Hierarchy of responses to resource pulses in arid and semi-arid ecosystems

In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?     

Recruitment patterns in Antarctic Peninsula shelf sediments: evidence of decoupling from seasonal phytodetritus pulses

Summer phytoplankton blooms on the West Antarctic Peninsula (WAP) shelf result in episodic deposition of labile food material for benthic detritivores. This summer deposition is thought to enhance benthic recruitment of macro- and megafauna. To explore seasonality in benthic recruitment, juvenile invertebrates (>100 μm) were collected in a seasonal time series at three stations on the WAP continental shelf. 4,098 juveniles were collected (average densities 2,000–7,000 m⁻²), with polychaetes dominant (2,581 individuals). The majority of polychaetes showed evidence of enhanced recruitment prior to the summer bloom, although patterns varied across stations. Additional taxa showed recruitment peaks in summer, but again, patterns varied among stations. Based on observed patterns, polychaete taxa are classified as “seasonal”, “marginally seasonal”, and “non-seasonal” recruiters, with the latter two patterns predominating. The year-round presence of small juveniles in most taxa suggests that recruitment occurs continuously, with periodic enhancement. Year-round recruitment is consistent with the presence of a persistent “food bank” of labile organic material in WAP shelf sediments, allowing recruitment to be largely decoupled from seasonal bloom dynamics.     

Comparative responses of three fuel wood yielding plants to PEG-induced water stress at seedling stage

Three species of fast growing fuel wood yielding plants locally available (Acacia holosericea, Bauhinia variegata and Cassia siamea) were characterized in respect of their responses to water stress. Seedlings (25 days) of these species, exposed to two levels of water stress (−0.5 and −1.0 MPa) induced by PEG-6000 for 24 h, were analysed for relative water content (RWC) and the contents of chlorophyll, protein, soluble sugars and proline in leaves along with activities of catalase, peroxidase and superoxide dismutase (SOD). RWC was lower in stressed compared to the unstressed seedlings. However, stress-induced decline in RWC was lowest in B. variegata. Chlorophyll and protein contents declined with increasing levels of water stress, decline being least in B. variegata. Soluble sugar and proline contents increased under water stress particularly in B. variegata. The enzyme activity of catalase (EC-1.11.1.6), peroxidase (EC-1.11.1.7) and SOD (EC-1.15.1.1) decreased with increased levels of water stress. Such decline in the activity of these enzymes was least in B. variegata. Apparently, B. variegata is potentially the species most tolerant to water stress among these three fuel wood-yielding plants.     

Positive flow dependence in the initial colonization of a fouling community: results from in situ water current manipulations

During late winter/early spring (1992–1993), a series of 12 tapered channels were placed subtidally in a Long Island, New York (U.S.A) estuary to examine total fouling community development. Rather than relying upon flow-dependent correlations among spatially segregated sites, these channels directly accelerated or decelerated the existing tidal currents ± 50% (ambient flow speeds ranged from < 2 to > 50 cm/s). In response to experimentally manipulated water flows over five months, the arborescent hydroid Obelia longissima Pallas (1766) accumulated onto Tygon tubing in direct proportion to mean free-stream flow speed, whereas barnacle (Balanus eburneus Gould) initial settlement and cumulative recruitment along plywood ceilings increased three-fold and five-fold, respectively, with a long-term doubling of water speed. These positive flow-dependent results directly contrast those from previous experiments in the same channel apparatus which failed to document any flow enhancement on the growth of suspension-feeding bivalves.     

Natural succession of macroalgal-dominated epibenthic assemblages at different water depths and after transplantation from deep to shallow water on Spitsbergen

In the current study, we investigated the primary succession of seaweeds over different time periods at different water depths. Furthermore, we followed the succession of field-grown benthic communities of different successional age, developing on ceramic tiles, prior to and after transplantation from 8 to 0.5 m water depth. The transplantation simulated changes associated with the break up of sea-ice cover, e.g. light regime or wave exposure. For this purpose, we transplanted 12 and 21-month old communities, grown at 8 m water depth, together with a set of sterile tiles, onto rafts, floating in 0.5 m water depth. Our results describe for the first time the succession of macroalgal communities in the Arctic and give important insights into the effect of disturbance of differently aged communities. The primary succession at 0.5 m water depth was mainly driven by Bacillariophyta and filamentous green algae like Urospora sp. and Ulothrix implexa. Twelve-month old communities at 8 m water depth are dominated by members of the Ectocarpales (Phaeophyceae), like Pylaiella littoralis, P. varia, and Ectocarpus siliculosus and the green alga U. implexa, whereas the 21-month old community showed a higher cover of the green algal class Ulvophyceae and sessile invertebrates. After transplantation to near surface conditions, species composition of the communities changed, but this effect was differently strong between communities of different age.     

