Low temperature during infection limits Ustilago bullata (Ustilaginaceae, Ustilaginales) disease incidence on Bromus tectorum (Poaceae, Cyperales)

Ustilago bullata is frequently encountered on the exotic winter annual grass Bromus tectorum in western North America. To evaluate the biocontrol potential of this seedling-infecting pathogen, we examined the effect of temperature on the infection process. Teliospore germination rate increased linearly with temperature from 2.5 to 25°C, with significant among-population differences. It generally matched or exceeded host seed germination rate over the range 10-25°C, but lagged behind at lower temperatures. Inoculation trials demonstrated that the pathogen can achieve high disease incidence when temperatures during infection range 20-30°C. Disease incidence was drastically reduced at 2.5°C. Pathogen populations differed in their ability to infect at different temperatures, but none could infect in the cold. This may limit the use of this organism for biocontrol of B. tectorum to habitats with reliable autumn seedling emergence, because cold temperatures are likely to limit infection of later-emerging seedling cohorts.     

A snail prefers native over exotic freshwater plants: implications for the enemy release hypotheses

1. One of the most popular single‐factor hypotheses that have been proposed to explain the naturalization and spread of introduced species is the enemy release hypothesis (ERH). Most studies have considered how specialist consumers might affect the success of exotics, but the importance of generalist herbivores has received little attention 2. In this study, we compared the palatability of native and exotic freshwater plants in both phylogenetic pairings and in region‐wide assays, using a generalist consumer (the pond snail Radix swinhoei). 3. Our study indicated that, when plants were paired by taxonomic relatedness, the snail preferred native over exotic freshwater plants by a 2 : 1 ratio. The snail also preferred native over exotic plants when tested across 20 native and seven exotic species found growing in Liangzi Lake. 4. Snails consumed more biomass of native than exotic plants, providing support for the ERH and suggesting that exotic species have a potential advantage in most lakes of the middle and lower reaches of the Yangtze River in China, thus helping their invasion.     

Variation in Herbivore Damage to Invasive and Native Woody Plant Species in Open Forest Vegetation on Mahé, Seychelles

Enemy release of introduced plants and variation in herbivore pressure in relation to community diversity are presently discussed as factors that affect plant species invasiveness or habitat invasibility. So far few data are available on this topic and the results are inconclusive. We compared leaf herbivory between native and invasive woody plants on Mahé, the main island of the tropical Seychelles. We further investigated variation in leaf herbivory on three abundant invasive species along an altitudinal gradient (50–550 m a.s.l.). The median percentage of leaves affected by herbivores was significantly higher in native species (50%) than in invasive species (27%). In addition, the species suffering from the highest leaf area loss were native to the Seychelles. These results are consistent with the enemy release hypothesis (ERH). While the invasive species showed significant and mostly consistent variation in the amount of leaf damage between sites, this variation was not related to general altitudinal trends in diversity but rather to local variation in habitat structure and diversity. Our results indicate that in the Seychelles invasive woody plants profit from herbivore release relative to the native species and that the amount of herbivory, and therefore its effect on species invasiveness or habitat invasibility, may be dependent on local community structure and composition.     

Enemy release at range edges: do invasive species escape their herbivores as they expand into new areas?

Aims We test the hypothesis that invasive plant species at their range edges experience lower herbivory and allocate less to defense at the edge of an expanding range edge than from more central parts of their distribution, during secondary invasion in a new range. Invasive plants are often able to spread rapidly through new areas. The success of invasive species in new ranges is frequently attributed to enemy release in these new areas and associated evolutionary changes minimizing allocation to defense in favor of growth and reproduction. Enemy release could also explain rapid advances of invasive species upon arriving in new habitats. If invasive species accumulate enemies over time in a new location, then these species may experience a release from their enemies at expanding range fronts. Enemy release at these range fronts may accelerate range expansion.Methods We used populations of four woody invasive species within the invaded range, and four native control species. We quantified leaf herbivory and leaf physical defense traits at both range central and range edge locations, over two 1-month sampling periods, sampled 7 months apart.Important findings Herbivory at the range edge did not differ to the range center but patterns were not consistent across species. There was a trend for lower herbivory at the range edge for Lantana camara, which was reflected in lower leaf toughness. Overall, leaf toughness was greater at the range edge location across invasive and control species. Physical defenses were different among range locations in a few species, though most species show the same trend, suggesting higher herbivory pressures at the range edge location or differences may be due to climatic factors. Leaves of L. camara were significantly less tough at range edges, suggesting that some species can potentially escape their enemies at range edges. However, our results overall do not support the hypothesis that plants at the edge of their ranges experience reduced impact from their enemies.     

