Effects of nonnative fish and habitat characteristics on lentic herpetofauna in Yosemite National Park, USA

Nonnative fishes have been introduced into naturally fishless mountain lakes worldwide, often with negative consequences for native fauna. In this study, I used data collected during a census of native herpetofauna, nonnative trout, and habitat characteristics at all lentic water bodies in Yosemite National Park (n=2655) to quantify the effect of trout introductions and habitat on the distribution of four amphibian species and two reptile species. Impacts of trout on amphibians and reptiles were characterized using generalized additive models that included as predictor variables fish presence/absence, amphibian presence/absence (only in models for the two reptile species), elevation, water depth, littoral zone and shoreline substrate composition, relative survey date, and location. All species showed significant associations with habitat characteristics, and elevation and water depth appeared particularly important in influencing distributions. In addition, distributions of the mountain yellow-legged frog (Rana muscosa) and Pacific treefrog (Hyla regilla) were strongly negatively associated with the presence of nonnative trout while those of the Yosemite toad (Bufo canorus) and Sierra newt (Taricha torosa sierrae) were unrelated to trout presence. The distribution of the mountain garter snake (Thamnophis elegans elegans) was strongly negatively associated with the presence of nonnative trout and positively associated with the presence of native amphibians. Regression results for the Sierra garter snake (Thamnophis couchi couchi) were similar except that the direct effect of nonnative trout was considerably weaker. Together, these results indicate that trout introductions have resulted in considerable alteration of Yosemite's herpetofauna. Long-term studies will be necessary to determine whether removal of nonnative trout populations, where possible, would allow these impacts to be partially reversed.     

Patterns of fish invasions in the Great Plains of North America

A basic step toward controlling invasive species outbreaks is to predict which assemblages or habitats are most susceptible to invasion and effects of invading species on native communities. However, because predictive models may not work across a wide range of conditions it is important to examine processes associated with fish invasions across different regions. In this study, we use data from 949 sites in Oklahoma and Kansas to examine spatial patterns of species invasions in the Great Plains region of the US. Of these sites, 16 were intensively sampled between 1977 and 1994 and used to evaluate temporal patterns of species introductions. We found a trend of increasing introduced species richness without a decline in native species richness over 18 years. However, total abundance and community structure of native assemblages has changed at several of these sites during this time period. Because some of these changes occurred at sites with few or no introduced species, factors other than interactions with introduced fishes may have been responsible for this pattern. A stepwise multiple regression model that included human population size, native species richness, mean annual precipitation and drainage area explained 43% of the variation in the number of non-native species across all sites. Most introduced species in this region have native ranges within the Mississippi River basin and therefore are presumably adapted to regional environmental conditions. Great Plains streams have, to some extent, resisted invasion by introduced species; however, our analysis shows that proximity to dense human populations and importance to humans has largely structured species introductions and can partly explain spatial patterns of species invasions in this region.     

Effects of introduced fish on macroinvertebrate communities in historically fishless headwater and kettle lakes

Widespread fish introductions have led to a worldwide decline in the number of fishless lakes and their associated communities. Studies assessing effects of fish stocking on native communities in historically fishless lakes have been limited to high-elevation headwater lakes stocked with non-native trout. Little is known about the effect of fish stocking in historically fishless and hydrologically isolated lowland kettle lakes. We compared the effects of introduced fish on macroinvertebrate communities in kettle lakes stocked with centrarchids, salmonids, and cyprinids, and headwater lakes stocked with brook trout (Salvelinus fontinalis) in Maine, USA. Fish had significant effects on macroinvertebrate community structure in both lake types, with reduced species richness and abundances of taxa characteristic of fishless lakes. The effects of introduced fish were more pronounced in headwater lakes despite a less diverse fish assemblage than in kettle lakes. We attribute this to abundant submerged vegetation providing refuge from fish predation and reduced stocking frequency in kettle lakes. We assessed effects of stocking duration on macroinvertebrates in a subset of headwater lakes with known dates of trout introduction. Species richness and abundance of most taxa declined within 3 years following trout introduction; however, richness and abundance were least in lakes with long stocking histories (?40 years). Macroinvertebrates previously identified as fishless bioindicators were absent from all stocked lakes, indicating that trout rapidly eliminate these sensitive taxa. Conservation of this historically undervalued ecosystem requires protecting remaining fishless lakes and recovering those that have been stocked.     

