Movement, dispersal and refuge use of co-occurring introduced and native crayfish

1. Dispersal and habitat use are key elements in determining impacts of introduced species. We examined if an invasive crayfish species showed a different pattern of movement, dispersal and refuge use to that of a species that it displaces. 2. Fifteen adults of the introduced American signal crayfish, Pacifastacus leniusculus and 20 adult native white‐clawed crayfish Austropotamobius pallipes were radiotagged in an area of river where in 2003 they co‐occurred and the spatial behaviour and habitat use of the two species were examined. Subsequent surveys at the study site in 2005 recorded no A. pallipes and the density of P. leniusculus had increased. 3. Clear differences in the spatial behaviour of the two species were found. The median distance moved per day by signal crayfish was over twice that of white‐clawed crayfish, and dispersal from release locations was higher in signal crayfish. A similar range of refuges with a high degree of overlap was used by both species suggesting that the potential for competitive interaction between the two species exists. 4. The greater movement and dispersal by signal crayfish recorded here supports the hypothesis that invaders are better dispersers than the species that they displace. In addition, movements by signal crayfish may allow them to make better use of patchy resources than white‐clawed crayfish and this, coupled with similar microhabitat use, may contribute to the observed replacement.     

Invasion of the shelter snatchers: behavioural plasticity in invasive red swamp crayfish,Procambarus clarkii

1. In many freshwater systems, competition for shelter plays an important role in determining the persistence of both native and alien species. The red swamp crayfish, Procambarus clarkii, is currently invading the native habitat of the signal crayfish, Pacifastacus leniusculus, in southern Oregon, and interspecific competition for shelter may be driving the species replacement in this region. 2. We designed a 2 × 3 factorial mesocosm experiment, with shelter density and species combination as factors, to investigate shelter occupancy and resource competition. Contrary to our predictions, the two crayfish species are equal competitors for shelter. Further, the invasive P. clarkii modified its shelter occupancy behaviour in the presence of the native P. leniusculus and has broader microhabitat preferences. 3. Specifically, we found that P. clarkii alters shelter occupancy and space use patterns when the two species occurred together, such that shelter use was identical between P. clarkii and P. leniusculus in mixed‐species treatments. In such treatments, both species increased their use of shelters when shelter density increased. When P. clarkii was alone, however, individuals did not alter shelter use as a function of shelter density, whereas P. leniusculus exhibited similar density‐dependent behaviour in both mixed‐ and single‐species treatments. 4. In a complementary field survey, we employed an ‘epicentre‐based’ design to sample two field sites. We observed patterns of microhabitat use and breadth for each species similar to those in our mesocosm experiment: the invasive P. clarkii was more abundant across different habitats and used a broader range of microhabitats than the native P. leniusculus. As such, we found that P. clarkii was more abundant across both field sites than the P. leniusculus, occupying microhabitats within and beyond the preferred range of P. leniusculus. Both field sites were affected by urban development and agriculture. 5. The use of microhabitats by both species was similar in the laboratory and the field. This study confirms that P. clarkii individuals can, and do, successfully occupy microhabitats preferred by P. leniusculus in the Willamette Valley. The results from our study may be relevant to other freshwater systems inhabited by P. clarkii and contribute to the understanding of ‘niche opportunity’, a concept which defines the environmental conditions that promote biological invasions.     

Movement and dispersal of the invasive signal crayfish Pacifastacus leniusculus in upland rivers

1. The American signal crayfish Pacifastacus leniusculus, an invasive species widely introduced throughout Europe, is a major threat to native European crayfish species and is causing increasing concern because of its wide impact on aquatic ecosystems. 2. Whilst various control and management methods have been proposed, very little is known about the factors influencing dispersal and movements of signal crayfish. 3. Sixty‐four adult signal crayfish (carapace length 31.9–63.8 mm) were radiotagged in upland rivers in northern England, during four periods. Tracking was carried out at two sites, a low‐density establishing population and a high‐density established population. Tracking was carried out at both sites concurrently during midsummer (June to August 2002), during late summer (August to September 2001) at the low‐density population site and during autumn to winter (October to February 2000/01) at the high‐density population site. 4. Maximum movement occurred during midsummer. Temperature appeared to be a major factor influencing the timing and extent of movements between tracking periods. 5. The frequency distribution of the maximum distance moved upstream and downstream by radiotagged crayfish showed an inverse power relationship. The median maximal upstream and downstream distances moved were 13.5 m (range 0–283 m) and 15 m (range 0–417 m), respectively. There was a significant difference between the distributions of upstream and downstream ranges, with greater distances moved downstream. 6. All downstream movements made by crayfish appeared to be active movements and not the result of passive movement during periods of high discharge. There was no apparent influence of size, sex or density on the amount of movement recorded. 7. The study provides important information on the spatial and temporal behaviour of introduced crayfish in upland lotic systems. In contrast to lowland rivers, our results suggest that flow or gradient may influence the invasive potential of signal crayfish in an upstream direction in upland rivers.     

