Interspecific Interference Research Articles

Potential of habitat complexity for mitigating interference competition between native and non-native salmonid species

This study aimed to confirm the mitigation effect of structurally complex habitat on interspecific interference competition between native and non-native salmonid species. We evaluated the effects of habitat complexity (number and size of aggregates of large woody debris and length of undercut banks) and other habitat components that were related to the local abundance of salmonids (dimension and mean depth of pool, mean particle size, and mean current velocity) on the local abundance of native white-spotted char ( Salvelinus leucomaenis (Pallas, 1814)) in allopatry and sympatry with non-native brown trout ( Salmo trutta L., 1758). The field survey was conducted in a Japanese montane stream. The number of char in pool habitats in the allopatric area correlated positively with habitat size, i.e., pool dimension. The number of char in the sympatric area with trout was positively correlated with habitat complexity, while it was negatively correlated with number of trout. In this study, we found that structurally complex habitats might be effective in mitigating interspecific competition between native white-spotted char and non-native brown trout in a montane stream.

Ecosystem functioning in stream assemblages from different regions: contrasting responses to variation in detritivore richness, evenness and density

1. The diversity of species traits in a biological assemblage varies not only with species richness, but also with species evenness and organism density, which together influence the concentration of traits within functional guilds. Potential trait diversity at local scales is also constrained by the regional species pool. Implications of such variation for spatio-temporal variability in biodiversity-ecosystem functioning relationships are likely to be complex, but are poorly understood. 2. In microcosm experiments conducted at laboratories in Sweden, Ireland and Romania, we investigated effects of species richness, evenness and density of stream-living detritivores on two related processes: detritivore leaf-processing efficiency (LPE) and growth. Assemblage composition varied among laboratories: one taxonomic order (Plecoptera) was studied in Sweden, whereas two orders, encompassing wider trait variation, were studied in Romania (Trichoptera and Plecoptera) and Ireland (Trichoptera and Isopoda). 3. Relationships between density and both LPE and growth ranged from negative to positive across the study species, highlighting the potential for density-dependent variation in process rates to alter ecosystem functioning, but indicating that such effects depend on species identity. 4. LPE varied with species diversity in the two more heterogeneous assemblages, but whereas LPE in the Romanian study was generally enhanced as richness increased, LPE in the Irish study increased only in less-even polycultures dominated by particular species. Transgressive overyielding was detected in the Irish experiment, indicating complementary resource use and/or facilitation (complementarity). These mechanisms could not be distinguished from the selection effect in the Romanian study. 5. Growth was elevated in Romanian species mixtures, reflecting positive complementarity, but lower than expected growth in some Swedish mixtures was associated with negative complementarity, indicating interspecific interference competition. 6. Our results emphasize the potential importance of detritivore diversity for stream ecosystem functioning, but both the effects of diversity on the studied processes, and the mechanisms underlying those effects, were specific to each assemblage and process. Such variability highlights challenges in generalizing impacts of diversity change for functional integrity in streams and other ecosystems in which the occurrence of important species traits fluctuates over relatively small spatio-temporal scales.

Agonistic behaviour of Palaearctic passerine migrants at a stopover site suggests interference competition

Records of interspecific agonistic behaviour of Palaearctic passerine migrants from their Afrotropical wintering grounds are rare. There are, however, no detailed observations from stopover sites where individuals might concentrate and depress resources that are critical for fat-depleted birds in times of high energy demand. We recorded intraspecific and interspecific interactions of Palaearctic migrant passerine birds at Ouadâne, Mauritania, a stopover site in the Sahara desert. In spring 2003 we made casual records of all aggressive behaviour we observed, whereas in spring 2004 we used focal sampling of foraging birds to record the frequency of aggression. We found that interspecific interactions occurred more often than intraspecific interactions but their relative frequency differed between species. There were also mass-dependent species hierarchies. A comparison with other studies showed that in Willow Warblers the frequency of interspecific aggression was significantly higher at the stopover site than in the wintering grounds farther south. We cannot assess the adaptive advantage of agonistic behaviour, as our approach does not consider the allocation of aggressive interactions in relation to other activities and its influence on the birds' energy balance. Nevertheless, our observations suggest interspecific interference competition because inferior individuals are temporarily deprived of food, which may influence the fitness of migrants.

