2008 SOUTHEASTERN NATURALIST 7(1):101–110
A Dung Beetle Assemblage in an Urban Park in Louisiana
Meghan G. Radtke1,2,*, Chris E. Carlton3, and G. Bruce Williamson1
Abstract - We examined the dynamics of a dung beetle community over the course
of a year in a forested urban park in Baton Rouge, LA. Dung beetle volume per
trap-day and abundance peaked during March and the months of August through
November, with species richness highest during March. The subfamily Aphodiinae
dominated the community during the cold months, and Scarabaeinae dominated it
during the warm months. The relationship of these patterns to local temperature and
precipitation is discussed.
Introduction
Dung beetles (Scarabaeidae) are important contributors to decomposition
processes in the southeastern United States. In natural environments,
dung beetles utilize the excrement of large and small mammals and can
subsist on carrion, rotting fruit, and other decaying organic matter if necessary
(Gill 1991). However, most landscapes in the United States have been
modified by people, thus altering local organism composition. Of particular
interest to us are the dynamics of urban parks within large cities, a subject
infrequently addressed by dung beetle research (Carpaneto et al. 2005, Wallace
and Richardson 2005). Are sufficient food sources available to sustain
populations of dung beetles? How do species richness and biomass compare
with more pristine habitat? Previous studies (Carpaneto et al. 2005, Wallace
and Richardson 2005) have demonstrated that dung beetles perform an important
service by recycling dung from house pets in urban areas.
Our study documented the dynamics of a dung beetle community over
one year in an urban bottomland hardwood forest park in Baton Rouge,
LA. We provide monthly data on species’ presence, abundance, biomass,
and species richness. These data will be useful for future urban park studies
or comparisons with more natural areas. Furthermore, we present
volume-biomass equations for dung beetles found in bottomland/upland
hardwood forests.
Methods
Scarab monitoring
Highland Road Observatory Park is a city park located in Baton Rouge,
LA (30°20.698'N, 91°04.406'W). The park is a fragment of secondary forest
1Department of Biological Sciences, Louisiana State University, Baton Rouge,
LA 70803. 2Current address - Department of Environmental Quality, Remediation
Services Division, PO Box 4314, Baton Rouge, LA 70821-4314. 3Louisiana
State Agricultural Center, Louisiana State University, Baton Rouge, LA 70803.
*Corresponding author - kc7moc@cisaz.com.
102 Southeastern Naturalist Vol.7, No. 1
of approximately 32.5 ha on the edge of the city. The forest, classified as bottomland
hardwood, is prone to fl ooding after heavy rains, especially in the
spring. Although the Park is isolated and small, the occasional Odocoileus
virginianus (Boddaert) (white-tailed deer) is present, in addition to Procyon
lotor Linnaeus (raccoon) and Canis lupus familiaris Linnaeus (dog). We collected
dung beetles from November 2003 until October 2004. We used 8 to
10 pitfall traps, baited with dung of Sus scrofa L. (pig), spaced at least 20 m
apart. Traps consisted of a 500-ml plastic cup, 88 mm in diameter by 121 mm
in height, with a wooden or plastic covering suspended above the cup for rain
protection. At least 50 g of pig dung was placed in a 150-ml cup and attached
to the side of the “pitfall” cup. Traps were baited on the first day of a collecting
period, and the contents collected 5 days later. We trapped once a month over
the course of a year, with the exception of September 2004.
Beetle specimens, collected in the field, were frozen until they could be
measured in the lab. Directly measuring biomass is difficult because dried
specimens are extremely fragile. Therefore, we measured beetle volume,
which is highly correlated with beetle biomass (Radtke and Williamson
2005). To measure volume, we inserted a number two insect pin into the
elytra of the beetle and used the pin as a “handle” to completely submerge
the beetle in a beaker of distilled water resting on top of an electronic balance.
The change in weight on the balance corresponds to the beetle’s
volume (1g = 1 ml of water at sea level). This method has been used successfully
to make comparisons, either directly of volume among samples or
indirectly by first converting volume into biomass (Radtke and Williamson
2005, Radtke et al. 2006). We identified each species using the Louisiana
State Arthropod Museum (LSAM) reference collection and keys (Arnett
et al. 2002, Harpootlian 2001). Specimens were deposited in the LSAM,
Baton Rouge, LA. Climatological data was taken from Louisiana Office of
State Climatology monthly reports.