Spatial and temporal variability of the phenological seasons in Germany from 1951 to 1996

Various indications for shifts in plant and animal phenology resulting from climate change have been observed in Europe. This analysis of phenological seasons in Germany of more than four decades (1951–96) has several major advantages: (i) a wide and dense geographical coverage of data from the phenological network of the German Weather Service, (ii) the 16 phenophases analysed cover the whole annual cycle and, moreover, give a direct estimate of the length of the growing season for four deciduous tree species. After intensive data quality checks, two different methods – linear trend analyses and comparison of averages of subintervals – were applied in order to determine shifts in phenological seasons in the last 46 years. Results from both methods were similar and reveal a strong seasonal variation. There are clear advances in the key indicators of earliest and early spring (?0.18 to ?0.23 d y^?1) and notable advances in the succeeding spring phenophases such as leaf unfolding of deciduous trees (?0.16 to ?0.08 d y^?1). However, phenological changes are less strong during autumn (delayed by + 0.03 to + 0.10 d y^?1 on average). In general, the growing season has been lengthened by up to ?0.2 d y^?1 (mean linear trends) and the mean 1974–96 growing season was up to 5 days longer than in the 1951–73 period. The spatial variability of trends was analysed by statistical means and shown in maps, but these did not reveal any substantial regional differences. Although there is a high spatial variability, trends of phenological phases at single locations are mirrored by subsequent phases, but they are not necessarily identical. Results for changes in the biosphere with such a high resolution with respect to time and space can rarely be obtained by other methods such as analyses of satellite data.     

Invasion of Mediterranean benthic assemblages by red alga Lophocladia lallemandii (Montagne) F. Schmitz: Depth-related temporal variability in biomass and phenology

Lophocladia lallemandii (Montagne) F. Schmitz is an invasive red alga which has recently become widespread in the Mediterranean Sea. Temporal variability in biomass of L. lallemandii across a depth gradient (10-30 m depth), as well as invasive potential by means of its reproductive output and recruitment abilities was assessed. No variability was found across the depth gradient, suggesting a homogenous pattern of invasion between the sampled depths. In contrast, all variables displayed a clear temporal pattern with higher growth and reproductive output with increasing temperatures, both in shallow and deep populations. The high total and relative biomass values of L. lallemandii confirm the invasive nature of this species. Likely, the high potential reproductive output by means of tetrasporocysts (41.8 ± 9.34 cm⁻²), together with the high recruitment abilities (352 ind. m⁻²) explains the great colonization capacity of L. lallemandii which prevents any kind of manual eradication.     

Changes to water repellence of soil caused by the growth of white-rot fungi: studies using a novel microcosm system

A microcosm system is described which permits assessment of the progressive growth of filamentous fungi through soil. We report on its application to measure the effects of Coriolus versicolor and Phanerochaete chrysosporium upon the sorptivity and water repellence of a mineral soil, measured using a miniature infiltration device. Both fungal species caused moderate sub-critical repellence. Since the pore structure was unaffected, the repellence was probably due to hydrophobic substances of fungal origin. This is the first report of changes in soil repellence caused by the growth of potential xenobiotic bioremediating fungi. The potential consequences are discussed.     

Efficiency of endozoochorous seed dispersal in six dry-fruited species (Cistaceae): from seed ingestion to early seedling establishment

We combined laboratory and nursery experiments to analyse the effectiveness of sheep as endozoochorous seed dispersers of six native shrubby Cistaceae species collected in SE Spain (Helianthemum apenninum (L.) Mill., H. violaceum (Cav.) Pers., Fumana ericoides (Cav.) Grand., F. thymifolia (L.) Spach, Cistus monspeliensis L. and C. laurifolius L.), considering the main stages after seed ingestion, i.e. seed recovery, seed germination, seedling emergence and early seedling establishment. Seed recovery after gut passage was high (around 40%) for all the species, except F. thymifolia (12%). Most seeds (ca. 90%) were recovered within 48 h after ingestion for all the species, although seeds were still recovered up to 96 h after ingestion. Gut passage increased germination up to seven-fold compared to non-ingested seeds. Furthermore, seedling emergence from seeds contained in pellets was overall similar (intact pellets) to or higher (crumbled pellets) than emergence from seeds without dung. Survival of emerged seedlings and mass of seedlings after 20 days were not reduced by dung. Sheep act therefore as effective dispersers of these Cistaceae species by scattering seeds and promoting germination, while faeces do not hamper seedling establishment. We conclude that the interaction between herbivorous ungulates and these dry-fruited species may be considered a mutualism qualitatively similar to the mutualism between frugivorous vertebrates and fleshy-fruited plants.     