Interference between two floating-leaved aquatic plants: Nymphoides peltata and Trapa bispinosa

A 12-week microcosm experiment was conducted to identify the intraspecific and interspecific interference abilities of two floating-leaved aquatic plants Nymphoides peltata (Gmel.) O. Kuntze and Trapa bispinosa Roxburgh. N. peltata exerted a strong interspecific interferential effect on T. bispinosa, but the opposite was not apparent. There existed an intraspecific interferential effect of T. bispinosa on itself, although the interspecific interference of N. peltata was stronger. No apparent intraspecific interference was found among the individual plants of N. peltata. The removal of N. peltata had a positive impact on growth and performance of T. bispinosa, whereas no apparent impacts were found in the removal of T. bispinosa on growth and performance of N. peltata. Moreover, both the removal of N. peltata and T. bispinosa did not show apparent effects on growth of the remaining individuals of the species themselves. These results suggest that N. peltata has distinct interferential advantages over T. bispinosa and that this depends mainly upon the differences between the two species in morphology and life history.

Interspecific territoriality in Calopteryx damselflies: the role of secondary sexual characters

Interspecific territoriality is usually interpreted to result from interspecific interference competition, although it may also originate from mistaken species recognition. In the latter case, it may be based on similarity of secondary sexual characters. In the damselfly Calopteryx splendens , males have pigmented wing spots as a sexual character, and males with the largest spots resemble males of another species, Calopteryx virgo . Probably because of this resemblance, C. virgo males are more aggressive towards large- than small-spotted C. splendens males. We examined whether wing spot size of C. splendens males affects territorial interactions between the species. In a removal experiment, the number of territorial C. splendens increased after C. virgo males were removed. However, interspecific territoriality was incomplete since before the removal the territories of the species partially overlapped. Wing spot size of C. splendens seemed to affect interspecific territoriality: before the removal territorial and nonterritorial C. splendens males had similar spot sizes and the distance to the nearest C. virgo territory increased with wing spot size of C. splendens . In addition, after the removal of C. virgo , the relation between wing spot size and the distance to the nearest old C. virgo territory disappeared. Our results suggest that mistaken species recognition may account for the interspecific territorial behaviour in the two species. Furthermore, interspecific territoriality may cause negative selection on wing spot size and thus may explain character displacement in wing spot size of C. splendens males.

Retracted: Dual role for an allelochemical: (±)‐catechin from Centaurea maculosa root exudates regulates conspecific seedling establishment

Summary Plant allelopathic compounds may have other roles than interspecific interference. We investigated whether (±)‐catechin, a phytotoxin exuded from the roots of the exotic invader Centaurea maculosa (spotted knapweed), is also one of the factors that regulates C. maculosa recruitment. Adding activated carbon, which adsorbs organic compounds, to soil around C. maculosa adults in the field increased seedling density by 78% 25 days after sowing, and by 34% 32 days after sowing, suggesting that soil‐borne compounds inhibited or delayed recruitment. Analysis of field soils near mature C. maculosa revealed that they can contain exceptionally high (±)‐catechin concentrations (mean = 1.55 mg g−1 dry soil, with 60% of samples containing ≥ 1.0 mg (±)‐catechin g−1). In laboratory experiments, treatment with ≥ 1.0 mg (±)‐catechin mL−1 reduced seedling root elongation by > 50%, indicating that field concentrations are sufficient to inhibit C. maculosa recruitment. Provided that 10% methanol was used to maintain (±)‐catechin in solution for > 1 day, treatment with ≥ 1.0 mg mL−1 also reduced C. maculosa germination by > 70%. (±)‐Catechin maintained in solution with methanol did not significantly reduce C. maculosa seed survival, suggesting that inhibition of germination was due, at least in part, to delayed germination rather than to seed mortality. Depending on the solubility of (±)‐catechin in soil and on the duration of its effects on recruitment, C. maculosa may avoid intraspecific competition or regulate the timing of seedling establishment by reducing seedling growth or postponing germination in response to its own phytotoxin. Chemical regulation of C. maculosa recruitment, as demonstrated here, suggests a dual role of (±)‐catechin as an allelochemical and an autoinhibitor. The potential for a single plant root exudate to influence both inter‐ and intraspecific interactions emphasizes the complex effects that plant secondary metabolites may have on plant population and community structure.

