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Grassland botanical structure influences lek spatial organization in Gryllotalpa major S. (Orthoptera: Gryllotalpidae).

INTRODUCTION

Grasslands once dominated much of the North American landscape from Central Canada to Northern Mexico, covering nearly fifty percent of the land surface of over 400 million ha. Due to the rich loamy soils that form the basis of these grasslands, extensive agricultural development has rendered the tallgrass prairie ecosystem one of the most fragmented and endangered in all of North America, and anthropogenic forces continue to threaten many endemic grassland species (Axelrod, 1985; Howe, 1994; Joern and Keeler, 1995; Collins, 2000; Collins et al., 2002; Copeland, 2002). Small remnants of tallgrass prairie are now located discontinuously within the historic range in regions of Iowa, Illinois, Missouri and Arkansas, whereas larger remnants are found in the flint hills of Kansas and Oklahoma (Joern and Keeler, 1995; Brye et al., 2002; Copeland, 2002). Understanding how endemic species respond to habitat fragmentation and shifts in changes in land use is of vital importance to attempts to maintain biodiversity and ecological balance of these systems, but in many cases we have yet to learn how endemic species respond to the environmental stochasticity inherent to a naturally functioning tallgrass prairie ecosystem.

Historically, tallgrass prairie ecosystems have had high occurrences of fire and grazing (Axelrod, 1985). Fire is known to inhibit the expansion of woody shrubs and trees into tallgrass prairie along the prairie-forest ecotone (Abrams et al., 1986; Abrams and Hulbert, 1987; Collins, 2000; Briggs et al., 2002) while reducing the accumulation of detritus, releasing nutrients back into the soil and influencing the structure of insect communities (Knapp and Seastedt, 1986; Johnson and Matchett, 2001; Brye et al., 2002). Grazing by bison and other native herbivores likewise affects nutrient redistribution and influences the structure of above ground biomass that is characterized by the seasonal dieback of vegetation that may reach 2 m in height by the end of the growing season (Finck et al., 1993; Collins et al., 1998; Dennis et al., 1998; Fuhlendorf and Engle, 2001; Matlack et al., 2001; Fay, 2003; Harrison et al., 2003; Joern, 2004; Trager et at, 2004; Joern, 2005). These interactive disturbance effects within the ecosystem combine to produce a highly variable detritus layer and grass height matrix across the landscape. Since acoustically advertising animals such as orthopterans are known to space themselves to optimize the transmission and reception of their signals with respect to the above ground botanical elements that characterize the advertising site (Romer, 1993), of special interest is whether or not species that have coevolved in such a stochastic environment as the tallgrass prairie are actually able to respond to changes in above ground habitat structure.

Fire and grazing have been reported to affect insect populations primarily through disturbance and changes in grassland floral assemblages (Bock and Bock, 1991; Siemann and Haarstad, 1997; Kerstyn and Stiling, 1999; Swengel, 2001; Bieringer, 2002;Jonas et al., 2002; Fay, 2003; Joern, 2004; Vermeire et al., 2004; Joern, 2005). However, in some insect groups the removal of the detritus layer and the resulting changes in soil moisture, soil temperature and availability of refugia may be more important than any change in plant diversity (Knapp and Seastedt, 1986). Whereas we expect insect groups occupying tallgrass prairie to be well adapted to the effects of fire on the landscape, some species have been shown to actually prefer grassland regions in which fire is a frequent disturbance factor. For example, advertising male prairie mole crickets form mating aggregations on sites that have been more recently burned (Howard and Hill, 2007), and some cicada species prefer recently burned prairie sites, as well (Callaham et al., 2002). It is not known, however, how removal of the detritus layer and/or alteration of the above ground botanical structure as a result of fire and grazing affect behavior in these or other fire-tolerant species.