Spatial and temporal variability of surface water in the Kuroshio source region, Pacific Ocean, over the past 21,000 years: evidence from planktonic foraminifera

The Kuroshio Current is the major western boundary current of the North Pacific Ocean and has had a large impact on surface water character and climate change in the northwestern Pacific region. The Kuroshio Current becomes a distinctive surface flow in the Ryukyu Arc region after diverging from the North Equatorial Current and passing through the Okinawa Trough. Therefore, the Ryukyu Arc area can be called the Kuroshio source region. We reconstructed post-21-ka time–space changes in surface water masses in the Ryukyu Arc region using 15 piston cores which were dated by planktonic δ¹⁸O stratigraphy and AMS ¹⁴C ages. Our analysis utilized spatial and temporal changes in planktonic foraminiferal assemblages which were classified into the Kuroshio, Subtropical, Coastal, and Cold water groups on the basis of modern faunal distributions in the study region. These results indicate that the Kuroshio Current and adjacent surface water masses experienced major changes during: (1) the Last Glacial Maximum (LGM), and (2) the so-called Pulleniatina minimum event (PME) from 4,500 to 3,000 yr BP. The Kuroshio LGM event corresponds to severe global cooling and is marked by decreases in planktonic δ¹⁸O values and estimated sea-surface temperature (SST) with the dominance of the Cold water group of planktonic foraminifera. Cooling within the Kuroshio source region was enhanced during the LGM event because the Kuroshio Current was forced eastward due to the formation of a land bridge between Taiwan and the southern Ryukyu Arc which prohibited its flow into the Okinawa Trough. Except for the severe reduction and disappearance of the Pulleniatina group, no clear cooling signal was identified during the PME based on δ¹⁸O values, estimated SST values and variations in the composition of planktonic foraminiferal faunas. The PME assemblages are marked by high abundances of Neogloboquadrina dutertrei, a distinctive Kuroshio type species, along with other species assigned to the Coastal and Central water groups. Subtle ecological differences exist between Pulleniatina obliquiloculata and N. dutertrei; i.e. P. obliquiloculata exhibits lower rates of reproduction under conditions of lower primary productivity in the central Equatorial Pacific Ocean. El Niño-like conditions in the Equatorial Pacific Ocean result in lower rates of surface transport in the Kuroshio Current. In turn, this response triggers lower rates of primary productivity in central equatorial surface waters as well as in the upstream Kuroshio source region, ultimately resulting in a lower abundance of P. obliquiloculata. Thus, we interpret the PME as a possible proxy signal of El Niño-like conditions and enhancement of the El Niño Southern Oscillation climate system after the PME in the tropical and sub-tropical Pacific Ocean.     

Rubisco and some chaperone protein responses to water stress and rewatering at early seedling growth of drought sensitive and tolerant wheat varieties

Four wheat varieties differing in their drought tolerance were subjected to severe but recoverable water stress at seedling stage. Growth parameters, leaf water deficit (WD) and electrolyte leakage (EL) were used to evaluate the stress intensity and the extent of recovery. The physiological response of the varieties was quite similar under severe drought. Leaf protein patterns and levels of some individual proteins relevant to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) maintenance were studied in control, stressed and recovering plants by electrophoresis and immunoblotting. The bands representing Rubisco large subunit (RLS), N- and C-terminus of RLS, Rubisco activase (RA) and Rubisco binding protein (RBP, cpn 60), as well as the chaperone and proteolytic subunits of the Clp protease complex were identified using polyclonal antibodies. Under drought conditions RLS, Clp proteases and especially RBP were enhanced, whereas the RA band was only slightly affected. The drought tolerant varieties had higher RBP content in the controls and drought treated plants. Its concentration could be a potential marker for drought tolerance.     

