Does invasion by an alien plant species affect the soil seed bank?

Questions: How does invasion affect old‐field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old‐fields? Location: Menorca, Balearic Islands, Spain, western Mediterranean basin. Methods: We monitored seed germination in soils from old‐fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes‐caprae. We also added O. pes‐caprae bulbs to uninvaded soils to test O. pes‐caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation. Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes‐caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%). Conclusions: Despite invasion by O. pes‐caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.     

Does reduced heterozygosity influence dispersal? A test using spatially structured populations in an alpine ungulate

Despite having a profound effect on population dynamics, the reasons that animals disperse are poorly understood. Evolutionary explanations have focused on inbreeding and competition, where the potential cost of philopatry is negated through dispersal. Such scenarios lead to the prediction that less successful individuals preferentially disperse, termed 'fitness-associated dispersal'. Since heterozygosity is associated with fitness, we assessed whether dispersed animals had less observed heterozygosity (H(O)) than residents. We tested this prediction using both genetic and population-monitoring data of mountain goats (Oreamnos americanus). Individuals classified as dispersers through cross-assignment had the lowest mean H(O), followed by residents, and then admixed individuals. Dispersed individuals had 6.3 per cent less H(O) than their subpopulation of origin. In the long-term study of the mountain goat herd at Caw Ridge, Alberta, immigrants had the lowest H(O); however, the opposite pattern was seen in emigrants, which may be related to density dependence. This study is the first to provide empirical evidence that heterozygosity is associated with dispersal.     

Does seed dispersal limit initiation of primary succession in desert playas?

To investigate the initiation of primary succession in a cold-desert playa-dune complex, we studied the large-scale (2000 m) seed (diaspore) dispersal patterns at Mono Lake, California. Seeds of seven of the ten species reaching the barren playa had wind-dispersal adaptations. Rates of dispersal (numbers of seeds per square metre per day) were as much as three orders of magnitude lower on the playa than in the diverse dune vegetation. However, seed input appeared sufficient to reach potential safe sites on the playa, with a peak input of 66 ± 8 total seeds·m·d. The smooth playa surface, the virtual absence of aboveground barriers, and the high windspeed environment promote the long-distance dispersal of seeds (at least 1300 m for Chrysothamnus spp. and at least 700 m for Sarcobatus vermiculatus). The large spatial scale of sampling revealed a relatively high seed input onto the playa by the dominant pioneer species S. vermiculatus, despite the low abundance of parent vegetation in this region. All of these results implicate low rates of seed entrapment as an obstacle to establishment on this desert playa, rather than a lack of seed input.     

Endozoochorous dispersal of aquatic plants: does seed gut passage affect plant performance?

The ingestion of seeds by vertebrates can affect the germinability and/or germination rate of seeds. It is, however, unclear if an earlier germination as a result of ingestion affects later plant performance. For sago pondweed, Potamogeton pectinatus, the effects of seed ingestion by ducks on both germinability and germination rate have been previously reported from laboratory experiments. We performed an experiment to determine the effects of seed ingestion by ducks on germination, seedling survival, plant growth and asexual multiplication. Both at the start and end of the winter, seeds were fed to three captive shovelers (Anas clypeata) and planted outdoors in water-filled containers. Plant biomass and its allocation to vegetative parts (shoot and roots), tubers, and seeds were determined in autumn. More duck-ingested seeds than control (uningested) seeds germinated in early winter, but this difference disappeared for seeds planted in late winter, when the treatments were first stratified for 3 mo. None of the variables for measuring seedling survival and plant performance varied between treatments. Under our experimental conditions (no herbivory or competition), ingestion by ducks in early winter resulted in increased performance for seeds surviving gut passage due to enhanced seed germinability, without other costs or benefits for the seedlings.     

Does dispersal capacity matter for freshwater biodiversity under climate change?

