Biological Invasions

, Volume 11, Issue 3, pp 725–735

Does juvenile competition explain displacement of a native crayfish by an introduced crayfish?

Authors

  • Eric R. Larson
    • Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological SciencesUniversity of Arkansas
    • U.S. Geological Survey, Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological SciencesUniversity of Arkansas
Original Paper

DOI: 10.1007/s10530-008-9286-2

Cite this article as:
Larson, E.R. & Magoulick, D.D. Biol Invasions (2009) 11: 725. doi:10.1007/s10530-008-9286-2

Abstract

The coldwater crayfish Orconectes eupunctus is endemic to the Spring and Eleven Point Rivers of Arkansas and Missouri, and appears to have been displaced from a portion of its range by the recently introduced ringed crayfish Orconectes neglectus. We examined competition among juveniles as a potential mechanism for this crayfish species displacement through laboratory and field experiments. Orconectes eupunctus juveniles survived and grew in stream cages in their former range, implicating biotic interactions rather than habitat degradation in the displacement. Laboratory experiments revealed O. neglectus juveniles were dominant in the presence of limited food, whereas size rather than species determined occupancy of limited shelter. In a field competition experiment using stream cages, O. neglectus juveniles did not inhibit growth or reduce survival of O. eupunctus juveniles. Consequently, laboratory evidence of O. neglectus dominance did not correspond with competition under field conditions. Combined with previous studies examining the effects of O. neglectus on O. eupunctus, these results suggest that competition may not be a factor in this crayfish species displacement. Alternate mechanisms for the apparent displacement of O. eupunctus by O. neglectus, such as differential predation or reproductive interference, should be investigated.

Keywords

CompetitionCrayfishDominanceField experimentInvasive speciesJuvenileGrowth ratesLaboratory experiment

Introduction

The displacement of native crayfish by introduced crayfish is generally attributed to one, or a combination, of four mechanisms: competition, differential predation, reproductive interference or hybridization, and disease transmission (Lodge et al. 2000). Competition between invasive and native crayfish may be the most studied of these mechanisms, with invasive crayfish regularly documented to compete with native crayfish for food or shelter (Capelli and Munjal 1982; Garvey et al. 1994; Hill and Lodge 1999; Usio et al. 2001; Gherardi and Daniels 2004). However, many of these studies examine competition among sexually mature adult crayfish, with fewer studies on competitive interactions at the juvenile life history stage (but see: Söderbäck 1991, 1994; Rahm et al. 2005; Mazlum and Eversole 2005).

Competition among juvenile crayfish may be important in crayfish species displacements. In many streams, juvenile crayfish are the majority of crayfish populations throughout much of the year and have greater growth rates than adult crayfish (Muck et al. 2001; Evans-White et al. 2003). As a result of these growth rates, juvenile crayfish consume a high proportion of easily assimilated invertebrate or other animal matter (Momot 1995; Whitledge and Rabeni 1997). When access to animal matter is restricted, juvenile crayfish experience reduced growth and survival (Paglianti and Gherardi 2004). If invertebrates or other animal matter are a limited resource under natural conditions, competition for these food items among fast-growing juvenile crayfish could play an important role in displacement of native crayfish by invasive crayfish. Additionally, juvenile crayfish are vulnerable to predation by fish (Söderbäck 1994; Fortino and Creed 2007) and cannibalism due to their small size and frequent molt intervals (Mason 1970; Nyström 1994; Figler et al. 1999), and consequently competition for shelter may be important to survival at this life history stage.

The ringed crayfish Orconectes neglectus has been widely introduced within the United States. Native to eastern Colorado, Kansas, Nebraska, Oklahoma and western Arkansas and Missouri (Williams 1954), O. neglectus has become established in both the northwest and northeast United States (Bouchard 1977; Daniels et al. 2001), as well as in the Spring River drainage of eastern Arkansas and Missouri (Flinders and Magoulick 2005). The introduction of O. neglectus threatens an endemic crayfish species found in the Spring River drainage (Magoulick and DiStefano 2007). The coldwater crayfish Orconectes eupunctus was previously common in the West Fork and South Fork of the Spring River (Pflieger 1996), but is no longer collected from many sites where O. neglectus is now abundant (Rabalais and Magoulick 2006a). Orconectes eupunctus is recognized as imperiled by the Missouri Natural Heritage Program (2008) and threatened by the American Fisheries Society (Taylor et al. 2007). Understanding the mechanism or mechanisms behind the apparent displacement of O. eupunctus by O. neglectus is valuable for conservation of this endemic crayfish, and may have management implications for other introduced populations of O. neglectus.

