Invertebrate Predators Research Articles

Land-use effects on structural and functional composition of benthic and leaf-associated macroinvertebrates in four Andean streams

The replacement of native forests by pastures takes place widely in the Andes. The effects of such land-use change on aquatic assemblages are poorly understood. We conducted a comparative analysis of the effects of forest conversion to pastures on the taxonomic, structural, and functional composition of macroinvertebrates (benthic and leaf-associated) in montane and upper montane streams (ecosystem type) of the south Ecuadorian Andes. Taxonomic composition of benthic and leaf-associated macroinvertebrates was different between ecosystem type and land use. Also, major differences in the structural and functional composition of benthic and leaf-associated macroinvertebrates were mainly promoted by land use in both ecosystem types. Forested streams showed higher diversity than pasture streams, sustaining more shredder, scraper, and predatory invertebrates. We also observed differences in the macroinvertebrate communities between benthic and leaf-bag samples. Leaf bags had lower diversity and more collector invertebrates than benthic samples. This study highlights the large effect of riparian forest conversion to pasture land on macroinvertebrate communities, and the importance of using appropriate sampling techniques to characterize aquatic assemblages. We also recommend the maintenance and restoration of riparian vegetation to mitigate the effects of deforestation on stream communities and ecosystem processes.

Patterns and causes of oviposition in monarch butterflies: Implications for milkweed restoration

Effective habitat restoration requires an understanding of species habitat preferences and the associated mechanisms driving those preferences. We examined the patterns and causes of oviposition preference in the monarch butterfly, a rapidly declining species, in southwestern Ontario at both landscape and milkweed patch spatial scales. Additionally, we measured the abundance of invertebrate predators, parasitoids and parasites across these same spatial scales. Oviposition preference was dependent on both the size of the milkweed patch and the density of milkweed within the patch, as well as landscape type. Small (<16m2), low-density (0.1–2 milkweed per m2) milkweed patches in agricultural landscape had the highest egg density compared to all types of milkweed patches in non-agricultural and roadside landscapes. Medium-sized patches had the highest predator abundance. Variation in the abundance of parasitoids, and occurrence of parasites of monarch eggs and larvae did not appear to coincide with preferred egg laying habitats. Our results suggest that investing heavily in milkweed restoration in roadside habitats should be done cautiously. Instead, a better strategy may be for managers to develop incentive programs with landowners to plant and maintain milkweeds in agricultural landscapes, which could complement other pollinator initiatives or ecosystem service programs in agricultural landscapes that focus on increasing nectar availability. Our results have important implications for restoring milkweed as an approach to counteract monarch butterflies declines.

Plant diversity increases predation by ground‐dwelling invertebrate predators

AbstractGlobal declines in biodiversity have raised concerns over the implications of diversity loss for the functioning of ecosystems. Plant diversity loss has impacts throughout food webs affecting both consumer communities and ecosystem functions mediated by consumers. Effects of plant diversity loss on communities of invertebrate predators have been documented, yet little is known about how these translate into variations in predation rates. We measured predation rates along two plant diversity gradients in grassland experiments manipulating species richness and functional diversity. Measurements were conducted at two different heights (ground and vegetation) and in two different seasons (spring and summer), using three different types of baits. Our results show that overall predation rates increase with plant species richness, but effects are seasonally variable and are much more pronounced on the ground than in the vegetation. Plant functional diversity did not consistently affect predation rates in our experiments. Potential mechanistic explanations for an effect of plant diversity on predation include higher complementarity between predator species or reduced intraguild predation with increasing structural complexity at higher plant diversity. These results underline the importance of high local plant diversity for natural pest control.

