Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

Identifying impacts of exotic species on native populations is central to ecology and conservation. Although the effects of exotic predators on native prey have received much attention, the role of exotic prey on native predators is poorly understood. Determining if native predators actively prefer invasive prey over native prey has implications for interpreting invasion impacts, identifying the presence of evolutionary traps, and predator persistence. One of the world’s most invasive species, Pomacea maculata, has recently established in portions of the endangered Everglade snail kite’s (Rostrhamus sociabilis plumbeus) geographic range. Although these exotic snails could provide additional prey resources, they are typically much larger than the native snail, which can lead to lower foraging success and the potential for diminished energetic benefits in comparison to native snails. Nonetheless, snail kites frequently forage on exotic snails. We used choice experiments to evaluate snail kite foraging preference in relation to exotic species and snail size. We found that snail kites do not show a preference for native or exotic snails. Rather, snail kites generally showed a preference for medium-sized snails, the sizes reflective of large native snails. These results suggest that while snail kites frequently forage on exotic snails in the wild, this behavior is likely driven simply by the abundance of exotic snails rather than snail kites preferring exotics. This lack of preference offers insights to hypotheses regarding effects of exotic species, guidance regarding habitat and invasive species management, and illustrates how native-exotic relationships can be misleading in the absence of experimental tests of such interactions.     

Eating their way to the top? Mechanisms underlying the success of invasive insect generalist predators

Insect generalist predators have been introduced outside of their native range intentionally to improve biological control, or accidentally during commerce, and can subsequently become invasive. Invasive insect generalist predators (IIGP) have widespread impacts on invaded communities because they consume both herbivores and other predators. Also, they often reach higher densities than and displace similar native species. Reflecting the complexity of their ecological roles, a wide variety of mechanisms might contribute to invasive success by IIGP. These species often drive resources to lower levels than do natives, leading to intense resource competition and sometimes competitive exclusion of other predators. The broader range of resources used by many IIGP can heighten their competitive advantage, particularly when IIGP exploit modified habitats. In either case, IIGP improve herbivore suppression by depressing prey densities below pre-invasion levels. Coexistence among native and invasive generalists is fostered when species differ substantially in their niche requirements. In this case, a larger proportion of the total prey population is subject to attack post- than pre-invasion, which strengthens prey suppression. On the other hand, some IIGP feed heavily on other predators, at times leading to a weakening of prey suppression. Future research should continue to explore the roles of competition and niche partitioning on larger spatial scales, and in both the native and invasive ranges of IIGP. Additionally, combining data from empirical studies with theory might be an effective way to predict the spread and community impacts of IIGP invasions.     

Predation by gulls on crabs in rocky intertidal and shallow subtidal zones of the Gulf of Maine

Most organisms in intertidal areas are marine in origin; many have distributions that extend into the subtidal zone. Terrestrial predators such as mammals and birds may exploit these animals during low tide and can have considerable effects on intertidal food webs. Several studies have shown that avian predators are capable of reducing densities of sessile and slow-moving intertidal invertebrates but very few studies have considered avian predation on mobile invertebrate predators such as crabs. In this study, we investigated predation by Great Black-backed Gulls (Larus marinus Linnaeus) on three species of crabs (Cancer borealis Stimpson, Cancer irroratus Say, and Carcinus maenas Linnaeus). The study was at Appledore Island, ME (a gull breeding island) and 8 other sites throughout the Gulf of Maine, including breeding islands and mainland sites. On Appledore Island, intertidal and subtidal zones provided over one-third of prey remains found at gull nests, and crabs were a substantial proportion (∼ 30% to 40%) of the total remains. Similarly, collections of prey remains from intertidal areas indicated that crabs were by far the most common marine prey. C. borealis was eaten far more often and C. irroratus and C. maenas less often than expected at each site. Comparing numbers of carapaces to densities of crabs in low intertidal and shallow subtidal zones at each site, we estimated that gulls remove between 15% and 64% of C. borealis during diurnal low tides. The proportion of C. borealis eaten by gulls was independent of proximity to a gull colony. Approximately 97% of the outer coast of Maine is within 20 km of a breeding island. Thus, a lot of gull predation on crabs may occur throughout the Gulf of Maine during summer months. Crabs are important predators of other invertebrates; if predation by gulls reduces the number of crabs in intertidal and shallow subtidal areas, gulls may have important indirect effects on intertidal food webs.     

Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell‐thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator‐mediated selection on local snail populations.     

