Does river regulation increase the dominance of invasive woody species in riparian landscapes?

Aim A regional analysis was used to explore the influence of river regulation on the dominance of non‐native, invasive shrubs and trees. We addressed the following questions: (1) How do large dams affect hydrological parameters that influence riparian vegetation? (2) How do flow regimes affect the dominance of non‐native woody species? (3) How do changes in flow regimes affect the dominance of non‐native woody species? Location South‐western USA. Methods We sampled the canopy cover of woody species on 179 point bars along seven non‐dammed and thirteen dammed river segments. Wilcoxon rank sum tests were used to determine differences between flow parameters in dammed and non‐dammed rivers. We used correlation analyses and generalized linear model comparisons to examine associations of flow parameters and canopy cover of native (Populus and Salix) and non‐native (Tamarix and Elaeagnus) taxa. An index of flow alteration that was created using principal components analysis was regressed with vegetation cover. Results Tamarix cover was positively related to drainage area, flow constancy, August and May median flow and flow recession rate, but Elaeagnus cover was unrelated to flow variables. River segments with peak flows in late summer or high constancy had the highest Tamarix cover. Populus cover was positively influenced by low maximum temperatures and frequent high pulses. Flow alteration was negatively related to Populus cover and positively related to Tamarix cover. Total non‐native, Elaeagnus and Salix covers were not correlated with flow alteration. Main conclusions Rivers with a large drainage area and low flow variability are inherently more vulnerable to invasions. River regulation does not necessarily increase the cover of non‐native, invasive species. Instead, changes in flow allow proliferation of species that have life‐history traits suited to modified flow regimes. River restoration projects that aim to reinstate natural flow regimes should be designed with knowledge of native and non‐native species' life history strategies.     

Does mating behaviour affect connectivity in marine fishes? Comparative population genetics of two protogynous groupers (Family Serranidae)

Pelagic larval duration (PLD) has been hypothesized to be the primary predictor of connectivity in marine fishes; however, few studies have examined the effects that adult reproductive behaviour may have on realized dispersal. We assessed gene flow (connectivity) by documenting variation in microsatellites and mitochondrial DNA sequences in two protogynous species of groupers, the aggregate spawning red hind, Epinephelus guttatus, and the single‐male, harem‐spawning coney, Cephalopholis fulva, to ask whether reproductive strategy affects connectivity. Samples of both species were obtained from waters off three islands (Puerto Rico, St. Thomas and St. Croix) in the Caribbean Sea. Despite the notion that aggregate spawning of red hind may facilitate larval retention, stronger signals of population structure were detected in the harem‐spawning coney. Heterogeneity and/or inferred barriers, based on microsatellites, involved St. Croix (red hind and coney) and the west coast of Puerto Rico (coney). Heterogeneity and/or inferred barriers, based on mitochondrial DNA, involved St. Croix (coney only). Genetic divergence in both species was stronger for microsatellites than for mitochondrial DNA, suggesting sex‐biased dispersal in both species. Long‐term migration rates, based on microsatellites, indicated asymmetric gene flow for both species in the same direction as mean surface currents in the region. Red hind had higher levels of variation in microsatellites and lower levels of variation in mitochondrial DNA. Long‐term effective size and effective number of breeders were greater for red hind; estimates of θ_f, a proxy for long‐term effective female size, were the same in both species. Patterns of gene flow in both species appear to stem in part from shared aspects of larval and adult biology, local bathymetry and surface current patterns. Differences in connectivity and levels of genetic variation between the species, however, likely stem from differences in behaviour related to reproductive strategy.     

Does polyandry control population sex ratio via regulation of a selfish gene?

The extent of female multiple mating (polyandry) can strongly impact on the intensity of sexual selection, sexual conflict, and the evolution of cooperation and sociality. More subtly, polyandry may protect populations against intragenomic conflicts that result from the invasion of deleterious selfish genetic elements (SGEs). SGEs commonly impair sperm production, and so are likely to be unsuccessful in sperm competition, potentially reducing their transmission in polyandrous populations. Here, we test this prediction in nature. We demonstrate a heritable latitudinal cline in the degree of polyandry in the fruitfly Drosophila pseudoobscura across the USA, with northern population females remating more frequently in both the field and the laboratory. High remating was associated with low frequency of a sex-ratio-distorting meiotic driver in natural populations. In the laboratory, polyandry directly controls the frequency of the driver by undermining its transmission. Hence we suggest that the cline in polyandry represents an important contributor to the cline in sex ratio in nature. Furthermore, as the meiotic driver causes sex ratio bias, variation in polyandry may ultimately determine population sex ratio across the USA, a dramatic impact of female mating decisions. As SGEs are ubiquitous it is likely that the reduction of intragenomic conflict by polyandry is widespread.     

