Juvenile Population Densities Research Articles

Effects of Stage Structure on Coexistence: Mixed Benefits.

The properties of competition models where all individuals are identical are relatively well-understood; however, juveniles and adults can experience or generate competition differently. We study here less well-known structured competition models in discrete time that allow multiple life history parameters to depend on adult or juvenile population densities. A numerical study with Ricker density-dependence suggested that when competition coefficients acting on juvenile survival and fertility reflect opposite competitive hierarchies, stage structure could foster coexistence. We revisit and expand those results. First, through a Beverton-Holt two-species juvenile-adult model, we confirm that these findings do not depend on the specifics of density-dependence or life cycles, and obtain analytical expressions explaining how this coexistence emerging from stage structure can occur. Second, we show using a community-level sensitivity analysis that such emergent coexistence is robust to perturbations of parameter values. Finally, we ask whether these results extend from two to many species, using simulations. We show that they do not, as coexistence emerging from stage structure is only seen for very similar life-history parameters. Such emergent coexistence is therefore not likely to be a key mechanism of coexistence in very diverse ecosystems, although it may contribute to explaining coexistence of certain pairs of intensely competing species.

Field Performance of Winegrape Rootstocks and Fumigation during Establishment of a Chardonnay Vineyard in Washington

In Washington, most winegrapes are own-rooted Vitis vinifera, which is susceptible to the plant-parasitic nematodes Meloidogyne hapla and Xiphinema americanum. Using resistant rootstocks to manage nematodes has not been evaluated in Washington vineyards. A long-term vineyard trial was established to evaluate the effects of soil fumigation and rootstock genotype on M. hapla and X. americanum population dynamics and vine growth during vineyard establishment (first three years) in a replant scenario. Vines in an existing V. vinifera ‘Chardonnay’ vineyard were first treated with foliar glyphosate in fall 2014. Randomized areas within the vineyard were then either fumigated or not with drip-applied metam sodium. Following fumigation, vines were removed. In spring 2015, the vineyard was replanted to Chardonnay on the following rootstocks: 1103 Paulsen, 101-14 Millardet et de Grasset, Teleki 5C, and Harmony. Self-grafted and own-rooted Chardonnay were included. Fumigation reduced M. hapla soil second-stage juvenile (J2) population densities on own-rooted and self-grafted vines for only the first year after fumigation. One year after fumigation, the self-grafted and own-rooted vines had higher population densities of M. hapla J2 than rootstocks. All rootstocks supported measurable densities of M. hapla J2 but were poor hosts relative to V. vinifera. Fumigation effectively reduced population densities of X. americanum for up to 3.5 years. Fumigation also reduced early establishment pruning weights. Vines grown in fumigated areas had lower pruning weights through year 2; but rootstock was the bigger influence on pruning weights by year 3. This trial demonstrates that rootstocks have a more sustained impact on nematode re-establishment and subsequent vine health in a vineyard replant scenario than that of pre-plant fumigation.

Socially cued anticipatory adjustment of female signalling effort in a moth.

Juvenile population density has profound effects on subsequent adult development, morphology and reproductive investment. Yet, little is known about how the juvenile social environment affects adult investment into chemical sexual signalling. Male gumleaf skeletonizer moths, Uraba lugens, facultatively increase investment into antennae (pheromone receiving structures) when reared at low juvenile population densities, but whether there is comparable adjustment by females into pheromone investment is not known. We investigate how juvenile population density influences the 'calling' (pheromone-releasing) behaviour of females and the attractiveness of their pheromones. Female U. lugens adjust their calling behaviour in response to socio-sexual cues: adult females reared in high juvenile population densities called earlier and for longer than those from low juvenile densities. Juvenile density also affected female pheromonal attractiveness: Y-maze olfactometer assays revealed that males prefer pheromones produced by females reared at high juvenile densities. This strategic investment in calling behaviour by females, based on juvenile cues that anticipate the future socio-sexual environment, likely reflects a response to avoid mating failure through competition with neighbouring signallers.

