Competitive Release Research Articles

Trophic cascade induced by molluscivore predator alters pore‐water biogeochemistry via competitive release of prey

Effects of predation may cascade down the food web. By alleviating interspecific competition among prey, predators may promote biodiversity, but the precise mechanisms of how predators alter competition have remained elusive. Here we report on a predator-exclosure experiment carried out in a tropical intertidal ecosystem, providing evidence for a three-level trophic cascade induced by predation by molluscivore Red Knots (Calidris canutus) that affects pore water biogeochemistry. In the exclosures the knots' favorite prey (Dosinia isocardia) became dominant and reduced the individual growth rate in an alternative prey (Loripes lucinalis). Dosinia, a suspension feeder, consumes suspended particulate organic matter (POM), whereas Loripes is a facultative mixotroph, partly living on metabolites produced by sulfur-oxidizing chemoautotrophic bacteria, but also consuming suspended POM. Reduced sulfide concentrations in the exclosures suggest that, without predation on Dosinia, stronger competition for suspended POM forces Loripes to rely on energy produced by endosymbiotic bacteria, thus leading to an enhanced uptake of sulfide from the surrounding pore water. As sulfide is toxic to most organisms, this competition-induced diet shift by Loripes may detoxify the environment, which in turn may facilitate other species. The inference that predators affect the toxicity of their environment via a multi-level trophic cascade is novel, but we believe it may be a general phenomenon in detritus-based ecosystems.

Ecological Opportunity and Diversification in a Continental Radiation of Birds: Climbing Adaptations and Cladogenesis in the Furnariidae

Ecological theories of adaptive radiation predict that ecological opportunity stimulates cladogenesis through its effects on competitive release and niche expansion. Given that key innovations may confer ecological opportunity, we investigated the effect of the acquisition of climbing adaptations on rates of cladogenesis in a major avian radiation, the Neotropical bird family Furnariidae, using a species-level phylogeny. Morphological specializations for vertical climbing originated in the woodcreepers ∼23 million years ago, well before that adaptation occurred in woodpeckers (Picidae) or in other potential competitors in South America. This suggests that the acquisition of climbing adaptations conferred ample ecological opportunity to early woodcreepers. Nonetheless, we found that increases in speciation rates in Furnariidae did not coincide with the acquisition of climbing adaptations and that the relationship between the accumulation of climbing adaptations and rates of speciation was negative. In addition, we did not detect a diversity-dependent decline in woodcreeper diversification rates consistent with saturation of the trunk-climbing niche. These findings do not support the hypothesis that ecological opportunity related to trunk foraging stimulated cladogenesis in this radiation. The negative effect of climbing on diversification may be mediated by an indirect positive effect of climbing on dispersal ability, which may reduce speciation rates over evolutionary timescales.

From coexistence to competitive exclusion: can overfishing change the outcome of competition in skates (Chondrichthyes, Rajidae)?

Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998-2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998-2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas.

From coexistence to competitive exclusion: can overfishing change the outcome of competition in skates (Chondrichthyes, Rajidae)?

Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998-2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998-2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas.

Islands in the sky or squeezed at the top? Ecological causes of elevational range limits in montane salamanders

Many animal and plant taxa reach their highest endemism and species richness in montane regions. The study of elevational range limits is central to understanding this widespread pattern and to predicting the responses of montane species to climate change. Yet, because large‐scale manipulations of the distributions of most species are difficult, the causes of species’ elevational range limits (e.g. competitive interactions, physiological specialization) are poorly understood. Here, we harness the power of new mechanistic approaches to dissect the factors that underlie the elevational replacement of two salamander species in the Appalachian Highlands. Our results challenge the long‐held idea that competitive interactions drive the lower elevational range limits of montane species and that physiological stress prevents low‐elevation species from expanding to high elevations. We show that physiological constraints drive the lower elevational range limit of the montane‐endemic species, Plethodon jordani. Conversely, we find that competition with P. jordani prevents the low‐elevation species, P. teyahalee, from expanding its range to include higher‐elevation habitats. These results are broadly consistent with the biogeography and behavior of other montane species, suggesting that similar mechanisms underlie patterns of elevational zonation across a variety of taxa and montane regions. To the extent that our findings are taxonomically and geographically widespread, these results challenge the idea that competitive release at species’ lower elevational range limits is driving the downslope range shifts exhibit by some montane taxa. Instead, our results raise the sobering possibility that even small changes in climate might cause erosion of the ranges of many high‐elevation species.