Rabalais and Magoulick (2006b) examined competition among O. neglectus and O. eupunctus adult crayfish using field experiments. Orconectes eupunctus adults grew and survived in their former range, implicating biotic interactions rather than abiotic factors or habitat degradation in the apparent displacement, but O. neglectus did not inhibit the growth and survival of O. eupunctus when placed together in stream cages. However, growth rates of these adult crayfish were low and competition could not be dismissed as a factor in the apparent displacement. As previously described, juvenile crayfish exhibit high growth rates, are the majority of crayfish populations through much of the year, and may be vulnerable to limits on food or shelter. Consequently, we investigated the potential role of competition at the juvenile life history stage in the apparent displacement of O. eupunctus by O. neglectus.

We evaluated competition among O. neglectus and O. eupunctus juveniles using a combination of laboratory and field experiments. We confined O. eupunctus in stream cages in their former range to determine if these crayfish would grow and survive. We conducted laboratory experiments examining dominance and competition in the presence of limited food and shelter resources. We examined the impact of O. neglectus on O. eupunctus growth and survival by confining juveniles of both species in stream cages in the zone of overlap where O. neglectus and O. eupunctus occur together. These experiments were intended to determine if O. eupunctus juveniles would survive in their former range independent of interactions with O. neglectus juveniles, identify patterns of dominance and competitive ability among juveniles of these crayfish in a laboratory setting, and evaluate potential effects of O. neglectus on O. eupunctus juvenile growth and survival in the field.

Methods

We conducted field experiments examining the ability of juvenile O. eupunctus to grow and survive in their former range from July 19 through October 16 in 2005, and from July 12 through October 5 in 2006. In both years, we placed O. eupunctus in stream cages at an upstream site (Lat: 36° 30′ 54″, Long: 91° 51′ 21″) in the West Fork where these crayfish are no longer collected and at a downstream site (Lat: 36° 20′ 03″, Long: 91° 35′ 49″) in the lower South Fork where they remain abundant and O. neglectus are not collected. These sites were separated by approximately 67 river kilometers. We also placed O. neglectus juveniles in stream cages at the upstream site to compare growth rates between O. neglectus and O. eupunctus independent of competition. Orconectes neglectus juveniles were not placed at the downstream site to avoid the possibility of introducing this species beyond its current range in the Spring River drainage.

We used seven stream cages per species and site in 2005, and nine stream cages per species and site in 2006. Each stream cage contained a single juvenile crayfish. Stream cages were 40 cm long × 20 cm wide × 15 cm high, and were constructed of plexiglass with 3 mm plastic mesh on upstream and downstream ends of the cages. For substrate, we provided each stream cage with a single 9 cm × 20 cm brick paving tile and 250 ml volume of gravel and pebble from the stream bed. At both the upstream and downstream site, cages were placed in a single riffle because O. eupunctus and O. neglectus select these habitats in the Spring River drainage (Flinders and Magoulick 2005). We measured depth and current velocity at each cage on August 10, 2005 and August 5, 2006 and compared these habitat variables between sites with t-tests. Temperature was measured over the duration of the 2006 experiment using Onset Computer Corporation HOBO® temperature loggers placed in the vicinity of the upstream and downstream cages. We calculated cumulative degree days for the 2006 experiment by summing mean daily temperatures at each site. Cages were anchored to the substrate using concrete reinforcing rods, and the experiment was terminated in October of both years to prevent loss of cages to autumn floods.