Evidence of indirect impacts of introduced trout on native amphibians via facilitation of a shared predator

Hyperpredation occurs when non-native prey facilitate invasive predators, which then suppress native prey. Direct impacts of introduced fish on amphibians are well studied, but the role of fish in supporting shared predators has not been considered. We present evidence for indirect effects of trout on amphibians through snake predation. Analyses of the diet, distribution and density of the Pacific coast aquatic garter snake (Thamnophis atratus) relative to the sympatric common garter snake (Thamnophis sirtalis) in the Klamath Mountains of California suggest that trout introductions facilitated expansion of T. atratus by providing alternative prey. T. atratus diet included trout and amphibians whereas T. sirtalis preyed solely upon amphibians. The distribution and density of T. atratus matched that of introduced trout instead of native amphibians. Populations of T. atratus could reach high densities in the absence of high densities of amphibians. When the snakes opportunistically prey upon amphibians whose numbers are already directly impacted by trout, they can cause significant additional declines. When T. atratus was present in lake basins, native Cascades frogs (Rana cascadae) were rarer than in basins without T. atratus. This case differs from other hyperpredation studies because the two prey species also interact via intraguild predation. Given the worldwide practice of stocking fish into aquatic habitats, it is important to understand the consequences of the practice on food-web structure and ecosystem functioning. Bottom-up impacts of introduced predators should be considered as well as top-down so that managers can incorporate the range of ecosystem-level effects into conservation goals and decisions.     

No evidence of general decline in an amphibian community of Southern France

We investigated long term changes in an amphibian community in the Languedoc area of Southern France by comparing results of a survey of 56 ponds made in the early 1970s with results of a survey of the same localities in 2001. Based on the frequency of new occurrences and disappearances of species in the sample ponds, there is no sign of a general decline of amphibians in this area. Most species showed non-significant variation in frequency of occurrence or had increased. “Green frogs” (Rana perezi/ridibunda/kl. grafi) showed clear signs of decline, but this is a local phenomenon rather than a decline at the regional scale. Pelodytes punctatus probably declined also, possibly due to its sensitivity to fish introduction. The species richness in the ponds has also remained stable except in ponds where fish have been introduced. The only adverse impact of human activities that we could detect was thus the introduction of fish, which had a significantly adverse effect on amphibian species richness.     

Freshwater fish in Scotland in the 18th, 19th and 20th centuries

Information on the distribution of freshwater fish in Scotland, published in old accounts (particularly around 1790 and 1880), is compared with that of a recent survey (around1970). Several changes are evident over the 180 years concerned, the main one being a northward extension of the distribution of a number of species, several apparently new to Scotland. Some of this dispersal is probably natural, but the main factor appears to be introduction by man — both from further south in Great Britain and from abroad. A number of unsuccessfull introductions of other species have been made. The future fish fauna of Scotland is discussed in the light of past and present events.     

Factors resulting in decline of the endangered Sonoran topminnow Poeciliopsis occidentalis (Atheriniformes: Poeciliidae) in the United States

The endangered Sonoran topminnow Poeciliopsis occidentalis has steadily declined in distribution and abundance in the past several decades, and currently survives in the United States only in several isolated localities in southern Arizona. This reduction is correlated with, and primarily attributed to, habitat destruction, and introduction and establishment of mosquitofish Gambusia affinis and other exotic fishes. Topminnows have characteristically been reduced in number or replaced within a year or two of introduction of non-native fishes. Other native fishes have experienced similar declines after introduction of exotics, and much of the endemic western ichthyofauna may be vulnerable to extirpation in this manner. Predation by introduced fishes on natives appears to be a likely mechanism of replacement.     