Invasive grass reduces aboveground carbon stocks in shrublands of the Western US

Understanding the terrestrial carbon budget, in particular the strength of the terrestrial carbon sink, is important in the context of global climate change. Considerable attention has been given to woody encroachment in the western US and the role it might play as a carbon sink; however, in many parts of the western US the reverse process is also occurring. The conversion of woody shrublands to annual grasslands involves the invasion of non-native cheatgrass ( Bromus tectorum ) which in turn leads to increased frequency and extent of fires. We compared carbon storage in adjacent plots of invasive grassland and native shrubland. We scaled-up the impact of this ecosystem shift using regional maps of the current invasion and of the risk of future invasion. The expansion of cheatgrass within the Great Basin has released an estimated 8±3 Tg C to the atmosphere, and will likely release another 50±20 Tg C in the coming decades. This ecosystem conversion has changed portions of the western US from a carbon sink to a source, making previous estimates of a western carbon sink almost certainly spurious. The growing importance of invasive species in driving land cover changes may substantially change future estimates of US terrestrial carbon storage.     

Are there general patterns in plant defence against megaherbivores?

Field surveys were conducted to test whether plants deploy structural defences in ways that match the distribution of megaherbivores. In Western Australian scrublands, where adult plants are within the reach of megaherbivores, structural defences increased vertically and were deployed preferentially by adult plants. Conversely, in woodlands of Eastern Australia and California, where adult plants grow above the reach of megaherbivores, structural defences decreased vertically. Populations of closely‐related taxa on offshore islands exhibited significant reductions in defence in the absence of megaherbivores. The results also demonstrate that island plant taxa can evolve vertical changes in defence after colonizing continents, where they are exposed to megaherbivores. Overall, the results of the present study illustrate a complex array of spatial patterns in plant defence that match the distribution and foraging behaviour of large mammals. When interpreted alongside previous work demonstrating similar spatial patterns in other types of plant defence, the results may help to unify our understanding of how megaherbivores have shaped the evolution of plant form and function. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 38–48.     

EVOLUTION OF HOST-PARASITE DIVERSITY

Hosts and parasites often have extensive genetic diversity for resistance and virulence (host range). Qualitative diversity occurs when the success of attack is an all-or-nothing response that varies according to the genotypes of the host and parasite. Quantitative diversity occurs when the success of attack is a graded response that depends on additive genetic variation in the host and parasite. Community diversity occurs when parasites vary in the success with which they can attack different host species, leading to a mixture of specialists and generalists. I developed a series of models that classify components of host-parasite interactions according to whether they cause stabilizing or disruptive selection for resistance and virulence. Stabilizing selection reduces diversity by favoring a single optimal phenotype. Disruptive selection creates diversity by favoring a mixture of widely separated phenotypes. The evolution of maximal resistance and virulence are opposed by one of three forces: metabolic costs, frequency dependence, or negative genetic correlations among beneficial traits. The models predict that qualitatively inherited resistance and virulence traits typically cause greater diversity than quantitatively inherited traits. However, each natural system is composed of many stabilizing factors that reduce diversity and disruptive factors that promote diversity. I advocate a style of modeling in which families of related assumptions are compared by their equilibrium properties, and general conclusions from equilibrium properties are tested by complete dynamical analysis. The comparison among models highlights the need for empirical studies that compare levels of diversity among related host-parasite systems.     