Does the invasive Lupinus polyphyllus increase pollinator visitation to a native herb through effects on pollinator population sizes?

Invasive plants may compete with native species for abiotic factors as light, space and nutrients, and have also been shown to affect native pollination interactions. Studies have mainly focused on how invasive plants affect pollinator behaviour, i.e. attraction of pollinators to or away from native flowers. However, when an invasive plant provides resources utilized by native pollinators this could increase pollinator population sizes and thereby pollination success in natives. Effects mediated through changes in pollinator population sizes have been largely ignored in previous studies, and the dominance of negative interactions suggested by meta-analyses may therefore be biased. We investigated the impact of the invasive Lupinus polyphyllus on pollination in the native Lotus corniculatus using a study design comparing invaded and uninvaded sites before and after the flowering period of the invasive. We monitored wild bee abundance in transects, and visit rate and seed production of potted Lotus plants. Bumblebee abundance increased 3.9 times in invaded sites during the study period, whereas it was unaltered in uninvaded sites. Total visit rate per Lotus plant increased 2.1 times in invaded sites and decreased 4.4 times in uninvaded sites. No corresponding change in seed production of Lotus was found. The increase in visit rate to Lotus was driven by an increase in solitary bee visitation, whereas mainly bumblebees were observed to visit the invasive Lupinus. The mechanism by which the invasive increases pollinator visit rates to Lotus could be increased availability of other flower resources for solitary bees when bumblebees forage on Lupinus.     

Phylloplane fungi: an extrinsic factor of tetranychid population growth?

In a study on leaf-inhabiting tetranychid mites (Tetranychus urticae Koch–TSSM and Panonychus ulmi (Koch)–ERM) we investigated the effects of an extrinsic factor on the mites environment, namely phylloplane fungi. In a research orchard four trees were selected and treated with an aerosol application of a phylloplane fungus (Alternaria alternata) in a tap-water emulsion. Applications were made immediately after each sampling, with the exception of the last sample date. Two tap water controls for each treated tree were also sampled: a nearest neighbour (< 3 m from the treated trees) and a distant neighbour (> 30 m from the treated trees with other apple trees in between). Due to possible migration from the treated trees to near neighbours, the distant control best reflected normal orchard conditions. Eight samples were taken throughout the 1994 growing season; however, appreciable mite populations were only observed on the last four sample dates. On the treated trees, the ERM maintained a steady low population (less than ten per leaf) whereas the TSSM showed a population outbreak (up to 44 mites per leaf). Conversely, on the distant trees, the TSSM maintained a low population (less than ten per leaf) while the ERM showed an outbreak (up to 33 per leaf). Observing on a leaf by leaf basis, when tetranychids were present on a leaf, either one species dominated or the other, suggesting mutual competitive exclusion, the outcome of which was reversed to favour TSSM on trees that received an application of fungus. We concluded that the application of additional or supplemental amounts of A. alternata to apple leaves enhanced the population growth of TSSM compared to that of ERM. Possible mechanisms are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Faster or slower: has growth of eastern Baltic cod changed?

Recent environmental changes have influenced the ecology and biology of eastern Baltic cod. Declining somatic condition, maturation at smaller size and restricted size distribution of the population suggest that growth rates have decreased between the early 2000s and the 2010s. Extensive age estimation problems have until now precluded testing of this hypothesis. This study presents evidence for a decrease in somatic growth rate of Baltic cod. Temporal patterns of growth, condition and maturation were analysed based on two complementary analyses: length frequency mode progression derived from DATRAS bottom trawl survey data and known-age samples, where size at age was back-calculated from daily otolith growth patterns. In the known-age samples, growth was positively related to somatic condition at capture with maturity dependent differences. Immature individuals had experienced significantly lower growth and were in lower condition at capture than mature individuals. Growth rates in the known-age samples were estimated at 9.5, 7.8 and 5.7?cm per year for age classes 1, 2 and 3 respectively. Growth between age 2 and 3 decreased significantly from 8.8?cm in the 1997 year class to 7.6?cm in the 2010 year class. While the 2001 and 2004 known-age samples were representative for the population, the 2013 sample was biased towards individuals with a higher condition and growth. Complementary length frequency analysis following the length mode of fish from age 2 to age 3 confirmed growth estimates from the early 2000s, while suggesting a 37.5% lower growth in 2013 compared with 2005.     

