Crop diversification offers a promising solution to meet expanding global food demands while maintaining ecosystem services. Diversification strategies that use mixed planting to reduce pest damage (e.g. intercropping), termed ‘associational effects’ (AE) in the ecological literature, can decrease (associational resistance) or increase (associational susceptibility) herbivore abundance on a focal plant. While application of AE to agroecosystems typically reduces pest abundance, the range of outcomes varies widely. We conducted a meta‐analysis using 272 estimates of insect herbivore abundance on crops neighbored by a conspecific or heterospecific from 44 studies undertaken on six continents. We focus on four agricultural crops well represented from sites across the globe to test hypotheses related to understanding how herbivore traits (diet breadth, feeding guild, origin), plant traits (crop type, phylogenetic distance to neighbour) and environmental context (climate, experimental design) contribute to variation in the outcomes of AE. Overall, bicultures provided a strong reduction of insect abundance on the focal crop. Climate and interactions between herbivore traits, particularly diet breadth and origin, and plant traits or environmental context mediated the strength of AE. Bicultures provided the strongest reductions in insect abundance at low latitudes, and this effect decreased at higher latitudes but only for insects with certain traits. Abundance of generalist herbivores and globally distributed pests tended to be most strongly negatively affected by bicultures, under certain contexts, whereas specialist herbivores and native pests were less affected by neighbours.
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Abstract Synthesis and application . This meta‐analysis highlights that crop diversification schemes have an overall strongly beneficial effect of reducing pest abundance. However, there was also variability in the outcomes that is determined in part by the interactive effects of herbivore traits and environmental context. The results provide guidance for incorporating beneficial ecological interactions into integrated pest management strategies. -
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Abstract A paradigm in the plant defence literature is that defending against herbivores comes at a cost to growth, resulting in a growth–defence trade‐off. However, while there is strong evidence for growth–defence trade‐offs across species, evidence is mixed within species.
Several mechanisms can account for this equivocal support within species, but teasing them apart requires examining growth–defence relationships both within and among populations, an approach seldom employed.
We examined correlations between plant biomass (growth) and terpene production (defence) within and among populations of
Monarda fistulosa , a perennial herb. We sampled populations from Montana and Wisconsin, regions that differ in resource availability characterized by different summer precipitation and associated abiotic conditions that influence plant productivity.We found negative, neutral and positive growth–defence correlations, depending on the scale examined. Negative correlations occurred across populations originating from divergent regions, positive correlations occurred across populations originating from within the high‐resource region and neutral correlations were found within single populations.
Collectively, these results challenge the general expectation of ubiquitous trade‐offs and support emerging views that resource availability (as it affects productivity) shapes the evolution of defence at different scales.
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Plain Language Summary can be found within the Supporting Information of this article. -
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Abstract Increasingly intensive strategies to maintain biodiversity and ecosystem function are being deployed in response to global anthropogenic threats, including intentionally introducing and eradicating species via assisted migration, rewilding, biological control, invasive species eradications, and gene drives. These actions are highly contentious because of their potential for unintended consequences. We conducted a global literature review of these conservation actions to quantify how often unintended outcomes occur and to elucidate their underlying causes. To evaluate conservation outcomes, we developed a community assessment framework for systematically mapping the range of possible interaction types for 111 case studies. Applying this tool, we quantified the number of interaction types considered in each study and documented the nature and strength of intended and unintended outcomes. Intended outcomes were reported in 51% of cases, a combination of intended outcomes and unintended outcomes in 26%, and strictly unintended outcomes in 10%. Hence, unintended outcomes were reported in 36% of all cases evaluated. In evaluating overall conservations outcomes (weighing intended vs. unintended effects), some unintended effects were fairly innocuous relative to the conservation objective, whereas others resulted in serious unintended consequences in recipient communities. Studies that assessed a greater number of community interactions with the target species reported unintended outcomes more often, suggesting that unintended consequences may be underreported due to insufficient vetting. Most reported unintended outcomes arose from direct effects (68%) or simple density‐mediated or indirect effects (25%) linked to the target species. Only a few documented cases arose from more complex interaction pathways (7%). Therefore, most unintended outcomes involved simple interactions that could be predicted and mitigated through more formal vetting. Our community assessment framework provides a tool for screening future conservation actions by mapping the recipient community interaction web to identify and mitigate unintended outcomes from intentional species introductions and eradications for conservation.
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Abstract Recent declines in wild bee populations have led to increases in conservation actions and monitoring of bee communities. Pan traps are a commonly used sampling method for monitoring bee populations due to their efficiency and low cost. However, potential biases inherent in different sampling techniques may result in misleading characterizations of bee communities across space and time.
In this paper, we examined how bee communities sampled using pan traps and aerial nets changed seasonally, and if they were affected by the availability of floral resources.
We found strong seasonal changes in the abundance, but not the richness, of bees captured in pan traps. Notably, we captured the fewest bees during weeks in spring when most flowering plant species were in bloom, suggesting that floral resource availability influences pan trap captures. We also compared patterns of bee abundance in pan traps to those captured by aerial netting. Bee richness in pans and nets was positively correlated, but relative abundances in pan and net samples were dominated by different bee genera. Furthermore, most genera decreased in pans with increasing floral richness, but patterns were mixed for nets. When using presence/absence data, rather than abundance, community composition was more similar between netted and pan‐trapped bee communities and changed less substantially across the floral richness gradient.
Overall, these differences led to sampling substantially different bee community compositions in pan traps versus nets, especially when using abundance‐based methods to characterize the bee community. By examining multiple years of intensive seasonal sampling of plant and bee communities, we document potential pitfalls with methods commonly used to sample bee communities.
We suggest that pan trapping and aerial netting provide similar estimates of bee species richness and community composition when using presence/absence data, but that practitioners should interpret pan‐trapped bee abundance data with caution especially when comparing bee communities between sites where plant communities may differ.
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In plants, chemical and mechanical traits are involved in the defense against environmental stressors. In the case of Mimosa plants, touch-sensitive responses through leaf-closure provide a unique form of mechanical defense. While past research has shown that the impacts of stressors such as competition or short-term drought may cause the chemical defenses in plants to increase or decrease, it is unknown how the interaction of these stressors may influence the efficiency of touch-sensitive responses. In this study, we hypothesized that the touch-sensitive responses of Mimosa strigillosa would strongly differ from regularly watered individuals when subjected to short-term drought. Additionally, we hypothesized that competition would hinder the touch-sensitive response, particularly from competitors in closely related taxa. To test these hypotheses, an experimental design was performed with three competition groups of Mimosa. In each competition group, half of the individuals were subjected to drought conditions. Plants were then subjected to a tactile stimulus in a greenhouse environment, with the degree of leaf closure being measured as an estimate of recovery. Drought was found to be a significant factor of both the rate of change of leaf closure, and the degree of leaf closure.more » « less
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