Contrasting plant adaptation strategies to latitude in the native and invasive range of Spartina alterniflora.

Biological invasions offer model systems of contemporary evolution. We examined trait differences and evolution across geographic clines among continents of the intertidal grass Spartina alterniflora within its invasive and native ranges. We sampled vegetative and reproductive traits in the field at 20 sites over 20° latitude in China (invasive range) and 28 sites over 17° in the US (native range). We grew both Chinese and US plants in a glasshouse common garden for 3 yr. Chinese plants were c. 15% taller, c. 10% denser, and set up to four times more seed than US plants in both the field and common garden. The common garden experiments showed a striking genetic cline of seven-fold greater seed set at higher latitudes in the introduced but not the native range. By contrast, there was a slight genetic cline in some vegetative traits in the native but not the introduced range. Our results are consistent with others showing that introduced plants can evolve rapidly in the new range. S. alterniflora has evolved different trait clines in the native and introduced ranges, showing the importance of phenotypic plasticity and genetic control of change during the invasion process.

Provenance-by-environment interaction of reproductive traits in the invasion of Spartina alterniflora in China.

Ecological invasions are facilitated by pre-adaptation and phenotypic plasticity, upon which evolution can act. The rapid invasion of the intertidal grass Spartina alterniflora in China during the last 36 yr is a test case for the roles of these mechanisms. A previous study of S. alterniflora in China found strong latitudinal clines in vegetative and sexual traits and concluded that most of this variation was due to phenotypic plasticity. Recent observations suggested provenance-by-environment interactions, and we employed common gardens at multiple latitudes as a test of this idea. Phenotypically, field plant height, which correlates strongly with biomass and other indices of vegetative performance in this species, showed a hump-shaped relationship across 10 sites, covering 19° of latitude; field seed set increased linearly with latitude. To assess the role of plasticity vs. genetic differentiation in these patterns, we grew plants from the ten field sites in three common gardens at low (20.9° N), mid (28.3° N), and high (38.0° N) latitudes to maturity, at 18 months. Plant height varied among common gardens, with the tallest plants at mid latitude, mirroring the field pattern, consistent with the previous study. Within the gardens, latitude of origin also affected plant height. Seed set varied among the gardens, with the greatest values at high latitudes, again mirroring the field pattern and indicating substantial plasticity. Evidence of evolution was found as increasing seed set with latitude among provenances within common gardens. However, the effect differed among common gardens, with the greatest slope in the high-latitude garden, lower slope in the mid-latitude garden, and no relationship in the low-latitude garden, indicating a provenance-by-environment interaction. The number of surviving plants also suggested a provenance-by-environment interaction; no relationship with latitude among provenances in the two southern gardens and increasing survival with latitude in the northern garden. Field seedling density was ~200-fold greater at high than at low latitude sites. The profuse seed germination and recruitment in the north could have created high selection intensity resulting in evolution of reproductive traits at high latitudes with the result that the mechanisms of the invasion differ with latitude.

Host selection by an insect herbivore with spatially variable density dependence.

Many species of phytophagous insects do not oviposit preferentially on plants that yield high offspring performance. One proposed explanation is that negatively density-dependent offspring performance would select for females that disperse eggs among plants to minimize competition. Recent work showing larval density dependence often varies substantially among plants suggests that ovipositing females should not only respond to the density of competitors but also to traits predictive of the strength of density dependence mediated by plants. In this study, we used field and greenhouse experiments to examine oviposition behavior in an insect herbivore that experiences density-dependent larval performance and variability in the strength of that density dependence among host-plant individuals. We found females moved readily among plants in the field and had strong preferences for plants that mediate weak offspring density dependence. Females, however, did not avoid plants with high densities of competitors, despite the fact that offspring performance declines steeply with density on most plants in natural populations. This means females minimize the effects of density dependence on their offspring by choosing plants that mediate only weak larval density dependence, not by choosing plants with low densities of competitors. Our results suggest that explaining the lack of positive preference-performance correlations in many systems may not be as simple as invoking density dependence. Resource selection behavior may depend not just on the presence or absence of density-dependent offspring performance but also on variation in the strength of offspring density dependence among sites within populations.

