Scirtid Beetles (Helodes pulchella), Leaf Litter, and Treeholes: Is There Evidence of Facilitation in the Field?
John Q. Burkhart, Leslie Smith, Shawn Villalpando, and Christopher J. Paradise
Southeastern Naturalist, Volume 6, Number 4 (2007): 597–614
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2007 SOUTHEASTERN NATURALIST 6(4):597–614
Scirtid Beetles (Helodes pulchella), Leaf Litter, and Treeholes:
Is There Evidence of Facilitation in the Field?
John Q. Burkhart1,2, Leslie Smith1,3, Shawn Villalpando1,4,
and Christopher J. Paradise1,*
Abstract - The primary resource of temperate forest treeholes is leaf litter, and different
insects specialize on particular stages of decay. Helodes pulchella (scirtid beetle)
takes part in a processing chain by shredding leaf litter and creating material for other
consumers. We hypothesized that variation in scirtid density and resources influences
the insect community. To test this, we manipulated scirtid beetle and resource densities
in field mesocosms. We used a two-factor design (3 litter levels by 3 scirtid densities),
and monitored insect communities from April 2004 to June 2005. We detected
no statistically significant effects of scirtids on leaf decay. However, during the first
season, species richness was higher in mid-summer in the presence of low scirtid density
compared to treatments with high scirtid densities or those with no scirtids. The
dominant species was Aedes triseriatus (eastern treehole mosquito) and its abundance
was unaffected by either scirtid or leaf litter. However, mean pupal mass of mosquitoes
was greater at low scirtid densities. Facilitation is suggested by the combination
of high mosquito densities and large pupal mass. Densities of Culicoides guttipennis
(ceratopogonid midge) were higher at intermediate and high resource levels without
scirtids, compared to treatments with any scirtids, suggesting a negative interaction
between midges and scirtids. We demonstrated strong, and not always positive, effects
of scirtid beetles on insect communities in water-filled treeholes.
The state and availability of resources may have wide-ranging effects at
both the population and community levels (Hunter and Price 1992, Naeem
1988, Srivastava and Lawton 1998, Walker et al. 1997), and may be dependent
upon processing chain interactions (Daugherty and Juliano 2002, Paradise
1999, Paradise and Dunson 1997). Processing chains, and other facilitative
interactions, have been hypothesized to affect community diversity and individual
populations (Bruno et al. 2003, Hacker and Gaines 1997, Jones et
al. 1997). During feeding, processors condition resources for consumers that
specialize on more advanced decay states (Heard 1994a). For instance, leaf
shredders, by processing leaf litter, leave behind smaller particulate organic
matter (Daugherty and Juliano 2002). A commensalism results as more resources,
distributed in a wider range of particle sizes, are made available to
1Department of Biology, Davidson College, Davidson, NC 28035-7118. 2Current
address - Department of Biology, University of South Alabama Mobile, AL, 36688-
0002. 3Current address - Graduate School of Oceanography, University of Rhode
Island, MERL Room 4, Box 9, 11 Aquarium Road, Narragansett, RI 02882. 4Current
address - ASU Box 12581, Appalachian State University, Boone, NC 28607. *Corresponding
author - firstname.lastname@example.org.
598 Southeastern Naturalist Vol. 6, No. 4
consumers through the actions of the processor (Heard 1994b, Paradise 1999,
Paradise and Dunson 1997). Processing chain amensalisms also may occur if
resources are abundant, if consumers are dependent upon physical or microorganism
processing, or if processors are highly efficient feeders (Heard 1994a).
Under these conditions, consumers are less dependent upon processors, and
foraging by processors has a net effect of reducing resources for consumers
(Heard 1994a). Processing-chain interactions have been reported in streams,
pitcher plants, Heliconia bracts, carcasses, treeholes, and bromeliads (Bradshaw
1983, Dieterich et al. 1997, Heard 1994b, Jonsson and Malmqvist
2005, Naeem 1990, Paradise 1999, Paradise and Dunson 1997, Schoenly
and Reid 1987, Seifert and Seifert 1979, Srivastava 2006). While several
of these experiments have shown that processors can increase the growth,
mass, or survival of other consumers, studies of the effects of a processor on
diversity and composition may lead to increased understanding of ecological
communities (Bertness and Calloway 1994).