The extent and impacts of ungulate translocations: South Africa in a global context

Despite the apparent risks of the introduction of non-indigenous ungulates to biodiversity, relatively little is known globally about the pathways of introduction, propagule pressure and realized impacts of ungulate introductions. These issues were examined here by investigating ungulate introductions to South Africa within a global context. Across countries globally, introduced ungulate richness is not related to indigenous ungulate richness, and several countries are clear outliers. South Africa is second only to the USA in the number of ungulate species introduced to date. Zoos have traded more ungulate species and individuals to non-zoo recipients than to other zoos, highlighting the tensions that exist between in situ and ex situ conservation goals. Introductions to, and extralimital introductions within South Africa have increased through time, with propagule pressure being highest in areas with high human population density. The long distances ungulates have been translocated raise concerns for genetic homogenization. Translocations of indigenous ungulate species extralimitally have significantly altered range sizes, typically to a greater extent than is expected from range shifts associated with global climate change. Although ungulate introductions and translocations are likely to have impacts on biodiversity, evidence for such impacts in South Africa, and elsewhere, is limited. Whilst arguments may be made for a precautionary approach to ungulate introductions, an evidence-based one is much more likely to deliver efficient and convincing conservation decision-making.     

The fish fauna of mountain streams in South Korean national parks and its significance to conservation of regional freshwater fish biodiversity

The fish fauna of mountain streams in the Jiri (440 km²) and Seorak (373 km²) National Park areas of South Korea was investigated from September 1998 to May 2001. A total of 5979 fish were collected and classified into 22 families and 60 species from both national parks. At Jiri, 30 species in 12 families of fishes were collected, while at Seorak there were 42 species in 17 families. Zacco temmincki [relative abundance (RA) 47.9%] was dominant at both parks. Subdominant species were Rhynchocypris kumkangensis (RA 10.8%), Zacco platypus (RA 9.4%), and Pungtungia herzi (RA 5.3%). A total of 20 species were found to be Korean endemic species, representing seven families. The relative abundance of Korean endemic species was higher at Seorak (30.5%) than Jiri (18.5%). One exotic species (Oncorhynchus mykiss) and two species translocated outside their native catchments (Hypomesus nipponensis and Coreoperca herzi) were collected. The proportion of Korean endemic freshwater fish species in both parks (33.3%) was higher than the overall proportion on the Korean peninsula (23.6%). Within the 14 South Korean mountain-area national parks as a whole, larger parks tend to have higher fish diversity. We conclude that South Korean national parks are important for conservation of the regional fish fauna, especially for endemic and endangered species. Current threats to conservation of fishes within South Korean national parks are identified and management solutions are suggested.     

Impact of fish introduction on planktonic food webs in lakes of the Patagonian Plateau

Patagonia fishless ponds have been stocked with fishes for recreational purposes since early in the 20th century. We carried out a summer plankton sampling in 18 Patagonian lakes; 12 fishless, 5 with introduced fishes and 1 with endemic fish fauna. The lakes are situated on a latitudinal gradient from 39° to 49°S. Zooplankton and phytoplankton composition, phytoplankton relative abundance, and zooplankton body size and mouthpart morphology were analyzed. Results showed differences between lakes with and without fishes; in the presence of fish zooplankton size spectrum tended to be narrower because of the disappearance of Daphnia and large centropagid copepods. Zooplankton composition changed: centropagid species richness decreased and rotifers dominated. Contrarily, in fishless lakes 3 or 4 centropagid species, differing markedly in body size and exploiting different food niches, were observed co-occurring. These changes in zooplankton seemed to cascade down to phytoplankton. Fish introduction increased the phytoplankton similarity in lakes even belonging to different basins in a latitudinal gradient. Indeed, cyanobacteria dominated only in lakes with introduced fishes. Probably the elimination of Daphnia favored cyanobacteria proliferation due to nutrient rebalance. As a consequence, water quality decreases and the value of sport fisheries is reduced. Fish introduction in Patagonia is a practice that should be re-evaluated by governments and NGOs due to its potentially negative impact on lakes and local economies.     