Host resistance reverses the outcome of competition between microparasites.

Predators and parasites can control the abundance or biomass of herbivores with indirect effects on producer communities and ecosystems, but the interplay of multiple natural enemies may yield unexpected dynamics. We experimentally examined interactions between two microparasites (entomopathogenic nematodes) isolated from sandy grassland soils of coastal California: Heterorhabditis marelatus (Heterorhabditidae) and Steinernema feltiae (Steinernematidae). Heterorhabditis marelatus drives trophic cascades by attacking root- and stem-boring ghost moth caterpillars (Hepialus californicus, Hepialidae), thereby indirectly protecting bush lupine shrubs (Lupinus arboreus, Fabaceae). Extensive field surveys demonstrated sympatric overlap in microhabitat use under lupine canopies and similar mean prevalence of the two nematode species. Using a response-surface design in the laboratory, we varied relative and absolute microparasite densities to test for competitive outcomes within an evolutionary naive host, larvae of the greater wax moth Galleria mellonella (Pyralidae), and within the native host Hepialus californicus. Independent of conspecific or interspecific density, S. feltiae dominated as expected over H. marelatus within the naive Galleria, but S. feltiae infected hosts at low frequency and showed lower reproductive fitness than H. marelatus within native Hepialus hosts. Contrary to studies that demonstrate the pairwise dominance of steinernematid over heterorhabditid species in laboratory hosts, host resistance to S. feltiae may provide a mechanism for coexistence of multiple microparasite species. We hypothesize that the ubiquitous field prevalence and rapid life history of S. feltiae imply its use of widespread, abundant but small-bodied hosts and indicate the lack of direct competition with H. marelatus in the Hepialus-Lupinus trophic cascade.

Metapopulation dynamics override local limits on long-term parasite persistence.

A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger-scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus, in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free-living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture. Our research uses long-term data on nematode incidence in combination with a landscape-level experiment to demonstrate the lack of a correlation between soil moisture and long-term persistence. A year-long experiment showed EPN mortality was weakly correlated with soil moisture among our study sites. Thirteen years of data, however, showed that colonization rates were highly correlated with long-term persistence. Sites with highest long-term persistence experienced the highest rates of rhizosphere colonization, extinction, and turnover. As a result, we concluded that metapopulation dynamics override limitations set by local and short-term abiotic conditions to determine long-term persistence in this parasite-driven trophic cascade.

Dynamics of a subterranean trophic cascade in space and time.

Trophic cascades, whereby predators indirectly benefit plant biomass by reducing herbivore pressure, form the mechanistic basis for classical biological control of pest insects. Entomopathogenic nematodes (EPN) are lethal to a variety of insect hosts with soil-dwelling stages, making them promising biocontrol agents. EPN biological control programs, however, typically fail because nematodes do not establish, persist and/or recycle over multiple host generations in the field. A variety of factors such as local abiotic conditions, host quantity and quality, and rates of movement affect the probability of persistence. Here, we review results from 13 years of study on the biology and ecology of an endemic population of Heterorhabditis marelatus (Rhabditida: Heterorhabditidae) in a California coastal prairie. In a highly seasonal abiotic environment with intrinsic variation in soils, vegetation structure, and host availability, natural populations of H. marelatus persisted at high incidence at some but not all sites within our study area. Through a set of field and lab experiments, we describe mechanisms and hypotheses to understand the persistence of H. marelatus. We suggest that further ecological study of naturally occurring EPN populations can yield significant insight to improve the practice and management of biological control of soil-dwelling insect pests.

Plant facilitation of a belowground predator.

