Sampling fossil floras for the study of insect herbivory: how many leaves is enough?

Sandra R Schachat,S Augusta Maccracken,Conrad C Labandeira

doi:10.5194/fr-23-15-2020

Abstract

Abstract. Despite the great importance of plant–insect interactions to the functioning of terrestrial ecosystems, many temporal gaps exist in our knowledge of insect herbivory in deep time. Subsampling of fossil leaves, and subsequent extrapolation of results to the entire flora from which they came, is practiced inconsistently and according to inconsistent, often arbitrary criteria. Here we compare herbivory data from three exhaustively sampled fossil floras to establish guidelines for subsampling in future studies. The impact of various subsampling routines is evaluated for three of the most common metrics of insect herbivory: damage type diversity, nonmetric multidimensional scaling, and the herbivory index. The findings presented here suggest that a minimum fragment size threshold of 1 cm2 always yields accurate results and that a higher threshold of 2 cm2 should yield accurate results for plant hosts that are not polyphyletic form taxa. Due to the structural variability of the plant hosts examined here, no other a priori subsampling strategy yields consistently accurate results. The best approach may be a sequential sampling routine in which sampling continues until the 100 most recently sampled leaves have caused no change to the mean value or confidence interval for damage type diversity and have caused minimal or no change to the herbivory index. For nonmetric multidimensional scaling, at least 1000 cm2 of leaf surface area should be examined and prediction intervals should be generated to verify the relative positions of all points. Future studies should evaluate the impact of subsampling routines on floras that are collected based on different criteria, such as angiosperm floras for which the only specimens collected are those that are at least 50 % complete.

Highlights

Plants serve as the foundation of many terrestrial ecosystems, and insects have herbivorized plants for hundreds of millions of years (Labandeira et al, 2013)
Auritifolia waggoneri and Taeniopteris spp. at Colwell Creek Pond (CCP) have similar damage type” (DT) diversity, which is higher than the DT diversity of Johniphyllum multinerve at South Ash Pasture (SAP)
Specimens are sampled sequentially from largest to smallest, or from most to least complete, and sampling ends either when the minimum size threshold has been reached or when the addition of 100 specimens to the dataset has had no effect on DT diversity and a minimum effect of the herbivory index

Summary

Introduction

Plants serve as the foundation of many terrestrial ecosystems, and insects have herbivorized plants for hundreds of millions of years (Labandeira et al, 2013). The fossil record provides data on plant–insect interactions that encompass far longer time spans than can be examined in laboratory studies, yielding insight into such timely issues as the response of insect herbivores to climate change (Currano et al, 2010). Insect herbivory on leaves and other plant organs has been categorized qualitatively using the “damage type” (DT) system (Labandeira et al, 2007). Studies of insect herbivory in the fossil record vary tremendously in their intensity of coverage. Many authors have taken intermediate approaches, for example, by documenting all DTs on a subset of leaves from various taxa (Filho et al, 2019), by documenting DTs for a specific behavior such as galling (Knor et al, 2013), or by categorizing feeding damage at a coarser scale than the DT system, such as the level of functional feeding group

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Discussion

Conclusion