Petrolisthes Armatus Research Articles

Size at the onset of morphological sexual maturity, sexual size dimorphism and relative growth of Petrolisthes armatus (Decapoda, Porcellanidae) from Northeastern Brazil

ABSTRACT Our study investigated the size at the onset of morphological sexual maturity, sexual size dimorphism, and relative growth in the green porcelain crab Petrolisthes armatus from a population in Northeastern Brazil. Specimens were collected monthly between February 2011 and January 2012 from the Baía do Pontal, Ilhéus, Bahia. We analysed 576 specimens, with females reaching the size at the onset of morphological sexual maturity at a carapace width of 4.7 mm and males at a carapace width of 5.83 mm. Sexual size dimorphism was confirmed, with females exhibiting a broader pleon and males a wider carapace and larger chelipeds. Positive allometry was observed in cheliped growth, with males exhibiting variation among juveniles and adults. Females showed ontogenetic distinctions in pleon width and cheliped dimensions. Our findings reveal that females and males of P. armatus exhibit different energetic investments in the development of secondary sexual characteristics, especially in cheliped growth for males and pleon enlargement for females. These results from a population of P. armatus in Northeastern Brazil contribute to a deeper understanding of reproductive strategies in females and males of the species.

Density-dependent predation and predator preference for native prey may facilitate an invasive crab’s escape from natural enemies

Native predators can confer biotic resistance through consumption of invasive prey. However, early in the invasion process, native predators may initially ignore an invader when it is rare and only increase consumption once it becomes abundant. Furthermore, the willingness of native predators to consume novel invasive prey may be influenced by the similarity of the invader to other native prey species that are favored or familiar. Here we examined whether a native predator (the common mudcrab, Panopeus herbstii) consumes the invasive filter-feeding crab, Petrolisthes armatus as a function of P. armatus abundance relative to native prey and the similarity of P. armatus to native prey. Using choice experiments, we quantified consumption of invasive P. armatus when its abundance was either rare, equal, or more abundant than native prey that were either taxonomically similar (crab, Eurypanoepus depressus) or dissimilar (mussel, Geukensia demissa) to the invader. We found that the absolute consumption of invasive P. armatus increased as its relative availability increased, but only in treatments where the alternative prey was a native crab. This suggests that prior experience of the native predator with a similar prey may prime the predator to consume more invasive P. armatus. A hierarchical Bayesian analysis determined that both native prey species were preferentially consumed by the native predator P. herbstii even when native prey were rare or equal in abundance to invasive P. armatus. These results suggest that density-dependent predation plus a persistent preference for native prey by native P. herbstii predators may help explain how P. armatus escapes its natural enemies.

Population structure and dynamics of Petrolisthes armatus (Gibbes, 1850) (Anomura: Porcellanidae) in an Amazon estuary

The aim of this study was to assess the environmental factors that modulate the spatiotemporal distribution of Petrolisthes armatus and analyze their population structure, dynamics and secondary production in an Amazon estuary. Monthly samplings were carried out over one year in the intertidal rocky outcrop considering the following environmental features; season (dry and rainy), areas (middle and lower estuary), banks (east and west), substrate with different rock sizes, and the upper and lower intertidal zones. Crab density was positively correlated with salinity and negatively correlated with temperature. Higher densities occurred during dry season, on the eastern bank, in the lower estuary, in the lower intertidal zone, and in the larger rock substrate. The high abundance of P. armatus indicates that this estuary is a suitable habitat for the development of this anomuran, and salinity and substrate were the most important factors driving population structure. High production reflects high density and biomass and the fast growth of this crab indicated that this filter feeder generates substantial production to the benthos. This study corroborated the hypothesis that the high metabolism of P. armatus combined with the high temperature in the Amazon region results in short life span (∼one year), high mortality rate (∼6 year −1), and high P/B ratio (∼4 year −1).