Defoliation of wild native box trees (Buxus sempervirens): Does box rust (Puccinia buxi) infection influence herbivory,survival and growth of the invasive Cydalima perspectalis?

The invasive box‐tree moth Cydalima perspectalis causes damage to horticultural box trees (Buxus spp.) in private gardens and parks in Europe and defoliation of large areas of European native box trees Buxus sempervirens, which grow in the understorey of deciduous forests. In some parts of their distribution area, wild native box trees are infected by the box rust Puccinia buxi, a fungus which does not occur in the native range of C. perspectalis. We examined whether the infection of P. buxi deters grazing by C. perspectalis in 31 wild box‐tree stands in forests in north‐western Switzerland and south‐western Germany. On average, 82.4% of the box trees were infested by P. buxi (among‐site variation 20%–100%), or 18.5% of their leaves. Linear mixed‐effect (LME) models revealed that the percentage of rust‐infected leaves was influenced by the local box‐tree density and the slope aspect of the forest site. 9.5% of the leaves showed grazing damage by C. perspectalis. LME showed that the percentage of leaves with grazing damage was affected by the elevation of the site and the percentage of rust‐infected leaves. The effect of box rust infection on food choice and larval performance (survival and growth rate) was examined in two experiments. In food‐choice tests, larvae of C. perspectalis did not show any preference for uninfected or rust‐infected box‐tree leaves and consumed similar amounts of leaf tissue of either type. However, when larvae were reared on leaves with three different degrees of P. buxi‐infection (and uninfected leaves as control), both survival and growth rate decreased with increasing degree of rust infection. This indicates that larval performance of C. perspectalis is reduced by the box rust. In the case of an outbreak of C. perspectalis, however, larvae may indiscriminately defoliate wild box trees.     

Does sea-grass biomass control the density of peracarids (Crustacea: Peracarida) in tropical lagoons?

We analyzed the time-space variation of the peracarid crustaceans that inhabit seagrasses of the Alvarado Lagoon System, Veracruz, Gulf of Mexico. The organisms were collected from 108 samples in six sites with Ruppia maritima beds (December 1992 to November 1994). The assemblage was composed of 11 species. Eight species of Amphipoda (Hourstonius laguna, Cerapus benthophilus, Apocorophium louisianum, Grandidierella bonnieroides, Leptocheirus rhizophorae, Gammarus mucronatus, Melita longisetosa and Haustorius sp.), one of Isopoda (Cassidinidea ovalis) and two of Tanaidacea (Discapseudes holthuisi and Leptochelia savignyi) were identified. Taxocoenosis, density and biomass of peracarids showed seasonal pulses related to R. maritima biomass, salinity variation, epicontinental affluent and inlets. The species C. ovalis, G. mucronatus, A. louisianum and D. holthuisi were dominant.     

Does climate control the northern range limit of eastern white cedar (Thuja occidentalis L.)?

Our aim was to test whether or not climate influence the northern distributional limit of eastern white cedar (Thuja occidentalis L.) by affecting its radial growth and regeneration. Twenty-two sites were selected along the bioclimatic gradient in Northwestern Québec, Canada. The gradient was divided into three zones based on the abundance of white cedar stands: continuous, discontinuous, and marginal zones. Radial growth and regeneration (numbers of sedds, saplings) were determined for each zone. Results showed that basal area increment (b.a.i.) was the same along the gradient. Seed production and germination did not differ between zones. One-year-old seedlings and small individuals (<30 cm) were less abundant in the northern sites than in stands from the continuous and discontinuous zones. More saplings were found in the northernmost sites (389/400 m² in the marginal and 354.7/400 m² in the discontinuous zones) than in the south (130.6/400 m²). Layering seemed to compensate for the low recruitment observed in the marginal sites. Recruitment of seedlings originating from sexual reproduction in the discontinuous zone was not different from the stands in the southern areas. Thus, a climatically driven decrease in recruitment cannot explain the observed decrease in white cedar abundance, which occurred at this latitude. Although seedlings were less abundant in isolated northern marginal stands, sexually based regeneration was still possible. Therefore, the direct effect of climate seems to only have a minor influence on white cedar northern distributional limit and other factors, such as natural disturbances, might better explain its actual boundary.     

Does plant diversity increase top–down control of herbivorous insects in tropical forest?