A multi-scale approach to facultative paedomorphosis of European newts (Salamandridae) in the Montenegrin karst: Distribution pattern, environmental variables, and conservation

Facultative paedomorphosis, a process in which newt larvae can opt for reproduction before or after metamorphosis, is geographically heterogeneous. Despite numerous ecological studies and recent evidence of declines in paedomorphic populations, however, no attempt to model environmental variables that explain the presence of paedomorphs has been made at a multi-scale level. Our aim was to fill this gap in studying three newt species (Lissotriton vulgaris, Mesotriton alpestris, and Triturus macedonicus) of the Montenegrin karst as model species. To this end, we used multivariate analysis on three scales of habitat: the breeding pond, the land use and the climatologic features. Results show that the study area is both an important hotspot for paedomorphosis and where intraspecific diversity is quickly disappearing (20-47% extirpation) because of fish introductions. Other habitat variables (water permanency, pH or the habitat origin) were shown to act on paedomorphosis but not consistently across species, confirming complexity of the evolutionary and ecological processes. This study appeals for more long-term and detailed landscape studies of polyphenisms, a neglected but promising topic, to better understand and protect alternative modes of development. Particularly, measures should be taken to identify hotspots of intraspecific diversity at a global scale and stop fish introductions before we reach a point of no-return.     

Distribution of non-native trout in Slovenia and their introgression with native trout populations as observed through microsatellite DNA analysis

In Slovenia, the Adriatic basin inhabited by native marble trout (S. marmoratus), and the Danubian basin inhabited by native Danubian lineage of brown trout (S. trutta) have been intensively affected by stocking with non-native trout strains. In order to assess spread of non-native strains and their introgression with native trout, a population study based on five microsatellite loci was applied across ten marble and ten brown trout populations, ranging from allegedly non-introgressed to heavily managed. On the basis of correspondence analysis, which revealed three clear groupings consisting of the Danubian and Atlantic lineages of brown trout and the marble trout, the alleles, characteristic of each grouping were identified and used for estimating genetic composition of each population according to the three possible origins. Among the wild populations, five marble and one brown trout populations were found to be pure; all the others were introgressed with exotic alleles (Atlantic and marmoratus alleles in the Danubian basin and Atlantic and Danubian in the Adriatic basin) that markedly dominate in intensively managed populations. As revealed by non-significant F_IS values, panmixia between native and introduced fishes has for the most part already been reached. Our research showed that it is not only marble trout whose identity is endangered in Slovenia but also the existence of autochthonous Danubian brown trout is critically compromised, which is new information to be taken into account for local trout conservation.     

Lake Kinneret (Israel) Ecosystem: Long-term Instability or Resiliency?

The Kinneret ecosystem has undergone significant man-made modifications. These include, among others: construction of the south dam (1932); drying of Hula Lake and swamps (1950s); salty springs diversion (1964); lake water salinity decline from 395 (1961) to 210 ppm chlorid; deepening of the Jordan River near Bnot-Yaacov bridge (1971) with consequent high loads of suspended matter; National Water Carrier (NWC) operation (1964); sewage removal and fishpond restriction in the catchment; implementation of the Hula Project (1994–1998); introduction of native and exotic fishes; an amplitude of water level fluctuations of 4.8 m. Natural fluctuations occurred as well: heavy floods with high nutrient loads in 1968/69 and 1991/92 and droughts in 1973, 1989–1991 and 1998–1999; 4.6 m water level fluctuations; low temperatures (12.3°C) and fish kill in 1971/72 and 1991/92; no mixing period in 1984; low (1975, 1996–1997) and high biomass of Peridinium (1994, 1998); N₂-fixing cyanobacterium bloom (1994, 1995); high stock of Lavnun (Acanthobrama) fishes (1990s); zooplankton biomass decline (1969–1993) and increase afterwards. Although man made changes and natural environmental changes were significant, the lake ecosystem did not deviate from the level of resilient fluctuations and the classification of long-term instability in Lake Kinneret is probably an overestimation. In this paper I use long-term (31 years) distribution data of plankton, nutrients, and water level in Lake Kinneret to distinguish between resiliency and instability.     

Do native species limit survival of reintroduced Atlantic salmon in historic rearing streams?