Helodes pulchella Guerin (scirtid beetle), a common resident of treehole
communities in the southeastern US, is a leaf shredder and a potentially
important processor (Barrera 1996, Paradise 2004). Particulate or dissolved
organic matter increases as scirtids shred leaf litter because not all particles
are consumed by scirtids; unconsumed particles become available for filterfeeders
and browsers (Daugherty and Juliano 2002, Paradise and Dunson
1997). As scirtids process litter, they also may increase the surface area
available for microorganism growth. This can be important because microbes
significantly add to the diet of detritivores in treeholes (Walker et al.
1991). In addition, fecal production by scirtids could increase resources for
other detritivores (Daugherty and Juliano 2003). Finally, dead scirtids themselves,
as they decay, could provide resources to microorganisms and other
insects (Daugherty et al. 2000). Southeastern US treeholes are dominated
numerically by larvae of Aedes triseriatus Say (eastern treehole mosquito;
Harlan and Paradise 2006, Lounibos 1983), a filter-feeder and browser that
consumes small particulate matter and microbes (Merritt et al. 1992). Other
common insects include Culicoides guttipennis Coquillet (ceratopogonid
midge), Mallota posticata Fabricius (syrphid), and Telmatoscopus albipunctatus
Williston (psychodid). These species are detritivores and consume
detritus in various stages of decay (Barrera 1988). In the laboratory, scirtid
beetles facilitate both eastern treehole mosquitos and ceratopogonid midges,
and positive impacts are greatest when resources are limiting (Paradise 1999,
2000; Paradise and Dunson 1997), as predicted by theory (Heard 1994a).
The purpose of this study was to test the hypothesis that scirtid beetles
increase treehole insect abundance and community diversity. These increases
are resource-dependent, as differences in leaf litter abundance have large
effects on diversity and individual species abundances (Paradise 2004, Schoener
1986) and because processing-chain interactions can change as resources
increase (Paradise 1999). We predicted that a processing-chain commensalism
between scirtids and other treehole detritivores would lead to increases
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 599
in insect diversity and abundance. Further, we predicted increases when resources
are limiting and scirtids are present, compared to when resources are
limiting and scirtids are either absent or very abundant (Heard 1994b). When
present at high densities, scirtids decrease growth rates and survival of detritivores
(Paradise 1999), and the net effect of high densities of scirtids when
resources are limiting is to reduce resource availability to detritivores. At
the community level, we thus predict lower diversity and overall abundance
(but higher relative abundance) of common species in treatments with high
densities of scirtids in comparison to treatments with lower densities, within
the same resource level. To test these predictions about processing-chain
interactions, we conducted a field mesocosm experiment in which leaf litter
and scirtid beetle densities were varied independently of one another. To test
the effects of these factors on colonization and community development, we
monitored populations within mesocosms over time.
We created mesocosms using 7.62-cm internal diameter PVC pipe cut
into 11-cm lengths. We use the term mesocosm to reflect the fact that these
containers are similar in size to natural treeholes. These mesocosms are
known to support the full range of treehole insects in our study area (Harlan
and Paradise 2006). We affixed fiberglass window screening to the inside of
the pipe by overlapping it beyond the two ends. The screen created a textured
inner surface and darkened the interior, promoting insect oviposition and allowing
scirtids to crawl to the top for air. We used silicon caulk to seal on an
end cap, which held the overlapping screen in place. At the top, we used a
PVC coupling that was sawed in half to seal the overlapping screen in place.
We used caulk to seal this coupling and the screen to the top of the mesocosm.
The total capacity of each mesocosm was approximately 540 ml. We
attached mesocosms in pairs to a frame with expandable polyurethane foam
(Great Stuff, Dow Chemical Co., Midland, MI). Frames were approximately
50 cm wide by 35 cm high by 25 cm deep and were constructed of 1.5-cm
PVC pipe. We flushed mesocosms several times in the laboratory over three
weeks to remove any volatile chemicals.
The frames were tied to trees at a height of less than 1 m throughout a 30-acre
hardwood forest on the Davidson College Ecological Preserve (DCEP,
Davidson, NC; 30°30'30"N, 80°49'45"W) in September 2003. We glued fi-
berglass window screen (2-mm mesh) to the top of each frame, about 25 cm
above the mesocosms, to reduce the amount and size of natural debris that
entered mesocosms. Finally, we wrapped each frame and the trunk of its tree
in 2.5-cm mesh chicken wire to exclude most vertebrates, and cut a hinged
door into the cage to allow researchers access (Paradise 2006).
