Paul M. Severns

The role of parasite release in invasion of the USA by European slugs

Several species of European slugs are invasive in the USA, threatening native species and damaging agricultural and horticultural crops. One possible explanation for the success of these invaders is parasite release. To test this hypothesis we collected European slugs in part of their native range (United Kingdom) and in the USA and compared prevalence, distribution and species richness of their nematode parasites. All slugs were dissected and examined for the presence of nematodes. In the UK, nematodes were present at 93% of study sites and 16.4% of all slugs examined were associated with nematodes whereas in the USA the respective figures were 34% of sites and 5.4% of slugs. Nematode species richness was greater in the UK with 12 species being found, seven of which were thought to be truly parasitic as opposed to being phoretic or necromenic. Nine species of nematode were found in the USA, four of which were truly parasitic. Four of the ten European slug species examined in the USA, were entirely free of truly parasitic nematodes whereas all were infected by nematodes in some sites in the UK. There was a significant difference in the prevalence of truly parasitic nematodes in five of these species when comparing their home versus invasive range. A significant difference in parasite prevalence was observed when comparing native and introduced slug species in the USA, however, this was not significant in the UK. Our data support a role for parasite release during the invasion of the USA by European slugs.

Exotic grass invasion impacts fitness of an endangered prairie butterfly, Icaricia icarioides fenderi

Fender’s blue butterfly is an endangered species restricted to fragmented, grassland remnants that are becoming increasingly dominated by tall, invasive grasses in western Oregon, USA. I performed a removal experiment to assess the impacts of structural degradation accompanying the invasion of Arrhenatherum elatius, tall oat grass, on butterfly fitness and fitness related behaviors. Clipping of A. elatius to native grass sward height resulted in 2.5–5 times as many eggs laid per leaf of host plant. Both male and female butterflies basked more frequently in areas removed of A. elatius inflorescences and upon encountering the treatment edge butterflies had a high rate of return into a large area removed of the grass inflorescences. Although butterfly behavior appeared to be affected by the change in sward height on the treatment edge, there was no evidence for the edge causing a disproportionate egg load. Invasion and dominance by A. elatius appeared to diminish host plant apparency which may result in overloading of eggs on conspicuous host plants, increased incidence of emigration, and a decrease in the likelihood of colonization because female butterflies appeared indifferent to larval resources beneath A. elatius inflorescences. Dominance of natural shortgrass prairies by tall stature grasses like A. elatius may be an insidious form of habitat degradation for grassland Lepidoptera worldwide, but it may go largely unnoticed because larval and adult resources can persist under the unnaturally tall grass canopy.

Habitat restoration facilitates an ecological trap for a locally rare, wetland-restricted butterfly

Abstract.  1. Ecological traps occur when organisms preferentially reproduce in low quality reproductive habitats because the co‐evolved cues associated with high quality habitat become re‐associated with low quality habitat due to anthropogenically mediated habitat changes. Although the proximate mechanisms underpinning nearly all empirical examples of ecological traps involve exotic species, agriculturalization, or artificial structures, habitat restoration may also facilitate ecological traps because it directly targets species assemblages with a long‐standing co‐evolutionary history.

Intraspecific chromosome number variation: a neglected threat to the conservation of rare plants.

The effectiveness of rare plant conservation will increase when life history, demographic, and genetic data are considered simultaneously. Inbreeding depression is a widely recognized genetic concern in rare plant conservation, and the mixing of genetically diverse populations in restoration efforts is a common remedy. Nevertheless, if populations with unrecognized intraspecific chromosome variation are crossed, progeny fitness losses will range from partial to complete sterility, and reintroductions and population augmentation of rare plants may fail. To assess the current state of cytological knowledge of threatened and endangered plants in the continental United States, we searched available resources for chromosome counts. We also reviewed recovery plans to discern whether recovery criteria potentially place listed species at risk by requiring reintroductions or population augmentation in the absence of cytological information. Over half the plants lacked a chromosome count, and when a taxon did have a count it generally originated from a sampling intensity too limited to detect intraspecific chromosome variation. Despite limited past cytological sampling, we found 11 plants with documented intraspecific cytological variation, while 8 others were ambiguous for intraspecific chromosome variation. Nevertheless, only one recovery plan addressed the chromosome differences. Inadequate within-species cytological characterization, incomplete sampling among listed taxa, and the prevalence of interspecific and intraspecific chromosome variation in listed genera, suggests that other rare plants are likely to have intraspecific chromosome variation. Nearly 90% of all recovery plans called for reintroductions or population augmentation as part of recovery criteria despite the dearth of cytological knowledge. We recommend screening rare plants for intraspecific chromosome variation before reintroductions or population augmentation projects are undertaken to safeguard against inadvertent mixtures of incompatible cytotypes.

