Experimental and observational evidence reveals that predators in natural environments do not regulate their prey: They are passengers,not drivers

Among both ecologists and the wider community there is a tacit assumption that predators regulate populations of their prey. But there is evidence from a wide taxonomic and geographic range of studies that predators that are adapted to co-evolved prey generally do not regulate their prey. This is because predators either cannot reproduce as fast as their prey and/or are inefficient hunters unable to catch enough prey to sustain maximum reproduction. The greater capacity of herbivores to breed successfully is, however, normally restricted by a lack of enough food of sufficient quality to support reproduction. But whenever this shortage is alleviated by a large pulse of food, herbivores increase their numbers to outbreak levels. Their predators are unable to contain this increase, but their numbers, too, surge in response to this increase in food. Eventually both their populations will crash once the food supply runs out, first for the herbivores and then for the predators. Then an “over-run” of predators will further depress the already declining prey population, appearing to be controlling its abundance. This latter phenomenon has led many ecologists to conclude that predators are regulating the numbers of their prey. However, it is the same process that is revealed during outbreaks that limits populations of both predator and prey in “normal” times, although this is usually not readily apparent. Nevertheless, as all the diverse cases discussed here attest, the abundance of predators and their co-evolved prey are both limited by their food: the predators are passengers, not drivers.     

Experimental evidence that sentinel behaviour is affected by risk

Sentinels are a conspicuous feature of some cooperative societies and are often assumed to provide benefits in terms of increased predator detection. Similar to other cooperative behaviours, variation in investment in sentinel behaviour should reflect variation in the benefits of such behaviour. However, evidence for this is inconclusive: to date experiments have manipulated the cost of sentinel behaviour, and considerations of changes in the benefits of sentinel activity on investment patterns are lacking. Here, we experimentally manipulated the benefits of sentinel behaviour in the cooperatively breeding pied babbler (Turdoides bicolor) to assess whether this had any impact on sentinel activity. We simulated the presence of an unseen predator using playbacks of heterospecific alarm calls, and the presence of an actual predator using a model snake. In both cases, the increase in perceived predation risk caused an increase in sentinel activity, demonstrating that investment in sentinel activity increases when the benefits are greater.     

Experimental evidence that women's mate preferences are directly influenced by cues of pathogen prevalence and resource scarcity

When choosing a mate, women are thought to face a trade-off between genetic and parental quality. Recent research suggests that this trade-off is influenced by environmental factors such as pathogen prevalence and resource scarcity, which affect the relative value of genetic and parental quality to offspring fitness. To further investigate these findings, the current study primed 60 women with pathogen prevalence, resource scarcity or an irrelevant threat, before administering a forced trade-off task that assessed mate preferences for traits thought to be indicative of genetic or parental quality. Women primed with pathogen prevalence revealed greater preferences for traits indicative of genetic quality at the expense of traits indicative of parental quality. The reverse was found for women primed with resource scarcity. These findings suggest that environmental factors may directly influence women's mate preferences owing to evolved plasticity, such that mate preferences are flexible in response to environmental factors.     

Experimental evidence that ecological effects of an invasive fish are reduced at high densities

Understanding the relationship between invasive species density and ecological impact is a pressing topic in ecology, with implications for environmental management and policy. Although it is widely assumed that invasive species impact will increase with density, theory suggests interspecific competition may diminish at high densities due to increased intraspecific interactions. To test this theory, we experimentally examined intra- and interspecific interactions between a globally invasive fish, round goby (Neogobius melanostomus), and three native species at different round goby densities in a tributary of the Laurentian Great Lakes. Eighteen 2.25 m² enclosures were stocked with native fish species at natural abundances, while round gobies were stocked at three different densities: 0 m^?2, 2.7 m^?2, and 10.7 m^?2. After 52 days, native fish growth rate was significantly reduced in the low density goby treatment, while growth in the high density goby treatment mirrored the goby-free treatment for two of three native species. Invertebrate density and gut content weight of native fishes did not differ among treatments. Conversely, gut content weight and growth of round gobies were lower in the high goby density treatment, suggesting interactions between round gobies and native fishes are mediated by interference competition amongst gobies. Our experiment provides evidence that invasive species effects may diminish at high densities, possibly due to increased intraspecific interactions. This is consistent with some ecological theory, and cautions against the assumption that invasive species at moderate densities have low impact.     