The prairie mole cricket Gryllotalpa major Saussure (Orthoptera: Gryllotalpidae) is an endemic to the tallgrass prairie ecosystem of the south central United States (Fig. 1). Largest of the North American cricket species (Walker and Figg, 1990), its populations have declined with the loss of suitable grassland habitat (Figg and Calvert, 1987), but remnant populations are known to occupy prairie fragments in Oklahoma, Arkansas, Kansas and Missouri (Vaughn et al., 1993). Upon emerging as adults in the late spring months of April and May in Oklahoma, males construct an acoustic calling chamber with a surface opening from their underground burrow complex and return to the calling chamber at dusk each evening to call if weather conditions are suitable (Walker and Figg, 1990; Hill, 1998). Males produce a long sequence of chirps varying from 1.7 to 2.9 per sec at a carrier frequency of about 2.0 kHz with harmonic overtones (Walker and Figg, 1990; Hill, 2000), and some aspects of the calling song are known to correlate with characteristics of the male's morphology (Howard and Hill, 2006). Males aggregate their burrows in lek arenas (Hill, 1999), and the calling song produced in the acoustic chamber has both airborne and substrate-borne components (Hill and Shadley, 1997). Females fly through the lek and select mates based upon unknown criteria (Walker and Figg, 1990; Hill, 1999; Howard and Hill, 2007). They drop to the ground and enter a male's burrow mouth, where he switches from the calling song to a courtship song (Hill, 2000). Females very likely use some mechanism at this close range in a second level of mate choice; however, aspects of courtship, mating, oviposition and juvenile development remain poorly understood or undescribed in the species (see Hill, 1999, 2000).

Male burrows (Fig. 2) are spatially aggregated in discrete leks within the prairie landscape, but even within leks the burrows are found distributed nonrandomly in clusters. Moreover, at a third level of scale, males are found in small groups of 2-3 neighbors within a space defined by about a 4 m radius (Hill, 1999). One hypothesis explaining this tertiary clustering is that these closest clumps of burrows are maintained by intermale signaling via substrate-borne vibrations (Hill and Shadley, 1997, 2001). This hypothesis predicts a dynamic interaction of attraction and repulsion that results in burrow spacing that promotes effective signal transmission within the habitat at a given point in time, while being constrained by male-male competition. This spacing should, thus, vary with seasonal and stochastic changes in above ground vegetational structure, as well as with population dynamics that contribute to competition for mates. However, we have not yet been able to classify most patterns of burrow distribution based on abiotic factors or intraspecies interactions. The pairs or triplets of burrows may be within a few meters of another pair or triplet, or widely spaced from others; however, spacing within the lek is not based on soil microhabitat features like pH, mineral or moisture content (Hill et al., in press). Males appear to exhibit preferences for certain soils (Vaughn et al., 1993; Hill et al., in press) and populations return to the general region of a previous lek arena in succeeding years (Hill, 1999), but they are not aggregating burrows in the only habitat available to them on a given site.

[FIGURE 1 OMITTED]

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Male bushcrickets, or katydids, advertising for mates in botanical environments in which their acoustic signals can rapidly attenuate are known to either position themselves at a height in the vegetation so as to minimize impedence effects, and/or to cluster more closely together in response to abbreviated detectability ranges (Romer, 1993). Additionally, the acoustic signal of advertising male orthopterans often reflects spectral or temporal adaptation to the structural elements commonly present in the acoustic environment through which the signal is being transmitted (Couldridge and von Staaden, 2004). In a variety of lek mating groups, advertising males often choose display sites that reduce the obstruction of acoustic and/or visual signals being transmitted to visiting females, and lek formation and size dynamics can be influenced by habitat characteristics of the lek site (Hoglund and Alatalo, 1995). Whereas the advertisement call of the male prairie mole cricket exhibits frequency adaptation for long range transmission in the grassland environment where the species is found, factors influencing the optimal placement of acoustic burrows and interburrow spatial dynamics within the lek arena remain unexplained.

Variation is also observed in the angle of the male prairie mole cricket's acoustic burrow with respect to the soil surface (see Fig. 2), and this variation is thought to be related to the male's attempts to optimize the sound field of his calling song. It is not known if this inherent variation in burrow design is related to or influenced by the botanical structure present on the lek site. The purpose of this study was to analyze the spatial organization and dip angles of burrows on 11 leks with respect to features of the surrounding prairie vegetation for a first look at biotic influences from outside the population of Gryllotalpa major. We predict that, in general, the variable spatial arrangements exhibited among males within the prairie mole cricket lek will be related to the heterogeneity of the above ground habitat, and specifically that (1) as structural complexity (measured in terms of grass height and total biomass) increases, intermale spacing will decrease and (2) acoustic burrow angles will increase as above ground structural complexity (i.e., grass height and biomass) increases as predicted by an acoustic impairment model of male spacing (Romer, 1993). Moreover, we suggest that this spatial variability may represent a form of adaptive plasticity that has evolved in response to environmental stochasticity in the tallgrass prairie ecosystem.

