Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common‐garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common‐garden experiment in the field and by documenting oviposition and feeding preference by specialist leaf beetles in a glasshouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the glasshouse and leaf damage in the common‐garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.     

Rethinking the common garden in invasion research

In common garden experiments, a number of genotypes are raised in a common environment in order to quantify the genetic component of phenotypic variation. Common gardens are thus ideally suited for disentangling how genetic and environmental factors contribute to the success of invasive species in their new non-native range. Although common garden experiments are increasingly employed in the study of invasive species, there has been little discussion about how these experiments should be designed for greatest utility. We argue that this has delayed progress in developing a general theory of invasion biology. We suggest a minimum optimal design (MOD) for common garden studies that target the ecological and evolutionary processes leading to phenotypic differentiation between native and invasive ranges. This involves four elements: (A) multiple, strategically sited garden locations, involving at the very least four gardens (2 in the native range and 2 in the invaded range); (B) careful consideration of the genetic design of the experiment; (C) standardization of experimental protocols across all gardens; and (D) care to ensure the biosafety of the experiment. Our understanding of the evolutionary ecology of biological invasions will be greatly enhanced by common garden studies, if and only if they are designed in a more systematic fashion, incorporating at the very least the MOD suggested here.     

Warming increases plant biomass and reduces diversity across continents,latitudes, and species migration scenarios in experimental wetland communities

Atmospheric warming may influence plant productivity and diversity and induce poleward migration of species, altering communities across latitudes. Complicating the picture is that communities from different continents deviate in evolutionary histories, which may modify responses to warming and migration. We used experimental wetland plant communities grown from seed banks as model systems to determine whether effects of warming on biomass production and species richness are consistent across continents, latitudes, and migration scenarios. We collected soil samples from each of three tidal freshwater marshes in estuaries at three latitudes (north, middle, south) on the Atlantic coasts of Europe and North America. In one experiment, we exposed soil seed bank communities from each latitude and continent to ambient and elevated (+2.8 °C) temperatures in the greenhouse. In a second experiment, soil samples were mixed either within each estuary (limited migration) or among estuaries from different latitudes in each continent (complete migration). Seed bank communities of these migration scenarios were also exposed to ambient and elevated temperatures and contrasted with a no‐migration treatment. In the first experiment, warming overall increased biomass (+16%) and decreased species richness (?14%) across latitudes in Europe and North America. Species richness and evenness of south‐latitude communities were less affected by warming than those of middle and north latitudes. In the second experiment, warming also stimulated biomass and lowered species richness. In addition, complete migration led to increased species richness (+60% in North America, + 100% in Europe), but this higher diversity did not translate into increased biomass. Species responded idiosyncratically to warming, but Lythrum salicaria and Bidens sp. increased significantly in response to warming in both continents. These results reveal for the first time consistent impacts of warming on biomass and species richness for temperate wetland plant communities across continents, latitudes, and migration scenarios.     

Costs and benefits of admixture between foreign genotypes and local populations in the field

Admixture is the hybridization between populations within one species. It can increase plant fitness and population viability by alleviating inbreeding depression and increasing genetic diversity. However, populations are often adapted to their local environments and admixture with distant populations could break down local adaptation by diluting the locally adapted genomes. Thus, admixed genotypes might be selected against and be outcompeted by locally adapted genotypes in the local environments. To investigate the costs and benefits of admixture, we compared the performance of admixed and within‐population F1 and F2 generations of the European plant Lythrum salicaria in a reciprocal transplant experiment at three European field sites over a 2‐year period. Despite strong differences between site and plant populations for most of the measured traits, including herbivory, we found limited evidence for local adaptation. The effects of admixture depended on experimental site and plant population, and were positive for some traits. Plant growth and fruit production of some populations increased in admixed offspring and this was strongest with larger parental distances. These effects were only detected in two of our three sites. Our results show that, in the absence of local adaptation, admixture may boost plant performance, and that this is particularly apparent in stressful environments. We suggest that admixture between foreign and local genotypes can potentially be considered in nature conservation to restore populations and/or increase population viability, especially in small inbred or maladapted populations.     