Experimental evidence that evolution by niche construction affects dissipative ecosystem dynamics

Evolution by niche construction occurs when organism-mediated modification of the environment causes an evolutionary response. Physicists have postulated that evolution in general, and evolution mediated via feedbacks between organisms and their environment in particular (i.e. evolution by niche construction), could increase the capacity of biological systems to dissipate free energy in an open thermodynamic system, and help them maintain a state far from thermodynamic equilibrium. Here, we propose using the bacterium Pseudomonas fluorescens (strain SBW25) as a model system to experimentally test theories in both evolutionary biology (e.g. niche construction) and physics (e.g. dissipative systems theory). P. fluorescens rapidly and predictably evolves multiple strategies for exploiting oxygen in unmixed culture flasks. This evolutionary dynamic is mediated by feedbacks between the modification of the oxygen gradient by P. fluorescens and the ecological and evolutionary responses of Pseudomonas to modified environmental conditions. To confirm this, we experimentally manipulated two aspects of the system that influence the strength of the feedback between P. fluorescens and oxygen gradients in the system. First, we inhibited the metabolism of the strain used to inoculate the cultures, and, second, we disturbed the formation of mats at the air–liquid interface. Overall, we found convincing experimental evidence of evolution by niche construction, and conclude that this study system is amenable to experimental investigations of both niche construction and dissipative systems theory.     

Ribeiroia ondatrae cercariae are consumed by aquatic invertebrate predators

Trematodes amplify asexually in their snail intermediate hosts, resulting in the potential release of hundreds to thousands of free-living cercariae per day for the life of the snail. The high number of cercariae released into the environment undoubtedly increases the probability of transmission. Although many individual cercariae successfully infect another host in their life cycle, most fail. Factors that prevent successful transmission of cercariae are poorly understood. Microcrustaceans and fish have been observed to eat cercariae of some species, although the possibility that predation represents a significant source of mortality for cercariae has been largely unexplored. We tested the cercariophagic activity of several freshwater invertebrates on Ribeiroia ondatrae, a trematode that causes limb deformities in amphibians. Individuals of potential predators were placed into wells of multiwell plates with 10-15 cercariae, and numbers of cercariae remaining over time were recorded and compared with numbers in control wells that contained no predators. Of the species tested, Hydra sp., damselfly (Odonata, Coenagrionidae) larvae, dragonfly (Odonata, Libellulidae), larvae, and copepods (Cyclopoida) consumed cercariae. In some cases, 80-90% of the cercariae offered to damselfly and dragonfly larvae were consumed within 10 min. In most cases, predators continued to consume cercariae at the same average rates when offered cercariae together with individuals of an alternate prey item. Hydra sp. ate fewer cercariae in these trials. Our findings suggest the need for field and laboratory studies to further explore the effects of predators on transmission of R. ondatrae to amphibian larvae. In addition, the results suggest that conservation of the biodiversity and numbers of aquatic predators may limit adverse impacts of trematode infections in vertebrate hosts.     

Experimental evidence that lignin-modifying enzymes are essential for degrading plant cell wall lignin by Pleurotus ostreatus using CRISPR/Cas9

Lignin-modifying enzymes (LMEs), which include laccases (Lacs), manganese peroxidases (MnPs), versatile peroxidases (VPs), and lignin peroxidases (LiPs), have been considered key factors in lignin degradation by white-rot fungi because they oxidize lignin model compounds and depolymerize synthetic lignin in vitro. However, it remains unclear whether these enzymes are essential/important in the actual degradation of natural lignin in plant cell walls. To address this long-standing issue, we examined the lignin-degrading abilities of multiple mnp/vp/lac mutants of Pleurotus ostreatus. One vp2/vp3/mnp3/mnp6 quadruple-gene mutant was generated from a monokaryotic wild-type strain PC9 using plasmid-based CRISPR/Cas9. Also, two vp2/vp3/mnp2/mnp3/mnp6, two vp2/vp3/mnp3/mnp6/lac2 quintuple-gene mutants, and two vp2/vp3/mnp2/mnp3/mnp6/lac2 sextuple-gene mutants were generated. The lignin-degrading abilities of the sextuple and vp2/vp3/mnp2/mnp3/mnp6 quintuple-gene mutants on the Beech wood sawdust medium reduced drastically, but not so much for those of the vp2/vp3/mnp3/mnp6/lac2 mutants and the quadruple mutant strain. The sextuple-gene mutants also barely degraded lignin in Japanese Cedar wood sawdust and milled rice straw. Thus, this study presented evidence that the LMEs, especially MnPs and VPs, play a crucial role in the degradation of natural lignin by P. ostreatus for the first time.     

