Ecology and Conservation of the Endangered American Burying Beetle (Nicrophorus americanus) MARKV. LOMOLINO,J. CURTIS CREIGHTON, GARY D. SCHNELL, AND DAVID L. CERTAIN

Department of Zoology, and Oklahoma Biological Survey, University of Oklahoma, Norman, OK 73019, U.S.A. email aa5577@uokmvsa.backbone.uoknor.edu

Abstract: Field studies were conducted on the western population of the endangeredAmerican burying beetle (Nicrophorus americanus) in Oklahoma and Arkansas to determine its habitat affinities. A common cause of declining populations is some specialized adaptation that makes it difficult to respond to a rapidly changing habitat. We evaluated the hypothesis that N. americanus is a habitat specialist in its search forfood, preferring mature forests with deep, humic soils. This hypothesis was rejected. Based on comparisons of niche breadth among syntopic congeners and niche overlap, N. americanus is relatively generalized in its use of a range of habitats when searching for food. It is likely that the generalist nature and the endangered status of N. americanus both derive from the fact that it is the largest member of its guild. In comparison to smaller species, N. americanus breeds on larger carcasses, which are more unpredictable in space and time. It is likely, therefore, that N. americanus must search over a larger area and greater diversity of habitats than its smaller congeners.

Ecologia y conservacion del escarabajo Nicrophorus americanus en peligro de extincion

Resumen: Se realizaron estudios de campo en la poblacion occidental del escarabajo Nicrophorus ameri- canus en peligro de extincion en Oklahoma y Arkansas, a efecto de deteminar sus afinidades de habitat. Una causa comun de laspoblaciones en declinacion es cierta adaptacion especializada que hace dificil responder a un habitat que cambia con rapidez. En este estudio, evaluamos la hip6tesis de que N. americanus es un especialista de haibitat en su buisqueda por alimento, prefiriendo bosques maduros con suelos hbumicos profundos. Esta hipo'tesisfue rechazada en base a comparaciones de la amplitud del nicho entre congeneres sintopicos y la superposicion de los nichos. N. americanus es una especie relativamente generalista en el uso de distintos habitats mientras busca alimento. Esposible que tanto la naturaleza generalista como el estado de amenaza de N. americanus deriven del hecho de que es el miembro mas grande de su gremio. En com- paracion con especies mas pequernas, N. americanus se reproduce en caddveres grandes, cuya disponibilidad en tiempo y espacio es mas impredecible. Por lo tanto, es posible que N. americanus deba buscar a lo largo de un acrea mas grande y de una mayor diversidad de habitats que sus congeneres mas pequenios.

Introduction

The American burying beetle (Nicrophorus america-

nus) is the largest member (body length = 27 to 35

mm) of a guild of beetles that breed and rear their

young on vertebrate carcasses (primarily mammals and

Paper submitted March 30, 1994; revised manuscript accepted June 7, 1994.

birds up to 300 grams). As with other burying beetles, N. americanus is a highly social species that provides biparental care to its young (Wilson, E. 0., 1971). Par- ents bury and maintain carcasses underground with se- cretions that seem to retard decay. Parents also feed their young by regurgitation and protect them from

predators (Bartlett 1988; Fetherston et al. 1990; Scott

1990; Trumbo 1990a). In contrast to other members of this carcass-burying

605

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606 Conservation Ecology ofNicrophorus americanus Lomolino et al.

guild, N americanus is now extremely rare. During this

century, the American burying beetle may have disap- peared from over 90% of its historic range (Fig. 1). In 1989 it was listed as an endangered species by the U.S. Fish and Wildlife Service (1991). Efforts to conserve

remaining populations of the American burying beetle have been hampered by a paucity of information. Des-

ignation of critical habitats, an integral part of the re-

covery process, is not possible without information on habitat affinities of the focal species. This information would facilitate surveys for undiscovered populations, and it is essential for managing habitats, assessing the

impact of alternative land-use strategies, and selecting optimal sites for reintroductions.

