Ex situ evaluation of impacts of invasive mosquitofish on the imperiled Barrens topminnow

Central to the protection of native species is an understanding of impacts of actual or potential invasive species and also the mechanisms through which those impacts are mediated. The introduction and spread of western mosquitofish, Gambusia affinis, into spring systems of the Barrens Plateau region of middle Tennessee is a concern for native species such as the Barrens topminnow, Fundulus julisia. We investigated whether mosquitofish might act as predators on early life stages of topminnows as well as affect the physiological well being of adults through aggressive interactions. A short-term, 24-h laboratory study with mosquitofish and topminnows demonstrated the vulnerability of young topminnow life stages to large mosquitofish predation and aggression. Survival of topminnow young, <16 mm total length (TL), was 0% and was attributed to predation by mosquitofish. Survival of juveniles, 20–30 mm TL, was 25%; juveniles mostly succumbed (post 24-h) to injuries inflicted by large mosquitofish. Adult topminnow survival was 100% but adults faced injury risk, primarily during the initial stages of their interaction with large mosquitofish. A long-term, 60-day laboratory study with syntopic and allotopic populations of adult topminnows and mosquitofish failed to detect any negative impacts on topminnows due to coexistence. Survival, growth, and fecundity of adult topminnows syntopic with mosquitofish were not different from the allotopic population, although injury risk in the form of fin damage was greater syntopically. Thus, predation and aggression towards young topminnows may be the primary mechanisms by which western mosquitofish jeopardize the persistence of native Barrens topminnows in the wild. Our results reemphasize the danger to native aquatic biodiversity of unregulated introductions of Gambusia species.     

Ground-truthing the impact of invasive species: spatio-temporal overlap between native least chub and introduced western mosquitofish

Western mosquitofish (Gambusia affinis) are one of the most destructive introduced species in the American West. The negative impact of introduced species on native taxa depends on their spatio-temporal overlap, which will determine the availability of refugia for native species. Experiments on the mechanisms underlying the interactions between introduced and native species rarely address habitat use, overlap, and refugia because individuals are confined to enclosures. In a previous study we used cages, microcosms, and aquaria to show that mosquitofish could prey on and out-compete native least chub (Iotichthys phlegethontis). In this study, we examined the spatio-temporal overlap between mosquitofish and least chub under natural conditions. We found periods of overlap and partitioning in the seasonal and diel habitat use of these species. Both species used shallow habitats during the day and night throughout the spring when least chub were spawning. Predation by adult mosquitofish on young least chub during the spring likely explains the reduction in least chub recruitment in the presence of mosquitofish. During the summer least chub avoided mosquitofish by exclusively using cooler habitats, or by occupying deeper, cooler habitats during the day when mosquitofish were active, and shallower, warmer habitats at night when mosquitofish were inactive. A shift to cooler habitats in the presence of mosquitofish may result in decreased growth and fecundity of least chub. However, we suggest that a reduction of warm shallow habitat may have a disproportionately greater negative impact on mosquitofish than species native to temperate regions, such as least chub. Habitat manipulations that reduce or eliminate warm habitats may ameliorate the harmful effects of mosquitofish and promote the long-term persistence of native taxa.     

Salinity mediates the competitive interactions between invasive mosquitofish and an endangered fish

The interplay of abiotic factors and competition has a long history in ecology, although there are very few studies on the interaction of salinity and competition in fish. Mosquitofish (Gambusia holbrooki) are among the most invasive fish worldwide, with well documented ecological impacts on several taxa such as amphibians and small native fish. It has been previously hypothesized, based on field observations, that salinity limits the invasive success of mosquitofish and provides a competitive refuge for Mediterranean cyprinodonts. We experimentally tested this hypothesis by examining the agonistic behaviour and food competition between mosquitofish and an endangered native cyprinodont (Aphanius fasciatus) at three salinities (0, 15, 25‰). Intraspecific aggressive behaviour for both species was not significantly affected by salinity. As salinity increased, mosquitofish decreased their aggressive behaviour towards cyprinodonts and captured less prey. In contrast, the cyprinodonts did not change their behaviour with different salinity treatments, with the possible exception of increased defensive acts in higher salinities, but captured more prey with increasing salinity because of the reduced efficiency of mosquitofish. Our study confirms previous field observations that salinity limits the invasive success of mosquitofish and provides one of the few experimental demonstrations that it may mediate behavioural and competitive interactions between fish species. Condition-specific competition of mosquitofish might be expected with other species and ecosystems worldwide and illustrates the importance of integrating biotic and abiotic factors in the study of interspecific interactions.     