We randomly assigned one of ten treatment combinations to each mesocosm.
We used three levels of leaf litter crossed with three levels of
scirtid density, all of which were within the ranges found in treeholes (Paradise
2004; Paradise, unpubl. data), with four replicates of each treatment
600 Southeastern Naturalist Vol. 6, No. 4
combination. Leaf-litter levels were 1, 5, and 10 grams of dried Quercus
rubra L. (red oak) leaves/L (hereafter low, intermediate, and high, respectively).
We collected medium-sized scirtid larvae (mostly 2nd instar) from a
large basal treehole present on the DCEP and added them at three densities:
0, 26, and 100 individuals/L (hereafter none, low, and high, respectively).
The tenth treatment consisted of four mesocosms supplied only with water,
giving a total of 40 mesocosms. The latter treatment was used to estimate
debris accumulation, which was limited to dust and small particles that could
fit through the 2-mm mesh covering the cages. The mass of particulate matter
in these mesocosms could be used to subtract from total litter mass of other
mesocosms to determine litter decay.
We collected leaves in September 2003 and added them to mesocosms in
early October 2003, along with a small aliquot of filtered treehole water and
≈500 ml of distilled water (which filled the mesocosms). We then covered
mesocosms with plastic sheeting held in place with cable ties, to prevent
oviposition. In late October 2003, scirtids were added at the appropriate
densities. Mesocosms were covered with no-see-um netting (0.5-mm mesh),
which allowed for gas exchange between the water column and the atmosphere
but prevented colonization by insects. We timed the addition of leaf
litter and scirtids to simulate the exposure of scirtids to leaf litter shortly
after natural leaf fall. The mesocosms remained in this state until they were
opened in early March 2004.
In early April 2004, we began monitoring for colonization of insect
larvae. Mesocosms were sampled every two to three weeks to determine
when colonization began, to monitor scirtid survival and maintain constant
water levels, and to eliminate any larvae of Toxorhynchites rutilus Coquillett
(predatory mosquito), as we were interested only in bottom-up effects.
Monitoring consisted of extracting 50 ml of water from a mesocosm using a
baster from just below the water surface. A second 50-ml sample was taken
subsequently from the bottom. In addition, observation of mesocosms allowed
us to detect any predatory mosquito larvae swimming to the surface.
We performed three complete censuses each in 2004 and 2005, which involved
removing all water and leaf litter from each mesocosm using basters and
forceps. All leaves were individually examined for insect larvae. The water was
spread out into pans, and carefully examined for insect larvae. All were identifi
ed to the lowest taxonomic level possible and counted. Some unknown larvae
were brought to the laboratory for examination or to rear to adulthood for identifi
cation. All other material, except for predatory mosquito larvae, was then
placed back into its original mesocosm. After the second census in July 2004,
we restocked scirtids, as needed, to their nominal levels.
We collected mosquito pupae from mid-June to mid-August 2004 every
three to seven days. This period coincided with emergence of the first
generation of mosquitoes to inhabit mesocosms. Pupae were collected by
catching them with a plastic pipet and returning them to the laboratory. We
froze, weighed, sexed, and identified emergent adults to species. We took
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 601
only up to three pupae from any single mesocosm during a collection bout,
as collecting all adults might negatively affect future colonization. Thus, individual
biomass of emergent mosquitoes was used as an indicator of habitat
productivity. Most (>90%) of the mosquito pupae collected were eastern
treehole mosquitos, so we used only those individuals for statistical analysis.
The remaining larvae were of Aedes albopictus (Skuse) (Asian tiger mosquito),
and those individuals were not analyzed. Any predatory mosquito
larvae observed were removed during these collections.
We terminated the experiment after we completed the final census in
early June 2005. We brought all leaf litter and coarse particulate matter back
to the laboratory. As we were not able to determine decay of leaves during
the experiment, this was our only method to assess leaf-litter condition, even
if mesocosms also contained debris that had fallen in during the experiment.
However, we determined that the material that collected in the four mesocosms
without leaf litter was all <1 mm in size. We rinsed leaf litter from
other mesocosms through a # 35 sieve, which allowed us to estimate the loss
of mass from the initial stock.