Interference between shatter cane (Sorghum bicolor) and soybean (Glycine max)

A field study was conducted to evaluate the effects of shatter cane (Sorghum bicolor) and soybean (Glycine max) densities on soybean seed yield and to quantify interspecific and intraspecific interference coefficients. The relations between soybean seed yield per plant and shatter cane density at different densities of soybean and also with soybean density at different densities of shatter cane are well described by the reciprocal equations. Soybean seed yield per unit area decreased with increasing shatter cane density. The highest yield loss (57%) was relative to 50 and 12 plants/m2 of soybean and shatter cane densities, respectively. Optimum soybean densities on the basis of maximum soybean seed yield per unit area at 0,4, 8, and 12 plants/m2 of shatter cane were achieved at 50, 34, 32, and 36 plants/m2, as estimated by the asymptotic or parabolic relations between soybean seed yield per unit area and soybean densities at different shatter cane densities. Shatter cane was a stronger competitor than soybean, as a shatter cane plant was equal to 2.5 soybean plants, based on soybean seed yield. In contrast, a soybean plant was equal to 0.18 of a shatter cane plant, on the basis of shatter cane biomass. It was, therefore, concluded that the superior competitor was mostly affected by intraspecific interference, but the weaker competitor was mostly affected by interspecific interference.

Analysis of interactions between the invasive tree-of-heaven (Ailanthus altissima) and the native black locust (Robinia pseudoacacia)

Invasive exotic plants can persist and successfully spread within ecosystems and negatively affect the recruitment of native species. The exotic invasive Ailanthus altissima and the native Robinia pseudoacacia are frequently found in disturbed sites and exhibit similar growth and reproductive characteristics, yet each has distinct functional roles such as allelopathy and nitrogen fixation, respectively. A four-month full additive series in the greenhouse was used to analyze the intraspecific and interspecific interference between these two species. In the greenhouse experiment, the inverse of the mean total biomass (g) response per plant for each species was regressed on the density of each species and revealed that the performance of the plants was significantly reduced by interspecific interference and not by intraspecific interference (p < 0.05). Other biomass traits such as root dry weight, shoot dry weight, stem dry weight, and leaf dry weight were also negatively affected by interspecific interference. Competition indices such as Relative Yield Total and Relative Crowding Coefficient suggested that A. altissima was the better competitor in mixed plantings. Ailanthus altissima consistently produced a larger above ground and below ground relative yield while R. pseudoacacia generated a larger aboveground relative yield in high density mixed species pots.

Establishment of Seeded Centipedegrass (Eremochloa ophiuroides) in Utility Turf Areas

Experiments were conducted to evaluate safety and effectiveness of herbicides during establishment of seeded centipedegrass. Centipedegrass tolerance to herbicides was evaluated at seeding and early postemergence. Imazapic at 105 g ai/ha, sulfometuron at 53 g ai/ha, or metsulfuron at 21 or 42 g ai/ha applied at seeding reduced centipedegrass ground cover compared with the nontreated. Imazapic at 18 or 35 g/ha or applications of atrazine or simazine at seeding did not reduce centipedegrass ground cover compared with the nontreated. Applications of chlorsulfuron plus mefluidide (7 + 140 g ai/ha) or metsulfuron at 21 or 42 g/ha applied 6 wk after seeding (WAS) centipedegrass (one-leaf to one-tiller growth stage) caused 20, 16, and 83% phytotoxicity, respectively, 56 d after treatment (DAT). Imazapic, sulfometuron, atrazine, or simazine applied 6 WAS caused &lt;15% phytotoxicity 56 DAT. When large crabgrass and centipedegrass were seeded together, large crabgrass emergence was reduced 41% when atrazine (1,100 g ai/ha) was applied at seeding. Centipedegrass tiller production was reduced with increasing amounts of crabgrass. However, centipedegrass tiller production and ground cover were higher when atrazine was applied because of reduced interspecific interference from large crabgrass. These data indicate that centipedegrass can be established more quickly if appropriate herbicides are used at seeding or shortly after seeding.

