Kenneth Schoenly

Dynamics of heterotrophic succession in carrion arthropod assemblages: discrete seres or a continuum of change?

SummaryPatterns of species change, computed day to day community similarity, polar ordination, and other statistics were used to discern successional trends and correspondence with named decay stages from eleven carrion arthropod studies. The carrion arthropod community develops primarily as a continuum of gradual change: rapid at first, slow during peak activity, and erratic in the final days as carcass resources become depleted. In only five studies the ordination analysis revealed recognizable clusters representing discrete faunal seres; none of these, however, completely supported a stage-based view of faunal succession. Collectively, the authors of published studies identified 29 decay stage boundaries; of these, only 14 were associated with major faunal changes. We found at least 27 additional episodes involving compositional differences in the fauna not recorded by the authors. Named decay stages may have descriptive utility in carrion studies. However, ecologists and forensic entomologists should be alerted to the inadequacies of decay stages in summarizing patterns of faunal succession in carrion arthropod investigations.

On the trophic relations of insects-a food-web approach

Trophic relations of 95 insect-dominated food webs from seven habitat categories are examined and discussed in relation to current ecological theory. We identify 13 salient patterns, of which fewer than half have been reported previously from other food-web studies. (1) Analysis of trophic links in the 61 community-type webs corroborate the well-known trend that trophic diversity of insects has increased over evolutionary time. (2) Numbers of saprophages and animal parasites follow a continuum ranging from grazer-based systems (plant-herbivore, gall) and parasitoid-poor systems (aquatic, carrion, phytotelmata, wood, dung) to habitats that are detritus-based (phytotelmata, rotting wood, carrion, dung) and parasitoid-rich (gall), respectively (3) Among trophic subsets of species that determine food-web shape, only proportions of bottom species in food chains are similar to values published previously. (4) Aquatic-terrestrial comparisons reveal that aquatic webs (including phytotelmata habitats) support a richer number of trophic modes, higher proportions of bottom species, more interactions per species, and a smaller proportion of specialist consumers than the latter. (5) Cannibalistic links are generally absent in most insect webs and uncommonly recorded in the remainder of these webs, a result consistent with other food-web studies. (6) Global patterns of connectance and link numbers per species in insect-rich webs are similar to other reports even in webs with fewer investigator-defined species. This finding suggests that web participants interact with each other in broadly similar ways and in small numbers (2 2 here vs. 1 99 elsewhere). (7) Global ratios of resources (prey) to consumers (predators) are smaller (1:2) than in other webs (7:8), which suggests that for every consumer in insect-rich systems roughly one less resource is included, on the average, in its diet (8) An earlier prediction of higher prey-to-predator ratios in sink webs than in community-type webs is generally supported here. (9) Monophagy is wide-spread across both insect consumers and habitats; conversely, most insect species appear to have no more than three local consumers. Aquatic webs are exceptions in the former case. (10) Most insect families (76%) are associated with only one habitat category; conversely, five families (e.g., ants, flesh flies) are habitat generalists, occurring in four habitat types (11) Tropical webs do not, on the average, have longer food chains than those in temperate or desert climates. (12) Upper limits in chain length vary from three in carrion systems to as many as seven in gall webs: this latter value exceeds suggested upper limits for insect feeding reported elsewhere. (13) Nearly half of all the insect taxa (families) have members that are top predators in certain habitats even after vertebrates are factored in. These latter two features underscore the important roles insects may play in natural communities, roles that reach beyond occupying merely the critical middle links in ecosystems or the occasional position of top predator in unusual places such as caves.