Reintroduction of extirpated populations creates a unique context that can exacerbate the effects of interactions among species. Thus, reintroduced populations may be particularly vulnerable to predators and competitors, including native species with which they historically coexisted. In this study, we evaluated the effect of native fishes on survival of reintroduced Atlantic salmon (Salmo salar) in the Connecticut River basin, where the native salmon population is extinct. Juvenile salmon are stocked annually in many Connecticut River tributaries. We sampled salmon reintroduction sites across tributaries with different fish communities to determine whether native fish reduce the success of salmon reintroductions (N = 19 site-years). Increased density of slimy sculpin (Cottus cognatus), a native generalist predator, was associated with reduced recruitment of reintroduced salmon. Salmon first-summer survival declined with increased sculpin density across sites, and low first-summer survival led to reduced densities of overyearling salmon the subsequent year. Hierarchical partitioning analysis showed that the negative relationship between sculpin and salmon was independent of potentially confounding variation in other fish community or habitat characteristics. Negative effects of native, historically-sympatric species, particularly generalist predators, can impede restoration of extirpated populations.     

Holoparasitic Cuscuta campestris suppresses invasive Mikania micrantha and contributes to native community recovery

Traditional biocontrol uses introduced non-native agents to control alien invasive species. Recently, using species of the invaded regions to control invaders has been proposed as a potential alternative to traditional biocontrol. To explore the effects of the obligate parasite Cuscuta campestris in the invaded regions on controlling the alien invasive weed Mikania micrantha and on recovering the heavily invaded communities in the coastal Guangdong Province in South China, we conducted a field survey in four sites on the Neilingding Island where Cuscuta has been artificially introduced to the Mikania-invaded communities for one to 4 years, and also three sites in Shenzhen, Dongguan and Haifeng where the Mikania-invaded communities have been naturally parasitized by Cuscuta for at least 5 years. Cuscuta effectively suppressed the growth and invasiveness of Mikania, leading to its decline. The restraint on Mikania by Cuscuta increased richness and diversity of native plants, contributing to native community recovery. Moreover, Cuscuta declined with decreasing Mikania, and its parasitism rate on Mikania was much higher than that on the native species, suggesting little non-target effects. The effects of Cuscuta varied little among sites of Shenzhen, Dongguan and Haifeng, but increased greatly with time of introduction on the Neilingding Island. Our results suggest that Cuscuta is an effective agent for controlling Mikania, and that enemies of the invaded regions may be a promising alternative to traditional biocontrol.     

Exotic fishes in fresh and brackish waters of Florida

At least 38 species and several hybrids of exotic fishes have been found in fresh and brackish waters of Florida. Twenty species and five hybrids are at present established in the form of reproducing populations. The ornamental aquarium fish industry has been the major source of these introductions. Several methods of introductions are considered and recommendations presented toward preventing repetitions in the future. Future dispersal of exotics by channelization projects should become part of environmental impact statements.     

Community-level impacts induced by introduced largemouth bass and bluegill in farm ponds in Japan

Largemouth bass Micropterus salmoides and bluegill Lepomis macrochirus have been introduced into many countries and have become cosmopolitan exotic species. However, only a few studies show their impact on introduced communities. To investigate their impact, we performed natural snapshot experiments in 15 farm ponds in Saitama prefecture, eastern Japan. We selected 10 and 5 small ponds in similar environmental conditions, but with and without exotics, respectively. The numbers of fish, crustaceans, and nymphal odonates were smaller in ponds where exotics were found and some species were considered to be locally extinct in several ponds. On the other hand, the numbers of benthic organisms, i.e. tadpoles, chironomids, chaoborids, and Oligochaeta were more abundant in ponds with exotics. These two groups of organisms were separated clearly on the first axis of DCA, which indicats that this difference was mainly induced by bass predation. This result suggests a trophic cascade in which top-down effects induced by exotics propagated to fish, crustaceans, and nymphal odonates directly and to some benthic organisms indirectly. Therefore, when one is to conserve native organisms, attention should be paid not only to direct negative effects, but also to indirect effects propagating to various trophic levels. Because farm ponds are typical Japanese small lentic systems having rich biodiversities and bass and bluegill have been shown to change farm pond communities widely, conservational treatments including eradication of exotic fish should be conducted immediately.     