Spring Wheat, Canola, and Sunflower Response to Persian Darnel (Lolium persicum) Interference

Integrated weed management practices, such as crop rotation and increased seeding rates, potentially improve weed management. Yet, few studies compare competitive interactions of weeds with different crops. This research quantified the impact of Persian darnel on spring wheat, canola, and sunflower yield across different seeding rates. Increasing crop density increased yield when Persian darnel affected crop yield early in physiological development. Crop yield loss was estimated to reach 83, 70, and 57% for spring wheat, canola, and sunflower, respectively, at high Persian darnel densities. Persian darnel reduced spring wheat yield by limiting the number of tillers per plant and seed per tiller; reduced canola yield by limiting the number of branches per plant, pods per branch, and seed per pod; and reduced sunflower yield by limiting the number of seed per plant. Persian darnel affected crop growth early in physiological development, indicating that interspecific interference occurred early in the growing season. Cultural and resource management aimed at reducing Persian darnel impact on resource availability and crop yield components will reduce Persian darnel impact on crop yield.

Interspecific interference behaviours by workers of the harvesting ant Messor capitatus (Hymenoptera Formicidae)

The different techniques employed by Messor capitatus workers to interfere with the activity of neighbouring heterospecific colonies are described for the first time. Observations and experiments conducted both in the field and in the laboratory allowed us to identify a behavioural task group of workers (defined as ‘sappers’) that have a direct detrimental effect on colony activity of heterospecific congeneric competitors. A rich repertory of interspecific interference behaviours are described. Sappers often gather around the entrance holes of other nests to limit or prevent (by threat displays, physical fighting, and nest-plugging with earth, pebbles or other materials) residents from emerging. All these strategies strongly affect the activity of the target colony that may be completely blocked as long as the sappers patrol the nest. Disturbance along foraging tracks as well as cleptobiosis (food robbing) at nest entrances have been recorded. M. capitatus workers are also very effective at monopolizing conspicuous food sources, chasing off workers of other species (even if they had first discovered the food), and inducing them to barricade themselves inside their nest until the newly-discovered food source has been completely depleted. The possible effects of M. capitatus interference strategies on the ecology of neighbouring colonies are discussed.

Prey switching in four species of carnivorous stoneflies

Summary1. Previous studies compared the functional responses to their prey, and both intraspecific and interspecific interference, in mature larvae of Dinocras cephalotes, Perla bipunctata, Isoperla grammatica and Perlodes microcephalus. The present study examines switching by larvae of the same species presented with different proportions of two contrasting prey types; larvae of Baetis rhodani and Chironomus sp. In each experiment, 200 prey were arranged in nine different combinations of the two prey types (20 : 180, 40 : 160, 60 : 140, 80 : 120, 100 : 100, 120 : 80, 140 : 60, 160 : 40, 180 : 20). Prey were replaced as they were eaten. A model predicted the functional response in the absence of switching and provided a null hypothesis against which any tendency for switching could be tested.2. No evidence for prey switching by Dinocras and Perla was obtained, both species showing a slight preference for Baetis over Chironomus. Prey switching occurred in Isoperla and Perlodes. As the relative abundance of one prey type increased in relation to the alternative, the proportion eaten of the former prey changed from less to more than expected from its availability, the relationship being described by an S‐shaped curve. Isoperla and Perlodes switched to a preference for Baetis when its percentage of the total available prey exceeded 57 and 42%, respectively. Equivalent values for Chironomus were 43 and 58% for Isoperla and Perlodes, respectively. Switching was strongest in Perlodes.3. Non‐switching in Dinocras and Perla was related to their feeding strategy, both species being more successful when using a non‐selective ambush strategy at dusk and dawn rather than a search strategy during the night. Both Isoperla and Perlodes used a search strategy. The smaller Isoperla fed chiefly at dusk and dawn, and preferred Chironomus larvae, whereas most of the larger Perlodes fed continuously from dusk to dawn and preferred Baetis larvae.