Interest in facilitative predator plant interactions has focused upon above-ground systems. Underground physical conditions are distinctive, however, and we provide evidence that bush lupine, Lupinus arboreus, facilitates the survival of the predatory nematode Heterorhabditis marelatus. Because H. marelatus is prone to desiccation and lupines maintain a zone of moist soil around their taproots even during dry periods, we hypothesized that dry-season nematode survival under lupines might be higher than in the surrounding grasslands. We performed field surveys and measured nematode survival in lupine and grassland rhizospheres under wet- and dry-season conditions. Nematodes survived the crucial summer period better under lupines than in grasslands; however, this advantage disappeared in wet, winter soils. Modeling the probability of nematode population extinction showed that, while even large nematode cohorts were likely to go extinct in grasslands, even small cohorts in lupine rhizospheres were likely to survive until the arrival of the next prey generation. Because this nematode predator has a strong top-down effect on lupine survival via its effect on root-boring larvae of the ghost moth Hepialus californicus, this facilitative interaction may enable a belowground trophic cascade. Similar cases of predator facilitation in seasonally stressful environments are probably common in nature.

Climate affects predator control of an herbivore outbreak.

Herbivore outbreaks and the accompanying devastation of plant biomass can have enormous ecological effects. Climate directly affects such outbreaks through plant stress or alterations in herbivore life-history traits. Large-scale variation in climate can indirectly affect outbreaks through trophic interactions, but the magnitude of such effects is unknown. On the California coast, rainfall in years during and immediately previous to mass lupine mortality was two-thirds that of years without such mortality. However, neither mature lupines nor their root-feeding herbivores are directly affected by annual variation in rainfall. By increasing soil moisture to levels characteristic of summers following El Niño/Southern Oscillation (ENSO) events, we increased persistence of a predator (the entomopathogenic nematode Heterorhabditis marelatus). This led to suppression of an outbreak of the herbivorous moth Hepialus californicus, indirectly protecting bush lupine (Lupinus arboreus). Our results are consistent with the marine-oriented Menge-Sutherland hypothesis (Menge and Sutherland 1987) that abiotic stress has greater effects on higher than on lower trophic levels. The mechanisms producing these results differ from those proposed by Menge-Sutherland, however, highlighting differences between trophic processes in underground and terrestrial/marine food webs. Our evidence suggests that herbivore outbreaks and mass lupine mortality are indirectly affected by ENSO's facilitation of top-down control in this food web.

Dispersal-Dependent Oviposition and the Aggregation of Parasitism.

The prediction that parasitoid foraging effort should increase with distance traversed to reach or to locate hosts has had little experimental attention. Consistent with a number of models of foraging behavior, we found that the per capita number of ovipositions by the minute fairyfly-egg parasitoid Anagrus sophiae increased significantly with dispersal distance to planthopper hosts in the field in experimental patches of many host eggs. In large continuous stands of cordgrass host plants, after dispersal of decimeters or less, female wasps laid approximately 18% of their average of 18.6 eggs. After dispersal to plants isolated 10 m from other cordgrass, they laid approximately 84%, and they laid virtually all of their eggs after dispersal of 250 m to experimental floating islands of cordgrass. The increased oviposition following dispersal tripled the CV2 index of aggregation of parasitism to a level theoretically sufficient to promote locally stable parasitoid-host dynamics in isolated patches. At the same time, the change in wasp behavior did not affect the relationship between parasitism and host density, which was consistently density independent. Our results suggest that increased foraging effort with distance traversed can counter Allee effects in colonization and increase spatial spread of populations of natural enemies.

Reduced herbivore resistance in introduced smooth cordgrass (Spartina alterniflora) after a century of herbivore-free growth.

