We investigate where bottom‐up and top‐down control regulates ecological communities as a mechanism linking ecological gradients to the geography of consumer abundance and biomass. We use standardized surveys of 54 North American grasslands to test alternate hypotheses predicting 100‐fold shifts in the biomass of four common grassland arthropod taxa—Auchenorrhyncha, sucking herbivores, Acrididae, chewing herbivores, Tettigoniidae, omnivores, and Araneae, predators. Bottom‐up models predict that consumer biomass tracks plant quantity (e.g. productivity and standing biomass) and quality (nutrient content) and that ectotherm access to food increases with temperature. Each of the focal trophic groups responded differently to these drivers: the biomass of sucking herbivores and omnivores increased with plant biomass; that of chewing herbivores tracked plant quality; and predator biomass did not depend on plant quality, plant quantity or temperature. The Exploitation Ecosystem Hypothesis is a top‐down hypothesis that predicts a shift from resource limitation of herbivores when plant production is low, to predator limitation when plant production is high. In grasslands where spider biomass was low, herbivore biomass increased with plant biomass, whereas bottom‐up structuring was not evident when spiders were abundant. Furthermore, neither predator biomass nor trophic position (via stable isotope analysis) increased with plant biomass, suggesting predators themselves are top‐down limited. Stable isotope analysis revealed that trophic position of the chewing herbivore and omnivore increased significantly with plant biomass, suggesting these groups increased scavenging and meat consumption in grasslands with higher carbohydrate availability. Taken together, our snapshot sampling documents gradients of food web structure across 54 grasslands, consistent with multiple hypotheses of bottom‐up and top‐down regulation.
Video surveillance reveals a community of largely nocturnal Danaus plexippus (L.) egg predators
Monarch butterfly (Danaus plexippus L.) declines in eastern North America have prompted milkweed host plant restoration efforts in non-agricultural grasslands. However, grasslands harbor predator communities that exert high predation pressure on monarch eggs and larvae. While diurnal monarch predators are relatively well known, no studies have investigated the contribution of nocturnal monarch predators. We used video cameras to monitor sentinel monarch eggs and fourth instars on milkweed in southern Michigan to identify predators and determine if nocturnally-active species impose significant predation pressure. We observed ten arthropod taxa consuming monarch eggs and larvae, with 74% of egg predation events occurring nocturnally. Taxa observed attacking monarch eggs included European earwigs (Forficula auricularia L.), tree crickets (Oecanthus sp.), lacewing larvae (Neuroptera), plant bugs (Miridae), small milkweed bugs (Lygaeus kalmii Stal), ants (Formicidae), spiders (Araneae: Salticidae and other spp.), harvestmen (Opiliones), and velvet mites (Trombidiformes: Trombidiidae). Larvae were attacked by ground beetles (Calleida sp.), jumping spiders (Araneae: Salticidae), and spined soldier bugs (Podisus maculiventris Say). Our findings provide important information about monarch predator-prey interactions that could be used to develop strategies to conserve monarchs through reducing predation on early life stages.