Associations Between Winter Temperatures and the Timing and Duration of Annual Larval Recruitment of a Non-native Anomuran Crab

The green porcelain crab, Petrolisthes armatus, a subtropical/tropical species, was first recorded on intertidal oyster reefs in South Carolina in 1995. Since then, its range has expanded north in a series of surges and setbacks that appear to be associated with particularly cold winters that may cause mortality. Petrolisthes armatus reproduces year-round in its native range whereas its reproductive season in the non-native range is limited to warmer months. Variation in the timing and duration of larval recruitment is not known for the non-native range and these factors may affect further range expansion. To determine whether P. armatus larval recruitment is related to temperature variation, in particular, the severity of winter temperatures, archival biweekly (every 2 weeks) larval samples collected 1994–2018 near its northern range limit in the North Inlet Winyah Bay National Estuarine Research Reserve (NERR) were examined. The goal was to determine whether the timing and duration of larval ingress varied over time and was related to temperature variation. Petrolisthes armatus larvae were first observed in samples collected in 1995. There was considerable interannual variability in the timing of the start of recruitment (coefficient of variation 29%), which ranged from the last week of March at the earliest to the last week of July at the latest, for all years but one. Earlier annual recruitment was positively correlated with a longer recruitment duration. The timing of first larval recruitment was also related to the severity of the preceding winter, with later larval recruitment occurring in years with several consecutive days of cold temperatures. Cold winter temperatures appear to affect overwintering populations and their ability to reproduce in the early spring. A decreasing frequency of severe winters along the coastal South Atlantic Bight may promote further poleward range expansion of P. armatus.

Residency and reproductive patterns of the green porcelain crab, Petrolisthes armatus, on an intertidal oyster reef in its non-native range

The green porcelain crab Petrolisthes armatus is an abundant invader on intertidal oyster reefs along the southeastern coast of the United States. Its range expansion appears to be thermally limited and is predicted to shift poleward as water temperatures rise. Little is known about how P. armatus' seasonal residency patterns affect its reproductive potential near its northern range limit. To study these factors, P. armatus was collected every two weeks over a 20-month period (April 2017 to November 2018) on an intertidal oyster reef within its non-native range in Charleston, SC. Crab abundance, size, sex, and ovigery status were recorded. In addition, a laboratory experiment was conducted to determine the time elapsed between broods for large vs. small adult females. Petrolisthes armatus abundances varied seasonally and were highest in the summers of both years. Adults were collected in April after a mild winter (2017), indicating overwintering occurred, but after a harsh winter (2018), no crabs were collected on the reef the following spring, with adults first collected in July. Peak ovigery rates reached 95% of adult female crabs in some summer months. Juvenile recruits matured and contributed to the reproductive population 6–8 weeks after first being collected in the intertidal oyster habitat. In the laboratory experiment, the median time between broods was 6 days (range 2–22 days). Females were observed with new broods immediately post-molt and also several days following their molt, suggesting mating occurs for females in intermolt and post-molt states. Study results indicate that overwintering success affects the timing of the start of the reproductive season, its duration, and total annual brood production. The laboratory observations suggest rapid sequential brood production occurs for this species in the non-native range. Brood production rates and evidence that females mate in intermolt and post-molt states are new information for this species and are consistent with previous observations of high reproductive output in the non-native range. Further poleward range expansion seems likely, in part due to high reproductive output and rapid recovery following harsh winters.

Turbidity alters estuarine biodiversity and species composition

Abstract Turbidity is widely regarded for modulating primary production and influencing the distribution of submerged aquatic vegetation. Although less well studied, turbidity can also have significant effects on trophic interactions and food webs by modifying light penetration and scattering, influencing foraging ability of visual-hunting predators such as fishes. By interfering with visual foragers, turbidity may shift food webs towards predators that forage with other sensory modalities (e.g. chemoreception and mechanoreception), consequently altering food web structure. We analysed turbidity effects on estuarine community composition and biodiversity in the Gulf of Mexico by analysing an 18-year fisheries-independent data set and assessing communities inhabiting contemporary oyster reefs (Crassostrea virginica). In the long-term data set, elevated turbidity was associated with decreased fish species richness and diversity and higher abundances of benthic species that rely more on chemoreception for foraging and predator avoidance (e.g. crabs). High turbidity may provide a predation refuge for crabs and other benthic organisms that visually oriented fish prey upon. On oyster reefs, crabs readily consume suspension-feeding organisms including newly settled oysters and porcelain crabs (Petrolisthes armatus). Both were significantly less abundant in high turbidity. Human practices that increase turbidity may indirectly influence trophic interactions, species distributions, ecosystem function, and biodiversity.

Sex, size, and prey caloric value affect diet specialization and consumption of an invasive prey by a native predator.