The signal crayfish is not a single species: cryptic diversity and invasions in the Pacific Northwest range of Pacifastacus leniusculus

1. We used historical sources, morphology‐based taxonomy and mtDNA sequence data to address questions about the signal crayfish Pacifastacus leniusculus. These included evaluating unrecognised cryptic diversity and investigating the extent to which P. leniusculus may have been introduced within its presumed native range in the Pacific Northwest region of North America. Our study builds and expands on Pacific Northwest phylogeographic knowledge, particularly related to patterns of glacial refugia for freshwater species. 2. Extensive collections (824 specimens) from British Columbia (Canada), Idaho, Nevada, Oregon and Washington (United States) were used to characterise P. leniusculus at the mitochondrial 16S rRNA gene. Genetic groups within the species were elucidated by phylogenetics and amova ; evolutionary relationships within the most common and diverse group were investigated using a statistical parsimony haplotype network, a nested amova , and tests of isolation by distance. Morphological measurements were used to relate findings of molecular analyses to three historically recognised P. leniusculus subspecies and characterise cryptic diversity by morphology. 3. We found substantial cryptic diversity, with three groups highly distinct from P. leniusculus in discrete geographic regions: the Chehalis River glacial refugium, Central Oregon and the Okanagan Plateau. Disjunct distributions of P. leniusculus relative to these cryptic groups and known patterns of Pleistocene glaciation and landscape evolution cast doubt on whether P. leniusculus is native to some areas such as coastal drainages of northern Washington and southern British Columbia. Morphological traits previously used to characterise P. leniusculus subspecies still persist but may be incapable of distinguishing P. leniusculus from newly discovered cryptic groups. 4. Cryptic diversity found within P. leniusculus highlights the pressing need for a thorough investigation of the genus Pacifastacus using data based on more extensive gene and taxon sampling. It also warrants conservation attention, as introductions of P. leniusculus within the Pacific Northwest may carry risks of hybridisation and introgression for cryptic groups. Owing to high genetic diversity and limited dispersal capacity relative to more vagile organisms like freshwater fish, crayfish of the genus Pacifastacus offer powerful potential insights into the geological history and phylogeography of the Pacific Northwest region. Finally, by shedding light on the long‐neglected native range of P. leniusculus, our results should also better inform our understanding of potential source populations for, and the ecology of, this important invasive species in regions including Europe, Japan and elsewhere in North America.     

Success and failure in a lotic crayfish invasion: the roles of hydrologic variability and habitat alteration

1. This paper examines the distribution, habitat relationships, and potential for spread of non‐native signal crayfish (Pacifastacus leniusculus) in streams of the Truckee River catchment, California, U.S.A. Crayfish associations with natural features and with impoundments and flow alteration were examined in a survey of 33 streams. Abundance changes were followed over 5 years, which included some of the highest and lowest flows on record, in three streams, two unregulated and one regulated. Movement of marked crayfish was studied in one 0.5 km stream reach just upstream from a reservoir. 2. Signal crayfish were most abundant in low‐gradient streams and were positively associated with proximity to reservoirs (both upstream and downstream). Crayfish were more likely to be found in regulated than unregulated sites, and did not occur in sites upstream of barriers, such as culverts, that separated them from reservoirs or lakes. 3. Crayfish declined in abundance in years following particularly intense and prolonged wet‐season spates, leading to a negative association between crayfish abundance and both peak discharge and duration of bankfull flows. 4. Crayfish moved distances of up to 277 m, and at rates of up to 120 m day^?1, suggesting significant dispersal ability. Larger crayfish moved greater distances and were more likely to move downstream. Female crayfish showed a pattern of upstream movement in early summer and downstream movement late in the summer, opposite the pattern found in two other studies. 5. These results suggest that natural or artificial gradient barriers and, in regulated systems, management of flow regimes to include bankfull or greater flows may help to control invasive crayfish in streams.     

