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Life History and Ecology of Cambarus halli (Hobbs)
Susan Dennard, James T. Peterson, and Edwin S. Hawthorne

Southeastern Naturalist, Volume 8, Number 3 (2009): 479–494

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2009 SOUTHEASTERN NATURALIST 8(3):479–494 Life History and Ecology of Cambarus halli (Hobbs) Susan Dennard1,3, James T. Peterson2,*, and Edwin S. Hawthorne1 Abstract - The life history of Cambarus halli, a crayfish endemic to the Tallapoosa River Basin, GA, was studied at four sites within the Tallapoosa River. Two sites had allopatric populations of C. halli, and two sites had populations of C. halli sympatric with C. englishi. Three age classes existed across sites. For Cambarus halli, total number of pleopodal eggs was positively related to carapace length, but egg size was only weakly positively related to carapace length. Cambarus halli were smaller across age classes at sympatric sites, but had greater growth rates than at allopatric sites. Cambarus halli density estimates were lower at sympatric sites, while proportions of reproductively active age-1 and age-2 individuals were higher at allopatric sites (63% vs. 33%). Introduction Crayfish are important in aquatic ecosystems, making up a large proportion of aquatic system biomass (Griffith et al. 1994, Rabeni 1992). They are critical in food webs as processors of leaf litter (Griffith et al. 1994, Huryn and Wallace 1987) and as important food for predatory fish (Probst et al. 1984, Roell and Orth 1993). Crayfish support recreational and commercial bait fisheries, serve as a popular human food (Taylor et al. 1996), and are used as bioindicators due to their sensitivity to organophosphates and carbamates, two widely used classes of pesticides (Hyne and Maher 2003). In North America, 122 of the 363 known species of crayfishes are either imperiled or extinct (Taylor et al. 2007). Effective crayfish conservation strategies need to be developed to protect and recover these important ecosystem components. However, the development of effective conservation measures for crayfish species requires information on reproductive biology and population dynamics. This information is often lacking, as basic distribution and life-history information is known for less than 40% of crayfish species in North America (Taylor et al. 2007). Taxa with small distributional ranges are considered particularly vulnerable to extirpation, because of habitat degradation and destruction, relative to more cosmopolitan species (Gilpin and Soule 1986). One such stream-dwelling crayfish species, Cambarus halli Hobbs, is endemic to the Tallapoosa River Basin, a small (12,121 km2) basin located in the northwest Georgia Piedmont (Brouchard 1978, Ratcliffe and DeVries 2004). Cambarus halli are similar in size (maximum carapace length = approximately 35–40 1Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602. 2US Geological Survey, Georgia Cooperative Fish and Wildlife Research Unit, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602.3Current address - Great Lakes Institute for Environmental Research, University of Windsor, Windsor, ON, Canada, N9B 1G6. *Corresponding author - 480 Southeastern Naturalist Vol. 8, No. 3 mm) and appearance (Freeman et al. 2003) to a congener, Cambarus englishi Hobbs and Hall. It is believed that C. halli prefers fl owing-water habitats and is often sympatric with C. englishi (Bouchard 1978, Freeman et al. 2003, Hobbs and Hall 1972). Allopatric C. halli reportedly use all habitat types, but are found primarily in non-riffl e habitats when they occur with C. englishi (Hobbs 1981). However, little is known about the reproductive and life-history characteristics of C. halli. Most life-history studies of crayfish in North America are based on studies of a single population located at single location (Muck et al. 2002). Here we studied the life-history characteristics of four populations of C. halli to evaluate how life history can vary in the presence of a sympatric congener. Thus, the goal of this study was to examine the life-history characteristics and ecology of C. halli, by completing the following objectives: 1) to determine the age-class structure, 2) to evaluate the chronology of reproductive events and estimate fecundity, 3) to evaluate the seasonal growth, 4) to determine the seasonal habitat use, and 5) to evaluate the potential infl uence of an abundant congener, C. englishi. Methods Study