Study On Grassland Grasshoppers Ecological Threshold

This thesis is reporting the post-doctor Study on the Locust and Grasshoppers Ecological Threshold. During the research, the definition of Keep Biodiversity Resource Index (K), the concept of Admit Subsistence Index (A), the measurement of Drought Factor (D) and the function of Compensation or Susceptibility Index (C) have been studied. Furthermore, the population dynamics of Oedaleus asiaticus and the estimation of Forage Loss (FL) have been studied. On the basis of these studies, a model of the Locust and Grasshoppers Ecological Threshold (ET or ECT) has been established.Keep Biodiversity Resource Index (K) reflects a necessarily preserved part of resource for keeping biodiversity in an eco-system. This part of resource is available to the living things in the eco-system for maintaining their population. K=αPP, in whichαis the coefficient of K which is depends on grassland types; PP is pasture productivity. K value is shared by diversified living things' populations. A value extremely in case that a single species bursts out in quantity can be calculated as a maximum value shared by the single species itself.Admit Subsistence Index (A) means a certain value of K(Keep Biodiversity Resource Index) to admit a certain quantity of kinds and populations of bio-species subsistent to maintain eco-system equilibrant. As a single species bursts out, it shares a certain population quantity by itself while the K resource is the maximum. Admit Subsistence Index is expressed as A, and A=K/FL·FL (Forage Loss) caused by a head of insect can be estimated as FL=I_1-3×SC_1-3+I₄×SC₄+I₅×SC₅×+Ia×SCa, in which I_1-3, I₄ and I₅ mains the intakes of 1st -3rd instar, 4th instar and 5th instar nymphs respectively, Ia mains the intakes of male or female adults, and SC mains subsistence rate of nymphs and adults. FL (Forage Loss) caused by mixed populations can be estimated as: FL_n =sum from i=1 to∞(FL_i×RA_i), in which n mains the number of locust orgrasshoppers population, RAi mains the population rate of insect i. The population dynamics of Oedaleus asiaticus and the estimation of Forage Loss (FL) have been studied with these methods, The measurement of Drought Factor (D) is based on the conclusion that drought is a key factor which infects grassland productivity, reported by a study of the research group. It has been proved that the total precipitation between the last October and next June is the critical factor. This thesis is trying to reflect the extent of drought by a series values within 0.1-1. With reference to the historical data of average precipitation and given that the historical average precipitation is 0.5, every 5 percentage point other 5% is 0.1 unit which mains the larger the value is the heavier the drought is. Then the measurement of Drought Factor (D) can be expressed as D = 0.5 + 2×((WH-Wt)/Wh), in which 0.5 is the drought coefficient of the drought factor based on historical average precipitation, W_h represents the historical average precipitation between the last October and next June, and W_h represents the average precipitation between the last October and next June in the same measuring year. As for the definition of the drought factor (D), it is different from the concept "G" reported by Lu Hui.Compensation or Susceptibility Index (C) reflects the effects of susceptibility or over-susceptibility of locust or grasshoppers on the grassland returning green in early spring or the grassland with a coverage rate lower than 30%, and the effects of compensation or super-compensation of locust or grasshoppers on the grassland at a low level of utilization or at a lower level of being damaged. These effects can be expressed as the formula (-1/LnR) in which R is the extent of plant coverage. (-1/LnR) and Drought Factor (D) are co-infected in result of effects of susceptibility or over-susceptibility of locust or grasshoppers and effects of compensation or super-compensation. Therefore, with the combination of (-1/LnR) and D, the Compensation or Susceptibility Index (C) can be measured by C=(-1/(D×Ln(r)))To search for the threshold of grasshoppers management on natural grassland, the ecological and economic views are both necessary to describe the threshold correctly. It is more important to reflect the objective facts. In accordance with Jiang Huaizhang's Economic Threshold Model: ET represents economic threshold; EC means effect of control; CC means cost of control; Y means yield of crops; P means price of products; SC means subsistence in control; (RY/PD) expresses a percentage of per insect's reduced yield to plant damage; PT means plant thresh-resistant factor; PS means plant stress factor; T means temperature factor. The plant thresh-resistant factor (PT) has considered the plant's effects of compensation or super-compensation, which are positive effects; and the plant thresh-resistant factor ((PT) has considered the plant's effects of susceptibility or over-susceptibility, which are negative effects. The temperature factor is actually an environmental factor; and temperature, light, moisture, precipitation and so on can positively or negatively effect on disasters. The estimate of forage loss caused by per insect (FL) equal to the yield (Y) by per insect caused forage loss (RY/PD) by subsistence (SC). The model can be simplified as ET=(CC/(EC×P×FL)×(PT/PS)×T, the theoretical basis above-mentioned is that the cost of control is equal to the saved loss. In the formula (PT/PS), PT means plant thresh-resistant factor; PS means plant stress factor. It can be described as (-1/(D×LnR)) in natural grassland. Therefore, the ecological threshold model can be defined as ET=((-1/(D×LnR)))×((K/FL_n)+CC(FL_n×P×EC)) which R means the grassland's projective coverage; K means the average productivity level onα% of grassland; FL_n means the per insect caused forage loss in the combined grasshoppers' population; CC means cost of control; EC means the effectiveness of control; P means price of products; D means drought factor. Finally, ET expresses Ecological Threshold, which is defined as ECT by some scholar. Considering to provide an easy reference basis for the management strategy, this thesis still introduces ET expressing Ecological Threshold. The model of Ecological Threshold has been statistically tested.