A test of the risk allocation hypothesis: tadpole responses to temporal change in predation risk

The risk allocation hypothesis predicts that temporal variationin predation risk can influence how animals allocate feedingbehavior among situations that differ in danger. We testedthe risk allocation model with tadpoles of the frog Rana lessonae,which satisfy the main assumptions of this model because theymust feed to reach metamorphosis within a single season, theirbehavioral defense against predators is costly, and they canrespond to changes in risk integrated over time. Our experiment switched tadpoles between artificial ponds with different numbersof caged dragonfly larvae and held them at high and low riskfor different portions of their lives. Tadpoles responded stronglyto predators, but they did not obey the risk allocation hypothesis:as the high-risk environment became more dangerous, there wasno tendency for tadpoles to allocate more feeding to the low-riskenvironment, and as tadpoles spent more time at risk, they didnot increase feeding in both environments. Our results suggestthat the model might be more applicable when the time spentunder high predation risk is large relative to the time requiredto collect resources.     

Sexual selection and the temporal separation of reproductive events: sperm storage data from reptiles, birds and mammals

The duration of sperm storage by females differs markedly between reptiles (maximum: 2555 d [7 years] and birds (maximum: 117d), with mammals showing both very short (< 1 d) and relatively long periods (maximum: 198 days). The females of many reptiles, probably all birds and some mammals have specialized structures for storing sperm, suggesting that selection for sperm storage has operated on females. Sperm storage, together with delayed implantation and delayed development, separates copulation from fertilization, and hence, copulation from the timing of birth. All three types of separation mechanism occur in mammals. Delayed implantation cannot occur in reptiles and birds because their eggs do not implant, but delayed development occurs in some reptiles. Comparisons among vertebrate classes suggest that sperm storage in the female tract may have an upper limit of a little over 100 days in endotherms: non-hibernating mammals have circumvented this limit by using delayed implantation as an uncoupling mechanism. A long reproductive cycle (up to four years) probably makes sperm storage obligatory for some reptiles. We consider existing hypotheses for prolonged sperm storage and the other reproductive delays, and propose a new hypothesis. Our hypothesis extends Sandell's (1990) hypothesis which states that in mammals delayed implantation has evolved to allow females to time both their copulation and birth seasons optimally. We propose that the other separation mechanisms, namely sperm storage and delayed development, have also evolved for this reason. Sandell suggested that the optimal time for females to copulate is when the opportunities to obtain the best quality male exist, and thus the dislocation of copulation and birth seasons has occurred through sexual selection. We propose that the other two separation mechanisms may also have evolved through sexual selection, in part at least. Of the three separation mechanisms, sperm storage has additional advantages for females in that it also allows them to modify their choice of male after copulation has occurred, through sperm competition. Thus we propose that: (1) all separation mechanisms have evolved when the optimal time for copulation is not compatible with either the optimal time for fertilization (birds) or the optimal time for birth, given a constant gestation period (reptiles, birds and mammals); (2) separation mechanisms have evolved through sexual selection enabling females to improve the quality of the male that fertilizes their eggs, either through pre-copulatory male-male competition (via any of the separation mechanisms) or through post-copulatory sperm competition (via sperm storage).     

Imbibition phases and germination response of Mimosa bimucronata (Fabaceae: Mimosoideae) to water submersion

Mimosa bimucronata is a pioneering tree that occurs predominantly in moist lowlands, floodplains and on margins of rivers and lakes in Latin America. The effect of submergence on seed germination in M. bimucronata was firstly studied. Patterns of water absorption by M. bimucronata seeds were investigated thereafter to assess the imbibition phases of scarified and unscarified seeds. The germination percentage was significantly higher in scarified than in unscarified seeds, and the velocity of seed germination also increased considerably in scarified seeds. Submergence duration did not significantly affect germination percentages of scarified and unscarified seeds. Therefore, seed viability after submersion suggests that M. bimucronata may display hydrochorous dispersal and also that seeds are able to germinate successfully in areas with frequent seasonal flooding. With respect to imbibition phases, phase II was very short or even absent for scarified and unscarified seeds; therefore, a plateau, where water absorption by seeds is established, was not observed. Finally, we verified that the passage from phase I to III was very tenuous and took a long time in seeds without scarification.     