Ecological genetics of vernalization response in Bromus tectorum L. (Poaceae)

BACKGROUND AND AIMS: Bromus tectorum (cheatgrass or downy brome) is an exotic annual grass that is dominant over large areas of former shrubland in western North America. To flower in time for seed production in early summer, B. tectorum plants generally require vernalization at winter temperatures, either as imbibed seeds or as established seedlings. METHODS: Variation in response to increasing periods of vernalization as seeds or seedlings for progeny of ten full-sib families from each of four B. tectorum populations from contrasting habitats was studied. KEY RESULTS: As vernalization was increased from 0 to 10 weeks, the proportion of plants flowering within 20 weeks increased, weeks to initiation of flowering decreased, and seed yield per plant increased, regardless of whether plants were vernalized as seeds or seedlings. Most of the variation was accounted for by differences among populations. Plants of the warm desert population flowered promptly even without vernalization, while those of the cold desert, foothill and montane populations showed incremental changes in response variables as a function of vernalization period. Populations differed in among-family variance, with the warm desert population generally showing the least variance and the cold desert population the most. Variation among populations and among families within populations decreased as vernalization period increased, whereas the non-genetic component of variance showed no such pattern. CONCLUSIONS: Variation in vernalization response was found to be adaptively significant and apparently represents the result of contrasting selection regimes on a range of founder genotypes.     

Geographic distribution patterns of small mammals in Swaziland in relation to abiotic factors and human land-use activity

Swaziland is a small, topographically diverse southern Africa country whose mammalian fauna has been poorly studied. The distributions of small mammals in Swaziland were surveyed at 39 localities. A total of 15 species of rodents and ten species of insectivores were captured during the survey. The number of species of insectivore captured at a locality was positively correlated with mean annual rainfall, while the number of rodent species captured was negatively correlated with altitude. The number of rodent species captured was lower on Swazi Nation Land compared with privately owned land or protected land (reserves). This was probably due to the radical habitat alteration that had taken place on Swazi Nation Land, inter alia overgrazing by livestock, cultivation of maize and deforestation. The effect of this habitat alteration on the number of rodent species was more pronounced in high-lying areas of Swaziland. This may have been due to the fact that a large number of the rodent species inhabiting high-lying areas require thick, tall grassland habitats, whereas many of the low-lying species prefer more open habitats with less grass cover. Since grazing acts to reduce grass cover, it is suggested that the species inhabiting high-lying areas would be more affected by overgrazing, than low-lying species.     

Surveillance for microbes and range expansion in house sparrows

Interactions between hosts and parasites influence the success of host introductions and range expansions post-introduction. However, the physiological mechanisms mediating these outcomes are little known. In some vertebrates, variation in the regulation of inflammation has been implicated, perhaps because inflammation imparts excessive costs, including high resource demands and collateral damage upon encounter with novel parasites. Here, we tested the hypothesis that variation in the regulation of inflammation contributed to the spread of house sparrows (Passer domesticus) across Kenya, one of the world''s most recent invasions of this species. Specifically, we asked whether inflammatory gene expression declines with population age (i.e. distance from Mombasa (dfM), the site of introduction around 1950). We compared expression of two microbe surveillance molecules (Toll-like receptors, TLRs-2 and 4) and a proinflammatory cytokine (interleukin-6, IL-6) before and after an injection of an immunogenic component of Gram-negative bacteria (lipopolysaccharide, LPS) among six sparrow populations. We then used a best-subset model selection approach to determine whether population age (dfM) or other factors (e.g. malaria or coccidian infection, sparrow density or genetic group membership) best-explained gene expression. For baseline expression of TLR-2 and TLR-4, population age tended to be the best predictor with expression decreasing with population age, although other factors were also important. Induced expression of TLRs was affected by LPS treatment alone. For induced IL-6, only LPS treatment reliably predicted expression; baseline expression was not explained by any factor. These data suggest that changes in microbe surveillance, more so than downstream control of inflammation via cytokines, might have been important to the house sparrow invasion of Kenya.