Nematode populations as affected by residue and water management in a long-term wheat-soybean double-crop system in eastern Arkansas

Soybeans (Glycine max L.) are one of the major row crops in the United States, particularly in Arkansas. The soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is one of the most damaging pests that can cause major economic losses in soybeans. Little is known concerning the combined effects of tillage [conventional tillage (CT) and no tillage (NT)], water management (irrigation and non-irrigation), residue burning (burn and no burn), and wheat residue level (high and low) on nematodes. The objective of this study was to evaluate the effects of the combination of agronomic practices (tillage, water management, burning, and residue level) on natural nematode population densities and change over the growing season in a long-term wheat (Triticum aestivum L.)-soybean rotation on a silt-loam soil in Arkansas. A suite of 10 nematode genera were measured in the top 10 cm in July, August, and October in 2017 and 2018. Averaged across all other treatment factors, SCN egg concentration was 17.9 times greater at the end of the growing season in October than in July and August. The SCN second-stage juvenile (J2) population densities were 3.4 times greater in the CT-burn than in the CT-no-burn and NT-burn treatment combinations. In contrast to under CT, SCN J2 population densities were 3.8 times greater in the NT-no-burn than under the NT-burn treatment combinations. Lance, lesion, spiral, and stunt nematode population densities were also affected by various field treatment combinations. Traditional (i.e., tillage, residue burning, and irrigation) and alternative (i.e., non-tillage, no burning, and dryland) wheat-soybean management practices can influence nematode population densities even in non-problematic fields, thus potentially affecting long-term strategies to maximize soybean profitability.

Benthic ctenophore (Order Platyctenida) reproduction, recruitment, and seasonality in south Florida

AbstractReproductive structures, modes, and seasonal patterns of size–class abundances are examined in two benthic platyctene (Family Coeloplanidae) ctenophore species present in dissimilar shallow marine environments in subtropical southeast Florida. Coeloplana waltoni, a minute (1–3 mm body length) epizoic associate of octocorals, occurs in exposed environments often under turbulent conditions, and Vallicula multiformis (2–10 mm) commonly occurs epiphytically on macroalgae in protected, calm‐water environments. Reproductive activity in C. waltoni is most active during the warm‐water summer season (June–October); gonadal development in V. multiformis occurs year‐round, and is most pronounced during sea‐warming periods in late spring (May) and late summer to early autumn (August–October), with release of cydippid larvae. Both species are hermaphroditic brooders, exhibiting paedogenesis (early gonadal development) at body lengths approximately one‐third (Coeloplana) to one‐sixth (Vallicula) of maximum adult size. Juvenile individuals (<0.6 mm) increased in abundance in C. waltoni during the summer reproductive period, and large (≥1 mm) pink‐colored individuals comprised 50% or more of samples from July through September. Seasonal abundance of gravid individuals and the timing of cydippid larval release in V. multiformis did not correspond closely with juvenile or adult population densities. Asexual fragmentation occurred in both ctenophore species, but was observed more frequently in individuals of V. multiformis. This asexual mode of reproduction probably accounted in part for the discordance between ctenophore abundances and larval recruitment events by sexual means. Morphological structures and behaviors associated with reproduction are described in this study. Uncommon images of reproductive products (gametes, embryos, larvae), spawning events, brooding, and asexual fragmentation are included, some for the first time in the published literature.

Occurrence and Development of the Cereal Cyst Nematode (Heterodera avenae) in Shandong, China

The cereal cyst nematode (CCN), Heterodera aveane, has been found in 16 provinces of China, including the Shandong winter-wheat-growing region. This study investigated the population dynamics of H. avenae in the winter wheat 'Jimai 22' and 'Tainong 18' for two consecutive years in the field. Soil and root samples were collected during the winter-wheat-growing season and H. avenae population densities and life-stages determined. H. avenae population dynamics in roots and soil of the two winter wheat cultivars were similar over the 2-year period. Second-stage juvenile (J2) population densities in wheat roots were greatest during booting stage (April), when mean soil temperature was between 11.8 and 14.4°C. Cysts in rhizosphere soil increased significantly when new cysts were formed after Zadoks (Z) 47 (booting stage). There was a peak in J2 population densities in soil during Z23 and Z30 (tillering and stem elongation, respectively) whereas J2 population densities were the lowest at Z13 (seedling stage). This research provides important information indicating that J2 populations in roots and soil increased after the wheat winter dormancy period. Knowledge of when different life stages of H. avenae occur in winter wheat in Shandong will provide valuable insights to enable the development of an integrated approach to manage this plant-parasitic nematode.