We used male crayfish exclusively in all experiments to control for potential differences in foraging, growth, and behavior between male and female crayfish (Berrill and Arsenault 1984; Hanson et al. 1990). We collected crayfish for the juvenile growth experiments by kick-seining in July, with all juvenile O. eupunctus collected from the downstream site in the South Fork and all juvenile O. neglectus collected from the upstream site in the West Fork. At the initiation of the experiment, crayfish ranged in size from 12 to 14 mm carapace length (CL), the typical size range for O. eupunctus and O. neglectus juveniles at this time of the year (Larson 2007). Crayfish assignment to stream cages was completely randomized.

Carapace length to 0.1 mm and wet weight to 0.1 g were measured for all crayfish in the 2005 experiment on July 19, August 10, August 26, September 9, September 24, and October 16. In 2006, we made measurements on crayfish on July 12, July 26, August 11, August 24, September 7, September 21, and October 5. Within each year we compared growth, measured by CL and wet weight, between species and site using repeated measures ANOVA. The null hypothesis in these tests was no significant difference in growth between species or site.

In late August and early September of 2005 and 2006, we conducted laboratory experiments examining dominance of juvenile O. eupunctus and O. neglectus in the presence of limited food and shelter. These experiments consisted of a series of trials pairing O. eupunctus and O. neglectus individuals in small experimental arenas with the limited resource. For both experiments, we used juvenile crayfish between 12 and 17 mm CL. Crayfish with damaged appendages or that had recently molted were not used in experiments. For both experiments we used naïve crayfish, with O. eupunctus collected by kick-seining at the downstream site in the lower South Fork and O. neglectus collected by kick-seining at the upstream site in the West Fork. We used naïve crayfish to represent O. eupunctus first encountering introduced O. neglectus, and to minimize the possibility of a crayfish in a trial having disproportionate social experience with the opposing species. Previous social experience can influence crayfish behavior and patterns of dominance (Daws et al. 2002; Bergman and Moore 2005).

Crayfish were maintained under similar laboratory conditions prior to both experiments. We kept crayfish in 37.8 l aquaria under natural light at room temperature (24°C). Crayfish density did not exceed six crayfish per aquarium, and O. eupunctus and O. neglectus were held in separate aquaria prior to experimental trials. We fed crayfish flake fish food ad libitum. Prior to both experiments, crayfish were allowed to acclimate to laboratory conditions for approximately one week before initiation of experimental trials.

We conducted the experiment to determine crayfish dominance in the presence of limited food from August 17 to 25, 2005. We ran each trial by pairing a single O. eupunctus juvenile with a single O. neglectus juvenile in a round, plastic experimental arena with a diameter of 22 cm and a depth of 8 cm. Because size is often a factor in crayfish dominance (Bovbjerg 1953; Ranta and Lindström 1993; Vorburger and Ribi 1999), three crayfish size treatments were used: an O. eupunctus size advantage (> 2 mm CL difference), an O. neglectus size advantage (> 2 mm CL difference), and no difference in size between the two crayfish. We ran twenty trials with no size advantage and twelve trials each of O. eupunctus and O. neglectus size advantage. Experiments were conducted in the daytime between 0900 and 1700 h.

Following simultaneous introduction, crayfish were acclimated to the arena for two hours, after which a 1 cm fragment of fresh earthworm (Lumbricus sp.) was introduced. Prior to each trial, crayfish had been fasted for a minimum of 24 h. Twenty minutes after introduction of the earthworm, we examined the arena to determine if one of the two crayfish had claimed the earthworm. The ability to claim an earthworm or earthworm fragment in the presence of a potential competitor has been used by other investigators as a measure of crayfish dominance (Hill and Lodge 1999; Gherardi and Cioni 2004). In this experiment, a crayfish with the worm fragment following twenty minutes was recorded as the dominant crayfish or “winner.” If no crayfish had the worm, the trial was recorded as a tie. Results were analyzed for each size treatment by a chi square test comparing the proportion of wins by species to an equal distribution of wins. Ties were excluded from analysis.

