| Individuals in the same population can differ in genetic, phenotype, or any other traits, which is known as intraspecific variation. These differences are widespread in natural populations, and have attracted increasing attention in genetics and evolutionary theory because they are the raw materials of evolution. However, community ecology has long put emphasize on interspecific variation whereas intraspecific variation has long been ignored. But the importance of intraspecific variation has gradually been revealed recently. More and more studies show that there are a lot of intraspecific trait variation in natural populations, and these intraspecific variation can affect the species niche breadth, and thus affect the ability to obtain resources, productivity and interspecific relationship of species. Furthermore, high extent of intraspecific variation has been proved to increase the capacity of diffusion of species, and increase stability of populations and communities in changing environments, reducing risk of extinction. Based on these arguments, involving intraspecific variation into classical theoretical framework of community ecology has been considered the key to solve the legacy problems in community ecology, and intraspecific variation has become a hot topic in community ecology.Nonetheless, many important issues on impact of intraspecific variation on community structure and function still remain unresolved, particularly, the impact of intraspecific variation on interspecific relationship and the response to it are still blank. For example,1) whether intraspecific variation could change the state of coexistence based solely on interspecific differences? If so, what conditions intraspecific variation need to meet to achieve stable coexistence? 2) Whose influence strength on individual growth is greater, intraspecific variation or interspecific variation? Would the relative strength change with growth stages? 3) Intraspecific variation contains variable phenotypic differences, then whether the extent of intraspecific variation itself would change with or response to the intensity of interspecific relationship? If there is a response, then are the extent of intraspecific variation and strength of the relationship between species positive correlated or negative correlated? Therefore, in view of the significant importance of intraspecific variation and interspecific relationship in coexistence of species and community assembly as well as other ecological processes, this study adopted theoretical model analysis, field seedling experiments and forest dynamic observation to answer the three specific questions, and explore the impact of intraspecific variation on individual growth and response to interspecific relationship.For problem 1, we started from the classical Lotka-Volterra interspecific competition model, and introduced intraspecific variation into the model by setting two different types of individuals for each species without losing the generality. Then we proved the stability of new competition system by verifying the instability of boundary equilibriums, and explored the conditions intraspecific variation need to meet to achieve stable coexistence. For problem 2 and 3, seedlings field experiment was manipulated in Tiantong region,6 common arbor species Cinnamomum camphora, Liquidambar formosana, Lithocarpus glaber, Cyclobalanopsis glauca, Castanopsis carlesii and Castanopsis sclerophylla were chose to plant. We set a mixed competition experiment with about seven hundred individuals belonging to six species above and a control experiment in which individuals grew alone. We measured the basal diameter and height of all individuals regularly as characteristic index and used them to calculate the amount of individual growth. Through staged analysis method, we excluded the effects of abiotic environment and neighborhood interference first, and then analyzed the effect of interspecific trait variation and intraspecific trait variation to detect their relative amount of contribution to individual growth. In addition, intraspecific variation coefficientCV, average competition index of species ICI, and relative interaction intensity RII were calculated to explore the relationship between interspecific competition intensity and extent of intraspecific variation. To compensate for lack of the control experiment in the number of species and individual growth stages, we selected the first and second survey data of Tiantong sample with an area of 20 hectares and the trait data as well as topography, soil and other environmental data within the scope of 5 hectares to do analysis similar to the seedlings field experiment. Based on all calculation and statistical analysis, the following conclusions are drawn:1. Intraspecific variation could change the status of species coexistence in which only interspecific variation was considered. When intraspecific differences between the two species overlap but not completely overlap, the vast majority of the boundary equilibrium in competitive system are not stable, that is, a certain degree of variation within a species can increase the probability of stable coexistence between species. The result also indicates that not any extent of intraspecific variation can promote species coexistence.2. In the seedling stage, the impact of intraspecific variation on hybrid individual growth is much smaller than interspecific variation. Although the relative importance of intraspecific variation in the vertical direction increased with the growth of seedlings, but the dominant position of interspecific variation kept still. In Tiantong natural community, according to analysis based on AIC weight, we found that both interspecific variation and intraspecific variation could affect individuals’growth greatly, and joint interpretation was 49%. The importance of interspecific variation was a little larger, but the difference between them was very small. Change of relative importance implies that the impact of intraspecific variation is likely to increase with individuals’growth and development.3.In seedlings field experiment, intraspecific variation coefficients of most species in mixed competition group are bigger than those in control group, implying that the relationship between species (especially competition) can increase the degree of intraspecific variation. Moreover, there is a negative correlation between the degree of intraspecific variation and the competitive ability of species, that is, when species is under larger competitive inhibition, variation in its kind would be greater. But this negative correlation has not been widely observed in natural communities.Although there are many deficiencies in this study, such as the theoretical analysis based on competitive system with only two species and two types of intraspecific variation, intraspecific trait variation types used in the experiment and species selected in control experiment were much limited, etc., but the previous results have filled a lot of blank in study of intraspecific variation. We propose the basic conditions for intraspecific variation to promote species coexistence, the relative strength of intraspecific and interspecific variation in influencing individual growth and the form of intraspecific variation response to interspecific relationship for the first time. The results show in many ways that intraspecific variation is a very significant difference and cannot be ignored, it may have influence on growth and breeding of populations in natural communities, the strength of interspecific interaction, the probability of species coexistence and other ecological processes. Meanwhile intraspecific variation itself may change with the intensity of interspecific relationship and in turn affect the relationship between species and probability of species coexistence. These findings provide some theoretical and experimental evidence for further exploring the impact of intraspecific variation on individual growth and the response to interspecific relationship. |
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