Effects of urbanization on California’s fish diversity: Differentiation, homogenization and the influence of spatial scale

Human development of freshwater ecosystems has lead to drastic changes in freshwater fish faunas, including the loss of many native species and the gain of non-natives. Typically conservation ecologists view these two opposing forces as contributing to biological homogenization, and consider homogenization as one of the principle negative consequences of urbanization. However, homogenization is only one outcome out of many that can result from the loss and gain of species. In particular, it is possible for invasions and extinctions to lead to differentiation; a process whereby two (or more) regions become less similar to one another through time. Using the freshwater fishes of California, we show that urbanization is highly positively correlated to both the endangerment of native fish and the invasion of non-native fish within watersheds. Despite this, the fish faunas of California’s watersheds have differentiated from one another through time. Furthermore, the degree of differentiation is positively correlated with measures of urbanization, which is contrary to expectation. We suggest that this result reflects: (1) the haphazard manner in which non-native fishes have been introduced into California watersheds, (2) the difficulty that both native and non-native fishes have in expanding their geographical ranges, and (3) the continued presence of vestiges of formerly distinct regional faunas. This pattern of differentiation among watersheds is likely a matter of scale, as previous work on freshwater fishes has demonstrated homogenization at both larger and smaller spatial scales. In addition the observed pattern is probably a short-term (temporal) phenomena and will disappear with continued invasion and extinction. We suggest that similar patterns may occur for other taxa that have limited natural dispersal abilities and that are idiosyncratically released as non-natives via human activities (e.g. herptiles).     

Atlantic salmon resources in the Northeastern United States and the potential effects of acidification from atmospheric deposition

The Atlantic salmon (Salmo salar) was formerly abundant in northeast coastal rivers in the United States from the Canadian border to the Connecticut River, and possibly as far south as the Delaware River. It was eliminated from most of its former range by a combination of overfishing, construction of dams impassable to migrating fish, and municipal and industrial pollution. Reproducing populations are now limited to a few rivers in Maine, but attempts are under way to reintroduce the species to some rivers where populations formerly existed. Most of the native Atlantic salmon rivers are low in acid neutralizing capacity and receive acidic precipitation. The third order streams are not now acidic; however, in some first and second order streams in Maine, pH episodically declines to 4.7 and Al increases to 350 μg g^?1. These conditions could be toxic to sensitive early life history stages of Atlantic salmon. Comparison of chemical conditions in two Maine rivers in 1980–1982 with those in 1969–1970 indicated that the streams have not become more acidic during this interval. Data on the sport catch of Atlantic salmon indicated that populations have generally remained stable or recently increased in the Maine rivers having naturally reproducing populations. The recent increase probably resulted from introductions of hatchery smolts to supplement natural reproduction, and the occurrence of strong year classes in 1978 and 1980. The population in one stream has declined significantly in recent years, but the cause of the decline is probably not related to acidic precipitation. Atlantic salmon resources in the U.S. have apparently not been adversely affected by atmospheric deposition at the present time.     

Altered drying regime controls distribution of endangered salamanders and introduced predators

Introduction of invasive species has been implicated in the decline of many native species. However, the mechanisms that underlie successful invasion and species replacement are often poorly understood. We tested the hypothesis that disruption of the natural disturbance regime can facilitate invasion of habitats, resulting in the exclusion of native species. In Arizona’s San Rafael Valley, cattle ponds designed to hold water year-round have replaced seasonal marshes, and this altered drying regime has facilitated invasion by disturbance-intolerant fish and bullfrogs that negatively affect native Sonoran tiger salamanders. We investigated the relationships among pond drying regime, presence of introduced fish and bullfrogs, and presence of tiger salamander populations in 42 ponds. Both fish and breeding bullfrog populations disappeared following pond drying. Pond drying in cattle ponds negatively affected salamanders, but not to the same extent that it affected fish and bullfrogs. Metamorphosed bullfrogs ate salamander larvae in laboratory and field experiments, and the risk of local extinction among salamander populations was increased by introduced fish. Once fish eliminated salamanders from an aquatic habitat, salamanders seldom reappeared unless fish were killed by drying. Simple models were developed to predict change in salamander, fish, and bullfrog distributions with changes in cattle pond drying frequency. The models show that decreases in pond drying frequency could negatively affect salamanders by leading to a community dominated by disturbance-intolerant fish and bullfrogs, but increased drying frequency could negatively affect salamander populations by preventing salamander breeding. These results suggest that manipulation or restoration of disturbance regimes may be a powerful tool in managing for native species threatened by biotic invasions.