Temperate equations
We developed a biomass-volume equation (Radtke et al. 2006) for
bottomland/upland hardwood forest scarabs using beetles collected from
Highland Road Observatory Park (Louisiana) and Homochitto National Forest
(Mississippi). Specimens were dried in a drying oven for at least 48 hours
at 50 °C. We measured their biomass three separate times on a top-loading
electronic balance (± 0.01 g) and then averaged the numbers. Equations were
derived using untransformed and log-transformed data. We log-transformed
the data because the variance in biomass and volume increased with larger
beetles. We used proc reg in SAS for our analyses (SAS Institute 2001).
Results
Scarab monitoring
We captured a total of 699 beetles and 12 species during 525 trap-days
(Table 1). Onthophagus hecate hecate (Panzer) dominated the collection
2008 M.G. Radtke, C.E. Carlton, and G.B. Williamson 103
Table 1. Species abundance and percent volume (in parentheses) for each month, total abundance, and average size and SD by species for Highland Road Observatory,
LA. “*” indicates the species with the largest proportion of volume for each month.
Total Average
Species Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Oct. abundance volume (ml)
Aphodius bicolor 29 6 20 4 0 0 0 0 0 0 0 59 0.001 ± 0.000
(0.2) (0.6) (4) (0.3)
A. nigritus 6 1 0 0 15 3 2 0 0 0 0 27 0.001 ± 0.000
(0.1) (0.1) (0.1) (0.1) (0.04)
A. rusicola 2 0 2 1 82 18 1 1 0 0 0 107 0.001 ± 0.000
(0.02) (0.4) (0.1) (0.3) (0.8) (0.02) (0.03)
Ateuchus histeroides Weber 0 0 0 0 3 1 0 0 0 0 0 4 0.029 ± 0.000
(0.3) (1)
Canthon viridis (Beauvois) 0 0 0 0 1 0 0 0 0 0 0 1 0.010
(0.03)
Copris minutus 0 1 4 3 2 0 3 0 0 0 11 24 0.120 ± 0.020
(8) (86)* (33) (0.9) (5) (10)
Deltochilum gibbosum gibbosum 1 0 0 0 0 1 2 0 0 0 0 4 2.060 ± 0.530
(21) (55)* (76)*
Dichotomius carolinus 2 0 0 0 10 0 0 0 0 2 4 18 2.660 ± 0.550
(23) (83)* (66)* (87)*
Onthophagus gazella F. 0 0 0 0 1 0 0 0 0 0 0 1 0.170
(0.6)
O. hecate hecate 134 20 1 16 77 21 22 55 6 62 6 420 0.050 ± 0.010
(54)* (91)* (11) (66)* (14) (43) (18) (99)* (100)* (34) (3)
O. orpheus Panzer 5 0 0 0 0 0 0 0 0 0 1 6 0.030 ± 0.010
(7) (0.1)
Pseudocanthon perplexus 0 1 0 0 23 0 1 3 0 0 0 28 0.010 ± 0.000
(1) (0.8) (0.2) (1)
104 Southeastern Naturalist Vol.7, No. 1
Figure 1. Dung beetle and abiotic dynamics of Highland Road Park (Baton Rouge,
LA). A) Volume per trap-day, B) Abundance per trap-day and species richness;
C) Minimum, maximum, and average temperatures for November 2003 through October
2004; and D) Monthly precipitation for 2003–2004 and the 30-year average.
2008 M.G. Radtke, C.E. Carlton, and G.B. Williamson 105
with 420 individuals. The number of individuals of other species ranged
from one to 107. The month of March yielded the highest volume per trapday,
abundance, and species richness. However, the months of August,
October, and November were also high in beetle volume, but abundance
did not strictly follow this pattern. Species richness generally mirrored the
dynamics of beetle abundance (Figs. 1A and 1B).
