Maintenance of two genetic entities by habitat selection

Summary In the laboratory, the two species of copepodsLepeophtheirus thompsoni andLepeophtheirus europaensis, ectoparasites of flatfishes, can meet and mate on at least one host species. In the wild however, these two species are found isolated on their sympatric hosts. Habitat selection theoretically represents a powerful enough mechanism to explain the maintenance of genetic heterogeneity in the wide sense. In this paper, the host colonization process is studied for both parasite species. It is shown that each parasite can develop and reach adult age on each host species. However,L. thompsoni is highly selective; it almost totally refuses to colonize hosts other than its natural one.Lepeophtheirus europaensis, on the contrary, readily infests turbot and brill in single-host experiments, but strongly prefers the brill when it has a choice. It appears that these two genetic entities are sympatrically maintained due to strong habitat selection. Such a pattern could theoretically only occur in a soft-selection context (density dependence). This point is discussed with respect to the different patterns in host use found in the geographical distribution of these parasites.     

Parasites, aggression and dominance in male upland bullies

The effects of the digenean trematode parasite Telogaster opisthorchis on aggression and competitive ability in male upland bullies ( Gobiomorphus breviceps ) were examined using mirror-image stimulation (MIS) techniques and dyadic contests for nest sites between pairs of males. Parasite load had no significant effect on male aggression or success in dyadic contests, nor was aggression a predictor of the likely winner of these contests. The results are discussed with relation to the role of parasites in male-male competition and to possible problems in using MIS techniques for studies on dominance and aggression in fish.     

EVOLUTION OF BRASSICA RAPA L. (CRUCIFERAE) POPULATIONS IN INTRA- AND INTERSPECIFIC COMPETITION

Populations of Brassica rapa were grown for three generations in each of two environments: intraspecific competition, with four surrounding Brassica rapa neighbors per pot, and interspecific competition, with two Raphanus sativus neighbors per pot. In each environment, the largest (by flower number) 10% of the plants were outcrossed and provided seeds for the next generation. As a control, a randomly chosen 10% of the plants in each environment were outcrossed to produce seed for the next generation. Each of these four treatments, the selected lines in intra- and interspecific competition and the corresponding control lines, was maintained for three generations. After a single generation of growth in a common, no-competition environment, replicate plants from each treatment were grown with no competition and with intra- and interspecific competition for determination of growth responses. After two generations of selection, flower number in the intraspecific-selection line had increased by more than 50% over that in the control line and by more than 19% over that under interspecific selection. After a common-environment generation, plants from the intraspecific-selection line were shown to have significantly faster growth in height and flower number as seedlings. Plants in the interspecific-selection line showed similar but nonsignificant trends. No differences in seed mass, emergence time, or photosynthetic rate were found between control and selected lines in either intra- or interspecific competition. Some differences between control and selected lines were noted in biomass allocation related to differences in phenology. The results demonstrate that performance in competitive environments can evolve through changes in plant development but that rates of evolution will differ in intra- and interspecific competition.     

PERSPECTIVE: DARWIN,PROGRESS, AND ECONOMIC PRINCIPLES

Darwin's concept of progress was an economic one, with the accumulation of adaptations having a strong analogy to technological innovations. This interpretation makes it easier to understand his differences with Lamarck and others whose views he considered metaphysically objectionable. It also clarifies his relationship to Malthus and such features of his theory as the episodic nature of evolutionary change. His position is consistent with modern theories of biotic invasions and long-term evolutionary trends. It also accords well with current efforts to produce a synthesis between biology and economics.     

Use and partition of space and resources by two coexisting Rhyacophila species (Trichoptera) in a high mountain stream

Life history, distribution, drift and food habits of two coexisting Rhyacophila species, Rhyacophila evoluta and Rhyacophila intermedia were studied in a high mountain stream in the Pyrénées.The life histories of both species took two years downstream and three years upstream. Several generations and a large range of larval instars permanently coexisted in the river.Slight temporal segregation of the life histories and differences in size and food habits of the coexisting larvae partially reduced the interspecific competition and allowed an optimal use of space. Instars IV of R. evoluta and V of R. intermedia, which had the most similar biological characteristics had the same microdistribution when they lived alone but used different habitats when they coexisted.     

The importance of intraspecific competition in a Littorina littorea population in the Wadden Sea

Two field experiments were carried out totest whether effects of intraspecific competition ina Littorina littorea population can be detectedin a short-term investigation. Different size classesof L. littorea showed no significant differencein preferences when offered four kinds of eitherpossible food or substrata (Fucus vesiculosus,Ulva lactuca, Carcinus maenas, brick).Large and medium winkles preferred Fucusvesiculosus, followed by Ulva lactuca. Deadshore crabs (Carcinus maenas) were the leastpreferred objects for all size classes. On the firstday of the experiment bricks were more attractive tosmall littorines than to larger ones. Considering allfour days, the same ranking occurred for all sizeclasses: Fucus vesiculosus > Ulvalactuca > brick > Carcinus maenas. The reactionofjuveniles to increased densities was examined using anin situ caging experiment on a mussel bed. Meshsize of the cages allowed adult densities to beincreased while juveniles could escape by passingthrough the meshes. However, there was no significantemigration of small winkles even from cages with 10 to20 times natural density of large individuals. Ofgreater importance was the original number of winklesat the site. The available resources on the musselbeds appear to be sufficient to maintain a highpopulation density. Intraspecific competition does notseem to play a major role in this L. littorea-population.     