Shaping carnivore communities by predator control: competitor release revisited

AbstractThe traditional way to deal with hunting interests is often focused on predator control. This approach requires a detailed understanding of predator responses to game management. Control methods can be non‐selective and, consequently, can potentially have a negative impact on non‐target predator species, affecting carnivore guild. In order to assess the potential risks associated with non‐selective predator control in the presence of intraguild competition, the classical Lotka–Volterra competition model was used, but with two additional factors: a linear factor for capture mortality (mortality caused by predator control) and a factor for vacated niche occupancy associated with immigration. Simulation in a scenario with three species revealed that one of them, the red fox Vulpes vulpes, which presented the highest intrinsic growth rate, can show population increases even under moderate control, whereas the population of European badger Meles meles, with a lower intrinsic growth rate, was reduced and, in some cases, may even become extinct. Lastly, the stone marten Martes foina presented a differential response, depending on the removal intensity and strategy employed. This behaviour is compatible with the results observed in a Mediterranean environment subjected to two types of game management: non‐selective predator control (culling) and no control. In areas with non‐selective control and moderate predator removal, the red fox population remained stable, while the stone marten and the European badger populations reduced markedly or disappeared.

Responses of house mice to the removal of mammalian predators and competitors

AbstractEfforts to eradicate multiple mammal pests from offshore islands and fenced mainland ‘habitat islands’ often fail to remove mice, and such failures can result in a dramatic change in the food‐web whereby the removal of larger mammal pests facilitates a population explosion of mice through predator and competitor release. We investigated the ecological responses of house mice to the removal of mammalian predators from a 500‐ha fenced sanctuary at Tawharanui, northern New Zealand. Data on population structure and body condition of mice trapped in 2007, in four habitat types within the sanctuary, were compared with baseline data collected in 2001, before mammal control operations commenced. We hypothesized that: (i) in the absence of mammalian predators mouse densities would increase in all habitat types that provide vegetation cover, and (ii) in the absence of mammalian competitors mice would become heavier due to greater access to food resources. Mouse densities were significantly higher in 2007 than in 2001 in three habitat types. The high density of mice in forest – where none were trapped prior to control – suggests a competitive release, in which mice profited from the removal of ship rats. No mice were caught in the presence of ship rats on a forest trap‐line at a control site outside the sanctuary. Mice trapped in 2007 were significantly heavier than those trapped in 2001, and significantly heavier than mice trapped at the control site. Greater access to food in the absence of competing and predatory mammals probably explains the heavier body weight of Tawharanui mice. There has been a significant change in the mammalian food‐web at Tawharanui, such that the house mouse is now the primary pest. A rapid and dramatic increase in mouse numbers is likely to adversely impact invertebrates and seedling recruitment, which in turn could affect ecosystem functions.

Ruderality in extreme‐desert cacti? Population effects of chronic anthropogenic disturbance on Echinocereus lindsayi

