INTERSPECIFIC LABORATORY COMPETITION OF THE RECENTLY SYMPATRIC SPECIES DROSOPHILA SUBOBSCURA AND DROSOPHILA PSEUDOOBSCURA

Abstract.—Drosophila subobscura and D. pseudoobscura are closely related species coexisting on the West Coast of North America, which was recently colonized by D. subobscura. In competition experiments with overlapping generations, D. subobscura is eliminated by D. pseudoobscura in a few generations at all four temperatures and two initial frequencies tested. Yet in one-species cultures, D. subobscura thrives at all experimental conditions. Single-generation competition experiments reveal lower survivorship and productivity of D. subobscura at all temperatures and frequencies. Productivity per female is dependent on the initial frequencies: greater for D. subobscura as its initial frequency becomes higher, but lower for D. pseudoobscura as its frequency becomes higher. Strains of D. subobscura from three disparate geographic origins yield similar results.     

FORAGING BEHAVIOUR OF TERRESTRIAL GASTROPODS: INTEGRATING FIELD AND LABORATORY STUDIES

The foraging range of adult snails, Helix aspersa, has beenstudied using radio-transmitters. Snails did not return to anexact roost site after foraging, but often remained in the foodpatch, or returned only to the general roost area. Time lapse video films of the slug Deroceras reticulatum madeunder infra-red lighting in arenas were analysed for tracklengthsand degree of turning, in order to simulate slug movements inan unbounded situation. The results suggest that many food itemsare found by random encounter. Slugs usually ate the first food item found, but often ignoredfood items encountered later. If food was scarce, the slugsfed almost every time. Electronic recordings of bites on a wheatflour pellet over 24 hours show that feeding is most intensein the first two and a half hours from starting to feed, andlater meals are both shorter and less regular. Starved slugsdiffered from fed slugs principally by taking a second mealshortly after the first. When given a choice of a more preferredfood (maize pellets) and a less preferred food (pea pellets)in different ratios, the slugs appeared to encounter pelletsat random, but they fed more from the preferred pellets unlessthe ratio was 1 maize: 7 pea. Starved slugs ate twice as muchas fed ones.     

A COEVOLUTIONARY ISOMORPHISM APPLIED TO LABORATORY STUDIES OF COMPETITION

Some empirical consequences of an isomorphism between the Lotka-Volterra competitive model and a coevolutionary competitive model are developed. In both the Lotka-Volterra and coevolutionary models, four competitive outcomes are possible: 1) species one wins, 2) species two wins, 3) indeterminate outcome, and 4) stable coexistence. These two models are isomorphic in the sense that the inequalities associated with a particular competitive outcome of the Lotka-Volterra model correspond in a one-to-one manner with similar inequalities associated with the same competitive outcome of the coevolutionary model. The inequalities of the Lotka-Volterra model involve the competition coefficients themselves, while the inequalities of the coevolutionary model involve the genetic variances and covariances of the competition coefficients. The isomorphism suggests some alternative interpretations of the results of classical laboratory studies of competition. The Lotka-Volterra (or ecological) hypotheses postulate that the competition coefficients are constant and that genetic considerations play no role in determining the competitive outcome. By contrast, the evolutionary hypotheses derived from the coevolutionary model postulate that the competition coefficients are variables and that the genetic variances and covariances of the competition coefficients determine the competitive outcome. The isomorphism is applied to competitive exclusion and coexistence, and to competitive indeterminacy in Tribolium. In particular, the evolutionary hypotheses isomorphic to the two classical explanations of competitive indeterminacy, the demographic stochasticity and genetic founder effect hypotheses, are constructed. The theory developed here and in a previous paper (Pease, 1984) provides one perspective on the relation among the Lotka-Volterra competition theory, quantitative genetics, competitive exclusion, the reversal of competitive dominance, coexistence, competitive indeterminacy in Tribolium, and experiments investigating the relation between genetic variability and the rate of evolution of fitness.     