Behavioral effects of invaders: alien crayfish and native sculpin in a California stream

Invasive crayfish have been shown to have negative impacts on a range of taxa, though the mechanisms for those effects have not always been evaluated. Signal crayfish (Pacifastacus leniusculus) in Sagehen Creek were associated with reduced growth rates and gut fullness of Paiute sculpin (Cottus beldingi) in earlier experiments. This paper assesses potential behavioral mechanisms of competition between the two species. I conducted experiments to determine crayfish effects on sculpin behavior and habitat use in a stream observation facility at the Sagehen Research Station, California, USA. Sculpin reduced their use of refuges and pools, shifted into higher-velocity microhabitats, and spent more time fleeing in the presence of crayfish. Crayfish used refuges, pools, and low-velocity habitats more than sculpin in either treatment. Both species were notably nocturnal, with most activity at dusk and night observations, although crayfish were more strongly so than sculpin. Detailed field surveys of lower Sagehen Creek found that potential refuges (unembedded rocks) were closely associated with total crayfish and sculpin numbers, suggesting that cover is at least sometimes limiting under natural conditions. By displacing sculpin from refuges and pools and increasing their activity rate, crayfish may increase the likelihood of predation on sculpin. Behavioral shifts in sculpin appear to be at least partly responsible for the reduced growth rates of sculpin in the presence of crayfish.     

Shelter competition between resident male red swamp crayfish Procambarus clarkii (Girard) and conspecific intruders varying by sex and reproductive status

The shelter defense dynamics of reproductively active (Form I) male red swamp crayfish Procambarus clarkii (Girard) were investigated by staging serial intrusions by male, maternal or non-maternal female conspecifics after one day of residence. The male residents showed a direct contest advantage only against non-maternal females, and won a significantly higher proportion of the encounters against non-maternal females than against either male or maternal female intruders. However, there was no significant difference in success against male or maternal female intruders. Further experiments against male intruders showed that increasing male prior residence to either 2 or 4 days did not significantly improve the residents' proportion of successful encounters. A final experiment revealed extremely low shelter fidelity in male P. clarkii, the most likely reason why male shelter defense against conspecific males and maternal females is not successful. These results, combined with those of past research, suggest that non-maternal females, juveniles and recently molted conspecifics are most vulnerable to predation and cannibalism, since males and maternal females easily out-compete non-maternal females for shelter, and adults out-compete juveniles. These findings may have implications for P. clarkii aquaculture and general management.     

Apparent competition between parasitoids mediated by a shared hyperparasitoid

Cocoons of the specialist parasitoid Cotesia melitaearum , which attacks the Glanville fritillary butterfly in the Åland islands of SW Finland, are parasitized by the generalist hyperparasitoid Gelis agilis . We added experimentally to the system a second host species for G. agilis , C. glomerata , with which C. melitaearum does not compete for resources. After the one-time addition of the second parasitoid the natural populations of C. melitaearum declined in the treatment, as predicted by the apparent competition theory.     

Differences in aggression,activity and boldness between native and introduced populations of an invasive crayfish