Root-knot nematodes affect annual and perennial weed interactions with chile pepper

A field microplot experiment was conducted in 1996 and 1997 to determine the influence of root-knot nematodes on intra- and interspecific interactions between chile pepper (chile) and spurred anoda and between chile and yellow or purple nutsedge (or both) using a substitution design. An additional objective was to determine the influence of London rocket, a winter annual and host plant for root-knot nematodes, on the inter- and intraspecific interactions between chile and spurred anoda. Twelve plant combinations were planted into paired 76-cm-diam microplots at a density of 24 plants per microplot each year. Each pair of microplots had one root-knot nematode–infested and one uninfested plot. One randomly selected plant pair or triplet from each plot was destructively sampled in June, July, August, and September each year. Data included leaf area, plant dry weights (leaf, stem, root or root plus rhizome, chile fruit, and nutsedge tuber), and nematode egg production from the belowground biomass of the different plant species within a 2,355-cm3sampled soil volume. Chile hosted the highest population of root-knot nematodes, followed by spurred anoda, purple nutsedge, and yellow nutsedge. Few root-knot nematode eggs were recovered from London rocket before incorporation into the microplots each spring. Root-knot nematode populations were higher in 1997 and, as a result, more interactions between nematodes and plant combinations were observed for chile. Spurred anoda and root-knot nematodes reduced chile shoot and root weights to levels not significantly different from zero in 1997. Yellow and purple nutsedge shoots, except for those from the original tuber, were removed throughout the season, and these species interfered less with chile. Spurred anoda was not affected by interspecific interference. Few interactions were observed between the spurred anoda plant combinations and root-knot nematodes. In June 1996, low populations of root-knot nematodes (&lt; 4,000 eggs per gram of root) stimulated spurred anoda growth, but higher populations in June 1997 (&gt; 30,000 eggs per gram of root) reduced spurred anoda growth. Prior presence of London rocket had little consistent influence on spurred anoda or chile. Yellow and purple nutsedge growth variables were affected by interference from chile and the other nutsedge species. Tuber number and tuber weight were higher when plants were infected with root-knot nematodes, particularly early in the season. The enhanced tuber production may increase early-season interference from nutsedge species under production conditions. The results suggest that the presence of root-knot nematodes influences plant interference, but the effect is species specific. Annual plants are affected differently compared with perennial nutsedges, possibly because of the continuous association between the perennials and the parasite.

Plants as seed traps: inter-specific interference with dispersal

Established plants may disrupt patterns of seed deposition by physically trapping dispersing seed. In this study we quantified seed trapping at very fine spatial scales, by measuring seed fall of two ericaceous dwarf shrub species at a range of distances from allospecific shrubs within a matrix of short grass vegetation. Next to allospecific bushes, seed deposition densities of Calluna vulgaris and Erica cinerea were between 7- and 20-fold higher than those in the short grass. This increase is comparable to deposition densities next to conspecific bushes, caused by local dispersal, which were 19–33-fold greater than the background seed rain. Seed densities decayed rapidly with distance from the allospecific bush, so that in general the bush’s influence on deposition extended <0.1 m. By causing localised, extreme peaks in seed density around established plants, seed trapping could have large effects on the fitness of plants dispersing seed and on the spatial pattern of recruitment across plant communities.