AbstractRuderal species, i.e., those that increase their numbers in the presence of disturbance, are not expected to occur in extreme environments. We test whether Echinocereus lindsayi, a cactus from an extreme desert, follows the ruderal trend observed in similar species from mild deserts, or, as theory suggests, it is a non ruderal. Contrary to expectations, its density and fraction of small individuals in the population increased with disturbance. This seemingly results from increased establishment, as it is nursed by rocks exposed by disturbance. A demographic model for two populations, one nearly pristine and another highly disturbed, showed that at the latter site recruitment was more frequent and likely. At the disturbed site the performance of E. lindsayi individuals was usually poor, except on favorable years. Then, competition release caused by disturbance apparently allowed for a better performance compared to the less disturbed site. Despite that this opportunistic behavior would suffice to maintain the population size stable, the large mortality produced by an insect outbreak in two of the four study years caused the population to diminish. In contrast, the population at the less disturbed site was near equilibrium. If the insect outbreak is associated to disturbance, E. lindsayi at the disturbed site would be already experiencing more disturbance than it tolerates. This agrees with the fact that no populations were found at greater disturbance intensities. While, contrary to our hypothesis, E. lindsayi is ruderal, this extreme desert species appears to tolerate far less disturbance than its counterparts from milder areas.

Extreme climatic events and vegetation: the role of stabilizing processes

AbstractCurrent climatic trends involve both increasing temperatures and climatic variability, with extreme events becoming more frequent. Increasing concern on extreme climatic events has triggered research on vegetation shifts. However, evidences of vegetation shifts resulting from these events are still relatively rare. Empirical evidence supports the existence of stabilizing processes minimizing and counteracting the effects of these events, reinforcing community resilience. We propose a demographic framework to understand this inertia to change based on the balance between adult mortality induced by the event and enhanced recruitment or adult survival after the event. The stabilizing processes potentially contributing to this compensation include attenuation of the adult mortality caused by the event, due to site quality variability, to tolerance, phenotypic variability, and plasticity at population level, and to facilitative interactions. Mortality compensation may also occur by increasing future survival due to beneficial effect on growth and survival of the new conditions derived from global warming and increased climatic variability, to lowered competition resulting from reduced density in affected stands, or to antagonistic release when pathogens or predators are vulnerable to the event or the ongoing climatic conditions. Finally, mortality compensation may appear by enhanced recruitment due to release of competition with established vegetation, for instance as a consequence of gap openings after event‐caused mortality, or to the new conditions, which may be more favorable for seedling establishment, or to enhanced mutualistic interactions (pollination, dispersal). There are important challenges imposed by the need of long‐term studies, but a research agenda focused on potentially stabilizing processes is well suited to understand the variety of responses, including lack of sudden changes and community inertia that are frequently observed in vegetation under extreme events. This understanding is crucial for the establishment of sound management strategies and actions addressed to improve ecosystem resilience under climate change scenarios.

Flying Squirrel Demography Varies between Island Communities with and without Red Squirrels

Recent studies in Southeast Alaska suggest the ecology of Glaucomys sabrinus differs from populations in the Pacific Northwest. In Southeast Alaska, densities were the highest reported for the species, populations were not as closely linked to old-forest attributes, and individuals had a more diverse diet that was less dependent on mycophagy. Pacific Northwest communities are comprised of several arboreal rodents; Southeast Alaska has a depauperate mammal fauna. I hypothesized that Southeast Alaska populations had a broader realized niche because of competitive release. The red squirrel (Tamiasciurus hudsonicus) is the only other arboreal squirrel and it is absent from the southern outer islands of Southeast Alaska's Alexander Archipelago. I compared demography and body mass of G. sabrinus on Prince of Wales Island to a population in sympatry with T. hudsonicus on a separate island (Mitkof). Home ranges were larger and population density, breeding female density, and juvenile recruitment of G. sabrinus were all lower in sympatry with T. hudsonicus. In a companion study, G. sabrinus on Prince of Wales Island used cavities for denning relatively more frequently than in sympatry with T. hudsonicus on Mitkof Island. Female G. sabrinus depend on cavities for natal dens, and breeding female and population densities are positively correlated with large snag or tree density. The presence of T. hudsonicus may influence G. sabrinus populations by limiting availability of cavities. Furthermore, variation in vertebrate assemblages among islands may influence realized niches of resident species, which manifest unique demographic profiles compared to populations of different ecological communities.

