Distribution and abundance of the Louisiana red swamp crayfish Procambarus clarkii Girard at Lake Naivasha,Kenya between 1987 and 1999

The abundance and habitat distribution of Procambarus clarkii, an exotic herbivore at Lake Naivasha, Kenya, was studied between 1987 and 1999. Its growth and morphometrics were similar to other locations in the world where the species have been studied. P. clarkii was found primarily amongst the floating, exotic vegetation where it achieved densities of over 500 m^–2 by the end of the study. It is suggested that these densities became possible because the habitat offers protection against predation and an adequate food supply of bacterial-enriched detritus exists in the absence of green plant material. Both adults and juveniles forage on the lake bed at night.     

The aquatic plant communities of the Lake Naivasha wetland,Kenya: pattern,dynamics and conservation

The spatial and temporal patterns of the wetland plant communities at Lake Naivasha over the past decade, and the past five years in particular, are discussed in relation to the major controlling factors. The four communities are:- the emergent swamp, dominated by Cyperus species; the floating raft, dominated by the aliens Salvinia molesta and Eichhornia crassipes; the floating-leaved plants, represented only by Nymphaea caerulea; and the submerged angiosperms, consisting of three species of Potamogetons — P. pectinatus, P. schweinfurthii, P. octandrus — together with Najas pectinata. The major factors affecting their dynamics are:- water level changes which influence agricultural clearance, introduced crayfish Procambarus clarkii, and interactions between communities such as the physical effects of mobile floating rafts. The value of the different communities to the ecological and economic value of the lake is highlighted, and the possible damage from future activities discussed.     

The smell of danger: chemical recognition of fish predators by the invasive crayfish Procambarus clarkii

1. Since avoiding predation can compromise animal fitness, prey are expected to respond to different predator species with an intensity appropriate to the level of risk. In fresh waters, the threat of predation is typically assessed by chemical cues, in particular by odours released by either injured/disturbed conspecifics (conspecific alarm odour) or predators (predator odours). Here, we used the most widely distributed crayfish in the world, the invasive North American Procambarus clarkii, to investigate the relative effectiveness of odours emitted by fish predators compared with conspecific alarm odour. We also tested whether P. clarkii is able to discriminate between fish predators of which it has ‘experience’ (either recent, via introduction to the same water body, or old, by sharing a native range), as well as between fish predators that pose low or high risk. 2. The study was carried out on introduced populations of P. clarkii from two sites, characterised by different fish assemblages: the Malewa River (a tributary of Lake Naivasha, Kenya) and Lake Trasimeno (Italy). Laboratory experiments consisted of three sequential phases (‘water’, ‘food’ and ‘smell’ phases) and five treatments. Treatments differed in the odour presented during the smell phase, i.e. no odour (plain water) and odours from either injured conspecifics or three fish species per site. Crayfish from the Malewa River population were confronted with the odours of largemouth bass (Micropterus salmoides), common carp (Cyprinus carpio) and tilapia (Tilapia zillii) (all introduced to Lake Naivasha but absent from the Malewa River), and those from the Lake Trasimeno population with the odours of the introduced largemouth bass and carp and the native chub (Squalius cephalus). Largemouth bass is the only predator that imposes a high risk to crayfish, and it also shares its native range with P. clarkii. We analysed the time spent by crayfish feeding, in locomotion and in adopting a raised or lowered posture. A reduction in the time spent feeding and in locomotion, and an increase in the time spent in the lowered posture were considered to indicate alarm. 3. Crayfish from both populations responded with a more pronounced reduction in feeding to conspecific alarm odour rather than to predator odours. Crayfish from the Malewa River reacted with the same intensity to the odours of the three fish species tested, whereas, in Lake Trasimeno, the odour of largemouth bass was significantly more threatening than the odours of the other two species. 4. Procambarus clarkii seems to perceive a general fish odour that alerts it to possible predation risk without the need of either a direct recent experience or via sharing a common native range. However, where they coexist with fish, crayfish become able to distinguish among species, adapting the intensity of their response to the effective risk. Our results confirm the relatively high learning capacity of P. clarkii reported in previous studies and suggest the existence of mechanisms that make predator recognition particularly efficient in this extraordinarily successful invader.     

