Variation in vegetation and seed bank in a Chilean semi‐arid community affected by ENSO 1997

Abstract. We followed plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall in two opposite‐facing slopes (mesic vs xeric) and an intervening, relatively wet, ravine of a typical creek in semi‐arid Chile. During the ENSO year 1997 cover of ephemerals increased in all three sites; 43% vs 8% on the xeric slope 75% vs 26% on the mesic slope and 57% vs 32% in the ravine. The number of species was almost three times higher on the xeric slope (27 vs 10 species), increased by 47% on the mesic slope (28 vs 19) and by 14% in the ravine (24 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 on the xeric slope, but no significant cover changes were found on the mesic slope or the ravine. On the xeric slope peak density of seeds was 4500/m² in 1996, while during the ENSO year it was 24000/m². On the mesic slope equivalent values were 3000 and 17000 seeds/m² while in the ravine figures were 8500 and 27000 seeds/m². The strong responses of ephemerals and seed bank to the ENSO‐driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing, recovering and probably maintaining ephemeral vegetation in this region.     

An underground index of rangeland degradation: cicadas in arid southern Africa

Summary The prediction that density of long-lived, underground herbivores (Cicadidae) is a function of rangeland condition was examined in arid shrublands in the Karoo, South Africa. It was found that the density of adult cicadas was correlated with cover of deep-rooted, perennial plants. Differences in perennial plant cover were independent of soil type and rock cover. On degraded rangelands, where perennial plants had been replaced by ephemerals and short-lived plants, cicada densities were significantly depressed. We concluded that vegetation transformation by domestic livestock is likely to affect invertebrate populations and their vertebrate predators.     

Is fencing enough? The short‐term effects of stock exclusion in remnant grassy woodlands in southern NSW

Summary Fencing remnant native vegetation has become a widespread activity for arresting declines in biodiversity in agricultural landscapes. However, few data are available on the effectiveness of this approach. The present study investigated the short-term effects of fencing to exclude livestock on dominant tree and shrub recruitment, plant species cover, litter and soil characteristics in remnant grassy woodlands in southern NSW. Vegetation and soil surveys were undertaken at 47 sites fenced by Greening Australia (NSW) for 2–4 years. Fenced and unfenced areas at each site were compared using split-plot sampling. Woodlands sampled were dominated by Yellow Box/Blakely's Red Gum ( Eucalyptus melliodora/Eucalyptus blakelyi ), Grey Box ( Eucalyptus microcarpa ) or White Cypress-pine ( Callitris glaucophylla ). Significantly higher numbers of tree recruits were found in the fenced sites, with tree recruitment found in 59% of fenced sites compared with 13% of unfenced sites. Fenced sites also had significantly greater cover of native perennial grasses, less cover of exotic annual species and less soil surface compaction. However, outcomes varied among woodland ecosystems and individual sites. Where tree recruitment occurred, there was significantly more tree recruitment where there was greater perennial grass cover and less regeneration where exotic annual grass cover or overstorey crown cover was dense. Few shrubs recruited in fenced or unfenced areas, reflecting the lack of mature shrubs in most sites. Fencing is an important first step for conserving threatened grassy woodlands, but more active management may be needed to enhance woodland recovery, particularly in sites where few or no recruits were found.
Key words bush regeneration, fencing, grazing exclusion, rehabilitation, woodland restoration.     

Vertebrate predators have minimal cascading effects on plant production or seed predation in an intact grassland ecosystem

The strength of trophic cascades in terrestrial habitats has been the subject of considerable interest and debate. We conducted an 8-year experiment to determine how exclusion of vertebrate predators, ungulates alone (to control for ungulate exclusion from predator exclusion plots) or none of these animals influenced how strongly a three-species assemblage of rodent consumers affected plant productivity. We also examined whether predator exclusion influenced the magnitude of post-dispersal seed predation by mice. Both ungulates and rodents had strong direct effects on graminoid biomass. However, rodent impacts on plant biomass did not differ across plots with or without predators and/or ungulates. Deer mice removed more seeds from seed depots on predator exclusion plots, suggesting trait-mediated indirect effects of predators, but this short-term behavioural response did not translate into longer-term impacts on seed survival. These results suggest that vertebrate predators do not fundamentally influence primary production or seed survival in our system.     