sites The Tallapoosa River fl ows south-southwest from its headwaters in Paulding County, GA, eventually joining the Coosa River to form the Alabama River (GA DNR 2002). Of the 12,121 km2 in the Tallapoosa River drainage, 15% is in Georgia (GA DNR 2002). The river basin is characterized by rolling hills and is almost entirely in the Upper Piedmont (GA DNR 2002). Four sites were chosen for this study based on an extensive survey of the basin (Freeman et al. 2003): one on Blalock Creek, one on Kiser Creek, and two areas on the mainstem of the Tallapoosa River (Fig. 1). Blalock and Kiser Creeks represented allopatric populations of C. halli, whereas the Tallapoosa River sites represented sympatric sites that contained both C. halli and C. englishi. Blalock Creek and Kiser Creek were second-order streams that emptied into Walker Creek and Holcomb Creek, respectively. The sites were approximately 250 m long and characterized by runs, riffl es, and pools. The substrate composition at each site was similar and was predominantly cobble and gravel, with some boulders, bedrock, and sand. The two sites on the mainstem of the Tallapoosa River were fourth-order, with the first located upstream of the Highway 27 crossing and the second located downstream of the Mount View Road crossing. Both mainstem sites were approximately 250 m long with varying combinations of runs, riffl es, and pools and also containing sand, silt, cobble, gravel, and bedrock substrate. Field sampling Sampling occurred between December 2001 and June 2003. At approximately monthly intervals, samples were used to determine phases of the reproductive cycle of C. halli, including fecundity, time of oviposition, hatching of young, and timing of the young’s departure from the female. A 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 481 1-m2 quadrat sampler, described by Rabeni (1985), with 6-mm netting and a 0.75-m bag was used to sample crayfish. Each study site was divided by mesohabitat (i.e., pool, riffl e, and run) and two to four replicates of each mesohabitat type were sampled. Within each mesohabitat, a minimum of 3 and maximum of 10 quadrat samples were taken to ensure the mesohabitat was thoroughly sampled. Captured crayfish were placed into buckets designated for the particular replicate mesohabitat in which they were captured, thus providing for easy release back into the same location. Semi-monthly sampling occurred during the months that oviposition, hatching of young, and departure of young from the female were thought to occur (approximately March to May). At least 50 individuals of the target species were collected during each sampling period; if fifty were not collected via the quadrat sampler, backpack electrofishing or simple hand capture was used to supplement. Data collected for each crayfish at each mesohabitat included: species, carapace length (CL; from tip of rostrum to posterior border of the thoracic region), and sex. Additionally, reproductive capacity was recorded, i.e., if males were Form I or II and if females had active glair. Glair was identified as the white cement substance located around the base of each pleopod that attaches the eggs to the pleopods (Stephens 1952a). Captured crayfish then were released back into the same mesohabitat unit from which they were originally sampled. However, approximately 10 randomly selected female crayfish per sampling period were kept and preserved in 70% Figure 1. The location of the Tallapoosa River Basin, GA and the four C. halli study sites indicated by gray circles. 482 Southeastern Naturalist Vol. 8, No. 3 ethanol to facilitate fecundity estimation. Following sampling of crayfish, water chemistry parameters (temperature, dissolved oxygen, conductivity, and turbidity) were also measured with calibrated meters and recorded. During the summer of 2001 and 2002, mesohabitat availability was estimated at each study site at basefl ow discharge. The location of each mesohabitat was recorded with a GPS unit. The boundaries of each mesohabitat were then delineated using the depth-based criteria of McKenney (2001) and the dimensions measured using a tape measure. The relative amount of each mesohabitat was estimated for each site by summing the area of each mesohabitat type and dividing by the total study site area (i.e., the total area of all the habitats measured within each site; Table 1). Analysis and laboratory procedures Fecundity was determined by dissecting ovaries from the subsample of