Figure 678). Care must be taken, however, when pooling fossil middens. First, it is not always possible to pool middens through time as the number of middens in a given series can vary from only five to a couple of hundred [START_REF] Betancourt | A 22,000-year record of monsoonal precipitation from Northern Chile's Atacama Desert[END_REF][START_REF] De Porras | Calibrating the pollen signal in modern rodent middens from northern Chile to improve the interpretation of the late Quaternary midden record[END_REF][START_REF] Elias | Paleoenvironmental significance of late Quaternary insect fossils from packrat middens in South-Central New Mexico[END_REF]Elias andVandevender, 1990, 1992;[START_REF] Hall | Late Quaternary and modern arthropods from the Ajo mountains of Southwestern Arizona[END_REF][START_REF] Hall | Late quaternary arthropod remains from Sonoran Desert packrat middens, southwestern Arizona and northwestern Sonora[END_REF][START_REF] Hall | Arthropod history of the Puerto Blanco mountains, Organ Pipe Cactus National Monument, Southwestern Arizona[END_REF][START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF][START_REF] Latorre | A vegetation history from the arid prepuna of northern Chile (22-23 degrees S) over the last 13,500 years[END_REF][START_REF] Mujica | Late Quaternary climate change, relict populations and present-day refugia in the northern Atacama Desert: a case study from Quebrada La Higuera (18 degrees S)[END_REF]. Second, abrupt changes in community structure and composition could go undetected when pooling middens over a large temporal window. Although a single fossil midden appropriately reflect the composition of arthropod assemblages (our study, [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] Elias | Fossil insect evidence for late Quaternary climatic change in the Big Bend region, Chihuahuan desert, Texas[END_REF], we argue that pooling middens may reduce the noise from the signal of past climate conditions when sufficient material is available.

Sampling biases of modern and fossil surveys

Another potential source of variability in live-dead agreements may come from our sampling design of live communities. Indeed, we sampled live communities over a limited period of time (October-December 2016) and only in a single year. Both pitfalls and sticky traps caught crawling and flying insects to a similar extent (Figure 1), and with high efficiency (Figure 2). However, fleas and ticks, for instance, were never found in modern communities despite being abundant in death assemblages. Setting traps for a longer period of time (or at different years) may help catch more taxa (e.g., fleas, ticks) in modern communities, thus increasing the similarity in species composition between modern communities and fossil assemblages (i.e., less variable and higher live-dead agreements). However, it would probably not change the fact that Coleoptera dominates both local live communities and death assemblages (modern, dead and fossil) of the Atacama Desert (see [START_REF] Jerez | Diversidad y patrones de distribución geográfica de insectos coleópteros en ecosistemas desérticos de la región de Antofagasta, Chile[END_REF][START_REF] Pizarro-Araya | Diversidad taxonómica de los artrópodos epígeos de la Región de Atacama (Chile): Estado del conocimiento[END_REF] for modern studies of the epigean fauna in the region of Atacama, Chile), or those in North America [START_REF] Elias | Observations on the Taphonomy of Late Quaternary Insect Fossil Remains in Packrat Middens of the Chihuahuan Desert[END_REF][START_REF] Hall | Late quaternary arthropod remains from Sonoran Desert packrat middens, southwestern Arizona and northwestern Sonora[END_REF][START_REF] Van Devender | Fossil arthropods from the Sierra Bacha, Sonora, Mexico[END_REF].

Another important sampling consideration is related to fossil midden weights. The indurated midden weight, washed weight (i.e., after dissolving the amberat), the percent weight loss in washing can greatly vary amongst middens, and potentially influence the number of arthropod taxa recovered [START_REF] Van Devender | Holocene arthropods from the Sierra Bacha, Sonora, Mexico, with emphasis on beetles (Coleoptera)[END_REF]. These variations in midden weights can be attributed to the relative proportion and composition of accumulated debris (e.g., sands, gravels, plant remains, and fecal pellets). To avoid relying on midden weight we standardized our protocol to the amount of time spent while screening washed and then dried materials (~3 hours; [START_REF] Latorre | Vegetation invasions into absolute desert: a 45,000 yr rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22°-24°S)[END_REF]. Our series of 46 middens yielded 639 specimens belonging to 43 taxa. In comparison, several series of 8 to 191 middens from the late Quaternary (<45,000 yr BP) located in the Sonoran and Chihuahuan desert have yielded from 17 to 85 taxa [START_REF] Elias | Insect fossil evidence of late Quaternary environments in the Northern Chihuahuan desert of Texas and New Mexico-Comparison with the paleobotanical record[END_REF][START_REF] Hall | Late Quaternary and modern arthropods from the Ajo mountains of Southwestern Arizona[END_REF][START_REF] Hall | Late quaternary arthropod remains from Sonoran Desert packrat middens, southwestern Arizona and northwestern Sonora[END_REF][START_REF] Hall | Arthropod history of the Puerto Blanco mountains, Organ Pipe Cactus National Monument, Southwestern Arizona[END_REF][START_REF] Van Devender | Fossil arthropods from the Sierra Bacha, Sonora, Mexico[END_REF][START_REF] Van Devender | Holocene arthropods from the Sierra Bacha, Sonora, Mexico, with emphasis on beetles (Coleoptera)[END_REF]. In those studies, however, the midden weight was not published, if recorded, and no standardized protocol was used, precluding meaningful comparisons of species richness amongst studies. The study of arthropod remains from fossil middens is in its infancy, and we advocate for the use of a standardized protocol in future studies.