Endangered species tend to have large bodies and narrow niches (Diamond 1984; Martin & Klein 1984; Vrba 1984; Owen-Smith 1988; Stevens 1992). Anderson

(1982a) attributed the decline of N americanus to the coincident pattern of deforestation in North America. He based this conclusion on the assumption that N. americanus is similar in habitat requirements to N ger- manicus of Europe and N concolor of Japan and China. Each of these species is the largest member of their

guild and requires relatively large carcasses (50 to 300

grams; Kozol et al. 1988; J. C. Creighton, personal ob-

servation). Anderson (1982a) held that the dependence on larger carcasses for breeding restricts these species to ecosystems with deeper soils. Thus, N. germanicus N concolor, and, presumably, N americanus should be restricted to mature forests with open understories and

deep, humic soils (see also Muths 1991). Recent studies on N americanus, however, do not

support Anderson's (1982a) hypothesis. Preliminary studies by Creighton et al. (1993) suggested that Amer- ican burying beetles in Oklahoma occur in both grass- land and upland forests; they tend to avoid bottomland forests. Similarly, Kozol et al. (1988) reported that N americanus is broadly distributed across available hab- itats on Block Island, Rhode Island (shrub thickets to

grazed fields). The apparent generalist nature of N americanus on Block Island, however, may be an arti- fact of this insular environment (Crowell 1983). Be- cause of the low diversity of predators and competitors on islands, insular populations often exhibit ecological release, occurring in a broad variety of habitats consid- ered atypical for populations on the mainland (Crowell 1962; Grant 1971; Case 1975; Cox & Ricklefs 1977; Lomolino 1984).

Distributions of other Nicrophorus species on main- land sites of North America appear to be strongly influ-

Present distribution --

Historic distribution A z l (1880 to 1940)

Historic locality record 0

Figure 1. Range collapse of American burying beetle (N. americanus; after US. Fish and Wildlife Service 1991). Site A refers to western population studied here (1 = Camp Gruber, 2 = Fort Chaffee). Site B refers to eastern

population studied by Kozol et at (1988). Status of American burying beetle at site C is unclear, but it is prob- ably tenuous because only six individuals were captured in 1992 and three in 1993 (Ratcliffe &Jameson 1992; K Frazier, personal communication 1993).

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Lomolino et al. Conservation Ecology of Nicrophorus americanus 607

enced by interspecific competition (Pirone & Sullivan

1980; Anderson 1982b; Shubeck 1983; Wilson & Fudge 1984; Wilson & Knollenberg 1984; Wilson et al. 1984; Trumbo 1990a, 1990b). Because of its large size, N. americanus feeds on large as well as small carcasses, and it may compete with several vertebrate and inver- tebrate species (Kozol et al. 1988).

The purpose of this study was to determine the range of habitat affinities of the American burying beetle. Spe- cifically, we evaluated the hypothesis that N. america- nus is a habitat specialist in its search for food, prefer- ring mature forests with deep, humic soils. Our study was conducted on a local scale, focusing on a known

population of the American burying beetle to determine whether individuals were randomly distributed among available habitats. This study was limited to the western

population of the American burying beetle in Oklahoma and Arkansas. The habitat distribution of these beetles

may differ substantially from that of the population on Block Island. Because we studied mainland populations with many potential competitors, we also examined niche overlap among members of the burying beetle

guild.

Methods

Study Sites and Trapping

Field studies were conducted during 1992 at two sites: Fort Chaffee, Arkansas, and Camp Gruber, Oklahoma

(Fig. 1). These sites are approximately 85 km apart and were known to be inhabited by N americanus, along with several other species of burying beetles. Both sites include a diverse array of habitats ranging from open grasslands to bottomland forests and mixed deciduous forests dominated by Quercus stellata, Q. marilandica,

Carya texana, and Ulmus alata (Rice & Penfound

1959; Kuchler 1964; Rice 1965). Trapping was conducted from June 17 to September

5 at Fort Chaffee and from June 17 to September 14 at

Camp Gruber. We trapped in 72 locations (plots) at Fort Chaffee and in 70 at Camp Gruber. These plots were

randomly positioned within each of the principal vege- tative strata at Fort Chaffee and were uniformly posi- tioned within each vegetative strata at Camp Gruber. The number of plots within each stratum was propor- tional to its aerial coverage (determined by analysis of satellite imagery).

At each plot, eight pitfall traps were set at 20-meter intervals along transect lines. These traps (0.7-liter plas- tic cups, 15 cm in depth) were set for three days, baited with aged chicken, and covered with a plastic dome to

protect beetles from rain and excessive heat. We checked traps daily before 1000 hours, rebaited them as

necessary, and identified all burying beetles captured to

species. The N. americanus were sexed and character-

ized as teneral or fully sclerotized based on the relative softness of the exoskeleton, sharpness of the mandibles, and brightness of coloration. Each beetle was perma- nently marked by cutting a 3-mm triangular notch in the

posterior portion of its left elytron. Marking allowed us to exclude recaptures from analysis of habitat affinities.