The influence of forest management on the abundance and diversity of hoverflies in commercial plantations of Sitka spruce: the importance of sampling in the canopy

Abstract The majority of Sitka spruce plantations in the UK are managed by clear-cutting and replanting, which leads to the formation of relatively large blocks (5–30 ha) of even-aged, closely spaced trees. These stands are considered to be poor in terms of general biodiversity and vulnerable to outbreaks of insect pests and diseases. In contrast, stands managed using alternative silvicultural systems, such as shelterwood or group selection systems, contain a wider range of tree ages and sizes and have the potential to support greater numbers of other species. If this includes the natural enemies (predators, parasitoids) of pest species, then adopting alternative silvicultural systems might help to reduce current and future pest impacts. To determine whether the greater structural complexity of stands managed using alternative silvicultural systems is associated with an increase in insect diversity and an increase in the abundance of predatory species, we sampled hoverflies (Diptera: Syrphidae) in 5 shelterwood stands, 6 group selection stands and 13 even-aged stands of Sitka spruce at 4 forest sites in mid and north Wales. The larvae of aphidophagous hoverflies are important invertebrate predators in spruce plantations, and hoverfly species richness is often used as an indicator of general invertebrate diversity. Hoverflies were sampled at ground level and in the canopy using yellow pan traps and flight interception traps. 7323 adult hoverflies of 74 species were captured, the majority of these in traps in the canopy. Total hoverfly abundance and the abundance of aphidophagous species were significantly higher ( P

Increasing habitat complexity on seawalls: Investigating large‐ and small‐scale effects on fish assemblages

The construction of artificial structures in the marine environment is increasing globally. Eco‐engineering aims to mitigate the negative ecological impacts of built infrastructure through designing structures to be multifunctional, benefiting both humans and nature. To date, the focus of eco‐engineering has largely been on benefits for benthic invertebrates and algae. Here, the potential effect of eco‐engineered habitats designed for benthic species on fish was investigated. Eco‐engineered habitats (“flowerpots”) were added to an intertidal seawall in Sydney Harbour, Australia. Responses of fish assemblages to the added habitats were quantified at two spatial scales; large (among seawalls) and small (within a seawall). Data were collected during high tide using cameras attached to the seawall to observe pelagic and benthic fish. At the larger spatial scale, herbivores, planktivores, and invertebrate predators were generally more abundant at the seawall with the added flowerpots, although results were temporally variable. At the smaller spatial scale, certain benthic species were more abundant around flowerpots than at the adjacent control areas of seawall, although there was no general pattern of differences in species density and trophic group abundance of pelagic fish between areas of the seawall with or without added habitats. Although we did not find consistent, statistically significant findings throughout our study, the field of research to improve fish habitat within human‐use constraints is promising and important, although it is in its early stages (it is experimental and requires a lot of trial and error). To advance this field, it is important to document when effects were detected, and when they were not, so that others can refine the designs or scale of habitat enhancements or their study approaches (e.g., sampling protocols).

Mammal‐induced trophic cascades in invertebrate food webs are modulated by grazing intensity in subalpine grassland

Even though mammalian herbivores can exert strong indirect effects on other animals by altering the vegetation, the study of trophic cascades retains a focus on apex predators and their top-down forces. Bottom-up trophic interaction chains induced by mammalian herbivores, particularly in invertebrate food webs, remain largely unexplored. We tested whether effects of mammalian herbivores on the vegetation ricochet back up several trophic levels of the invertebrate food web. We further tested two alternative hypotheses: the strength of herbivore-induced indirect interactions either increases with plant productivity because of a concurrent higher grazing intensity, or it decreases because of a higher plant tolerance to grazing. We progressively excluded large, medium and small herbivorous mammals from replicated plots of 6m2 in productive, intensively grazed short-grass vegetation and less productive, less intensively grazed tall-grass vegetation of subalpine grasslands. We measured vegetation quantity, quality, structure and composition, and determined the abundance of invertebrate herbivores, detritivores, omnivores and predators. We used structural equation modelling to test vegetation-mediated cascading effects of the different mammalian herbivores across different trophic groups of invertebrates. In the short-grass vegetation, mammals caused changes in vegetation quantity and thickness. These changes directly affected detritivorous and predatory invertebrate abundance, yet indirectly affected predatory and omnivorous invertebrates through a bottom-up trophic cascade via changes in herbivorous invertebrate abundance. In the tall-grass vegetation, mammal-induced changes in vegetation quality and composition affected detritivorous invertebrates and in turn omnivorous invertebrates, but these cascading effects were weaker than those in the short-grass vegetation. Smaller mammals were at least as important as large mammals in structuring the invertebrate food web. Our results demonstrate that differently sized mammalian herbivores can trigger trophic cascades in the local invertebrate food web. Our findings further support the hypothesis that herbivore-induced indirect interactions are stronger in more productive systems because of higher foraging intensity, as opposed to the hypothesis that a higher grazing tolerance of plants should dampen herbivore-induced indirect interactions in productive systems.