The Sabah Biodiversity Experiment: a long-term test of the role of tree diversity in restoring tropical forest structure and functioning

Relatively, little is known about the relationship between biodiversity and ecosystem functioning in forests, especially in the tropics. We describe the Sabah Biodiversity Experiment: a large-scale, long-term field study on the island of Borneo. The project aims at understanding the relationship between tree species diversity and the functioning of lowland dipterocarp rainforest during restoration following selective logging. The experiment is planned to run for several decades (from seed to adult tree), so here we focus on introducing the project and its experimental design and on assessing initial conditions and the potential for restoration of the structure and functioning of the study system, the Malua Forest Reserve. We estimate residual impacts 22 years after selective logging by comparison with an appropriate neighbouring area of primary forest in Danum Valley of similar conditions. There was no difference in the alpha or beta species diversity of transect plots in the two forest types, probably owing to the selective nature of the logging and potential effects of competitive release. However, despite equal total stem density, forest structure differed as expected with a deficit of large trees and a surfeit of saplings in selectively logged areas. These impacts on structure have the potential to influence ecosystem functioning. In particular, above-ground biomass and carbon pools in selectively logged areas were only 60 per cent of those in the primary forest even after 22 years of recovery. Our results establish the initial conditions for the Sabah Biodiversity Experiment and confirm the potential to accelerate restoration by using enrichment planting of dipterocarps to overcome recruitment limitation. What role dipterocarp diversity plays in restoration only will become clear with long-term results.

Niche overlap in allotopic and syntopic populations of sexually interacting ground‐hopper species

Abstract There is accumulating evidence that sexual interactions among species (reproductive interference) could have dramatic effects for species’ coexistence. It has been shown that the fitness of individuals can be substantially reduced as a consequence of reproductive interference. This might subsequently lead to displacement of a species (sexual exclusion). On the other hand, some evolutionary and ecological mechanisms might enable species to coexist, such as the divergence of mate recognition systems (reproductive character displacement), habitat partitioning, clumped dispersion patterns or different colonization capabilities. We have previously shown that the two ground‐hopper species Tetrix subulata and Tetrix ceperoi interact sexually in the laboratory as well as in the field. At sites where both species co‐occur niche overlap was high, suggesting that coexistence is maintained by different niche breadths rather than by habitat partitioning. To test the hypothesis that habitat partitioning does not contribute to species’ coexistence, we examined whether allotopic and syntopic populations of these two species differ in niche overlap (competitive release). Our results show that niche overlap is higher in syntopic than in allotopic populations, suggesting that the site‐specific habitat structure (heterogeneity) has a stronger influence on microhabitat utilization than the presence of heterospecifics. Hence, our data do not support the hypothesis that habitat partitioning plays a substantial role for the coexistence of these sexually interacting species.

Elevated size and body condition and altered feeding ecology of the grouper Cephalopholis argus in non-native habitats

