Expansive reed populations-alien invasion or disturbed wetlands?

Interaction between Nitrogen, Phosphorus, and Invasive Alien Plants

Plant invasion is significantly affected by environmental factors in the recipient habitats and affects the stability and sustainable development of society. The invasiveness of alien plants may be increased by anthropogenic-mediated disturbances, such as fluctuations in nutrients caused by excessive emissions of nitrogen (N) and phosphorus (P). To improve our understanding of the interactions between N and P fluctuations and invasive alien plants, the current report focuses on the biogeochemical behavior of N and P among invasive alien plants, native plants, and the soil within the plant–soil ecosystem. Our research, together with a synthesis of the literature, shows that fluctuations in N and P resources provide more opportunities and competitiveness for plant invasion. At the same time, the biogeochemical cycles of N and P are promoted because of their efficient and increased utilization and rate of release by invasive alien plants. However, there is no consensus on whether the N and P compositions of invasive species are different from those of the natives in their habitat. Quantitative studies that compare N and P contents in plant, litter, and soil between native plant communities and invaded communities on a global scale are an indispensable area of research focus for the future.

The genic view of hybridization in the Anthropocene

Human impact is noticeable around the globe, indicating that a new era might have begun: the Anthropocene. Continuing human activities, including land-use changes, introduction of non-native species and rapid climate change, are altering the distributions of countless species, often giving rise to human-mediated hybridization events. While the interbreeding of different populations or species can have detrimental effects, such as genetic extinction, it can be beneficial in terms of adaptive introgression or an increase in genetic diversity. In this paper, I first review the different mechanisms and outcomes of anthropogenic hybridization based on literature from the last five years (2016-2020). The most common mechanisms leading to the interbreeding of previously isolated taxa include habitat change (51% of the studies) and introduction of non-native species (34% intentional and 19% unintentional). These human-induced hybridization events most often result in introgression (80%). The high incidence of genetic exchange between the hybridizing taxa indicates that the application of a genic view of speciation (and introgression) can provide crucial insights on how to address hybridization events in the Anthropocene. This perspective considers the genome as a dynamic collection of genetic loci with distinct evolutionary histories, giving rise to a heterogenous genomic landscape in terms of genetic differentiation and introgression. First, understanding this genomic landscape can lead to a better selection of diagnostic genetic markers to characterize hybrid populations. Second, describing how introgression patterns vary across the genome can help to predict the likelihood of negative processes, such as demographic and genetic swamping, as well as positive outcomes, such as adaptive introgression. It is especially important to not only quantify how much genetic material introgressed, but also what has been exchanged. Third, comparing introgression patterns in pre-Anthropocene hybridization events with current human-induced cases might provide novel insights into the likelihood of genetic swamping or species collapse during an anthropogenic hybridization event. However, this comparative approach remains to be tested before it can be applied in practice. Finally, the genic view of introgression can be combined with conservation genomic studies to determine the legal status of hybrids and take appropriate measures to manage anthropogenic hybridization events. The interplay between evolutionary and conservation genomics will result in the constant exchange of ideas between these fields which will not only improve our knowledge on the origin of species, but also how to conserve and protect them.

Phylogenetic diversity shapes salt tolerance in Phragmites australis estuarine populations in East China

Estuaries are dynamic and selective environments that provide frequent opportunities for the turnover of Phragmites australis populations. We studied Phragmites genetic diversity patterns in three of the major deltas of China, viz. the Yellow River, the Yangtze and the Liaohe, in relation to Phragmites global phylogeography and soil salinity. We found that two distantly related P. australis haplotypes, each with intercontinental distribution, co-occur in these deltas in China. One is European Phragmites (Haplotype O) and is related to P. japonicus; the other (Haplotype P) has its range in East Asia and Australia and is related to the Asian tropical species P. karka. The two haplotypes have differing salt tolerance, with Haplotype O in areas with the highest salinity and Haplotype P in areas with the lowest. Introgressed hybrids of Haplotype P with P. karka, and F1 hybrids with Haplotype O, have higher salt tolerance than Haplotype P. Phylogenetic diversity appears as the factor that better explains population structure and salinity tolerance in these estuaries. Future research may explain whether the two P. australis haplotypes evolved in East Asia, and East Asia is a center of Phragmites diversity, or are introduced and a threat to P. japonicus and P. karka.

Testing the Enemy Release Hypothesis on tall-statured grasses in South Africa, using Arundo donax, Phragmites australis, and Phragmites mauritianus as models

The Enemy Release Hypothesis (ERH) predicts that introduced plant species can escape herbivory and therefore have a competitive advantage over native plants, which are exposed to both generalist and specialist natural enemies. In this study, the ERH was explored using the invasive alien species, Arundo donax and two native tall-statured grasses, the cosmopolitan Phragmites australis and African endemic Phragmites mauritianus in South Africa. It was predicted that A. donax would have reduced species richness of herbivores compared with the native Phragmites spp., that it would be devoid of specialist herbivores and would thus be experiencing enemy escape in the adventive range. The herbivore assemblages were determined from both field surveys and a literature review. The assumptions of the ERH were for the most part not met; 13 herbivores were found on A. donax compared with 17 on P. australis and 20 on P. mauritianus. Arundo donax had two specialist herbivores from its native range, and shared native herbivores with Phragmites spp. Although A. donax had reduced species richness and diversity compared with that found in the native distribution, it has partially re-acquired a herbivore assemblage which is similar to that found on analogous native species. This suggests that enemy release may not fully explain the invasive success of A. donax in South Africa.