We examined crayfish competition for limited shelter from August 29-September 12, 2006. The same experimental arenas were used as in the 2005 limited food experiments, but each arena contained a single 3 cm long × 1.5 cm high PVC half-tube shelter. As with the previous experiment, three size treatments were considered: O. eupunctus size advantage (>2 mm CL difference), O. neglectus size advantage (>2 mm CL difference), and no difference in size between the two crayfish species. Additionally, trials were conducted using O. eupunctus or O. neglectus alone to evaluate shelter occupancy in the absence of a competitor. We ran fourteen replicates of each of the five possible treatments. Crayfish were acclimated to the experimental arenas overnight and shelter occupancy was recorded hourly from 0900 to 1700 ho the following day. We used Mann–Whitney U-tests to examine differences in mean shelter occupancy between O. eupunctus and O. neglectus in the competition treatments, and to compare mean shelter occupancy by each species in the competition treatments to their mean shelter occupancy in the absence of a competitor. While shelters were intended to be small enough to prevent crayfish from sharing, some shelter sharing did occur in several trials. When shelter sharing occurred, shelter occupancy was recorded for both species and used in the above analysis.

Because dominance in crayfish is related to size (Bovbjerg 1953; Ranta and Lindström 1993; Vorburger and Ribi 1999), we anticipated that the larger crayfish would claim the earthworm fragment or occupy the shelter for the majority of the time regardless of species. Consequently, superior competitive ability by a species would be demonstrated by claiming the earthworm or occupying the shelter when at the same size or when at a size disadvantage relative to the opposing species.

We conducted the field experiment of competition between O. neglectus and O. eupunctus juveniles from July 12 through October 6, 2006. Competition between these species was evaluated using stream cages in a single riffle (Lat: 36° 27′ 23″, Long: 91° 51′ 48″) in the zone of overlap where O. eupunctus and O. neglectus occur together in the South Fork. We compared interspecific competition between O. eupunctus and O. neglectus relative to intraspecific competition within O. eupunctus using three treatments: three O. eupunctus per cage, six O. eupunctus per cage, and three O. eupunctus and three O. neglectus per cage. Each of the three treatments was replicated ten times and all crayfish were 12–14 mm CL at the initiation of the experiment. Stream cages were 0.5 m wide by 0.5 m long by 0.15 m high, and were supplied with three 9 cm × 20 cm brick paving tiles and 500 ml volume of gravel and pebble from the streambed as substrate. Cages were wood frames covered with 3 mm plastic mesh. Juvenile crayfish density within these stream cages (12–24 crayfish m2) fell within the observed range for the Spring River drainage (Flinders and Magoulick 2005), but was generally higher than mean O. eupunctus and O. neglectus densities in zone of overlap riffles, which ranged from 1–12 crayfish m2 (Rabalais and Magoulick 2006a). Cages were placed mid-riffle on a 3 cage wide × 10 cage long grid with treatment randomized within the grid. Prior to introduction of crayfish, cages were left in the stream for a week for colonization of food sources.

Following introduction of crayfish to stream cages, we measured CL and wet weight on July 26, August 10, August 25, September 8, September 22, and October 6. Dead or missing crayfish were recorded and replaced with similar-sized crayfish of the same species to maintain experimental densities. Missing crayfish were assumed to be mortalities that had been cannibalized, as crayfish of these sizes did not escape from cages in either preliminary experiments or the experiment of O. eupunctus growth in its former range. Replacement crayfish were marked with a uropod clip to differentiate them from original crayfish. Only those crayfish that were originally stocked were used for mean CL, mean wet weight and statistical tests. We tested for differences in growth of O. eupunctus by treatment using repeated-measures ANOVA on CL and wet weight, and we tested for differences in total mean percent mortality of O. eupunctus by treatment using a Kruskal–Wallis nonparametric ANOVA. Evidence of O. neglectus interspecific competition with O. eupunctus would require statistically significant reduced growth or survival of O. eupunctus relative to the high density O. eupunctus treatment (i.e., does interspecific competition with O. neglectus exceed intraspecific competition?). All analyses were performed in SYSTAT with significance determined at α = 0.05.