females that were kept from each sampling period (following Stephens 1952b). Eggs on pleopods were also counted. Mean egg diameter for pleopod eggs was determined by measuring ten randomly selected eggs with a dissecting scope; if the total number of eggs was less than ten, all eggs were measured. Linear regression analysis (Neter et al. 1996) was used to compare the total number of ovarian and pleopod eggs and mean size of pleopod eggs to carapace length, and the precision of model coefficients was assessed by calculating 90% confidence intervals (CI). Crayfish density in each sampled replicate mesohabitat was estimated by dividing the total number of crayfish collected in the mesohabitat by the total number of quadrat samples. Age-class structure was determined with monthly length-frequency histograms, a method that was shown to be at least 80% accurate (Momot 1967). Because each size class was not distributed equally throughout mesohabitats, mesohabitat-specific density estimates were extrapolated by multiplying the estimated density of each 1-mm size group by the mesohabitat availability proportion shown in Table 1. Increases in the modal carapace length for an age group through time can be used to estimate crayfish growth (Baker et al. 2008, Hamr and Berrill 1985, Muck et al. 2002). We used a linear regression analysis (Neter et al. 1996) to examine the relationship between age class-specific modal carapace length and sample month, and interpreted the slope of the relationship as the monthly growth rate. Because newly hatched C. halli (age 0) were not recruited to our sampling gear until June (see Results), we coded months beginning with June = 0 and ending with May = 11. We examined the relationship between C. halli Table 1. Total surface area and mesohabitat composition of the four study sites expressed as a percentage of the total surface area. Pool Riffl e Run Study site Area (m2) (% area) (% area) (% area) Blalock Creek 1370.9 28.1 32.3 39.6 Kiser Creek 1175.1 36.3 32.2 31.5 Tallapoosa River at Highway 27 2073.8 15.7 17.3 67.0 Tallapoosa River at Mount View Road 2010.0 11.6 20.5 67.9 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 483 monthly growth rate and crayfish age, and the presence of C. englishi by including predictor variables for age and sympatry. The three C. halli age classes (see Results) were categorized by assigning binary variables (0, 1) to age 0 and age 2 crayfish, with age 1 crayfish retained as the baseline. Sympatric and allopatric sites were similarly coded using a binary variable, with sympatric sites coded as 1 and allopatric sites coded as 0. An information-theoretic approach, described by Burnham and Anderson (2002), was used to evaluate the relative plausibility of models relating sympatry and age to C. halli growth rate. We constructed a global model (with all predictors) and a subset of 14 candidate models representing hypotheses about the relative effects of age and sympatry on growth (Table 2a). Our primary hypothesis of interest was the infl uence of C. englishi presence on growth and size at age. Candidate models also included a quadratic term for time of year to account for likely nonlinearities in growth. To assess the plausibility of each candidate model, we calculated AIC with the small-sample bias adjustment (AICc; Hurvich and Tsai 1989) and Akaike weights, as described in Burnham and Anderson (2002). We also computed model-averaged parameter estimates of the individual coefficients and standard errors (Burnham and Anderson 2002) for the predictor variables that occurred in one or more candidate models with weights within 10% of the largest weight (i.e., the bestapproximating model). The precision of model-averaged coefficients was assessed by calculating 90% CI. Goodness-of-fit was assessed for the global model by examining residual and normal probability plots (Neter et al. 1996). Dependence among samples collected at a site was examined by conducting an analysis of variance (ANOVA) on the residuals from the global model, with significant differences indicating spatial dependence (i.e., autocorrelation). The information-theoretic approach described above also was used to evaluate the relative fit of various candidate models relating crayfish density estimates to habitat, season, age class, and sympatry. Similar to above, we constructed a global model (with all predictors) and a subset of 12 candidate models representing hypotheses about the relative effects of