Functional response and multiple predator effects of two generalist predators preying on Tuta absoluta eggs.

Interactions among invertebrate predators could affect a pest suppression. The hemipteran species Macrolophus pygmaeus (Rambur) and Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae) are natural enemies of several pests in agroecosystems and coexist in tomato crops in Mediterranean countries. By using the multiplicative risk model (MRM) and the substitutive model, the multiple predator effects (MPEs) on prey suppression were calculated when two individuals of the predators foraged at the same densities on South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), eggs. Egg consumption increased with increasing egg density and the two predators exhibited a type III functional response. Predation rates were strongly affected by prey density. Using the MRM, we found risk reduction at intraspecific treatments at high prey density. Applying the substitutive model, we detect risk enhancement at interspecific treatments at high egg density. At low prey densities, most of the interactions were independent, whereas at high densities most interactions were not independent and resulted in prey risk reduction, indicating antagonism between the individuals involved. We also showed that N. tenuis is a more competitive predator species for T. absoluta eggs than M. pygmaeus; however, combination of the two predator species will lead to better pest suppression at high T. absoluta population densities. © 2017 Society of Chemical Industry.

Energetic solutions of Rock Sandpipers to harsh winter conditions rely on prey quality

Rock SandpipersCalidris ptilocnemishave the most northerly non‐breeding distribution of any shorebird in the Pacific Basin (upper Cook Inlet, Alaska; 61°N, 151°W). In terms of freezing temperatures, persistent winds and pervasive ice, this site is the harshest used by shorebirds during winter. We integrated physiological, metabolic, behavioural and environmental aspects of the non‐breeding ecology of Rock Sandpipers at the northern extent of their range to determine the relative importance of these factors in facilitating their unique non‐breeding ecology. Not surprisingly, estimated daily energetic demands were greatest during January, the coldest period of winter. These estimates were greatest for foraging birds, and exceeded basal metabolic rates by a factor of 6.5, a scope of increase that approaches the maximum sustained rate of energetic output by shorebirds during periods of migration, but far exceeds these periods in duration. We assessed the quality of their primary prey, the bivalveMacoma balthica, to determine the daily foraging duration required by Rock Sandpipers to satisfy such energetic demands. Based on size‐specific estimates ofM. balthicaquality, Rock Sandpipers require over 13 h/day of foraging time in upper Cook Inlet in January, even when feeding on the highest quality prey. This range approaches the average daily duration of mudflat availability in this region (c. 18 h), a maximum value that annually decreases due to the accumulation of shore‐fast ice. Rock Sandpipers are likely to maximize access to foraging sites by following the exposure of ice‐free mudflats across the upper Cook Inlet region and by selecting smaller, higher qualityM. balthicato minimize foraging times. Ultimately, this unusual non‐breeding ecology relies on the high quality of their prey resources. Compared with other sites across their range,M. balthicafrom upper Cook Inlet have relatively light shells, potentially the result of the region's depauperate invertebrate predator community. Given the delicate balance between environmental and prey conditions that currently make Cook Inlet a viable wintering area for Rock Sandpipers, small variations in these variables may affect the suitability of the site in the future.

Evolution of a predator-induced, nonlinear reaction norm.