Results

In both 2005 and 2006, O. eupunctus juveniles grew in their former range (Fig. 1). In 2005, there was no significant difference between upstream and downstream O. eupunctus growth as measured by either CL (F = 0.520, P = 0.760) or wet weight (F = 1.125, P = 0.358). Additionally, there was no significant difference between O. eupunctus and O. neglectus growth at the upstream site by CL (F = 1.343, P = 0.260), but O. eupunctus juveniles grew more by weight than O. neglectus juveniles (F = 3.027, P = 0.017) in 2005. In 2006, O. eupunctus at the upstream site grew larger by both CL (F = 23.857, P < 0.001) and wet weight (F = 33.695, P < 0.001) than O. eupunctus at the downstream site. Orconectes eupunctus and O. neglectus did not differ signficantly at the upstream site in either CL (F = 0.833, P = 0.548) or wet weight (F = 0.639, P = 0.699) in 2006.
https://static-content.springer.com/image/art%3A10.1007%2Fs10530-008-9286-2/MediaObjects/10530_2008_9286_Fig1_HTML.gif
Fig. 1

Mean (SE) CL by date for juvenile O. eupunctus and O. neglectus crayfish at upstream West Fork and downstream South Fork stream cages from summer 2005 (top) and summer 2006 (bottom). A single O. eupunctus juvenile died at the upstream site in 2005 and a single O. eupunctus juvenile died at the downstream site in 2006, and these mortalities were omitted from repeated-measures ANOVA. Growth by weight followed the same general pattern as CL, although upstream O. eupunctus weighed significantly more than upstream O. neglectus in 2005 (F = 3.027, P = 0.017)

Stream cages at the downstream site were in significantly deeper (t = –9.008, P < 0.001) and faster flowing (t = −4.727, P < 0.001) water than upstream cages in 2005, and deeper water (t = −12.530, P < 0.001) in 2006. There was no significant difference in current velocity between upstream and downstream cages in 2006 (t = −1.156, P = 0.264). Mean depth at the downstream cages was 0.20 m (±0.03SD) in 2005 and 0.19 m (±0.02 SD) in 2006. Mean depth at the upstream cages was 0.11 m (±0.02 SD) in 2005 and 0.10 m (±0.01) SD in 2006. Mean current velocity at the downstream cages was 0.41 m/s (±0.11 SD) in 2005 and 0.34 m/s (±0.06 SD) in 2006. Mean current velocity at the upstream cages was 0.19 m/s (±0.10 SD) in 2005 and 0.29 m/s (±0.11 SD) in 2006. Additionally, stream temperature differed between the downstream and upstream site in 2006. Cumulative degree days were 2,048°C for the upstream site and 1,787°C for the downstream site, and the upstream site generally exhibited a greater range in daily temperature.

In the laboratory experiment examining dominance in the presence of limited food, O. neglectus was significantly more likely to claim the worm when holding a size advantage or when sizes were equal between the two species, and neither crayfish species was significantly more likely to claim the worm when O. eupunctus had a size advantage (Table 1). Orconectes eupunctus never claimed the worm when sizes were equal between the two species or when O. neglectus held a size advantage, while O. neglectus was able to claim the worm in approximately half of non-tie trials where O. eupunctus held a size advantage.
Table 1

Results of 2005 laboratory experiment of competition for limited food with wins, χ2 values, and P-values for O. neglectus size advantage, O. eupunctus size advantage, and no size difference treatments

 