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- Award ID(s):
- 1832042
- NSF-PAR ID:
- 10165190
- Date Published:
- Journal Name:
- Journal of Insect Conservation
- ISSN:
- 1366-638X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Schmidt-Jeffris, Rebecca (Ed.)Abstract Invasive black and pale swallow-worts (Vincetoxicum nigrum (L.) Moench, and Vincetoxicum rossicum Kelopow), which are related to milkweeds, can act as ecological traps for monarch butterflies (Danaus plexippus L. (Lepidoptera: Nymphalidae)) as they lay eggs on them that fail to develop. A recently approved biological control agent against swallow-worts, Hypena opulenta Christoph, occupies the same feeding guild on swallow-worts as monarch larvae and could be perceived as a competitor to monarchs. We tested how the presence of this defoliating moth on swallow-worts may influence monarch host selection. In a two-year field experiment, we placed pale swallow-wort plants that were either infested with H. opulenta or noninfested as well as common milkweed (Asclepias syriaca L.), into monarch habitats to assess oviposition rates. In the laboratory, monarchs were either given a choice or not between milkweeds and black swallow-worts with or without H. opulenta. While monarchs strongly preferred common milkweed in the field, up to 25% of the eggs we observed were laid on pale swallow-wort, without preference for swallow-wort with (10.7%) or without (14.3%) H. opulenta. In laboratory choice and no-choice tests, monarchs did not lay any eggs on black swallow-wort, likely because of the long-term laboratory rearing on common milkweeds. Our results confirm that pale swallow-wort may act as an oviposition sink to monarchs in Michigan as well. Since the biological control program is still in its infancy, the nature of interactions between monarchs and H. opulenta may change as the biocontrol agent becomes more widespread.more » « less
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null (Ed.)Background Oviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies ( Danaus plexippus ) rely on milkweeds ( Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies. Methodology/Principal Findings We first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca . We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa , and (2) A. verticillata and A. incarnata . Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35–40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20–30% each) but more eggs on regrown plants (40–60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance. Conclusions/Significance Overall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage.more » « less
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Abstract Herbivorous insects have evolved many mechanisms to overcome plant chemical defences, including detoxification and sequestration. Herbivores may also use toxic plants to reduce parasite infection. Plant toxins could directly interfere with parasites or could enhance endogenous immunity. Alternatively, plant toxins could favour down‐regulation of endogenous immunity by providing an alternative (exogenous) defence against parasitism. However, studies on genomewide transcriptomic responses to plant defences and the interplay between plant toxicity and parasite infection remain rare. Monarch butterflies (
Danaus plexippus ) are specialist herbivores of milkweeds (Asclepias spp.), which contain toxic cardenolides. Monarchs have adapted to cardenolides through multiple resistance mechanisms and can sequester cardenolides to defend against bird predators. In addition, high‐cardenolide milkweeds confer monarch resistance to a specialist protozoan parasite (Ophryocystis elektroscirrha ). We used this system to study the interplay between the effects of plant toxicity and parasite infection on global gene expression. We compared transcriptional profiles between parasite‐infected and uninfected monarch larvae reared on two milkweed species. Our results demonstrate that monarch differentially express several hundred genes when feeding onA. curassavica andA. incarnata , two species that differ substantially in cardenolide concentrations. These differentially expressed genes include genes within multiple families of canonical insect detoxification genes, suggesting that they play a role in monarch toxin resistance and sequestration. Interestingly, we found little transcriptional response to infection. However, parasite growth was reduced in monarchs reared onA. curassavica , and in these monarchs, several immune genes were down‐regulated, consistent with the hypothesis that medicinal plants can reduce reliance on endogenous immunity. -
Abstract Diverse and robust predator communities are important for effective prey suppression in natural and managed communities. Ants are ubiquitous components of terrestrial systems but their contributions to natural prey suppression is relatively understudied in temperate regions. Growing evidence suggests that ants can play a significant role in the removal of insect prey within grasslands, but their impact is difficult to separate from that of nonant predators. To test how ants may contribute to prey suppression in grasslands, we used poison baits (with physical exclosures) to selectively reduce the ant population in common garden settings, then tracked ant and nonant ground predator abundance and diversity, and removal of sentinel egg prey for 7 wk. We found that poison baits reduced ant abundance without a significant negative impact on abundance of nonant ground predators, and that a reduction in ant abundance decreased the proportion of sentinel prey eggs removed. Even a modest decrease (~20%) in abundance of several ant species, including the numerically dominant Lasius neoniger Emery (Hymenoptera: Formicidae), significantly reduced sentinel prey removal rates. Our results suggest that ants disproportionately contribute to ground-based predation of arthropod prey in grasslands. Changes in the amount of grasslands on the landscape and its management may have important implications for ant prevalence and natural prey suppression services in agricultural landscapes.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s10841-020-00248-w