Shell utilization,predation pressure,and thermal stress in panamanian hermit crabs: An interoceanic comparison

Shell preference patterns of two common hermit crabs from hard bottom reef flats on the Caribbean coast of Panama are examined in relation to the predation pressures and physical stresses of their habitat. Clibanarius antillensis Stimpson lives in the high intertidal habitat and minimizes exposure to predators by seeking refuge during high tides. It prefers high-spired shells which maximize protection from thermal stress. Calcinus tibicen Herbst avoids tidal emersion and prefers low-spired shells which enhance resistance to the predators common on Caribbean reef flats.The results are compared with similar results from the tropical eastern Pacific Bay of Panama. Shell-crushing predation on Caribbean hermit crabs is suggested to differ quantitatively and qualitatively from predation on hermit crabs in the Bay of Panama. Predation on hermit crabs in the Bay of Panama is more intense and effects larger individuals than predation on Caribbean reef flat hermit crabs. In addition, shell-crushing predation on hermit crabs in the Bay of Panama is primarily from teleost fish predators (Diodon spp.), while predation on Caribbean hermit crabs is primarily by bottom-dwelling crustaceans.Differences in predation pressures and tidal regimes between the Caribbean and Pacific coasts of Panama are reflected in the shell preferences and behavior of hermit crabs from the two areas.     

Parasites alter host phenotype and may create a new ecological niche for snail hosts

By modifying the behaviour and morphology of hosts, parasites may strongly impact host individuals, populations and communities. We examined the effects of a common trematode parasite on its snail host, Batillaria cumingi (Batillariidae). This widespread snail is usually the most abundant invertebrate in salt marshes and mudflats of the northeastern coast of Asia. More than half (52.6%, n=1360) of the snails in our study were infected. We found that snails living in the lower intertidal zone were markedly larger and exhibited different shell morphology than those in the upper intertidal zone. The large morphotypes in the lower tidal zone were all infected by the trematode, Cercaria batillariae (Heterophyidae). We used a transplant experiment, a mark-and-recapture experiment and stable carbon isotope ratios to reveal that snails infected by the trematode move to the lower intertidal zone, resume growth after maturation and consume different resources. By simultaneously changing the morphology and behaviour of individual hosts, this parasite alters the demographics and potentially modifies resource use of the snail population. Since trematodes are common and often abundant in marine and freshwater habitats throughout the world, their effects potentially alter food webs in many systems.     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.     

Hierarchical Levels of Seed Predation Variation by Introduced Beetles on an Endemic Mediterranean Palm

Seed predators can limit plant recruitment and thus profoundly impinge the dynamics of plant populations, especially when diverse seed predators (e.g., native and introduced) attack particular plant populations. Surprisingly, however, we know little concerning the potential hierarchy of spatial scales (e.g., region, population, patch) and coupled ecological correlates governing variation in the overall impact that native and introduced seed predators have on plant populations. We investigated several spatial scales and ecological correlates of pre-dispersal seed predation by invasive borer beetles in Chamaerops humilis (Arecaceae), a charismatic endemic palm of the Mediteranean basin. To this end, we considered 13 palm populations (115 palms) within four geographical regions of the Iberian Peninsula. The observed interregional differences in percentages of seed predation by invasive beetles were not significant likely because of considerable variation among populations within regions. Among population variation in seed predation was largely related to level of human impact. In general, levels of seed predation were several folds higher in human-altered populations than in natural populations. Within populations, seed predation declined significantly with the increase in amount of persisting fruit pulp, which acted as a barrier against seed predators. Our results revealed that a native species (a palm) is affected by the introduction of related species because of the concurrent introduction of seed predators that feed on both the introduced and native palms. We also show how the impact of invasive seed predators on plants can vary across a hierarchy of levels ranging from variation among individuals within local populations to large scale regional divergences.     

The Role of Rhythmic Systems in the Adaptation of Fiddler Crabs to the Intertidal Zone

Fiddler crabs inhabiting the intertidal zone must adapt theiractivity to both the day-night cycle and the cycle of the tides.The tidal cycle imposes on the animals a rhythmic alternationbetween terrestrial and marine periods of existence. At thesame time the crabs are influenced by the day-night cycle, sincethey exhibit specific diurnal and nocturnal habits. Moreover,the interaction of daily and tidal rhythms may give rise tosemi-monthly variations in activity. It is now well established that persistent daily and tidal rhythmsin physiological processes underlie rhythmic variations in thebehavior of crabs in the field. The present paper reviews morerecent studies, which have compared the persistent rhythms incrabs from different tidal and non-tidal regions. Rhythmic patternsrecorded in the laboratory are found to be closely correlatedwith tidal conditions in the native habitat of the crabs. Ithas also been shown that the persistent pattern can be modifiedby transplanting crabs to the intertidal zone of another coastwhere they are exposed to tidal cycles which differ in formfrom those in the original habitat.     