Apparent power-law distributions in animal movements can arise from intraspecific interactions

Lévy flights have gained prominence for analysis of animal movement. In a Lévy flight, step-lengths are drawn from a heavy-tailed distribution such as a power law (PL), and a large number of empirical demonstrations have been published. Others, however, have suggested that animal movement is ill fit by PL distributions or contend a state-switching process better explains apparent Lévy flight movement patterns. We used a mix of direct behavioural observations and GPS tracking to understand step-length patterns in females of two related butterflies. We initially found movement in one species (Euphydryas editha taylori) was best fit by a bounded PL, evidence of a Lévy flight, while the other (Euphydryas phaeton) was best fit by an exponential distribution. Subsequent analyses introduced additional candidate models and used behavioural observations to sort steps based on intraspecific interactions (interactions were rare in E. phaeton but common in E. e. taylori). These analyses showed a mixed-exponential is favoured over the bounded PL for E. e. taylori and that when step-lengths were sorted into states based on the influence of harassing conspecific males, both states were best fit by simple exponential distributions. The direct behavioural observations allowed us to infer the underlying behavioural mechanism is a state-switching process driven by intraspecific interactions rather than a Lévy flight.

Persistence models for mark-recapture

The stable of models available for analyzing mark-recapture data (Otis et al. Wild Momogr 66:135, 1978) includes those having behavioral characteristics, time variation, heterogeneity, along with combinations of those characteristics. This paper proposes use of a series of models based on the persistence model of Ramsey and Usner (Biometrics 59:331–339, 2003). We show that persistence can be modeled in combination with behavior and with time variation. We apply the persistence model to situations in which capture occasions are not equally-spaced in time. Two case studies illustrate the use of these extended persistence models.

Losing a battle but winning the war: moving past preference–performance to understand native herbivore–novel host plant interactions

Introduced plants can positively affect population viability by augmenting the diet of native herbivores, but can negatively affect populations if they are subpar or toxic resources. In organisms with complex life histories, such as insects specializing on host plants, the impacts of a novel host may differ across life stages, with divergent effects on population persistence. Most research on effects of novel hosts has focused on adult oviposition preference and larval performance, but adult preference may not optimize offspring performance, nor be indicative of host quality from a demographic perspective. We compared population growth rates of the Baltimore checkerspot butterfly, Euphydryas phaeton, on an introduced host, Plantago lanceolata (English plantain), and the native host Chelone glabra (white turtlehead). Contrary to the previous findings suggesting that P. lanceolata could be a population sink, we found higher population growth rates (λ) on the introduced than the native host, even though some component parameters of λ were higher on the native host. Our findings illustrate the importance of moving beyond preference–performance studies to integrate vital rates across all life stages for evaluating herbivore–host plant relationships. Single measures of preference or performance are not sufficient proxies for overall host quality nor do they provide insights into longer term consequences of novel host plant use. In our system, in particular, P. lanceolata may buffer checkerspot populations when the native host is limiting, but high growth rates could lead to crashes over longer time scales.