Habitat Studies

Habitat characteristics were recorded from September 25 to 28 for each plot at Camp Gruber and September 28 to 30 for those at Fort Chaffee. The same team of two researchers conducted all habitat studies. At each plot, we positioned a 0.25-m2 sample frame midway between

adjacent pairs of the eight trap locations (total of seven frame locations).

At each of the seven positions, we recorded the pres- ence of grass, herbaceous plants, moss, rocks, litter, shrubs less than 2 meters in height, shrubs over 2 meters in height, and trees (woody vegetation with a diameter at breast height of more than 10 cm). We re- corded percentage of canopy closure (using a spherical densiometer) and soil depth at the central and two end

Fort Chaffee

3.0

1.0 - -

> d00 C 0 0

-4. 0 00 0 90 0 00

- -1.0 o00 0 0 of0 0 010 0

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

Forest development --->

4.0 Camp Gruber 4.0

I 2.0

o 0.0 00

' -2.0

CD, -4.01 I--

-2.0 -1.0 0.0 1.0 2.0

Forest development --->

Figure 2. Distribution of trapping plots relative to forest development (principal component I) and shrub cover (principal component II). Numbers

along axes refer to component scores from principal- components analyses conducted on habitat vari- ables.

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608 Conservation Ecology ofNicrophorus americanus Lomolino et al.

stations. At the central station, we also recorded the distance to nearest forest (for grassland sites) or open area (for forested sites) with a rangefinder. If these mea- sures exceeded the limit of the rangefinder (180 m), then the distance was recorded as 180 meters. At plots composed of a mixture of scattered trees and open ar-

eas, distance to forest and grassland was reported as zero.

A soil sampler was used to measure soil depth and to take samples at 51 plots at Fort Chaffee. An independent research team analyzed samples for percentage compo- sition of sand, silt, and clay.

Statistical Analyses

Trapping results and associations with habitat variables were subjected to separate but identical analyses for the Fort Chaffee and Camp Gruber plots. Principal- components analysis (using the correlation matrices) was used to summarize habitat variation among plots. All variables were first standardized to a mean of 0.0 and

a standard deviation of 1.0. Results of these analyses were similar for the two study sites. The first principal component, which accounted for 27.0% and 34.1% of the variation at Fort Chaffee and Camp Gruber, respec- tively, was a measure of forest development (compo- nent loadings exceeded 0.5 for canopy closure, litter, distance to forest, and distance to grassland). The sec- ond principal component, which accounted for 14.9% and 16.4% of the variation at Fort Chaffee and Camp Gruber, respectively, was largely a measure of shrub cover (high component loadings for shrub cover and

rocks). At both sites, the third principal component ac- counted for a minor portion of the overall variance

(10.8% and 8.6% at Fort Chaffee and Camp Gruber, respectively) and was not used in analyses of habitat affinities. This result simplified statistical analyses and allowed descriptions of trapping plots and habitat affin- ities over a two-component space (Fig. 2). Component scores were calculated using standardized data, and the scores were then normalized to have a zero mean and unit variance.

Table 1. Results of burying beetle (Nicrophorus) trapping studies at Camp Gruber, Oklahoma, and Fort Chaffee, Arkansas, during 1992.

Habitat^ N. americanusb Other Nicrophorus Speciesc Habitat"

Group n F.M S:T ORBI MARG TOME PUST CARO SAYI Trap-Nightsd

Fort Chaffee a 22 11:10 9:13 0 5 0 0 0 0 108.5 b 126 75:50 27:99 8 33 3 0 1 0 460.5 c 154 71:64 54:100 38 281 1 0 4 0 1048.0 d 132 79:47 43:89 26 2 1 0 0 0 589.0 e 2 1:1 0:2 2 0 0 0 0 0 51.5 f 127 57:70 39:88 82 2 1 1 0 0 502.0 g 37 19:17 12:25 188 2 5 0 0 0 605.5 h 4 3:1 1:3 196 0 0 0 0 0 191.5 i 10 6:4 3:7 97 0 2 3 0 0 309.5