We determined the species richness (minus scirtids), Shannon-Weiner diversity
(minus scirtids), percent dominance of the most common species, and
densities of scirtids and the two most common insect larvae (eastern treehole
mosquitoes and ceratopogonid midges). Because species richness estimates are
biased by rare species, high values indicate presence of numerous rare species.
To provide another indication of effects on species composition, we also calculated
and performed statistical analysis on diversity (as -H = Σ (pi * ln (pi)),
where pi = the proportion of species i individuals in the community) and percent
dominance. Each of these might be influenced by presence of processors.
We analyzed the proportion of leaf-litter mass lost using a two-factor analysis
of variance (ANOVA) in MINITAB (Version 13.31 for Windows), and
graphically examined the change in leaf-litter dry mass. For mass of eastern
treehole mosquito adults, we used a nested repeated measures ANOVA, using
individual mosquitoes as a random-effects factor nested within mesocosms,
and leaf-litter and scirtid densities as fully crossed fixed-effects factors.
For species richness, diversity, and densities of scirtids, mosquitoes, and
ceratopogonid midges, we used profile analysis to test for effects of scirtid
density and leaf litter level over time. Scirtids mostly were absent from
mesocosms in 2005, so we analyzed response variables from 2004 only. In
profile analysis, differences and averages at consecutive time points become
transformed variables in two-way MANOVAs with leaf litter and scirtid
densities as fixed-effects factors (von Ende 2001). Profile analysis allowed
us to compare responses of any one variable over time without the variance
problems associated with repeated measures ANOVAs that use time as a factor
(von Ende 2001). Tests for interactions between time and other factors
were done by comparing differences at sequential sampling points, while
tests for main effects were performed by using the averages of successive
time points. We adjusted α to 0.01 for the profile analyses (experiment-wise
602 Southeastern Naturalist Vol. 6, No. 4
α of 0.05 ÷ 5 profile analyses). All data were tested for univariate normality
and homoscedasticity, and transformations were used where appropriate.
Leaf litter and scirtids
We found a significant leaf-litter effect on change in dry mass. Intermediate
and high leaf-litter treatments lost about half of their dry mass, but low leaflitter
treatments lost 75–85% of their dry mass (Fig. 1; F = 31.09; df = 2, 26; P <
0.001). This is a valid estimate of mass lost, as there was no coarse particulate
matter recovered from mesocosms with no leaf litter. We found no effect of
scirtid density or the interaction between scirtid and leaf-litter densities on
proportion of leaf mass lost (scirtids: F = 0.14; df = 2, 26; P = 0.87; interaction:
F = 0.27; df = 4, 26; P = 0.89). These results indicate that changes in leaf-litter
dry mass were caused primarily by scirtid-independent processing.
We found a significant leaf-litter effect on scirtid densities. Decline in
scirtid densities was highest at low resources and high scirtid densities (100
individuals/L). Only in high leaf-litter mesocosms did scirtid densities remain
near nominal by the time of the first census, while densities were lower
than nominal in low and intermediate leaf-litter treatments (Table 1, Fig. 2).
At the time of our first census (June 2004), we found few scirtids remaining
in low and intermediate leaf-litter mesocosms, and thereafter scirtid densities
did not vary greatly in those treatments, and remained at a consistently
Figure 1. Proportional loss of dry mass from initial dry mass measured in October
2003 to final dry mass measured in June 2005. N = 4 for each mean in each graph.
Sc/L = scirtid beetles per liter.
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 603
low level. Prior to that, however, sampling confirmed that numerous scirtids
had survived the winter in the mesocosms.
We cumulatively found larvae or pupae belonging to 10 dipteran
species. Major families observed were Culicidae (mosquitoes), Ceratopogonidae
(biting midges), Psychodidae (moth flies), and Syrphidae
(hover flies). Mosquito species included the eastern treehole mosquito, the
Asian tiger mosquito, Orthopodomyia signifera (Coquillett) (white-lined
moquito), and the predatory mosquito. For species richness tallies, we
Table 1. Statistical results of profile analysis MANOVAs on 2004 censuses. All densities were
log-transformed. For scirtids, only low and high densities of scirtids were used as levels in the
MANOVA because we attempted to track changes in densities of the original cohort added. Pvalues
< 0.01 were deemed significant (marked with *). LL = leaf-litter density, scirtid = scirtid
density, and λ = Wilk’s λ.