Aggressiveness, along with foraging voracity and boldness, are key behavioral mechanisms underlying the competitive displacement and invasion success of exotic species. However, do aggressiveness, voracity and boldness of the invader depend on the presence of an ecologically similar native competitor in the invaded community? We conducted four behavioral assays to compare aggression, foraging voracity, threat response and boldness to forage under predation risk of multiple populations of exotic signal crayfish Pacifastacus leniusculus across its native and invaded range with and without a native congener, the Shasta crayfish P. fortis. We predicted that signal crayfish from the invaded range and sympatric with a native congener (IRS) should be more aggressive to outcompete a close competitor than populations from the native range (NR) or invaded range and allopatric to a native congener (IRA). Furthermore, we predicted that IRS populations of signal crayfish should be more voracious, but less bold to forage under predation risk since native predators and prey likely possess appropriate behavioral responses to the invader. Contrary to our predictions, results indicated that IRA signal crayfish were more aggressive towards conspecifics and more voracious and active foragers, yet also bolder to forage under predation risk in comparison to NR and IRS populations, which did not differ in behavior. Higher aggression/voracity/boldness was positively correlated with prey consumption rates, and hence potential impacts on prey. We suggest that the positive correlations between aggression/voracity/boldness are the result of an overall aggression syndrome. Results of stream surveys indicated that IRA streams have significantly lower prey biomass than in IRS streams, which may drive invading signal crayfish to be more aggressive/voracious/bold to acquire resources to establish a population.     

Competition for nitrogen between Australian native grasses and the introduced weed Nassella trichotoma

BACKGROUND: and Aims Nassella trichotoma is an unpalatable perennial grass weed that invades disturbed native grasslands in temperate regions of south-eastern Australia. This experiment investigated whether elevated N levels, often associated with disturbance, increases the competitiveness of N. trichotoma relative to C3 and C4 native Australian grasses. METHODS: A pot experiment investigated competitive interactions between four native grasses, two C3 species (Microlaena stipoides and Austrodanthonia racemosa) and two C4 species (Themeda australis and Bothriochloa macra), and N. trichotoma at three different N levels (equivalent to 0, 60 and 120 kg ha-1) and three competing densities (zero, one and eight neighbouring plants), using an additive design. KEY RESULTS: All native grasses were competitive with N. trichotoma at low N levels, but only M. stipoides was competitive at high N. High densities of native grasses (8:1) had a major competitive effect on N. trichotoma at all N levels. The competitive ranking of native grasses, across all N levels, on N. trichotoma was: M. stipoides>A. racemosa>B. macra>T. australis. The C3 species were generally more competitive than the C4 species and C4 grasses were not inherently more productive at low N levels, in contrast to the results of other studies. CONCLUSION: To resist invasion from N. trichotoma, these native grasses need to be maintained at a high density and/or biomass. The results do not support the theory that species such as N. trichotoma, with high tissues density, are always less competitive than those of low tissue density; in this case competitiveness depended on N levels. The ability of N. trichotoma to accumulate biomass at a higher rate than these native grasses, helps to explain why it is a major weed in disturbed Australian native grasslands.     

Competition for space between introduced brown trout (Salmo trutta L.) and a native galaxiid (Galaxias vulgaris Stokell) in a New Zealand stream

Increasing circumstantial evidence indicates that the introduction of brown trout ( Salmo trutta L.) to New Zealand has caused a widespread decline in native fish populations but few of the underlying mechanisms have been investigated. The possibility of spatial competition was investigated by comparing the microhabitat used by native Galaxias vulgaris Stokell (Family Galaxiidae) that were sympatric and allopatric with brown trout. A range of microhabitat variables was measured from random locations where G. vulgaris were present in the Shag River during the day. G. vulgaris preferred coarse substrates, using them as resting places, but showed no other microhabitat preferences. This pattern of microhabitat use did not change in the presence of brown trout although galaxiid densities were considerably lower. Experiments in in situ stream channels confirmed that competition for space does not occur during the day even at high galaxiid densities. This situation changed dramatically at night, however, with G. vulgaris spending significantly more time in slower areas when trout were present. G. vulgaris feeds on drifting invertebrates, so brown trout could affect the galaxiids deleteriously by forcing them to occupy less profitable feeding positions. Interspecific competition for space, perhaps combined with competition for food and predation by trout, could explain declines in G. vulgaris populations.     

Competition for food between an introduced crayfish and two fishes endemic to the Colorado River basin

Synopsis Crayfish are not native to the Colorado River basin (CRB), however they are now established in portions of the mainstem and in many tributaries. I used density manipulation experiments in a laboratory setting to determine intra- and interspecific competition for food between Orconectes virilis, an aggressive polytrophic crayfish now common in the CRB, and two native fishes: Gila chub, Gila intermedia, and flannelmouth sucker, Catostomus latipinnis. I tested each fish species in separate trials. Growth of Gila chub decreased when animal densities increased, however they were more affected by intraspecific competition than by crayfish presence. In contrast, growth of flannelmouth suckers was more affected by crayfish than by intraspecific competition. Crayfish growth was not significantly altered by presence of either fish. Crayfish thus reduced fish growth by competition for food, but the effect differed markedly between the two species. An erratum to this article can be found at .     