Total 614 322:264 188:426 637 325 13 4 5 0 3866.0

Trapping successd 0.159 0.165 0.084 0.003 0.001 0.001 0.000

Camp Gruber a 38 23:15 10 12 11 0 15 0 270.5 b 18 10:8 48 5 6 0 0 2 271.5 c 9 3:6 14 1 3 0 0 1 65.0 d 60 34:26 369 0 22 1 0 0 494.5 e 26 18:8 821 0 11 4 0 0 369.0 f 37 13:24 836 0 14 6 0 1 413.0 g 3 3:0 112 0 0 0 0 0 48.0 h 14 10:4 57 0 1 1 0 3 47.5 i 8 6:2 31 0 0 1 0 0 40.5 j 2 1:1 183 0 0 4 0 0 61.5

Total 215 121:94 2481 18 68 17 15 7 2081.0

Trapping successd 0.103 1.192 0.009 0.033 0.008 0.007 0.003

aData presented by habitat categories based on cluster analysis of vegetative characteristics (see Methods and Fig. 2). b N. americanus results listed as number of individuals (n), ratio offemales to males (F:M), and ratio offully sclerotized to teneral adults (S:T). N. americanus were not aged at Camp Gruber, and some individuals were not sexed at Fort Chaffee. Trap success expressed as number of individualsper trap-night Adult body length (mm) = 27-35for N. americanus, 16-24 mm for N. orbicollis, 15-23 mm for N. marginatus, 15-20 mm for N. tomentosus, 15-23 for N. pustulatus, 15-25 mm for N. carolinus, and 16-24 for N. sayi cSpecies codes: CARO = N. carolinus; MARG = N. marginatus; ORBI = N. orbicollis; PUST = N. pustulatus; SAYI = N. sayi and TOME = N. tomentosus d Trap-nights = number of nights trapped minus 1.0 for each disturbed trap and minus 0.5 for each trap without bait in the morning. Trapping success = number of beetles caught per trap-night.

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Lomolino et al. Conservation Ecology of Nicrophorus americanus 609

To assess habitat affinities and potential niche segre- gation among burying beetles, plots were assigned to 1 of 10 groups based on habitat characteristics. Habitat

groups were formed by subjecting the component scores described above for individual plots to the KMEANS cluster-analysis procedure. SYSTAT was used for both principal-components and cluster analyses (SYSTAT Inc. 1992).

Trapping effort was calculated as total number of trap- nights at each plot minus the number of traps that were

completely disturbed and minus 0.5 times the number of traps missing bait when they were checked in the

morning. Continued disturbance of one plot at Fort Chaffee resulted in its deletion from further analyses. Because this plot represented the only one in its habitat

category, analyses at Fort Chaffee data are based on spe- cies distributions over nine of the ten habitat clusters.

Niche breadth was calculated by comparing the pro- portion of individuals (Pb) to the proportion of trap- nights (pt) conducted in each habitat. Maximal niche breadth is obtained when these values are equal over all habitats (when Pb = Pt). Therefore, we tested the null

hypothesis that beetles were randomly distributed

among available habitats using a measure of specific overlap (Ludwig & Reynolds 1988:115). Specific over-

lap (SO) was calculated using the following formula:

r r

SO = exp t [pjiln(Pi)] - E[pbjln(pb)] j (1)

where r = number of habitat categories, Pbj = propor- tion of individuals captured in habitat j, and py = pro- portion of trap-nights conducted in habitat j

Overlap measures were also used to test for niche

segregation between burying beetle species. Specific overlap was used to test for pairwise segregation be- tween species, and adjusted generalized overlap was used to test whether the entire guild exhibited identical habitat affinities. Adjusted generalized overlap (GO) was calculated as

- s r

GO = exp L?{nj[ln(cj) - ln(p)]}/T , (2) L i j

where s = number of species, r = number of habitat

categories, cj = proportion of individuals of all species in habitatj, ny = number of individuals of species i in habitat category j, py = proportion of individuals of

species i in habitat categoryj and T = total number of individuals. Both specific and generalized niche overlap measures range from 0.0 (no overlap) to 1.0 (complete overlap). The null hypothesis of complete overlap was tested using statistics that were distributed as chi-square (see Ludwig & Reynolds 1988:116).

Spearman rank correlations were used to test for as- sociations of beetles with soil characteristics-percentage of sand, silt, and clay). The three soil fractions are, of course, highly correlated (absolute values of Spearman rank correlation > 0.6). Soil fractions were not signifi- cantly correlated (r > 0.07,p > 0.05) with the first habitat component score (a measure of forest development), al- though percentage of sand and percentage of silt were significantly correlated with component II (a measure of understory shrub cover; r = - 0.43 and 0.48 for compo- nent II on sand and silt, respectively, p < 0.01).

