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1.
Introduced populations of Guekensia demissa occur on the west coast of North America. They have been reported in San Francisco Bay, four southern California wetlands, and in Estero de Punta Banda (EPB), Baja California Norte, Mexico. We randomly sampled benthic invertebrates in four habitat types within EPB: marsh, channel, mudflat and pan. Geukensia demissa was the most abundant bivalve in the wetland at EPB. It was significantly associated with the native cordgrass, Spartina foliosa, and occurred at higher average densities in vegetated marsh sites (24/m2) and Spartina-dominated tidal channels (35/m2), compared to mudflat (0/m2), and pan (0/m2) sites. We estimated that the total biomass of this invader was over four times that of the next most abundant bivalve, Tagelus spp., in EPB. We examined G. demissa for parasites and found that only a few native parasites colonized this introduced host at very low prevalences and intensities. We performed bird surveys to determine the habitat overlap and potential impact of this mussel on the EPB population of light-footed clapper rails (Rallus longirostrus levipes), an endangered species in the United States. The high abundance of G. demissa in EPB, its presence in clapper rail habitat, and its known effects on salt marsh habitat in it’s native range, warrant further investigations of the impact of this invader in EPB and elsewhere.  相似文献   

2.
Numbers of light-footed clapper rails Rallus longirostris levipes, an endangered bird inhabiting southern California salt marshes, have substantially declined from historic levels. RAPD (randomly amplified polymorphic DNA) analysis was employed to assess the genetic variability within and among four of the largest remaining light-footed clapper rail populations. A single, larger population of the endangered Yuma clapper rail Rallus longirostris yumanensis was used for comparison. A total of 325 RAPD primers were tested on DNA from a subset of five clapper rails composed of a single representative for each of the four light-footed clapper rail populations and a representative for the single Yuma clapper rail population. Of the 1338 amplified bands (loci) surveyed in these five representative birds, approximately 1% were polymorphic, indicating the level of differentiation across all loci is quite low. Nine primers yielding these 16 polymorphic bands were used to analyse 48 individuals from five populations. Five of these bands were polymorphic in both subspecies, six were polymorphic only within the light-footed clapper rails, and five were polymorphic only within the Yuma clapper rail samples. Considering the few bands that were polymorphic among the light-footed clapper rail populations, a surprisingly high level of population differentiation (GST= 0.28) was found. This is in accord with the results of AMOVA analyses which show that a fairly high percentage of the limited variability among the rails is due to either differences between subspecies or differences between the light-footed rail populations. Because inbreeding depression is suspected and overall genetic distances between populations are low, movement of light-footed clapper rails from larger populations into smaller ones might be considered as a management strategy. Employing RAPDs as one of a series of assays is useful in revealing the population structure of genetically depauperate species.  相似文献   

3.
Many authors have referred to the important role of vegetation in the consolidation of salt marsh sediments, but experiments previously carried out by us have shown results that do not always agree with these statements. In other words, the type of salt marsh surface coverage is not the main factor that contributes to the consolidation of sediments. To test this hypothesis different Portuguese salt marsh stations (species/unvegetated areas) from two sites, Tagus estuary (Corroios and Pancas) and Ria de Aveiro (Barra and Verdemilho), were compared to evaluate their influence on suspended matter deposition on the salt marsh surface. A short-term sedimentation study was performed within stands of Spartina maritima, Halimione portulacoides, Sarcocornia perennis subsp. perennis and unvegetated areas, by analysing the deposition of sediment material on nylon filters anchored to the marsh surface. Numerical results obtained from hydrodynamic models coupled to a Lagrangean module implemented for the Ria de Aveiro and the Tagus Estuary, namely the root-mean square velocity (V rms) and residual velocity of tides, were also used. Average sedimentation rates (mean value between the different surface cover in a salt marsh) showed a seasonal trend more or less defined but with significantly different values between sites and salt marshes. Sedimentation rates varied between marshes: there are significant differences between Pancas and the other three marshes, but only significant differences in sedimentation rates between Spartina and Sarcocornia. Despite the important role of vegetation in the consolidation of salt marsh sediments, our results suggest that, the position of stations and related abiotic conditions in the salt marshes are determining factors of variation to take into account in the studies related with the stabilization and survival of salt marshes facing sea level rise. Handling editor: P. Viaroli  相似文献   

4.
Although the impact of plant invasions on benthic communities, especially burrowing crabs, has received increasing attention, the results from past studies are mixed. The exotic plant Spartina alterniflora has become the most abundant species in the salt marshes of the Yangtze River estuary since it was first found just over a decade ago, but its effects on crabs in the salt marshes is largely unknown. To examine whether the invasions of this exotic plant affected native crabs, we compared the biomass and abundance of the dominant burrowing crab Sesarma dehaani in an exotic Spartina marsh, native Phragmites australis marsh and mudflats of the Yangtze River estuary, China. To explain the differences of S. dehaani populations between different habitats, feeding preference of S. dehaani for Spartina and Phragmites was investigated. Results showed crab abundance and biomass in the Spartina marsh were significantly greater than those in the Phragmites marsh and mudflats. Soil water content and plant community characteristics in the Spartina marsh also significantly differed in the Phragmites marsh and mudflats. Moreover, the feeding preference experiment showed that crabs consumed Spartina more than twice as much as Phragmites. In summary, this study showed that Spartina provided compatible habitats for native crab S. dehaani through offering suitable food source and moderate environmental conditions.  相似文献   