METHODS

The study was conducted on two disjunct tallgrass prairie sites in northern Oklahoma where Gryllotalpa major populations have been documented. The Nature Conservancy's Tallgrass Prairie Preserve (36[degrees]49'N, 96[degrees]23'W) in north central Oklahoma is the largest continuous tract of tallgrass prairie remaining in North America, currently encompassing about 16,000 ha with approximately 80% tallgrass prairie vegetation and 20% post oak/ blackjack oak woodlands. The long term land management plan for this property includes the use of prescribed burning, bison and cattle grazing, and mowing to facilitate the restoration of the site to a functional tallgrass prairie landscape (Hamilton, 1996). White Oak Prairie in Craig County, Oklahoma (36[degrees]37'N, 95[degrees]16'W) has been surveyed during March-June since 1993 as part of a long term conservation project. This prairie remnant of about 61 ha is mowed and periodically burned, but it has never been ploughed or grazed by cattle. Beginning in April 2005 and continuing through the spring reproductive season of 2006, historic G. major chorusing sites at these two locations were surveyed to locate active mating aggregations.

When a calling aggregation was found, the burrow of each calling male was located and flagged. Meteorological data (air and soil temperature, relative humidity, dew point, barometric pressure, wind speed and sky conditions) were recorded for each lek site. An attempt was made to procure a tape recording of the advertisement call of every male located, but due to the combination of short calling season and large size of the study sites this was not always feasible. Geographic coordinates of the acoustic burrows of calling males were documented at the conclusion of the season using a Trimble Pro XRS unit with a TSC1 asset recorder (Navigation Limited, Sunnyvale, CA, USA), which is accurate to submeter level in most cases. Positional dilution of precision values remained below 3.0 at all times during which points were recorded, and real time differential correction of the data insured that the burrow coordinates utilized in calculating the spatial arrangements of the display arenas were resolved beyond nearest neighbor distances observed in the display arenas. The field survey spatial data were then exported into ArcMap 9.0 GIS software (ESRI, 2004) to be integrated into a GIS project.

During the 2005 and 2006 reproductive seasons 11 active lek sites containing the burrows of 300 advertising males were located at the two study sites. A lek site is rather easily delineated in the prairie landscape as a discrete aggregation of burrows separated spatially from other clusters of advertising males (see Fig. 1). At each burrow within the 11 leks, ten grass height measurements ([H.sub.g]) were taken around the perimeter of a 0.25 [m.sup.2] quadrat placed around the burrow such that the burrow opening served as the central point of the quadrat. The mean of these ten measurements was used to characterize the grass height ([H.sub.g]) at that particular burrow. Grass height values (cm) for each burrow were then used to calculate a mean grass height for each lek site. The dip angle (or slope with respect to the horizontal of the ground surface of the most exterior wall of the burrow's acoustic horn; see Fig. 2) of each burrow was measured using a modified dial caliper. An effort was made to procure the dip angle for each burrow located; however, in some cases the burrow was sealed, washed out by a rainfall event or otherwise damaged before this measurement could be taken. The data for these damaged burrows were not included in the analysis. At each of the 11 lek sites, ten above ground botanical biomass samples ([B.sub.a]) were taken at random locations within the perimeter of the lek, Within a 0.25 [m.sup.2] quadrat all above ground botanical material (living or dead) was removed to soil level using electric grass clippers (Black and Decker Corp., Hunt Valley, MD, USA). Samples were then air dried for approximately 2 wk before oven drying for 24 h at 95 C, The dry mass (g/[m.sup.2]) of each sample was then determined using a Denver Instruments APX-203 top loading balance (Denver, Colorado, USA) and the mean of the ten samples calculated. This value was used to characterize the above ground botanical biomass ([B.sub.a]) value for each lek site.