Temporal Variation in Food Web Structure: 16 Empirical Cases

Analysis of food web structure and temporal dynamics is essential to understanding energy flow and population dynamics of species, and may contribute to conservation, wildlife management, and disease and pest control. This report synthesizes all the observational studies of food web dynamics to which we have access. Most published food webs are static and cumulative: they depict information gathered over many occasions. A web observed over a single, relatively short time period is time specific. Here we analyze the relation between cumulative and time—specific versions of webs in 16 published cases. Fourteen of the 16 webs are from detritus—based habitats that harbor large fractions of arthropod species: carcasses, tree fluxes, felled logs, treeholes, dung pads, and an acidic pond. The other two webs describe soybean fields and the arctic tundra. These webs are presented here in a consistent format and are analyzed in four ways. First, we quantified temporal trends and levels of variation in nine web properties: the percentages of species in the web that are top predators (%T), intermediate species (%I), and basal species (%B); the ratio of number of prey species to number of predator species (P); the mean chain length (µ); the product of species richness and connectance (S × C); and the numbers of total species, newly arriving species, and local extinctions. In most webs %I and %T fluctuated widely; the latter generally increased in time or remained constant, while the former correspondingly decreased or remained constant. Since the number of basal species usually varied little, changes in %B were inversely associated with changes in species richness over successional and seasonal time scales. Predictable changes in P, µ and S x C accompanied the changes in %B, %I, and %T. The numbers of total species, new arrivals, and local extinctions displayed no consistent increasing or decreasing trends. Second, we compared cumulative and time—specific webs from the same habitat to determine which properties, if any, of time—specific webs might be predicted from cumulative webs. In cumulative webs, P, µ, and %T came closest to the median of the values from time—specific webs, followed by %I, S x C, and %B Cumulative webs, which usually appear in general ecology textbooks, overestimate S x C and underestimate %B relative to time—specific versions. In five studies cumulative webs were completed when the last or next—to—last samples were taken; additional sampling in these cases would probably have uncovered more species. Third, opportunistic species were removed from four time—specific webs to determine how these species influenced web structure. Removing one top—feeding opportunistic species from each web caused a dramatic rise in %T, small reductions in %I, S × C, µ, and P, and a negligible rise in %B. A single opportunistic species, even though it makes only rare and brief visits to a habitat, can dramatically reshape web structure. Fourth, properties of cumulative and time—specific versions of the 16 food webs were compared to properties of cumulative webs in two published web catalogs. The cumulative versions of the 16 webs grossly resemble the cumulative webs in both prior catalogs, but the median S × C is greater and the median %B is lower in the 16 cumulative webs than in either prior catalog. Even for these two statistics, the median value for the 16 cumulative webs falls well within the range of variation of both prior catalogs. The time—specific webs in the 16 cases differ from those of the two prior catalogs somewhat more than do the cumulative webs. Comparisons between time—specific and cumulative versions of a web, one system at a time, are more sensitive than rough comparisons between collections of webs because the methods used to define species and links are (usually) consistent within a study.

A food web approach to evaluating the effect of insecticide spraying on insect pest population dynamics in a Philippine irrigated rice ecosystem

Summary 1. Data from a 645-taxa Philippines-wide food web and multiple regression models were used to predict population fluctuations of insect pests in a rice field. Independent variables of pest models included the biomass of rice plants in the field, the abundance of each pest, and the abundances of five highly correlated enemies of the pest, all as functions of time. 2. To test the ability of the models to reveal effects of insecticide spraying, a rice field at the International Rice Research Institute (IRRI) in the Philippines was divided into deltamethrin-sprayed and unsprayed plots. Data on the abundance of seven pest species (Nephotettix virescens, Recilia dorsalis, Sogatella furcifera, Nilaparvata lugens, Hydrellia philippina, Ne. nigropictus and Cofana spectra) and their natural enemies (predators and parasitoids) were collected during the dry season of 1990. 3. Spraying insecticide disorganized the population dynamics of insect species feeding in the IRRI field. Multiple regression models were less able in the sprayed plot than in the unsprayed plot to forecast the population fluctuations of pest species on the basis of various numbers and combinations of independent variables. For example, current pest abundance, by itself, was a significant predictor of future pest abundance for four of the seven pests (Ne. virescens, R. dorsalis, S. furcifera, H. philippina) in the unsprayed plot, but significant fits were found for only two pests (R. dorsalis, H. philippina) in the insecticide-sprayed plot. 4. In the unsprayed plot, independent variables were significant predictors of future pest abundance in four of seven initial models compared to one of seven models in the sprayed plot. Step-wise removal of independent variables in the models enhanced their forecasting power in both the sprayed and unsprayed plots, but significant models in the unsprayed plot nearly always outnumbered those in the sprayed plot. 5. In the unsprayed plot, Ne. virescens retained five of seven independent variables as significant predictors, compared to four for S. furcifera, three for C. spectra, and one for the remaining four pests. Classical models that contain one or two species as independent variables may not be sufficient to forecast future abundances of some Philippine rice pests in unsprayed and sprayed plots. 6. In general, models that included interaction terms and either the presence or absence of sprays among the independent variables did not improve the forecasting power of models in either the sprayed or unsprayed plot. 7. The methods developed here for studying the impact of spraying on the organization of arthropod communities in rice fields could be applied to other interventions