Fine‐tuning of a mowing regime, a method for the management of the invasive plant, Ambrosia artemisiifolia, at different population densities

Common ragweed (Ambrosia artemisiifolia) is an invasive annual plant with highly allergenic pollen. Its spread in introduced and native ranges often occurs on roadsides, where it builds stable and rapidly growing populations. The most sustainable way of controlling the population size of this species is to prevent seed production in order to deplete the soil seed bank. Populations on roadsides are submitted to regular mowing management, which can even exacerbate the situation by inducing resprouting after cutting or by accidentally spreading seeds along the road. The population density in the juvenile stages of development could play an important role in the success of cutting regimes, as it might influence the resprouting capacity of this plant. The influence of the juvenile population density and of seven cutting regimes, differing in the timing and frequency of cuts, on easily measurable reproductive traits was investigated in a glasshouse experiment. The cutting regimes had a strong influence on the reproductive success and on the phenology of the development stages of ragweed. The population density in the juvenile stages did not play a role in further phenological development, but did influence the reproductive traits. The reproduction of ragweed can be lowered by locally adapted combinations of the timing and frequency of mowing. As the optimal management option for the reduction of both the male and female flowers, the authors suggest a first cut just before the start of male flowering, followed by subsequent cuts every 3–4 weeks.

Effects of levonorgestrel‐quinestrol (EP‐1) treatment on Mongolian gerbil wild populations: a case study

Rodent pest population outbreaks occur frequently in grassland ecosystems in northern China. The Mongolian gerbil (Meriones unguiculatus) is a dominant pest rodent which is distributed across the semi-desert grasslands of Inner Mongolia, China. In 2009, we studied the contraceptive effect of levonorgestrel-quinestrol (EP-1), concentration 50 ppm, on a wild Mongolian gerbil population. The one-off contraceptive treatment was compared with a control group using a semi-monthly live trapping method in the Ordos Semi-desert Grassland Region of Inner Mongolia. The results show that juveniles were not recruited in spring in the treatment group. Ratios of juveniles in the control and treatment groups showed significant semi-monthly differences from spring to summer (one-way ANOVA, F2, 14 = 7.53, P < 0.05). Between both groups, annual fluctuations of juvenile and total population densities were significantly different respectively (F2, 14 = 4.64, P < 0.05; F2, 18 = 7.72, P < 0.05). The contraceptive EP-1 delayed the normal reproductive pattern of Mongolian gerbil populations. This suppressed birth rates of gerbil populations, reduced their densities, and changed their age structures. The period of EP-1 baiting should be extended but it could be an ideal method for controlling Mongolian gerbil populations during each breeding season.

Influence of historical exploitation and recovery of biological resources on contemporary status of Margaritifera margaritifera L. and Salmo salar L. populations in Northwestern Russia

The influence of the historical exploitiation and recovery of biological resources on populations of freshwater pearl mussel and salmon was studied in the Kozha river (Onega River Basin). The freshwater pearl mussel was characterized by a low population density (0.09 ind./m2, on the average) and number of recruits. The role of the anthropogenic factor in the population is negligible; the effect of the pearl fishing is not observed. The dramatic decrease in the number of Atlantic salmon in the Onega River Basin in the late 20th century is a major reason for the slowed down reproduction of the freshwater pearl mussel. The second factor is the reconstruction of the Onega salmon hatchery in 1984, which led to a decrease in the juvenile salmon population density below the critical level on spawning and nursery grounds. As a result, mollusc reproduction has stopped. An artificial increase in the number of recruits must be developed in order to preserve the salmon population in the Onega River Basin. It is also necessary to forbid salmon fishing, including fishing migratory paths (with the exception of trapping for hatchery needs and traditional harvesting performed by locals in historical settlements).

Interactions between field populations of the potato cyst nematode Globodera pallida and Rhizoctonia solani diseases of potatoes under controlled environment and glasshouse conditions

Abstract This research elucidates interactions between Globodera pallida and Rhizoctonia solani (AG3) by comparing disease development in potato plants inoculated with G. pallida and R. solani. The effects of juvenile population density and time of infestation on R. solani disease were investigated in controlled environments. Plants were inoculated with a range of densities of second-stage juveniles (J2) of G. pallida (800, 4000, 10 000, 20 000 and 40 000 J2/plant) and R. solani at 2, 4 and 6 weeks after planting and a single treatment of G. pallida density and R. solani alone. The experiment demonstrated that greatest disease incidence and severity occurred when high population densities of G. pallida were applied at an early stage of plant development. To determine if this effect could explain differences in R. solani disease incidence and severity following interactions with individual field populations of G. pallida, a glasshouse experiment was conducted using two populations of G. pallida with known hatching characteristics (fast and slow hatching). The experiment revealed that a combination of a fast-hatching population with R. solani caused significantly more R. solani disease incidence and severity than a population of G. pallida with slower hatching characteristics.