Inducible, anti-predator traits are a classic example of phenotypic plasticity. Their evolutionary dynamics depend on their genetic basis, the historical pattern of predation risk that populations have experienced and current selection gradients. When populations experience predators with contrasting hunting strategies and size preferences, theory suggests contrasting micro-evolutionary responses to selection. Daphnia pulex is an ideal species to explore the micro-evolutionary response of anti-predator traits because they face heterogeneous predation regimes, sometimes experiencing only invertebrate midge predators and other times experiencing vertebrate fish and invertebrate midge predators. We explored plausible patterns of adaptive evolution of a predator-induced morphological reaction norm. We combined estimates of selection gradients that characterize the various habitats that D. pulex experiences with detail on the quantitative genetic architecture of inducible morphological defences. Our data reveal a fine scale description of daphnid defensive reaction norms, and a strong covariance between the sensitivity to cues and the maximum response to cues. By analysing the response of the reaction norm to plausible, predator-specific selection gradients, we show how in the context of this covariance, micro-evolution may be more uniform than predicted from size-selective predation theory. Our results show how covariance between the sensitivity to cues and the maximum response to cues for morphological defence can shape the evolutionary trajectory of predator-induced defences in D. pulex.

Predators do not spill over from forest fragments to maize fields in a landscape mosaic in central Argentina.

South America is undergoing a rapid and large‐scale conversion of natural habitats to cultivated land. Ecosystem services still remain important but their level and sustainability are not known. We quantified predation intensity in an Argentinian agricultural landscape containing remnants of the original chaco serrano forest using artificial sentinel prey. We sought to identify the main predators and the effect of landscape configuration and maize phenology on predation pressure by invertebrate and vertebrate predators in this landscape. The most common predators were chewing insects (50.4% predation events), birds (22.7%), and ants (17.5%). Overall predation rates in forest fragments (41.6% per day) were significantly higher than in the surrounding maize fields (21.5% per day). Invertebrate predation was higher inside and at the edge of forest fragments than within fields, and did not change with increasing distance from a fragment edge, indicating a lack of spillover from the native habitat remnants to the cultivated matrix at the local scale. Distance from a continuous forest had a positive impact on predation by invertebrates and a negative impact on vertebrate predation.

Trophic niches, diversity and community composition of invertebrate top predators (Chilopoda) as affected by conversion of tropical lowland rainforest in Sumatra (Indonesia).

Conversion of tropical rainforests into plantations fundamentally alters ecological niches of animal species. Generalist predators such as centipedes (Chilopoda) may be able to persist in converted ecosystems due to their ability to adapt and switch to alternative prey populations. We investigated variations in community composition and trophic niches of soil and litter living centipedes in a range of ecosystems including rainforests, jungle rubber agroforests, and rubber and oil palm monocultures in two landscapes in Sumatra, Indonesia. Including information on environmental factors in the soil and litter habitat, we explored drivers shaping ecological niches of soil living invertebrate predators in one of the world’s hotspots of rainforest conversion. Conversion of rainforests into agroforests and plantations was associated with a marked change in the composition of centipede communities. However, irrespective of major differences in habitat characteristics, changes in total abundances were small and the overall diversity and biomass of centipedes was similar in each of the systems investigated, suggesting that the number of ecological niches for this group of predators remains unchanged. By using stable isotope analysis (15N and 13C), we investigated trophic niche shifts of the centipede community; lower δ13C values of centipedes in oil palm plantations as compared to other ecosystems suggests that centipedes switch from decomposer prey to other prey, presumably understory associated herbivores, due to reduced availability of litter associated prey species. The results suggest that the ability to utilize alternative prey is a key feature enabling invertebrate predators to persist in ecosystems undergoing major structural changes due to anthropogenic land use change.

Noxious newts and their natural enemies: Experimental effects of tetrodotoxin exposure on trematode parasites and aquatic macroinvertebrates

The dermal glands of many amphibian species secrete toxins or other noxious substances as a defense strategy against natural enemies. Newts in particular possess the potent neurotoxin tetrodotoxin (TTX), for which the highest concentrations are found in species within the genus Taricha. Adult Taricha are hypothesized to use TTX as a chemical defense against vertebrate predators such as garter snakes (Thamnophis spp.). However, less is known about how TTX functions to defend aquatic-developing newt larvae against natural enemies, including trematode parasites and aquatic macroinvertebrates. Here we experimentally investigated the effects of exogenous TTX exposure on survivorship of the infectious stages (cercariae) of five species of trematode parasites that infect larval amphibians. Specifically, we used dose-response curves to test the sensitivity of trematode cercariae to progressively increasing concentrations of TTX (0.0 [control], 0.63, 3.13, 6.26, 31.32, and 62.64 nmol L−1) and how this differed among parasite species. We further compared these results to the effects of TTX exposure (0 and 1000 nmolL−1) over 24 h on seven macroinvertebrate taxa commonly found in aquatic habitats with newt larvae. TTX significantly reduced the survivorship of trematode cercariae for all species, but the magnitude of such effects varied among species. Ribeiroia ondatrae – which causes mortality and limb malformations in amphibians – was the least sensitive to TTX, whereas the kidney-encysting Echinostoma trivolvis was the most sensitive. Among the macroinvertebrate taxa, only mayflies (Ephemeroptera) showed a significant increase in mortality following exogenous TTX exposure, despite the use of a concentration 16x higher than the maximum used for trematodes. Our results suggest that maternal investment of TTX into larval newts may provide protection against certain trematode infections and highlight the importance of future work assessing the effects of newt toxicity on both parasite infection success and the palatability of larval newts to invertebrate predators.

The facilitation of the native bluegill sunfish by the invasive bighead carp

Abstract Communities of organisms are shaped by a complex suite of positive and negative species interactions. Ecological phenomena like biological invasions typically evoke notions of negative effects on native communities. Yet, negative effects within specific food‐web components can also have positive feedbacks that manifest elsewhere within the food web. We designed an experiment to evaluate the direct and indirect effects of invasive bighead carp (Hypophthalmichthys nobilis: Cyprinidae) on planktonic and benthic invertebrates and the growth and survival of juvenile bluegill sunfish (Lepomis macrochirus: Centrarchidae). Our experiment indicated that the presence of bighead carp indirectly facilitated bluegill growth and survival. The underlying processes driving predator–predator facilitation stemmed from a combination of indirect interactions occurring through invertebrates in planktonic and vegetated habitats. Individual bluegill consumed more cladocerans and predatory macroinvertebrates in the presence of bighead carp. The presence of bighead carp appears to have indirectly influenced interactions among cladocerans, invertebrate predators, and bluegill. Understanding the cumulative direct and indirect effects of bigheaded carp on aquatic ecosystems requires the documentation of both negative and positive effects. Although one taxon benefitted from the presence of an invader, our findings demonstrate that this response occurred because bigheaded carp caused imbalances within the food web. We urge that future studies consider a priori how positive and negative interspecific interactions shape the structure of food webs.

Small vertebrate granivores reduce seedling emergence in native tallgrass prairie restoration

AbstractHigh seed cost and low rates of establishment make tallgrass prairie restorations challenging and expensive endeavors. Typical seedling emergence rates in prairie restorations are approximately 10% and the causes of seed mortality are poorly understood. In this study, we examined the impact of small vertebrate granivores on prairie restoration by comparison of seedling emergence in open (sham) versus closed exclosures at three newly restored sites. To assess other causes of seed loss, we also tracked seed fates at one prairie restoration site. We coated seeds of four prairie species with fluorescent dye, placed them under closed exclosures, and monitored their fate (emerging seedling, partially germinated, nongerminated/viable, and nongerminated/nonviable) over a 5‐month period. On average, 9.6 more seedlings/m2 emerged in the closed than the opened exclosures, suggesting that small vertebrate granivores reduce seedling emergence in prairie restoration. Granivores influenced the composition of the emerging community but did not preferentially consume large‐seeded species. In the seed‐tracking experiment, we found that greater than 70% of seeds were lost within 30 days of sowing, that seed recovery and viability both decreased with time in soil, and that seed fates differed between species. Collectively, our results indicate that small vertebrate granivores are an important cause of seed loss in prairie restoration, but unidentified belowground (e.g. fungal decomposition, invertebrate predation) and environmental (wind, rain) factors account for a greater proportion of total seed loss. Until these causes of seed loss are better understood, high seed costs will persist and continue to impede prairie restoration.