age, season, mesohabitat type, and the presence of C. englishi on the density of C. halli (Table 2b). We defined seasons as: winter (January–February), spring (March–May), summer (June–September), and fall (October–November). The seasons were categorized using binary variables (0, 1) that were assigned to spring, fall, and winter, with summer retained as the baseline. Mesohabitat types were similarly coded for pool and riffl e habitats, with runs as the baseline. The crayfish density candidate models then were fit using linear regression, their plausibility evaluated by calculating AICc and Akaike weights, and goodness-of-fit assessed by examining residuals from the global model, as described above. Results Sampling We collected 1434 crayfish (n = 150 samples) at Blalock Creek and 367 crayfish (n = 38 samples) at Kiser Creek, both allopatric sites. Cambarus 484 Southeastern Naturalist Vol. 8, No. 3 Table 2. Biological interpretation of predictors used in candidate models of the factors infl uencing (a) C. halli modal carapace length and (b) estimated C. halli density. Asterisks indicate interaction between predictor variables. Two-way interactions and quadratic terms were only included in candidate models containing associated main effects. Predictor variables Biological interpretation (hypothesis) (a) C. halli modal carapace length Time Change in carapace length through time represents growth rate. Time2 The increase in growth rate within age class is non-linear and decreases through time. Sympatric sites Size of C. halli across age groups is different at sympatric sites due to interaction with C. englishi Age 0, age 2 Carapace length of C. halli differs among age classes. Time*sympatric sites Time2 *sympatric sites Growth rate of C. halli differs at sympatric sites due to interaction with C. englishi. Time*age 0, time*age 2 Time2*age 0, time2*age 2 C. halli growth rate varies with age class. Time*sympatric sites*age 0, time*sympatric sites*age 2 Time2*sympatric sites*age 0, time2*sympatric sites*age 2 The infl uence of sympatric C. englishi on crayfish growth rate differs with age class. (b) Estimated C. halli density Sympatric sites The density of C. halli across ages and habitat types differs at sympatric sites due to interaction with C. englishi. Fall, winter, spring The density of C. halli differs among seasons. Age 0, age 2 The density of C. halli differs among age classes. Pool, riffl e The density of C. halli differs among habitat types, due to differences in habitat use. Sympatric sites*age 0, sympatric sites *age 2 The density of C. halli infl uence of sympatric C. englishi on varies with age. Sympatric sites*pool, Sympatric sites *riffl e Habitat use of all age classes differs in sympatric sites due to interactions with C. englishi. Fall*pool, fall*riffl e, winter*pool, winter*riffl e, Habitat use of C. halli differs among seasons. spring*pool, spring*riffl e Age 0*pool, age 0*riffl e, age 2*pool, age 2*riffl e Habitat use of C. halli differs among age classes. Sympatric sites*fall*age 0, sympatric sites*winter*age 0, Seasonal habitat use varies in the presence of C. englishi, through aggression, competition, sympatric sites*spring*age 0, sympatric sites*fall*age 2, or predation throughout the stream. sympatric sites*winter*age 2, sympatric sites*spring*age 2 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 485 halli was collected with Cambarus latimanus LeConte, Cambarus striatus Hay (Hay Crayfish), and Procambarus spiculifer LeConte in Blalock Creek and with C. latimanus in Kiser Creek. Sampling at Kiser Creek was not conducted for the entire sampling period and was stopped prematurely on April 2, 2002 due to site-access issues. At the sympatric sites, Tallapoosa at Highway 27 and Tallapoosa at Mount View Road, we collected 582 crayfish (n = 69 samples) and 652 crayfish (n = 81 samples), respectively. At both Tallapoosa sites, C. halli was collected with C. englishi, C. latimanus, and P. spiculifer. Age-class structure Hatching began in March and April at all sites (see below); however, the young (henceforth defined as age 0) did not recruit to the quadrat gear until June, when the smallest individuals were approximately 5 mm in length (Fig 2). Examination of carapace length-frequency distributions suggested the presence of 3 age classes: age 0 (newly hatched that year), age 1, and age 2 (Fig. 2). Graduation from age 0 to age 1 occurred in April/May at approximately 19 mm, for both sympatric and allopatric