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1 June 2007 Litter disturbance and trap spatial positioning affects the number of captured individuals and genera of crickets (Orthoptera: Grylloidea)
Carlos F. Sperber, Luiz G. S. Soares, Marcelo R. Pereira
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Abstract

There are several factors that may affect sampling with pitfall traps. Here we test the hypothesis that the mere walking of the researcher proximate to the traps could cause an increment in the capture of crickets. This would occur if the walking provoked vibration in the litter, to which crickets showed a jumping response, thus falling into the pitfall traps. We mounted 126 traps in 14 groups of nine. The traps within a group were positioned in three parallel rows of three traps each, one meter apart from each other. Each group of nine traps was separated from the other groups by at least 5 m. Each group of nine traps was submitted to one of seven levels of disturbance frequency. Exposure time was 7 d for all traps. Treatments (disturbance frequencies) were allocated randomly among trap groups. For the data analyses we adjusted mixed-effects polynomial models. We captured 723 cricket individuals, distributed in 10 genera, most in the nymphal stage. As expected, the number of captured individuals, as well as the number of genera, increased with disturbance frequency. However this response was not linear: at higher disturbance frequencies there was a decrease in captures. There was also an effect of trap positioning within each group: central traps were more affected by disturbance than peripheral ones, while peripheral traps captured more individuals and genera in the absence of disturbance. Therefore we recommend areas near pitfall traps not be visited during the trapping period. Alternatively, to enhance sampling efficiency, the researcher may do programmed visiting to the trapping area, but this must be rigorously designed to provoke exactly the same disturbance for all traps. Enhancing the distance among traps will augment efficiency in capturing individuals and capture larger cricket diversity. Further studies of the interaction between methodology and cricket behavior will refine our ability to design and interpret pitfall studies.

Introduction

Pitfall traps have been used extensively to sample ground-dwelling arthropods (Southwood 1978). Pitfalls have been more commonly used for carabid beetles (Briggs 1960; Luff 1968, 1975; Digweed et al. 1995) and ants (Sackmann & Farj-Brener 2006; Borgelt & New 2006; Ribas et al. 2005; Schoereder et al. 2004a, b), but also for mole crickets (Tsurikov 2006, Barbara & Buss 2006, Adjei et al. 2003), and true crickets (Carmona et al. 1999, Simpson et al. 1992, Ribas et al. 2005, Velez & Brockmann 2006). Crickets have sometimes been used to test ecological hypotheses (Ribas et al. 2005, Mendes & Sperber 2003), insofar as they are the most common Orthoptera in tropical forest litter (Desutter-Grandcolas 1995).

Though pitfall trapping remains the most widely used and practical method available for sampling epigaeic arthropods, it incorporates many possible biases (Adis 1979, Digweed et al. 1995). It is known that pitfall catches can be affected by trap size and shape (Luff 1975, Adis 1979, Spence & Niemelä 1994), type of preservative (Luff 1968, Sperber et al. 2003), physical structure of the environment (Greenslade 1964, Durkis & Reeves 1982), and time of trap disposition (Niemelä et al. 1990).

Catches are often highest soon after pitfall traps are inserted into the soil, which has been termed a “digging-in effect” (Greenslade 1973). In a previous work, Mendes and Sperber (2003, pers. obs.) observed that pitfall traps collected twice within a disposition period of five days, captured more crickets than traps collected only once during the same period. This led to the hypothesis that crickets could be responding to the vibration of the litter substrate, provoked by researchers walking to the traps. Traps collected twice would recieve a greater frequency of substrate vibration than traps collected once, (the latter visited only on trap insertion into the soil and on final trap collection). In response to substrate vibration the crickets would jump, falling into the nearby pitfall traps. The same process could be generating a “digging-in” effect on these organisms. The work reported here aimed to test the prediction that cricket captures in pitfall traps, provoked by walking among the traps, would increase with the disturbance frequency.

Methods

Study area and experimental design.—The study took place in an Atlantic forest remnant (secondary submontaneous semideciduous forest), called Mata da Biologia (Jardim Botânico da UFV) in Viçosa, Minas Gerais State, Brazil (long 20°45′S, lat 42°50′W) in April 2006. We mounted 126 pitfall traps: translucent plastic vials, of 10-cm diameter and 10-cm depth. The traps were positioned in groups of nine (Fig. 1), in a total of 14 groups. Each group was placed in the middle of a 7 × 7-m quadrat, subjected to a particular experimental disturbance frequency (treatment), with two replicates per disturbance level (