Response variable Source of variation λ df P
Log H. pulchella density
LL 0.43 4, 34 0.005*
Scirtid 0.94 2, 17 0.616
LL x scirtid 0.84 4, 34 0.551
LL x date 0.94 4, 34 0.907
Scirtid x date 0.98 2, 17 0.863
LL x scirtid x date 0.89 4, 34 0.738
Log mosquito density
LL 0.97 4, 52 0.949
Scirtid 0.94 4, 52 0.816
LL x scirtid 0.71 8, 52 0.304
LL x date 0.89 4, 52 0.549
Scirtid x date 0.98 4, 52 0.973
LL x scirtid x date 0.82 8, 52 0.724
Log ceratopogonid midge
density LL 0.69 4, 52 0.042
Scirtid 0.77 4, 52 0.142
LL x scirtid 0.84 8, 52 0.801
LL x date 0.87 4, 52 0.445
Scirtid x date 0.62 4, 52 0.010*
LL x scirtid x date 0.84 8, 52 0.787
Insect species richness
LL 0.75 4, 52 0.105
Scirtid 0.82 4, 52 0.261
LL x scirtid 0.66 8, 52 0.183
LL x date 0.92 4, 52 0.721
Scirtid x date 0.63 4, 52 0.010*
LL x scirtid x date 0.62 8, 52 0.119
LL 0.74 4, 52 0.088
Scirtid 0.75 4, 52 0.107
LL x scirtid 0.89 8, 52 0.930
LL x date 0.88 4, 52 0.481
Scirtid x date 0.71 4, 52 0.067
LL x scirtid x date 0.94 8, 52 0.990
604 Southeastern Naturalist Vol. 6, No. 4
counted all mosquitoes as a single species unless we positively identified
more than one species (early instar eastern treehole mosquito and Asian
tiger mosquito are difficult to distinguish).
The insect community was dominated numerically by mosquitoes, with
ceratopogonid midges as the second most commonly found taxonomic
group. Mosquito larvae (not counting predatory mosquito) were found in
100% of all counts. When present, mosquitoes made up 69.4% of all larvae.
In 85% of counts, across all combinations of leaf litter and scirtid density,
mosquitoes were the dominant taxon. Mosquito populations peaked in June
and steadily declined through the summer and fall of 2004 (Figs. 3a, b, c).
However, we found no effect of leaf-litter or scirtid densities on the densities
of mosquitoes (Table 1).
Figure 2. Scirtid
time in each of
a. Scirtid densities
densities in intermediate
densities in high
N = 4
for each mean in
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 605
Although both eastern treehole mosquito and Asian tiger mosquito pupae
were collected, Asian tiger mosquito sample sizes were insufficient for parametric
analysis (<9% of 570 pupae collected). Eastern treehole mosquito
adult mass demonstrated a sex-specific response to scirtid treatment. We
Figure 3. Mosquito (eastern treehole mosquito and Asian tiger mosquito) and
ceratopogonid midge densities over time in each of nine treatment combinations.
Because eastern treehole mosquitos and Asian tiger mosquitos are difficult to identify
to species in early instars, they were pooled together. a. Mosquito densities in low
leaf-litter mesocosms. b. Mosquito densities in intermediate leaf-litter mesocosms.
c. Mosquito densities in high leaf-litter mesocosms. d. Midge densities in low leaflitter
mesocosms. e. Midge densities in intermediate leaf-litter mesocosms. f. Midge
densities in high leaf-litter mesocosms. N = 4 for each mean in each graph.
606 Southeastern Naturalist Vol. 6, No. 4
found a significant leaf-litter by scirtid interaction for mass of adult males
collected from mid-June to mid-August 2004, but no significant effects on
female mass for the same period (Table 2). Low scirtid densities at low and
high leaf-litter produced significantly larger males than high scirtid densities
at low and high leaf-litter (Fig. 4). This does not appear to be a mosquito
density-dependent effect; low scirtid/high leaf-litter mesocosms in June had
statistically equivalent mosquito densities as high scirtid/high leaf-litter
mesocosms, and yet produced larger male mosquitoes.
