One of the most cited hypotheses explaining the inordinate success of a small proportion of introduced plants that become pests is the 'natural enemies hypothesis'. This states that invasive introduced plants spread rapidly because they are liberated from their co-evolved natural enemies. This hypothesis had not been properly tested until recently. Previous reviews on this topic have been narrative and vote counting in nature. In this review, we carried out quantitative synthesis and meta-analysis using existing literature on plants and their herbivores to test the different components of the enemy release hypothesis. We found supporting evidence in that (1) insect herbivore fauna richness is significantly greater in the native than introduced ranges, and the reduction is skewed disproportionally towards specialists and insects feeding on reproductive parts; and (2) herbivore damage levels are greater on native plants than on introduced invasive congeners. However, herbivore damage levels are only marginally greater for plants in native than in introduced ranges, probably due to the small numbers of this type of study. Studies quantifying herbivore impacts on plant population dynamics are too scarce to make conclusions for either comparison of plants in native vs introduced ranges or of co-occurring native and introduced congeners. For future research, we advocate that more than two-way comparisons between plants in native and introduced ranges, or native and introduced congeners are needed. In addition, the use of herbivore exclusions to quantify the impacts of herbivory on complete sets of population vital rates of native vs introduced species are highly desirable. Furthermore, three-way comparisons among congeners of native plants, introduced invasive, and introduced non-invasive plants can also shed light on the importance of enemy release. Finally, simultaneously testing the enemy release hypothesis and other competing hypotheses will provide significant insights into the mechanisms governing the undesirable success of invasive species.

First, we report the results of the longest-known field study (9 years) to examine the effects of elevated carbon dioxide (CO 2 ) on leaf miner densities in a scrub-oak community at Kennedy Space Center, Florida. Here, the densities of all leaf miner species (6) on all host species (3) were lower in every year in elevated CO 2 than they were in ambient CO 2 . Second, meta-analyses were used to review the effects of elevated CO 2 on both plants (n 5 59 studies) and herbivores (n 5 75 studies). The log of the response ratio was chosen as the metric to calculate effect sizes. Results showed that elevated CO 2 significantly decreased herbivore abundance (À21.6%), increased relative consumption rates (116.5%), development time (13.87%) and total consumption (19.2%), and significantly decreased relative growth rate (À8.3%), conversion efficiency (À19.9%) and pupal weight (À5.03%). No significant differences were observed among herbivore guilds. Host plants growing under enriched CO 2 environments exhibited significantly larger biomass (138.4%), increased C/N ratio (126.57%), and decreased nitrogen concentration (À16.4%), as well as increased concentrations of tannins (129.9%) and other phenolics. Effects of changes on plant primary and secondary chemistry due to elevated CO 2 and consequences for herbivore growth and development are discussed.

The potential harmful effects of non—indigenous species introduced for biological control remain an important unanswered question, which we addressed by undertaking a literature review. There are few documented instances of damage to non—target organisms or the environment from non—indigenous species released for biological pest control, relative to the number of such releases. However, this fact is not evidence that biological control is safe, because monitoring of non—target species is minimal, particularly in sites and habitats far from the point of release. In fact, the discovery of such impacts usually rests on a remarkable concatenation of events. In addition to trophic and competitive interactions between an individual introduced species and a native one, many effects of introduced species on ecosystems are possible, as are numerous types of indirect interactions. Predicting such impacts is no mean feat, and the difficulty is exacerbated by the fact that introduced species can disperse and evolve. Current regulation of introduced biological—control agents, particularly of entomophages, is insufficient. At the very least, strong consideration should be given to the likely impact of both the pest and its natural enemy on natural ecosystems and their species, and not only on potential costs to agriculture, silvi—culture, and species of immediate commercial value.

We used sets of meta‐analyses to review the evidence of sex‐biased herbivory in dioecious plants as well as intersexual differences in plant characteristics that might affect herbivores. Thirtythree studies were reviewed to assess the effects of plant sex on herbivore abundance, survivorship and damage imposed to host plants, whereas 54 studies were reviewed for the effects of plant sex on plant morphological, physiological, and nutritional characteristics. The standardized mean difference between males and females was chosen as the measure to calculate effect sizes. Male plants exhibited significantly higher numbers of herbivores (d++=1.074) and significantly higher herbivory measured in terms of plant damage (d++=0.577) compared to female plants. Effects of plant sex on herbivore abundance were stronger for folivores and gallformers compared to the other guilds, whereas effects of plant sex on herbivore damage were stronger for flower predators compared to browsers and folivores. No difference in herbivore survivorship was observed between sexes. Male plants exhibited significantly more leaves (d++=0.202), larger leaves (d++=0.91), fewer flowers (d++=−0.89) and longer stems (d++=0.614) than female plants. Although male plants exhibited significantly lower concentrations of secondary compounds (d++=−0.209) and other defenses (d++=−0.53) than female plants, no difference in nutrient concentration, such as foliar nitrogen, was observed between sexes.

I examined case studies of density-related parasitism in insect host-parasitoid systems reported in the literature. For 171 examples, the frequency of density dependence was 25% (n = 43), of inverse density dependence 23% (n = 39), and of independence 52% (n = 89). This is a considerably lower frequency of density dependence than previously reported in other reviews (Lawton and McNeill 1979, Lessens 1985, but see Dempster 1983. Biological features of case histories gave few clues as to the expected frequency of density dependence. Such features included host order, life stage attacked, parasite group, native vs. introduced status of hosts or parasites, number of attacking parasitoid species, mobile vs. sedentary hosts, and monophagous vs. polyphagous hosts or parasitoids. Biotic and abiotic factors may, however, provide good reasons for a low overall frequency of density dependence.

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There is much interest in assessing the relative importance of ''top-down'' (from the trophic level above) vs. ''bottom-up'' (from the trophic level below) effects in ecological communities. This study compares the strength of top-down effects of parasites and bottom-up effects of plants on herbivores.We established populations of sea-oxeye daisy, Borrichia frutescens, and gall-making flies, Asphondylia borrichiae, on 12 separate off-shore islands in Pinellas County, Florida (USA). High levels of parasitism were obtained by adding high densities of parasitized galls, and high levels of host-plant nitrogen were obtained by adding fertilizer. We established three replicates of each of the following treatments: low parasitism, low nitrogen; low parasitism, high nitrogen; high parasitism, low nitrogen; and high parasitism, high nitrogen.There was a significant interaction of parasitism and plant nitrogen: parasitism was important only on high-nitrogen plants where galls were abundant. Thus, our results contribute new insights into top-down vs. bottom-up effects: bottom-up forces set the stage on which top-down forces may act.