Role of nitrogen fixing bacteria in the invasion success of weedy Australian Acacias

Several Australian Acacias have become invasive when introduced into new areas within Australia. These invasive Acacias may have a detrimental effect on native flora and induce changes to below-ground microbial composition. Previous studies have shown that the invasive success of Acacias can be at least partly attributed to their ability to associate with N-fixing rhizobial bacteria. This study assessed the role of rhizobia in the invasion success of four Acacia species (A. cyclops, A. longifolia, A. melanoxylon, A. saligna) and closely related species Paraserianthes lophantha in their nonnative environment within Australia. Presence or absence of rhizobia in soils in both the native and introduced ranges of each species, as well as infectivity and efficacy, were estimated using the Most Probable Number (MPN) method to determine numbers of rhizobial cells/g-1 of soil. A glasshouse experiment had shown that there are significant differences in aboveground biomass between plants grown in soils from their native compared with non-native environment. The outcome of this study will enhance our understanding of the role of plant-soil interactions in Acacia invasion into novel ranges in Australia and also contribute towards a larger global framework of studies on the invasion ecology of non-native Acacias

Invasive legumes encounter similar soil fungal communities in their non-native and native ranges in Australia

Acacias are globally one of the most successful invaders, but relatively little is known about the role of soil microbial communities, particularly fungi, in their invasion success. We assessed intra- and interspecific variation in the soil fungal communities of five legumes native to either south-east (Acacia longifolia, Acacia melanoxylon – invasive in the south-west) or south-west (Acacia cyclops, Acacia saligna, Paraserianthes lophantha – invasive in the south-east) Australia across their Australian non-native and native ranges. Soil samples were collected from around the roots (up to 5 cm) of each legume species from multiple populations and soil fungal communities were amplified and 454 pyrosequenced. Results revealed a significant interaction between host species and geographic location (i.e., south-east vs south-west) suggesting that among-population differences in fungal communities were largely driven by geographic-scale variation between south-east and south-west Australia. However, pairwise analyses of range effects within species showed that soil fungal communities were not different between non-native and native range populations for any host species except for A. cyclops, suggesting that this species may be more of a generalist than the other legumes studied. Pairwise analyses between species revealed considerable variation in soil fungal communities. Our work suggests that these legume species, introduced into novel ranges across the continent, whether south-east or south-west, generally associate with similar soil fungal communities, indicating that soil fungal communities in non-native ranges are unlikely to have constrained or facilitated the invasion success of these legumes

Specialists or generalists? Nitrogen fixing bacterial communities of invasive acacias in Australia

Although some Australian acacias are amongst the most notable invaders world-wide, information on the relative role of soil biota, particularly beneficial microbial communities such as rhizobia, in their invasion success remains elusive. We examined the nitrogen fixing bacteria (henceforth NFB, focus on rhizobia) in native and introduced populations across New South Wales, Victoria, South Australia and Western Australia of four weedy Acacia spp. and a sister-taxa Paraserianthes lophantha using 454 sequencing to identify the NFB community composition and diversity in the rhizophere and nodules. We hypothesized that if these acacias are specialists, they will associate with similar NFB subsets in their nodules across the continent and are likely to be constrained in the introduced populations if compatible microorganisms are absent from the rhizosphere. However, if these acacias are generalists, they will successfully form relationships with novel NFB in the introduced range populations and are unlikely to be constrained by the absence of suitable soil mutualists. Results showed that overall the rhizosphere NFB communities were different across the continent (south-east vs south-west), while similar across the ranges (native vs introduced) in the nodules of the host species. The most dominant rhizobial taxa in the rhizosphere and nodules were slow-growing Bradyrhizobium. These results suggest that acacias are specialists that predominantly associate with Bradyrhizobium in their nodules across native and introduced populations. However, this does not translate into an invasion constraint since Bradyrhizobium is common in the soils across the south-eastern and south-western populations of these acacias

Mutualisms are not constraining cross-continental invasion success of Acacia species within Australia

Aim: Studying plant-soil interactions of introduced species in different parts of their global range could assist in managing biological invasions by elucidating the level of host specificity of key mutualists. We assessed the role of the soil microbial community (with an emphasis on symbiotic nitrogen-fixing bacteria, collectively termed rhizobia) in determining cross-continental invasion success of five woody legume species. Location: Australia. Methods: For each species, we compared growth of plants in soils from their native and non-native ranges using a glasshouse study, a soil dilution method (most probable number) and T-RFLP to assess rhizobial abundance and community composition, respectively. Results: Acacia longifolia was the only species that had significantly larger above-ground biomass when grown in soils from its non-native range. Rhizobial abundance was equally high across species and ranges, indicating plants are unlikely to be limited by soil rhizobial abundance in non-native ranges. Acacia cyclops, A. saligna and Paraserianthes lophantha formed associations with different rhizobial communities in non-native vs. native range soils. Acacia longifolia and A. melanoxylon associated with similar rhizobial communities in their native and non-native ranges, suggesting that rhizobia may have been accidentally introduced into their novel range with seeds or seedlings. Main conclusions: Invasive success of these five legume species is not constrained by the abundance of rhizobia in novel ranges for established legume populations, at least within Australia. Although differences in rhizobial community composition were evident between the native and non-native ranges for three of the five species, these were not associated with differences in plant growth. Increased above-ground biomass of A. longifolia when grown in soil from its non-native range suggests that invasive success of this species may be associated with differences in the non-rhizobial components of soil microbial communities in the novel range. This information could assist in management practises by facilitating a more instructive and effective screening for invasiveness

Improving the durability and efficiency of plant resistance deployment using eco-evolutionary modelling

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