5.
Invasive species frequently degrade habitats, disturb ecosystem processes, and can increase the likelihood of extinction of imperiled populations. However, novel or enhanced functions provided by invading species may reduce the impact of processes that limit populations. It is important to recognize how invasive species benefit endangered species to determine overall effects on sensitive ecosystems. For example, since the 1990s, hybrid Spartina (Spartina foliosa × alterniflora) has expanded throughout South San Francisco Bay, USA, supplanting native vegetation and invading mudflats. The endangered California clapper rail (Rallus longirostris obsoletus) uses the tall, dense hybrid Spartina for cover and nesting, but the effects of hybrid Spartina on clapper rail survival was unknown. We estimated survival rates of 108 radio-marked California clapper rails in South San Francisco Bay from January 2007 to March 2010, a period of extensive hybrid Spartina eradication, with Kaplan–Meier product limit estimators. Clapper rail survival patterns were consistent with hybrid Spartina providing increased refuge cover from predators during tidal extremes which flood native vegetation, particularly during the winter when the vegetation senesces. Model averaged annual survival rates within hybrid Spartina dominated marshes before eradication (? = 0.466) were greater than the same marshes posttreatment (? = 0.275) and a marsh dominated by native vegetation (? = 0.272). However, models with and without marsh treatment as explanatory factor for survival rates had nearly equivalent support in the observed data, lending ambiguity as to whether hybrid Spartina facilitated greater survival rates than native marshland. Conservation actions to aid in recovery of this endangered species should recognize the importance of available of high tide refugia, particularly in light of invasive species eradication programs and projections of future sea-level rise.  相似文献   

6.
In order to examine the influence of differences in food conditions on gut characteristics in Porcellio scaber, pH-manipulated and microbially inoculated leaf litter from three different tree species were offered. Microbial activity was clearly influenced by the pH levels of the leaves. Analyses of the pH levels in the gut indicated the ability of P. scaber to buffer the pH value in the intestinal tract to about 5.5–6.0 in the anterior hindgut, and to about 6.0–6.5 in the posterior hindgut. The pH levels of the gut sections remained in this range, within a range of food pH from 4.0 to 7.5, no matter what kind of leaves the animals were fed. Homeostatic responses to changes in food pH guarantee optimized digestion of leaf litter. However, when the pH level of the litter dropped below 3.5, P. scaber was not able to maintain the pH conditions in the gut. Furthermore, microorganisms colonizing the litter biased the pH level in the anterior hindgut where digestive processes mainly take place. These results indicate a decline of litter quality with regard to the nutrition of terrestrial isopods, caused by acidification and consequently reduced microbial activity. Accepted: 19 July 1997  相似文献   

7.
Spartina alterniflora, a species vegetating on inter-tidal flats that was introduced from the eastern coast of United States, has become a hot topic, focusing on its invasion within local species in the coastal zone of China. Impacts of S. alterniflora on the inter-tidal macrobenthos community in the Jiangsu coastland are addressed by comparing the macrobenthos characteristics in a mudflat and in a four-year-old Spartina salt marsh that had earlier been a mudflat. During the period October 2002–July 2003, we studied the distribution pattern and diversity of macrobenthos, and discussed their correlation with environmental factors caused by Spartina vegetation. The results showed that a total of 43 macrobenthos species were found, mainly consisting of Mollusca, Crustacea, and Annelida. Ten macrobenthos species were found in the Spartina salt marsh, and 36 species were found in the mudflat. Life forms and functional groups of macrobenthos in the Spartina salt marsh were obviously distrinct from that of the mudflat. The study showed that macrobenthos diversity in the Spartina salt marsh decreased, and the community structure altered obviously, whereas the biomass showed no differences in different seasons. Statistical analysis demonstrated that seasonal change of macrobenthos diversity in the Spartina salt marsh negatively related to content of sediment organic matter, total N, bulk density, height and biomass of Spartina vegetation, and positively related to the density of Spartina. All these differences suggested the obvious effects of the Spartina vegetation on the Jiangsu inter-tidal benthic macroinvertebrate ecology. Furthermore, the investigation also showed that the niche of the native macrobenthos living in the mudflat has been transferred down, seaward, due to the invasion of Spartina in our study site.  相似文献   

8.
Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messor andrei, the black seed-harvesting ant, on a serpentine grassland plant community in northern California. First, to determine if selective seed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundances of all species on these plots and on control plots were measured before and after exclosure. Second, to determine if M. andrei nest mounds affect plant community composition, we compared plant species abundances on and off nest mounds. M. andrei deposit large amounts of organic matter on their nest mounds over a foraging season, so mounds may alter the edaphic environment. The exclusion of seed-harvesting activity did not cause changes in the plant community. Nest mounds had a strong effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, produced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentine grassland. Received: 31 March 1997 / Accepted: 6 May 1997  相似文献   

9.
10.
Major electrolytes and nitrogenous excretory products were analysed in the blood plasma, ureteral urine and cloacal urine of juvenile Alligator mississippiensis and Crocodylus porosus in fresh and hypoosmotic salt water (206 mosmol · l−1). Both species coped well with saline water, showing little (Alligator) or no (Crocodylus) change in plasma composition. Comparisons of renal-cloacal function point to major differences in their osmoregulatory physiology. The cloaca of C. porosus is a very active osmoregulatory organ in salt and fresh water, contributing to water conservation and NaCl excretion through the lingual salt glands. In contrast, the cloaca of Alligator has little impact on the composition of excreted urine. It seems likely that A.␣mississippiensis is largely constrained to a renal response to osmotic and ionic stress while C. porosus is able to call on a more complex mix of renal response, post-renal modification of urine in the cloaca, and excretion of excess NaCl through the salt glands. The results support the idea that there are deep-seated differences in the osmoregulatory physiology of alligatorids and crocodylids (Eusuchia), an understanding of which should provide valuable insights into their evolution and zoogeography. Accepted: 7 September 1996  相似文献   

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