Can separation along the temperature niche axis promote coexistence between native and invasive species?

Aim Western mosquitofish (Gambusia affinis) have been linked with the decline of native fish and amphibians throughout the world. Separation along the temperature niche axis may promote the long‐term coexistence of introduced western mosquitofish, with native species in temperate regions. Recent research has shown that western mosquitofish can reduce the recruitment of native least chub (Iothichthys phlegethontis) endemic to the Bonneville Basin. We tested the hypotheses that cold temperatures (≤ 15 °C in the summer, freezing winters) would: (1) reduce the aggressive and predatory effects of western mosquitofish on least chub, and (2) eliminate the overwinter survival and recruitment of western mosquitofish while having little effect on least chub recruitment. Location Bonneville Basin of Utah, USA. Methods We used short‐term tests in the laboratory at the level of individuals and manipulated temperature (warm, cold and seasonal treatments) in long‐term experiments using mesocosms at the population level. Results Cold temperatures (≤ 15 °C) reduced the aggression and predation of western mosquitofish on least chub at the level of individuals. At the population level, however, cool summers (≤ 15 °C) eliminated recruitment in both species because they required warm summers (c. 20–30 °C) to survive freezing winters. Although least chub had an overwinter advantage in survival (75% least chub, 45% western mosquitofish), it was overwhelmed by the rapid reproduction of western mosquitofish as temperatures increased in the summer. Main conclusions Studies at the level of populations are necessary to understand the ultimate effects of introduced species on native taxa. Separation along the temperature niche axis was not sufficient to promote coexistence between these species in habitats with warm summers (c. 30 °C). Although coexistence may be possible in habitats with cool summers (≤ 20 °C) and freezing winters, the ability of niche separation to promote long‐term coexistence between native and introduced species may ultimately depend on their respective rates of evolution. Long‐term coexistence may not be possible if introduced species can adapt to new environmental conditions faster than native species can evolve mechanisms to reduce their harmful effects.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade

Summary We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m² enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m³), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.     

Trophic Cascades Uncoupled in a Coastal Marsh Ecosystem

The Mediterranean population of the exotic eastern mosquitofish Gambusia holbrooki (Agassiz 1859) (Osteichthyes, Poeciliidae) has been held responsible for causing eutrophication due to zooplankton removal and phytoplankton enhancement, however no experimental evidence exists of this. To test this allegation, an enclosure experiment was conducted in spring in an oligohaline coastal marsh. The manipulation of fish density had profound effects on zooplankton, whose density greatly decreased when the occurrence of mosquitofish increased. Cladocerans and ostracods were more affected by mosquitofish than cyclopoid copepods, whilst rotifer density was not modified. Changes in zooplankton density did not cascade to lower trophic levels as no differences were observed between the chlorophyll concentration in fish and fish-less enclosures. This is because zooplankton was dominated by species with low filter-feeding rates, such as small cladocerans. In consequence, the total macrophyte standing crop was not affected. The only benthic macroinvertebrate species whose density increased in the absence of eastern mosquitofish was the mud snail P. acuta. Higher numbers of snails explain why the standing crop of the filamentous green algae Oedogonium sp. decreased in fish-less enclosures. The density of chironomid midge larvae did not increase in fish-less enclosures, because eastern mosquitofish forage on them mainly during summer, when zooplankton has already been depleted; nor were damselflies, probably because they are too large. Nitrogen concentration decreased after fish exclusion, but phosphorus concentration remain unchanged. In conclusion, it was found that the eastern mosquitofish affect zooplankton of the Mediterranean oligohaline lagoons considerably, but they do not enhance phytoplankton growth, because the system is bottom-controlled by submerged macrophytes.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Interaction between the introduced eastern mosquitofish and two autochthonous Spanish toothcarps

