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61.
In California and Oregon, two recently described oomycete forest pathogens, Phytophthora nemorosa and P. pseudosyringae, overlap in their host and geographic ranges with the virulent P. ramorum, causal agent of “sudden oak death.” Epidemiological observations, namely broader geographic distribution and lack of landscape-level mortality, led to the hypothesis they are native to this region, whereas multiple lines of evidence indicate P. ramorum is exotic to North America. We used AFLP analysis to measure genetic variability in the homothallic P. nemorosa and P. pseudosyringae and to evaluate the hypothesis of endemism. We analysed 39 P. nemorosa and 48 P. pseudosyringae isolates (29 American and 19 European) from throughout their geographic and host ranges. In the US, both P. nemorosa and P. pseudosyringae have a dominant AFLP clone with several closely related variants. There is no evidence that genetic diversity is partitioned by host or location in P. nemorosa, but the US P. pseudosyringae clonal lineage is largely nested within a more genetically variable European group. Though the absence of highly variable sampled source populations does not allow us to determine whether each species is native or introduced in the western US with certainty, the results are most consistent with the hypothesis that both are introduced — P. pseudosyringae perhaps from Europe. Invasive Phytophthora species are increasingly being implicated in emergent forest diseases, highlighting the need to identify and characterize both native and previously unknown introduced forest Phytophthoras. 相似文献
62.
Species richness and density of understory plants were investigated in eight 1 ha plots, distributed one each in undisturbed
and disturbed tropical evergreen, semi-evergreen, deciduous and littoral forests of Little Andaman island, India, which falls
under one of the eight hottest hotspots of Biodiversity in the world viz. the Indo-Burma. One hundred 1 m−2 quadrats were established in each 1 ha plot, in which all the understory plants (that include herbs, undershrubs, shrubs
and herbaceous climbers) were enumerated. The total density of understory plants was 6,812 individuals (851 ha−1) and species richness was 108 species, representing 104 genera and 50 families. Across the four forest types and eight study
plots, the species richness ranged from 10 to 39 species ha−1. All the disturbed sites harbored greater number of species than their undisturbed counterparts. Herbs dominated by species
(63%) and density (4,259 individuals). The grass Eragrostis tenella (1,860 individuals; IVI 40), the invasive climber Mikania cordata (803; IVI 20) and the shrub Anaxagorea luzonensis (481; IVI 17.5) were the most abundant species. Poaceae, Asteraceae, Acanthaceae, Orchidaceae and Euphorbiaceae constituted
the species-rich families represented by 6 species each. The species-area curves attained an asymptote at 0.8 ha level except
in sites DD and DL, indicating 1 ha plot is not sufficient to capture all the understory species in disturbed forests. The
alien weeds formed about one-fourth of the species richness (31 species; 28%) and density (1,926 individuals; 28.3%) in the
study sites, indicating the extent of weed invasion and the attention required for effective conservation of the native biodiversity
of the fragile island forest ecosystem. 相似文献
63.
This study examines the impact of the exotic nitrogen-fixing legume Melilotus officinalis (L.) Lam. on native and exotic species cover in two Great Plains ecosystems in Badlands National Park, South Dakota. Melilotus is still widely planted and its effects on native ecosystems are not well studied. Melilotus could have direct effects on native plants, such as through competition or facilitation. Alternatively, Melilotus may have indirect effects on natives, e.g., by favoring exotic species which in turn have a negative effect on native species.
This study examined these interactions across a 4-year period in two contrasting vegetation types: Badlands sparse vegetation
and western wheatgrass (Pascopyrum smithii) mixed-grass prairie. Structural equation models were used to analyze the pathways through which Melilotus, native species, and other exotic species interact over a series of 2-year time steps. Melilotus can affect native and exotic species both in the current year and in the years after its death (a lag effect). A lag effect
is possible because the death of a Melilotus plant can leave an open, potentially nitrogen-enriched site on the landscape. The results showed that the relationship between
Melilotus and native and exotic species varied depending on the habitat and the year. In Badlands sparse vegetation, there was a consistent,
strong, and positive relationship between Melilotus cover and native and exotic species cover suggesting that Melilotus is acting as a nurse plant and facilitating the growth of other species. In contrast, in western wheatgrass prairie, Melilotus was acting as a weak competitor and had no consistent effect on other species. In both habitats, there was little evidence
for a direct lag effect of Melilotus on other species. Together, these results suggest both facilitative and competitive roles for Melilotus, depending on the vegetation type it invades. 相似文献
64.
Debra Ayres Frederick J. Ryan Eva Grotkopp John Bailey John Gaskin 《Biological invasions》2009,11(5):1175-1187
Tumbleweeds (Salsola species, section Kali) are road side and rangeland pest plants throughout the 48 contiguous states in the US. Three described tumbleweed species
and two undescribed Salsola taxa occur in California. The known species are Russian thistle, Salsola tragus, introduced from Eurasia in the 1800s, Russian barbwire thistle, S. paulsenii, which grows in the desert regions of California, and is also native to Eurasia, and the recently identified S. kali subspecies austroafricana, possibly native to South Africa. Our goals were to investigate karyology, genome size, and molecular genetic affinities
of the described species and the other taxa within their ranges in California using recently developed microsatellite loci,
dominant nuclear DNA markers (RAPD and ISSR), and DNA sequence data. Chromosome counts and genome size assessments made with
flow cytometry were compared. These analyses indicated that one undescribed taxon is a new allopolyploid hybrid between S. tragus and S. kali subspecies austroafricana, and the other undescribed taxon appears to be a complex hybrid involving all three described species. The invasion potentials
for the hybrid taxa are unknown. Tumbleweeds are the focus of biological controls efforts but the identification of suitable
agents for the hybrid taxa may be problematic because of the large amount of genetic variability encompassed within this evolving
Salsola complex. 相似文献
65.