The implications of accepting untestedhypotheses: a review of the effectsof purple loosestrife (Lythrum salicaria)in North America

The acceptance of poorly tested hypotheses has adverse scientific consequences, and may have adverse ecological and social consequences. The hypothesis that purple loosestrife (Lythrum salicaria) has deleterious effects on North American wetlands is an example. We traced the history of purple loosestrife and its control in North America and found little scientific evidence consistent with the hypothesis that purple loosestrife has deleterious effects. The most commonly cited study of the effects of purple loosestrife on native flora and fauna produced inconclusive results. The general acceptance of this hypothesis, however, has resulted in the introduction of nonindigenous insects for biological control. Efforts to control purple loosestrife may be misplayed and may have long-term ecological consequences if loosestrife does not have the impact it is believed to have. The acceptance of this hypothesis using scientific justifications may affect future scientific credibil ity. Careful evaluation of the precautionary principle is necessary when considering the control of nonindigenous organisms.     

Quantifying the effects of distance and conspecifics on colonization: experiments and models using the loosestrife leaf beetle, Galerucella calmariensis

The ability of an insect to disperse to new habitat patches is difficult to quantify, but key to the establishment and persistence of populations. In this study, we examined dispersal of the phytophagous chrysomelid beetle, Galerucella calmariensis, which is currently being introduced into North America for the biological control of purple loosestrife (Lythrum salicaria), an aggressive wetland weed. We used a mark, release, and recapture approach to determine how rates of colonization of host patches by this beetle are influenced by the distance of the patch from the source of dispersers, and by the presence of conspecifics at the patch. We released color-coded beetles at six distances from a long, linear patch of purple loosestrife that was divided into segments with and without conspecifics. We observed initial flight directions as beetles left the release points and collected all beetles that settled at the target patch. We found a bias in initial flight toward the target for distances up to 50 m. Over the 7 days of the experiment, beetles arrived at the target from all release points, including the farthest release point, 847 m away. G. calmariensis was strongly attracted to conspecifics when settling after dispersal; 86% of the 582 recovered beetles came from the segments inhabited by conspecifics. The probability of an individual arriving at the patch declined steeply with release distance. This relationship fits a model in which beetles move in a random direction and stop if they intercept the target patch, and where beetles are lost at a constant rate with distance travelled. The dispersal and patch-colonizing behavior of G. calmariensis is likely to have important consequences for the biological control program against purple loosestrife. Received: 23 January 1996 / Accepted:30 September 1996     

Biological control of the wetlands weed purple loosestrife (Lythrum salicaria) in the Pacific northwestern United States

Purple loosestrife (Lythrum salicaria) is an Eurasian perennial hydrophyte that has become naturalized in wetlands and in and along waterways throughout temperate North America. The ecological integrity of such areas is threatened by rapidly forming monotypic infestations that displace valued flora and diminish critical fish and wildlife habitat. The inability of physical, cultural, and chemical methods to provide adequate control of the weed has led to the development of an insect-based biological control program. The first field releases of the bud and leaf feeding beetles, Galerucella calmariensis and G. pusilla, and a root-mining weevil, Hylobius transversovittatus, were made in the United States and Canada in 1992. A total of 4740 Galerucella spp. adults were released in central Washington during 1992 and 1993 at eight sites and 471 H. transversovittatus egg inoculations were made in 1993 at three locations. Establishment of both Galerucella spp. was confirmed and Hylobius colonization was achieved.     