SIZE‐CORRECTION AND PRINCIPAL COMPONENTS FOR INTERSPECIFIC COMPARATIVE STUDIES

Phylogenetic methods for the analysis of species data are widely used in evolutionary studies. However, preliminary data transformations and data reduction procedures (such as a size‐correction and principal components analysis, PCA) are often performed without first correcting for nonindependence among the observations for species. In the present short comment and attached R and MATLAB code, I provide an overview of statistically correct procedures for phylogenetic size‐correction and PCA. I also show that ignoring phylogeny in preliminary transformations can result in significantly elevated variance and type I error in our statistical estimators, even if subsequent analysis of the transformed data is performed using phylogenetic methods. This means that ignoring phylogeny during preliminary data transformations can possibly lead to spurious results in phylogenetic statistical analyses of species data.     

海洋卡盾藻与中肋骨条藻和锥状斯氏藻种间竞争研究

研究了不同起始细胞密度与硅酸盐浓度对海洋卡盾藻(Cm)与中肋骨条藻(Sc)及锥状斯氏藻(St)之间种间竞争的影响,利用竞争抑制参数对相互间的竞争关系进行了分析,并引入体积竞争抑制系数的概念。结果表明:海洋卡盾藻的环境容量不受起始细胞密度(100—3600cells/mL)的影响,但随着起始接种密度的增加,达到最大细胞密度和进入稳定生长期的时间均提前。在硅缺乏的条件下,中肋骨条藻仍能保持一定生长,并能在与海洋卡盾藻之间的种间竞争中保持数量优势,但硅酸盐浓度的增加有利于其种间竞争。初始细胞密度和接种比例对3种赤潮藻类之间的种间竞争影响明显,总体来说海洋卡盾藻在竞争中处于劣势。当海洋卡盾藻细胞密度远远高于中肋骨条藻时(Cm:Sc=6:1),海洋卡盾藻才能在与中肋骨条藻的竞争中取胜;而在与海洋卡盾藻共培养条件下,锥状斯氏藻具有一定竞争优势,其对海洋卡盾藻的体积抑制系数(α’)是后者(β’)的27—100倍。但在所有处理中均没有绝对优胜者,竞争结果都会出现不稳定的平衡状态。       

OTHER PAPERS: INTERFERENCE AND RESOURCE COMPETITION IN TWO LAND SNAILS: ADULTS INHIBIT CONSPECIFIC JUVENILE GROWTH IN FIELD AND LABORATORY

Interference and resource competition by adults inhibited growthrates of conspecific juveniles of the land snail species Mesodonthyroidus and Neohelix albolabris in separate field and laboratoryexperiments, but not in laboratory experiments on Anguispiraalternata. In 1 m² field cages at near-natural densities underambient food and water conditions, juvenile M. thyroidus apparentlycompeted with adults for food or water or both resources, growingmore slowly when living with two conspecific adults, but beingunaffected by adult presence when food and water were augmented.Neohelix albolabris juveniles were similarly unaffected in fieldcages by presence of two adults when food and water were augmented.In contrast, interference, not resource competition, apparentlyexplained growth inhibition in laboratory cages at densitiesconsiderably greater than natural densities, with non-limitingfood and moisture; both M. thyroidus and N. albolabris juvenilesgrew more slowly as conspecific adult number increased fromzero to three. (Received 17 July 1995; accepted 11 November 1996)     

LABORATORY DIAGNOSIS OF RESPIRATORY DISEASES: PCR VERSUS SEROLOGY

We present a comparison between serology and genetic detection of three bacterial pathogens causing lower respiratory tract infection (LRI). We evaluated serology and PCR for the detection of Mycoplasma pneumoniae, Bordetella pertussis and Chlamydia pneumoniae from 1712 nasopharyngeal and serum samples. For 856 nasopharyngeal samples, average PCR time was 7.2 days, the parallel serum samples was 13 days. Automated extraction of nucleic acids reduces average PCR time to 6.7 days.     