O. neglectus advantage

No advantage

O. eupunctus advantage

O. neglectus w/worm

8

11

3

O. eupunctus w/worm

0

0

4

χ2

8.00

11.00

0.14

P-Value

0.005

0.001

0.708

Neither w/worm

4

9

5

Total trials

12

20

12

In the laboratory experiment of competition for limited shelter, the larger crayfish was dominant as measured by shelter occupancy regardless of species (Fig. 2). In the O. neglectus size advantage treatment, mean shelter occupancy by O. eupunctus was significantly reduced relative to shelter occupancy in the absence of a competitor (Mann–Whitney test; U = 170.0, P = 0.001). This was also the only treatment of the competition trials where mean shelter occupancy by O. neglectus was significantly greater than mean shelter occupancy by O. eupunctus (U = 26.5, P = 0.001). In the equal size (U = 75.0, P = 0.280) and O. eupunctus size advantage (U = 86.5, P = 0.588) treatments, O. eupunctus shelter occupancy did not differ significantly from that of O. eupunctus alone. Similarly, mean shelter occupancy by O. neglectus was only significantly less than shelter occupancy in the absence of a competitor in the treatment where O. eupunctus held a size advantage (U = 34.0, P = 0.003). This was the only treatment of the competition trials where mean shelter occupancy by O. eupunctus was significantly greater than mean shelter occupancy by O. neglectus (U = 168.0, P = 0.001). In the equal size (U = 66.0, P = 0.131) and O. neglectus size advantage (U = 109.5, P = 0.591) treatments, O. neglectus shelter occupancy did not differ significantly from that of O. neglectus alone. Where size between O. eupunctus and O. neglectus was equal, there was no significant difference in mean shelter occupancy between the two species (U = 126.0, P = 0.178).
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Fig. 2

Results of 2006 laboratory experiment of competition for limited shelter with mean (SE) shelter occupancy by O. eupunctus and O. neglectus juveniles in five treatments: O. eupunctus alone, O. eupunctus size advantage, no size advantage, O. neglectus size advantage, and O. neglectus alone. Significance in shelter occupancy between O. neglectus and O. eupunctus where together and relative to each crayfish alone noted by asterix

In the field competition experiment, O. neglectus did not reduce O. eupunctus growth relative to either the low density or high density O. eupunctus treatments (Fig. 3). There was no significant difference in O. eupunctus growth between treatments as measured by either carapace length (F = 0.647, P = 0.799) or weight (F = 0.712, P = 0.738). There was a significant difference in total O. eupunctus percent mortality between treatments (K-W = 6.246, P = 0.044). However, O. eupunctus mortality was highest in the 6 O. eupunctus treatment (23%) and lowest in the 3 O. eupunctus and 3 O. neglectus treatment (10%).
https://static-content.springer.com/image/art%3A10.1007%2Fs10530-008-9286-2/MediaObjects/10530_2008_9286_Fig3_HTML.gif
Fig. 3

Mean (SE) CL by date for O. eupunctus juveniles from 2006 stream cage competition experiments in the South Fork Spring River. Growth by weight followed the same general pattern as CL

Discussion

Our results suggest that the displacement of O. eupunctus from its former range in the upper South Fork Spring River is not explained by competition at the juvenile life history stage with the introduced crayfish O. neglectus. Orconectes eupunctus juveniles grew and survived in their former range, implicating biotic rather than abiotic factors in the displacement, and O. neglectus juveniles were dominant over O. eupunctus juveniles in the presence of limited food under laboratory conditions. However, other investigators have urged discretion in translating laboratory behavioral experiments to crayfish interactions under natural conditions (Capelli and Munjal 1982; Bergman and Moore 2003; Gherardi and Daniels 2004). This is relevant to our results, as O. neglectus juveniles did not impair growth or survival of O. eupunctus juveniles in stream cages. Combined with previous studies examining competition between O. eupunctus and O. neglectus (Rabalais and Magoulick 2006a, b), our results suggest that a mechanism other than competition with the introduced crayfish O. neglectus is responsible for the displacement of O. eupunctus from its former range.

Verification of O. eupunctus growth and survival in its former range is important because habitat degradation has been a factor in population declines of other crayfish species (U.S. Department of the Interior Fish and Wildlife Service 1986). The Spring River drainage is predominantly agricultural pasture and oak (Quercus sp.) and hickory (Carya sp.) forest with no urban areas, and it seems likely that stream water quality has changed little since the mid 1980s when O. eupunctus was abundant in the West Fork and upper South Fork. We observed robust growth of O. eupunctus juveniles at the upstream West Fork site relative to O. neglectus juveniles and O. eupunctus juveniles in their remaining range at the downstream South Fork site. Rabalais and Magoulick (2006b) also found growth and survival of O. eupunctus adults confined in stream cages in the West Fork. As a result, the introduction and spread of O. neglectus remains the suspected cause of the observed O. eupunctus range contraction.

