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不同载畜率对典型草原蝗虫多度及多样性的影响
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作者 李颖 沈婷婷 +2 位作者 谭瑶 karl behrendt 韩国栋 《草原与草业》 2025年第4期15-23,共9页
本研究在以克氏针茅(Stipa krylovii)为建群种的典型草原为实验地,自2021年起,设置了3种不同载畜率梯度绵羊(Ovis aries)放牧试验,分别为不放牧(CK,0羊单位·hm^(-2)·a^(-1))、适度放牧(MG,6羊单位·hm^(-2)·a^(-1))... 本研究在以克氏针茅(Stipa krylovii)为建群种的典型草原为实验地,自2021年起,设置了3种不同载畜率梯度绵羊(Ovis aries)放牧试验,分别为不放牧(CK,0羊单位·hm^(-2)·a^(-1))、适度放牧(MG,6羊单位·hm^(-2)·a^(-1))和重度放牧(HG,9羊单位·hm^(-2)·a^(-1)),调查并分析了在放牧高峰期(7月—8月)不同载畜率下的蝗虫多度及多样性。结果表明:样地的优势种蝗虫为亚洲小车蝗(Oedaleus decorus asiaticus)、宽须蚁蝗(Myrmeleotettix palpalis)、华北雏蝗(Chorthippus brunneus huabeiensis)和毛足棒角蝗(Dasynippus barbipes)4种。随着载畜率的增加,华北雏蝗的多度降低,宽须蚁蝗、毛足蝗角蝗、亚洲小车蝗在MG的多度均显著低于CK和HG(P<0.05)。蝗虫多度与植物Simpson指数、植物Shannon-Wiener指数呈显著正相关,蝗虫多度与植物高度呈显著负相关。研究表明:随着载畜率的增大,植物生物量逐渐减小,蝗虫多度先减小后增大,在MG时最低,蝗虫的Shannon-Wiener指数和Pielou指数随载畜率的增大而先增大后减小,Simpson指数随着载畜率的增加而减小,因此,适度放牧有利于控制蝗虫多度处于低密度水平。 展开更多
关键词 蝗虫 载畜率 典型草原 植物功能群
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Asymmetric Price Transmission Analysis of the International Soybean Market
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作者 G.M. Barboza Martignone Dimitrios Paparas karl behrendt 《Agricultural Sciences》 CAS 2023年第3期317-334,共18页
This study analyzed the asymmetric price transmission in the international soybean market, using data from the US (Chicago Futures), European (Rotterdam), Brazilian (Paranaguá), Argentinian (Rosario Futures and R... This study analyzed the asymmetric price transmission in the international soybean market, using data from the US (Chicago Futures), European (Rotterdam), Brazilian (Paranaguá), Argentinian (Rosario Futures and Rosario Spot), and Chinese (Spot and Futures) markets. The study looked at the price transmission between these markets over a period of almost 10 years, from September 2009 to May 2019. The Phillips-Perron unit root test was used to determine the order of integration of the time series. The Engle-Granger cointegration test failed to find any evidence of cointegration between the Chinese and Argentinian markets with any others of the international markets. The lack of cointegration was associated with highly government intervened markets. The cointegration and threshold test proposed by Enders and Siklos, succeeded in rejecting the Null hypothesis and finding cointegration among the series after structural breaks had been taken into account. The BDS test for nonlinearity showed that most of the time series were nonlinear, which prompted the investigation to look into nonlinear modelling. To evaluate asymmetric price transmission, the study used the Threshold autoregressive (TAR) model and the momentum threshold model (MTAR). The Argentine and Chinese markets were primarily suspected of exhibiting asymmetric price transmission due to structural government intervention. However, the test results failed to reject the null hypothesis and revealed asymmetric price transmission between these markets and the international market. As expected, the results found no evidence of asymmetric price transmission in the Paranaguá, Rotterdam, and Chicago markets. Hence, it can be concluded that symmetric price transmission is more prevalent in the global soybean market than asymmetric price transmission. 展开更多
关键词 Agricultural Economics ECONOMETRICS Price Transmission Soybean Market Asymmetric Price Transmission TAR MTAR BDS
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