Apple replant disease: the assessment and results of seedling bio-assays of growth responses to soil fumigation with chloropicrin

The effects of chloropicrin fumigation of field soils before replanting apple are frequently predicted from bio-assays using pot-grown apple seedlings. The results of 506 such bio-assays carried out during 12 years are discussed with reference to modifications which were made to the test method. The interpretation of the most recent and valid bio-assay results is considered in relation to the different purposes for which such tests may be conducted. When all test soils were amended with phosphorus (to compensate for the eradication of vesicular-arbuscular mycorrhizal fungi by fumigation) the proportion of old orchard soils yielding economically significant growth responses increased from 39% to 68%. When, additionally, plant growth was assessed by shoot fresh weight (rather than shoot length) the proportion further increased to 82%. The traditional method of expressing growth responses to soil sterilisation, as proportionate growth increases, was found to be subject to skewing effects of non-biological fertility factors; this feature was avoided when results were given as actual growth increases. The latter value therefore is considered preferable when assessing the relative disease status of soils for etiological purposes (as opposed to predicting the cost-benefits of field soil treatment). The data indicated that disease severity was on average diminished in the most acidic soils (pH 4.8–6.1), but did not support the generalisation that disease severity increases progressively with soil pH.     

Photosynthetic activity of US biomes: responses to the spatial variability and seasonality of precipitation and temperature

We examined the response of the normalized difference vegetation index, integrated over the growing season (gNDVI) to mean precipitation, maximum temperature (T_max), and minimum temperature (T_min) over an 11‐year period (1990–2000) for six biomes in the conterminous United States. We focused on within‐ and across‐biome variance in long‐term average gNDVI, emphasizing the degree to which this variance is explained by spatial gradients in long‐term average seasonal climate. Since direct measurements of ecosystem function are unavailable at the spatial and temporal scales studied, we used the satellite‐based gNDVI as a proxy for net photosynthetic activity. Forested and nonforested biomes differed sharply in their response to spatial gradients in temperature and precipitation. Gradients in mean spring and fall precipitation totals explained much of the variance in mean annual gNDVI within arid biomes. For forested biomes, mean annual gNDVI was positively associated with mean annual and seasonal T_min and T_max. These trends highlight the importance of the seasonal components of precipitation and temperature regimes in controlling productivity, and reflect the influence of these climatic components on water balance and growing‐season length. According to the International Panel on Climate Change (IPCC) (2001) increases in temperature minima and fall precipitation have contributed the dominant components of US increases in temperature and precipitation, respectively. Within the range of conditions observed over the study region, our results suggest that these trends have particularly significant consequences for above‐ground plant productivity, especially for Grassland, Open Shrubland, and Evergreen Needleleaf Forest. If historical climatic trends and the biotic responses suggested in this analysis continue to hold, we can anticipate further increases in productivity for both forested and nonforested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation.     

The effect of early human contact and the separation method from the dam on responses of beef calves to humans

The objectives of this experiment were to evaluate the effect of early human contact and of the separation method from the dam on the future relationships of calves with humans, and to investigate the relationship between dam responses and calf responses. Thirty-three Salers calves aged 2–4 days old and reared outdoors were split into 3-week treatments balanced according to sex and birth dates. Group 1 (long separation from the dam and human contact: LS + H; n = 11) underwent 8 h of separation from the dam per day and 5 min of individual stroking; group 2 (short separation from the dam and human contact: SS + H; n = 11) underwent 1 h of separation and the same amount of human contact as LS + H calves. Group 3 (short separation from the dam and no human contact: SS − H; n = 11) was a control group undergoing the same duration of separation as SS + H but without stroking. At 3, 15 and 45 weeks of age, the calves were tested in a standard arena test (AT) where they were successively left alone (2 min), left with a stationary human (5 min), and left with a human approaching and touching them (2 min). At 15 and 45 weeks, the calves were also tested with the standard docility test (DT: test of restraint). The dams were also tested with DT 2 months before calving. Data analysis via Mann–Whitney tests and Spearman's correlations showed no significant effect of the duration calves were separated from their dams. Just after treatment at 3-week of age, calves given stroking (LS + H and SS + H) were more motionless and more willing to accept human contact (AT: touching) than control calves (SS − H, P < 0.01). At 45 weeks of age, calves given stroking spent also significantly more time (P < 0.05) motionless with the approaching human compared to non-stroked calves (SS − H), suggesting a persistent effect. However, this effect was not reproduced on the other behavioural criteria recorded (e.g., duration of human contact or docility score). In the different tests and at the different ages, the docility scores of the dams were significantly correlated (up to 0.7, P < 0.01) with behaviour towards humans shown by stroked calves but not non-stroked calves (SS − H). Our results suggest that additional human contact at early age, but not duration of the separation from the dam, could be beneficial for the human–animal relationship, but only for calves born to docile dams.