We compared resistance to insect herbivory in two introduced populations of smooth cordgrass (Spartina alterniflora) differing in their history of herbivory. One population in Willapa Bay, Washington, has spread in the absence of herbivory for more than a century, while another population in San Francisco, California, was introduced 20 years ago and is fed upon by the Spartina-specialist planthopper, Prokelisia marginata. The planthopper is a sap-feeder common on the Atlantic and Gulf coasts of North America, where smooth cordgrass is native. Smooth cordgrass plants from Willapa Bay (WB), San Francisco Bay (SFB), and Maryland (the source of the SFB introduction) were exposed to P. marginata herbivory over two consecutive summers in a common greenhouse environment, and their growth was compared with that of control plants that were grown herbivore-free. The planthoppers had relatively little effect on the growth of SFB plants, with plants exposed to herbivores averaging 77% and 83% of the aboveground biomass of herbivore-free controls after the first and second season of herbivory, respectively. The growth of plants from Maryland was similarly little-affected by the planthoppers, with the plants exposed to herbivores averaging near 100% of the biomass of herbivore-free controls after two seasons. In contrast, the growth of the WB plants was greatly reduced by the planthopper, with the plants exposed to planthopper herbivory averaging only 30% and 12% of the aboveground biomass of herbivore-free controls after the first and second seasons of herbivory, respectively. By the end of the second season of herbivory, 37% of the WB plants exposed to herbivory had died, while none of the SFB plants exposed to herbivores had died. Among WB clones, there was variation in resistance; one WB clone suffered 0% mortality while another suffered 100% mortality when exposed to herbivores. Short-term herbivory experiments with the putative founder clone for the WB population suggested that the WB founder was similar to the more resistant WB clones in its susceptibility to planthopper herbivory. Nitrogen analyses of green leaf tissue indicated that WB plants, including the WB founder clone, averaged 70% more total leaf nitrogen than SFB and Maryland plants. In a planthopper choice experiment, more planthoppers were observed on WB plants than SFB plants after 95 days of exposure to herbivory. Planthopper preference for WB plants may have contributed to the lower resistance of WB plants to herbivory; however, even before planthoppers had become more abundant on the WB plants, the proportion of leaves with 50% or more dead tissue averaged significantly greater on the WB plants, suggesting a difference between populations in tolerance to herbivory as well. Multiple factors, including a founder effect, further loss of herbivore tolerance, and herbivore preference for WB plants, appear to account for the reduced planthopper resistance in the WB population.

Hybridization between invasive Spartina densiflora (Poaceae) and native S. foliosa in San Francisco Bay, California, USA.

Rapid evolution in contemporary time can result when related species, brought together through human-aided introduction, hybridize. The significant evolutionary consequences of post-introduction hybridization range from allopolyploid speciation to extinction of species through genetic amalgamation. Both processes are known to occur in the perennial cordgrass genus, Spartina. Here we report the existence of a third recent Spartina hybridization, discovered in 2002, between introduced S. densiflora and native S. foliosa in San Francisco Bay, California, USA. We used nuclear and chloroplast DNA analysis and nuclear DNA content with chromosome counts to examine plants of morphology intermediate between S. densiflora and S. foliosa in a restored marsh in Marin County, California. We found 32 F(1) diploid hybrids and two triploid plants, all having S. densiflora and S. foliosa as parents; there is also evidence of a genetic contribution of S. alterniflora in some hybrids. None of these hybrids set germinable seed. In 2007 we found a hybrid over 30 miles away in a marsh where both parental species occurred, suggesting hybridization may not be a localized phenomenon. The presence of diploid and triploid hybrids is important because they indicate that several avenues existed that may have given rise to a new allopolyploid species. However, such an event is now unlikely because all hybrids are targets of eradication efforts.

Soil mediates the interaction of coexisting entomopathogenic nematodes with an insect host.

We tested for soil substrate effects on the movement and infectivity of naturally co-occurring entomopathogenic nematodes Steinernema feltiae and Heterorhabditis marelatus, alone and in combination. We manipulated the presence and bulk density of soil and added Galleria mellonella baits within capped and perforated 15mL centrifuge tubes. Sampling tubes were then deployed in situ into field and laboratory settings as experimental traps for infective juveniles. In comparisons with standard soil collections from Lupinus arboreus rhizospheres, sampling tubes were equally sensitive to the presence of H. marelatus and more sensitive to S. feltiae. In laboratory microcosms, both EPN species infected Galleria at high frequencies in tubes lacking soil and in the absence of heterospecifics. Infection frequency of S. feltiae was unaffected by the presence of H. marelatus, but it declined with higher soil bulk density inside tubes. In contrast, detectable infection frequency by H. marelatus was reduced only marginally by the presence of soil but severely by the presence of S. feltiae. Thus, the presence of soil in tubes reversed the identity of dominant species infecting Galleria in tubes, an effect magnified when soils were compacted. Moreover, S. feltiae rarely moved into tubes lacking Galleria baits, whereas H. marelatus colonized unbaited tubes 4- to 5-fold more frequently than S. feltiae. In situ, sampling tubes acted as filters to reduce interference and contamination by fungal pathogens common in field soils. The method allows precision sampling with minimal soil disturbance while protecting bait insects from scavengers. Manipulation of tube design may allow selective sampling of EPN species, depending on the abiotic characteristics of soils, and the biology, behavior, and interspecific interactions of coexisting species.