Degree of host susceptibility in the initial disease outbreak influences subsequent epidemic spread

Disease epidemics typically begin as an outbreak of a relatively small, spatially explicit population of infected individuals (focus), in which disease prevalence increases and rapidly spreads into the uninfected, at-risk population. Studies of epidemic spread typically address factors influencing disease spread through the at-risk population, but the initial outbreak may strongly influence spread of the subsequent epidemic.We initiated wheat stripe rust Puccinia striiformis f. sp. tritici epidemics to assess the influence of the focus on final disease prevalence when the degree of disease susceptibility differed between the at-risk and focus populations.When the focus/at-risk plantings consisted of partially genetic resistant and susceptible cultivars, final disease prevalence was statistically indistinguishable from epidemics produced by the focus cultivar in monoculture. In these experimental epidemics, disease prevalence was not influenced by the transition into an at-risk population that differed in disease susceptibility. Instead, the focus appeared to exert a dominant influence on the subsequent epidemic.Final disease prevalence was not consistently attributable to either the focus or the at-risk population when focus/at-risk populations were planted in a factorial set-up with a mixture (~28% susceptible and 72% resistant) and susceptible individuals. In these experimental epidemics, spatial heterogeneity in disease susceptibility within the at-risk population appeared to counter the dominant influence of the focus.Cessation of spore production from the focus (through fungicide/glyphosate application) after 1.3 generations of stripe rust spread did not reduce final disease prevalence, indicating that the focus influence on disease spread is established early in the epidemic.Synthesis and applications. Our experiments indicated that outbreak conditions can be highly influential on epidemic spread, even when disease resistance in the at-risk population is greater than that of the focus. Disease control treatments administered shortly after the initial outbreak within the focus may either prevent an epidemic from occurring or reduce its severity.

Seedling population size and microhabitat association in Lupinus oreganos A. Heller var. kincaidii C.P. Sm. (Fabaceae) a threatened plant of western Oregon grasslands

Lupinus oreganus A. Heller var. kincaidii C.P. Sm. (Fabaceae) is a federally listed Threatened, endemic, perennial species of western Oregon grasslands and is the primary host plant for the Endangered Fender’s blue butterfly (Plebejus icarioides fenderi Macy [Lepidoptera: Lycaenidae]). For effective conservation and restoration, determining the habitat characteristics that are related to natural seed germination is necessary, yet unknown, for Kincaid’s lupine. In 6 populations of Kincaid’s lupine, generally < 1% of the estimated seed cohort became seedlings the following year, and pre-dispersal seed predation by weevil (Tychius lineellus Le Conte [Coleoptera: Curculionidae]) larvae can substantially reduce the estimated seedling population size. The likelihood of seedling presence increased with a decreasing amount of thatch but no relationship between lupine plant density and seedling likelihood was detected. Kincaid’s lupine seedlings were found in habitats with ample exposure to light, such as beds of moss and in areas of grass < 10 cm (4 in) tall. Land managers may encourage larger germinant populations of Kincaid’s lupine by decreasing the amount of thatch; controlling the abundance of exotic, rhizomatous grasses; and targeting habitats with short-stature bunch grasses for restoration (seed sowing).

DOES STANDING WATER AND PREDATOR PRESENCE STRUCTURE A WETLAND TERRESTRIAL MOLLUSC COMMUNITY

Impacts of co-occurring biotic and abiotic environmental variables on terrestrial mollusc communities have rarely been studied. In wetlands, terrestrial molluscs can drown or become stranded by rising water levels and they are often prey for predaceous beetles and small mammals. I used coverboard traps over a three year period to study the biotic and abiotic gradients that may structure a wetland prairie terrestrial mollusc community in western Oregon, USA. A non-metric multidimensional scaling (NMDS) ordination indicated that shrews (Sorex vagrans) and predaceous beetles were the primary gradients associated with the terrestrial mollusc community. Snails were associated with the presence of shrews, slugs were allied with predaceous beetles, and the vectors representing the shrew and predaceous beetle gradients were opposed, suggesting that beetles preyed upon snails while the shrews consumed both beetles and slugs. These hypothesized predator-prey relationships among molluscs, shrews, and beetles coincide with studies of Sorex gut contents and predaceous carabid beetle life history. The mollusc community was not associated with the water cover gradient in the NMDS ordination, but small mammal burrows and predaceous beetles were found in greater abundance in unflooded mound habitats. This suggests that standing water may temporally concentrate both predators and prey in a shared unflooded environment during the rainy season. The high metabolic demands of shrews and an apparent preference for both beetle and molluscan prey suggest that shrews may exert a strong top-down effect on invertebrate communities.