sites; graduation from age 1 to age 2 also occurred in April/May at approximately 30 mm, for both sympatric and allopatric sites. Carapace length-frequency distributions suggested that age-2 individuals were either dying in October/November or indistinguishable from age 1. Reproductive biology Both male and female C. halli were in a nonreproductive state from June to September (Fig. 3). By October, approximately 16.8% of C. halli were Form I (reproductive) males. Female C. halli were becoming reproductively active by January, as indicated by the presence of glair, and they began to extrude eggs in late March and early April. Water temperatures during this time varied between sites, ranging from 10 to 14 °C at Blalock Creek, where C. halli was allopatric, and from 10 to 17 °C at the Tallapoosa sites, where C. halli and C. englishi co-occured. The maximum number of ovigerous females occurred in May, when water temperatures reached 19 °C. In June 2002, all males were in a nonreproductive (Form II) state. However, some reproductive (Form I) males were collected in June 2003. The smallest sexually mature female (i.e., females with glair or bearing eggs) for C. halli was 13 mm, collected March 4, 2003 at the Tallapoosa at Mount View Road site. The smallest sexually mature male (i.e., Form I) for C. halli was 12 mm, collected October 17, 2002 at Blalock Creek. The proportion of age-1 and age-2 reproductive individuals were greater at allopatric sites (0.63; 90% CI: 0.38–0.86) compared to sympatric sites (0.33, 90% CI: 0.14–0.51). At all sites, hatching began in March and April (Fig. 3). Eggs were only measured and counted for C. halli at Blalock Creek. The total number of pleopod eggs carried by females ranged from 65 to 217. Linear regression models indicated that the number of pleopod eggs was positively related to female carapace length (Fig. 4a); carapace length accounted for 486 Southeastern Naturalist Vol. 8, No. 3 Figure 2. An example of C. halli carapace length-frequency histogram from April– June 2002. 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 487 83.7% of the variation in the number of pleopod eggs. Mean egg diameter ranged from 2.280 to 2.476 mm. The egg diameter, however, was only weakly positively related to female carapace length (Fig. 4b), which accounted for only 0.8% of the variation in egg diameter. The carapace-length parameter estimate for relating egg diameter was imprecise, as the confidence interval was wide and included zero (Table 3). Seasonal growth The examination of residuals from the global linear models relating crayfish age and the presence of C. englishi to C. halli modal carapace length indicated no departures from normality and no detectable spatial autocorrelation. The best-approximating model of C. halli carapace length was the global model that contained time, age, the sympatric indicator variable, and the interactions: time by sympatric site, time by age, and time by sympatric site by age. This model was 2 times more likely than the next-best approximating model Figure 3. Reproductive state of mature male and female C. halli during February 2002 to June 2003. Percentage occurrence is the percentage of crayfish in a reproductive state out of all mature individuals found in that sampling period. Table 3. Parameter estimates, standard error (SE), and upper and lower 90% confidence limits for models relating carapace length (mm) to total number of pleopod eggs and pleopod egg size (mm) for C. halli. Parameter Estimate SE Lower Upper Total number of pleopod eggs Intercept -137.859 24.617 -178.231 -97.488 Carapace length 8.667 0.797 7.360 9.975 Pleopod egg size Intercept 2.361 0.243 1.962 2.760 Carapace length 0.002 0.008 -0.011 0.015 488 Southeastern Naturalist Vol. 8, No. 3 that did not contain the time by sympatric site by age interaction. There was no support for a non-linear relationship between time and carapace length within age class because models containing the quadratic term for time were among the poorest approximating. The model-averaged parameter estimates indicated that carapace length was positively related to time and that age-1 crayfish at allopatric sites grew 2.39 mm per month, whereas age-0 and age-2 growth rates were lower than the age-1 growth rate (Table 4). The sympatric site parameter estimate indicated that C. halli were on average 2.07 mm smaller at sympatric sites compared to allopatric sites. However, the growth rates of age-0 and age-2 crayfish at sympatric sites were on average, 0.58 and 0.97 mm greater than allopatric sites, respectively. The parameter estimates for the time by sympatric site interaction Figure 4. Relation of female C. halli size to (a) total number of pleopod eggs, and (b) pleopod egg size in May and June of 2003 at Blalock Creek. 