Geographic expansion of the invasive red crayfish <Emphasis Type="Italic">Procambarus clarkii</Emphasis> (Girard, 1852) (Crustacea: Decapoda) in Mexico

The red crayfish Procambarus clarkii, which is native to southcentral USA and northeastern Mexico, has been successfully introduced into several countries around the world. This study documents the geographic expansion of the exotic red crayfish in Mexico and discusses the consequence of a greater propagation of this species in Mexican inland waters. New state records of this crayfish in the Baja California peninsula and in the states of Durango and Sinaloa indicate its progressive dispersion. The propagation of P. clarkii in Mexico has been caused mainly by human introduction, but it is also facilitated because of the species’ tolerance to an ample range of environmental conditions. Because of the invasive capability of P. clarkii, we suspect that this exotic species is competing for habitat and food with native freshwater shrimp of the genus Macrobrachium in many sites of northern Mexico.     

Length–weight relationships of fish species in the freshwater rift valley lakes of Kenya

This study reports length–weight relationships for 10 species of fish encountered in the only two freshwater lakes wholly in the Rift Valley of Kenya, Naivasha and Baringo. In Lake Naivasha, none of the species analysed was native to the lake. Two were introduced species non‐native to East Africa (Micropterus salmoides, Cyprinus carpio), with three native to East Africa but not to the lake (Tilapia zillii, Oreochromis leucostictus, Barbus paludinosus). In Lake Baringo, all species were native (Oreochromis niloctictus baringoensis, Labeo cylindricus, Barbus intermedius autralis, Barbus lineomaculatus, Clarias gariepinus). Specimens were collected by gill netting in September 2004 and October 2005. With the exception of B. lineomaculatus, the b values in the relationship W = aL^b were between 2.90 and 3.22.     

Two hundred years of a diverse Daphnia community in Lake Naivasha (Kenya): effects of natural and human-induced environmental changes

1. We used fossil diapausing eggs extracted from ²¹⁰Pb‐dated sediment cores to reconstruct historical changes in the Daphnia community of Lake Naivasha, a climate‐sensitive lake in Kenya which over the past 200 years has experienced a series of well‐documented natural and anthropogenic environmental changes. 2. Contiguous sampling and analysis of four cores yielded ephippial capsules of eight Daphnia species. Only two of these had been recorded previously in live collections from Lake Naivasha, and one species is new to science. The four more common species (Daphnia barbata, D. laevis, D. magna, and D. pulex) show striking differences in abundance patterns and population dynamics through time. Four other species (D. lumholtzi, D. curvirostris, D. longispina s.l., and Daphnia sp. nov. type Limuru.) appear to have been present only occasionally. Nevertheless, between 1895 and 1915 seven species of Daphnia inhabited Lake Naivasha simultaneously. 3. Despite considerable natural environmental change associated with climate‐driven lake‐level fluctuations, the Daphnia community of Lake Naivasha has been severely affected by human activities over the past century, especially the introduction of exotic fishes and water‐quality changes because of agricultural soil erosion. The recent reappearance of large‐bodied Daphnia species (D. magna, D. barbata, D. lumholtzi, Daphnia sp. nov. type Limuru) after 20–110 years of absence can be explained by their release from fish predation, following a dramatic increase in turbidity caused by excess clastic sediment input from eroded catchment soils. The small‐bodied species D. laevis has fared less well recently, presumably because the benefit of lowered predation pressure is counteracted by more pronounced negative effects of increased turbidity on this species and loss of submerged macrophyte beds which formerly served as predation refuge. 4. Our results suggest that, despite considerable environmental instability and the absence of specialised zooplanktivores, top‐down control of fish on large zooplankton is important in Lake Naivasha. Predation pressure from fish has led to clear‐cut shifts in local Daphnia species composition, but failed to drive the larger taxa to extinction.     

Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha)

Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.     

Population structure and secondary productivity of Limnodrilus hoffmeisteri (Claparede) and Branchiura sowerbyi Beddard in the profundal zone of Lake Naivasha,Kenya

Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological functioning have had to wait until enough was known about its structure. The energetics of the lake has only been studied to date at primary producer level. Following the identification of the invertebrate components of the littoral and profundal benthos, this study was initiated. The absence of native fish species in Lake Naivasha, combined with a fishery based only on three, introduced species, added an applied dimension to the work. The introduction of additional fish species which will utilize unexploited ecological niches has been suggested. The benthic invertebrates form one such niche. Two oligochaetes dominated the community, Branchiura sowerbyi Beddard and Limnodrilus hoffmeisteri Claparade. The former had a productivity of 7.4 g m^–2 ann^–1 (as dry weight), the latter 0.6. These figures are not particularly high and do not support the introduction of a new fish species on their own.     

Predator–prey interactions of Procambarus clarkii with aquatic macroinvertebrates in single and multiple prey systems

Understanding the interspecific interactions of Procambarus clarkii with other aquatic macroinvertebrates will help to unveil the mechanisms and processes underlying biological invasiveness. The purpose of this study was to investigate predator–prey interactions of two ontogenic phases of P. clarkii with native and exotic species of aquatic macroinvertebrates at a single and multiple prey level. We performed laboratory experiments to determine the consumption and the behavioral responses of Chironomus riparius, Physa acuta and Corbicula fluminea to P. clarkii. The presence of P. clarkii significantly affected the abundance of C. riparius and P. acuta, but not of C. fluminea whether prey species were provided singly or simultaneously. The consumption of C. riparius by P. clarkii was higher than P. acuta for both crayfish sizes and situations (single/multiple prey systems) and C. fluminea was never consumed. Physa acuta was the only species that exhibited an anti-predator behavior to P. clarkii. Our results show that P. clarkii can have strong consumptive and trait effects on aquatic macroinvertebrate prey at a single and multiple prey level, resulting in differential impacts on different prey species. This study clarifies some aspects of the predator–prey interactions between P. clarkii and native as well as other exotic macroinvertebrate species that have invaded freshwater biocenosis worldwide.     

The dynamics and ecology of exotic tropical species in floating plant mats: Lake Naivasha,Kenya

The floating water fern Salvinia molesta has occurred in Lake Naivasha since the early 1960s and during this period has obstructed fishing activities and navigation. In recent years the extent of Salvinia has declined markedly. Since 1988, a second floating weed, Eichhornia crassipes (water hyacinth), has colonised the lake. Salvinia formed large mats around the edges, bays and lagoons and had very few plant or invertebrate species associated with it. Mats of E. crassipes, however, support a greater abundance and variety of animal and plant species, which shows a relationship with mat size. This seems to have facilitated plant succession but without a zonation typical of the classic hydroseral sequence for the lake.     

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant,but the plant does not reciprocally facilitate the herbivore

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Comparison of trophic function between the globally invasive crayfishes <Emphasis Type="Italic">Pacifastacus leniusculus</Emphasis> and <Emphasis Type="Italic">Procambarus clarkii</Emphasis>