Results We captured 829 American burying beetles during these studies (614 and 215 at Fort Chaffee and Camp Gruber, respectively; Table 1). In addition to N. amer- icanus, five other species were trapped at Fort Chaffee and six at Camp Gruber (Table 1). N orbicollis was the most common species at both sites, whereas N sayi was recorded only at Camp Gruber.

Niche breadths were calculated for the three most common species at each site (N. orbicollis, N. ameri- canus, and N marginatus at Fort Chaffee, and N. or- bicollis, N americanus, and N. tomentosus at Camp Gruber; Table 2). At both sites, N. americanus exhibited the broadest niche, which, at Fort Chaffee was 84% and at Camp Gruber 93% of that exhibited for a complete generalist. N. americanus was the only species recorded in all habitat categories at the two trap sites (Table 1).

Habitat affinities of N. americanus were not signifi- cantly different (p > 0.05) between sexes or between teneral and fully sclerotized adults. At Fort Chaffee the adjusted general overlap was 0.982 between sexes and 0.983 between age classes. At Camp Gruber the value between sexes was 0.923.

As with N. americanus, N. tomentosus also exhibited a relatively broad niche, but N. orbicollis and N. mar-

Table 2. Niche breadths of the three most common species of burying beetles at Fort Chaffee and at Camp Gruber during 1992.a

Fort Chaffee Camp Gruber

Nicrophorus Niche Niche Species Breadth Ub Breadth Ub

N. americanus 0.844 208.66c 0.925 33.58c N. orbicollis 0.512 852.19c 0.707 1721.60c N. marginatus 0.402 596.78c N. tomentosus 0.903 13.90a

aNiche breadth calculated as specific overlap between distribution of individuals and distribution of trapping effort within each hab- itat category (specific overlap ranges from 0.0 for perfect specialists to 1.0 for perfect generalists; see Ludwig & Reynolds 1988:115). We tested the hypothesis that niche breadth (specific overlap) was sig- nificantly less than 1.0. b U-statistic distributed as chi-square with n - 1 degrees offreedom (here n equals number of habitat categories; df = 8 at Fort Chaffee and 9 at Camp Gruber). Cp < 0.001. dp > 0.05.

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610 Conservation Ecology of Nicrophorus americanus Lomolino et al.

ginatus were highly specialized in their habitat affinities

(Table 2). This is reflected in habitat selectivity, which was calculated as the proportion of individuals trapped in a given habitat type minus the proportion of trap- nights in that habitat. Theoretically, habitat selectivity can vary from -1.0 for maximal avoidance to 1.0 for maximal preference. Habitat selectivity was notably dif- ferent among the four most common Nicrophorus spe- cies at both Fort Chaffee (Fig. 3) and Camp Gruber (Fig. 4). At Fort Chaffee N. americanus was broadly distrib- uted across habitats, with habitat selectivity values rang- ing only from -0.10 to 0.09 (Fig. 3a); there was no

particular pattern with respect to forest development or shrub cover. In contrast, N. orbicollis showed a definite

preference for more wooded sites (Fig. 3b), and N mar-

ginatus was concentrated in one group of plots charac- terized as being unwooded, with an intermediate level of shrub cover (Fig. 3c). For N. tomentosus, the number of captures was relatively small at Fort Chaffee (Table 1); no pattern was evident in habitat selectivity relative to forest development and shrub cover (Fig. 3d).

At Camp Gruber the patterns of habitat selectivity (Fig. 4) were very similar to those at Fort Chaffee (Fig. 3). Habitat selectivity values for Nt americanus at Camp

a) N. americanus_ b) N. orbicollis

0.6 0.6

0.4 0.4 h

*Io.\ ~.1 ~rest~ 1 -

i 'i |/ir r^\~~3 3 -02 .mar s ) to

to forest development (principal component I) and shrub cover (principal component II). Larger values

"0*4 ^<~- f v, 1 ci eOe/

on vertical axis.

c) N. marginatus d) N.tomentosus

0.6 c 0.6

I 0.21 r)~~~~ T 0.2 "2

-~~~~~~0.40 '^-,-^,~~~~~~ /~- 0.0

-0.2~Fort 2haffee

on vertical axis.