Experimental evidence that source genetic variation drives pathogen emergence

A pathogen can readily mutate to infect new host types, but this does not guarantee successful establishment in the new habitat. What factors, then, dictate emergence success? One possibility is that the pathogen population cannot sustain itself on the new host type (i.e. host is a sink), but migration from a source population allows adaptive sustainability and eventual emergence by delivering beneficial mutations sampled from the source''s standing genetic variation. This idea is relevant regardless of whether the sink host is truly novel (host shift) or whether the sink is an existing or related, similar host population thriving under conditions unfavourable to pathogen persistence (range expansion). We predicted that sink adaptation should occur faster under range expansion than during a host shift owing to the effects of source genetic variation on pathogen adaptability in the sink. Under range expansion, source migration should benefit emergence in the sink because selection acting on source and sink populations is likely to be congruent. By contrast, during host shifts, source migration is likely to disrupt emergence in the sink owing to uncorrelated selection or performance tradeoffs across host types. We tested this hypothesis by evolving bacteriophage populations on novel host bacteria under sink conditions, while manipulating emergence via host shift versus range expansion. Controls examined sink adaptation when unevolved founding genotypes served as migrants. As predicted, adaptability was fastest under range expansion, and controls did not adapt. Large, similar and similarly timed increases in fitness were observed in the host-shift populations, despite declines in mean fitness of immigrants through time. These results suggest that source populations are the origin of mutations that drive adaptive emergence at the edge of a pathogen''s ecological or geographical range.     

Further evidence that sperm nuclear proteins are necessary for embryogenesis

We have recently presented evidence that the structural integrity of the mouse sperm nuclear matrix may be necessary for the proper unpackaging of sperm DNA for participation in embryogenesis. It is likely that the sperm nuclear matrix contributes to the organisation of the sperm DNA and its disturbance can seriously damage the paternal genome or its expression. In this work, we confirm our previous data and further suggest that even very subtle changes in the sperm nuclear structure may have a significant impact on embryo development. As reported previously, dithiothreitol (DTT) in the presence of an ionic detergent, ATAB, destabilized the nuclear matrix as measured by the halo assay, and oocytes injected with these nuclei failed to develop. We also discovered that omitting the protease inhibitor PMSF from the buffers used to extract spermatozoa prevented sperm injected into oocytes from participating in development. The organization of DNA into loop domains by the nuclear matrix in these nuclei appeared normal, as measured by the halo assay. Oocytes injected with sperm nuclei that had been washed with ATAB in the presence of phenylmethylsulphonyl fluoride (PMSF) but in the absence of DTT resulted in live births. Neither DTT treatment nor the absence of PMSF would be expected to disrupt the integrity of the paternal DNA. The data therefore suggest that even very subtle alterations in the structural proteins of the nucleus are enough to deprive sperm DNA of the ability to contribute to embryonic development.     

Experimental evidence that high levels of inbreeding depress sperm competitiveness

The effects of inbreeding on sperm quantity and quality are among the most dramatic examples of inbreeding depression. The extent to which inbreeding depression results in decreased fertilization success of a male’s sperm, however, remains largely unknown. This task is made more difficult by the fact that other factors, such as cryptic female choice, male sperm allocation and mating order, can also drive patterns of paternity. Here, we use artificial insemination to eliminate these extraneous sources of variation and to measure the effects of inbreeding on the competitiveness of a male’s sperm. We simultaneously inseminated female guppies (Poecilia reticulata) with equal amounts of sperm from an outbred (f = 0) male and either a highly (f = 0.59) or a moderately inbred (f = 0.25) male. Highly inbred males sired significantly fewer offspring than outbred males, but share of paternity did not differ between moderately inbred and outbred males. These findings therefore confirm that severe inbreeding can impair the competitiveness of sperm, but suggest that in the focal population inbreeding at order of a brother–sister mating does not reduce a male’s sperm competitiveness.     

