高阶作用对植物-传粉者-植食动物集合群落续存的影响

高阶作用通常指一个物种对另外两个物种之间相互作用强度的影响,对物种共存、群落构建及生物多样性具有重要影响。在集合种群水平上考虑了植食动物对动植物传粉关系造成的高阶作用,以及植食动物对传粉者的间接作用。通过分析基本生态过程,建立植物-传粉者-植食动物的集合群落模型,模型清楚地展示高阶作用和间接作用,可以用来研究它们对集合群落稳定性和续存的影响。结果表明:(1)互惠关系在集合群落尺度上会引起双稳态现象,说明了群落动态对初始条件的依赖性;(2)正高阶作用能够扩大集合群落双稳态的参数范围,负高阶作用和间接作用缩小它的参数范围,但都不会从本质上改变双稳态现象;(3)正高阶作用能够降低集合群落的灭绝阈值,增加集合群落稳定时的占有率,有利于集合群落续存,而负高阶作用和间接作用不利于续存。研究结果说明高阶和间接作用对调节多物种系统动态和物种共存具有重要作用。

Effects of soil nitrogen levels on growth and defense of the native and introduced genotypes of alligator weed

Plants growing in nutrient-rich environment are predicted to be less defended than conspecifics under nutrient limitation.However,less is known about the effects of nutrient levels on tolerance and induced resistance,and whether the effects differ between native and introduced populations of invasive plants.We performed a greenhouse experiment with introduced(the USA)and native(Argentina)genotypes of Alternanthera philoxeroides in order to study the effects of soil nitrogen levels on plant growth,constitutive and herbivore(Agasicles hygrophila)-induced chemical defense,and herbivory tolerance.We measured total biomass,elongation rate(as proxy of growth rate),carbon and nitrogen,and the concentration of triterpenoid saponins(defensive chemicals)in leaves and roots.Constitutive resistance(+33%higher leaf triterpenoid saponins in control treatment at low nitrogen level)and tolerance[less decreased total biomass after herbivory treatment(−24%and−15%for high and low nitrogen levels)]were favored at lower nitrogen level,while induced resistance was favored at higher nitrogen level(+24%increased leaf triterpenoid saponins after herbivory treatment at high nitrogen level).Constitutive resistance and tolerance exhibited trade-offs with growth rate,while induced resistance positively correlated with growth rate.Additionally,the introduced genotypes had−6%lower content of leaf carbon in the presence of herbivores than the native genotypes at low nitrogen level,but such difference was absent at high nitrogen level.Our results indicate that soil nitrogen levels influence the preference of different defensive strategies of plant,and interweave with herbivory to determine the performance of introduced genotypes.

Responses of morphological and physiological traits to herbivory by snails of three invasive and native submerged plants

The submerged plant species Carolina fanwort(Cabomba caroliniana)has become a dominant invasive aquatic plant in the Lake Taihu Basin(LTB)in China.Introduced species may escape their original specialist enemies and encounter fewer enemies in their new environment.They were assumed to have suffered less herbivory than native species as they are relatively unpalatable(the enemy release hypothesis[ERH]).The objective of this study was to compare the responses of C.caroliniana with those of co-occurring native species to herbivory from native herbivores.We conducted a mesocosm experiment to record the responses of C.caroliniana and two commonly co-occurring native submerged plant counterparts,water thyme(Hydrilla verticillata)and Eurasian watermilfoil(Myriophyllum spicatum),to herbivory by two native generalist gastropod snails,Radix swinhoei and Sinotaia quadrata.Plant morphological traits(total biomass,shoot/root[S/R]biomass ratio and relative growth rate[RGR])and physiological traits(leaf total nonstructural carbohydrate[TNC],lignin,and cellulose)were recorded.The snail S.quadrata rarely influenced the plant traits of the three submerged plants.With the increasing numbers of R.swinhoei treatments,most of the plant traits of H.verticillata and M.spicatum changed,while those of C.caroliniana showed a relatively stable fluctuation.This result indicates that C.caroliniana is more resistant to herbivory by the snail R.swinhoei,which is consistent with the ERH hypothesis.This finding indicates that herbivorous snail species contributes to the invasion of C.caroliniana,which potentially alters the species composition of submerged plants in the plant community.