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 489 suggested that the growth rate for age-1 C. halli at sympatric sites was slightly greater than at allopatric sites; however, this estimate was unreliable, because the confidence interval was wide and included zero (Table 4). Habitat use and density estimates An examination of the normal probability plot of residuals from the global linear models relating age, habitat, season, and the presence of C. englishi to C. halli density estimates indicated that the residuals departed from expected (i.e., the plots were curvilinear rather than linear). To normalize these data, we natural log transformed the data and re-fit the candidate models. The ANOVA of residuals from the global C. halli density model fit to the transformed data indicated no detectable dependence among samples within sites (P = 0.27). The most plausible model of C. halli density contained season, age, mesohabitat type, the sympatric indicator variable, and the interactions age by sympatric site and mesohabitat type by sympatric site, and was 1.4 times more likely to predict density than the next-best approximating model, the global model. Density estimates of C. halli were greatest during the summer months and lowest during the spring (Table 5). Density estimates were also lower at sites where they were sympatric with C. englishi, with the greatest differences between density estimates at allopatric and sympatric sites occurring for age-1 C. halli. The parameter estimates for mesohabitat type and mesohabitat type by sympatric site interaction indicated that density estimates of all age classes of C. halli were generally greatest in runs and pools at allopatric sites, but were lower in pools and runs and greatest in riffl es at sympatric sites (Table 5). The remaining parameter estimates were unreliable for interpretation, as the confi- dence intervals were wide and included zero. Discussion Reproductive characteristics of C. halli in the Tallapoosa River Basin were similar to other stream-dwelling crayfish, such as Orconectes luteus Creaser (Golden Crayfish) in the Missouri Ozarks (Muck et al. 2002). Table 4. Model-averaged parameter estimates, standard errors (SE), and upper and lower 90% confidence limits for composite linear regression model of C. halli carapace length. Time (month = zero) begins at June and ends in May; age 1 was used as the baseline age class in the regression. Upper Lower Parameter Estimate SE (90% CI) (90% CI) Intercept 15.705 1.114 17.531 13.879 Time 2.391 0.270 2.834 1.947 Sympatric sites -2.067 1.210 -0.083 -4.051 Age 0 -7.957 1.279 -5.859 -10.054 Age 2 14.252 1.809 17.219 11.286 Time*sympatric sites 0.223 0.322 0.751 -0.304 Time*age 0 -0.663 0.365 -0.063 -1.262 Time*age 2 -2.447 0.777 -1.172 -3.721 Time*sympatric sites*age 0 0.580 0.265 1.014 0.146 Time*sympatric sites*age 2 0.972 0.557 1.885 0.059 490 Southeastern Naturalist Vol. 8, No. 3 Reproductive timing for O. luteus was comparable to C. halli, with O. luteus females extruding eggs in late March and early April, eggs hatching in April, and independence of young in the summer (Muck et al. 2002). Growth rates of the three O. luteus age classes examined by Muck et al. (2002) were comparable to C. halli; for example, age-0 C. halli reached 19 mm after one year of growth, while young-of-year O. luteus grew to approximately 20 mm before graduating to age 1 (Muck et al. 2002). Fecundity for C. halli differed slightly from O. luteus. Muck et al. (2002) found that both number of pleopod eggs and mean egg diameter increased with increasing carapace length of the female. We observed a relationship between number of eggs and carapace length, but not between egg diameter and carapace length. Fecundity for C. halli somewhat refl ected the life-history strategy of an r-selected species: maturity at a young age (3% of age-0 crayfish were reproductively active at the end of their first summer), rapid growth rates (19 mm in the first year, and 11 mm more the second year), and large numbers of offspring (more than 200 for a female with CL of 35 mm). However, it is not as fecund as invasive species, such as Procambarus clarkii Girard (Red Swamp