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Lomolino et al. Conservation Ecology ofNicrophorus americanus 611

a) N. americanus b) N. orbicollis

0.6 0.6

0.4 0.4

0 -2 0 - 2v

0.2 D 0.2

c nr3 3 1 -0.21 I r I 2 -0.21 I \ 1 /^2

-0.4 O '

-0.4 0 -1 -1 -1 - l

5^^<! 0

' - -

0ror -e^0 1 /-2 -2 Orestc (eres 3rste v1eop te 2 -/3 2e 7

-3

c) N. marginatus d) N. tomentosus

0.6 -r' 0.6

0.4 0.4

picted in Fig0.2. 0.2

-0.2 2 -0.2v 02

-0.4 -0.4

'/ore't 1 resto? --

i eo t

2 -3 e e n

Camp Gruber

picted in Fig. 2.

Gruber were nearly uniform across groups of plots, ranging from - 0.06 to 0.05 (Fig. 4a). N. orbicollis was concentrated in more-forested plots, particularly those with increased shrub cover (Fig. 4b). N. marginatus was found only at plots with few trees (Table 1), with the species showing marked habitat selectivity for very open plots with intermediate shrub cover (Fig. 4c). N. tomentosus was broadly distributed across habitat

groups, with values for habitat selectivity from - 0.04 to

0.09. Overall, these findings for the two sites are con- sistent with those of previous studies that reported N. tomentosus as a habitat generalist and N. marginatus and N. orbicollis as strongly associated with meadows and forests, respectively (Anderson 1982b; Wilson & Knollenberg 1984; Wilson et al. 1984; Kozol et al. 1988; Trumbo 1990b).

Analyses of distributions of all species revealed highly significant niche segregation, especially at Fort Chaffee.

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612 Conservaton Ecology of Nicrophorus americanus Lomolino et al.

The adjusted general overlap of the six species at Fort Chaffee was only 0.449, less than one-half of its theoret- ical maximum of 1.0 (V = 1,492.8, df = 40,p < 0.001; see Ludwig & Reynolds 1988:116). The overlap of the seven species at Camp Gruber, albeit higher than that at Fort Chaffee, was also significantly different from 1.0

(GO = 0.735, V = 626.19, df = 54, p < 0.001). Comparisons between species revealed significant

niche segregation (p < 0.01) for all but one species pair (N tomentosus on N. americanus at Camp Gruber: Ta- ble 3). Niche segregation was highest (low overlap) be- tween the habitat specialists (N. orbicollis and N. mar-

ginatus at Fort Chaffee; Table 3). At both sites, N. americanus exhibited the greatest pairwise overlaps.

Tests of associations between soil characteristics and N americanus did not support Anderson's (1982a) sug- gestion that this species is restricted to habitats with

deep soils. The trapping success of N americanus was not significantly correlated with soil depth at either site

(r = -0.107 at Fort Chaffee and -0.011 at Camp Gru-

ber; Spearman rank correlations, n = 72 and 70). The trapping success of N. americanus, however, was

correlated with the three soil fractions (percentage of

sand, silt, and clay; Fig. 5). Trapping success increased with percentage of sand and decreased with percentage of silt and clay (r = 0.43, -0.39 and -0.43, respec- tively). While these trends were significant (p < 0.01), they included considerable variation. At Fort Chaffee N. americanus tended to avoid soils with less than 40% sand and over 50% silt and 20% clay (Fig. 5). In con-

trast, N. orbicollis and N marginatus were not signifi- cantly associated with soil fractions at Fort Chaffee (p >

0.05).

Discussion and Conclusion

For most guilds, larger species tend to feed on larger prey, to occupy a greater diversity of habitats, and to

dominate in interference competition, but they may suf- fer from exploitative competition from smaller species (Ashmole 1968; Rosenzweig 1968; Schoener & Gorman 1968; Hespenheide 1971; Werner 1974; Wilson 1975; Zaret 1980; Gittleman 1985; duToit & Owen-Smith 1989). Because larger prey (here, carcasses of birds and mammals) are less abundant than smaller prey (Peters 1983; Brown & Maurer 1987; Lawton 1990; Damuth 1991), larger guild members also require larger home ranges. It is likely that these and other important impli- cations of body size combine to account for the scarcity and proneness to extinction of giant mammals and flightless birds-the vertebrate megafauna. At less than 2 grams, N. americanus may be an invertebrate equiv- alent of a member of the endangered megafauna. It is the largest member of a guild of species that specialize on rare and unpredictable resources, vertebrate carcasses. In contrast to other guild members, N. americanus must range over a larger area and a greater diversity of habi- tats to find suitable carcasses.