In 2006, O. eupunctus juveniles grew larger by both CL and wet weight at the upstream West Fork site than at the downstream South Fork site. Rabalais and Magoulick (2006b) also observed inter-annual variation in crayfish growth rates, with poor growth by weight observed in adult male O. eupunctus at the upstream West Fork site in 2003 but not in 2002. Variation in stream invertebrate growth rates has been related to factors including temperature and food availability (Morin and Dumont 1994; Frutiger and Buergisser 2002). Inter-annual variation of these factors may be responsible for observed differences in juvenile crayfish growth rates between 2005 and 2006. We did not measure food resource availability at study sites. Water temperatures were greater at the upstream study site in 2006 than at the downstream study site, but temperature data was not available from 2005. Additionally, depth and current velocity was not consistent between cages at the upstream and downstream study sites due to innate habitat differences between the smaller, shallower West Fork and the larger, deeper South Fork. Stream size and permanence has been documented to be a factor in crayfish distributions in the Spring River drainage (Flinders and Magoulick 2003). While the difference in O. eupunctus juvenile growth rates between 2005 and 2006 is interesting, growth of O. eupunctus juveniles matched growth of O. neglectus juveniles at the upstream site in both years and exceeded growth of O. eupunctus at the downstream site in 2006, indicating that the upstream West Fork site remains suitable habitat for O. eupunctus juveniles.

Our laboratory trials of dominance and competition provided conflicting results depending on which limited resource was considered. Crayfish size rather than species determined shelter occupancy, and consequently neither crayfish species was dominant in the presence of limited shelter. If these results are consistent with behavior in the wild, we would not expect O. neglectus juveniles to exclude like-sized O. eupunctus juveniles from shelter. However, crayfish that hatch earlier in the year or have greater juvenile growth rates may achieve a size advantage with implications for dominance interactions (Rabeni 1985). Life history sampling for O. neglectus and O. eupunctus in 2005 and 2006 did not identify differences in reproductive timing or juvenile growth rates that would provide juveniles of either crayfish species with a size advantage (Larson 2007). In the limited food experiment, O. neglectus always claimed the worm in trials with a “winner” when either holding a size advantage or when both crayfish were equal size. Orconectes eupunctus was unable to claim the worm in significantly more trials than O. neglectus while holding a size advantage. These results may indicate dominance of O. neglectus over O. eupunctus in the presence of limited food, which could have important implications for the growth and survival of O. eupunctus juveniles where they are sympatric with O. neglectus. Other researchers have found similar laboratory dominance trials to correspond with fitness consequence for crayfish in mesocosm experiments and natural populations (Garvey et al. 1994; Hill and Lodge 1999).

However, crayfish dominance behavior in the laboratory does not always translate into competition under more natural conditions. Capelli and Munjal (1982) observed dominance of the invasive crayfish Orconectes rusticus over the native crayfishes Orconectes virilis and Orconectes propinquus for limited shelter in the laboratory, but O. rusticus did not displace O. virilis and O. propinquus from preferred substrate in mesocosms. Bergman and Moore (2003) found that general patterns of crayfish agonistic behavior were similar between the laboratory and the field, but that fights over limited resources in the wild were shorter, less intense, and less likely to result in behaviors that increased crayfish vulnerability to predation. Additionally, crayfish detection of food can be influenced by water flow and hydrodynamics (Moore and Grills 1999; Keller et al. 2001). Both O. eupunctus and O. neglectus are riffle and run-dwelling crayfish in the Spring River drainage (Flinders and Magoulick 2005), but our experimental arenas lacked water circulation or directional flow. Finally, Willman et al. (1994) found the invasive crayfish O. rusticus was more likely to respond to odor, whether from food or predators, than native crayfish, and consequently attributed superior olfaction to the invasive species. While the ability of O. neglectus juveniles to claim the worm fragment more frequently than O. eupunctus juveniles in my experiments could demonstrate dominance, it could also be caused by better acclimation of O. neglectus to artificial laboratory conditions or superior olfaction by this species.