Higher trophic level interactions are key mediators of ecosystem functioning in tropical forests. A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underlying such effects. The 'enemies hypothesis’ states that predators exert more effective top–down control of herbivorous insects with increasing plant diversity. Support for this hypothesis has been found in temperate forests and agroecosystems, but remains understudied in tropical forests. We compared incidence of attacks of different natural enemies using artificial caterpillars in a tropical forest landscape and investigated the role of plant community structure (i.e. species richness, composition and density), and the role of forest fragmentation (i.e. patch size, edge distance and canopy openness) on predation intensity. Plant community effects were tested with respect to three vegetation strata: trees, saplings and herbs. Observed predation was substantially due to ants. Predation rates increased with plant species richness for trees and herbs. Density of saplings, herb cover and herb species composition were important factors for predation. No significant patterns were found for fragmentation parameters, suggesting that forest fragmentation has not altered predation intensity. We conclude that in tropical forests, top–down control of herbivorous insects in the understory vegetation is affected by a combination of plant diversity, plant species composition and structural features of the plant community.     

Does apple replant disease affect the soil patch selection behaviour and population growth of Collembolans?

Apple replant disease (ARD) is common to all major apple-growing regions in the world. It occurs when new apple trees are replanted on sites where previously the same or closely related crop species were grown. Biotic (fungi, bacteria and nematodes) and abiotic soil factors (poor soil structure, nutrition) contribute to the development and severity of ARD. However, the aetiology of ARD and effects on higher trophic levels are still unknown. In that sense, Collembola might play an important role, since they are one of the dominant mesofauna groups in many soils. They act as decomposer, fungivores and predators, representing different trophic levels in soil food webs. Therefore, any effect of ARD on the occurrence of Collembola could have ecological impacts on the soil quality and health. Here, we examined the colonization behaviour of two Collembolan species, Folsomia candida and Sinella curviseta, in choice tests and population growth tests using Apple Replant Diseased soil (ARD) and non-ARD soil samples from different field sites and standardized laboratory bioassays. Additionally, Collembola behaviour was quantified by continuous video observations to investigate short-term behavioural changes. Results showed that both Collembolan species significantly preferred colonization of the non-ARD soils compared with ARD soils, independent of the origin of the soil samples or specific disinfection treatments. Moreover, the detailed video analysis of the foraging behaviour indicates rapid colonization of soil samples and low dispersal rates. Most likely, volatile compounds and to a lesser extent feeding stimulants play a vital role for the colonization process for both Collembolan species. Finally, results showed negative effects of ARD on population growth of both Collembolan species already after an 8-week period, implying strong nutritional deficiencies in ARD affected soils. The hypothesis that ARD causing microorganisms directly affected orientation, colonization and population development of Collembola is discussed.     

Boom to bust? Implications for the continued rapid growth of the eastern Australian humpback whale population despite recovery

Despite heavy overexploitation and near extirpation, some populations of large whales are recovering. Monitoring their recovery has important implications for conservation, management and our understanding of population dynamics and recovery in large mammals. The eastern Australian population of humpback whales was hunted to near-extirpation by the early 1960s. Despite this, the population started to recover, and structured surveys were initiated in the 1980s. These surveys comprise one of the longest and most consistent series of surveys of a population of whales in the world. Collectively, they have demonstrated a rapid recovery of the population with a long-term average rate of increase of 10.9% per annum. Here, we present the results of the last three surveys, conducted in 2007, 2010 and 2015. The 2015 survey shows that the population is essentially recovered, with abundance estimated at 24,545 whales (95% confidence interval 21,631–27,851), and yet continues to grow at a rapid rate. Modeling the rate of growth and abundance suggests that either the whales are heading for a higher than expected abundance of at least 40,000 whales or that an irruption may occur with models suggesting a peak in whale abundance in 2021–2026. Understanding the possible future scenarios of this population is critical to its management. This situation also presents a rare opportunity to study in detail the growth of a well-defined population of large mammal as it recovers from severe depletion.     

Does Rubisco control the rate of photosynthesis and plant growth? An exercise in molecular ecophysiology

