Biosciences

Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.

^^ investigated the effects of prescribed burning on the composition, abundance, species richness and diversity of oak savanna arthropod communities in a replicated, large-scale, 30-y experiment. We employed four sampling methods over 3 y and caught 11,215 arthropods of 551 species. Species had varied and often negatively correlated short vs, longterm responses to burning. In the years savannas were burned, species richness and abundance of arthropods, especially Homoptera and Lepidoptera, were reduced and species compositions were sometimes more similar. However, despite its major effect on vegetation, frequency of burning did not affect arthropod abundance and species richness, but sometimes caused savannas to be similar in species or taxonomic order composition. The Shannon diversity index was unaffected by burning. On the whole, prescribed burns necessary to maintain grasslands and savannas do not appear to be harmful to, or to greatly impact, the arthropod fauna.

Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.

^^ investigated the effects of prescribed burning on the composition, abundance, species richness and diversity of oak savanna arthropod communities in a replicated, large-scale, 30-y experiment. We employed four sampling methods over 3 y and caught 11,215 arthropods of 551 species. Species had varied and often negatively correlated short vs, longterm responses to burning. In the years savannas were burned, species richness and abundance of arthropods, especially Homoptera and Lepidoptera, were reduced and species compositions were sometimes more similar. However, despite its major effect on vegetation, frequency of burning did not affect arthropod abundance and species richness, but sometimes caused savannas to be similar in species or taxonomic order composition. The Shannon diversity index was unaffected by burning. On the whole, prescribed burns necessary to maintain grasslands and savannas do not appear to be harmful to, or to greatly impact, the arthropod fauna.

Invasive plants are often more vigorous in their introduced ranges than in their native ranges. This may re¯ect an innate superiority of plants from some habitats or an escape from their enemies. Another hypothesis proposes that invasive plants evolve increased competitive ability in their introduced range. We present the results of a 14-year common garden experiment with the Chinese Tallow Tree (Sapium sebiferum) from its native range (Asia), place of introduction to North America (Georgia) and areas colonized a century later (Louisiana and Texas). Invasive genotypes, especially those from recently colonized areas, were larger than native genotypes and more likely to produce seeds but had lower quality, poorly defended leaves. Our results demonstrate signi®cant post-invasion genetic differences in an invasive plant species. Postintroduction adaptation by introduced plants may contribute to their invasive success and make it dif®cult to predict problem species.

1. The evolution of increased competitive ability (EICA) hypothesis proposes that exotic plants often become invasive because they have evolved reduced allocation to defence and increased allocation to growth and reproduction in response to low herbivore loads. Previous studies with Chinese tallow tree Sapium sebiferum support EICA predictions of invasive North American ecotypes displaying rapid growth, poorly defended leaves and considerable compensation after defoliation. In contrast, native Asian ecotypes are comparatively slow growing with highly defended leaves and are negatively impacted by mechanical leaf damage. The effects of root damage on the different ecotypes are unknown.

Invasive plants are often more vigorous in novel habitats than in their native ranges. Sapium sebiferum (Chinese tallow tree) is a major invader of habitats in the southern United States. Long-term common garden experiments in Texas and Hawaii (USA) with S. sebiferum genotypes from its native range (Asia) and from areas in North America where it is invasive suggest that post-introduction evolutionary changes may contribute to its invasiveness in Texas. In Texas, where there was uniformly low herbivory, fast-growing, poorly defended invasive North American genotypes outperformed slower growing, better defended native Asian genotypes in common garden plots. In Hawaii, in contrast, an Asian herbivore, Adoretus sinicus (Chinese rose beetle) is abundant, and S. sebiferum is not invasive. In Hawaiian common garden plots, A. sinicus caused greater damage to North American genotypes, and Asian genotypes were competitively superior. Our results suggest that exotic plants freed from herbivory can evolve greater competitive ability, allowing them to become much more abundant than in their native ranges. This did not seem to occur, though, if herbivores from the native range were abundant.

The Evolution of Increased Competitive Ability (EICA) hypothesis proposes that exotic plants may become superior competitors in their introduced ranges because they have evolved reduced defense allocation and increased growth in response to low herbivory. Long-term common garden studies with Chinese tallow tree (Sapium sebiferum), a problematic invader in the southeastern United States, support EICA predictions of invasive ecotypes displaying rapid growth with poorly defended leaves, whereas native ecotypes are slower growing and have nitrogen rich, highly defended leaves. We conducted two full-factorial experiments designed to assess the effects of leaf herbivory and soil fertility on growth of Sapium seedlings derived from seeds obtained in the species' native Chinese range and introduced range along the Texas Gulf Coast. The first was a field study employing simulated defoliation to examine herbivory tolerance, while the second was a pot study using grasshopper bioassays to measure the compensatory regrowth of native and introduced Sapium ecotypes. In accordance with EICA predictions, introduced Texas ecotypes compensated for mechanical and natural defoliation regardless of resource conditions while growth of native Chinese ecotypes was significantly reduced as a result of both simulated and natural herbivory damage. These results demonstrate that invasive Sapium ecotypes have a greater capacity to compensate for herbivory damage than native Sapium ecotypes. The differences between Sapium ecotypes are consistent with the mechanisms of EICA and other findings demonstrating that invasive ecotypes have become herbivory tolerant due to post-introduction changes in tissue quality.

The evolution of increased competitive ability (EICA) hypothesis predicts that release from natural enemies in the introduced range favors exotic plants evolving to have greater competitive ability and lower herbivore resistance than conspecifics from the native range. We tested the EICA hypothesis in a common garden experiment with Sapium sebiferum in which seedlings from native (China) and invasive (USA) populations were grown in all pairwise combinations in the native range (China) in the presence of herbivores. When paired seedlings were from the same continent, shoot mass and leaf damage per seedling were significantly greater for plants from invasive populations than those from native populations. Despite more damage from herbivores, plants from invasive populations still outperformed those from native populations when they were grown together. Increased competitive ability and higher herbivory damage of invasive populations relative to native populations of S. sebiferum support the EICA hypothesis. Regression of biomass against percent leaf damage showed that plants from invasive populations tolerated herbivory more effectively than those from native populations. The results of this study suggest that S. sebiferum has become a fastergrowing, less herbivore-resistant, and more herbivore-tolerant plant in the introduced range. This implies that increased competitive ability of exotic plants may be associated with evolutionary changes in both resistance and tolerance to herbivory in the introduced range. Understanding these evolutionary changes has important implications for biological control strategies targeted at problematic invaders.

Trade-offs associated with maintaining herbivory resistance and herbivory tolerance are frequently inferred in plant life histories. Invasive success for many non-native plants is often attributed to novel resistance that repels native herbivores. Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) is a non-native invader that threatens ecosystems throughout the southeastern United States, including imperiled coastal prairie regions. Low herbivore loads due to costly resistance are generally assumed to give Sapium a competitive advantage over native plants. We predicted that if Sapium experienced higher damage levels it would show significant decreases in growth and reduced ability to compete with native prairie vegetation. We conducted full-factorial, paired greenhouse and field experiments designed to assess the effects of simulated leaf herbivory on Sapium growth in the presence of prairie vegetation at different levels of light and nitrogen. Contrary to our expectations, neither low-intensity, chronic defoliation nor high-intensity, acute defoliation negatively affected Sapium seedlings in any resource combination in either experiment. These studies reveal that Sapium possesses considerable phenotypic plasticity, and herbivory tolerance is a newly appreciated trait that likely contributes to its invasive potential.