Invasive alien insects represent a clear but variable threat to biodiversity

Strength of Enemy Release From Parasitoids Is Context Dependent in the Invasive African Fig Fly, Zaprionus indianus

Understanding the mechanisms underlying the success of biological invasions is essential to employ effective prediction and management strategies. Escape from natural enemies in invaded regions (enemy release hypothesis, ERH) and increased competitive ability are hallmarks of invasive species; however, these two processes are rarely studied within the same context. Here, we examined the effect of enemy release on the competition outcomes of a successful invasive insect pest in North America, the African fig fly (Zaprionus indianus). Parasitoid wasps such as Leptopilina heterotoma that parasitize drosophilid larvae may seek out established species with known host suitability over a novel species, so we hypothesized Z. indianus may have low susceptibility to parasitoids, giving them a competitive advantage over co-occurring drosophilids. We tested this hypothesis by comparing the adult emergence rates from Z. indianus larvae reared alone or in competition with Drosophila hydei or D. simulans larvae in the presence and absence of parasitoid wasps under low and high larval densities. At low larval densities, Z. indianus emerged at equal rates to D. hydei but outcompeted D. simulans, and these outcomes were not affected by parasitoids. However, at high densities, the addition of parasitoids shifted competition outcomes in favor of Z. indianus, suggesting enemy release provides a competitive advantage under some circumstances. These results indicate that the strength of enemy release in Z. indianus is widely dependent on contextual factors such as density and competitor species. This study emphasizes how a community approach to testing the ERH is vital as the overall interpretation of the presence and strength of enemy release differed between intraspecific and interspecific experiments. Further investigation of how these results apply to field environments could offer insight into how Z. indianus alters ecosystems and how productive biological control may limit the spread of Z. indianus.

Strength of enemy release from parasitoids is context-dependent in the invasive African Fig Fly, Zaprionus indianus

Understanding the mechanisms underlying the success of biological invasions is essential to employ effective prediction and management strategies. Escape from natural enemies in invaded regions (enemy release hypothesis, ERH) and increased competitive ability are hallmarks of invasive species; however, these two processes are rarely studied within the same context. Here, we examined the effect of enemy release on the competition outcomes of a successful invasive insect pest in North America, the African fig fly (Zaprionus indianus). Parasitoid wasps such as Leptopilina heterotoma that parasitize drosophilid larvae may seek out established species with known host suitability over a novel species, so we hypothesized Z. indianus may have low susceptibility to parasitoids, giving them a competitive advantage over co-occurring drosophilids. We tested this hypothesis by comparing the adult emergence rates from Z. indianus larvae reared alone or in competition with Drosophila hydei or D. simulans larvae in the presence and absence of parasitoid wasps under low and high larval densities. At low larval densities, Z. indianus emerged at equal rates to D. hydei but outcompeted D. simulans, and these outcomes were not affected by parasitoids. However, at high densities, the addition of parasitoids shifted competition outcomes in favor of Z. indianus, suggesting enemy release provides a competitive advantage under some circumstances. These results indicate that the strength of enemy release in Z. indianus is widely dependent on contextual factors such as density and competitor species. This study emphasizes how a community approach to testing the ERH is vital as the overall interpretation of the presence and strength of enemy release differed between intraspecific and interspecific experiments. Further investigation of how these results apply to field environments could offer insight into how Z. indianus alters ecosystems and how productive biological control may limit the spread of Z. indianus.

Counteractive effects of predator invasion and habitat destruction on predator-prey systems

Alien species invasion and habitat destruction are among the primary threats to native animal communities, particularly for native predator-prey systems. However, when predator invasion and habitat destruction co-occur, it remains unclear whether their respective threats to native systems compensate each other or accumulate, as well as how these effects respond to the different characteristics of predator invasion and habitat destruction. In this study, we developed a spatially explicit simulation model with one prey species and one predator species and exposed it to invasive predators and habitat destruction with different properties. The results revealed the following insights: (1) Habitat destruction can compensate threats to native predator-prey systems from global predator invasion only when native predators possess predation capability similar to those of the invaders. In other scenarios, cumulative effects arise from predator invasion and habitat destruction. (2) Low levels of habitat destruction occurring at a faster rate, in conjunction with a substantial number of global invasive predators being present, can better compensate their respective threats to native predator-prey systems than the other scenarios. These findings provide valuable insights into situations where habitat destruction and alien species invasion coincide. They raise the question of whether we can leverage the interaction between them to reduce threats to biodiversity.

Global indicators of the environmental impacts of invasive alien species and their information adequacy

Monitoring the extent to which invasive alien species (IAS) negatively impact the environment is crucial for understanding and mitigating biological invasions. Indeed, such information is vital for achieving Target 6 of the Kunming-Montreal Global Biodiversity Framework. However, to-date indicators for tracking the environmental impacts of IAS have been either lacking or insufficient. Capitalizing on advances in data availability and impact assessment protocols, we developed environmental impact indicators to track realized and potential impacts of IAS. We also developed an information status indicator to assess the adequacy of the data underlying the impact indicators. We used data on 75 naturalized amphibians from 82 countries to demonstrate the indicators at a global scale. The information status indicator shows variation in the reliability of the data and highlights areas where absence of impact should be interpreted with caution. Impact indicators show that growth in potential impacts are dominated by predatory species, while potential impacts from both predation and disease transmission are distributed worldwide. Using open access data, the indicators are reproducible and adaptable across scales and taxa and can be used to assess global trends and distributions of IAS, assisting authorities in prioritizing control efforts and identifying areas at risk of future invasions. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.