Top predator control of plant biodiversity and productivity in an old-field ecosystem

Abstract Predators can have strong indirect effects on plants by altering the way herbivores impact plants. Yet, many current evaluations of plant species diversity and ecosystem function ignore the effects of predators and focus directly on the plant trophic level. This report presents results of a 3‐year field experiment in a temperate old‐field ecosystem that excluded either predators, or predators and herbivores and evaluated the consequence of those manipulations on plant species diversity (richness and evenness) and plant productivity. Sustained predator and predator and herbivore exclusion resulted in lower plant species evenness and higher plant biomass production than control field plots representing the intact natural ecosystem. Predators had this diversity‐enhancing effect on plants by causing herbivores to suppress the abundance of a competitively dominant plant species that offered herbivores a refuge from predation risk.     

Secondary seed dispersal of Erodiophyllum elderi,a patchily distributed short‐lived perennial in the arid lands of Australia

We investigated secondary dispersal of propagules of Erodiophyllum elderi (Asteraceae), a short‐lived perennial plant growing in small patches in the arid lands of southern Australia. In spite of its importance for population dynamics, secondary dispersal is a little understood process. We monitored the dispersal of 2280 large woody capitula (seed heads) released in six E. elderi patches for 9 months. Colour‐coded seed heads were located at night using UV light and their distance and direction from the release point were measured. Over the 9‐month period, more seed heads moved, and those that did, moved further in areas with high herbivore activity. Overall dispersal distance across the ground was limited to less than 30 m. Dispersal patterns were related to the topographical slope at the release site: seed heads moved further, and more dispersed on steeper slopes unless the steep slopes had sandy soil in which case seed heads were buried, caught or there was reduced sheet water flow limiting their dispersal potential. After several months, seed head dispersal virtually ceased as seed heads became stuck in the debris and soil after heavy rains or further dispersal became unlikely when seed heads reached locally low‐lying areas. Secondary dispersal patterns suggest two distinctly different influences associated with the presence of herbivores: the direct movement of seed heads by trampling from sheep (an introduced herbivore) and the indirect effect of a reduced standing biomass from grazing. Reduced vegetation cover allows seed head redistribution via sheet water flow during large rainfall events.     

Vegetation change following removal of keystone herbivores from desert grasslands in New Mexico

Abstract. Responses of plant communities to mammalian herbivores vary widely, due to variation in plant species composition, herbivore densities, forage preferences, soils, and climate. In this study, we evaluated vegetation changes on 30 sites within and adjacent to the Sevilleta National Wildlife Refuge (SNWR) in central New Mexico, USA, over a 20‐yr period following removal of the major herbivores (livestock and prairie dogs) in 1972–1975. The study sites were established in 1976, and were resampled in 1986 and 1996 using line transect methods. At the landscape scale, repeated measures ANOVA of percentage cover measurements showed no significant overall net changes in total perennial plant basal cover, either with or without herbivores present; however, there was an overall increase in annual forbs and plant litter from 1976 to 1996. At the site scale, significant changes in species composition and dominance were observed both through time and across the SNWR boundary. Site histories varied widely, with sites dominated by Bouteloua eriopoda being the most dynamic and sites dominated by Scleropogon brevifolius being the most persistent. Species‐specific changes also were observed across multiple sites: B. eriopoda cover increased while Gutierrezia sarothrae greatly decreased. The non‐uniform, multi‐directional changes of the sites' vegetation acted to prevent detection of overall changes in perennial vegetation at the landscape level. Some sites displayed significant changes after removal of herbivores, while others appeared to respond primarily to climate dynamics. Certain species that were not preferred by livestock or prairie dogs, showed overall declines during drought periods, while other preferred species exhibited widespread increases during wetter periods regardless of herbivore presence. Therefore, the vegetation dynamics cannot be attributed solely to removal of herbivores, and in some cases can be explained by short‐ and long‐term fluctuations in climate. These results emphasize the variety of responses of sites with differences in vegetation to mammalian herbivores under otherwise similar climatic conditions, and illustrate the value of site‐ and landscape‐scale approaches to understanding the impacts of plant‐herbivore interactions.     

