Does Intermediate Disturbance Hypothesis explain mechanisms of biodiversity decline in agricultural lands? Perennial plants play key roles in maintaining herbivorous insect diversity

Background/Question/Methods

In recent decades, biodiversity decline due to land-use changes in agricultural landscapes is a central theme across the globe. Land-use changes including intensification and abandonment have caused biodiversity loss in semi-natural grasslands providing herbivorous insects with resources. Although many studies have described the decline of herbivorous insect diversity in semi-natural grasslands, only a few studies examined the mechanisms. In agricultural lands, it has been proposed that biodiversity decline can be explained by the Intermediate Disturbance Hypothesis. However, how disturbance regime correlates with insect diversity decline has not been studied enough.

The purpose of this study is to examine the mechanisms of herbivorous insects decline in semi-natural grasslands by focusing on resource (plant community) changes through land-use changes. We compared the diversity of butterflies, grasshoppers and plants among three land-use types: traditional (intermediately disturbed), intensive (highly disturbed) and abandoned (less disturbed) agricultural fields. We then analyzed the relationships between butterfly/grasshopper and their resources (host plant, vegetation height and aboveground biomass). In the analyses, we divided plants/insects into several functional groups, and examined response to resource changes. This study was conducted at 124 plots (one plot-area: 5x50m) in semi-natural grasslands around three types of rice fields in Japan. 

Results/Conclusions

We found diversity of perennial plant, butterfly and grasshopper species was the highest in the traditional fields whereas diversity of annual plant species in traditional fields was as high as in intensive fields. The relationship between perennial plant diversity and disturbance (mowing) frequency indicated by vegetation height was shown as a unimodal pattern, while annual plant species monotonically decreased with decreasing disturbance frequency. Most functional groups of butterflies tended to prefer grasslands with high diversity of host and/or flowering perennials. All grasshopper groups had significantly positive correlations with perennial plant diversity. Grasshoppers exhibited various responses to vegetation height and aboveground biomass depending on functional groups.

Our results demonstrated that herbivorous insect diversity had stronger relationships with perennial plant diversity than annual plant diversity. Thus, herbivorous insect diversity decline in semi–natural grasslands was caused by the perennial plant decline, which was well explained by the Intermediate Disturbance Hypothesis. This study will contribute to a better understanding of mechanisms of biodiversity decline due to land-use changes in agricultural lands.