Genetic relationships of the marine invasive crab parasite <Emphasis Type="Italic">Loxothylacus panopaei</Emphasis>: an analysis of DNA sequence variation,host specificity,and distributional range

Host specificity is a key variable of the niche breath of parasites that can be an important determinant of a parasite’s ability to invade new areas. There is increasing evidence that many parasite species may comprise a variety of genetically variable lineages, which differ in host specificity and geographic range. In this study, we (1) explored the extent of diversity in the invasive parasitic barnacle Loxothylacus panopaei (Rhizocephala) infecting mud crabs (2) examined the geographic origin for the invasive lineage and (3) assessed if further southward spread of the parasite may be impeded. Along the US Atlantic coast, L. panopaei infects different hosts in its invaded range (Chesapeake Bay to north of Cape Canaveral) compared to one portion of the native range in Southeast Florida. This difference was reflected in genetic lineages on two independent loci, mitochondrial cytochrome oxidase I and nuclear cytochrome c. Both loci were concordant in that they showed one lineage infecting crabs of the genus Panopeus in the native range and one lineage infecting Eurypanopeus depressus and Rhithropanopeus harrisii hosts in the invaded range and in the Gulf of Mexico, thus indicating Gulf of Mexico populations as the most likely source of introduction into Chesapeake Bay. Interestingly, the nuclear marker resolved an additional lineage of parasites infecting panopeid hosts in the native range. All three parasite lineages were well supported, but a decision about species status must await further analyses. Since its introduction in the 1960s, the invasive L. panopaei lineage has expanded its range southward along the US Atlantic coast, now almost reaching the northern limit of native Panopeus-infecting lineages at Cape Canaveral, Florida. We hypothesized that parasite-free E. depressus in Southeast Florida, living in sympatry with infected panopeid populations, might be resistant to infection by the invasive lineage. Our infection experiments rejected this hypothesis, suggesting that any impediment to further southward range expansion might be expected from temperature regimes of the subtropical zoogeographic region south of Cape Canaveral.     

Biogeographic variation in behavioral and morphological responses to predation risk

The expression of prey antipredator defenses is often related to ambient consumer pressure, and prey express greater defenses under intense consumer pressure. Predation is generally greater at lower latitudes, and antipredator defenses often display a biogeographic pattern. Predation pressure may also vary significantly between habitats within latitudes, making biogeographic patterns difficult to distinguish. Furthermore, invasive predators may also influence the expression of prey defenses in ecological time. The purpose of this study was to determine how these factors influence the strength of antipredator responses. To assess patterns in prey antipredator defenses based upon geographic range (north vs. south), habitat type (wave-protected vs. wave-exposed shores), and invasive predators, we examined how native rock (Cancer irroratus) and invasive green (Carcinus maenas) crab predators influence the behavioral and morphological defenses of dogwhelk (Nucella lapillus) prey from habitats that differ in wave exposure across an ~230 km range within the Gulf of Maine. The expression of behavioral and morphological antipredatory responses varied according to wave exposure, geographic location, and predator species. Dogwhelks from areas with an established history with green crabs exhibited the largest behavioral and morphological antipredator responses to green crabs. Dogwhelk behavioral responses to rock crabs did not vary between habitats or geographic regions, although morphological responses were greater further south where predation pressure was greatest. These findings suggest that dogwhelk responses to invasive and native predators vary according to geographic location and habitat, and are strongly affected by ambient predation pressure due to the invasion history of an exotic predator.     

Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

Juvenile Chinese mitten crabs (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the Thames estuary: distribution,movement and possible interactions with the native crab <Emphasis Type="Italic">Carcinus maenas</Emphasis>

The Chinese mitten crab, Eriocheir sinensis is a successful invader. Whilst non-breeding adult E. sinensis have been associated with the destruction of riverbanks, little is known about the ecology of this species in its invaded areas. This is especially true of the juveniles which are a key migratory stage. Intertidal surveys along the Thames estuary indicated an increase in the population since the 1990s. Juvenile E. sinensis were abundant in the sampled upper tidal region of the Thames, refuging under boulders in the intertidal at low tide. Seasonal differences in sampled populations were observed, with a significantly lower abundance of crabs found during winter compared to summer. Mark-recaptures indicated movement in the intertidal occurring during high tide, with an influx of new crabs evident after a single tidal cycle. Endogenous rhythms were also apparent, with peaks in activity occurring corresponding with night-time high tide conditions. E. sinensis juveniles can successfully exclude similar sized native Carcinus maenas from shelters in the laboratory, regardless of which species originally inhabited the shelter. This may have implications for native estuarine Carcinus populations, which can rely on such intertidal shelters for refuges.     