Ceratopogonid midge larvae were found in 86.7% of samples, and when
present, this midge made up 28.7% of all larvae and was the dominant species
in 15% of all counts. Midge densities were significantly affected by
the scirtid density by date interaction (Table 1). There was a trend towards
higher midge densities with increasing leaf-litter levels (Figs. 3d, e, f). At
both intermediate and high leaf-litter levels, midge densities were highest
when no scirtids were present, and at high leaf-litter, there was an inverse
relationship between scirtid and midge densities (Fig. 3f). Midge densities
in different scirtid density treatments converged over time, leading to the
significant interaction with date.
Psychodids were found in 22.2% of counts, and appears to have two generations
per year, as they were present in June and September, and almost
completely absent from mesocosms in July. Densities of this larva were less
than 1.0 individual/L, except in mesocosms with intermediate leaf-litter
levels and high scirtid densities in September (2.5 ± 1.6 (SE) individuals/L).
Very few psychodids were found in low leaf-litter mesocosms, regardless of
Larvae syrphids were found only in June 2004 in low densities, in only 2.9%
of all samples. They were never found in intermediate leaf-litter mesocosms,
and only rarely in low leaf-litter mesocosms, with the highest densities in high
leaf-litter mesocosms (0.03 ± 0.01 [SE] inidividuals/L in low leaf-litter and
0.26 ± 0.18 [SE] individuals/L in high leaf-litter mesocosms). There was no
systematic trend in syrphid distribution across scirtid densities. Other larvae
belonging to the families Dolichopodidae (Systenus sp.), Ephydridae, and Syrphidae
(Myiolepta sp.) also were observed, all in less than 3% of all samples.
Table 2. Results of repeated-measures ANOVA on mean pupal mass. Pupal data transformed
and pooled by month. LL = leaf litter level, and scirtid = scirtid density. Statistically significant
effects are marked with *.
Response variable Effect F df P
Log male mass LL 0.12 2, 50 0.886
Scirtid 1.55 2, 50 0.218
LL * scirtid 3.27 4, 50 0.015*
Individual (LL * scirtid) 0.58 50, 95 0.983
Log female mass LL 0.76 2, 74 0.472
Scirtid 1.11 2, 74 0.333
LL * scirtid 1.51 4, 74 0.205
Individual (LL * scirtid) 2.12 74, 107 <0.001*
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 607
Figure 4. Mean adult eastern treehole mosquito mass in each of nine treatments. a.
Adult male mass. Letters above bars denote statistical equivalence as determined by
post-hoc pair-wise comparisons. b. Adult female mass. There were no significant effects
of scirtid density (Sc/L) or leaf-litter level on female mass.
608 Southeastern Naturalist Vol. 6, No. 4
Mesocosms typically contained between one and four insect species at
any time. No mesocosm contained more than 50% of the regional species
pool, except one mesocosm with high leaf-litter and high scirtid density
in June 2004 that contained 5 species. The median local species richness
was 2, and mesocosms contained 2 or 3 species 84.2% of the time. Scirtid
density and date interacted significantly to affect insect species richness
Figure 5. Insect species richness and diversity over time in each of nine treatments. a.
Species richness in low leaf-litter mesocosms. b. Species richness in intermediate leaflitter
mesocosms. c. Species richness in high leaf-litter mesocosms. d. Diversity in low
leaf-litter mesocosms. e. Diversity in intermediate leaf-litter mesocosms. f. Diversity in
high leaf-litter mesocosms. N = 4 for each mean in each graph.
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 609
(Table 1; Figs. 5a, b, c). In July, there was approximately 1 more species per
mesocosm on average in intermediate and high leaf-litter mesocosms with
low scirtid densities, than in mesocosms with the same amount of leaf litter
and either no scirtids or high scirtid densities (Figs. 5b, c). In previous and
subsequent censuses, this trend was either absent or reversed. There were no
leaf-litter effects on species richness and no significant effects of any kind
on diversity in 2004 (Table 1). Diversity was low, being on average less than
0.8, presumably due to the dominance by mosquitoes and midges. Diversity
was slightly lower in the presence of scirtids in September than in their absence,
but was not statistically significant (Figs. 5d, e, f).