Experimental evidence that corticosterone affects offspring sex ratios in quail

Recent studies have shown that some species of birds have a remarkable degree of control over the sex ratio of offspring they produce. However, the mechanism by which they achieve this feat is unknown. Hormones circulating in the breeding female are particularly sensitive to environmental perturbations, and so could provide a mechanism for her to bias the sex ratio of her offspring in favour of the sex that would derive greatest benefit from the prevailing environmental conditions. Here, we present details of an experiment in which we manipulated levels of testosterone, 17beta-oestradiol and corticosterone in breeding female Japanese quail (Coturnix coturnix japonica) using Silastic implants and looked for effects on the sex ratio of offspring produced. Offspring sex ratio in this species was significantly correlated with faecal concentrations of the principal avian stress hormone, corticosterone, and artificially elevated levels of corticosterone resulted in significantly female-biased sex ratios at laying. Varying testosterone and 17beta-oestradiol had no effect on sex ratio alone, and faecal levels of these hormones did not vary in response to corticosterone. Our results suggest that corticosterone may be part of the sex-biasing process in birds.     

Experimental evidence that wildflower strips increase pollinator visits to crops

Wild bees provide a free and potentially diverse ecosystem service to farmers growing pollination‐dependent crops. While many crops benefit from insect pollination, soft fruit crops, including strawberries are highly dependent on this ecosystem service to produce viable fruit. However, as a result of intensive farming practices and declining pollinator populations, farmers are increasingly turning to commercially reared bees to ensure that crops are adequately pollinated throughout the season. Wildflower strips are a commonly used measure aimed at the conservation of wild pollinators. It has been suggested that commercial crops may also benefit from the presence of noncrop flowers; however, the efficacy and economic benefits of sowing flower strips for crops remain relatively unstudied. In a study system that utilizes both wild and commercial pollinators, we test whether wildflower strips increase the number of visits to adjacent commercial strawberry crops by pollinating insects. We quantified this by experimentally sowing wildflower strips approximately 20 meters away from the crop and recording the number of pollinator visits to crops with, and without, flower strips. Between June and August 2013, we walked 292 crop transects at six farms in Scotland, recording a total of 2826 pollinators. On average, the frequency of pollinator visits was 25% higher for crops with adjacent flower strips compared to those without, with a combination of wild and commercial bumblebees (Bombus spp.) accounting for 67% of all pollinators observed. This effect was independent of other confounding effects, such as the number of flowers on the crop, date, and temperature. Synthesis and applications. This study provides evidence that soft fruit farmers can increase the number of pollinators that visit their crops by sowing inexpensive flower seed mixes nearby. By investing in this management option, farmers have the potential to increase and sustain pollinator populations over time.     

Experimental evidence that refuges delay insect adaptation to Bacillus thuringiensis

Theoretical projections suggest that refuges from exposure can delay insect adaptation to environmentally benign insecticides derived from Bacillus thuringiensis, but experimental tests of this approach have been limited. We tested the refuge tactic by selecting two sets of two colonies of diamondback moth (Plutella xylostella) for resistance to B. thuringiensis subsp. aizawai in the laboratory. In each set, one colony was selected with no refuge and the other with a 10 per cent refuge from exposure to B. thuringiensis subsp. aizawai. Bioassays conducted after nine selections were completed show that mortality caused by B. thuringiensis subsp. aizawai was significantly greater in the refuge colonies than in the no-refuge colonies. These results demonstrate that the refuges delayed the evolution of resistance. Relative to a susceptible colony, final resistance ratios were 19 and 8 for the two no-refuge colonies compared to 6 and 5 for the refuge colonies. The mean realized heritability of resistance to B. thuringiensis subsp. aizawai was 0.046 for colonies without refuges, and -0.002 for colonies with refuges. Selection with B. thuringiensis subsp. aizawai decreased susceptibility to B. thuringiensis toxin Cry1Ab, but not to Cry1C or B. thuringiensis subsp. kurstaki. Although the ultimate test of refuges will occur in the field, the experimental evidence reported here confirms modelling results indicating that refuges can slow the evolution of insect resistance to B. thuringiensis.     

