Herbivory may promote a non-native plant invasion at low but not high latitudes

Long period exposure to serious cadmium pollution benefits an invasive plant (Alternanthera philoxeroides) competing with its native congener (Alternanthera sessilis)

Many aggressive plants possess high tolerance to heavy metals, but little is known about their invasiveness at heavy metal polluted sites. We performed a greenhouse experiment to examine the impacts of Cd (0, 10, 30, 60, and 100 mg kg^-1) and inter-specific competition on the reproductive capability of an invasive plant, Alternanthera philoxeroides, and its native congener, Alternanthera sessilis. We also examined the population dynamics of both native and invasive species in a simulated field experiment. Compared with A. philoxeroides, native A. sessilis was a stronger competitor as measured by vegetative growth, sexual reproduction, and dominance status in a mixed culture. However, A. philoxeroides showed great plasticity in root mass ratio that was positively affected by inter-specific competition and high Cd levels. Such high root allocation might allow for delayed growth of A. philoxeroides rhizomes as the relative cover of A. philoxeroides to A. sessilis in the field experiment gradually increased and > 1 after nine months culture, especially at high Cd treatment. Our results suggest that the invasiveness of A. philoxeroides is highly context- and time-dependent. In severely polluted environments, clonal propagation of A. sessilis is likely inhibited by the synergistic negative effects of inter-specific competition and heavy metal pollution, and a long time co-existence of these two competing species would facilitate the colonization of invasive plant.

Latitudinal variation in the diversity and composition of various organisms associated with an exotic plant: the role of climate and plant invasion

Climate and plant invasion can shape biotic communities at large spatial scales. Yet, how diverse groups of organisms associated with an invasive plant change simultaneously with latitude and the roles of climate and plant invasion remains unclear. We conducted a field survey of plants (native vs exotic), soil fungi (pathogenic, saprotrophic, arbuscular mycorrhiza fungi (AMF) and ectomycorrhizal (EcM) fungi) and arthropods (herbivores, predators and detritivores) associated with the invasive plant Alternanthera philoxeroides at 49 sites spanning 14 latitudinal degrees in China. Results showed that diversity and composition of these functional groups changed differently with latitude, partially due to their specific responses to climate, invasion of A. philoxeroides and other biotic environments. Moreover, A. philoxeroides invasion and/or composition of other plants, rather than climate, predicted the diversity and richness of major functional groups and partly explained variance in composition of putative fungal pathogens. Our results suggest that climate and plant invasion could affect the diversity and composition of diverse groups of organisms simultaneously and their relative importance might vary among functional groups. Thus, it is necessary to explore latitudinal patterns and underlying drivers of diverse groups of organisms simultaneously to improve our ability to predict and mitigate threats posed by plant invasion and climate change.

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60° than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors.