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1. Biogeographic variation of distance‐dependent effects in an invasive tree species

   https://doi.org/10.1111/1365-2435.13306  

   Yang, Qiang ; Ding, Jianqing ; Siemann, Evan ; Godoy, ed., Oscar ( March 2019 , Functional Ecology)

   Plant pathogens and herbivores can maintain forest diversity by reducing survival of tree seedlings close to conspecifics. However, how biogeographic variation in these natural enemies affects such distance‐dependent processes is unknown. Because invasive plants escape ecologically important enemies when introduced to a new range, distance‐dependent mortality may differ between their native and introduced ranges.

   Here, we test whether the invasive treeTriadica sebiferaescaped distance‐dependent mortality when introduced to the United States from China, and examine the roles of natural enemies in native and introduced ranges. In both the United States and China, we performed field surveys along with field and greenhouse experiments with field‐collected soils and soil sterilization treatments.

   In field surveys and the field experiment, insect damage onT. sebiferaseedlings decreased with distance to conspecific trees in the native range (China), but damage was low at all distances in the introduced range (United States). In the greenhouse experiment testing the effects of soil pathogens,T. sebiferaseedling mortality decreased with soil distance from conspecific trees in both ranges but distance‐independent mortality was higher in native range soils.

   Our findings indicate that both insect herbivores and the soil biota contribute to distance‐dependent effects onT. sebiferain its native range. They suggest, however, that plants may more readily escape herbivore than soil biota distance‐dependent effects when introduced to a new range and so herbivores, rather than soil pathogens, contribute more strongly to biogeographic variation in distance‐dependent effects. These results highlight the importance of considering species biogeographic variation in distance‐dependent effects and teasing apart the roles that different natural enemies play when studying species coexistence, community diversity and biological invasions.

    

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2. Forest cover and proximity decrease herbivory and increase crop yield via enhanced natural enemies in soybean fields

   https://doi.org/10.1111/1365-2664.13732  

   González, Ezequiel ; Landis, Douglas A. ; Knapp, Michal ; Valladares, Graciela ; Isaac, ed., Marney ( August 2020 , Journal of Applied Ecology)

   Non‐crop habitats are essential for sustaining biodiversity of beneficial arthropods in agricultural landscapes, which can increase ecosystem services provision and crop yield. However, their effects on specific crop systems are less clear, such as soybean in South America, where the responses of pests and natural enemies to landscape structure have only recently been studied.

   Here, we analysed how native forest fragments at local and landscape scales influenced arthropod communities, herbivory and yield in soybean fields in central Argentina. To do this, we selected soybean fields located in agricultural landscapes with varying proportions of forest cover. At two distances (10 and 100 m) from a focal forest fragment, we sampled natural enemy and herbivore arthropods, and measured soybean herbivory and yield. We focused on herbivore diversity, abundance of key soybean pests in the region (caterpillars and stink bugs), and their generalist and specialist natural enemies.

   Higher abundance of predators, lower herbivory rates and increased yield were found near forests, while overall forest cover in the landscape was positively related with parasitoid and stink bug abundance, soybean yield, and negatively with herbivory. Moreover, yield was positively linked to richness and abundance of generalist and specialist enemies and independent of herbivory according to piecewise Structural Equation Models.

   Synthesis and applications. Our results show positive effects of native forests on biodiversity and yield in soybean crops, highlighting the need for conservation of forest fragments in agricultural landscapes. Moreover, the relation between natural enemies and crop yield suggests that Chaco forests support a diverse and abundant community of natural enemies that can provide sustained levels of ecosystem services and result in positive effects for farmers.

    

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3. Introduced mangroves escape damage from marine and terrestrial enemies

   https://doi.org/10.1002/ecy.3604  

   Davidson, Timothy M. ; Smith, Celia M. ; Torchin, Mark E. ( January 2022 , Ecology)