Impacts of invasive species may manifest most strongly if these organisms are highly distinct functionally from the native species they often replace. Yet, should we expect functional differences between native and invasive species of generalist organisms like freshwater crayfish? Some existing evidence has pointed to native and invasive crayfish species as ecologically equivalent. We contribute to this literature by comparing the trophic niches of the globally invasive crayfishes Pacifastacus leniusculus and Procambarus clarkii, by applying carbon and nitrogen stable isotope analyses to replicated allopatric (alone) and sympatric (together) lake populations in western Washington State, USA, where P. clarkii has been recently introduced and P. leniusculus is presumed native. Our study corrected for potential inherent differences in lake food webs as a consequence of lake abiotic or biotic characteristics using random effects in linear mixed effects models. We found that although overall trophic niche size or area of these species was not significantly different, P. leniusculus was significantly higher in trophic position than P. clarkii when also accounting for the effects of body size, sex, and lakes as random effects. This pattern of increased trophic position of P. leniusculus over P. clarkii was conserved over time in one sympatric lake for which we had data over multiple years. Cumulatively, our findings point to trophic differences between the globally cosmopolitan crayfishes P. leniusculus and P. clarkii, particularly when accounting for the ways that ecosystem context can affect food web structure of communities and the trophic resources available to these consumers.     

Introduced deer reduce native plant cover and facilitate invasion of non-native tree species: evidence for invasional meltdown

Invasive species are a major threat to native communities and ecosystems worldwide. One factor frequently invoked to explain the invasiveness of exotic species is their release in the new habitat from control by natural enemies (enemy-release hypothesis). More recently, interactions between exotic species have been proposed as a potential mechanism to facilitate invasions (invasional meltdown hypothesis). We studied the effects of introduced deer on native plant communities and exotic plant species on an island in Patagonia, Argentina using five 400 m² exclosures paired with control areas in an Austrocedrus chilensis native forest stand. We hypothesized that introduced deer modify native understory composition and abundance and facilitate invasion of introduced tree species that have been widely planted in the region. After 4 years of deer exclusion, native Austrocedrus and exotic Pseudotsuga menziesii tree sapling abundances are not different inside and outside exclosures. However, deer browsing has strongly inhibited growth of native tree saplings (relative height growth is 77% lower with deer present), while exotic tree sapling growth is less affected (relative height growth is 3.3% lower). Deer significantly change abundance and composition of native understory plants. Cover of native plants in exclosures increased while cover in controls remained constant. Understory composition in exclosures after only 4 years differs greatly from that in controls, mainly owing to the abundance of highly-browsed native species. This study shows that introduced deer can aid the invasion of non-native tree species through negatively affecting native plant species.     

Aspects of the ecology of the prickly sculpin,Cottus asper Richardson,a persistent native species in Clear Lake,Lake County,California

Synopsis The biology of the prickly sculpin was investigated in Clear Lake, Lake County, California in order to determine how it has persisted in the face of introductions of numerous exotic species when most other native species have declined in abundance or have become extinct. Sculpins over 15 mm SL inhabited all types of benthic habitats in the lake, while post larval sculpins were pelagic when the postlarvae of exotic species were absent. The feeding ecology of sculpins was distinct from the other fishes in the lake in that they fed largely on amphipods and chironomid midge larvae regardless of the time of year, time of day, or habitat. Sculpins were uncommon in the stomachs of piscivorous fishes, except juvenile largemouth bass (Micropterus salmoides). It is concluded that prickly sculpins have persisted in Clear Lake in part because they are ecologically distinct from the exotic species and are not preyed upon by them to any great extent, and in part because they have managed to survive other man-related perturbations of this ecosystem.     

Environmental gradients determining the distribution of benthic macroinvertebrates in Lake Takkobu, Kushiro wetland, northern Japan

