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Effects of simulated climate change on the abundance of an exotic weevil, Cyrtepistomus castaneus
Bryan Marbert (ASU ) and Paul Hanson (ORNL)
Contact Information: Price ([email protected]), Williams ([email protected])
Emily Price and Ray S Williams, Department of Biology, Appalachian State University, Boone, NC
Fig. 1 TDE forest floor, pitfall trap and soli/litter sample
Lab Logo
Figure 2. Abundance (mean + SE) of Cyrtepistomus castaneus by treatment and slope position.
Figure 4. Regression of Oi and Oe horizon parameters by weevil abundance
Figure 5. Total abundance of oak and red maple in treatments byslope position.
INTRODUCTION Invasive species can pose a threat to the organisms that are
native to an area by outcompeting or displacing them. Originally from Asia, Cyrtepistomus castaneus is a tree-feeding beetle whose range now includes the eastern United States and Canada. This generalist feeds on several tree hosts but prefers oak and maple. It is considered a pest species due to the damage to leaves and roots. Climate change in the form of precipitation alteration could change the abundance of invasive herbivores such as C. castaneus via effects on host plants or physical characteristics of the forest floor important for its reproduction. The Throughfall Displacement Experiment (TDE) at Oak Ridge National Laboratory allowed us to ask if long-term alteration of precipitation in forests could affect an exotic weevil. In a related study, it was found that removing water from one area and transferring it to another shifted the Carabid beetle community and that these shifts were due in part to alterations in forest floor parameters. This complimentary study provides insight into possible changing dynamics of an exotic insect as climate change occurs in large-scale forest ecosystems in the future.
The TDE (1993-2006) examined long-term precipitation alterations in an intact hardwood forest. It consisted of three treatments in 80m X 80m plots; (1) dry, where approx. 33% of natural precipitation was collected in canopy troughs and transported via pvc pipes to a wet treatment (2), where water was dripped onto the forest floor, and (3) unaltered manipulation of precipitation (ambient). The site consisted of upper and lower slope positions, allowing for the examination of landscape-level effects. Three times in 2005, ground-dwelling arthropods were collected using pitfall traps in 10 8m X 8m plots per treatment (total of 150 traps). Data on physical parameters of the forest floor were collected in plots, including litter mass and chemistry measured as carbon:nitrogen ratio and carbon-based phenolics. The primary focus of an overall larger experiment was to determine the effects of precipitation alteration on beetles in the family Carabidae. That study found significant effects of treatment on beetle abundance and tribe-level evenness (data not shown). Each measure was lower in dry plots, due in part to the positive correlations between abundance and soil water, which was higher in wet plots. Higher litter mass in the drought treatment contributed to lower ground beetle abundance, showing that physical changes to the forest floor due to altered precipitation affected this important insect taxa. Complimenting this investigation, weevils in the species C. castaneus were quantified from traps. Abundance was compared using ANOVA, with treatment and slope position as main effects and forest floor physical characteristics correlated using linear regression. Prevalence of tree species, including oaks and red maple, was calculated from mapped tree species positions.
METHODS
RESULTSIn this investigation, the mean abundance of the exotic weevil C. castaneus was affected by both treatment (P < 0.003) and slope position (P < 0.001; Fig. 2). At the level of plot, weevils were most abundant in the wet treatment and in all treatments higher in abundance at the upper slope position. Absolute abundance of the weevil showed similar results, where it is evident that weevils were found in much lower numbers at the lower slope (Fig 3).
Physical characteristics of the forest floor found related to Carabid beetles (litter mass and soil moisture) were unrelated to the abundance of C. castaneus in plots (data not shown). Instead, chemical constituents of the Oi and Oe layers predicted weevil abundance. Both tannic acid equivalents and the ratio of carbon:nitrogen (C:N) were positively related to the abundance of this exotic species (Fig. 4). In the Oi horizon the ratio of C:N was highest in dry plots at the upper slope (44.5 + 3., mean + se) compared to the wet (37.6 + 2.4) and ambient (37.4 + 2.2) treatments. Overall C:N of the Oi horizon was lower at the lower slope. For phenolics expressed as tannic acid equivalents, this measure was both higher at the upper slope position (P < 0.01) and in dry (13.4 + 1.3) compared to wet (10.9 + 0.8) plots.
OBJECTIVES There were two primary objectives in this study:
•Quantify the abundance of C. castaneus collected in pitfall traps in wet, ambient and dry treatments at two slope positions.
•Relate abundance to chemical and physical parameters and prevalence of preferred tree species in plots.
Figure 3. Total abundance of Cyrtepistomus castaneus by treatment, slope position and slopes combined.
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Potential oak host tree species for C. castaneus were more prevalent in the wet plots and the higher slope position (Fig. 5). Red maple, a possible host for the weevil, was overall more common in wet than either ambient or dry plots and with the exception of the dry treatment, more abundant upslope (Fig. 5).
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A significant correlation between oak species and weevil abundance was identified, with the average number of beetles per plot increasing with average oak abundance (Fig. 6). No relationship between red maple abundance and weevils was found (Fig. 6), suggesting that in terms of potential host plants and their contribution to observed beetle responses to treatments, oaks were more important.
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CONCLUSIONSOur data demonstrates that although the abundance of this exotic weevil was different between treatments, a clear explanation for the mechanism is elusive. A lack of clear relationships between weevil abundance and abiotic factors of the habitat known to affect other beetle taxa, along with the prevalence of host plants being highest in plots with higher weevil abundance, suggest that observed differences were due more to host plant availability than alteration in the habitat as precipitation is manipulated. While plausible, this explanation does not explain the large disparity in weevil abundance in upper compared to lower slopes, where host plants were also found in large number (Fig. 5). The positive relationship between C. castaneus and chemical parameters of the organic horizons Oi and Oe are intriguing because both of these measures were higher in the dry treatment upslope. Clearly they did not directly affect the increased abundance of the weevil (which reproduces in the litter) in wet plots. Possibly in concert with possible effects on host plant quality, which were not addressed in this study, factors such as litter C:N and phenolics could explain our observations. A study targeting the weevil specifically would be needed to fully examine the possible role of habitat alteration at ground level due to precipitation alteration on above-ground processes.
Figure 6. Regression of mean oak and maple abundance with weevils.
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ACKOWLEDGMENTS
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r2 = 0.21
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r2= 0.23