Global realized niche divergence in the African clawed frog Xenopus laevis

Although of crucial importance for invasion biology and impact assessments of climate change, it remains widely unknown how species cope with and adapt to environmental conditions beyond their currently realized climatic niches (i.e., those climatic conditions existing populations are exposed to). The African clawed frog Xenopus laevis, native to southern Africa, has established numerous invasive populations on multiple continents making it a pertinent model organism to study environmental niche dynamics. In this study, we assess whether the realized niches of the invasive populations in Europe, South, and North America represent subsets of the species’ realized niche in its native distributional range or if niche shifts are traceable. If shifts are traceable, we ask whether the realized niches of invasive populations still contain signatures of the niche of source populations what could indicate local adaptations. Univariate comparisons among bioclimatic conditions at native and invaded ranges revealed the invasive populations to be nested within the variable range of the native population. However, at the same time, invasive populations are well differentiated in multidimensional niche space as quantified via n‐dimensional hypervolumes. The most deviant invasive population are those from Europe. Our results suggest varying degrees of realized niche shifts, which are mainly driven by temperature related variables. The crosswise projection of the hypervolumes that were trained in invaded ranges revealed the south‐western Cape region as likely area of origin for all invasive populations, which is largely congruent with DNA sequence data and suggests a gradual exploration of novel climate space in invasive populations.     

Condition status and parasite infection of Neogobius kessleri and N. melanostomus (Gobiidae) in their native and non-native area of distribution of the Danube River

The success of introduced species is often facilitated by escape from the effects of natural predators and parasites. Introduced species can profit from this favourable situation, attaining higher population densities and greater individual sizes in novel areas. In this study, somatic condition and parasite infection were compared between native and non-native populations of Neogobius kessleri Günther; introduced only within the interconnected Danube and Rhine River system, and N. melanostomus (Pallas); widely introduced throughout several river systems in Europe and North America. Higher values of Fulton’s condition factor were observed in non-native populations of both goby species. Neogobius melanostomus attained higher gonadosomatic index values in non-native populations, indicating potential increased investment in reproduction in its new area. A lower splenosomatic index was observed in non-native populations, especially in N. melanostomus. Parasite infracommunity richness and mean abundance were higher in N. kessleri in both native and non-native populations, suggesting higher susceptibility of N. kessleri to these parasites. Non-native populations of both hosts showed higher infra-community richness as a result of acquiring parasites native to the new area, but lower parasite abundance. Differences in success of the introduction and establishment in new areas between the two fish species may be associated with a relatively low parasite infection rate and a higher gonadosomatic index in non-native populations of N. melanostomus in comparison to N. kessleri.     

Predation on 0-group and older year classes of the bivalve Macoma balthica: interaction of size selection and intertidal distribution of epibenthic predators

The bivalve Macoma balthica is a common species in the Wadden Sea and North Sea. Juveniles temporarily use nurseries in the high intertidal. To explain this nursery use, predation pressure was examined for both juvenile and adult Macoma at low and high tidal flats. The study was carried out in the eastern Dutch Wadden Sea.Shrimps Crangon crangon, adult crabs Carcinus maenas, gobies Pomatoschistus and juvenile flatfish were more abundant and larger on low than on high tidal flats, but 0-group Carcinus was more abundant on the high tidal flats. Crangon and 0-group Carcinus stomachs frequently contained Macoma remains. These predators selectively preyed on small 0-group Macoma, both in the field and in laboratory experiments. The effect of predation by epibenthic animals and birds, on the low and high tidal flats, was examined in exclosure experiments (2 mm mesh). There was no effect of epibenthos exclosure on adult Macoma. For 0-group Macoma, densities were higher in exclosures than in the controls where predators had normal access. The density reduction by epibenthic predators was much larger in the low than in the high intertidal. We found no effect of bird predation on densities of 0- and 1+group Macoma.Thus, 0-group Macoma is under high predation pressure by epibenthos in the low intertidal, especially by shrimps, while they are relatively safe in the high intertidal. However, most of the shellfish outgrow their epibenthic predators during their first summer. Therefore, it becomes safe for the bivalves to redistribute to locations where epibenthic predators are abundant, during their first winter. On the other hand, it did not become clear from this study why many of the larger Macoma leave the high intertidal. Concluding, the nursery use of Macoma-spat in the high intertidal is probably, at least partly, an adaptation to avoid epibenthic predation.     