Functional responses of contrasting seed predator guilds to masting in two Mediterranean oak species

The predator satiation hypothesis poses that synchronous and variable seed production during masting events increases seed escape through seed predator satiation. The success of this strategy depends upon the type of consumer functional response, in this case defined as the change in seed consumption rate by a predator as a function of change in seed density. Type II (where the proportion of seed consumed is highest at low levels of seed availability) and type III (where the proportion of seed consumed is highest at some intermediate level of seed availability and then declines towards zero) functional responses describe negative density‐dependence and indicate predator satiation. The type of function response should be contingent upon herbivore traits: type II responses are predicted for dietary specialist predators with low mobility, and type III responses are predicted for highly mobile, dietary generalist predators. Surprisingly, most studies have not evaluated whether functional responses vary among seed predator guilds. Here we describe the functional responses at population and individual tree level of highly mobile generalist (birds and rodents) and less mobile specialist (insects) pre‐dispersal seed predators attacking acorns of two sympatric oaks (Quercus suber and Q. canariensis) over a 10‐year period. Our results showed that in most cases specialist seed predators exhibited the predicted type II functional response at both the individual tree and population level for both oak species. However, generalist seed predators did not exhibit the predicted type III response; instead, they also exhibited a type II response at the individual tree and population level for both oak species. By independently assessing the effects of multiple seed predators associated with the same host tree species, our work highlights the influence of herbivore traits on the outcome of plant–seed predator interactions in masting species, and thus furthers our understanding of the ecological and evolutionary mechanisms underlying masting behaviour.     

Temporal and micro-spatial patterning of seedling establishment. Consequences for patch dynamics in the southern Monte,Argentina

We compared the temporal and micro-spatial patterning of seedling emergence and establishment of two cohorts of perennial grasses and shrubs in the southern Monte, Argentina. Samplings were carried out in two contrasting areas (grazed and non-grazed) during four years. We found lower densities of emerged and established seedlings of perennial grasses in the grazed relative to the non-grazed area. Conversely, emerged seedlings of shrubs did not vary between the grazed and the non-grazed area and densities of established shrub seedlings were higher in the grazed than in the non-grazed area. We only found differences between cohorts in seedling emergence of perennial grasses. These differences could be associated with the amount of precipitation in the year previous to the emergence. Both in the grazed and non-grazed area, seedlings of perennial grasses were concentrated at the periphery of plant patches. We defined a plant patch as a discrete unit of the spatial pattern of vegetation surrounded by, at least, 20 cm of bare soil from the nearest neighbour patch. Emergence in perennial grasses was more frequent at the southern/western patch-periphery than at other patch-periphery locations. Established seedlings of perennial grasses, however, were homogeneously distributed throughout patch periphery. Emergence in shrubs was more frequent at the centre and periphery of patches than at inter-patch microsites. In contrast, established seedlings of shrubs were homogeneously distributed among microsites. Our results suggests that differential seedling establishment between life forms is the outcome of complex biotic and abiotic interactions and feedbacks at the patch level between seedlings and established plants. Both life forms appear to have a different role in the origin, dynamics, and maintenance of spotting vegetation. Because of the ability to establish both at inter-patch and patch microsites, shrubs could be identified as colonizers or initiators of small plant patches in bare soil or they may contribute to increase the cover and size of pre-existing plant patches. Both processes would be promoted in grazed areas. Due to the ability to establish at patch peripheries, perennial grasses would contribute to the isodiametric growth of pre-existing patches, preferentially in non-grazed areas. This revised version was published online in June 2006 with corrections to the Cover Date.     

Herbivore species richness, composition and community structure mediate predator richness effects and top-down control of herbivore biomass

Changes in predator species richness can have important consequences for ecosystem functioning at multiple trophic levels, but these effects are variable and depend on the ecological context in addition to the properties of predators themselves. Here, we report an experimental study to test how species identity, community attributes, and community structure at the herbivore level moderate the effects of predator richness on ecosystem functioning. Using mesocosms containing predatory insects and aphid prey, we independently manipulated species richness at both predator and herbivore trophic levels. Community structure was also manipulated by changing the distribution of herbivore species across two plant species. Predator species richness and herbivore species richness were found to negatively interact to influence predator biomass accumulation, an effect which is hypothesised to be due to the breakdown of functional complementarity among predators in species-rich herbivore assemblages. The strength of predator suppression of herbivore biomass decreased as herbivore species richness and distribution across host plants increased, and positive predator richness effects on herbivore biomass suppression were only observed in herbivore assemblages of relatively low productivity. In summary, the study shows that the species richness, productivity and host plant distribution of prey communities can all moderate the general influence of predators and the emergence of predator species richness effects on ecosystem functioning.     