The role of invertebrates in seedling establishment in a heterocarpic plant, Atriplex sagittata

AbstractSeed heteromorphism is a marked character of many Chenopodioideae (Amaranthaceae). Seed morphs differ in dormancy, germination and seedling biology, but differences in their predation have not yet been studied. Atriplex sagittata produces small black dormant and large brown non‐dormant seeds. In this study, the timing of seed release and seedling establishment were ascertained, and their consumption by invertebrates (carabids, isopods and slugs) was studied. Seeds dispersed in the autumn passed the winter on the ground surface, protected from invertebrate predation by low temperatures. In the following vegetative season, ungerminated black seeds exposed to predation on ground surface were preferred by a large carabid species, Pseudoophonus rufipes. Some black and all brown seeds escaped predation by germinating in early spring. The seedlings were little endangered by carabids and isopods but were preferred by an invasive slug, Arion vulgaris, the feeding of which can exterminate seedlings at places in which slugs are abundant. Invertebrate predation is important factor of seed and seedling mortality of A. sagittata and seed heteromorphism modifies its intensity and timing.

Response of adult dragonflies to artificial prey of different size and colour.

Aposematism is an evolved, cross-species association between a preys’ unprofitability and the presence of conspicuous signals. Avian predators have been widely employed to understand the evolution of these warning signals However, insect predators are abundant, diverse, and highly visual foragers that have been shown to be capable of learned aversion. Therefore, it is likely that their behaviour also shapes the nature of anti-predator traits. In this study, we evaluated the rates of attack of a community (13 species) of mature adult dragonflies (Odonata) on artificial prey of varying size (2.5–31 mm lengthwise) and colour pattern (black, black/yellow striped). The relative attack rates of dragonflies on prey increased as prey size decreased, but there was no evidence that the attack rates by dragonflies were affected by prey colour pattern and no evidence for an interaction between colour pattern and size. To investigate prey selection by specific predator species under field conditions, we compared the time to attack distributions of black-painted prey presented to two common dragonflies: Leucorrhinia intacta and the larger, Libellula pulchella. We found that the two dragonfly species, as well as the two sexes, had different foraging responses. L. pulchella was more likely to attack larger prey, and females of both species more likely to attack prey than males. Collectively, our results indicate that dragonflies are highly size selective. However, while the nature of this selectivity varies among dragonfly species, there is little evidence that classic black/yellow warning signals deter attack by these aerial invertebrate predators.

Interactive effects of land use, temperature, and predators determine native and invasive mosquito distributions

Abstract Land‐use and climate change could alter the distribution of both native and exotic mosquitoes by changing abiotic and biotic characteristics of freshwater habitats. We initially studied the influence of land use on standing water habitats, and the subsequent effects on native and exotic mosquito and mosquito‐predator presence. Associated with abiotic habitat characteristics, mosquito‐predator richness was highest in aquatic habitats of natural land uses (forest and grassland), and lowest in human‐modified land uses (pastoral and urban), and the opposite relationship was true for mosquito presence. Based on the outcome of the field survey we investigated the potential effects of climate‐induced habitat warming and drying on interactions between invasive Aedes notoscriptus and native Culex pervigilans mosquitoes and native invertebrate predators affected by the land‐use gradient. Predator presence, which is directly affected by both climate and land‐use change, influenced both mosquito survivorship and behaviour. We found predation rates increased with temperature, but the magnitude of changes depended on both predator and prey identity. In general, higher temperatures increased mosquito pupation rates, but also made mosquitoes more susceptible to predation. Although invasive A. notoscriptus were more susceptible to predation, increased temperature resulted in shorter life cycles, thereby reducing the net effect of increased predation caused by higher temperatures. Overall, our results suggest interactions between temperature, land use and predator identity will be important in determining mosquito distributions, and will likely differ between mosquito species. Such interactions and species‐specific responses will be particularly important if environmental changes facilitate range expansion of invasive mosquito species that are vectors for disease.