Regardless of why O. neglectus juveniles claimed limited food more frequently in the laboratory than O. eupunctus juveniles, O. neglectus juveniles did not impair growth or survival of O. eupunctus juveniles under more natural conditions in stream cages. There were no significant intraspecific or interspecific effects on growth of O. eupunctus juveniles in stream cages. Additionally, intraspecific effects on mortality of O. eupunctus juveniles were greater than interspecific effects. These results suggest that juvenile crayfish were not food-limited in the Spring River drainage at natural densities, and that competition for food between O. neglectus and O. eupunctus juveniles may not be a factor in the observed O. eupunctus range contraction. While enclosure experiments have well-documented limitations and artifacts, many of these artifacts, such as high densities and restricted movement of study organisms, are generally suspected to over-state evidence of competition (Gurevitch et al. 1992; Englund and Olsson 1996).

Other studies examining competition at the juvenile life history stage as a mechanism for crayfish species displacement have found results ranging from major to no significance. Mazlum and Eversole (2005) found that the rapid displacement of Procambarus clarkii by Procambarus acutus acutus in aquaculture ponds was attributable to competition among juveniles, with P. acutus acutus growing faster and experiencing less mortality than P. clarkii. Söderbäck (1994) found interspecific competition for shelter at the predation-vulnerable juvenile life history stage contributed to the displacement of the native crayfish Astacus astacus by the invader P. leniusculus in Swedish lakes. In an earlier study, Söderbäck (1991) found juvenile crayfish were as aggressive as adults, with the invasive crayfish P. leniusculus dominant over A. astacus. However, Rahm et al. (2005), working on the St. Francis River of Missouri, did not find juveniles of the invasive crayfish Orconectes hylas to be behaviorally dominant over the native crayfishes Orconectes quadruncus and Orconectes peruncus under laboratory conditions. Although not studying invasive crayfish, Fortino and Creed (2007) found no evidence of competition for food among juvenile crayfish in an Appalachian stream, and instead attributed distributional patterns of two crayfish species to differential juvenile susceptability to predation. As with competition between adult crayfish, competition at the juvenile life history stage is a potentially important mechanism in crayfish species displacements, but may not be relevant in all displacements. However, the fast-growing, abundant, and predation and cannibalism-vulnerable juvenile life history stage should not be neglected when examining competition as a potential mechanism of native crayfish displacement.

When considered in the context of previous research, our results indicate that competition is not an important factor in the apparent displacement of O. eupunctus by O. neglectus from the West Fork and upper South Fork Spring River. Not only did Rabalais and Magoulick (2006b) find no evidence of competition among adults of these two species using stream cages, but field sampling at the upstream, zone of overlap, and downstream study sites found that O. neglectus did not displace O. eupunctus from its preferred habitats at either the juvenile or adult life history stage (Rabalais and Magoulick 2006a). Similarly, a life history study of these two species found growth of O. eupunctus juvenile crayfish was comparable between zone of overlap and downstream sites, and that O. neglectus juveniles did not hatch earlier in the year, grow faster, or hold a size advantage over O. eupunctus juveniles (Larson 2007). Although competition cannot be altogether dismissed, these studies cumulatively suggest that a mechanism other than competition with O. neglectus is responsible for the O. eupunctus range contraction in the Spring River drainage.

Future investigations into the role of O. neglectus in the apparent displacement of O. eupunctus may focus on mechanisms such as differential predation, reproductive interference and hybridization (Butler and Stein 1985; Garvey et al. 1994; Perry et al. 2001). Identifying mechanisms responsible for the apparent displacement of O. eupunctus by O. neglectus is important to the conservation of endemic Spring River crayfish, and may also be informative for researchers and managers interested in other introduced populations of O. neglectus. While not as prominent an invader as O. rusticus or P. clarkii, the presence of O. neglectus in the northwest and northeast United States may represent an emerging invasive species with the potential to impact native aquatic organisms, as it has apparently done in the Spring River drainage of Arkansas and Missouri.

Acknowledgements

This research was supported by a grant from the Arkansas Game and Fish Commission. Comments from A.V. Brown, R.J. DiStefano, G.R. Huxel, D.M. Lodge and two anonymous reviewers improved the quality of this manuscript. We thank all landowners who provided river access during this study. The Arkansas Cooperative Fish and Wildlife Research Unit is supported by the Arkansas Game and Fish Commission, University of Arkansas, U.S. Geological Survey, and the Wildlife Management Institute.

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© Springer Science+Business Media B.V. 2008