Comparative Performance and Complementarity of Four Sampling Methods and Arthropod Preference Tests from Human and Porcine Remains at the Forensic Anthropology Center in Knoxville, Tennessee

Abstract Comparative performance and complementarity tests of four arthropod sampling methods (aerial netting, hand collection, pitfall traps, and sticky traps), used by forensic entomologists in death investigations, training workshops, and research trials, were conducted from simultaneously placed human and porcine subjects inside the Forensic Anthropology Center at the University of Tennessee, Knoxville, TN. A secondary aim investigated the widely held claim that pig carcasses are reliable surrogates for human corpses. Over a 35-d period in summer 1989, >72,000 invertebrates from three subjects (one human, two pigs) were sampled of which 93% were members of the forensically important (FI) fauna. Performance tests revealed that hand collections, when performed by an experienced forensic entomologist, consistently yielded the largest fraction of FI arthropods from the total invertebrate catch, followed by aerial netting, sticky traps, and pitfall traps, regardless of subject. Pitfall traps and hand collections were broadly effective at sampling both fly and beetle populations, whereas aerial netting and sticky traps mostly targeted flies. The best two-method combination, based on the highest combined catches of FI taxa, were hand collections and pitfall traps, regardless of subject. Between-subject comparisons revealed negligible preference by FI arthropods for human over pig remains. Insofar as our limited comparisons allow with only three study subjects, these results validated the concept of transferability of “best practices” from one subject to another and confirmed the claim that pig carcasses (of 23–27-kg starting mass) can substitute for human corpses in research and training programs, at least for summer-exposed and unconcealed remains in the first 5 wk postmortem.

An experimental field protocol for investigating the postmortem interval using multidisciplinary indicators.

This article proposes an experimental field protocol for investigating the postmortem interval using specially designed apparatus and human and pig cadavers. We further propose that this goal can only be achieved by a multidisciplinary group, comprised of forensic entomologists, pathologists, and anthropologists. The apparatus and collecting methods described by the authors establish the means by which data can be collected on several fronts simultaneously: the sequential arrival and variety of insects in the decay process, the character and manner of soft tissue decomposition, the sequence and nature of bone exposure and order of disarticulation of skeletal remains, and the influence of climate and season on decay rates and arthropod succession. A central feature of this protocol involves the construction and use of a dual-functioning insect trap that allows separate but simultaneous capture of arriving and emerging populations while successional and decompositional processes of the cadaver are left intact. Results of trap performance tests in an arid climate and preliminary arthropod data collected from field-exposed pig carcasses are presented. The use of this protocol could provide important and badly needed baseline data for both medical investigators and law enforcement personnel, information that is critical to understanding the causes, manner, and time of death, which the law requires to be ascertained.

Food web dynamics of irrigated rice fields at five elevations in Luzon, Philippines