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 439:203-212 (2011) - DOI: https://doi.org/10.3354/meps09338 Elevated size and body condition and altered feeding ecology of the grouper Cephalopholis argus in non-native habitats Amanda L. Meyer1,2,*, Jan Dierking1,3,*,** 1Department of Zoology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA 2Present address: Pacific Reefs National Wildlife Complex, US Fish and Wildlife Service, Honolulu, Hawaii 96850, USA 3Present address: Leibniz Institute of Marine Sciences (IFM-GEOMAR), Evolutionary Ecology of Marine Fishes Group, 24105 Kiel, Germany *The 2 authors contributed equally to this study**Email: jdierking@ifm-geomar.de ABSTRACT: In 1956, the shallow-water grouper Cephalopholis argus was introduced from Moorea (French Polynesia), where grouper diversity (14 species) is high, to the Main Hawaiian Islands (MHI), where only 2 rare native deep-water groupers occur. In this non-native environment, the species has flourished and has become the dominant apex predator on many reefs. In the present study, a comparison of non-native populations of C. argus in the MHI with native populations in Moorea showed that mean total length (32.0 vs. 26.9 cm), mass (722 vs. 326 g), growth, and body condition were each significantly elevated in the MHI. In addition, while an ontogenetic shift towards larger prey occurred in both locations, it was faster and more consistent in Moorea than in the MHI. As a result, while small C. argus of comparable size in the 2 locations consumed similar-sized prey, large C. argus in Moorea consumed significantly longer and deeper-bodied prey than their counterparts in the MHI. This pattern was unrelated to the size distributions of available prey and may thus reflect stronger intra- and interspecific competition for small prey in Moorea. Although ecological release in a broader sense (i.e. a combination of predator release, parasite release, and competitive release) may play a role, the most direct explanation for the observed differences between C. argus in native habitats in Moorea (with many competing grouper species) and non-native habitats in the MHI (few competitors) would be competitive release (here used in the sense of benefits resulting from the reduction of interspecific competition). KEY WORDS: Competitive release · Ecological release · Interspecific competition · Niche shift · Invasive species · Peacock hind · Main Hawaiian Islands · Moorea (French Polynesia) Full text in pdf format PreviousNextCite this article as: Meyer AL, Dierking J (2011) Elevated size and body condition and altered feeding ecology of the grouper Cephalopholis argus in non-native habitats. Mar Ecol Prog Ser 439:203-212. https://doi.org/10.3354/meps09338 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 439. Online publication date: October 20, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Deciphering complex relationships between apparently unrelated species

Rabbits are considered by biologists to be among the worst invasive species. Their impact on invaded ecosystems, epitomized in Australia and on islands worldwide (e.g. North, Bullock & Dulloo, 1994; Williams et al., 1995; Burbidge & Moris, 2001; Garzon-Machado et al., 2010), has earned them a well-deserved place in the infamous Invasive Species Specialist Group’s ‘100 of the Worst Invasive Alien Species’ list. Rabbits are capable of profoundly altering communities, ecosystems and landscapes by sapping biodiversity and biomass from the basic trophic levels of often fragile ecosystems. However, they also have more complex, and hitherto seldom documented effects on higher trophic levels (Zedler & Black, 1992; Priddel, Carlile & Wheeler, 2000; Oliver, LuqueLarena & Lambin, 2009; Denyer, Hartley & John, 2010). In the context of a few, but loud voices attempting to question the importance of alien invasive species (but see Lambertini et al., 2011; Lockwood, Hoopes & Marchetti, 2011), the paper by Brodier et al. (2011) is well timed. It reminds us that restoration ecology and the fight against biodiversity loss are possible, in particular on islands, as they generally have many endemic and threatened species. This paper also highlights two very important, yet previously overlooked points. The first is the importance of longterm follow-up after restoration programmes. The second is the need to consider the indirect (and consequently often hidden) effects that alien invasive species can have on seemingly unconnected species. By monitoring the populations, several petrel species and their avian predator (the brown skua, Catharacta skua) following the removal of rabbits, Brodier et al. (2011) show that rabbits compete for space with blue petrels, Halobaena caerulea. Following rabbit eradication, the competition for burrows was removed and the blue petrel population increased up to eightfold in a few generations. Brown skua also increased their survivorship in response to the increase in its main prey, the blue petrel. Interestingly, the population of another petrel, the Antarctic prion, Pachyptila desolata, was also impacted by the rabbits. Following the eradication, the Antarctic prion population declined fourfold (to exclusion) in areas of deep soil and remained stable where the soil was too shallow for blue petrel (and rabbits). The resulting system, illustrated in Fig. 1, shows that one unexpected, and up to now unreported, effect of rabbits was to erode the niche partitioning of nesting seabirds on the island. Removing the rabbits led to both a competitor release (of the blue petrel, freed from the competition with rabbits) and to an apparent competitor release (of the shared predator, the brown skua, boosted by increased blue petrel availability). The combination of direct and indirect interactions restricted Antarctic prions to shallow soil areas, less favoured by the blue petrel. This neat system shows how alien invasive species can have effects beyond the trophic level constituting their resources. The contrast between the spectacular recovery of the blue petrel, the increased survival but stable number of skua breeding pairs and the decrease of the Antarctic prion also illustrates well the difficulties to forecast outcomes that conservation program may face even in relatively simple trophic webs. This is especially so in the current context of global climate change, which already affects subantarctic islands (Chapuis, Frenot & Lebouvier, 2004; Dowding et al., 2009), and which could have played a role in the system, for example, in the delayed recovery of the skua. Monitoring population densities, survival and predator diet, every year, on this remote, far reaching island, for long after the rabbit eradication might have been seen as a risky bet. As it turned out, the bet paid off, in the best currency ever: knowledge and understanding. Deciphering the indirect effects a terrestrial herbivore can have on a community of seabirds with which they have few interactions if any, is crucial in our battle against alien invasive species (Bergstrom et al., 2009; Genovesi, 2011). It demonstrates very well that if we were to ‘judge alien species only on their effects’, we would miss all but the tip of the iceberg and would allow important but unapparent, indirect effects of alien invasive species on native ecosystems. One regret from this study is that the monitoring effort conducted after rabbit eradication was not also implemented Animal Conservation. Print ISSN 1367-9430