Acid Toxicity Levels in Nova Scotian Rivers Have Not Declined in Synchrony with the Decline in Sulfate Levels

The Atlantic salmon (Salmo salar) resource of eastern Canada is impacted by acid rain in the Southern Upland (Atlantic Coast) area of Nova Scotia. Salmon runs in this area have become extinct in 14 rivers, are severely impacted in 20 rivers, and lightly impacted in 15 rivers. Water chemistry and fish communities in nine Southern Upland salmon rivers were studied from 1982 to 1996 as part of the effort to monitor the effects of the emission controlprograms in Canada and the United States. There hasbeen no statistically significant change in total ioncontent of Southern Upland river water, but there wasa significant decline in sulfate levels that was balanced by an increase in organic anions, and declines in calcium and magnesium that were balanced by increases in sodium and potassium. A geochemical scenario is proposed to account for these chemical changes. River water pH levels showed no overall linear trend, but at borderline toxicity sites the year-to-year variations in pH were correlated withchanges in juvenile salmon population densities. Tenfish species were collected, but none showed anysignificant overall time trend in population density.Fish species diversity was positively correlated with pH.

Food ration and condition affect early survival of the coral reef damselfish, Stegastes partitus.

The supply of larvae is a major determinant of population and community structure in coral reef fishes. However, spatial and temporal variation in condition (i.e. quality) of potential recruits, as well as their density (i.e. quantity), may influence survival and growth of juveniles. We conducted an experiment to test whether recent feeding history could affect growth, condition and post-recruitment survival in a Caribbean damselfish, Stegastes partitus. Fish were collected soon after settlement, and fed either low or high rations in aquaria for 7 days. Fish fed the high ration grew faster in aquaria and were in a better condition (higher total lipids and Fulton's condition factor) at the end of the feeding period. Subsequently, we released 50 fish in 25 pairs (one fish subjected to low rations, the other to high rations) on a Bahamian coral reef and monitored survival for 10 days. Survivorship of high-ration fish was double that of low-ration fish (80 vs 40% over 10 days). However, low-ration fish that survived 10 days were of similar condition and grew at similar rates to high-ration fish, suggesting that short-term ration differences may not persist in surviving fish. Laboratory experiments showed that low-ration fish were taken by piscivorous fishes before high-ration fish, indicating that differential predation may account for survival differences. This study highlights the potential of feeding history and condition to affect the relationship between patterns of larval arrival at reefs, and subsequent juvenile and adult population densities.

Agricultural and Municipal Compost Residues for Control of Root-Knot Nematodes in Tomato and Pepper

The efficacy of olive (Olea europea) pomace (OP), chicken litter (CL), and municipal compost residues (MCR) as soil amendments for control of root-knot nematode (Meloidogyne javanica) was studied in shadehouse and field microplot experiments. Results from a shadehouse experiment with green pepper (Capsicum annum) demonstrated that plants in soil amended with MCR, OP, and MCR+OP had lower numbers of M. javanica in the roots than those grown in unamended control soil. Ammonia ted OP (treated with NH3) was not as effective as the other amendments for nematode control. Amended soils exhibited higher esterase activity than control soil and numbers of nematodes per gram of root were inversely related to soil esterase activity. In a field microplot experiment with tomato (Lycopersicon esculentum) highest yields in soil fumigated with methyl bromide and infested with the nematode after fumigation were obtained with CL+OP amendments; highest yields in nonfumigated soil also infested with the nematode corresponded to treatments with MCR+OP, CL+OP, and CL. Most amendments in the microplot experiment were suppressive of M. javanica. Numbers of M. javanica juveniles in unamended, methyl bromide-treated soil were higher than in unamended unfumigated soil at harvest. Esterase activity was higher in amended soils than those without amendments irrespective of whether the soils had been treated with methyl bromide or not. Numbers of nematodes per gram of root were inversely related to soil esterase activity in unfumigated soil but not in soil treated with methyl bromide. Juvenile population densities in soil were inversely related to esterase activity in both fumigated and unfumigated soil. However, the model describing the relationship between the two variables indicated a more rapid decline in nematode numbers with increasing enzymatic activity for unfumigated soil than in methyl bromide-treated soil.

Soil and Plant Factors Associated with Sudden Death Syndrome of Soybean

Sudden death syndrome (SDS) of soybean (Glycine max [L.] Merr.) is a soilborne disease of increasing importance in high-yield environments. This study was conducted to determine which combinations of soil fertility parameters and soybean cyst nematode (Heterodera glycines Ichinoe) (SCN) second stage juvenile (J2) population levels were associated with SDS disease severity. Also, the effect of SDS disease severity on leaf nutrient content and on soybean yield and seed germination were determined. These studies were conducted at the Pine Tree Station, in Colt, on a Crowley milt loam (fine, montmorillonitic, thermic Typic Albaqualfs) over 3 yr with the soybean cultivar Lee 74 [...]