Ecological theory suggests that processing chains may play a large
role in structuring communities (Bruno et al. 2003, Heard 1994a). We thus
predicted positive, facilitative impacts of scirtid beetles on treehole detritivore
populations and diversity under conditions of low resources and low
processor density; facilitation is most likely to occur when resources are
limiting and efficiency of processing is low due to low density of shredders
(Hacker and Gaines 1997, Heard 1994a, Paradise 1999, Paradise and
Dunson 1997). Scirtids may be necessary to change leaf litter to a state
available for other detritivores to consume (Daugherty and Juliano 2002,
Heard 1994b). Positive effects of scirtid beetles on growth of eastern
treehole mosquitos and ceratopogonid midges under conditions of low
leaf-litter resources have been observed (Paradise 1999, Paradise and
Dunson 1997). Scirtids did not have a significant effect on litter mass in
this study, even though experimental evidence suggests that scirtids can
increase availability of fine particulate organic matter (Daugherty and Juliano
2002, 2003). However, it is likely that their activity facilitated growth
of microbes, on which other detritivores feed (Kaufman et al. 2000). This
may have been one of the mechanisms that produced the observed effects.
Studies of the effects of scirtid foraging on microorganism populations are
needed to confirm this. Differences in scirtid activity during the winter and
spring may have been enough to alter the response of communities with
different scirtid densities. Despite the decline in scirtid densities over time,
we demonstrated scirtid effects on ceratopogonid midge densities, the mass
of adult eastern treehole mosquitos, and species richness. In the case of
both species richness and adult eastern treehole mosquito size, our predictions
were consistent with our findings; with ceratopogonid midge, our
predictions did not match our observations.
Densities of ceratopogonid midges were highest in mesocosms in which
scirtids were absent, the effect being more pronounced in the high leaf-litter
condition. The effect occurred during peak midge densities in May and June
(Harlan and Paradise 2006, Paradise 2004), while scirtids had not yet declined
in density, suggesting a strong negative interaction between scirtids
and midges. Midges either did not survive in the presence of high densities
610 Southeastern Naturalist Vol. 6, No. 4
of scirtids, or females avoided ovipositing in those habitats, both of which
would explain their absence in high scirtid-density mesocosms. If females
did not avoid ovipositing in habitats with scirtids, then midge larvae either
competed with scirtids for resources or were consumed by scirtids. Midges
tend to crawl in and around sediment and detritus (Barrera 1988, Paradise
2004), creating a situation in which these small, early instar larvae could be
accidentally consumed by foraging, shredding scirtids. However, Paradise
and Dunson (1997) found no clear effect of scirtids on survival of midges
in laboratory mesocosms, suggesting oviposition avoidance in the field,
not low survival of hatching larvae. Midge oviposition appears also to be
quite sensitive to other conditions, including presence of the top predator
(L. Smith and C.J. Paradise, unpubl. data).
Eastern treehole mosquito pupae were larger in the presence of low densities
of scirtids, although this was not consistent across all leaf-litter levels.
Mosquito mass is important for the survival of the mosquitoes in which both
females and males experience greater fitness benefits from achieving a larger
size either due to their ability to produce more eggs, or by increasing their
longevity (Benjamin and Bradshaw 1994, Livdahl 1984, Lounibos et al.
1993). Adult mass attained by mosquitoes is density-and resource-dependent
(Walker et al. 1997), yet we found some of the largest mosquitoes in treatments
with low levels of leaf litter and high densities of mosquitoes—exactly
where we would expect to see reductions in mosquito mass in the absence
of facilitation. Large mosquitoes emerging from containers with low levels of
leaf litter, high densities of mosquitoes, and low densities of scirtid processors
provides strong evidence of facilitation. Treatment conditions in which we expected
facilitation to occur, either from theory or past laboratory experiments
(Daugherty and Juliano 2002; Paradise 1999, 2000; Paradise and Dunson
1997), but where we did not observe facilitation here, can likely be attributed
to other biotic interactions also affecting growth and density of mosquitoes
in this complex community. These exceptions aside, we still demonstrate a
facilitative effect on mosquito growth under some conditions.
Communities were dominated by eastern treehole mosquito and ceratopogonid
midges, and dominance was not affected by either leaf litter
or scirtid density. Diversity was predicted to increase with either resources
or facilitative effects of processors, reflecting, in part, a predicted decrease
in dominance and an increase in diversity of available resources (Armbruster
et al. 2002, Hacker and Gaines 1997, Jones et al. 1997). Here, diversity was
not affected by either scirtid densities or leaf litter, due to the continued
high dominance of mosquitoes and midges, and also by the lack of effect of
scirtids on leaf litter. However, species richness was affected by the scirtids,
at least during some times.