1. Freshwater ecosystems appear to be sensitive to even minor climatic shifts, and the dendritic nature of rivers as well as patchy distribution of habitats within the terrestrial landscape could limit the ability of species to track suitable climate conditions. Although the importance of dispersal is recognised in theory, there is great uncertainty when quantifying the capacity of species to shift their distributions in response to climate change.
2. The influence of dispersal capacity on species’ vulnerability to climate change was assessed, using the modelled projections of 527 freshwater species in New South Wales (NSW), Australia. Species’ future ranges were calculated by iteratively identifying colonisation of accessible habitats and loss of suitable habitats within network models. The accessibility of new habitats was based on a given dispersal mode (aquatic, semi‐terrestrial and aerial). The relative impact of dispersal parameters on projected range were evaluated alongside other known sources of uncertainty (climate and emissions scenarios, modelling algorithm and biological group), analysed collectively in a generalised additive mixed‐model, and spatially to locate regions of NSW where projections are associated with the most uncertainty.
3. Our simulations (1.4 million scenario combinations) suggest at least a third of species will lose more than half their range under climate change. Nevertheless, we emphasise the broad uncertainty that any average encapsulates. Dispersal capacity only had a minor impact on projected range shifts relative to other modelling assumptions but the network‐pathways and maps of uncertainty have value for conservation planning at large scales. Projected range losses initially decreased rapidly as dispersal rates increased but the benefits are reduced above 2–3 km year^?1. Taxa restricted to dispersal within the stream network (aquatic) were more vulnerable to climate change than taxa with semi‐terrestrial or aerial dispersal and maps of variation due to dispersal mode and rate indicate where habitat connectivity would be most beneficial.
4. This study demonstrates the breadth of uncertainties that challenge plans for improving ecosystem adaptation under climate change and highlights where in the landscape those differences were consistent. We emphasise the need for freshwater conservation studies to be ecologically representative, to focus on broad‐scale connectivity for taxa that can move between catchments, and an accessible network of refugia for taxa with more limited dispersal.

     

Does a foliar endophyte improve plant fitness under flooding?

Although endophytic fungi are ubiquitous in plants, their full range of ecological effects has yet to be characterized, particularly in non-agronomic systems. In this study, we compared the responses of two congeneric bluegrass species to flooding. Both plant species co-occur in subalpine zones of the Rocky Mountains. Marsh bluegrass (Poa leptocoma) commonly hosts a vertically transmitted fungal endophyte (Epichloë sp.) and naturally grows in wetter conditions than does nodding bluegrass (Poa reflexa), which lacks an epichloid endophyte. We investigated the novel hypothesis that endophyte symbiosis promotes host fitness under flooded conditions, contributing to niche differentiation between the two bluegrass species. We used a factorial greenhouse experiment to test whether endophyte presence improved survival, growth, or reproduction of P. leptocoma under flooded versus non-flooded edaphic conditions by experimentally removing the endophyte from half of the plants. We compared P. leptocoma responses to those of the endophyte-free congener. In contrast to expectations generated from the natural distributions of the two plant species, endophyte presence was more beneficial to P. leptocoma under ambient soil moisture than under flooding. Increased benefits of symbiosis in drier soils are consistent with studies of other grass endophytes. Flooded soils also unexpectedly improved the growth of P. reflexa more than that of the wet habitat specialist, P. leptocoma. While our results demonstrate an overall benefit of fungal symbiosis in this system, ecological factors other than flooding per se likely underlie the observed geographical distributions of these congeneric grasses in nature.     

Does the morphology of animal foraging pits influence secondary seed dispersal by ants?

Secondary seed dispersal by ants (myrmecochory) is an important process in semi‐arid environments where seeds are transported from the soil surface to an ant nest. Microsites from which ants often remove seeds are the small pits and depressions made by native and exotic animals that forage in the soil. Previous studies have demonstrated greater seed retention in the pits of native than exotic animals, but little is known about how biotic factors such as secondary seed dispersal by ants affect seed removal and therefore retention in these foraging pits. We used an experimental approach to examine how the morphology of burrowing bettong (Bettongia lesueur), greater bilby (Macrotis lagotis), short‐beaked echidna (Tachyglossus aculeatus) and European rabbit (Oryctolagus cuniculus) foraging pits and ant body size influenced ant locomotion and seed removal from pits along an aridity gradient. Ants took 3.7‐times longer to emerge from echidna pits (19.6 s) and six‐times longer to emerge from bettong pits (30.5 s) than from rabbit pits (5.2 s), resulting in lower seed removal from bettong pits than other pit types. Fewer seeds were removed from pits when cages were used to exclude large body‐sized (>2 mm) ants. Few seeds were removed from the pits or surface up to aridity values of 0.5 (humid and dry sub‐humid), but removal increased rapidly in semi‐arid and arid zones. Our study demonstrates that mammal foraging pit morphology significantly affects ant locomotion, the ability of ants to retrieve seeds, and therefore the likelihood that seeds will be retained within foraging pits.     

Biotic resistance to plant invasion in grassland: Does seed predation increase with resident plant diversity?

