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991.
Vertebrate herbivores as diverse as ungulates, geese, and rabbits preferentially feed on plants that have previously experienced
herbivory. Here, we ask whether smaller grassland “cryptic consumers” such as voles (Microtus ochrogaster and M. pennsylvanicus) preferentially clip (cut stems for access to leaves or seeds) or avoid previously clipped individuals of two tallgrass prairie
species (Desmanthus illinoensis and Echinacea purpurea) within a growing season. Further, we ask how these plants respond to repeated clipping within a growing season, and whether
the effects of this herbivory last into the subsequent growing season. Voles preferentially clipped stems of D. illinoensis and E. purpurea plants that had been previously clipped. The exception was indiscriminant clipping of stems of E. purpurea late in the growing season when its achenes, a favorite vole food, ripened. For D. illinoensis, repeated clipping resulted in a 59% reduction in biomass, 42% lower ratio of reproductive to vegetative biomass, and 57%
fewer seeds produced per plant compared with unclipped plants. These effects lasted into the following growing season in which
plants were protected from voles. In contrast, the only effect of repeated clipping for E. purpurea was that the number of achenes per plant was substantially reduced by three episodes of clipping. This effect did not carry
over to the next growing season. Such differences in D. illinoensis and E. purpurea response to repeated stem clipping by voles offer insights into how these small rodents can effect major changes in composition
and dominance in grassland communities. 相似文献
992.
Adult sex ratios (ASRs) and population size are two of the most fundamental parameters in population biology, as they are
the main determinants of genetic and demographic viability, and vulnerability of a population to stochastic events. Underpinning
the application of population viability analysis for predicting the extinction risk of populations is the need to accurately
estimate parameters that determine the viability of populations (i.e. the ASR and population size). Here we demonstrate that
a lack of temporal information can confound estimation of both parameters. Using acoustic telemetry, we compared differences
in breeding durations of both sexes for a giant Australian cuttlefish Sepia apama breeding aggregation to the strongly male-biased operational sex ratio (4:1), in order to estimate the population ASR. The
ratio of breeding durations between sexes was equal to the operational sex ratio, suggesting that the ASR is not strongly
male-biased, but balanced. Furthermore, the short residence times of individuals at the breeding aggregation suggests that
previous density-based abundance estimates have significantly underestimated population size. With the current wide application
of population viability analysis for predicting the extinction risk of populations, tools to improve the accuracy of such
predictions are vital. Here we provide a new approach to estimating the fundamental ASR parameter, and call for temporal considerations
when estimating population size. 相似文献
993.
Plants infected with vertically transmitted fungal endophytes carry their microbial symbionts with them during dispersal into
new areas. Yet, whether seed-borne endophytes enhance the host plant’s ability to overcome colonisation barriers and to regenerate
within invaded sites remains poorly understood. We examined how symbiosis with asexual endophytic fungi (Neotyphodium) affected establishment and seed loss to predators in the invasive annual grass Lolium multiflorum (Italian ryegrass) across contrasting successional plots. Italian ryegrass seeds with high and low endophyte incidence were
sown into three communities: a 1-year-old fallow field, a 15-year-old grassland, and a 24-year-old forest, which conformed
to an old-field chronosequence in the eastern Inland Pampa, Argentina. We found that endophyte infection consistently increased
host population recruitment and reproductive output. Endophyte presence also enhanced aerial biomass production of ryegrass
in a low recruitment year but not in a high recruitment year, suggesting that symbiotic effects on growth performance are
density dependent. Endophyte presence reduced seed removal by rodents, although differential predation may not account for
the increased success of infected grass populations. Overall, there was no statistical evidence for an endophyte-by-site interaction,
indicating that the fungal endosymbiont benefitted host establishment regardless of large differences in biotic and abiotic
environment among communities. Our results imply that hereditary endophytes may increase the chances for host grass species
to pass various ecological filters associated with invasion resistance across a broad range of successional habitats. 相似文献
994.
995.
996.
