Impacts Of Forest Conversion On Understory Insect Diversity And Its Seasonal Variation

Background Anthropogenic habitat disturbance is a significant threat to biodiversity,and forest conversion is a common form of habitat disturbance.Among them,logging is a common forest management practice,but the intensity of forest conversion also varies due to the different purposes of logging.When a relatively intact mature forest is converted to other forms of forest,it usually has a negative impact on plant diversity.However,we know little(such as for a specific order or for a specific season)about how different forest conversions affect the diversity of insects that often depend on plants.This knowledge is important because insects are among the most diverse and abundant taxa in the entire animal kingdom and contribute significantly to ecosystems.For example,insects can influence the ecosystem by moderating ecological processes such as pollination,decomposition,and cycling;and providing resources for human medicine,biological control,and other services.However,insect diversity is currently experiencing a significant decline,so a detailed understanding of the impact of forest conversion on insect diversity is essential for forest information,forest management,conservation planning and maintaining ecosystem stability,in addition to protecting insect diversity.My research area was located in Tiantong National Forest Park in Ningbo,Zhejiang Province.The area has three forest types with different conversion intensities:mature forest with almost no human disturbance,shrublands formed by natural regeneration after selective logging,and a plantation forest formed by planting artificially selected species after large-scale logging.Thus,the three forest types provide different conversion intensities and an ideal environment for carrying out the effects of forest conversion on insect diversity and its seasonal dynamics.Objective This study was based on three different conversion types of forests in Tiantong National Forest Park,Zhejiang Province,where insect samples were collected for four consecutive seasons.The collected sample data were used for statistical modeling to quantify the response of insect diversity and species composition among different conversion types and sampling seasons,respectively.This study provides a preliminary investigation of insect diversity in three different conversion types of forests in the Tiantong area,which can deepen the understanding of the insect resource base in the area.In addition,this study will deepen our understanding of how insect community diversity and species composition respond to forest conversion and seasonal changes,and provide some direction for studying potential drivers,thus providing a solid theoretical foundation for forest ecosystem management and insect diversity conservation.Specifically,this study asks the following questions:(1)How do insect communities(diversity and composition)respond to different types of forest conversion(shrublands and plantation versus intact mature forests)?(2)Do insect communities’(diversity and composition)responses to different forest conversion types vary among seasons(spring,summer,fall,and winter)and insect groups under consideration?(3)Do forest floor habitat heterogeneity and plant diversity vary among forests with different conversion histories? How do forest floor habitat heterogeneity and plant diversity relate to insect diversity?Methods In this study,sample sites representing three forest conversion types(mature forest,shrubland,and plantation forest)were selected in the Tiantong area of Ningbo.And four replicates were selected for each forest conversion type,with a linear distance of no less than 200 m between each replicate.The selected sample sites were sampled for plant diversity,environmental factor indicators,and insects,respectively.Regarding insect diversity,two methods(Malaise and Baited pitfall traps)were used to collect insect samples for four seasons from July 2021 to May 2022.Then the insects were classified by morphological species after collection to obtain the species composition and diversity(abundance,richness,evenness,and Shannon’s diversity)of insects.Regarding plants,woody as well as non-woody plants were surveyed and identified to species level at the 10 m × 10 m sample level to quantify plant species composition and diversity.Regarding environmental factors,collections at the 20 m × 20 m sample level included data on exposed mineral soil(% area without any ground cover),moss cover(%),ant mounds(%),rocks(%),dead wood(%),grass cover(%),and depressions(%).The heterogeneity of forest floor habitats at the sample site was described as the mean value of five 20 m × 20 m sample squares.Based on the data obtained above,several analyses were conducted as follows:(1)Repeated measures analysis of variance analyses: A repeated-measures ANOVA was conducted to assess the effect of forest conversion intensity on insect diversity(overall,Lepidoptera,Coleoptera,Diptera,Hemiptera,and Blattaria)and whether its effect would be influenced by the sampling season.Before conducting the ANOVA,I checked the normality of the residuals of the data and the homogeneity of the variance across groups.If the results of the repeated-measures ANOVA indicated a significant effect of forest conversion,seasonality,or their interactions,a Tukey posthoc analysis was performed to determine the differences further.