Seed predation impacts heavily on plant populations and community composition in grasslands. In particular, generalist seed predators may contribute to biotic resistance, i.e. the ability of resident species in a community to reduce the success of non-indigenous plant invaders. However, little is known of predators’ preferences for seeds of indigenous or non-indigenous plant species or how seed predation varies across communities. We hypothesize that seed predation does not differ between indigenous and non-indigenous plant species and that seed predation is positively related to plant species diversity in the resident community. The seed removal of 36 indigenous and non-indigenous grassland species in seven extensively or intensively managed hay meadows across Switzerland covering a species-richness gradient of 18–50 plant species per unit area (c. 2 m²) was studied. In mid-summer 2011, c. 24,000 seeds were exposed to predators in Petri dishes filled with sterilized soil, and the proportions of seeds removed were determined after three days’ exposure. These proportions varied among species (9.2–62.5%) and hay meadows (17.8–48.6%). Seed removal was not related to seed size. Moreover, it did not differ between indigenous and non-indigenous species, suggesting that mainly generalist seed predators were active. However, seed predation was positively related to plant species richness across a gradient in the range of 18–38 species per unit area, representing common hay meadows in Switzerland. Our results suggest that generalist post-dispersal seed predation contributes to biotic resistance and may act as a filter to plant invasion by reducing the propagule pressure of non-local plant species.     

How important is long‐distance seed dispersal for the regional survival of plant species?

Long‐distance seed dispersal is generally assumed to be important for the regional survival of plant species. In this study, we quantified the importance of long‐distance seed dispersal for regional survival of plant species using wind dispersal as an example. We did this using a new approach, by first relating plant species’ dispersal traits to seed dispersal kernels and then relating the kernels to regional survival of the species. We used a recently developed and tested mechanistic seed dispersal model to calculate dispersal kernels from dispersal traits. We used data on 190 plant species and calculated their regional survival in two ways, using species distribution data from 36,800 1 km²‐grid cells and 10,754 small plots covering the Netherlands during the largest part of the 20th century. We carried out correlation and stepwise multiple regression analyses to quantify the importance of long‐distance dispersal, expressed as the 99‐percentile dispersal distance of the dispersal kernels, relative to the importance of median‐distance dispersal and other plant traits that are likely to contribute to the explanation of regional survival: plant longevity (annual, biennial, perennial), seed longevity, and plant nutrient requirement. Results show that long‐distance dispersal plays a role in determining regional survival, and is more important than median‐distance dispersal and plant longevity. However, long‐distance dispersal by wind explains only 1–3% of the variation in regional survival between species and is equally important as seed longevity and much less important than nutrient requirement. In changing landscapes such as in the Netherlands, where large‐scale eutrophication and habitat destruction took place in the 20th century, plant traits indicating ability to grow under the changed, increasingly nutrient‐rich conditions turn out to be much more important for regional survival than seed dispersal.     

Does host plant quality affect the oviposition decisions of an omnivore?

Optimal oviposition theory predicts a positive relationship between female preference for oviposition hosts and offspring performance. Interspecies effects on oviposition preference have been widely investigated, especially for herbivores. However, intraspecies variation, such as nitrogen content, might also influence female preference for oviposition hosts and subsequent offspring performance. To evaluate this possibility, we investigated the oviposition preference of a zoophytophagous omnivore and the development and survival of its nymphs on a single species of host plant that varied in nitrogen content. In choice and no‐choice experiments without prey, female omnivores were allowed to oviposit on plants that had been fertilized using 4 rates of nitrogen fertilizer (39, 78, 156, and 311 mg/L nitrogen) for 72 h. After 72 h, the most females were found on tomato plants receiving high concentrations of nitrogen fertilizer and more eggs were laid on those plants. First instar nymphs developed more rapidly on high‐nitrogen plants and third instar nymphs developed faster on low‐nitrogen plants. Plant nitrogen did not affect nymph survival to the adult stage, or the probability of survival over time. Although female omnivores did discriminate between potential oviposition hosts based on plant nitrogen, their choices did not significantly impact nymph development or survival. This is the first study to show that intraspecies variation in nitrogen content between plants affects the oviposition preference of female omnivores, but not offspring performance.     

Does seed retention potential affect the distribution of plant species in highly fragmented calcareous grasslands?