Clonal diversity of the marine trematode Maritrema novaezealandensis within intermediate hosts: the molecular ecology of parasite life cycles

We quantified the clonal diversity of the New Zealand marine trematode Maritrema novaezealandensis (n = 1250) within Zeacumantus subcarinatus snail (n = 25) and Macrophthalmus hirtipes crab (n = 25) intermediate hosts using four to six microsatellite loci, and investigated the potential biological and physical factors responsible for the observed genetic patterns. Individual snails harboured one to five trematode genotypes and 48% of snails were infected by multiple parasite genotypes. Overall, the number of parasite genotypes did not increase with snail size, but was highest in intermediate-sized snails. Significantly larger numbers of parasite genotypes were detected in crabs (relative to snails; P < 0.001), with 16-25 genotypes recovered from individual crabs. Although crabs are typically infected by small numbers of cercariae sourced from many snails, they are occasionally infected by large numbers of cercariae sourced from single snails. The latter cases explain the significant genetic differentiation of trematode populations detected among their crab hosts (F(ST) = 0.009, P < 0.001). Our results suggest that the timing of infection and/or intraspecific competition among parasite clones within snails determine(s) the diversity of parasite clones that snails harbour. The presence of a large number of infected snails and tidal mixing of cercariae prior to infection results in crabs potentially harbouring hundreds of parasite genotypes despite the crabs' territorial behaviour.     

Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades

Although invasive species often resemble their native counterparts, differences in their foraging and anti-predator strategies may disrupt native food webs. In a California estuary, we showed that regions dominated by native crabs and native whelks have low mortality of native oysters (the basal prey), while regions dominated by invasive crabs and invasive whelks have high oyster mortality and are consequently losing a biologically diverse habitat. Using field experiments, we demonstrated that the invasive whelk’s distribution is causally related to a large-scale pattern of oyster mortality. To determine whether predator–prey interactions between crabs (top predators) and whelks (intermediate consumers) indirectly control the pattern of oyster mortality, we manipulated the presence and invasion status of the intermediate and top trophic levels in laboratory mesocosms. Our results show that native crabs indirectly maintain a portion of the estuary’s oyster habitat by both consuming native whelks (density-mediated trophic cascade) and altering their foraging behavior (trait-mediated trophic cascade). In contrast, invasive whelks are naive to crab predators and fail to avoid them, thereby inhibiting trait-mediated cascades and their invasion into areas with native crabs. Similarly, when native crabs are replaced with invasive crabs, the naive foraging strategy and smaller size of invasive crabs prevents them from efficiently consuming adult whelks, thereby inhibiting strong density-mediated cascades. Thus, while trophic cascades allow native crabs, whelks, and oysters to locally co-exist, the replacement of native crabs and whelks by functionally similar invasive species results in severe depletion of native oysters. As coastal systems become increasingly invaded, the mismatch of evolutionarily based strategies among predators and prey may lead to further losses of critical habitat that support marine biodiversity and ecosystem function. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Consumptive and non-consumptive effects of an invasive marine predator on native coral-reef herbivores

Invasive predators typically have larger effects on native prey populations than native predators, yet the potential roles of their consumptive versus non-consumptive effects (CEs vs. NCEs) in structuring invaded systems remains unclear. Invasive lionfish (Pterois volitans) may have ecosystem-level effects by altering native fish grazing on benthic algae that could otherwise displace corals. Lionfish could reduce grazing by decreasing the abundance of herbivorous fishes (CEs), and/or the predation risk posed by lionfish could alter grazing behavior of fishes (NCEs). To test for these CEs, we manipulated lionfish densities on large reefs in The Bahamas and surveyed fish populations throughout June 2009–2011. In July 2011, NCEs of lionfish were measured by observing fish grazing behavior on algal-covered substrata placed in microhabitats varying in lionfish presence at different spatial scales, and quantifying any resulting algal loss. Lionfish reduced small herbivorous fish density by the end of the 2010 summer recruitment season. Grazing by small and large fishes was reduced on high-lionfish-density reefs, and small fish grazing further decreased when in the immediate presence of lionfish within-reefs. Lionfish had a negative indirect effect on algal loss, with 66–80 % less algae removed from substrata in high-lionfish-density reefs. Parrotfishes were likely driving the response of herbivorous fishes to both CEs and NCEs of lionfish. These results demonstrate the importance of considering NCEs in addition to CEs of invasive predators when assessing the effects of invasions.