We found between 1 and 5 species in any one community, which is similar
to other studies in both natural treeholes and mesocosms (Harlan and Paradise
2006, Paradise 2004, Yanoviak 1999). Small differences in local species richness,
in communities in which maximum local richness is ≤5, represent large
2007 J.Q. Burkhart, L. Smith, S. Villalpando, and C.J. Paradise 611
proportional changes in species richness (Yanoviak 1999). In July, richness
in treatments with intermediate or high leaf-litter and low scirtid densities
remained high, while richness in other treatments declined. By September
2004, richness had dropped in the low scirtid-density treatment and had risen
proportionately in the no and high scirtid-density treatments. This result was
probably caused by rare species (most likely M. posticata, or T. albipunctatus)
colonizing or persisting in low scirtid-density mesocosms through July and
emerging before September. Individuals of these same or different species
then colonized no and high scirtid-density mesocosms later and were counted
in September. The increased presence, but low densities of rare insects
explains the patterns in species richness, dominance, and diversity. In communities
dominated by one or two species, an increase in the number of rare
species might have no impact on diversity if dominance is maintained by one
or two common species.
The maintenance of rare species in treehole metacommunities probably
comes about from adaptations to extreme habitats, habitat generality, variation
in life cycles, and high vagility (Holyoak et al. 2005). For instance, the
psychodid T. albipunctatus, one of the more frequently observed rare species,
can be found in a wide range of habitats (Hribar et al. 2004), and is not
an obligate treehole breeder, allowing a metapopulation to be maintained
even if it is outcompeted within treeholes by dominant species. Other rare
species also may be facultative treehole breeders or may be found only at
certain times of the year, such as the syrphid M. posticata. To determine the
factors that affect species richness in treeholes, it is critical to determine
when a given species is more likely to be found.
In addition to effects of scirtid densities, we predicted positive effects
of leaf litter on treehole insect communities based on theory and previous
research (Fish and Carpenter 1982, Léonard and Juliano 1995, Naeem 1990,
Paradise 2004, Sota 1996). Specifically, we expected to observe higher
densities and diversity of insect larvae, because greater availability of litter
leads to greater habitat heterogeneity and space. Habitat heterogeneity
significantly impacts species richness in phytotelmata communities and
accounts for a high proportion of variation in species richness within these
systems (Armbruster et al. 2002, Paradise 2004, Yanoviak 1999). This effect
was not as strong within our mesocosms because natural phytotelmata have
a wider range of heterogeneity, including variation in factors such as water
volume and container size, factors that we held constant (Armbruster et al.
2002, Kitching 2000, Paradise 2004, Sota et al. 1994). The wider range of
resource conditions in natural habitats may then interact with the effects
of scirtids on species richness that we demonstrated.
Previous research (Paradise 1999, Paradise and Dunson 1997) showed
that scirtid beetles can have a significant positive impact on populations
of detritivores within the communities of which they are a part. We found
that positive effects of scirtid beetles in naturally colonized container communities
were more prevalent when scirtid densities were low, as compared
612 Southeastern Naturalist Vol. 6, No. 4
to when they were high or when scirtids were absent from the community.
Effects were neither always present nor facilitative, as predicted, and leaflitter
effects were absent or moderated by scirtid presence. In our experiment,
scirtid beetles positively affected species richness and negatively impacted
the ceratopogonid midge population, although the effects were temporally
variable. Scirtid beetles also positively affected the size of the dominant
species in the treehole, the eastern treehole mosquito. Thus, we conclude
that scirtids do affect treehole communities, possibly by creating conditions
favorable to increased richness of rare species and increased mass of the
dominant species. The effects we observed were conditional and mediated
by other factors, such as resource abundance, lifecycles of the other treehole
inhabitants, time of year, and dominance by single species.
We thank Duncan Berry, Charlie Chrisawn, Nicole Harlan, Lauren Harshaw,
Benjamin Kegan, Stella Kenyi, Benjamin Kittinger, Lindsay Nakaishi, and Kate Williams
for assistance in the field and with data management, and Davidson College
for permission to work on the Davidson College Ecological Preserve. This research
was supported by NSF grant DEB-0315208 to C.J. Paradise, NSF-REU grant DBI-
0139153 to the Davidson College Biology Department, and a Davidson College
Faculty Study and Research grant to C.J. Paradise. The experiments conducted comply
with the current laws of the United States of America.
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