Long-distance dispersal is a crucial factor in the life-cycle of plants, especially in our modern, highly fragmented landscapes. Because natural herds of large animals have disappeared and grazing practices have been abandoned, important potential vectors for seed dispersal over large distances may have been lost. In the context of the re-establishment of grazing management for nature conservation purposes, it is therefore important to gain insight in the ability of grazing animals to act as seed dispersal vectors. Whereas local dispersal mainly occurs through standard vectors typically described based on morphological adaptations of the diaspore, large herbivores act as non-standard seed dispersers. Therefore, traditional dispersal classes are loosing scientific relevance and continuous predictors of dispersal potential have been proposed. Here, we explored whether dispersal related plant traits, including the "seed retention potential", could explain the distribution patterns of 180 plant species over 64 fragmented semi-natural calcareous grasslands in Belgium. The distribution of habitat specialist plant species was strongly determined by the degree of isolation of the grasslands. Interestingly, species distribution patterns were clearly linked with a species' potential to migrate through large grazers, as quantified by its retention potential: species producing seeds with high retention capacity were less affected by habitat isolation. Categorical dispersal classes based on seed morphology did not explain a species' response to fragment isolation. Although seed retention potential outperformed simple seed dimensional traits, plant height, which is an indicator of epizoochorous attachment potential, was even more important. Therefore we suggest further extension of the epizoochorous retention potential model by incorporating basic ecological mechanisms that effectively contribute to successful dispersal events.     

Can an Imidazole Be Formed from an Alanyl-Seryl-Glycine Tripeptide under Possible Prebiotic Conditions?

The five-membered heterocyclic imidazole group, which is an essential component of purines, histidine and many cofactors, has been abiotically synthesized in different model experiments that attempt to simulate the prebiotic environment. The evolutionary significance of imidazoles is highlighted not only by its presence in nucleic acid components and in histidine, but also by experimental reports of its ability to restore the catalytic activity of ribozymes. However, as of today there are no reports of histidine in carbonaceous chondrites, and although the abiotic synthesis of His reported by Shen et al. (1987, 1990a) proceeds via an Amadori rearrangement, like in the biosynthesis of histidine, neither the reactants nor the conditions are truly prebiotic. Based on the autocatalytic biosynthesis of 4-methylidene-imidazole-one (MIO), a cofactor of some members of the amino acid aromatic ammonia-lyases and aminomutases, which occur via the self-condensation of a simple Ala-Ser-Gly motif within the sequence of the enzymes, we propose a possible prebiotic synthesis of an imidazolide.     

Induced plant defenses breached? Phytochemical induction protects an herbivore from disease

Although wound-induced responses in plants are widespread, neither the ecological nor the evolutionary significance of phytochemical induction is clear. Several studies have shown, for example, that induced responses can act against both plant pathogens and herbivores simultaneously. We present the first evidence that phytochemical induction can inhibit a pathogen of the herbivore responsible for the defoliation. In 1990, we generated leaf damage by enclosing gypsy moth larvae on branches of red oak trees. We then inoculated a second cohort of larvae with a nuclear polyhedrosis virus (LdNPV) on foliage from the damaged branches. Larvae were less susceptible to virus consumed on foliage from branches with increasing levels of defoliation, and with higher concentrations of gallotannin. Defoliation itself was not related to any of our chemistry measures. Field sampling supported the results of our experiments: death from virus among feral larvae collected from unmanipulated trees was also negatively correlated with defoliation. In 1991, defoliation and gallotannin were again found to inhibit the virus. In addition, gallotannin concentrations were found to be positively correlated with defoliation the previous year. Compared with previous results that demonstrated a delecterious effect of induction on gypsy moth pupal weight and fecundity, the inhibition of the virus should confer an advantage to the gypsy moth. Since leaf damage levels increase as gypsy moth density increases, and since leaf damage inhibits the gypsy moth virus, there is the potential for positive feedback in the system. If phytochemical induction in red oak can inhibit an animal pathogen such as LdNPV, it suggests to us that induction in red oak is a generalized response to tissue damage rather than an adaptive defense against herbivores.     

Rodent‐mediated plant seed dispersal: What happens to the seeds after entering the gaps with different sizes?

In general, it is accepted that gap formation significantly affects the placement of scatter‐hoarded seeds by small rodents, but the effects of different forest gap sizes on the seed‐eating and scatter‐hoarding behaviors of small rodents remain unclear. Thus, we examined the effects of a closed‐canopy forest, forest edge, and gaps with different sizes on the spatial dispersal of Quercus variabilis acorns and cache placement by small rodents using coded plastic tags in the Taihang Mountains, China. The seeds were removed rapidly, and there were significant differences in the seed‐eating and caching strategies between the stand types. We found that Q. variabilis acorns were usually eaten after being removed from the closed‐canopy forest and forest edges. By contrast, the Q. variabilis acorns in the forest gap stands were more likely to be scatter‐hoarded. The dispersal distances of Q. variabilis acorns were significantly longer in the forest gap plots compared with the closed canopy and forest edge plots. However, the proportion of scatter‐hoarded seeds did not increase significantly as the gap size increased. In small‐scale oak reforestation projects or research, creating small gaps to promote rodent‐mediated seed dispersal may effectively accelerate forest recovery and successional processes.     