Effects of herbivory and competition on an introduced plant in decline

Anthemiscotula was introduced to Denmark 500 years ago, and its distribution is presently limited and in decline. A manipulative field experiment was performed to investigate the effects of native plant competitors and native invertebrate herbivores on its performance. Generally, both herbivory and competition treatments had great impact, and when both factors were operating, the effects were additive for all variables except plant height. Although A. cotula showed plasticity in growth, resource allocation and flowering timing, it was unable to adjust to competition and compensate for losses due to herbivory sufficiently to ensure and restore its achene production. This vulnerability, combined with improved cereal cleaning techniques and thus fewer reintroductions of A. cotula seeds, may be the cause of its current decline. A. cotula responded to herbivory by prolonging its flowering period, a “bet-hedging” strategy. In Denmark this strategy is unreliable since risks of sub-optimal conditions are much greater in August–October. Received: 12 December 1997 / Accepted: 14 October 1998     

Resistance in introduced populations of a freshwater snail to native range parasites

Introduced species provide an opportunity to examine responses to novel ecological conditions, in particular to the absence of co-evolved enemies. Introduced populations could evolve lower investment in resistance or could down-regulate their immune system as a plastic response to enemy absence. The response might have consequences for the success of introduced species. Assuming a trade-off between resistance and traits related to demographic success, an evolved change or reallocation from resistance could increase the chances of invasions. On the other hand, introduced populations could have increased resistance as a correlate of greater vigour and competitive ability among successful invaders [Sampling Bias hypothesis (SBH)]. These hypotheses make different predictions about investment in resistance in introduced populations. Using a New Zealand clonal snail (Potamopyrgus antipodarum), we examined the resistance of three introduced genotypes (one from the US and two from Europe) to several populations of a native range parasite (Microphallus sp.). One genotype (Euro A) was resistant to all native range parasite populations, consistent with the SBH. However, two remaining genotypes (Euro C and US 1) were less susceptible to parasite populations that were allopatric to their source populations. Furthermore, resistance of one genotype (US 1) collected from the introduced range was indistinguishable from its resistance when collected from the range of the parasite. Hence, there was no evidence for decreased resistance in the absence of native enemies, which is inconsistent with hypotheses that envision reduced allocation to resistance or a trade-off between competitive ability and resistance.     

Evidence for asymmetrical intraguild predation between native and introduced Anolis lizards

Since its introduction, Anolis sagrei (Sauria: Polychrotidae) has been replacing native A. carolinensis in Florida and native A. conspersus in Grand Cayman Island as the common anole of urban environments and other open habitats. To assess the likelihood that predation of juvenile native anoles by A. sagrei adults is an important interaction in this process, the propensities for intraguild predation and cannibalism were assessed for A. sagrei and A. carolinensis in Florida and for A. sagrei and A. conspersus in Grand Cayman. Predation experiments were conducted in cages, using freshly captured lizards, in which adult males of each species were presented with conspecific and heterospecific juveniles. Adult A. sagrei were (1) significantly more likely to eat juveniles than were adult A. carolinensis or A. conspersus, and (2) significantly more likely to eat heterospecific than conspecific juveniles, whereas adult A. carolinensis and A. conspersus were not. Thus, the propensity for intraguild predation is asymmetrical in favor of introduced A. sagrei in Florida and Grand Cayman. Further study is needed, however, to determine the importance of intraguild predation under field conditions. Received: 14 July 1999 / Accepted: 6 March 2000     

Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed

Background and Aims: While invasive species may escape from natural enemies in thenew range, the establishment of novel biotic interactions withspecies native to the invaded range can determine their success.Biological control of plant populations can be achieved by manipulationof a species' enemies in the invaded range. Interactions weretherefore investigated between a native parasitic plant andan invasive legume in Mediterranean-type woodlands of SouthAustralia. Methods: The effects of the native stem parasite, Cassytha pubescens,on the introduced host, Cytisus scoparius, and a co-occurringnative host, Leptospermum myrsinoides, were compared. The hypothesisthat the parasitic plant would have a greater impact on theintroduced host than the native host was tested. In a fieldstudy, photosynthesis, growth and survival of hosts and parasitewere examined. Key Results: As predicted, Cassytha had greater impacts on the introducedhost than the native host. Dead Cytisus were associated withdense Cassytha infections but mortality of Leptospermum wasnot correlated with parasite infection. Cassytha infection reducedthe photosynthetic rates of both hosts. Infected Cytisus showedslower recovery of photosystem II efficiency, lower transpirationrates and reduced photosynthetic biomass in comparison withuninfected plants. Parasite photosynthetic rates and growthrates were higher when growing on the introduced host Cytisus,than on Leptospermum. Conclusions: Infection by a native parasitic plant had strong negative effectson the physiology and above-ground biomass allocation of anintroduced species and was correlated with increased plant mortality.The greater impact of the parasite on the introduced host maybe due to either the greater resources that this host providesor increased resistance to infection by the native host. Thisdisparity of effects between introduced host and native hostindicates the potential for Cassytha to be exploited as a controltool.     

Effect of density on competition between wild and hatchery‐reared Atlantic salmon for shelter in winter

The effect of varying the density of hatchery‐reared Atlantic salmon Salmo salar on the ability of single wild fish to occupy a shelter is assessed. Although there was strong density‐dependence on sheltering overall, the ability of wild Atlantic salmon parr to occupy a shelter was not affected by the presence of hatchery‐reared fish even when outnumbered by four to one. These findings illustrate a competitive asymmetry for shelter in favour of the wild fish at the densities tested.     

Behavioural and growth differences between experienced and naïve populations of a native crayfish in the presence of invasive rusty crayfish

1. Species invasions are a leading threat to native species and ecosystems. How populations of native species respond to the presence of invasive species will ultimately determine their long-term persistence.
2. In this study, we capitalise on a unique opportunity to compare the behaviour and growth of naïve and experienced virile crayfish ( Orconectes virilis Hagen) populations in the presence of invasive rusty crayfish ( O. rusticus Girard ) . In behavioural trials, experienced O. virilis (coexisted with O. rusticus for >30 years) showed more aggressive behaviour than their naïve counterparts. Naïve O . virilis retreated from aggressive interactions with O. rusticus 38% more often than experienced O . virilis. Experienced O. virilis spent 39% more time occupying shelter than naïve O. virilis.
3. There were also differences in O. rusticus behaviour: O. rusticus spent 24% more time occupying shelter with naïve O. virilis relative to experienced O. virilis .
4. In field mesocosm experiments with O. rusticus , naïve O . virilis declined in body mass by 1% while experienced O. virilis' body mass increased by 6%, thus highlighting the potential population-level implications of the previously observed behavioural effects.
5. Our work demonstrates significant behaviour and growth differences between naïve and experienced O. virilis in the presence of invasive O. rusticus. Whether this difference is the result of phenotypic plasticity or evolution by natural selection remains to be determined. Either way, this area of inquiry has implications for managing native populations and species in an increasingly invaded world.     

Evidence for competition between carnivorous plants and spiders

Several studies have demonstrated that competition between disparate taxa can be important in determining community structure, yet surprisingly, to our knowledge, no quantitative studies have been conducted on competition between carnivorous plants and animals. To examine potential competition between these taxa, we studied dietary and microhabitat overlap between pink sundews (Drosera capillaris) and wolf spiders (Lycosidae) in the field, and conducted a laboratory experiment examining the effects of wolf spiders on sundew fitness. In the field, we found that sundews and spiders had a high dietary overlap with each other and with the available arthropod prey. Associations between sundews and spiders depended on spatial scale: both sundews and spiders were found more frequently in quadrats with more abundant prey, but within quadrats, spiders constructed larger webs and located them further away from sundews as the total sundew trapping area increased, presumably to reduce competition. Spiders also constructed larger webs when fewer prey were available. In the laboratory, our experiment revealed that spiders can significantly reduce sundew fitness. Our findings suggest that members of the plant and animal kingdoms can and do compete.