Effects of environmental variables on the distribution of benthic macroinvertebrates inhabiting sediments were studied at 25 sites along the shoreline of Lake Takkobu in the Kushiro wetland of northern Japan in summer 2003. During the last decade, the lake’s status has undergone a drastic shift from clear water dominated by submerged macrophytes to turbid water dominated by phytoplankton. The canonical correspondence analysis showed that four environmental variables explained the significant variation in the macroinvertebrate species composition: submerged plant biomass, bottom sediment organic matter content (OMC), distance from the mouth of the Takkobu River, and bottom-layer pH. Five species of Chironomidae [Chironomus sp. (except plumosus group), Psectrocladius sp., Corynoneura sp., Parachironomus sp. arcuatus group, and Zavreliella sp.] occurred in sites with relatively lower pH and a high submerged plant biomass, whereas three species of Tubificidae (Tubifex tubifex, Aulodrilus limnobius and Aulodrilus sp.) and two of Chironomidae (Nanocladius sp. and Monodiamesa sp.) occurred in sites with high pH and little vegetation. The three Tubificidae species also preferred organic-rich sediments. Irrespective of aquatic vegetation, Sphaerium sp. (Bivalvia) and Monodiamesa sp. (Chironomidae) occurred in low-OMC sites, whereas Tanypus sp. (Chironomidae) preferred high-OMC sites. The number of macroinvertebrate taxa showed the highest correlation with the number of submerged plants, suggesting that macroinvertebrate species richness was related mostly to submerged plant species diversity in this lake. The quantity and species richness of submerged plants and OMC are thus important determinants of the community structure of macroinvertebrates inhabiting sediments in Lake Takkobu.     

Concentrations of Trace Metals in Dominant Aquatic Plants of the Lake Provala (Vojvodina, Yugoslavia)

The trace metal (Fe, Mn, Zn, Cu, Ni, Pb, Cd, Sr, and Cr) contents in the most common submerged and floating aquatic plants Ceratophyllum demersum L., Myriophyllum spicatum L., and Nymphoides flava Hill. of Provala Lake were evaluated. Considerable higher contents of iron, manganese, zinc, nickel, lead and strontium were found in submerged species than in the floating ones. The presence of cadmium and lead in plant tissues points to a certain degree of lake water pollution.     

Restoration of wetland vegetation using soil seed banks: lessons from a project in Lake Kasumigaura, Japan

The restoration of degraded wetland ecosystems and the recovery of wetland biodiversity are important global issues. Generally, wetland restoration projects include activities to recover vegetation. A promising revegetation technique is one in which soil seed banks are utilized as the source of plant recolonization. Using such a technique, a pilot project to restore lakeshore vegetation was launched at Lake Kasumigaura, Japan, in 2002. In the project, lake sediments containing the seed banks were spread thinly (∼10 cm) on the surfaces of artificial lakeshores, which were constructed in front of concrete levees and had microtopographic variations. In total, 180 species, including six endangered or vulnerable species and 12 native submerged plants that had disappeared from the above-ground vegetation of the lake, were recorded in five recreated lakeshores (total area, 65,200 m²) during the first year of the restoration. The distribution of each restored species at the sites suggested the importance of microtopographic relief for recolonizing species-rich lakeshore vegetation. Furthermore, the origin of the source seed banks affected the species composition of the restored vegetation. On the other hand, the restoration sites were subject to exotic plant invasions. Here, we report lessons learned from the Lake Kasumigaura restoration project as a contribution to the establishment of ecologically sound revegetation techniques.     

The rise of potentially toxin producing cyanobacteria in Lake Naivasha,Great African Rift Valley,Kenya

Lake Naivasha, an important inland water ecosystem and a crucial freshwater resource in the Great African Rift Valley, has displayed clear signals of degradation in recent decades. We studied the phytoplankton composition and biomass levels in the period 2001–2013 and noted a progressive increase in the occurrence of potentially toxic cyanobacteria. Analyses for the presence of cyanotoxins such as microcystins (MC), cylindrospermopsin (CYN) and anatoxin-a (ATX-a) were carried out on samples collected in 2008–2013. Among the cyanotoxins tested, low concentrations of MC were detected in the lake. This is the first record of the occurrence of MC in Lake Naivasha. For the first time, molecular phylogenetic investigations of field clones of cyanobacteria from Lake Naivasha were carried out to establish the taxa of the dominant species. Amplification of the aminotrasferase (AMT) domain responsible for cyanotoxin production confirmed the presence of the mcyE gene belonging to the microcystin synthesis gene cluster in field samples containing Microcystis and Planktothrix species. These findings suggest that toxin producing cyanobacteria could become a threat to users of this over-exploited tropical lake in the near future.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.