   The enemy release hypothesis (ERH) posits that introduced species often leave their enemies behind when introduced to a new range. This release from enemies may allow introduced species to achieve higher growth and reproduction and may explain why some invaders flourish in new locations. Red mangroves (Rhizophora mangle) were introduced to Hawaiʻi from Florida over a century ago. Because Hawaiʻi has no native mangroves, the arrival ofR. manglefundamentally changed the structure and function of estuarine shorelines. While numerous enemies affect red mangroves in their native range (tropical America), in Hawaiʻi, mangroves apparently experience little herbivory, which may explain why introduced mangroves are so productive, fecund, and continue to spread. In this study, we compared the effects of enemies in native and introduced populations of brackish red mangroves (R. mangle) in 8–10 sites in the native range (Florida, Belize, and Panama) and introduced range of mangroves (Hawaiʻi). At each site, we measured the (1) occurrence of enemies using timed visual surveys, (2) occurrence of damage to different mangrove structures (leaves, apical buds, dead twigs, roots, propagules, and seedlings), and (3) rate of propagule herbivory using tethering experiments. Consistent with the ERH, we found an order of magnitude less damage and fewer enemies in introduced than native mangrove sites. While introduced mangroves harbored few enemies and minimal damage, native mangroves were affected by numerous enemies, including leaf‐eating crabs, specialist bud moths, wood‐boring insects and isopods, and propagule predators. These patterns were consistent across all plant structures (roots to leaves), among marine and terrestrial enemies, and across functional groups (browsers, borers, pathogens, etc.), which demonstrates enemy escape occurs consistently among different functional groups and via trophic (e.g., herbivores) and non‐trophic (e.g., root borers) interactions. Our study is among the first biogeographical enemy release studies to take a comprehensive approach to quantifying the occurrence of damage from a broad suite of marine and terrestrial taxa across an array of wetland plant structures. Understanding how natural enemies alter this key foundation species will become increasingly relevant globally as mangroves continue to invade new regions through intentional plantings or range expansion driven by climate change.

    

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4. Natural Enemies and the Maintenance of Tropical Tree Diversity: Recent Insights and Implications for the Future of Biodiversity in a Changing World

   https://doi.org/10.3417/2020591  

   Comita, Liza S. ; Stump, Simon M. ( September 2020 , Annals of the Missouri Botanical Garden)

   null (Ed.)

   Over the past five decades, many studies have examined the Janzen-Connell hypothesis, which posits that host-specific natural enemies, such as insect herbivores and fungal pathogens, promote plant species coexistence by providing a recruitment advantage to rare plant species. Recently, researchers have been exploring new and exciting angles on plant-enemy interactions that have yielded novel insights into this long-standing hypothesis. Here, we highlight some empirical advances in our understanding of plant-enemy interactions in tropical forests, including improved understanding of variation in plant species’ susceptibility to enemy effects, as well as insect and pathogen host ranges. We then review recent advances in related ecological theory. These theoretical studies have confirmed that specialist natural enemies can promote tree diversity. However, they have also shown that the impact of natural enemies may be weakened, or that natural enemies could even cause species exclusion, depending on enemy host range, the spatial extent of enemy effects, and variation among plant species in seed dispersal or enemy susceptibility. Finally, we end by discussing how human impacts on tropical forests, such as fragmentation, hunting, and climate change, may alter the plant-enemy interactions that contribute to tropical forest diversity. 

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5. Invaders responded more positively to soil biota than native or noninvasive introduced species, consistent with enemy escape

   https://doi.org/10.1007/s10530-022-02919-y  

   Liu, Yu ; Zheng, Yu−Long ; Jahn, Lydia V. ; Burns, Jean H. ( February 2023 , Biological Invasions)

   Understanding the mechanisms governing biological invasions has implications for population dynamics, biodiversity, and community assembly. The enemy escape hypothesis posits that escape from enemies such as herbivores and predators that were limiting in the native range helps explain rapid spread in the introduced range. While the enemy escape hypothesis has been widely tested aboveground, data limitations have prevented comparisons of belowground mechanisms for invasive and noninvasive introduced species, which limits our understanding of why only some introduced species become invasive. We assessed the role of soil biota in driving plant invasions in a phylogenetic meta−analysis, incorporating phylogeny in the error structure of the models, and comparing live and sterilized conditioned soils. We found 29 studies and 396 effect size estimates across 103 species that compared live and sterilized soils. We found general positive effects of soil biota for plants (0.099, 95% CI = 0.0266, 0.1714), consistent with a role of soil mutualists. The effect size of soil biota among invaders was 3.2× higher than for natives, the strength of effects was weaker for older conditioning species with a longer introduced history, and enemy escape was stronger for distant relatives. In addition, invasive species had a weaker allocation tradeoff than natives. By demonstrating that the net effect of soil biota is more positive for invasive than native and noninvasive introduced species, weakens over time since introduction, and strengthens as phylogenetic distance increasing, we provide mechanistic insights into the considerable role of soil biota in biological invasions, consistent with the predictions of the enemy escape hypothesis. 

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