The effects of flooding,plant traits,and predation on purple loosestrife leaf‐beetles

Classical biological weed control is based on the premise that introducing specialized natural enemies from the native range re‐establishes herbivore control of plant invaders, ultimately leading to negative population growth rates. Evidence from past biocontrol programs suggests that herbivores are not solely responsible for shaping plant demography. Diverse environmental conditions in the introduced range may not only affect demography, but also influence top‐down control of target plants. We investigated how flooding affects impacts of predators (top‐down) and plant quality (bottom‐up) on performance of two leaf‐beetles, Galerucella calmariensis L. and Galerucella pusilla Duftschmid (Coleoptera: Chrysomelidae: Galerucini), released in North America as biocontrol agents of purple loosestrife, Lythrum salicaria L. (Lythraceae). Predation and flooding regime have been linked to low leaf‐beetle recruitment at sites where insects failed to attain outbreak populations. Predator exclusion experiments at adjacent flooded and non‐flooded sites indicated a positive effect of flooding on leaf‐beetle survival for all developmental stages, whereas predator exposure had little effect. There was no difference in predation rates at sites with successful or failed purple loosestrife control, questioning the importance of predation in limiting growth and impact of these biocontrol agents’ populations. Effect of flooding on purple loosestrife quality was evaluated in a common garden study where plants were grown under different flooding treatments. Plants grown in flooded soil had higher water content and lower tannic acid concentration than plants grown in well‐drained soil. Consistent with field observations, leaf‐beetle oviposition rate and survival were higher on flooded plants. Results indicate that both bottom‐up and top‐down forces operate on Galerucella populations, yet their relative strength is mediated by flooding regime. Ignoring intricacies of plant‐herbivore and trophic interactions in the introduced range appears to be a major handicap for the improvement of weed biocontrol programs.     

Evidence for a shift in life-history strategy during the secondary phase of a plant invasion

We investigated the correlated response of several key traits of Lythrum salicaria L. to water availability gradients in introduced (Iowa, USA) and native (Switzerland, Europe) populations. This was done to investigate whether plants exhibit a shift in life-history strategy during expansion into more stressful habitats during the secondary phase of invasion, as has recently been hypothesized by Dietz and Edwards (Ecology 87(6):1359, 2006). Plants in invaded habitats exhibited a correlated increase in longevity and decrease in overall size in the transition into more stressful mesic habitats. In contrast, plants in the native range only exhibited a decrease in height. Our findings are consistent with the hypothesis that secondary invasion is taking place in L. salicaria, allowing it to be more successful under the more stressful mesic conditions in the invaded range. If this trend continues, L. salicaria may become a more problematic species in the future.     

Reconstructing the spread of invasive plants: taking into account biases associated with herbarium specimens

Aim To reconstruct the spread of invasive wetland species using herbarium specimens and to develop a method that accounts for the biases associated with this type of historical record. Location Southern Québec. Methods The temporal and spatial distribution of herbarium specimens of vascular plants was examined. Six invasive species that are mainly found in wetlands were compared with five native, non‐expanding hydrophytes. The cumulative number of locations was plotted against time to construct invasion curves. For native species, this ‘invasion curve’ indicates the spatiotemporal distribution of the sampling for herbarium specimens of wetland plants. It also represents the history of the state of knowledge of the distribution of native species. Deviation from the native species invasion curve can be used to describe the spread of invasive species. For every year on record, the cumulative number of locations of each exotic species was divided by the cumulative number of locations of all native species, and the proportions obtained were plotted against time. Periods of invasiveness were then delineated. Results During the twentieth century, two periods of intensive herbarium specimen collection in Québec can be clearly distinguished, i.e. from 1930 to 1940 and from 1950 to 1985. Several periods of invasiveness were delineated for Butomus umbellatus L. (1922–35), Hydrocharis morsus‐ranae L. (1957–96), Lycopus europaeus L. (1963–2000), Lythrum salicaria L. (1890–1905, 1923–46), Phragmites australis (Cav.) Trin. ex Steudel (1963–84) and Rorippa amphibia (L.) Bess. (1929–34, 1943–52, 1971–79). Main conclusions Accounting for sampling biases associated with herbarium specimens is important in order to objectively delineate periods of invasiveness for exotic species. The spread of wetland vascular plant species can be reconstructed using herbarium specimens, even when the phenomenon is rapid, but the reconstruction is certainly more accurate when the invasion occurred during an active sampling period. The highly variable sampling effort for herbarium specimens in Québec and in other regions strongly cautions against using herbarium data without correction procedures.