PERFORMANCE OF TADPOLES FROM THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX: INTERACTIONS WITH POND DRYING AND INTERSPECIFIC COMPETITION

The performance of three genotypes (LL, LR, RR) of tadpoles resulting from the hybrid mating system of Rana lessonae (phenotype L, genotype LL) and Rana esculenta (phenotype E, genotype LR) was determined in artificial ponds. The effects of interspecific competition and pond drying on growth, development, and survival of tadpoles were used to measure the performance of genotypes and the relative fitness of offspring. Among the three genotypes, tadpoles from the homogametic mating RR had the lowest survival, growth, and development under all environmental conditions. Body size of the LL and LR genotype tadpoles at metamorphosis was reduced by competition and pond drying. Days to metamorphosis were also higher for the LL and LR genotype tadpoles in competition ponds. The proportion of individuals metamorphosing of each genotype was differentially lowered by competition and pond drying. The LL genotype produced more metamorphs than the LR genotype in the constant water level ponds, but the LR genotype produced more in drying ponds. In competition ponds, the LR genotype produced more metamorphs than the LL genotype, but the LL genotype produced more metamorphs in ponds without competition. The RR genotype produced no metamorphs in any of the experimental environments. Increased performance of LR offspring from the heterogametic mating, in harsh conditions, and reduced performance of RR offspring from the homogametic mating, even under favorable conditions, relative to the parental genotype (LL) suggests that the population dynamics of this hybridogenetic system is strongly dependent on mate choice in mixed populations and the subsequent pond environment females select for oviposition and larval development.     

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.     

INTRAPOPULATION DIFFERENTIATION IN ANNUAL PLANTS. III. THE CONTRASTING EFFECTS OF INTRA- AND INTERSPECIFIC COMPETITION

The roles of intraspecific and interspecific competition in producing differentiation within populations of Veronica peregrina were studied in two populations under controlled, greenhouse conditions. In nature, each population spans an environmental gradient across the center and sides of a temporary, vernal pool in California. Individuals at the center are subjected to intense intraspecific competition produced by high densities (to 30 seedlings/cm²) generated by quasi-simultaneous germination (90% of seeds germinate in one week). Individuals at the periphery are subjected to interspecific competition with grasses, which shade out the Veronica 4–6 weeks after the onset of winter growth. I predicted that 1) when grown under immediate intraspecific competition in the greenhouse, offspring of plants from the central subpopulation (C) would perform better (i.e., grow larger and produce more seeds) than those from the periphery (P) and that 2) when grown under delayed interspecific competition provided by Agrostis tenuis and Lollium multiflorum, offspring of plants from the periphery would perform better than those from the center. Both predictions were confirmed. The center-periphery differences were pronounced and statistically significant in an undisturbed population (V-2), while in a population disturbed by yearly plowing (V-3), the differences tended to be consistent with those in V-2 but seldom significant. Distribution of variability tended to be positively skewed and/or leptokurtic in subpopulations grown under “foreign” competition (i.e., intraspecific for P plants and interspecific for C plants) but was normally distributed following exposure to “familiar” competition. Timing of competition affected many results. There were four additional significant differences between the central and peripheral subpopulations. 1) Germination rate: the faster rate in central plants can be advantageous under immediate intraspecific competition. The slower rate in peripheral plants can be advantageous under conditions of erratic and unpredictable soil moisture. 2) Response to nutrient competition: central plants were more sensitive to N-deficiency and peripheral plants were more sensitive to P-deficiency. 3) Allocation of biomass: central plants allocated a greater proportion of biomass to seeds, while peripheral plants allocated a greater proportion of biomass to leaves under all growing conditions. 4) Root elongation: at the seedling stage, central plants have longer roots, while at the adult stage, peripheral plants have longer roots (but not more root mass). Most components of this complex pattern of differentiation are interpretable in an adaptive context. Other results defy simple explanations and underline the importance of phenotypic plasticity, which was pronounced in the competition experiments.