Fragment size affects plant herbivory via predator loss

Fragmentation and resultant changes in patch size are predicted to alter species diversity and community composition, yet the consequences of these differences for species interactions are poorly understood. Theory predicts that predators are more sensitive to fragmentation than their prey, resulting in greater predator loss in small patches. Predator loss, in turn, is predicted to 1) increase herbivory rates overall, and 2) cause herbivores to shift feeding from plants that act as refugia to those that are preferred forage. We tested these predictions in an old‐field community using two experiments. The first was a large‐scale experiment that included hundreds of arthropod species in fragments of various sizes, and used goldenrod and switchgrass to assess herbivory. Our second experiment manipulated densities of a focal predator species and a focal prey species to determine if changes in densities, rather than other characteristics of fragments, were sufficient to cause the trends observed in the first experiment. We found that predator densities declined in small fragments whereas herbivore densities showed the opposite trend. Total herbivory mirrored herbivore densities by increasing in small patches, and this mean increase was driven by large increases in goldenrod herbivory but declines in switchgrass herbivory. Experimental manipulation of densities confirmed that herbivores preferentially feed on goldenrod, and that predators depress herbivory on goldenrod but have a negligible effect on switchgrass. Our results suggest that fragmentation alters trophic interactions by causing declines in predator densities and increases in herbivore densities, but that feeding preferences of herbivores may generate unequal impacts among plant species.     

The hard lives of trees in African savanna—Even without elephants

The ongoing loss of large trees and densification of shrubs are two prevalent processes that take place in African savannas, with profound consequences for their structure and function. We evaluated herbivore impacts on savanna woody communities using a long-term exclosure experiment in the Kruger National Park, South Africa, with three treatments: the exclusion of large mammals only (i.e. elephant and giraffe), exclusion of all herbivores larger than a hare, and areas open to all herbivores. We asked three questions: (1) How did variable exclusion of herbivores affect woody density and structure across the catena (i.e. riparian, sodic and crest vegetation)? (2) Did the exclusion of herbivores result in unique woody species composition? (3) Did herbivore exclusion result in a higher proportion of palatable species? After 17 years, we found that herbivores mainly affected the heights and densities of existing species, rather than leading to turnover of woody species assemblages. Although densities of individuals increased in the full exclosure (350 ha⁻¹), the change was more moderate than expected. By contrast, mixed mega-and meso-herbivores decreased the number of trees and shrubs (decreases of 780 ha⁻¹) via a variety of physical impacts. Meso-herbivores alone, on the other hand, had less impact on individual density (i.e. no change), but limited average height growth and canopy dimensions in certain habitat types. Where elephants are present, they are effective at reducing the density of woody stems to the point of counteracting woody encroachment, but at the same time are actively preventing the persistence of large trees (>5 m) as well as preventing trees from recruiting to larger size classes. However, the lack of massive recruitment and woody cover increases with elephant exclusion, especially for more preferred species, suggests that factors beyond elephants, such as dispersal limitation, seed predation, and drought, are also acting upon species.     

Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert

Long-term (1977–90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.     

Entrapped carrion increases indirect plant resistance and intra‐guild predation on a sticky tarweed

Many plants employ indirect defenses against herbivores; often plants provide a shelter or nutritional resource to predators, increasing predator abundance, and lessening herbivory to the plant. Often, predators on the same plant represent different life stages and different species. In these situations intraguild predation (IGP) may occur and may decrease the efficacy of that defense. Recently, several sticky plants have been found to increase indirect defense by provisioning predatory insects with entrapped insects (hereafter: carrion). We conducted observational studies and feeding trials with herbivores and predators on two sticky, insect‐entrapping asters, Hemizonia congesta and Madia elegans, to construct food webs for these species and determine the prevalence of IGP in these carrion‐provisioning systems. In both systems, intraguild predation was the most common interaction observed. To determine whether IGP was driven by resource abundance, whether it reduced efficacy of this indirect defense and whether stickiness or predator attraction was induced by damage, we performed field manipulations on H. congesta. Carrion supplementation led to an increase in predator abundance and IGP. IGP was asymmetric within the predator guild: assassin bugs and spiders preyed on small stilt bugs but not vice versa. Despite increased IGP, carrion provisions decreased the abundance of the two most common herbivores (a weevil and a mealybug). Overall seed set was driven by plant size, but number of seeds produced per fruit significantly increased with increasing carrion, likely because of the reduction in the density of a seed‐feeding weevil. Observationally and experimentally, we found that carrion‐mediated indirect defense of tarweeds led to much intraguild predation, though predators effectively reduced herbivore abundance despite the increase in IGP.     