Experimental Evidence that Sexual Displays are Costly for Nest Survival

Nest predation is one of the most significant limitations for successful breeding of tropical passerines. Thus, parental strategies may include choosing appropriate nest sites and behaving in ways that minimize predation. Habitat characteristics that may influence nest success include degree of nest concealment, proximity to habitat edge, plant architecture as well as several others cited in the literature. However, few studies have examined display behavior as a factor that could also influence nest survival. We experimentally tested whether sexual motor displays served as a cue for visually oriented predators to locate artificial nests in a population of blue‐black grassquits Volatinia jacarina, a Neotropical passerine that exhibits a complex sexual display and is subjected to elevated rates of nest predation. We also evaluated the effect of nest substrate on survival. Predation rate was higher for nests within territories of displaying males relative to areas without displaying males and for nests placed in shrubs relative to grasses. Predation increased sharply in the third experimental replicate, at the end of the breeding season, which suggests that predators may develop a search image for nests or may become more abundant during specific periods of the season. Avian predators appear to be the most important nest predators. Results suggest that there may be a trade‐off between the increase in fitness derived from sexual displays of males to attract potential mates and the decrease owing to predation of active nests within their territories.     

Genetic evidence that the multiple apolipoprotein A-1 isoforms are encoded by a common structural gene

We have used a genetic structural variation of apolipoprotein A-I in mice to examine the origin of the multiple charge isoforms of the plasma protein. Apolipoprotein A-I translated in vitro from hepatic or enteric mRNA revealed that the genetic variation simultaneously alters the charge of the protein produced by both tissues. The variation also shifted the charge of the entire family of isoforms found in plasma or translated in vitro. These results indicate that the protein produced by different tissues as well as the multiple isoforms are all derived from a common structural gene by processing.     

Group A human rotavirus genomics: evidence that gene constellations are influenced by viral protein interactions

Group A human rotaviruses (HRVs) are the major cause of severe viral gastroenteritis in infants and young children. To gain insight into the level of genetic variation among HRVs, we determined the genome sequences for 10 strains belonging to different VP7 serotypes (G types). The HRVs chosen for this study, D, DS-1, P, ST3, IAL28, Se584, 69M, WI61, A64, and L26, were isolated from infected persons and adapted to cell culture to use as serotype references. Our sequencing results revealed that most of the individual proteins from each HRV belong to one of three genotypes (1, 2, or 3) based on their similarities to proteins of genogroup strains (Wa, DS-1, or AU-1, respectively). Strains D, P, ST3, IAL28, and WI61 encode genotype 1 (Wa-like) proteins, whereas strains DS-1 and 69M encode genotype 2 (DS-1-like) proteins. Of the 10 HRVs sequenced, 3 of them (Se584, A64, and L26) encode proteins belonging to more than one genotype, indicating that they are intergenogroup reassortants. We used amino acid sequence alignments to identify residues that distinguish proteins belonging to HRV genotype 1, 2, or 3. These genotype-specific changes cluster in definitive regions within each viral protein, many of which are sites of known protein-protein interactions. For the intermediate viral capsid protein (VP6), the changes map onto the atomic structure at the VP2-VP6, VP4-VP6, and VP7-VP6 interfaces. The results of this study provide evidence that group A HRV gene constellations exist and may be influenced by interactions among viral proteins during replication.     

Experimental evidence that ground‐disturbance benefits Woodlark Lullula arborea

To determine whether ground‐disturbance increased Woodlark Lullula arborea abundance, we examined responses over 3 years to four treatments varying in establishment method (shallow‐ or deep‐cultivated) and complexity (homogeneous or ‘complex‐mosaics’ comprising fallow and recently cultivated subplots), plus controls, replicated across the largest lowland grass‐heath in the UK. Abundance increased through the study and was higher on plots closer to woodland and across all treatments. Within complex‐mosaics, Woodlark preferentially used recently cultivated subplots over 1‐ or 2‐year‐old fallows. Regardless of treatment detail, providing suitable foraging habitat within c. 45 m of woodland, through annual ground‐disturbance, can increase Woodlark abundance within lowland grass‐heaths characterized by closed swards.