In aquaria, adults and young-of-the-year (YOY) of both Spanish toothcarp Aphanius iberus and Valencia toothcarp Valencia hispanica received more aggression in the presence of eastern mosquitofish Gambusia holbrooki . YOY of both species also showed decreased feeding rates, as did V. hispanica adults. Adult V. hispanica also were more aggressive themselves and had their reproductive behaviour disrupted. Cases of eastern mosquitofish predation on small A. iberus were also observed. The occurrence and intensity of these potentially harmful effects, however, seemed to be modulated by water temperature, reproductive condition, relative size of the interacting fishes and species-specific habitat preferences. Thus, aggression almost disappeared at low water temperatures. Valencia hispanica and eastern mosquitofish used similar vertical positions in the water column and this apparently increased the frequency of their interactions. In contrast, A. iberus preferred positions closer to the bottom than the other two species and this vertical segregation appeared to lead to less direct interaction with eastern mosquitofish. In an outdoor mesocosm experiment, under semi-natural conditions, the presence of eastern mosquitofish resulted in an almost 70% reduction in the number of o.spring produced by A. iberus . The addition of extra, non-breeding males of A. iberus had no significant effect. YOY A. iberus from cages with eastern mosquitofish or extra males were significantly smaller (c. 1 mm, or 8% shorter) than those from cages without additional fish. Results for V. hispanica were not conclusive, as very few YOY were recovered. In contrast, eastern mosquitofish did not affect the survivorship of breeding adults of both native species.     

Ontogenetic Variation in the Diurnal Food and Habitat Associations of an Endemic and an Exotic Fish in Floodplain Ponds: Consequences for Niche Partitioning

In floodplain ponds with low piscivore abundance, both endemic Midgley's gudgeons, Hypseleotris sp. 5, and exotic mosquitofish, Gambusia holbrooki, showed significant ontogenetic variation in the use of food and space. Small gudgeons were generally associated with surface and benthic habitats, then restricted their distribution to benthic habitats at a size of approximately 24mm (standard length). The ontogenetic variation in mosquitofish habitat use was less discrete, and could be described as a tendency for larger individuals to be associated with the bottom of the littoral macrophyte beds than with the surface of the macrophyte beds or surface of the limnetic zone. Small gudgeons exhibited high spatial overlap with mosquitofish within the surface habitats of the ponds. All size-class/species comparisons showed significant partitioning of food resources, however, the diets of small gudgeons and mosquitofish were very similar. Therefore, juvenile gudgeons may have to pass through a similar spatial and trophic niche to introduced mosquitofish before recruiting to the adult stage. Possible mechanisms driving the ontogenetic variation in gudgeon and mosquitofish habitat use are discussed. This paper demonstrates that ontogenetic niche shifts at fine spatial scales can affect our interpretation of interactions between native and introduced fishes.     

Behavioral responses of American toad and bullfrog tadpoles to the presence of cues from the invasive fish, Gambusia affinis

The introduction of non-native predators is thought to have important negative effects on native prey populations. The susceptibility of native prey to non-native or introduced predators may depend on their ability to respond appropriately to the presence of these non-native predators. We conducted a laboratory based behavioral experiment to examine the response of American toad (Bufo americanus) and bullfrog (Rana catesbeiana) tadpoles to the presence of cues from the introduced mosquitofish (Gambusia affinis), a potential tadpole predator. Neither the American toad tadpoles nor the bullfrog tadpoles responded behaviorally to the presence of mosquitofish cues. If tadpoles are unable to respond to the presence of mosquitofish cues appropriately, then their ability to avoid predation by mosquitofish may be compromised and this may contribute to the impacts of mosquitofish on some tadpole populations.     

Impact of Holoshestes heterodon Eigenmann (Pisces: Characidae) on the plankton community of a subtropical reservoir: the importance of predation by Chaoborus larvae

We analyzed the effects of planktivorous Holeshestes heterodon Eigenmann (Characidae) predation on the plankton community of a small subtropical reservoir, using four enclosures (volume about 17.5 m³), open to the sediment, established in the littoral zone. Two enclosures were stocked with fish (mean TL 5.7 cm), at a density of about 4–5 fish m^–3 (approx. 8 g m^–3), whereas two remained fishless. The experiment lasted a little longer than one month. In the fish enclosures, most Crustacea and Chaoborus larvae remained scarce, probably as a result of visually selective fish predation. In both fishless enclosures, Chaoborus larvae became abundant. However, in only one of these did large individuals become relatively numerous; this discrepancy in the demographic structure of the Chaoborus populations between the two fishless enclosures is unexplained. Only in the fishless enclosure without appreciable numbers of large Chaoborus did densities of Crustacea increase greatly. It is suggested that in the enclosure containing large Chaoborus individuals, crustacean populations were prevented from developing due to predation pressure, while the small Chaoborus larvae of the other enclosure could not readily consume these prey. Rotifers were low in abundance in the absence of fish, probably as a consequence of Chaoborus predation. Phytoplankton density increased in all four enclosures, due probably to the lack of water flow. Only in the fishless enclosure with high densities of crustaceans did phytoplankton abundance decrease markedly at the end of the experiment, perhaps because of grazing losses.     