Coleomegilla maculata (Coleoptera: Coccinellidae) predation on pea aphids promoted by proximity to dandelions

The impact of a predator on its prey may depend on the presence of other species in the community. In particular, if predators are attracted to areas containing one prey species, another prey species may suffer greater predation if it occurs in the same areas. If the predator is omnivorous, this may occur even if one prey species is an animal and the other is a plant. We investigated the role of local dandelion densities on the impact of the predator Coleomegilla maculata on pea aphids in alfalfa fields. At small spatial scales, increased dandelion densities were associated with high C. maculata densities, presumably because these omnivorous ladybird beetles aggregated to pollen resources. In turn, the high C. maculata densities were associated with low aphid densities, presumably because of increased predation. We used laboratory cages to simulate C. maculata foraging in two adjacent patches of alfalfa, one with dandelions and one without. As in the field, the laboratory experiment showed that C. maculata aggregated to alfalfa interspersed with dandelions, which resulted in increased predation on aphids on alfalfa. This study demonstrates that a pollen-producing plant can indirectly decrease nearby herbivore densities by attracting an omnivorous predator.     

Plant damage from and defenses against ‘cryptic’ herbivory: A guild perspective

Abstract

Despite considerable interest in the factors affecting trophic cascades in terrestrial systems, there has been relatively little attention paid to the importance of the herbivore-plant link in explaining why some systems “cascade” (have strong top-down effects on plant survival and population growth) and others “trickle” (have top-down effects on plant damage, but little effect on plant fitness). This is despite the fact that herbivore guild identity has long been recognized as a major force affecting herbivore-plant interactions. We address the potential importance of herbivore guild identity in determining the strength of tritrophic interactions by reviewing literature concerning plant damage from and induced defenses against two “cryptic” herbivore guilds, predispersal seed predators and root/stem borers. Although both guilds are capable of strongly affecting plant fitness, the impact of root/stem borers on plants in natural systems seems far greater than that of predispersal seed predators. The large impact of root/stem borers occurs via their disruption of plant vascular systems, while a variety of factors (safe-site-limited plant populations, long-lived seed banks, temporal plant escape, etc.) each seem important in explaining the smaller effect of predispersal seed predators. While the lack of attention to herbivore guilds is understandable, given the (by necessity) single-species focus of much trophic cascade research, we suggest that predator suppression of root/stem borers and predispersal seed predators will, respectively, yield strong versus weak top-down effects on plant fitness. The potential tritrophic consequences of herbivore feeding mode highlight the importance of research on varied predator-herbivore chains that share a common basal resource.     

Positive effects of shrubs on plant species diversity do not change along a gradient in grazing pressure in an arid shrubland

Facilitative or positive interactions among species are driven mainly by the environmental amelioration or protection from grazing provided by nurse plants. Some studies have suggested that protection from grazing is inconsequential in water-limited environments because of low herbivore densities and their grazing effects. Others, however, argue that herbivores have a major effect on semi-arid plant communities, and that protection from grazing is a significant factor driving positive plant–plant interactions in such environments. We identified a gradient in grazing pressure in a semi-arid shrubland in south-eastern Australia along which we compared soil condition, incident radiation and plant composition beneath two nurse shrub species with open (shrub-free) interspaces. Our aim was to assess the degree of microclimatic amelioration provided by both shrubs, and changes in the interactions (intensity, importance and frequency) between both nurse shrubs and their understorey species, and their effects on species richness at the community level. Both the relative interaction intensity (RII) and interaction importance (I_imp) indices of plant–plant interactions were generally positive and independent of grazing pressure. Soil beneath both nurse plants had significantly greater indices of nutrient cycling and infiltration, and contained more C and N than soil in the open. Almost twice as many species occurred under the canopies of both shrubs (44 species) than in the open (23 species), and the composition of species differed significantly among microsites. Fifty-four percent of all perennial plant species occurred exclusively under shrubs. Our results suggest that environmental amelioration is a stronger driver of the facilitatory effect of shrubs on their understorey species than protection from grazing. Our conclusions are based on the fact that the substantial effect of plant–plant interactions on plant species richness was largely independent of grazing pressure. Irrespective of the underlying mechanism for this effect, our study illustrates the ecological role of shrubs as refugia for understorey plants in semi-arid environments and cautions against management practices aimed at reducing shrub populations.     