Interspecific interference and the functional response of four species of carnivorous stoneflies

Summary1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae ofPerlodes microcephalus,Isoperla grammatica,Dinocras cephalotesandPerla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae.Baetislarvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae.2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest forPerlodes,slightly higher forIsoperla,and much higher forDinocrasandPerla. It was not affected significantly either by predator density or the identity of the competing species.3. Attack rate also varied between species and decreased with predator density. This decrease was slight forPerlodes,and also forDinocrasandPerlain competition withIsoperla. The decrease inDinocrasandPerlawas similar to that for intraspecific interference.4. The decrease in attack rate was described by a convex curve forPerlodeswith the other three species and forDinocras/PerlawithIsoperla,but by a concave curve (negative power function) forIsoperlacompeting with the other three species, and for bothDinocrasandPerlain competition withPerlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate.5. A comparison with previous results for intraspecific interference showed that the latter was dominant forPerlodesin all contests and forDinocrasorPerlacompeting withIsoperla,whilst interspecific interference dominated forIsoperlain all contests and forDinocrasandPerlacompeting withPerlodes. Both types of interference were applicable to competition betweenDinocrasandPerla.Isoperlawas the least, andPerlodesthe most, aggressive of the four species withDinocrasandPerlaintermediate.

Analysis of Spatial Patterns and Spatial Association between the Invasive Tree-of-Heaven (Ailanthus altissima) and the Native Black Locust (Robinia pseudoacacia)

Invasive exotic plants can persist and successfully spread within ecosystems and negatively affect the recruitment of native species. The exotic invasive Ailanthus altissima and the native Robinia pseudoacacia are frequently found in disturbed sites and exhibit similar growth and reproductive characteristics, yet each has distinct functional roles such as allelopathy and nitrogen fixation, respectively. Spatial point pattern analysis of trees in a silvicultural experiment was used to analyze the potential intraspecific and interspecific interference between these two species by looking at their individual spatial distribution and their spatial association. Analysis of spatial point patterns in the field with Ripley's K indicated that A. altissima and R. pseudoacacia were positively associated with each other along the highly disturbed skid trails in the majority of the field sites. Robinia pseudoacacia was clumped in the majority of the sites, whereas A. altissima was randomly distributed in most sites. Locally, increased disturbances could lead to more opportunities for A. altissima to invade, interact with R. pseudoacacia and other native species and potentially have an effect on the native plant community.

Food niche overlap among neotropical frugivorous bats in Costa Rica

Food habits of 15 species of frugivorous bats were studied at La Selva Biological Station, Costa Rica. Eight hundred and fifty-four (854) fecal samples and 169 samples from fruit parts and seeds discarded by bats beneath feeding roosts were analyzed. During eight months of study, 47 fruit species consumed by bats were identified. Five plant genera (Cecropia, Ficus, Piper Solanum, and Vismia) constituted 85% of all plants found in fecal samples. Feeding niche breadth differed significantly among the six most common species of frugivorous bats (Artibeus jamaicensis, Carollia sowelli, C. castanea, C. perspicillata, Dermanura sp., and Glossophaga commissarisi). All species, except for Dermanura sp., showed a diet dominated by one or two plant species. This suggests a pattern of resource partitioning at a generic level, in which Carollia consumed mainly Piper, Artibeus consumed Ficus and Cecropia, and Glossophaga consumed Vismia. Cluster analysis revealed higher values of food niche overlap in congeneric species than among species of different genera. Results show that if food is a limiting factor, mechanisms other than trophic selection must reduce interspecific interference or competition for food in this frugivorous bat guild.

