Effects Of Habitat Fragmentation And Spatial Isolation On The Population Characters Of The Endangered Plant Euonymus Chloranthoides Yang

Euonymus chloranthoides Yang is an endangered species endemic to Chongqing. It mainly distributs in several scienic spots and its populations have been threatened by severe habitat fragmentation and increasing spatial isolation because of strong human disturbance. The main purpose of this paper is to study the effects of habitat fragmentation and spatial isolation on population dynamics,. reproduction traits,seed germination characters,morphological differentiation and population genetic characters of this species. The main results are as follows:1. The age structure and survival curve of populations in Mt. Jingyun and Mt. Jinggong show that the ratio of seedlings declines with the population size. The indivudaul number of every age class of Banzigou population, the isolated smallest population, fluctuates most severely. Which indicates this population can't keep steady when environment fluctuates. The more severely the population fluctuates, the higher the extinction risk are. The survival curve and life table of populations show that the high seedling death rate happens to the big populations perhaps because of severe infra-specific competition and inbreeding depression, which indicates that the seedling period is one of the weakest stages in this species' life history. The individual longevity declines with their population size. A long longevity helps to reproductive potential store and population stabilization. Time sequence analysis shows that all the populations of E. chloranthoides are declining populations, and that most populations decline faster with smaller size of their population. The results of Leslie matrix analysis show that bigger the populations are, slower the populations decline. To some extent, population density may delay the population's decline. In total, habitat fragmentation and spatial isolation do harm to population increase.2. The populations in Mt. Beiwenquan and Mt. Jigong show the character that the average fruit number per individual increases when the population size decreases, however, the average fruit number per individual and average seed number per fruit of Banzigou population are both the smallest. We found that pollination insects of E. chloranthiodes were very scarce and the fruit set rate was very low in natural population through wild investigation. We also found that fruit set rate could be improved obviously by additional pollination, which means that there exists severe pollen limitation during the reproductive process . Small populations and populations with low density are not very attractive to the pollination insects, so the pollen limitation and inbreeding depression will be even more severe for the small populations and low density populations. The P1 population in Beiwenquan is in a better reproductive condition maybe because of a high density of reproductive individuals, which indicates that population density has the function of regulating reproduction.The germination experiment of seeds produced by different pollination treatment shows that the seeds via cross pollination within population have the highest germination rate, and the seeds by self pollination and cross pollination between populations ( populations between Mt. Jinyun and Mt. Jigong) show low germination rate. Germination rate of the seeds by cross pollination between populations is low maybe because long time of spatial isolation among populations leads to outbreeding depression.Fecundity tables based on the seed number show that if seed germination rate and seedling survival rate are not considered, then smaller populations grow faster than bigger populations. But fecundity tables based on the annual seedling number show that the bigger populations grow faster than the smaller ones. This means that after habitat fragmentation, small populations try to adapt to the new conditions by increasing the reproductive investment in order to fulfill their population regeneration, in fact, due to the inevitable inbreeding depression, small populations exhibit a low population growth rate.However, from fecundity tables based on both the seed number and the annual seedling number we can see that the population growth rate of Banzigou population is the smallest among all the populations. Perhaps this is because the Banzigou population is too small to increase its reproduction investment and regulate reproduction.Reproduction value tables of all populations also show that to some extent, small populations have higher reproduction investments than bigger populations.3. Seed germination characteristics of 7 spatially isolated populations of E. chloranthoides were studied in the laboratory condition. The differences of the seed germination rates among 7 populations were found to be extremely notable. The seed germination rates of the studied populations are positively correlated with their population sizes, while show no correlation with the soil parameters of their original habitats. The differences of the seed germination rates between big populations and small populations reach the extremely notable level. The average seed weights of 7 populations are not copiously correlated both with their population size and with soil parameters in the habitats, which indicates that the average seed weight of a studied population is relatively stable and less influenced. The low seed germination rate in smaller populations may mainly due to the inbreeding depression, and/or loss of genetic diversity in the isolated, smaller populations, while not due to the environmental factors. The study suggests that we should take both the non-genetic, demographic factors and the genetic factors into consideration and enlarge the population to maintain higher seed germination rate and restore the normal regenerations among the smaller and isolated populations of this endangered plant.4. Study of morphological differentiation of the wild populations shows that morphological differentiation among populations reflects a model of isolation by distance, but morphological differentiation of seedlings grown under the same condition does not strictly shows the model of isolation by distance. The morphological traits of P2 population in Beiwenquan diverge from that of P3 population in Beiwenquan obviously though they come from the same park and their positions are very near today. Maybe it is because of genetic drift during reproduction which induces the small population of P2 diverge from other populations.5. Three populations in Beiwenquan and Banzigou population were selected to study the effects of habitat fragmentation and spatial isolation on population genetic traits of E. chloranthoides. The results of ISSR experiment show that the population genetic diversity is high. the G_ST is 0. 4062 which means most of the genetic diversity exists within the population, and a small part of genetic diversity exists among populations. The genetic diversity of Banzigou population, the smallest one, is the lowest. The gene flow between Banzigou population and other populations is also the lowest. The genetic diversity of P2 population in Beiwenquan is the lowest among the three populations in Beiwenquan even though it has higher gene flow with other populations. This may suggests that the strong gene flow can not change the genetic effects of inbreeding depression because P2 population is a very small one. P4 population in Beiweiquan is not the biggest one, but its genetic diversity is the highest among the populations studied. A lot of factors can affect population genetic diverstity and structure, for example, mating system, insect's pollination bias, gene flow, preexisting subpopulation structure and founder effect. Maybe some of these factors or their combination lead to P4 population's higher genetic diversity.6. In short, habitat fragmentation and isolation do harm to population increase, reproduction, genetic diversity of E. chloranthoides Yang. Populations have the self regulating ability when population is above a certain minimum population size or the isolation is within a certain extent, but for a small population with severe isolation, population self regulating ability is destroyed. The dispersal ability of E. chloranthoides Yang is weak because its pollination insects are very scarce, and the seed is big. Which lead to this species' sensitivity to habitat fragmentation and isolation even within a small area.7. The habitat of E. chloranthoides Yang should be protected from further loss and fragmentation, which is the first step to protect this species. Because of severe pollen limitation during the reproductive process of E. chloranthoides Yang, it is important to improve the attraction of habitat to pollination insects by many ways, including increasing the habitat species diversity. If there is no outcrossing depression among populations, the level of gene flow should be restored to its historical level. It is a weak stage from seeds to successful seedlings in the life history of E. chloranthoides Yang, so seeds germination and seedings growth can be finished in the laboratory before replanting the seedlings to its initial populations. If it is truly difficult to restore the small wild populations, then ex-situ conservation should be implemented according to the theories of population genetics.