Species composition and dominance changed throughout the year. In
terms of actual numbers, Aphodius bicolor Say, A. nigritus F., A. rusicola
Melsheimer, and Pseudocanthon perplexus (LeConte) were high in abundance
and dominated the scarab community from January through March
(Figs. 2A and 2B). Onthophagus hecate hecate was present all year round,
but was most dominant during the hottest months of the year, even though
its abundance was sometimes quite low (Fig. 2C). We examined community
dominance by looking at beetle volume and found that O. hecate hecate
was dominant for five months, Dichotomius carolinus (L.) for three months,
Deltochilum gibbosum gibbosum (F.) for two months, and Copris minutus
(Drury) for one month (Table 1). All other species were generally more random
in their presence in the community and did not show any discernable
pattern; however, most species were present during the month of March.
Average monthly temperatures were coldest from December through
February and warmest from June through September. In general, the
abundance of O. hecate hecate mirrored the increases and decreases in temperature
(Fig. 1C). During the study, monthly precipitation showed peaks
in February, May through June, and October, although 30-year averages
exhibited relatively constant monthly rainfall. With the exception of the
summer months, total beetle volume seemed to shift up or down in a positive
response to rainfall (Figs. 1A and 1D).
Temperate equations
For the biomass-volume regressions, we measured 201 beetles from 9
species in 5 genera. We constructed two highly significant equations (P <
0.0001) with untransformed data explaining 86% of the variation and logtransformed
data explaining 95% of the variation (Table 2).
Discussion
Scarab monitoring
Over the course of a year, we found that dung beetle volume, abundance,
and species richness changed by season. We investigated possible
correlations of these changes with average monthly temperature and
Table 2. Regressions for temperate scarabs. Both equations are significant at the P < 0.0001
level and with one degree of freedom.
Equation R2 F
Untransformed y = 0.19x + 0.007 0.86 1180
Log-transformed y = 0.82x - 0.71 0.95 3438
106 Southeastern Naturalist Vol.7, No. 1
rainfall. Although evidence exists in the literature that dung beetle composition
changes with abiotic factors and our study found support for
Figure 2. Dung beetle demographics at Highland Park (Baton Rouge, LA). A) Abundance,
B) Dominance of small beetles, and C) Abundance and percent of the scarab
community composition for O. hecate hecate.
2008 M.G. Radtke, C.E. Carlton, and G.B. Williamson 107
some of these conclusions, we caution against any definitive conclusions
based solely on this study. We collected data for 12 months, but
temperature and precipitation can be highly variable from year to year
necessitating the need for several more years of data. Furthermore, we
cannot directly compare our study to others because those performed
at similar latitudes in North America were not in bottomland hardwood
forests (Howden and Scholtz 1986, Nealis 1977). With these caveats, we
suggest the following patterns and explanations for the dung beetles collected
at Highland Road Park.
Species composition varied by season, mostly by subfamily. Species of
Aphodiinae were only present during the cold months and dominated the
dung beetle community during January, whereas most species of Scarabaeinae
were collected during the warmer months, mainly March through
November. The aphodiine species tolerate cold conditions and dominate
northern temperate climates, although they are equally species rich in the
tropics. They are small, generally less than 13 mm in length, and reproduce
directly in dung pats (Hanski and Cambefort 1991a). Scarabaeine species
thrive in warm climates, and many diapause during cold and dry months.
They are larger in size than members of Aphodiinae and either bury dung
below the pat or roll it away for burial and subsequent nesting. Scarabaeines,
because of their size and resource relocation strategies, easily out-compete
the Aphodiinae for resources during seasons where both subfamilies are active
(Finn and Gittings 2003, Hanski and Cambefort 1991a).
We observed two peak periods of dung beetle abundance and volume:
March, and August through November. Onthophagus hecate hecate made up
36 to 97% of total individuals collected during the peak periods and dominated
collections during the months of August (97%) and November (75%).
Aphodius rusicola (38%) represented the highest proportion of beetles in
March, and C. minutus (50%) was dominant in October. The degree of dominance
by individual species varied between the March and fall collections.
The fall months had lower absolute numbers of individuals (22–179), and
during this time, members of a single species accounted for at least 50% of
the collection. March had a much higher number of individuals (214), and
two species, A. rusicola and O. hecate hecate (36%), shared dominance during
this month.