Competition between stink bug and heliothine caterpillar pests on cotton at within-plant spatial scales

Outbreaks of non-target pests associated with transgenic Bt cotton threaten the economic and ecological benefits of the technology in cotton-producing countries. In the southeastern USA, stink bug pests, namely Nezara viridula L. and Euschistus servus Say (both Heteroptera: Pentatomidae), have recently become severe problems associated with Bt cotton, requiring continued insecticide use. However, the causes of non-target pest outbreaks remain unclear. Release from competition with Bt-susceptible pest species, in addition to other mechanisms, may contribute to increased stink bug populations in Bt cotton. We investigated the competitive interactions between the two stink bug species and the Bt-susceptible pests Helicoverpa zea (Boddie) and Heliothis virescens Fabricius (both Lepidoptera: Noctuidae) on non-Bt cotton. We tested for competition effects on stink bug growth rates in no-choice experiments at two spatial scales: a single cotton boll and a branch with multiple developing bolls. Although caterpillars of the two species had equivalent effects on resource availability, they had distinct effects on stink bug growth rates. Fourth instar H. zea reduced growth rates of both stink bug species by 60% when caged on a single cotton boll and reduced growth rates of only E. servus by 36% when caged on a cotton branch. In contrast, H. virescens had no effect on stink bug growth rates. Resource competition was apparent in the interactions between H. zea and E. servus, but interference competition may have contributed to the interactions as well. Competitive release of stink bug populations in Bt cotton is possible, and should be more likely for E. servus than for N. viridula. Understanding the causes of non-target pest outbreaks in Bt cotton will contribute to improved environmental risk assessments of future releases of Bt cotton and related transgenic crops.

Shrub island effects on a high-altitude forest cutover in the eastern Tibetan Plateau

The roles of woody-plant islands are well documented in low-altitude regions, but research related to such shrub effects in high-altitude regions is scant. Four common shrub species (Cerasus trichostoma, Ribes glaciale, Rosa omeiensis and Salix sphaeronymphe) in a high-altitude forest cutover of the eastern Tibetan Plateau, were chosen to evaluate the effects of both species and size of shrub islands on microhabitats, herbaceous communities and woody seedling regeneration. Total 86 shrubs with different sizes were investigated; The shrub size, herb community structure and species composition, litter, soil nutrient and microclimate parameters beneath the shrub canopies were also measured. All shrubs significantly ameliorated microclimates, increased content of soil organic matter and total nitrogen, both grass and forb species richness, and litter cover and biomass, and promoted woody seedling recruitment (richness and number), but decreased cover and biomass of the herbaceous community beneath them. These effects were greater for larger shrubs, and also varied among shrub species with different crown architectures. We also found differences in species-dependency of the shrub effect for the responses of the herbaceous and woody seedling species, suggesting that shrubs also indirectly facilitate forbs and seedling regeneration through competition release of grasses. We conclude that shrub-island effects are size- and species-dependent. In order to accelerate natural succession and restoration in alpine cutovers, shrub island preservation and their effective utilization as reforestation microhabitats should be integrated into vegetation management procedures.