Spatial characteristics for each of the 11 leks was calculated with ArcMap 9.0 GIS software (ESRI, 2004) using the Hawth's Spatial Ecology extension (Beyer, 2004). Once all points for each of the 11 leks were imported into an ArcGIS project, the extension was used to calculate the area ([m.sup.2]) of each lek as defined by a minimum convex polygon (MCP) consisting of all of the burrows within the lek. Additionally, the extension calculated the population density within each lek (mole crickets/[m.sup.2]), the interburrow distances (distance between the focal male and each of its neighbors within the lek; in m), the nearest neighbor distances (m) and the furthest neighbor distances (m) for all burrows within the lek. The mean interburrow distance, the mean nearest neighbor distance and mean furthest neighbor distance values for each lek were determined by calculating the arithmetic mean of the summed distances for each burrow for each of these three parameters. Data were tested for normality, and then for the level of association between grass height ([H.sub.g]) and above ground botanical biomass ([B.sub.a]), population density and the five spatial variables (SigmaStat for Windows, ver. 2.0, Jandel Scientific, San Rafael, CA, USA). The Pearson Product Moment test was used for normally distributed data, and the Spearman Rank Order Correlation was used for data sets that failed the Kolmogorov-Smirnov test of normality.

RESULTS

Mean grass height ([H.sub.g]; measured in cm above the soil) values at the 11 lek sites surveyed in Osage and Craig Counties ranged from 6.48-36.30 cm (mean = 18.93 cm, SE = 3.22), with mean above ground botanical biomass values for the study sites ([B.sub.a]; measured in g/[m.sup.2]) ranging from 10.60-825.92 g/[m.sup.2] (mean = 403.84 g/[m.sup.2], SE = 80.95). Analyses of the data generated by this study indicate a significant positive correlation between [H.sub.g] and [B.sub.a] ([r.sub.s] = 0.78, n = 11, P < 0.01). As would he expected, above ground botanical biomass increases as grass height increases (Table 1). Neither [B.sub.a] ([r.sub.s] = 0.26, n = 11, P = 0.22) nor [H.sub.g] ([r.sub.s] = 0.15, n = 11, P = 0.33) was significantly correlated with lek population, and neither [B.sub.a] ([r.sub.s] = 0.08, n = 11, P = 0.41) nor [H.sub.g] was significantly correlated with lek population density ([r.sub.s] = 0.16, n = 11, P = 0.32). Only lek area shared a relationship with the number of advertising males at a site ([r.sub.s] = 0.61, n = 11, P = 0.02); as the number of advertising males increased within a lek, a corresponding increase in lek display area was observed. A significant positive correlation was observed between [H.sub.g] and two spatial parameters of the leks (Fig. 3): mean interburrow distance ([r.sub.s] = 0.55, n = 11, P = 0.04), and mean furthest neighbor distance ([r.sub.s] = 0.62, n = 11, P = 0.02). A significant positive correlation was also observed between [B.sub.a.] and three spatial parameters of the lek (Fig. 4): mean nearest neighbor distance ([r.sub.s] = 0.56, n = 11, P = 0.04), mean furthest neighbor distance (r = 0.54, n = 11, P = 0.04), and mean interburrow distance (r = 0.54, n = 11, P = 0.04). Burrow angle (Fig. 5) exhibited a significant positive correlation with [H.sub.g] ([r.sub.s] = 0.36, n = 129, P < 0.01), but not with [B.sub.a] (r = 0.52, n = 11, P = 0.10).

[FIGURE 3 OMITTED]

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DISCUSSION

Prior work with Gryllotalpa major has characterized the aggregation of burrows from which males produce their sexual advertisement songs as a lek arena (Hill, 1999). It is known that advertising males are more likely to construct their burrows on recently burned sites that may afford a more suitable acoustic display arena (Howard and Hill, 2007). It is also known that the species is more likely to be found within the broader grassland landscape in soils with higher silt content, and that silt may be required for burrow construction (Vaughn et al., 1993). At the closest distances within a lek site, only the influences of male-male interactions have been confirmed to influence spacing. Males respond to a simulated substrate vibrational component of another male's calling song at distances characteristic of nearest neighbor spacing in the field (Hill and Shadley, 2001), and there is a negative correlation between richness of harmonic content in a male's song and the distance to his nearest calling neighbor (Hill, 1998). This then leads to the prediction that these interactions include attraction at greater nearest neighbor distances and repulsion when burrows are constructed too closely together. Although this prediction has yet to be tested and is not under consideration in this study, we do know that males respond to the vibrational component and ignore the airborne element of the calling song in interactions with a nearest neighbor during advertisement (Hill and Shadley, 1997, 2001).

