Exotic plants accumulate and share herbivores yet dominate communities via rapid growth

Herbivores may facilitate or impede exotic plant invasion, depending on their direct and indirect interactions with exotic plants relative to co-occurring natives. However, previous studies investigating direct effects have mostly used pairwise native-exotic comparisons with few enemies, reached conflicting conclusions, and largely overlooked indirect interactions such as apparent competition. Here, we ask whether native and exotic plants differ in their interactions with invertebrate herbivores. We manipulate and measure plant-herbivore and plant-soil biota interactions in 160 experimental mesocosm communities to test several invasion hypotheses. We find that compared with natives, exotic plants support higher herbivore diversity and biomass, and experience larger proportional biomass reductions from herbivory, regardless of whether specialist soil biota are present. Yet, exotics consistently dominate community biomass, likely due to their fast growth rates rather than strong potential to exert apparent competition on neighbors. We conclude that polyphagous invertebrate herbivores are unlikely to play significant direct or indirect roles in mediating plant invasions, especially for fast-growing exotic plants.

Environmental factors affecting the success of exotic plant invasion in a wildland-urban ecotone in temperate South America

Urbanization is one of the main causes driving changes in biodiversity patterns and it is regarded as a major threat to native biota. Successful exotic plant invasion depends on invasiveness and invasibility. Invasiveness is related to the characteristics of exotic plants and invasibility to the features of the sites. The objective of this study was to identify the invasibility environmental factors affecting the success of exotic plant invasion in a wildland-urban ecotone of the central region of Argentina (Potrero de los Funes Village, San Luis). Fifty phytosociological inventories were recorded in an area of 700 ha during spring and summer seasons (2013–2015). Abundance-coverage values of plants and environmental variables such as soil characteristics, anthropogenic disturbance, and altitude of the sites were assessed. Soil moisture, electrical conductivity (EC), acidity (pH), organic matter content, and nitrates were determined as part of the soil analysis. A Nonmetric Multidimensional Scaling analysis was used to identify the possible relationship between abundance-coverage of the vegetation and environmental variables. Abundance-coverage of exotic plants was positively influenced by anthropogenic disturbance and nitrate levels, and negatively affected by altitude. However, no significant correlation was found between percentage of exotic plants and pH, EC, or soil moisture. Thus, urbanization and touristic activities influenced the success of exotic plant invasion.

Post-fire invasion in Torres del Paine Biosphere Reserve: the role of seed tolerance to heat

Biological invasions are one of the major causes of biodiversity loss. Anthropogenic alteration in fire regimes has promoted plant invasion in several regions and ecosystems worldwide, especially in those where fires are historically rare. This is the case of the UNESCO Biosphere Reserve Torres del Paine National Park (TPNP), located in southern South America. In this study, we hypothesised that the seeds of native species of the TPNP do not resist fire-related high soil temperatures as do exotic plants, which could facilitate the spread of exotic plant invasion in post-fire environments. To test this hypothesis, we exposed seeds of plant species collected from the TPNP to different heat doses (60°C, 5min; 90°C, 5min; 90°C, 10min; 120°C, 5min) and compared their final germination and viability with those of unheated seeds. The study was conducted on five native species and three aliens inhabiting the TPNP. Almost all the seeds of native species died after exposure to temperatures higher than 60°C, whereas seeds of the alien species resisted higher heat doses. The on-site resistance of the aliens (through fire-tolerant soil seed banks) and the fire sensitivity of native seeds contribute to our understanding on post-fire invasion in the TPNP.