Herbivory by an introduced Asian weevil negatively affects population growth of an invasive Brazilian shrub in Florida

The influence of warming and biotic interactions on the potential for range expansion of native and nonnative species

Plant species ranges are expected to shift in response to climate change, however, it is unclear how species interactions will affect range shifts. Because of the potential for enemy release of invasive nonnative plant species from species-specific soil pathogens, invasive plants may be able to shift ranges more readily than native plant species. Additionally, changing climatic conditions may alter soil microbial functioning, affecting plant-microbe interactions. We evaluated the effects of site, plant-soil microbe interactions, altered climate, and their interactions on the growth and germination of three congeneric shrub species, two native to southern and central Florida (Eugenia foetida and E. axillaris), and one nonnative invasive from south America (E. uniflora). We measured germination and biomass for these plant species in growth chambers grown under live and sterile soils from two sites within their current range, and one site in their expected range, simulating current (2010) and predicted future (2050) spring growing season temperatures in the new range. Soil microbes (microscopic bacteria, fungi, viruses and other organisms) had a net negative effect on the invasive plant, E. uniflora, across all sites and temperature treatments. This negative response to soil microbes suggests that E. uniflora's invasive success and potential for range expansion are due to other contributing factors, e.g. higher germination and growth relative to native Eugenia. The effect of soil microbes on the native species depended on the geographic provenance of the microbes, and this may influence range expansion of these native species.

Lesser leaf herbivore damage and structural defense and greater nutrient concentrations for invasive alien plants: Evidence from 47 pairs of invasive and non-invasive plants

Empirical evidence of enemy release is still inconsistent for invasive alien plant species, although enemy release is the key assumption for both the enemy release hypothesis (ERH) and the evolution of increased competitive ability hypothesis (EICA). In addition, little effort has been made to test this assumption in terms of defense investment using a multi-species comparative approach. Using a phylogenetically controlled within-study meta-analytical approach, we compared leaf herbivore damage, structural defenses and nutrients between 47 pairs of invasive versus native and/or non-invasive alien plants in China. The invasive relative to the co-occurring native or non-invasive (native and non-invasive alien) plants incurred lesser leaf herbivore damage, had lesser leaf concentrations of cellulose, hemicellulose, lignin and carbon, lesser leaf density and carbon or lignin to nitrogen ratio but greater nutrients, which may facilitate success of the invasive plants. The lesser structural investment did not result in lesser leaf construction costs for the invaders, which may be associated with their greater leaf nitrogen concentration. However, the invasive plants were not significantly different from the non-invasive alien plants in any trait. Our results provide strong evidence for ERH, also are consistent with EICA, and indicate that enemy release may be an important factor in alien plant invasions.

Census timing alters stage duration distributions in matrix population models

Matrix population models are widely used to study the dynamics of stage-structured populations. A census in these models is an event monitoring the number of individuals in each stage and occurs at discrete time intervals. The two most common methods used in building matrix population models are the prebreeding census and postbreeding census. Models using the prebreeding and postbreeding censuses assume that breeding occurs immediately before or immediately after the censuses, respectively. In some models such as age-structured models, the results are identical regardless of the method used, rendering the choice of method a matter of preference. However, in stage-structured models, where the duration of the first stage of life varies among newborns, a choice between the prebreeding and postbreeding censuses may result in different conclusions. This is attributed to the different first-stage duration distributions assumed by the two methods. This study investigated the difference emerging in the structures of these models and its consequence on conclusions of eigenvalue and elasticity analyses using two-stage models. Considerations required in choosing a modeling method are also discussed.

Effects of native diversity, soil nutrients, and natural enemies on exotic invasion in experimental plant communities

Many factors can promote exotic plant success. Three of these factors-greater pressure from natural enemies on natives, increased soil nutrient supply, and low native species richness-may interact during invasions. To test for independent and interactive effects of these drivers, we planted herbaceous perennial communities at two levels of native richness (monocultures and five-species polycultures). We then factorially manipulated soil nutrient supply and access to these communities by aboveground foliar enemies (fungal pathogens and insect herbivores), and allowed natural colonization to proceed for four years. We predicted that nutrient addition would increase exotic success, while enemy exclusion and increasing native richness would reduce exotic success. Additionally, we expected that enemy exclusion would reduce the benefits of nutrient addition to exotic species most in species-poor communities, and that this effect would be weaker in species-rich communities. In total, we found no evidence that nutrient supply, enemy access, and native richness interacted to influence exotic success. Furthermore, native richness had no effect on exotic success. Instead, nutrient addition increased, and enemy exclusion decreased, exotic success independently. As predicted, enemy exclusion reduced exotic success, primarily by slowing the decline in abundance of planted native species. Together, these results demonstrate that multiple drivers of exotic success can act independently within a single system.

Release from herbivory does not confer invasion success for Eugenia uniflora in Florida

One of the most commonly cited hypotheses explaining invasion success is the enemy release hypothesis (ERH), which maintains that populations are regulated by coevolved natural enemies where they are native but are relieved of this pressure in the new range. However, the role of resident enemies in plant invasion remains unresolved. We conducted a field experiment to test predictions of the ERH empirically using a system of native, introduced invasive, and introduced non-invasive Eugenia congeners in south Florida. Such experiments are rarely undertaken but are particularly informative in tests of the ERH, as they simultaneously identify factors allowing invasive species to replace natives and traits determining why most introduced species are unsuccessful invaders. We excluded insect herbivores from seedlings of Eugenia congeners where the native and invasive Eugenia co-occur, and compared how herbivore exclusion affected foliar damage, growth, and survival. We found no evidence to support the ERH in this system, instead finding that the invasive E. uniflora sustained significantly more damage than the native and introduced species. Interestingly, E. uniflora performed better than, or as well as, its congeners in terms of growth and survival, in spite of higher damage incidence. Further, although herbivore exclusion positively influenced Eugenia seedling survival, there were few differences among species and no patterns in regard to invasion status or origin. We conclude that the ability of E. uniflora to outperform its native and introduced non-invasive congeners, and not release from insect herbivores, contributes to its success as an invader in Florida.