Crayfish), whose average-sized females produce Table 5. Model-averaged parameter estimates, standard errors (SE), and upper and lower 90% confidence limits (CI) for composite linear regression model of estimated C. halli density. Parameter Estimate SE Upper CI Lower CI Intercept 1.366 0.098 1.526 1.206 Sympatric sites -0.901 0.100 -0.736 -1.065 Fall -0.202 0.106 -0.029 -0.375 Winter -0.241 0.132 -0.024 -0.457 Spring -0.176 0.090 -0.029 -0.324 Age 0 -0.378 0.081 -0.245 -0.511 Age 2 -0.349 0.081 -0.216 -0.481 Pool 0.099 0.109 0.277 -0.080 Riffl e -0.841 0.156 -0.585 -1.096 Sympatric sites*age 0 0.401 0.122 0.601 0.202 Sympatric sites*age 2 0.340 0.119 0.536 0.145 Sympatric sites*pool -0.080 0.048 -0.002 -0.159 Sympatric sites*riffl e 0.535 0.112 0.719 0.351 Fall*pool 0.056 0.164 0.326 -0.213 Fall*riffl e 0.164 0.164 0.434 -0.105 Winter*pool 0.158 0.179 0.452 -0.137 Winter*riffl e 0.256 0.179 0.550 -0.039 Spring*pool 0.080 0.127 0.290 -0.129 Spring*riffl e 0.202 0.127 0.411 -0.007 Age 0*pool -0.218 0.127 -0.010 -0.426 Age 0*riffl e 0.124 0.127 0.332 -0.084 Age 2*pool -0.079 0.127 0.129 -0.287 Age 2*riffl e 0.172 0.127 0.380 -0.037 Sympatric sites*fall*age 0 0.106 0.180 0.400 -0.189 Sympatric sites*winter*age 0 0.503 0.292 0.983 0.024 Sympatric sites*spring*age 0 -0.285 0.143 -0.051 -0.519 Sympatric sites*fall*age 2 -0.197 0.179 0.097 -0.491 Sympatric sites*winter*age 2 -0.182 0.292 0.297 -0.661 Sympatric sites*spring*age 2 0.225 0.142 0.458 -0.009 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 491 400 pleopodal eggs (Gherardi 2006) compared to C. halli that produced on average 127 eggs. The density estimates of C. halli were significantly lower at sites where they were sympatric with C. englishi compared to those without C. englishi. Previous studies have suggested that the density of stream-dwelling macroinvertebrates is related to local stream habitat characteristics (Allan 1995), the types and amounts of nutrients (Allan 1995), and species interactions (Bovbjerg 1970). Of these factors, we believe that habitat characteristics were probably not responsible for the observed differences in C. halli density estimates for several reasons. First, the streams were physically similar with riffl e-pool-run sequences and large amounts of cobble and gravel substrate. Second, C. halli density estimates were lower across all mesohabitat types, except riffl e habitats at the sympatric sites. Third, all of the study sites were located relatively close together, had similar climatic conditions (e.g., temperature, precipitation), and fl owed through similar geologies, which suggests similar groundwater nutrient inputs. We believe that the differences in C. halli density could be due to differences in terrestrial nutrient inputs, species interactions, or a combination of these two factors. Identifying the exact mechanism, however, requires an understanding of how these factors infl uence the production of crayfish and other macroinvertebrates. Low-order (small) streams rely on allocthonous nutrient inputs for consumer communities; however, as the river broadens, energy inputs change, allowing for autochthonous production (Vannote et al. 1980). Because crayfish are processors of leaf litter and detritus (Griffith et al. 1994, Huryn and Wallace 1987), C. halli require some type of coarse particulate organic matter (CPOM) input—usually provided by streamside vegetation. Blalock Creek and Kiser Creek are lower order, smaller systems than the Tallapoosa River sites. The fact that low-order streams are typically associated with a higher CPOM resource base could, in part, be responsible for the higher density estimates at the allopatric Blalock and Kiser creeks. In addition to nutrient differences between sites, we believe interactions with C. englishi could be responsible for the lower density estimates at the sympatric sites. Crayfishes that use the same habitats are often limited to one or two species, and closely related species usually exhibit disjunct distributions (Rabeni 1985). Both C. halli and C. englishi reportedly prefer stream habitats, suggesting that these two species often interact and may compete for resources (Bouchard 1978, Freeman et al. 2003, Hobbs and Hall 1972). We found evidence that C. halli did alter their habitat use in the presence of C. englishi. Both species also use leaf litter and detritus as their food base (Hobbs and Hall 1972), which suggest that there is strong potential for competition of resources at the