Explaining the Geographic Pattern of Plant Invasion in 67 Nature Reserves in China

Biological invasion is a serious threat to biodiversity and ecosystem function in nature reserves. However, the knowledge of the spatial patterns and underlying mechanisms of plant invasions in nature reserves is still limited. Based on a recent dataset on both invasive and native plants in 67 nature reserves of China, we used correlation, regression, and variation partitioning methods to statistically assess the relative roles of the “human activity,” “biotic acceptance,” and “environmental heterogeneity” hypotheses in explaining the geographic pattern of plant invasion. A total of 235 invasive plant species were compiled from 67 nature reserves. The high explanatory power of the human activity variables supported the human activity hypothesis. The biotic acceptance hypothesis was weakly supported since no significant correlations between climatic variables and invasion levels were found when the effects of the other factors were controlled. The environmental heterogeneity hypothesis was partially supported, since the number of native plants, representing environmental heterogeneity at fine-scale explained remarkable proportion of spatial variance of invasive plants but not that of the proportion of invasive plants. We predict that nature reserves with high plant diversity affected by rapid economic development and increasing temperature will face a serious threat of exotic plant invasion. In conclusion, our results provide crucial clues for understanding geographic variance of plant invasion in China’s nature reserves and spatial risk assessment.

Abiotic conditions shape the relationship between indigenous and exotic species richness in a montane biodiversity hotspot

Montane ecosystems are more prone to invasions by exotic plant species than previously thought. Besides abiotic factors, such as climate and soil properties, plant-plant interactions within communities are likely to affect the performance of potential invaders in their exotic range. The biotic resistance hypothesis predicts that high indigenous species richness hampers plant invasions. The biotic acceptance hypothesis, on the other hand, predicts a positive relationship between indigenous and exotic species richness. We tested these two hypotheses using observational data along an elevational gradient in a southern African biodiversity hotspot. Species composition data of indigenous and exotic plants were recorded in 20 road verge plots along a gradient of 1775–2775 m a.s.l. in the Drakensberg, South Africa. Plots were 2 × 50 m in size and positioned at 50 m elevational intervals. We found a negative correlation between indigenous and exotic richness for locations with poorly developed mineral soils, suggesting biotic resistance through competitive interactions. A strong positive correlation for plots with very shallow soils at high elevations indicated a lack of biotic resistance and the possibility of facilitating interactions in harsher environments. These results suggest that biotic resistance is restricted to the lower and mid elevations while biotic acceptance prevails in presence of severe abiotic stress, potentially increasing the risk of plant invasions into montane biodiversity hotspots.

Regional climate and local-scale biotic acceptance explain native–exotic richness relationships in Australian annual plant communities

Native and exotic species richness is expected to be negatively related at small spatial scales where individuals interact, and positive at larger spatial scales as a greater variety of habitats are sampled. However, a range of native–exotic richness relationships (NERRs) have been reported, including positive at small scales and negative at larger scales. We present a hierarchical metacommunity framework to explain how contrasting NERRs may emerge across scales and study systems, and then apply this framework to NERRs in an invaded winter annual plant system in southwest Western Australia. We analysed NERRs at increasing spatial scales from neighbourhoods (0.09 m 2 ) to communities (225 m 2 ) to metacommunities (greater than 10 ha) within a multilevel structural equation model. In contrast to many previous studies, native and exotic richness were positively related at the neighbourhood scale and were not significantly associated at larger scales. Heterogeneity in soil surface properties was weakly, but positively, associated with native and exotic richness at the community scale. Metacommunity exotic richness increased strongly with regional temperature and moisture availability, but relationships for native richness were negative and much weaker. Thus, we show that neutral NERRs can emerge at larger scales owing to differential climatic filtering of native and exotic species pools.

STATUS OF THE INVASION OF A POACEAE SPECIES IN TROPICAL SEMIARID RESERVOIRS

ABSTRACT The species of the Urochloa genus, exotic and infesting in Brazilian waters, are known to be invasive and dominant, occupying from humid, shallow areas, and irrigation canals to margins of deep reservoirs. This paper hypothesis that less depth reservoirs have higher infestation rate and higher biomass of U. arrecta. The objectives were to measure the percentage of occurrence of exotic macrophyte U. arrecta in 40 ecosystems from the Mamanguape basin (Paraíba, Brazil) and determine the infestation of the species in two reservoirs. The acquired data were geo-referenced with the ArcGIS software (v. 9.3). A covariance analysis was performed using the R program (The R project is Statistical Computing). The results showed large spatial distribution of the species, indicating that reservoirs may act as steppingstones in the landscape, in a regional scale. The hypothesis of biotic acceptance is seen as a relevant factor in explaining the presence of the species with low percentage of occurrence in 37 out of the 40 sampled ecosystems, being observed only in areas prone to the colonization of native and naturalized macrophytes, in banks and points of lower declivity, in both spatial scales studied. Thus, factors such as environmental instability (promoted by intermittent or prolonged desiccation of the habitat), shadowing and declivity of the reservoirs synergistically acted on exotic and native species.