Escaping the control of natural enemies is thought to heavily influence the establishment success and impact of non-native species. Here, we examined how the profitability of alternative prey in combination with the presence of a competitor and predator aggressive behavior explain individual differences in diet specialization and the consumption of the invasive green porcelain crab Petrolisthes armatus by the native mud crab predator Panopeus herbstii. Results from bomb calorimetry estimates show that invasive P. armatus has high caloric value relative to alternative native prey. Laboratory assays indicated that specialization and consumption of invasive P. armatus was mostly exhibited by large, female P. herbstii, but the presence of a competitor and predator aggressiveness did not influence diet and the consumption of P. armatus. Thus, intrinsic factors (e.g., sex and body size) seem to explain consumption of P. armatus and dietary specialization in P. herbstii, more generally. Although there are still many predator individuals that do not consume P. armatus, the proportion of individuals that have begun to specialize on P. armatus suggests that for some, it has become more profitable relative to alternative native prey. Given the high caloric value of P. armatus, we suggest that it is likely that differences in the cost of its consumption, including attack, capture, and handling times relative to alternative prey, determine its net profitability to individual predators.

Spatial and Temporal Associations Between Native Crabs and the Invading Green Porcelain Crab, Petrolisthes armatus, Throughout Its Northernmost Invaded Range

Increasing ocean temperatures have led to poleward range expansions of many marine organisms. The green porcelain crab, Petrolisthes armatus, was first reported on intertidal oyster reefs near Charleston, South Carolina in the mid-1990s, an expansion from its tropical to sub-tropical native range. In order to investigate the relative abundances of this introduced crab and a portion of the native crab community, resident crab assemblages were sampled on intertidal oyster reefs across four seasons from 2015 to 2016. Sampling occurred at five locations extending from Savannah, Georgia where P. armatus is now well-established, to Wilmington, North Carolina, where its inter-annual presence is intermittent. Petrolisthes armatus was the numerically dominant crab species at all locations except Wilmington, where the species was not collected in the winter. Differences between northern and southern sample locations in the taxonomic composition of crab assemblages were most pronounced in the summer (June–September) and fall (October–January), with dissimilarities largely attributed to high abundances of P. armatus at southern locations. Crab abundances of all species peaked in the summer and were lowest in the winter (February–March). Petrolisthes armatus was less abundant toward the northern range edge, whereas native crabs did not differ in abundance throughout the sampling range. Throughout the study area, adult P. armatus sex ratios shifted from unbiased or female-biased in the summer to male-biased in the fall. Adults were collected in the spring (April–June), indicating they had overwintered. While native crab densities (Panopeus herbstii, Eurypanopeus depressus) stayed relatively constant across locations, P. armatus densities varied greatly, suggesting that resources, such as shelter and food, are not limiting factors for the coexistence of these native and invading crabs.

Does predator-driven, biotic resistance limit the northward spread of the non-native green porcelain crab, Petrolisthes armatus?

Biotic resistance by native predators can limit the geographic range and abundance of non-native species following introduction into an ecosystem. Here we tested the hypothesis that the strength of predation pressure varies with latitude and limits the abundance and northward expansion of the non-native green porcelain crab, Petrolisthes armatus, whose northern range is also hypothesized to be limited by physical tolerances to cold temperatures. We quantified the predation risk of P. armatus across 400 km of the crab’s invasive range along the coastline of the southeastern US. In addition, we measured the density of large P. armatus, habitat quality, and other environmental factors that may affect the crab’s predation risk. Finally, we conducted a size-selective predator exclusion experiment to determine the predator species and size classes that may be consuming P. armatus. Results indicated that neither the density of large P. armatus nor its predation risk varied systematically with latitude. Instead, variation in predation risk was best explained by local site-level differences in habitat quality, the density of large P. armatus, and the mean abundance of predators. The predator exclusion experiment indicated that both small and large size classes of predators are capable of equally strong rates of predation on P. armatus. Together, our results suggest that although native predators readily consume P. armatus, they do not provide biotic resistance against its northward expansion. Instead, it seems likely that other latitudinally differential factors like low winter temperatures that decrease P. armatus survival are more influential in limiting the crab’s northern expansion.

Chaetotaxy and setal diversity of grooming legs in species of porcelain crabs (Crustacea: Anomura: Porcellanidae)

The morphology of the fifth pereiopods was studied under scanning electron microscopy in ten species of porcelain crabs for chaetotaxy and setal diversity, namelly Megalobrachium pacificum, Megalobrachium roseum, Pachycheles grossimanus, Petrolisthes armatus, Petrolisthes tuberculatus, Pisidia brasiliensis, Pisidia longicornis, Polyonyx gibbesi, Porcellana platycheles and Porcellana sayana. Six setal types were identified: simple, pappose, sickle-shaped serrate, straight serrate, club-shaped and tooth-like cuspidate. Porcelain crabs can differ in the fifth pereiopod setal morphology, chaetotaxy and setal density, even among species within the same genus. The absence of sexual dimorphism of the grooming legs in porcelain crabs suggests that grooming eggs requires no particular grooming apparatus in females and that male and female are equal in grooming efficiency.

Seasonal differences in egg size in three species of crabs from a tropical upwelling zone

ABSTRACTEgg size and offspring size are fundamentally important aspects of the life histories of all animals. However the impact of environmental conditions on intraspecific variation in egg size of marine invertebrates is poorly documented. Here we followed three species of intertidal crabs Xanthodius sternberghii, Petrolisthes armatus and Clibanarius albidigitus to understand how seasonal environmental variation in temperature and salinity associated with seasonal upwelling impacts egg size. Ovigerous females of both P. armatus and C. albidigitus were found year round, while X. sternberghii has a limited reproductive season, with ovigerous females found only between November and February. In all three species more than half of the variation in egg size was attributable to variation among broods from different females. Eggs collected during the dry, upwelling season were significantly larger than those collected during the wet, non-upwelling season. Multiple regression analysis showed that average egg size from each brood was significantly negatively correlated with temperature for all three species. Egg size was also negatively correlated with salinity in P. armatus when we controlled for temperature. Overall these results support the idea that changes in environmental temperature caused by seasonal upwelling play a significant role in generating seasonal differences in egg size.

Phylogeography of Petrolisthes armatus, an invasive species with low dispersal ability

Theoretically, species with high population structure are likely to expand their range, because marginal populations are free to adapt to local conditions; however, meta-analyses have found a negative relation between structure and invasiveness. The crab Petrolisthes armatus has a wide native range, which has expanded in the last three decades. We sequenced 1718 bp of mitochondrial DNA from native and recently established populations to determine the population structure of the former and the origin of the latter. There was phylogenetic separation between Atlantic and eastern Pacific populations, and between east and west Atlantic ones. Haplotypes on the coast of Florida and newly established populations in Georgia and South Carolina belong to a different clade from those from Yucatán to Brazil, though a few haplotypes are shared. In the Pacific, populations from Colombia and Ecuador are highly divergent from those from Panamá and the Sea of Cortez. In general, populations were separated hundreds to million years ago with little subsequent gene flow. High genetic diversity in the newly established populations shows that they were founded by many individuals. Range expansion appears to have been limited by low dispersal rather than lack of ability of marginal populations to adapt to extreme conditions.

Spacing and movement in the green porcelain crab Petrolisthes armatus: evidence for male competition and mate guarding

The green porcelain crab, Petrolisthes armatus, is a common invasive species on inter-tidal oyster reefs in the South Atlantic Bight whose behavior is largely unknown. We assessed the effects of the presence of opposite-sex conspecifics on adult crab spacing at a low density to infer potential mate acquisition behaviors. Adult crabs held in all-male, all-female, and mixed-sex treatments were observed twice daily over five days to determine nearest-neighbor (NN) distances and relative activity. Males in the mixed-sex treatment with the shortest NN distances (NN1) were most commonly (1) closest to females, and, when compared to NN1 males in the all-male treatment, were (2) closer to their NN and (3) had lower activity levels. Female movement did not differ between treatments. Percent ovigerous females at the end of trials were higher in the mixed-sex treatment, reflecting mating activity, which when observed, involved inter-molt females. Mate guarding, rather than pure-searching behavior, was the mating behavior most consistent with observed spacing and movement patterns.