[FIGURE 5 OMITTED]

The results of this study suggest for the first time that variability in spacing of Gryllotalpa major burrows within a lek may be related to specific botanical attributes of the grasslands in which these leks are located. However, our prediction that increased structural complexity above ground would be somehow associated with closer spaced burrows based on an acoustic impairment model was not supported. The acoustic impairment model stipulates that advertising males will position themselves in such a way as to optimize signal transmission and broadcast range, so that males should cluster acoustic burrows more closely together when displaying in environments where signal transmission, identification or localization might be impaired. Instead, male prairie mole cricket burrows were spaced farther apart in lek settings with greater above ground botanical structure. Two spatial characteristics of the leks sites were significantly correlated with grass height (mean interburrow distance and mean furthest neighbor distance), whereas the dry mass quantity of above ground botanical biomass at the lek site was significantly correlated with three indicators of intermale spacing (mean interburrow distance, mean nearest neighbor distance and mean furthest neighbor distance) (Table 1).

The findings from this study are consistent with the acoustic advantage explanation invoked by Howard and Hill (2007), and yet suggest that Gryllotalpa major male mating aggregations also display a form of spatial plasticity with respect to the actual burrow locations based upon habitat variables found at the lek site. While not entirely understood, this spacing behavior in males may make them easier for searching females to locate and thus provide an energetic advantage to potential mates in an acoustic environment made more complex by the heterogeneity of the tallgrass prairie landscape (Romer, 1993; Romer and Bailey, 1998). Structural impedances can impair signal perception and reduce the ability of the signal target to accurately locate the signaler (Naguib and Wiley, 2001), and a male may gain a competitive advantage by distancing his burrow from his nearest neighbor in such a way as to ensure a female attracted to his signal does not encounter either structural obstacles or another signaler's burrow while navigating toward his calling song. In this regard, a G. major male advertising for searching females may in fact be ascribing in some respects to the essence of the Romer model by positioning himself within the available habitat of the lek site not merely to increase broadcast range, but in such a way as to increase the probability of female success in navigating to his burrow. Additional information about the sound field is necessary for a fuller understanding of why burrows are spaced as they are and to successfully model female choice in this species (Forrest and Raspet, 1994).

The data collected in this study regarding the variation in the burrow angles of the acoustic chamber of advertising male prairie mole crickets (see Fig. 2) did support our original prediction. The acoustic horus in lek settings where grass heights are greater tend to have greater dip angles; however, this relationship fell short of significance when considering above ground botanical biomass. This suggests, however, that male prairie mole crickets are able to adjust the dip angle of the burrow opening in response to some aspects of the above ground habitat structure. The acoustic horn of the male's burrow amplifies his signal, and we suspect that the dip angle of the horn contributes in some way to characteristics of the sound field produced above the burrow. The steeper angle of the broadcasting chamber at sites with taller grasses could provide the caller with a signaling advantage over a burrow constructed with a shallower angle, since females are flying just above the height of the existing grasses as they make their final pass through an aggregation of calling males (Howard and Hill, 2006). Vegetation acts as a low pass filter (Latimer and Sippel, 1987) and pieces of the call (Romer and Lewald, 1992), as well as the higher frequencies that attenuate with distance, may be differentially affected in settings with dense botanical structure (Romer, 1993). Therefore, adjusting the dip angle as the vegetation changes height with the progressing growing season would increase the probability that a signal broadcast to flying females would be directed more toward the actual flight path and would potentially lose less information through interference than one propagated at the same angle irrespective of changes in the above ground vegetation. The height of the grass is, thus, a better predictor of dip angle than is the total mass of vegetation.