Predator–prey interaction between greenhead ants and processionary caterpillars is mediated by chemical defence

The predatory greenhead ant, Rhytidoponera metallica, readily attacks some insects but not the processionary caterpillar, Ochrogaster lunifer. This urticarial lepidopteran larva is covered in long hairs and has numerous smaller, detachable barbed setae present on the third instar and older larvae. These setae are a health risk to humans and other mammals. In this study, we tested whether O. lunifer larvae are defended against invertebrate predators, using R. metallica as an example. Field experiments at R. metallica nests and laboratory olfactometer experiments were used to assess possible behavioural, morphological and chemical defence mechanisms. We found that a volatile chemical component from O. lunifer larvae inhibited attack/approach by R. metallica. This odour was associated with O. lunifer cuticular components: larval hairs, setae and exoskeleton. Behavioural defence was not found to inhibit R. metallica attack. Furthermore, despite the dense hairs and setae covering O. lunifer larvae, these did not act as a physical barrier to ant attack. Our study demonstrates the key role of chemical ecology in invertebrate interactions involving such hairy caterpillars. Future isolation of the deterrent chemical(s) produced by O. lunifer larvae may offer novel possibilities for managing ant behaviour.

Increase in predator-prey size ratios throughout the Phanerozoic history of marine ecosystems.

The escalation hypothesis posits that predation by increasingly powerful and metabolically active carnivores has been a major driver of metazoan evolution. We test a key tenet of this hypothesis by analyzing predatory drill holes in fossil marine shells, which provide a ~500-million-year record of individual predator-prey interactions. We show that drill-hole size is a robust predictor of body size among modern drilling predators and that drill-hole size (and thus inferred predator size and power) rose substantially from the Ordovician to the Quaternary period, whereas the size of drilled prey remained stable. Together, these trends indicate a directional increase in predator-prey size ratios. We hypothesize that increasing predator-prey size ratios reflect increases in prey abundance, prey nutrient content, and predation among predators.

Linking riparian shade and the legacies of forest management to fish and vertebrate biomass in forested streams

AbstractDetermining the factors that limit abundance and biomass of fish is fundamental to effective fisheries management. In streams, pool availability, cover, and habitat complexity often limit fish—particularly salmonids—and many restoration efforts are directed toward addressing physical habitat factors. However, the availability of prey, and the factors that influence prey abundance, can also influence the abundance, biomass, and growth of fish and other consumers. Both habitat and prey availability can be influenced by characteristics of the riparian forest in headwaters. In this study, we evaluate how variables associated with stream habitat, primary production, and macroinvertebrate biomass account for variability in the biomass of cutthroat trout and total vertebrates (fish and salamanders) across a series of paired stream reaches with contrasting forest structure. Each of nine stream pairs consisted of an old‐growth reach and a reach bordered by 40‐ to 60‐yr‐old second‐growth riparian forest. We evaluated relationships between response and explanatory variables for each forest structure type using correlation analysis, regression analysis, andAICc model comparison analyses. We also conducted correlation and regression analyses on within‐stream reach pair differences across the nine study streams. Canopy openness, nitrate concentration, periphyton chlorophyllaaccrual, total invertebrate biomass, predatory invertebrate biomass, cutthroat trout biomass, and total vertebrate biomass were all positively correlated with each other, while temperature was negatively correlated with biotic variables. Within reach pairs, canopy openness emerged as the strongest correlate with top predators, with differences in canopy openness explaining 84% of the variation in vertebrate biomass differences in the paired analysis. Other habitat metrics were poorly correlated with invertebrate, fish, and salamander biomass for all analyses. Overall, these results suggest that for the stream reaches surveyed here, resource availability—as regulated through bottom‐up, autotrophic pathways—is a dominant control on fish and other consumers. This highlights the importance of food resource limitation for fish and top predators in headwater streams, and illustrates how differences in canopy structure can affect bottom‐up drivers of stream food webs.