Does isoprene protect plant membranes from thermal shock? A molecular dynamics study

The question of why plants release isoprene when heat stressed has been continuously debated for more than half a century. In this work we use molecular dynamics simulation techniques to directly investigate the interaction between isoprene and a model phospholipid membrane in atomic detail. It is found that isoprene partitions preferentially in the center of the membrane and in a dose dependent manner enhances the order within the membrane without significantly changing the dynamical properties of the system. At a concentration of 20 mol% isoprene (16 isoprene molecules per 64 lipid molecules) the effect of the addition of isoprene on the membrane order is equivalent to a reduction in temperature of 10 K, rising to a reduction of 30 K at 43 mol% isoprene. The significance of the work is that it provides for the first time direct evidence that isoprene stabilizes lipid membranes and reduces the likelihood of a phospholipid membrane undergoing a heat induced phase transition. Furthermore it provides a clear mechanistic picture as to why plants specifically utilize isoprene for this purpose.     

Does pre-dispersal seed predation limit reproduction and population growth in the alpine clonal plant Geum reptans?

We studied the impact of the seed damaging gall midge larva Geomyia alpina on its perennial alpine host plant Geum reptans. We analysed the effect of seed predation on reproduction by seeds, i.e. seed number, seed mass, and seed viability and on growth and clonal propagation of non-protected plants in comparison to plants protected from predation by an insecticide. Additionally, we assessed the consequences of seed predation for population growth using matrix projection modelling. Seed predation resulted in a decrease in total seed mass per flower head by 23.8% in non-protected plants (P < 0.05). Individual seed mass decreased with increasing infestation intensity (P < 0.05). Seed number remained unaffected because the sucking feeding behaviour by gall midge larvae does not evoke seed abortion. Percent germination of seeds from non-protected plants was reduced by 97.9% compared to seeds from protected plants. According to reduced seed viability, modelling revealed a decrease in population growth rate from λ = 1.055 to λ = 1.041. Predation did neither influence total plant biomass nor biomass fractions. But stolon dry-weight of non-protected plants increased by 24.1% (P < 0.05), which may indicate a trade-off between sexual reproduction and clonal propagation. Our results demonstrate that despite substantial reduction of viable seeds, predation by gall midge larvae only slightly affected population growth of G. reptans suggesting that in this alpine species, persistence by longevity and clonal propagation can balance potential seed losses by predation, at least for local population growth.     

Does isoprene protect plant membranes from thermal shock? A molecular dynamics study

The question of why plants release isoprene when heat stressed has been continuously debated for more than half a century. In this work we use molecular dynamics simulation techniques to directly investigate the interaction between isoprene and a model phospholipid membrane in atomic detail. It is found that isoprene partitions preferentially in the center of the membrane and in a dose dependent manner enhances the order within the membrane without significantly changing the dynamical properties of the system. At a concentration of 20 mol% isoprene (16 isoprene molecules per 64 lipid molecules) the effect of the addition of isoprene on the membrane order is equivalent to a reduction in temperature of 10 K, rising to a reduction of 30 K at 43 mol% isoprene. The significance of the work is that it provides for the first time direct evidence that isoprene stabilizes lipid membranes and reduces the likelihood of a phospholipid membrane undergoing a heat induced phase transition. Furthermore it provides a clear mechanistic picture as to why plants specifically utilize isoprene for this purpose.     

Does alary dimorphism imply dispersal dimorphism in the waterstrider,Gerris remigis?

ABSTRACT.

• 1 The hypothesis that wing dimorphism reflects dimorphism for dispersal ability is tested in two populations of stream-dwelling waterstriders, Gerris remigis Say, in southern Quebec, Canada.
• 2 Movements were assessed directly by recaptures of marked adults, and indirectly by comparisons of residence times and patterns of disappearance of macropterous and apterous morphs. Sampling was done weekly, over a period of 26 months, and 4828 adult G.remigis were individually marked during this time.
• 3 Movements of >100 m were very rare for both morphs. The movement patterns of the two morphs were very similar, and differed only over the winter, when no macropters moved upstream. This latter observation suggests that macropters may be less successful than apterous individuals at moving overland, or rowing against a current at this time of year.
• 4 Residence times and patterns of disappearance of the two morphs were very similar during all seasons.
• 5 These results suggest that macropterous G.remigis do not, in general, disperse using any means unavailable to the apterous morph, and that macropters in this species should not be simply categorized as long-distance dispersers.