Signal of past climate conditions

Once the taphonomy of how arthropod remains are preserved over space and time are better understood, midden series can reveal the true biological signal of arthropod community response to environmental changes. We show that artefacts can induce some variability among middens, which could be reduced by pooling three successively occurring middens. In this context, temporal variations in live-dead agreements in species composition showed three abrupt declines around ~10,100 cal yr BP, ~3,300 cal yr BP, and in the last 1,700 cal yr BP (three pooled middens; Figure 7). An explanation for such declines can in part be due to onsets of intense aridity, such as has been attributed to the strong decline in the diversity of the Chihuahuan Desert insect fauna was also observed in the last 2,500 14 C years, using fossil records from packrat middens [START_REF] Elias | Late Quaternary zoogeography of the Chihuahuan desert insect fauna, based on fossil records from packrat middens[END_REF]. In addition, those declines were also highlighted by temporal variations in Shannon diversity, and to a lesser extent Simpson diversity (Figure 9). This suggests that relatively rare species probably were more impacted than abundant ones. With the occurrence of one midden every ~308 ± 56.6 cal yr BP, on average, over the last 13,000 cal yr BP (Table A), the midden series evaluated in this study provides a robust paleoenvironmental reconstruction of the arthropod faunas over the entire Holocene and key insights into the community-environment relationship to be made over the same period of time.

CONCLUSIONS

To date, this study presents the first explicit assessment of the ecological fidelity and spatiotemporal resolution of arthropod fossil assemblages from South American rodent middens.

Three main conclusions can be drawn from our analyses: (1) although live-dead agreements and diversity indices are highly variable among samples, they consistently suggest that an average fossil midden better captures the structure and composition of living communities than species richness per se; (2) the spatial resolution of arthropod death assemblages is restricted to the close vicinity of rodent shelters; and (3) the effect of time and time averaging on death assemblages is limited. Altogether, we argue that pooling three successive middens may represent an optimal trade-off between the temporal resolution of death assemblages and inter-midden variations in community structure and composition when trying to detect potential influences of past environmental conditions. We hope this study will stimulate a renewed interest in midden records to disentangle the spatiotemporal variations in complex assemblages (including floristic and faunal assemblages). Regarding the high occurrence of rodent middens in arid environments worldwide, unraveling these paleoecological archives could provide invaluable insights into the natural and human-induced changes in ecosystems. abundances. Local and regional data are pooled. Note the axis break on the y-axis. See Table B for the list of codes and associated taxa identities.

Figure 2 :

 2 Figure 2: Rarefaction curves of effective species richness for all data pooled by data type (A), and

Figure 3 :

 3 Figure 3: Live-dead agreement of Simpson's turnover index (A), Spearman rank correlation (B),

Figure 4 :

 4 Figure 4: Comparison of relative order abundances pooled by data type. Local (green); regional

Figure 5 :

 5 Figure 5: Rarefaction curves of effective species richness for restricted datasets pooled by data

Figure 6 :

 6 Figure 6: Live-dead agreement of Simpson's turnover index (A, B), Spearman rho (C, D) and

Figure 7 :

 7 Figure 7: Live-dead agreement of Simpson's turnover index (A, B), Spearman rho (C, D) and

Figure 8 :

 8 Figure 8: Species richness (A), Shannon (B) and Simpson (C) diversity indices of fossil middens

Figure 9 :

 9 Figure 9: Species richness (A), Shannon (B) and Simpson (C) diversity indices of fossil middens

  Figure 1 851

  Figure 7 863

TABLES Table 1 .

 TABLES1 Rarefied richness (S±95%CI; n=240) and mean Pielou's evenness (PIE±SE) for samples pooled by data type.

		S	PIE
	Local	29.43±3.24	0.83±0.02
	Regional	28.02±2.57	0.74±0.05
	Dead	28.71±3.04	0.92±0.01
	Fossil	32.38±3.13	0.86±0.02

Table 2 .

 2 Percentages of "live species" (above diagonal) and "dead species" (below diagonal) found in each data type. Trap-type standardized catch of each modern taxon compared with uncorrected

		Local	Regional	Dead	Fossil
	Local	-	38.78	30.23	28.57
	Regional	35.19	-	22.22	20.45
	Dead	26.53	21.74	-	40.0
	Fossil	21.82	17.31	35.82	-

ACKNOWLEDGMENTS

We are grateful to Dr Mario Elgueta, Director of the entomology department of the "Museo Nacional de Historia Natural" in Santiago (Chile) and to Dr Álvaro Zúñiga-Reinoso for their assistance in identifying fossil remains. Thanks to Dr Maria L. Carrevedo for her assistance in the field and Carol Berrios for preliminary identifications of midden arthropod faunas. OD received a postdoctoral fellowship provided by the Biology Department of Rutgers University-Camden. GABV received a PhD fellowship provided by CONICYT (21160182; Chile). This study was supported in part by Apoyo de Financiamiento Basal (AFB) 170008 to the IEB (to CL).

DATA AVAILABILITY

Data supporting this manuscript will be provided in Dryad.

AUTHORS'S CONTRIBUTION

CONFLICTS OF INTERESTS

We have no conflicts of interests.

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