The above-water food webs of arthropod communities in irrigated rice fields on Luzon Island, Philippines, were studied over the growing season at five sites (Los Banos, Cabanatuan, Bayombong, Kiangan, Banaue) ranging in elevation from 22 m to 1524 m. Arthropod populations were vacuum-sampled at roughly weekly intervals from the date after seedlings were transplanted to flowering at each site. Site- and time-specific webs were constructed from a 687-taxa cumulative Philippines web and time-series of species present. Taxonomic composition, food web structure, and arthropod phenology were broadly similar across different sites. Arthropod abundance was inversely associated with altitude across the five sites, but numbers of taxa and links and six food web statistics showed no obvious increasing or decreasing trend with altitude. The rise of taxa, links and mean food chain length over the growing season at each site reflected an increase in plant size with age and, at some sites, an orderly accumulation of newly arriving herbivore, predator, parasitoid and omnivore species. At each site, herbivores built up faster than predators and parasitoids, and predators arrived faster than parasitoids; the difference between the latest and earliest sampling dates of first arrivals, averaged over the five sites, was 38, 63 and 73 days for herbivores, predators and parasitoids, respectively. Site-to-site consistencies in food web properties and first arrivals suggest that such patterns may be influenced more by crop age than by geography or altitude. Sampled predator, parasitoid and omnivore taxa potentially encountered only a subset of their lifetime prey and predator species at any particular time in the rice field. Prey lists cumulated over time may underestimate the temporal specificity of predation by potential biological control agents. Research opportunities linking rice food webs and integrated pest management with East Indies biogeography are proposed.

Sampling flies or sampling flaws? Experimental design and inference strength in forensic entomology.

ABSTRACT Forensic entomology is an inferential science because postmortem interval estimates are based on the extrapolation of results obtained in field or laboratory settings. Although enormous gains in scientific understanding and methodological practice have been made in forensic entomology over the last few decades, a majority of the field studies we reviewed do not meet the standards for inference, which are 1) adequate replication, 2) independence of experimental units, and 3) experimental conditions that capture a representative range of natural variability. Using a mock case-study approach, we identify design flaws in field and lab experiments and suggest methodological solutions for increasing inference strength that can inform future casework. Suggestions for improving data reporting in future field studies are also proposed.

Quantifying the Impact of Insecticides on Food Web Structure of Rice-Arthropod Populations in a Philippine Farmer’s Irrigated Field: A Case Study

Rice is grown on 10% of the arable land worldwide and provides one-third of the total calorie supply for two-thirds of the human population (International Rice Research Institute, 1991; Food and Agriculture Organization, 1992). In the middle to late 1960s, the widespread adoption of new high-yield, pest-resistant rice cultivars, nitrogenous fertilizers, and pesticides brought significant increases in rice production (popularly called The Green Revolution (Barker et al., 1985)) in south and southeast Asia. Extensive irrigation systems were built in Asia to capitalize on the insensitivity to the photoperiod of the new cultivars. Eliminating the former fallow period during the dry season removed one form of pest protection and brought new pest problems (Litsinger, 1993). Major outbreaks in the 1970s of the rice brown plant hopper, Nilaparvata lugens (Stal), a secondary rice pest before 1964, were traced to overuse of insecticides (Kenmore, 1980; Kenmore et al., 1984; Heinrichs and Mochida, 1984). The pest problems led to integrated pest management (IPM) strategies for tropical rice in the late 1970s that emphasized host plant resistance, cultural practices, and biological control, minimizing the use of pesticides.

Carcass Enrichment Does Not Alter Decay Rates or Arthropod Community Structure: A Test of the Arthropod Saturation Hypothesis at the Anthropology Research Facility in Knoxville, Tennessee

Abstract In a test of an arthropod saturation hypothesis, we asked if the 30-yr history of carcass enrichment at the Anthropology Research Facility, Knoxville TN, has altered carcass decay rates or community structure of sarcosaprophagous arthropods, compared with three local nonenriched sites. Over a 12-d period in 1998, using pitfall traps and sweep nets, we sampled a total of 81,000 invertebrates from freshly euthanized pigs (Sus scrofa L.) placed in these sites. From this number, we sorted 69,286 forensically important (sarcosaprophagous) arthropods. The community structure of these organisms, as measured by species and individuals accumulation curves, rarefaction, and nonparametric correlation, was comparable in all four sites in taxonomic similarity, colonization rates, aerial species richness, and ranked abundances of forensically important taxa on a per carcass basis. Measures of carcass decay rate, remaining carcass weight (%) and periodic weight loss, also were similar. In most cases, carcass surface temperatures and maggot mass temperatures were also statistically indistinguishable. Probability-based results and posthoc power analyses of these variables led us to conclude that the sarcosaprophagous arthropod community of the Anthropology Research Facility is representative of surrounding sites.