Predation,competition, and co-occurrences of Boeckella and Calamoecia (Copepoda: Calanoida) in Western Australia

Patterns of co-occurrences of Boeckella and Calamoeciafound in Western Australia are documented. Patterns are analyzed inrelation to size of organisms and to geographical distributions oforganisms. Deviations from randomness in the number ofmulti-species assemblages suggest that biotic interactionsinfluence co-occurrences, but size relationships in severalco-occurring pairs indicate that competition for resources is nota factor influencing co-occurrence. Laboratory experimentsdemonstrate that the large Boeckella triarticulata consumesimmature stages of the small Calamoecia tasmanica butprovides a partial refuge for C. tasmanica when theplanktivorous fish Gambusia, which consumes Boeckellamore rapidly than Calamoecia, is present. These resultsindicate that co-occurrence patterns may be related to theintensity of predation by fish: in the absence of fish predation,Boeckella can prevent invasion of Calamoecia, whilemoderate predation by fish will reduce Boeckella density,consequently allowing Calamoecia to invade.     

Influence of fish on the breeding of the red-necked grebe Podiceps grisegena (Boddaert, 1783)

I determined whether selection of breeding sites by thered-neckedgrebe Podiceps grisegena (Boddaert, 1783) was affectedbycompetition for food with fish. During 1987–89, ten ponds inScania, southern Sweden were investigated, five with(breedingsites) and five without breeding red-necked grebe(non-breedingsites). I quantified the biomass of fish and macrozoans(here:larger invertebrates and small vertebrates) and measured thephysical and chemical conditions of the ponds.Breeding sites had significantly lower relative fish abundancethannon-breeding sites. They were either devoid of fish or hadonly nine-spined sticklebackPungitius pungitius. The biomass (dry weight) ofmacrozoans,many of which represent a food resource for grebes, was 16timesgreater at breeding than at non-breeding sites. The physicalandchemical conditions of the ponds did not differsignificantly.These results are consistent with the hypothesis thatcompetitionfor macrozoans occurs between red-necked grebe and fish andthat adense population of many fish species caused red-necked grebestoavoid otherwise acceptable breeding sites.     

Relationship between Meloidogyne incognita and Rotylenchulus reniformis as Influenced by Soybean Genotype

The effect of soybean genotype on competition between Meloidogyne incognita race 2 (Mi) and Rotylenchulus reniformis (Rr) was evaluated in greenhouse and microplot replacement series experiments. Soil in pots containing seedlings of ''Davis'' (susceptible to Mi) or ''Buckshot 66'' (resistant to Mi) was infested with 1,000 vermiform individuals in the following Mi:Rr ratios: 0:0, 100:0, 75:25, 50:50, 25:75, or 0:100. After 91 days, the relative nematode yields (number of nematodes in mixed culture divided by the number in nonmixed culture) of each species were calculated based on soil and root nematode populations expressed as nematodes per gram of dry root tissue. To define the relationship between the two species, calculated relative nematode yields were compared with a theoretical noncompetition model using lack-of-fit regression. In the greenhouse, Mi populations on ''Davis'' were stimulated in the presence of Rr. In microplots, low Mi and Rr population densities likely resulted from severe galling and destruction of feeder roots that probably occurred early in the season. Enhanced susceptibility to Mi was not observed on ''Buckshot 66'', which remained resistant to Mi even when colonized by Rr. Host resistance is a key factor in determining the nature of the relationship between Mi and Rr.     

Direct and indirect effects of plant genetic variation on enemy impact

Abstract.

• 1 The Tritrophic and Enemy Impact concepts predict that natural enemy impact varies: (a) among plant genotypes and (b) may depend on the abundance of heterospecific herbivores, respectively. I tested these predictions using three herbivore species on potted, cloned genotypes of Salik sericea Marshall in a common garden experiment.
• 2 Densities of the leaf miner (Phyllonorycter salicifoliella (Chambers)) and two leaf galling sawflies (Phyllocolpa nigrita (Marlatt) and Phyllocolpa eleanorae Smith and Fritz) varied significantly among willow clones, indicating genetic variation in resistance.
• 3 Survival and natural enemy impact caused by egg and larval parasitoids and/or unknown predators differed significantly among willow clones for each of the three herbivore species, indicating genetic variation in survival and enemy impact.
• 4 Survival of Phyllonorycter was negatively density-dependent among clones.
• 5 Survival of Phyllonorycter and Phyllocolpa eleanorae were positively correlated with densities of heterospecific herbivores among clones and parasitism of these species were negatively correlated with densities of the same heterospecific herbivores among clones.
• 6 At least for Phyllonorycter this positive correlation may suggest either facilitation of survival between herbivore species, which do not share natural enemies, or an apparent interaction caused by host plant genetic variation.
• 7 Among clones, egg parasitism of Phyllocolpa eleanorae was weakly positively correlated with density of Phyllocolpa nigrita. Since these species share the same Trichogramma egg parasitoid, this interaction could support the hypothesis of apparent competition.