Abiotic factors,competition or predation: what determines the distribution of young crayfish in a watershed?

In the headwaters of the New River in western North Carolina, the dominant crayfish species changes with increasing stream size. One transition occurs between third- and fourth-order streams. Cambarus chasmodactylus is the dominant species in third-order tributaries. Orconectes cristavarius is the dominant species in the fourth-order South Fork of the New River. While adult C. chasmodactylus are present in the South Fork, the young-of-the-year (YOY) of this species are absent despite evidence of reproduction. In this study we evaluated the factors that may be responsible for the absence of C. chasmodactylus YOY from the South Fork. A field experiment was used to evaluate the role of abiotic factors and competition with YOY O. cristavarius. There was no significant effect of either of these factors on mortality or growth of C. chasmodactylus YOY. The growth rate of O. cristavarius was 3× faster than that of C. chasmodactylus in this experiment. Since neither abiotic factors nor competition appeared responsible for the exclusion of C. chasmodactylus YOY from the South Fork we evaluated the potential importance of selective predation by rock bass (Ambloplites rupestris), a fish species that is common in the South Fork but virtually absent in the tributaries. In a laboratory experiment, C. chasmodactylus YOY experienced significantly higher mortality than O. cristavarius YOY in the presence of rock bass. Field observations and a laboratory experiment suggest that the two crayfish species differ in their anti-predator behaviors. Cambarus chasmodactylus was less likely to swim when initially disturbed and swam shorter distances than O. cristavarius. The differences in escape behavior and growth rate may contribute to the differences in the vulnerability of the two species to rock bass predation. Our results suggest that intense predation pressure exerted by the rock bass may contribute to the virtual exclusion of C. chasmodactylus YOY from the fourth-order South Fork. Handling editor: J. Trexler     

Direct and indirect effects of predation on mosquitofish behavior and survival

Predation can have strong direct and indirect effects on the behavior of prey. We investigated whether predation by chain pickerel (Esox niger) caused adult eastern mosquitofish (Gambusia holbrooki) to alter their habitat use and whether pickerel predation influenced survival of adult and neonate mosquitofish. The number of adult mosquitofish using the riskier of three habitats was lowest when two predators occupied the risky habitat, intermediate in the treatment with one predator, and highest when no predators occurred there. More mosquitofish neonates survived high predation treatments than treatments lacking pickerel. We conclude that pickerel predation causes adult mosquitofish to shift to refuge habitats. The pattern of neonate survival suggests that adult habitat use may create a refuge from cannibalism for neonate mosquitofish, resulting in higher neonate survival in treatments with more pickerel. Hence, pickerel predation has a direct effect on adult mosquitofish behavior and a strong indirect effect on neonate survival. Both interspecific and intraspecific predation can effect prey populations and can interact to produce important indirect effects.     

Separating the effects of intra- and interspecific age-structured interactions in an experimental fish assemblage

We documented patterns of age-structured biotic interactions in four mesocosm experiments with an assemblage of three species of co-occurring fishes from the Florida Everglades, the eastern mosquitofish (Gambusia holbrooki), sailfin molly (Poecilia latipinna), and bluefin killifish (Lucania goodei). These species were chosen based on their high abundance and overlapping diets. Juvenile mosquitofish and sailfin mollies, at a range of densities matching field estimates, were maintained in the presence of adult mosquitofish, sailfin mollies, and bluefin killifish to test for effects of competition and predation on juvenile survival and growth. The mesocosms held 1,200 l of water and all conditions were set to simulate those in Shark River Slough, Everglades National Park (ENP), USA. We placed floating mats of periphyton and bladderwort in each tank in standard volumes that matched field values to provide cover and to introduce invertebrate prey. Of 15 possible intra- and interspecific age-structured interactions, we found 7 to be present at the densities of these fish found in Shark River Slough marshes. Predation by adult mosquitofish on juvenile fish, including conspecifics, was the strongest effect observed. We also observed growth limitation in mosquitofish and sailfin molly juveniles from intra- and interspecific competition. When maintained at high densities, juvenile mosquitofish changed their diets to include more cladocerans and fewer chironomid larvae relative to low densities. We estimated size-specific gape limitation by adult mosquitofish when consuming juvenile mosquitofish and sailfin mollies. At high field densities, intraspecific competition might prolong the time period when juveniles are vulnerable to predation by adult mosquitofish. These results suggest that path analysis, or other techniques used to document food-web interactions, must include age-specific roles of these fishes.     