Christa P. H. Mulder M. Nicole Grant-Hoffman David R. Towns Peter J. Bellingham David A. Wardle Melody S. Durrett Tadashi Fukami Karen I. Bonner 《Biological invasions》2009,11(7):1671-1688
Introduced rats (Rattus spp.) can affect island vegetation structure and ecosystem functioning, both directly and indirectly (through the reduction
of seabird populations). The extent to which structure and function of islands where rats have been eradicated will converge
on uninvaded islands remains unclear. We compared three groups of islands in New Zealand: islands never invaded by rats, islands
with rats, and islands on which rats have been controlled. Differences between island groups in soil and leaf chemistry and
leaf production were largely explained by burrow densities. Community structure of woody seedlings differed by rat history
and burrow density. Plots on islands with high seabird densities had the most non-native plant species. Since most impacts
of rats were mediated through seabird density, the removal of rats without seabird recolonization is unlikely to result in
a reversal of these processes. Even if seabirds return, a novel plant community may emerge. 相似文献
66.
The coldwater crayfish Orconectes eupunctus is endemic to the Spring and Eleven Point Rivers of Arkansas and Missouri, and appears to have been displaced from a portion
of its range by the recently introduced ringed crayfish Orconectes neglectus. We examined competition among juveniles as a potential mechanism for this crayfish species displacement through laboratory
and field experiments. Orconectes eupunctus juveniles survived and grew in stream cages in their former range, implicating biotic interactions rather than habitat degradation
in the displacement. Laboratory experiments revealed O. neglectus juveniles were dominant in the presence of limited food, whereas size rather than species determined occupancy of limited
shelter. In a field competition experiment using stream cages, O. neglectus juveniles did not inhibit growth or reduce survival of O. eupunctus juveniles. Consequently, laboratory evidence of O. neglectus dominance did not correspond with competition under field conditions. Combined with previous studies examining the effects
of O. neglectus on O. eupunctus, these results suggest that competition may not be a factor in this crayfish species displacement. Alternate mechanisms for
the apparent displacement of O. eupunctus by O. neglectus, such as differential predation or reproductive interference, should be investigated. 相似文献
67.
Biotic and abiotic factors that influence the avian distribution in a dry zone wetland was investigated by studying the distribution
of Asian Openbill (Anastomus oscitans), Cotton Pygmy-goose (Nettapus coromandelianus) and Pheasant-tailed Jacana (Hydrophasianus chirurgus) in Anavilundawa Ramsar sanctuary in Sri Lanka in 2006. Their distribution was recorded in Anavilundawa, Suruwila and Maiyawa
reservoirs, their catchments and respective paddy fields, through line transects. The floral cover of surface water was recorded
by floating quadrates. Water lily (Nymphea spp.) was the dominant flora in Anavilundawa reservoir, invasive water hyacinth (Eichhornia crassipes) in Suruwila reservoir and a native lotus (Nelumbo nucifera) in Maiyawa reservoir, respectively. Anavilundawa area had a higher distribution of birds than in the other two areas. Among
the three species observed, Asian Openbill distribution was mainly restricted to Anavilundawa whereas Pheasant-tailed Jacana
was present in all three areas. Nesting of Asian Openbill occurred only in Anavilundawa while nesting of Pheasant-tailed Jacana
was only observed in Maiyawa. Among the three species recorded, Cotton Pygmy-goose numbers were the least. Asian Openbill
preferred dead trees and trees with no leaves for nesting and perching over live trees. It is concluded that the spread of
invasives such as Water hyacinth and Salvinia (Salvinia molesta) forming mats over surface water have reduced the abundance of habitat specialists like Cotton Pygmy-goose. 相似文献
68.
Understanding how exotic invasive species are spread is fundamental for ecology and conservation biology. Human transport
has become one of the primary modes of dispersal for exotic species. We examined how the multiple queen, or polygyne, social
form of the fire ant Solenopsis invicta is spread along roadsides in Tallahassee, Florida, USA. We then determined the likely source of this expanding population,
which was a central soil depot. A survey of road maintenance practices in counties of several southeastern states and Texas
revealed that the use of a central soil depot is a common practice. Road maintenance therefore may be the primary source for
the establishment of new polygyne fire ant populations in this region and elsewhere. Control efforts focused on the soil depots
will help to limit further spread of polygyne fire ants and perhaps other invasive organisms, particularly invasive weeds. 相似文献
69.
70.
We examined whether the residual effects on soil caused by the invasion of Carpobrotus edulis, common iceplant, would inhibit the reestablishment of a native plant species. Carpobrotus edulis interacts both directly by suppressing the growth and establishment of other plants and indirectly by altering soil chemistry.
We tested whether the residual effects of C. edulis resulted in lowered germination, survival, growth, and reproduction of Gilia millefoliata, a rare dune annual. We compared G. millefoliata planted in plots previously occupied by C. edulis to G. millefoliata planted in plots that previously had native vegetation. Each plot received three treatments: seed, transplant, and unplanted,
and were censused every three weeks until senescence. Carpobrotus edulis had strong negative effects on the germination, survival, growth, and reproduction of G. millefoliata. C. edulis lowers soil pH and increases organic content due to the recalcitrance of tissue to decomposition, which may have evolved
as a mechanism to facilitate recolonization and invasion. 相似文献