Breeding white clover for increased ability to compete with associated grasses

Greater white clover (Trifolium repens L.) competitive ability can increase the forage quantity and quality of clover-grass mixed stand (MS) cropping. The present work assessed the relationship of clover competitive ability with morphophysiological traits, and the efficiency of direct and indirect selection procedures for increasing clover dry matter (DM) yield in MS. Some 165 large-leaved clover genotypes representative of Italian Ladino germplasm and 13 clover genotypes from an elite small-leaved population were clonally evaluated for one year in pure stand (PS) and in MS environment with Italian ryegrass (Lolium multiflorum Lam.) and tall fescue (Festuca arundinacea Schreb.) that reproduced the level of interspecific interference met by the clover over a multi-year crop cycle. Results for all genotypes and their subsets indicated that the level of leaf size variation among tested genotypes can affect the results. Wider variation decreased the relative importance of clover genotype×evaluation environment interaction, because competitive ability was mainly correlated with traits (longer petiole; larger leaflet) contributing to scale foraging in MS and yielding ability in PS. Competitive ability tended to be more closely associated with other traits (high stolon density; plasticity of petiole elongation) contributing to foraging ability, i.e. the ability to make fine-scale exploitation of light and nutrients in MS, within germplasm of similar leaf size. Leaf size variation also affected the relative efficiency of selection procedures estimated from expected responses to clonal selection. Results for all entries indicated direct selection in MS as 23% more efficient than selection in PS based on a selection index including three traits associated with competitive ability (petiole length; stolon density; leaflet size) which, in turn, was predicted to be 16% more effective than selection in PS based on DM yield. The advantage in relative efficiency of direct selection was almost doubled when considering only large-leaved germplasm. In a second experiment, evaluating 16 clover half-sib progenies of elite large-leaved parents for 2 years in PS and in MS with Italian ryegrass, tall fescue and cocksfoot (Dactylis glomerata L.), direct selection was estimated to be over three-fold and 68% more effective than indirect selection in PS based on DM yield and the selection index, respectively, based on expected responses to genotypic selection. The selection index may improve the efficiency of preliminary, PS-based clonal selection. Further evaluation of elite genotypes and/or their polycross progenies in MS is recommended for final selection.

Seedling Growth and Interference of Diffuse Knapweed (Centaurea diffusa) and Bluebunch Wheatgrass (Pseudoroegneria spicata)1

Growth and growth rate characteristics of diffuse knapweed and bluebunch wheatgrass seedlings were evaluated under two temperature regimes, 10 and 16 C, and two moisture regimes, − 0.01 and − 0.03 MPa, in an environmental chamber. Intra- and interspecific interference were evaluated at 10 C and at either − 0.01 or − 0.03 MPa. Root–leaf ratios for bluebunch wheatgrass were greater than for diffuse knapweed at − 0.01 MPa. Under moist conditions, root penetration was greater for diffuse knapweed than for bluebunch wheatgrass roots; penetration was equal in warm or dry conditions. Wheatgrass root length was greater than knapweed length under dry conditions. Intraspecific interference was greater than interspecific interference for both species. Leaf area data and root–leaf ratios suggest that diffuse knapweed is the faster-growing species under moist conditions. Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) Scribn. & Smith; diffuse knapweed, Centaurea diffusa Lam. #3 CENDI. Additional in...

Exclusion by interference competition? The relationship between red and arctic foxes.

The distribution of many predators may be limited by interactions with larger predator species. The arctic fox in mainland Europe is endangered, while the red fox is increasing its range in the north. It has been suggested that the southern distribution limit of the arctic fox is determined by interspecific competition with the red fox. This has been criticised, on the basis that the species co-exist on a regional scale. However, if the larger red fox is superior and interspecific competition important, the arctic fox should avoid close contact, especially during the breeding season. Consequently, the distribution of breeding dens for the two species would be segregated on a much smaller spatial and temporal scale, in areas where they are sympatric. We tested this hypothesis by analysing den use of reproducing arctic and red foxes over 9years in Sweden. High quality dens were inhabited by reproducing arctic foxes more often when no red foxes bred in the vicinity. Furthermore, in two out of three cases when arctic foxes did reproduce near red foxes, juveniles were killed by red foxes. We also found that breeding arctic foxes occupied dens at higher altitudes than red foxes did. In a large-scale field experiment, red foxes were removed, but the results were not conclusive. However, we conclude that on the scale of individual territories, arctic foxes avoid areas with red foxes. Through interspecific interference competition, the red fox might thus be excluding the arctic fox from breeding in low altitude habitat, which is most important in years when food abundance is limited and competition is most fierce. With high altitude refuges being less suitable, even small-scale behavioural effects could scale up to significant effects at the population level.