Despite the misconception that prairies are extensive "oceans of grass" marked by botanical and structural homogeneity, healthy grassland ecosystems are profoundly patchy in nature (Abrams and Hulbert, 1987; Collins, 2000; Fuhlendorf and Engle, 2001; Collins et al., 2002). Influenced by seasonally shifting disturbance factors such as fire and grazing (Axelrod, 1985; Finck et al., 1993; Joeru and Keeler, 1995; Johnson and Matchett, 2001; Harrison et al., 2003; Trager et al., 2004), grasslands in general and tallgrass prairie specifically have presented faunal occupants with habitat distributions best described as a shifting mosaic (Harrison et al., 2003; Fuhlendorf and Engle, 2004). Tallgrass prairie endemics such as C, ryllotalpa major have evolved behaviors that allow the species to thrive in a landscape where botanical heterogeneity is the rule rather than the exception. The spatial plasticity observed in the mating aggregations of male prairie mole crickets may be one example of how the species is able to respond to predictable seasonal and annual variability, as well as environmental stochasticity brought about by prairie fires and grazing.

Acknowledgments.--This project was supported by funding from the National Science Foundation Graduate Research Fellowship Program, and the Oklahoma EPSCoR NSF Scholars Program. We wish to thank Bob Hamilton of The Tallgrass Prairie Preserve, the Pawhuska Field Office of The Nature Conservancy, and Wallace Olsen and the Kelly Ranch for access to the research sites and facilities, and Wayne Isaacs and Brad Asbill of the Cherokee Nation Environmental Programs Office for GIS technical support. We thank Drs. V. H. Hutchison, R. Reeder, K. Sublette, B. Tapp and H. Wells for additional assistance with this study. Additional field assistance was provided by Mindy Baker, Carrie Hall, Emmalea Howard, Jarrett Hutton and Charlotte Sanderson. We finally thank the editor and two anonymous readers for comments made in review that have helped to strengthen this manuscript.

SUBMITTED 24 APRIL 2008

ACCEPTED 9 SEPTEMBER 2008

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DANIEL R. HOWARD (1) AND PEGGY S. M. HILL

University of Tulsa, Department of Biological Science, 600 South College Avenue, Tulsa, Oklahoma 74104

(1) Present address: Department of Biological Science, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4 Canada; Telephone: (416)287-7433; FAX: (416-287-7676); e-mail: dhoward@utsc.utoronto.ca
TABLE 1.--Correlation table of grass height ([H.sub.g]), above
ground botanical biomass ([B.sub.a]), lek population and density,
five spatial characteristics of the prairie mole cricket leks
surveyed and burrow dip angle

                                       Grass height ([H.sub.g])

Grass height (cm)                 [r.sub.s] = 1.00, n = 11, P = 0.00
Botanical biomass (g/[m.sup.2])   [r.sub.s] = 0.78, n = 11, P < 0.01
Lek population (#)                [r.sub.s] = 0.15, n = 11, P = 0.33
Lek area([m.sup.2])               [r.sub.s] = 0.24, n = 11, P = 0.24
Lek density (#/[m.sup.2])         [r.sub.s] = 0.16, n = 11, P = 0.32
Mean NN distance (m)              [r.sub.s] = 0.39, n = 11, P = 0.12
Mean FN distance (m)              [r.sub.s] = 0.62, n = 11, P = 0.02
Mean interburrow dist. (m)        [r.sub.s] = 0.53, n = 11, P = 0.04
Burrow dip angle (degrees)        [r.sub.s] = 0.36, n = 129, P < 0.01

                                     Botanical biomass ([B.sub.a])

Grass height (cm)                 [r.sub.s] = 0.78, n = 11, P < 0.01
Botanical biomass (g/[m.sup.2])   [r.sub.s] = 1.00, n = 11, P = 0.00
Lek population (#)                [r.sub.s] = 0.26, n = 11, P = 0.22
Lek area([m.sup.2])               [r.sub.s] = 0.22, n = 11, P = 0.26
Lek density (#/[m.sup.2])         [r.sub.s] = 0.08, n = 11, P = 0.41
Mean NN distance (m)              [r.sub.s] = 0.56, n = 11, P = 0.04
Mean FN distance (m)              [r.sub.s] = 0.54, n = 11, P = 0.04
Mean interburrow dist. (m)        [r.sub.s] = 0.54, n = 11, P = 0.04
Burrow dip angle (degrees)        [r.sub.s] = 0.52, n = 11, P = 0.10
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Author:Howard, Daniel R.; Hill, Peggy S.M.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
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