Insights of the life history in the porcellanid crab Petrolisthes armatus (Gibbes, 1850) (Crustacea: Anomura: Porcellanidae) from the Southwestern Atlantic coast

Petrolisthes armatus (Gibbes, 1850) life history was evaluated based on growth and reproduction. Specimens were captured at Ubatuba (Brazil), identified, sexed and measured: carapace (CL, length; CW, width), larger cheliped propodus (PL, length; PW, width; and PH, height), and 2th and 5th abdominal somite (AW2 and AW5, width). Relative growth was studied using CL as independent variable and its relation with other variables (dependents), to estimate puberty size and morphological maturity. Fecundity was represented by number of eggs vs. body size CL, with a better fit using a power function. Considering the 257 specimens analyzed, males were larger than females, and the overall sex ratio was 1:1, with some differences among size classes. Maturity (puberty size) was revealed by: AW2 × CL (males: 8.6 mm CL; females: 7.6 mm CL); and AW5 × CL (7.1 and 7.6 mm CL, respectively). Reproduction was continuous, but more intense in rainy season and recruitment occurring in dry season. Fecundity of this species was 228 ± 163 eggs, with better fit by a power function (R2 = 0.72). Maturity size and growth differed when a pristine area (Ubatuba) was compared with a polluted area (São Sebastião), in the same Brazilian area (São Paulo State).

Reproductive performance of the marine green porcelain crab Petrolisthes armatus Gibbes, 1850 in its introduced range favors further range expansion

Reproductive performance of the marine green porcelain crab Petrolisthes armatus Gibbes, 1850 in its introduced range favors further range expansion

Composition of decapod larvae in a northeastern Brazilian estuarine inlet over a full tidal cycle

ABSTRACT. Composition changes of decapod larvae in a neotropical estuarine inlet (Catuama inlet, northeast Brazilian coast) were studied in a complete tidal cycle in August 2001. Sampling was performed at three stations at two or three depth levels that were located along a transect across the inlet. The collections were performed in 3 h intervals under a neap tide regime. Samples were taken with a pump and the water was filtered through 300 μm plankton net. 27 planktonic decapod taxa were identified, which included the first larval stages of the coastal and oceanic shrimps Lucifer faxoni and Acetes americanus, besides larvae of the estuarine and coastal decapods Pinnixa spp., Uca spp., Petrolisthes armatus , Upogebia spp. and Alpheidae, which were the most frequent taxa. All of the planktonic larval stages were observed for most of the decapods, suggesting that the probable exportation from the estuary is to the inner adjacent shelf area, instead of the outer oceanic area. Keywords: decapod larvae, meroplankton, tidal changes, neotropical estuary, northeastern Brazil.

Species-specific responses of resident crabs to vertical habitat complexity on intertidal oyster reefs

Habitat vertical complexity is an important physical feature of marine systems that can influence factors such as predator–prey interactions and recruitment. We examined how vertical structure on intertidal Eastern oyster Crassostrea virginica reefs affected resident crab distributions by deploying trays of varying rugosity at two sites in Charleston, South Carolina. Trays were collected after fifteen weeks and oyster spat and resident macrofauna were counted and measured. Densities of oyster spat and the filter-feeding, invasive, green porcelain crab Petrolisthes armatus were strongly positively associated with rugosity. Flatback mud crab Eurypanopeus depressus densities were positively associated with rugosity and with scorched mussel Brachiodontes exustus densities. The common mud crab Panopeus herbstii exhibited a more even distribution across oyster rugosity treatments and was the only species that commonly occurred in the low rugosity treatment. Differences in crab sizes between sites were attributable to abundances of smaller individuals and may reflect spatial variation in recruitment potential. Differences in crab size distributions among treatments were caused by more relatively large crabs in the high rugosity treatments, where greater vertical structural complexity provided more refuge for larger individuals. The relationships between rugosity and faunal densities were consistent at both sites, indicating that the rugosity metric can be used to estimate changes in spat and associated faunal densities when vertical habitat complexity on oyster reefs is reduced, for instance, through harvesting or sedimentation.