(2)Permutational multivariate analysis of variance(Per MANOVA)tests,nonmetric multidimensional scaling ordinations: To assess whether insect species composition(overall,Lepidoptera,Coleoptera,Diptera,Hemiptera,and Blattaria)differed between forest conversion intensities and sampling seasons,while a non-metric multidimensional scale analysis was performed to visualize the data.(3)One-way ANOVA: To test whether plant diversity and near-site habitat heterogeneity indices differed by forest conversion intensity,I conducted a one-way ANOVA.Plant diversity or forest floor habitat heterogeneity indices were used as response variables,and forest conversion was used as a predictor variable.The normality of residuals and homogeneity of variances were checked before the analysis,and these assumptions were met.When one-way ANOVA results were significant,I conducted a Tukey post-hoc analysis to further clarify specific differences.(4)Linear regression analysis: To explore the relationship between insect diversity and forest floor habitat heterogeneity or between plant diversity and insect diversity.Results(1)Forest conversion impacted species richness,evenness,and abundance of overall insects(all orders combined).Among them,insect abundance and richness were highest in moderately disturbed shrublands,showing a single-peaked pattern.Concerning seasonal variation,all insect diversity indices peaked in warm and rainy summers and were lowest in cold winters.Seasonal variation significantly affected all the overall insect diversity indices,and the interaction of forest conversion and seasons also significantly affected all the diversity indices.These results suggest that the response of insect diversity to forest conversion is influenced by seasons.(2)The diversity of individual orders(Lepidoptera,Coleoptera,Diptera,Hemiptera,and Blattaria)showed a generally consistent pattern with overall insect diversity regarding the effects of forest conversion;however,there was considerable variation among orders as well.Forest conversion had significant or marginal effects on more than half of the diversity indicators but no significant effects on a small number of diversity indicators(mainly evenness).For example,there was no significant effect on evenness for Lepidoptera,Coleoptera,Hemiptera,and Blattaria insects.Seasonal variation in the diversity of individual taxa was found to have a generally consistent pattern-highest in summer,decreasing in autumn,reaching a minimum in winter,and increasing again in spring.(3)The species composition of insects(overall,Lepidoptera,Coleoptera,Diptera,Hemiptera,and Blattaria)differed significantly across forest conversion intensities and seasons.(4)Forest floor habitat heterogeneity and plant diversity differed significantly between forest conversion types-highest in shrublands with moderate disturbance intensity.For overall insects(all orders combined),forest floor habitat heterogeneity showed a significant positive correlation with its abundance and species diversity;plant diversity showed a significant positive correlation with its abundance,and no significant correlations were found for other diversity indicators.For all the insect groups I focused(Lepidoptera,Coleoptera,Diptera,Hemiptera,and Blattaria),many order’s abundance and richness were positively correlated with forest floor habitat heterogeneity,and only a few order’s evenness and Shannon’s diversity(evenness of Coleoptera,evenness and Shannon’s diversity of Diptera)were negatively correlated with forest floor habitat heterogeneity.No significant correlations with plant diversity were found for the other diversity indices,except for the positive correlations between the species richness of Coleoptera and plant diversity.Conclusions Both forest conversion and seasonal dynamics had significant effects on insect diversity and species composition(overall,Lepidoptera,Coleoptera,Diptera,Hemiptera and Blattaria),and the response of insect diversity to forest conversion was seasonally influenced.Plant diversity and forest floor habitat heterogeneity peaked in shrublands with moderate disturbance intensity and these shrublands also supported maximum insect diversity,suggesting that plant diversity and forest floor habitat heterogeneity are among the important drivers of insect diversity.Maintaining high plant diversity and promoting habitat heterogeneity can thus be among the important considerations while planning insect conservation.