The fitness of drug-resistant malaria parasites in a rodent model: multiplicity of infection.

Malaria infections normally consist of more than one clonally replicating lineage. Within-host interactions between sensitive and resistant parasites can have profound effects on the evolution of drug resistance. Here, using the Plasmodium chabaudi mouse malaria model, we ask whether the costs and benefits of resistance are affected by the number of co-infecting strains competing with a resistant clone. We found strong competitive suppression of resistant parasites in untreated infections and marked competitive release following treatment. The magnitude of competitive suppression depended on competitor identity. However, there was no overall effect of the diversity of susceptible parasites on the extent of competitive suppression or release. If these findings generalize, then transmission intensity will impact on resistance evolution because of its effect on the frequency of mixed infections, not because of its effect on the distribution of clones per host. This would greatly simplify the computational problems of adequately capturing within-host ecology in models of drug resistance evolution in malaria.

Aggregative Behavior is Not Explained by an Allee Effect in the Walnut Infesting Fly, Rhagoletis juglandis

Component Allee effects are considered to be a driving force in the origin and maintenance of aggregative behavior. In this study, we examine whether a pattern of active host reuse by the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), involves an Allee effect. We examined how the density of clutches deposited within a fruit, the temporal pattern in which successive clutches are deposited and the spatial distribution of clutches over a fruit's surface influences survival to pupation and pupal size. Within the density range used in this experiment (1 to 7 clutches), increases in larval density strongly reduced pupal weight but not larval survival to pupation. The temporal staggering of clutches into a host strongly reduced offspring survival and, probably owing to competitive release, increased the pupal weight of survivors. Offspring survival and pupal weight were affected relatively little by whether two clutches were deposited within the same oviposition punctures or were evenly spaced. In contrast, in three-clutch treatments offspring survival was higher when clutches were placed within the same oviposition cavity. However, pupal weights did not significantly increase when clutches were placed together and this relatively higher survival rate was not greater than that associated with hosts that contained fewer clutches. The results of the study failed to provide evidence of an Allee effect. We put forward a scenario under which females appear to reuse larval hosts to maximize their own reproductive success, albeit at the expense of the per capita fitness of their offspring.

Climate influences parasite-mediated competitive release

Parasitism is believed to play an important role in maintaining species diversity, for instance by facilitating coexistence between competing host species. However, the possibility that environmental factors may govern the outcome of parasite-mediated competition has rarely been considered. The closely related amphipods Corophium volutator and Corophium arenarium both serve as second intermediate host for detrimental trematodes. Corophium volutator is the superior competitor of the two, but also suffers from higher mortality when exposed to infective trematode stages. Here, we report parasite-mediated competitive release of C. arenarium in an intertidal habitat, in part triggered by unusually high temperatures linked to the North Atlantic climate oscillation (NAO). The elevated temperatures accelerated the transmission of cercariae from sympatric first intermediate hosts (mud snails) to amphipods, causing a local collapse of the parasite-sensitive C. volutator population and concordant increase in the abundance of the competitively inferior C. arenarium.

Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals

Invasive species are frequently the target of eradication or control programmes to mitigate their impacts. However, manipulating single species in isolation can lead to unexpected consequences for other species, with outcomes such as mesopredator release demonstrated both theoretically and empirically in vertebrate assemblages with at least two trophic levels. Less is known about the consequences of species removal in more complex assemblages where a greater number of interacting invaders increases the potential for selective species removal to result in unexpected changes in community structure. Using a replicated Before-After Control-Impact field experiment with a four-species assemblage of invasive mammals we show that species interactions in the community are dominated by competition rather than predation. There was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.