ESTRUCTURA DE LA COMUNIDAD EPIFAUNAL ASOCIADA A COLONIAS DE VIDA LIBRE DEL HIDROCORAL MILLEPORA ALCICORNIS LINNAEUS 1758 EN BAHÍA PORTETE, CARIBE COLOMBIANO

The epifaunal community associated with free-living colonies of Millepora alcicornis, the dominant coral species in Portete Bay, was characterized from twenty living colonies randomly collected within a shallow Thalassia testudinum seabed. The three-dimensional growth colonies sheltered 15 families, 15 genera and 18 mobile invertebrate species. The 97% of individuals was represented by decapods crustaceans specially adapted as to live in association with the host. Within them, the porcelanid Petrolisthes armatus, representing 79% of the epifauna (961 individuals), was exclusively found in a juvenile stage suggesting that M. alcicornis is mainly used by this species as a nursery zone. The epifaunal composition is partially related to coral living tissue whereas the principal simbionts size is correlated with the spatial separation of coral branches. In spite of being slightly diverse in comparison with the epifaunal community found in other coral species, the epifauna found in M. alcicornis is of great interest and peculiarity by their association with the free-living form of this coral.

A multiple habitat restoration strategy in a semi-enclosed Florida embayment, combining hydrologic restoration, mangrove propagule plantings and oyster substrate additions

Abstract Habitat loss and disturbance are ranked globally as the greatest threats to biodiversity. Development and coastal population growth are the leading causes for habitat losses. Recently, the restoration of marine habitats has increased, especially with the goal of increasing non-consumptive ecosystem services derived from mangrove and submerged aquatic vegetation (SAV) along with biogenic oyster reefs. Habitats reside in landscapes dominated by multiple species. Rather than focusing on a single habitat such as oysters or mangroves or SAV, we took an approach restoring multiple adjacent habitats to accelerate restoration in a Florida embayment that had been significantly degraded prior to the restoration of natural tidally generated flows. After a multiple habitat die-off, a project was initiated in 2006 to reintroduce tidal flushing. The re-introduction of tidal flushing, however, did not result in immediate recovery of mangrove shorelines or oyster-dominated reefs. There was a lack of mangrove propagule production and significant substrate limitation in areas with appropriate salinity, sediment and tidal flows. From 2009–2012, red mangrove (Rhizophora mangle) propagules were collected (over 500,000) and planted for a total area of 3.24 ha. From 2009–2010, five intertidal reefs were constructed by adding bagged and fossil shell (54 MT) for Crassostrea virginica larvae to recruit onto totaling over 779 m2. Monitoring of planted mangrove versus unplanted shorelines demonstrated that prop root and drop root densities were higher where propagules were planted (28 m−2) versus unplanted (2.3 m−2). Oyster densities and mean sizes (multiple year classes) at new and natural reefs were measured after 8, 12, and 24, and 36 months. An initial settlement pulse was observed in the first 8 months followed by an increase in the density of greater than 1-year old oysters. Xanthid crab densities (Eurypanopeus depressus and Panopeus spp.) in restored reefs and natural reefs were similar, while Petrolisthes armatus densities were lower in restored reefs. Whole reef seston filtration rates over restored reefs were −26 to 157 L m−2 h−1 when measured at 4, 15, 28, and 40 months. A multiple habitat approach may be useful in accelerating the natural ecological succession, especially if the project site has reached a degraded, alternate ecological state. These results suggest a multiple habitat approach can be useful in providing non-provisioning ecosystem services to a Florida embayment.

Individual variation in predator behavior and demographics affects consumption of non-native prey

Non-native species can serve as a prey resource for native predators. Yet because there is often no shared evolutionary history between the predator and prey, individuals within a predator population may vary greatly in their willingness to consume a recently introduced, yet profitable prey. Here, we measured individual variation in diet, behavior, and demographic traits of the native predatory mud crab, Panopeus herbstii , and evaluated how these traits influenced an individual’s consumption of a recently introduced, non-native crab, Petrolisthes armatus , using both simultaneous and no-choice assays. These same individual predatory mud crabs were also assayed to quantify their antipredator reaction and exploratory behavior. Results indicated significant variation in the diets of individual predators with 45% specializing on native mussels, 14% specializing on non-native Petrolisthes , and the remainder eating multiple prey species. When given a choice of alternative prey, individual Panopeus predators that consumed a larger proportion of Petrolisthes were female, smaller, and more likely to flee in response to predators. When given no choice of alternative prey, Petrolisthes was consumed more frequently by Panopeus that were female and less exploratory. We suggest that individuals that more readily consume non-native Petrolisthes may be attempting to reduce competition with conspecifics that are larger, more aggressive, exploratory, and male. Our results suggest that at least initially following invasion, adoption of a non-native prey species into the diet of a native predator may not occur universally within the population. Such nonuniform predation pressure could contribute to the non-native prey’s release from natural enemies.