Lees (Proc Zool Soc Lond 121:759–772, 1952) concluded that the ixodid tick Ixodes ricinus grows endocuticle during the slow but not during the rapid, phase of engorgement, a conclusion supported by Andersen and
Roepstorff (Insect Biochem Mol Biol 35:1181–1188, 2005) for the same species. In this study analysis of dimensional data and cuticle weight measurements from female ixodid ticks
(Amblyomma hebraeum) were used to test this hypothesis. Both approaches showed that endocuticle growth continues during the rapid phase, tapering
to zero at a fed/unfed weight ratio of ~60. Of the total mass of cuticle in the engorged tick 32–43% was formed during the
rapid phase. We demonstrate that if cuticle growth stopped at the end of the slow phase, there would not be sufficient cuticle
to account for the thickness of cuticle observed at the end of engorgement. This finding is consistent with prior studies
of Rhipicephalus (Boophilus) microplus, and with a dimensional analysis of the cuticle thickness data of Lees for I. ricinus, in contradiction to his conclusion from an analysis of tick cuticle weight measurements. An examination of cuticle weight
measurements for I. ricinus by Andersen and Roepstorff similarly supports the finding of cuticle growth during the rapid phase. All ixodid ticks undergo
major body expansion, typically tenfold or more, during a rapid phase of engorgement and require sufficient cuticle at the
end of that process to contain their body. The fact that cuticle grows during the rapid phase of engorgement in three species
suggests that this is a general characteristic of the family Ixodidae. 相似文献
997.
The OSPAR Eutrophication Strategy requires assessment of eutrophication to be based on the ecological consequences of nutrient
enrichment and not just on nutrient enrichment alone, i.e. finding reliable evidence for accelerated growth of algae and higher
forms of plant life caused by anthropogenic nutrient enrichment, leading to undesirable disturbance. Fully flushed marine
waters of England and Wales (salinity >30) were assessed against OSPAR’s harmonised criteria of nutrient concentration and
ratios, chlorophyll concentrations, phytoplankton indicator species, macrophytes, dissolved oxygen (DO) levels, incidence
of fish kills and changes in the zoobenthos, using region specific thresholds. None of the thirteen assessment areas, including
six nutrient enriched areas, exhibited evidence for undesirable disturbance. This paper details the methods and the overall
outcome of the assessment. It presents evidence that undesirable disturbance caused by nutrient enrichment was not detected
in English and Welsh marine waters assessed under the OSPAR procedure. The main reasons for the lack of eutrophication problems,
such as the underwater light climate limiting the accelerated growth of algae, which might otherwise result from nutrient
enrichment, are discussed. 相似文献
998.
999.
1000.
Spatial variability in nutrient concentration and biofilm nutrient limitation in an urban watershed 总被引:1,自引:0,他引:1
Nutrient enrichment threatens river ecosystem health in urban watersheds, but the influence of urbanization on spatial variation
in nutrient concentrations and nutrient limitation of biofilm activity are infrequently measured simultaneously. In summer
2009, we used synoptic sampling to measure spatial patterns of nitrate (NO3
−), ammonium (NH4
+), and soluble reactive phosphorus (SRP) concentration, flux, and instantaneous yield throughout the Bronx River watershed
within New York City and adjacent suburbs. We also quantified biofilm response to addition of NO3
−, phosphate (PO4
3−), and NO3
− + PO4
3− on organic and inorganic surfaces in the river mainstem and tributaries. Longitudinal variation in NO3
− was low and related to impervious surface cover across sub-watersheds, but spatial variation in NH4
+ and SRP was higher and unrelated to sub-watershed land-use. Biofilm respiration on organic surfaces was frequently limited
by PO4
3− or NO3
− + PO4
3−, while primary production on organic and inorganic surfaces was nutrient-limited at just one site. Infrequent NO3
− limitation and low spatial variability of NO3
− throughout the watershed suggested saturation of biological N demand. For P, both higher biological demand and point-sources
contributed to greater spatial variability. Finally, a comparison of our data to synoptic studies of forested, temperate watersheds
showed lower spatial variation of N and P in urban watersheds. Reduced spatial variation in nutrients as a result of biological
saturation may represent an overlooked effect of urbanization on watershed ecology, and may influence urban stream biota and
downstream environments. 相似文献