Phoresy of the entomopathogenic nematode Heterorhabditis marelatus by a non-host organism, the isopod Porcellio scaber.

Entomopathogenic nematodes are widespread in nature and commonly used in the biological control of insect pests. However, we understand little about how these organisms disperse. We show in a laboratory setting that the entomopathogenic nematode Heterorhabditis marelatus is phoretically dispersed by a non-host organism, the isopod Porcellio scaber. These species both inhabit tunnels excavated in the roots and lower stems of bush lupine (Lupinus arboreus) by the nematodes' primary prey, larvae of the ghost moth Hepialus californicus. Phoretic dispersal via P. scaber may play a role in the metapopulation dynamics of this nematode.

Widening the window of persistence in seasonal pathogen-host systems.

Local instability of exploiter-victim systems is well-known in both theory and in nature. Victims can be too sparse to support exploiter reproduction (under-exploitation) or they can be too readily driven to extinction (over-exploitation). Exploiters of seasonal resources face the additional challenge of surviving periods when victims are rare or unavailable. We formulate a fully stochastic model of highly seasonal pathogen-host dynamics and explore the interactions between an entomopathogenic nematode and its lepidopteran host. Our model suggests that if nematode populations experience the high rates of mortality predicted by short-term laboratory experiments, the paired threats of under- and over-exploitation should preclude the long-term persistence of this exploiter-victim system. We measured nematode mortality rates in the field and found that long-term mortality is lower than that predicted by short-term experiments. Incorporation of this new data into our model produces long-term persistence of local nematode populations across a range of initial nematode densities.

Pollen limitation causes an Allee effect in a wind-pollinated invasive grass (Spartina alterniflora).

It is usually assumed that pollen availability does not limit reproduction in wind-pollinated plants. Little evidence either supporting or contradicting this assumption exists, despite the importance of seed production to population persistence and growth. We investigated the role of pollen limitation in an invasive estuarine grass (Spartina alterniflora), with a manipulative pollen supplementation and exclusion experiment in areas of high population density and at the low-density leading edge of the invasion. We also quantified pollen deposition rates on stigmas and pollen traps along a windward to leeward gradient. We found pollen impoverishment at the low-density leading edge of a large invasion, causing an 8-fold reduction in seed set. We found 9-fold more pollen on stigmas of high-density plants than on those of low-density plants. Pollen deposition rates on stigmas and traps did not increase downwind of low-density plants but did increase downwind of high-density plants and dropped off precipitously across a gap that lacked pollen donors. The delay of appreciable numbers of seed caused by pollen limitation persists for decades until vegetative growth coalesces plants into continuous meadows, and this Allee effect has slowed the rate of spread of the invasion.

Seasonally limited host supply generates microparasite population cycles.

Cycles in biological populations have been shown to arise from enemy-victim systems, delayed density dependence, and maternal effects. In an initial effort to model the year-to-year dynamics of natural populations of entomopathogenic nematodes and their insect hosts, we find that a simple, nonlinear, mechanistic model produces large amplitude, period two population cycles. The cycles are generated by seasonal dynamics within semi-isolated populations independently of inter-annual feedback in host population numbers, which differs from previously studied mechanisms. The microparasites compete for a fixed number of host insect larvae. Many nematodes at the beginning of the year quickly eliminate the pool of small hosts, and few nematodes are produced for the subsequent year. Conversely, initially small nematode populations do not over-exploit the host population, so the surviving hosts grow to be large and produce many nematodes that survive to the following year.