Volume per trap-day peaks were largely the result of Scarabaeinae
rather than Aphodiinae because of body-size differences. In particular,
D. carolinus was the most influential volume contributor. As one of the
largest species collected, having a few individuals in a monthly collection
drastically increased the volume per trap-day. As functional efficiency of
dung beetles may be related to their overall size (Larsen et al. 2005), early
spring and fall may have the highest rates of dung degradation caused
by the presence of D. carolinus. In temperate forests, these times of year
correspond with an increase in food sources for mammals. Early spring
offers an abundance of flowers and new leaf growth that is rich in nitro108
Southeastern Naturalist Vol.7, No. 1
gen, phosphorus, and other nutrients (Nolet et al. 2005). Forage quality
decreases during the summer, but food becomes abundant again in the
fall as fruits and nuts reach maturity. Mammals depend on these times of
elevated food availability; thus, their activity increases and reproductive
events are timed accordingly (Côté and Festa-Bianchet 2001, Nolet et al.
2005). In addition to increasing the mammal dung supply, the presence
of copious quantities of decaying flowers and fruit could directly supplement
the dung beetle diet as well (Gill 1991).
Dung beetle abundance and volume per trap-day peaks may be explained
in part by temperature and precipitation requirements. Dung
beetles have the ability to diapause during seasons of the year that are
unfavorable for survival. Factors that may induce this behavior are rainfall
(too much or too little), temperature (too high or too low), resource
availability, and interspecific competition (Hanski and Cambefort 1991b).
The rainfall in February combined with the warm temperatures in March
may have created optimal conditions for the already active aphodiine
populations to expand and for the inactive scarabaeines to emerge from
dormancy. The drop in scarabaeine populations in the summer may have
been caused by higher than favorable temperatures as well as a reduction
in mammal activity. Dung beetle abundance and volume peaked again in
the fall as resources and environmental conditions once again became optimal
for activity.
The presence of O. hecate hecate in every month of the collection period
suggests it is a hardy species that tolerates a variety of temperature
and moisture conditions. It is small, ranging in size from 0.021 to 0.066 ml,
and the large numbers collected make it an important contributor to volume
per trap-day, even in months when it was not the dominant contributor
(Table 1). Onthophagus hecate hecate does not show a seasonal abundance
pattern (Fig. 2C). Interspecific competition, possibly with some of the larger
scarabaeines, may control population increases and declines during the
warm months of the year; whereas biotic conditions may control the success
of the species during the winter. Despite these fl uctuations, O. hecate hecate
clearly dominated the dung beetle community from May through August and
then again in November and December (Fig. 2C).
Studies of urban dung beetle communities are important because they
can indicate ecological changes in the local environment (Spector 2006).
Carpaneto et al. (2005) documented changes in the dung beetle community
of an urban park in Rome when dung resources changed from sheep
dung to primarily dog dung. Species richness decreased from 19 to 9
species, whereas the abundance of dung beetles increased by seven fold.
Wallace and Richardson’s (2005) study of an urban dung beetle community
in Austin, TX documents 9 species that heavily rely on dog dung as
a resource. Demographic studies like this one and Wallace and Richardson’s
(2005) are important baseline studies for future comparisons. As
2008 M.G. Radtke, C.E. Carlton, and G.B. Williamson 109
humans are constantly modifying their surroundings, monitoring dung
beetle communities can provide insight into the larger ecological changes
that are occurring.
Temperate equations
Both the untransformed and log-transformed equations relating beetle
biomass and volume were highly significant, as has been seen in Neotropical
studies (Radtke and Williamson 2005, Radtke et al. 2006). The log-transformed
equation explained the relationship better than the untransformed
equation, probably because of the increased variance in measurement in
large beetles. We recommend these equations be used in ecological studies
or assessments, especially where field conditions may prevent access to drying
ovens or other specialized equipment.
Acknowledgments
We thank Joshua Dyke and Jeremy Gerald for their assistance in the field and
Christena Gazave for laboratory work. The project was funded by Louisiana State
University BioGrads and in part by the National Science Foundation. This publication
has been approved by the Director of the Louisiana Agricultural Experiment
Station as manuscript number 07-26-0422.
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