Experiments are described in which tobacco (Nicotiana tabacum L.) transformed with antisense rbcS to decrease expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) was used to evaluate the contribution of Rubisco to the control of photosynthetic rate, and the impact of a changed rate of photosynthesis on whole plant composition, allocation and growth. (1) The concept of flux control coefficients is introduced. It is discussed how, with adequate precautions, a set of wild-type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficient. (2) The flux control coefficient of Rubisco for photosynthesis depends on the short-term conditions. It increases in high light, or low CO₂. (3) When plants are grown under constant irradiance, the flux control coefficient in the growth conditions is low (<0.2) at irradiances of up to 1000μmol quanta m⁻² s⁻¹. In a natural irradiance regime exceeding 1500μmol quanta m⁻² s⁻² over several hours the flux coefficient rose to 0.8–0.9. It is concluded that plants are able to adjust the balance between Rubisco and the remainder of the photosynthetic machinery, and thereby avoid a one-sided limitation of photosynthesis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisco had a higher flux control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewed as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A change in the rate of photosynthesis did not automatically translate into a change in growth rate. Several factors are identified which contribute to this buffering of growth against a changed photosynthetic rate. (6) There is an alteration in whole plant allocation, resulting in an increase in the leaf area ratio. The increase is mainly due to a higher leaf water content, and not to changes in shoot/root allocation. This increased investment in whole plant leaf area partly counteracts the decreased efficiency of photosynthesis at the biochemical level. (7) Plants with decreased Rubisco have a lower intrinsic water use efficiency and contain high levels of inorganic cations and anions. It is proposed that these are a consequence of the increased rate of transpiration, and that the resulting osmotic potential might be a contributory factor to the increased water content and expansion of the leaves. (8) Starch accumulation in source leaves is decreased when unit leaf photosynthesis is reduced, allowing a more efficient use of the fixed carbon. (9) Decreased availability of carbohydrates leads to a down-regulation of nitrate assimilation, acting via a decrease in nitrate reductase activity.     

Skeletons, noise and population growth: the end of an old debate?

Population dynamics models remain largely deterministic, although the presence of random fluctuations in nature is well recognized. This deterministic approach is based on the implicit assumption that systems can be separated into a deterministic part that captures the essential features of the system and a random part that can be neglected. But is it possible, in general, to understand population dynamics without the explicit consideration of random fluctuations? Here, we suggest perhaps not, and argue that the dynamics of many systems are a result of interactions between the deterministic nonlinear skeleton and noise.     

Modeling control of Common Carp (Cyprinus carpio) in a shallow lake–wetland system

Wetlands Ecology and Management - The introduction of Common Carp (Cyprinus carpio) into North American waterways has led to widespread alteration of aquatic ecosystems. Control of this invader has...     

Does population structure and growth of an intertidal pulmonate snail reflect environmental conditions within a small estuary?

The aim of this research was to determine if population attributes and growth of an estuarine pulmonate could be used as bioindicators of environmental conditions in New Zealand estuaries. We quantified the size distribution of the deposit feeding Amphibola crenata in areas which contrasted in contaminant inputs. Within a small estuary, location and tidal level significantly affected A. crenata density. The highest mean abundances were found at the most contaminated site, close to the waste treatment discharge point, compared with lower densities at two river sites and close to the estuary mouth. The population structure was site-specific with juveniles present in historically contaminated areas. Growth of A. crenata held in experimental cages for 6 weeks was highest for medium length individuals from close to the waste discharge point and least for individuals from the estuary mouth. Sediment surface microalgal biomass (chlorophyll a), representing the potential food supply for A. crenata was greatest at the least contaminated site, and lowest at the most polluted site. The abundance, population structure and growth rate of A. crenata are attributes that may be used as ecological bioindicators, reflecting the complex environmental conditions within New Zealand estuaries.     

The invasive gall wasp Quadrastichus erythrinae (Hymenoptera: Eulophidae) in South America: is classical biological control needed?

The invasive gall wasp Quadrastichus erythrinae, a pest of Erythrina plants, is recorded for the first time in South America in Espírito Santo, Brazil. Research on Q. erythrinae and its potential natural enemies in this region is required to determine the need for classical biological control of this invasive pest.     

Filling of a wetland (Seine estuary,France): natural eutrophication or anthropogenic process? A sedimentological and geochemical study of wetland organic sediments

Hydrobiologia - For over a century the Seine estuary has been highly affected by human activities, resulting in a reduction of the surface of wetland habitat. Several ponds of the Vernier Marsh,...     

Cellular senescence: A reflection of normal growth control,differentiation, or aging?

Normal cells, with few exceptions, cannot proliferate indefinitely. Cell populations--in vivo and in culture--generally undergo only a limited number of doublings before proliferation invariably and irreversibly ceases. This process has been termed the finite lifespan phenotype or cellular senescence. There is long-standing, albeit indirect, evidence that cellular senescence plays an important role in complex biological processes as diverse as normal growth control, differentiation, development, aging, and tumorigenesis. In recent years, it has been possible to develop a molecular framework for understanding some of the fundamental features of cellular senescence. This framework derives primarily from the physiology, genetics, and molecular biology of cells undergoing senescence in culture. Our understanding of senescence, and the mechanisms that control it, is still in its infancy. Nonetheless, recent data raise some intriguing possibilities regarding potential molecular bases for the links between senescence in culture and normal and abnormal growth control, differentiation, and aging.