Relationships between litterfall rate,litter mass and decomposition rate in Eucalyptus forests in southeastern Australia

Measurements of annual litterfall rate and mean annual litter mass for various indigenous open-forests are listed, along with the consequent decomposition constant (k) and mean annual rainfall. Using data only from sites considered valid for comparison (i.e. all those with a sclerophyllous plant understorey, not recently burnt, grazed or invaded by an exotic decomposer, and where for accuracy of assessment the litter layer has been sampled on several occasions) a quadratic relationship between litterfall rate and litter mass was significant, with decomposition rates at sites increasing with increasing litterfall rate. The existence of such a relationship may be useful for future studies: if litter data obtained lie appreciably outside the values on the line of quadratic regression, it would be worthwhile checking the accuracy of litterfall and mass estimates, site past history and the validity of the decomposition constant estimate. There was a significant positive linear relationship between the decomposition constant and mean annual rainfall at sites where rainfall was within the range 600–1800 mm y^-1, the general limits for open-forest site development. However, more data are required to increase the predictive value of relationship.     

Responses of two species of heathland rodents to habitat manipulation: Vegetation density thresholds and the habitat accommodation model

The abundance of two native rodent species, Rattus lutreolus and Pseudomys gracilicaudatus, has been shown to correlate with vegetation density in coastal wet heath. Fox's habitat accommodation model relates relative abundances of such small mammal species to heathland vegetation regeneration following disturbance. Implicit in the model is recognition that it is successional changes in vegetation, not time per se, that drives the responses of small mammal species along a regeneration axis. Using a brush‐cutter we deliberately removed approximately 85% of vegetation around trapping stations and recorded significant reductions in the abundance of both P. gracilicaudatus (an earlier‐stage colonizing species) and R. lutreolus (a late seral‐stage species). A significant decrease in the abundance of only the latter had been demonstrated previously when 60–70% of the vegetation had been removed. Following the brush‐cutting both species re‐entered the mammalian secondary succession at different times, first P. gracilicaudatus followed by R. lutreolus after the vegetation cover thresholds of each species had been reached. The impact of this habitat manipulation experiment was to produce a retrogression of the small mammal succession, experimentally demonstrating causality between changes in vegetation density and subsequent small mammal habitat use.     

Geotactic Behavior of Chlamydomonas

SYNOPSIS The normal negative geotaxis (motility oriented against gravity) of Chlamydomonas is an energy-dependent response that requires coordinated flagellar activity. It is evident from quantitative assays that the rate of geotaxis is steady, and slow relative to the average swimming speed.
Geotaxis is inhibited when the horizontal swimming path is less than 200 μm, suggesting that normal geotactic reorientation maneuvers involve long gradual turns. Videomicrographic tracking of cells confirms that such turns are common. In contrast, contact-reorientations generate random cell orientations. When collision frequencies increase, geotaxis in inhibited. The mechanism of normal geotactic orientation, then, depends on long slow reorientation maneuvers (from net downward to net upward vectors) that require hundreds of micrometers of free swimming space. Mechanisms of geotaxis that would require passive reorientation or sedimentation, or rapid active responding, are excluded.
Unusually dense populations sediment with atypical rapidity, probably due to formation of functionally aggregated subpopulations.
Sodium azide causes an inhibition of orientation behavior that is selective relative to its effects on general motility. Evidence presented suggests that active physiologic mechanisms for geotaxis should be reconsidered.     

Periphyton responses to a hydraulic gradient in a regulated river in New Zealand

1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1–0.5m depth, 0.1–1.5ms^?1 velocity, and 0.01–1.5 Froude number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis herculeana having the highest relative biovolumes. 2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co-dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid-velocity range (velocity = 0.8–1.2ms^?1). 3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1–2mm) at low velocities and Fr (in pool habitats), to thick (approximately 10mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash-free dry mass (APDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient. 4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms^?1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions. 5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient-poor streams.     

Population dynamics of two species of hyraxes in the Matobo National Park, Zimbabwe

Populations of the syntopic rock hyrax, Procavia capensis (Pallas), and yellow-spotted rock hyrax, Heterohyrax brucei (Gray), in the 42,400-ha Matobo National Park (MNP), Zimbabwe, were characterized from 1992 to 1996. The Procavia:Heterohyrax species ratio varied significantly ( P < 0.01) across years from 1.43:1.00 to 0.74:1.00. Estimates of abundance (31,114–39,869 Procavia and 21,833–46,619 Heterohyrax ) and densities (0.73–0.94 Procavia ha⁻¹ and 0.51–1.10 Heterohyrax ha⁻¹) indicated that populations had experienced a considerable decline since 1978, with drought suspected as the principal cause. The Heterohyrax population fluctuated more than the Procavia population. Although sex ratios of individuals trapped and collected in 1991–92 did not differ from 1:1 for either species, female Procavia nearly outnumbered males ( P < 0.10) among the prey of black eagles ( Aquila verreauxii Lesson), suggesting the possibility of divergence in the social organization and vigilance of the two species. In each year of the study, pups comprised < 30% of individuals in the two populations after the interspecifically synchronous annual birth event in March–April, and evidence exists for an effect by drought on fecundity. Females appear to breed, on average, no more than every other year. We estimated juvenile (0–1 year of age) mortality at 52.4–61.3% for Heterohyrax and 59.6–75.6% for Procavia . Mean mass (±1 SE) of Procavia adults was 3.4±0.2 kg and of Heterohyrax adults, 2.4±0.1 kg. Procavia had a significantly ( P < 0.01) greater condition index (body mass/total length) than Heterohyrax . Adults are especially important reservoirs of biomass for the diverse and numerous predators of the MNP, accounting for > 84% of hyrax biomass intraspecifically.     

Trait‐mediated effects of predation across life‐history stages in container mosquitoes

1. It was determined if the predatory midge Corethrella appendiculata Grabham imposes a fitness cost in a native mosquito, Ochlerotatus triseriatus Say, and an invasive mosquito, Aedes albopictus Skuse. The hypothesis that decreased activity of immature prey in the presence of predator cues is associated with life history costs through all life cycle stages was tested. 2. In experiment 1, individual larvae of O. triseriatus or A. albopictus were raised in the presence or absence of predation cues at two resource levels. Prey were video recorded to detect behavioural responses and to measure development time, size at emergence, and adult longevity. In experiment 2, prey populations were reared in similar environments and the frequency of predator cue additions was varied. 3. Only O. triseriatus reduced its activity in the presence of predation cues. Predation cues were associated with longer immature development times and shorter adult life spans in O. triseriatus, whereas in A. albopictus, the cues were associated with a larger size of emerging adults. 4. In the present study, it was found that behavioural modifications during the larval stage can affect mosquitoes through multiple stages of their complex life cycle. The species‐specific behavioural differences are probably attributable to the longer evolutionary history O. triseriatus has with predators, relative to the invasive A. albopictus.     

Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast- ( E. saligna ) and slow-growing ( E. sideroxylon ) seedlings exposed to preindustrial (290), current (400) or projected (650 μL L⁻¹) CO₂ concentration ([CO₂]) and to current or projected (current +4 °C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO₂] than to elevated temperature, while E. saligna responded similarly to elevated [CO₂] and elevated temperature; (2) the transition from preindustrial to current [CO₂] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO₂] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO₂], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.