Sheep grazing and rodent populations: evidence of negative interactions from a landscape scale experiment

Inter-specific competition, facilitation and predation influence herbivore assemblages, but no study has experimentally explored the interactions between large ungulates and small rodents. In a fully replicated, landscape scale experiment, we manipulated densities of domestic sheep in mountain pastures in Norway. We then determined population growth and densities of rodents by live trapping in each of the areas with different sheep densities. We found that the (summer) population growth rate and autumn density of the field vole (Microtus agrestis) was lower at high sheep density. This provides the first experimental evidence of negative interactions between an ungulate and small rodent species. There was no effect on the bank vole (Clethrionomys glareolus), whose diet differs from sheep. Sheep density, therefore, potentially alters the pattern of inter-specific population synchrony amongst voles. Our study shows that negative interactions between large ungulates and small rodents may be species-specific and negative population consequences for the rodent population appear above threshold ungulate densities.Electronic supplementary material is available for this article at     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Identifying Environmental Conditions to Promote Species Coexistence: An Example with the Native Barrens Topminnow and Invasive Western Mosquitofish

Coexistence of a native and invasive species may be possible at certain conditions along an environmental gradient where the individual responses of each species are maximally apart. Water temperature may differentially affect the growth of a native cool-water species like the Barrens topminnow, Fundulus julisia, and an originally warm-water adapted western mosquitofish, Gambusia affinis, who is a recent invader in Barrens Plateau region of middle Tennessee. We measured the specific growth rate (SGR) of the two species separately in laboratory aquaria at 10, 15, 20 and 25 °C, representing a range of temperatures that occur in topminnow habitats throughout the year. Both species grew faster with increasing temperature and SGRs were highest at 25 °C. The interspecific difference in SGR was maximized at 15 °C. At this temperature, mean growth rate of topminnows was 0.78% per day, more than twice that of mosquitofish (0.38% per day). These results suggest that cool springhead habitats with a near-constant thermal environment of 15 °C throughout the year may provide a growth advantage to the Barrens topminnow over the mosquitofish. Other environmental, density-dependent, or behavioral factors not examined here may act along with temperature to mediate the coexistence of the topminnow and mosquitofish.     

Biotic resistance to an invasive spider conferred by generalist insectivorous birds on Hawai’i Island

A central problem for ecology is to understand why some biological invasions succeed while others fail. Species interactions frequently are cited anecdotally for establishment failure, but biotic resistance is not well supported by quantitative experimental studies in animal communities. In a 33-month experiment on Hawaii Island, exclusion of native and alien forest birds resulted in a 25- to 80-fold increase in the density of a single non-indigenous spider species (Theridiidae: Achaearanea cf. riparia). Caged plots held large aggregations of juveniles and more large-bodied individuals, suggesting potential reproductive individuals are more susceptible to bird predation. Most examples of biotic resistance involve competition for limiting resources among sessile marine animals or terrestrial plants. The present results show that generalist predators can limit the success of introductions, even on oceanic islands, generally assumed less resistant to invasion.     

Stage-structured interactions between seasonal and permanent residents of an estuarine nekton community

Estuarine assemblages of fishes and natant decapod crustaceans (i.e. nekton) comprise both permanent resident species and juveniles of coastal marine species that use estuaries primarily as nurseries. In an attempt to understand how the young of marine species successfully invade communities of permanent estuarine residents we studied potential interactions between two of the most abundant decapod crustaceans in nekton assemblages of the southeastern United States. Three years of quantitative samples from an intertidal marsh on Sapelo Island, Georgia showed that densities of the resident daggerblade grass shrimp Palaemonetes pugio were reduced during the time that juvenile white shrimp Penaeus setiferus used the estuary as a nursery. Results of a field enclosure experiment showed that white shrimp had no significant lethal or sublethal effects on adult grass shrimp. However, they did reduce survival of both juvenile and larval grass shrimp in laboratory experiments, suggesting the potential importance of a stage-dependent predatorprey interaction between the two shrimp species. The mortality rate of young grass shrimp in the presence of white shrimp was unaffected by grass shrimp density, but larvae (2.6–3.0 mm) suffered higher mortalities than did juveniles (5.0–15.0 mm). We suggest that the vulnerability of grass shrimp to predation by white shrimp is related to their molting cycle. The window of vulnerability opens more often for younger grass shrimp because they molt more frequently. When combined with losses due to other predators and competitors, the impact of early white shrimp cohorts on grass shrimp larvae and juveniles may prevent the resident species from maintaining its population at high densities, thereby freeing resources in the nursery for subsequent cohorts of juvenile white shrimp.