Dynamics of a plant-herbivore-predator system with plant-toxicity

A system of ordinary differential equations is considered that models the interactions of two plant species populations, an herbivore population, and a predator population. We use a toxin-determined functional response to describe the interactions between plant species and herbivores and use a Holling Type II functional response to model the interactions between herbivores and predators. In order to study how the predators impact the succession of vegetation, we derive invasion conditions under which a plant species can invade into an environment in which another plant species is co-existing with a herbivore population with or without a predator population. These conditions provide threshold quantities for several parameters that may play a key role in the dynamics of the system. Numerical simulations are conducted to reinforce the analytical results. This model can be applied to a boreal ecosystem trophic chain to examine the possible cascading effects of predator-control actions when plant species differ in their levels of toxic defense.     

The underappreciated role of life history in mediating the functional consequences of biodiversity change

Biodiversity is changing on both global and local scales, motivating research to understand the consequences of these changes for how communities and ecosystems function. Here, we explore the role of life history strategies in mediating biodiversity and ecosystem functioning. In particular, we evaluate how the composition, biomass (% cover), and richness of perennial (persistence ≥ 1 year) and ephemeral (persistence < 1 year) species change along a gradient of increasing seaweed species richness on a rocky shoreline. We show that the majority of biomass is comprised of perennial species, especially where overall richness is low, whereas the majority of species are ephemeral, especially where overall richness is high. We then present and discuss the results of an 18‐month field manipulation quantifying the factorial effects of tidal elevation, wave exposure, herbivore removals, thermal and desiccation stress amelioration, and nutrient additions on perennial versus ephemeral species. In particular, the diversity of ephemeral species was strongly affected, relative to perennial species, by tidal elevation, wave exposure, and herbivory; herbivores reduced diversity of ephemeral species relative to perennials. Relative to perennial cover, ephemeral cover was greater higher on the shore, in more wave‐exposed habitats, and where herbivores were removed, plots were unscreened, and/or nutrients were added. Thus, perennials and ephemerals responded differently to environmental context and experimental manipulation. We compared nitrate uptake and photosynthesis rates of ephemeral and perennial species and found that maximum nitrate uptake and photosynthesis rates of ephemerals were twice as high as those of perennials. These results highlight the disproportionate roles that ephemeral species play in mediating ecosystem‐level processes. In combination with our comparisons of the diversity and cover of perennial and ephemeral species along a biodiversity gradient, these results demonstrate the utility of incorporating life history traits into our efforts to understand the functional consequences of biodiversity change.     

Does aggregation benefit bark beetles by diluting predation? Links between a group‐colonisation strategy and the absence of emergent multiple predator effects

Abstract. 1. Aggregation in bark beetles (Coleoptera: Scolytidae) aids in mate attraction and resource procurement when colonising well‐defended plants; however, some species colonise primarily poorly defended plants, and intraspecific competition increases mortality. The hypothesis that decreased risk of predation was a potential benefit to aggregation in such circumstances was tested, using the pine engraver, Ips pini (Say) and its two major predators Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae). Both single‐ and multiple‐predator effects, across a range of prey densities, were tested. 2. Both male and female colonisation events increased with herbivore density, in an asymptotic fashion. 3. Predators decreased the number of colonisers in a density‐dependent manner, consistent with a type II functional response. 4. The proportional impact of predators decreased with increased herbivore colonisation densities. These findings indicate that predator dilution may be a viable benefit to aggregation. 5. Total emergence of the herbivore also increased with density, although the net replacement rate during one generation was independent of initial arrival density. This was likely due to larval predation, which negates potential relationships between per capita reproductive success and establishment density. 6. Each predator species decreased I. pini's net replacement rate by approximately 42%, and their combined effect was approximately 70%. 7. Overall, these predators modified their prey's establishment and adult mortality relationships in additive manners. This is somewhat surprising, given the potential for emergent effects due to interactions between multiple predators foraging within a common habitat. The persistence of additivity, rather than risk reduction or enhancement to the prey, may increase the predator‐swamping benefit to aggregation for this herbivore. 8. The effects of these predators are substitutable, and likely exert equivalent selective pressures to mask signals at the whole‐plant level.