sympatric sites. Other crayfish species were also collected at the allopatric and sympatric study sites, but we believe that it was unlikely that they competed with C. halli. The differences in habitat preferences between C. halli and C. latimanus, C. striatus, and P. spiculifer account for the coexistence of these species at the study sites. Cambarus latimanus prefers small burrows in the stream bottom or between rocks alongside the stream (Yarbrough 1973), C. striatus are primary burrowers 492 Southeastern Naturalist Vol. 8, No. 3 that tend to remain in their burrows continuously (Hobbs 1981), and P. spiculifer are reported to be habitat generalists (Ratcliffe and Devries 2004), though we primarily found them in slow-fl owing, edge areas with vegetation. Interaction between C. halli, C. latimanus, C. striatus, and P. spiculifer was unlikely due to resource partitioning. The primary infl uence of C. englishi on C. halli appears to occur during early life-history stages. As expected, allopatric C. halli populations exhibited demographic patterns consistent with the life-history strategy of an r-selected species: many young-of-year were present shortly after hatching, followed by sharp decreases in estimated density (i.e., low survival) till age-1. Sympatric C. halli, however, exhibited a different pattern than expected with low estimated densities of hatchlings, followed by a slight decrease in estimated density till age-1. We hypothesize that the different patterns can be explained by two potential mechanisms: (1) reproduction of C. halli was lower at sympatric sites or (2) age-0 C. halli had a higher mortality rate before gear recruitment at the sympatric sites. The observed lower proportion of reproductively active adults at the sympatric sites is consistent with the first mechanism. Support for the second mechanism could not be evaluated with the existing data. However, the smaller decreases in estimated density from age-0 to age-2 at sympatric sites compared to allopatric sites suggests that sympatric age 0 individuals that were large enough to be collected generally had higher survival than their counterparts at the allopatric sites. Further, the greater growth rates of age-0 and age-1 C. halli observed at sympatric sites suggests that competition for resources (food) may have been lower than at allopatric sites. Both of these lines of evidence suggest that the mortality of age-0 crayfish that occurred before they were recruited to the sampling gear may not have been higher at sympatric sites. Clearly further study is needed to determine the mechanisms, and we recommend that these studies focus on reproductive dynamics and very early life stages when age-0 crayfish are smaller than 5 mm carapace length. The results of our study suggest that C. halli life history was similar to other stream-dwelling crayfish, with larger females focusing energy into the production of more eggs, rather than larger, higher-quality eggs. Hobbs (1981) reported that allopatric C. halli use all habitat types, but are primarily found in non-riffl e habitats when they occur with C. englishi. However, we found that C. halli preferred pools and runs over riffl e habitats at allopatric sites for all age classes. Differences in estimated density among sites were likely caused by differences in terrestrial nutrient inputs and by species interactions, though identifying the exact mechanism requires further research. For instance, the life history of C. englishi must be known, specifically its reproductive biology, growth, and habitat use, to determine the specific interactions occurring between C. halli and C. englishi. Because the counties containing the Tallapoosa River Basin will likely become part of Metro-Atlanta in the near future, understanding the ecological roles of both C. halli and C. englishi will be crucial for conservation of these species. Both species are listed as being “of special concern” by the Georgia Natural Heritage Program, and C. englishi is also on Georgia’s state 2009 S. Dennard, J.T. Peterson, and E.S. Hawthorne 493 list of protected species. Effective conservation strategies require knowledge of reproductive biology, reproductive timing, growth, and seasonal habitat use. This study adds to our understanding of C. halli life history and provides a starting point for the formation of conservation strategies. Acknowledgments We thank crew members N. Banish, S. Craven, A. 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