While this study indicates that N. americanus is a generalist in its search for carcasses, it is likely that this species is more stenotopic when selecting sites for car- cass burial and breeding. Its success in locating car- casses, outcompeting other species, and, ultimately, breeding on a carcass may differ among habitats. Clearly, additional information on the ecology and breeding bi- ology of N. americanus is essential to conserving its populations in the wild. We are now conducting field manipulations to examine the effects of habitat condi- tions on the breeding success of this species.

The idiosyncratic nature of N americanus and, per- haps, of numerous other endangered species argues strongly for continued focal-species research as re- quired by the Endangered Species Act. Other ap- proaches certainly will be required to conserve biodi- versity, especially at community to landscape levels. Regardless of the level of focus, however, successful ap- proaches for conserving biodiversity should be guided by relevant theory but should be based on empirical

Table 3. Specific overlap and U-statistics of habitat affinities of the three most common species at Fort Chaffee and Camp Gruber.

Location/Species Specific Overlap (U-statistic)a

Fort Chaffee N. americanus N. orbicollis N. marginatus N. americanus 0.146(2359.3) 0.143(2388.4) N. orbicollis 0.175 (2224.0) 0.064 (1789.4) N marginatus 0.395 (603.2) 0.000(13,887.6)

Camp Gruber N. americanus N orbicollis N. tomentosus N. americanus 0.412 (380.9) 0.367 (431.2) N. orbicollis 0.560(2874.0) 0.080 (12,539.5) N tomentosus 0.903 (13.9)b 0.428(115.5)

Overlaps reported for species on horizontal against those on vertical (for example, overlap of N. americanus on N. orbicollis at Fort Chaffee = 0.146). Overlap ranges from 0.0 for complete niche segregation to 1.0 for complete overlap. U-statistic is distributed as chi-square with n - 1 degrees of freedom (see Ludwig & Reynolds 1988:116; df = 8 at Fort Chaffee and 9 at Camp Gruber). bSpecific overlap not significantly less than 1.0 (p > 0.05); otherwise, p < 0.01.

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Lomolino et al. Conservation Ecology ofNicrophorus americanus 613

1.00 U) 5 U)

o 0.75 0 o

U

c) 0.50 n C D o

-- o X 0.25 ? a E [ ] o o I- Do

0.00 0 20 40 60 80

Sand (%)

1.00 u)

u 0.75 0 o

U) 0.50 o

a 8~0o 0.25 oo

0.00 0 20 40 60 80

Clay (%)

1.00 U)

? 0.75

U) 0) 0.50 ' C Do

0.25 oo

0.00 ?m? O o 20 40 60 80

Silt (%)

Figure 5. Habitat distribution of N. americanus as a function of soil characteristics-percentage of sand, silt, and clay-at Fort Chaffee, Arkansas.

Trapping success calculated as number of individu- als captured at each plot divided by number of trap- nights at thatplot.

studies of the species, communities, or landscapes in

question.

Acknowledgments Michael Proctor, Marcus Nix, and Joel Klumpp assisted with field studies. Caryn Vaughn, Ian Butler, Daniel

Hough, and Rebecca Rudman provided advice and as- sistance with data management and related tasks. Fund-

ing for the field work was provided by Fort Chaffee and

Camp Gruber through the U.S. Army Construction En-

gineering Research Laboratories (USACERL), Cham-

paign, Illinois. Data on soils was provided by Timothy Blechl of USACERL. We thankJerry Sturdy and Ken Hoyt

for their assistance with coordinating research efforts at Fort Chaffee and Camp Gruber, respectively. Additional

funding for data analyses was provided by the Oklahoma

Department of Wildlife Conservation (ODWC) and the U.S. Fish and Wildlife Service (USFWS) through Section 6 of the Endangered Species Act, and we thank Mark

Howery and Harold Namminga of the ODWC, and Ken Frazier and Michael Amaral of the USFWS for their as- sistance. Ola Fincke, Paul Giller, John Jaenike, Steven

Juliano, Peter Leimgruber, Caryn Vaughn, and Andrew Smith provided useful comments on an earlier version of this manuscript.

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