Use of pitfall traps to investigate arthropod diversity.
Abstract
Species diversity is one of the essential components of ecosystem studies and it determination would require a suitable sampling technique according to the habitat and target species. The experimental procedure was carried out to determine the arthropod species diversity in three habitats, the grassland; ant mounds and under trees. The pitfall trap method was used whereby a ground borer was used to dig a hole into the ground, placing a sampling container with ethanol as the preservative and leaving open for two days. The ant mound showed the highest species richness due to abundance of resources and the grassland had the least due to limitation of resources in the open area although all the habitats had high species diversity. Evenness was high in the grasslands in comparison to all the other habitats which was attributed to the sampling area being larger than all the others which increased chances of capturing different arthropods equally. The pitfall trap method could be effectively used to capture all arthropods intended although limitations included: variation in habitat structure, seasonal and daily activity patterns, behaviour as well as size of target species.
Introduction
The increase in anthropogenic activity (large scale forestry, agricultural land-uses, climate change and exotic species introduction) impacts on ecosystems has led to a growing interest in the determination of the degree of damage to native species’ ecology (Work et al., 2002). Arthropods have often been used as ecological indicators due to their occupation of a large percentage of variant ecosystems since they account for an estimated 57% of all global species (Millennium Ecosystem Assessment, 2005) and represent over 80% of global species richness (Wilson, 1992). Their ability to occupy highly essential niches in the ecosystem (for example; pollination, nutrient cycling and natural eradication of pest species) (Pyle et al., 1981; Madden & Fox, 1997; Schowalter et al., 1998), quick responses to any ecological changes and short life cycles has led ease of their use as ecological indicators (Work et al., 2002).
Species diversity is the numerical measure of heterogeneity and distribution of species in a specific area from the species richness and species abundance (Lloyd & Gherlardi, 1964; Pielou, 1966). The study of the species diversity requires the use of suitable sampling methods which ensure cost effectiveness, suitability of habitat conditions of the target species as well as underlying biases of the method (Southwood, 1994; Gullan & Cranston, 2005). The pitfall trap method has often been used to capture ground dwelling arthropods. This consists of open sampling containers with a liquid preservative sunk into the ground (Barber, 1931) posing a few advantages which are: it is easy to use; inexpensive and allows a 24 hour operation (Clark & Blom, 1992). However, biases do arise with the use of this method as Digweed et al., (1995) and Schmidt et al., (2006) argued that factors such as trap material, shape, preservative used and the placement of the trap could influence the abundance of species collected.
The aim of the experimental procedure was to investigate the arthropod biodiversity in the vlei and how stratification of the vlei according to the type of habitat would affect the diversity noted. The efficiency of the pitfall trap-method was also investigated in relation to the abundance of species that were caught by the trap.
Materials and Methods
Three habitats were selected to investigate arthropod diversity and these were: the grassland, ant mounds and under trees. Using a ground borer, 30 pitfall traps were dug with 10 in the grassland, 10 on the various ant mounds and 10 under trees without ant mounds. One sampling bottle containing approximately 15ml of ethanol was placed in each of the holes dug and left open to capture small arthropods. These traps were a great distance apart and left in place at the sampling points for 2 days.
Results
The grassland habitat had six arthropod species in total with 131 total individuals recorded. The least number of individuals were of the beetles (6) whilst the most number of individuals in the sample were of the crickets (46) (Table 1).
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The ant mound habitat had 11arthropod species in total with 196 total individuals recorded. The least number of individuals collected were of the honey bee and fly (1) whilst the most number of individuals in the sample were of the ant spp 1 (83) (Table 2).
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The habitats under trees had 10 species in total with 94 total individuals recorded. The least number of individuals collected were of the two housefly species (1) whilst the greatest number of individuals in the sample were of the beetles (24) (Table 3).
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Discussion
The presence of inherent biases of pitfall traps (Southwood, 1994; Spence and Niemela, 1994) have led to the type of arthropods captured being determined by factors such as: the trap size (Brennan et al., 1999); habitat structure (Greenslade, 1964); seasonal and daily activity patterns of the target organisms (Niemela et al., 1992); behaviour as well as size of target species (Halsall & Wratten, 1988). The determining factors in the experimental procedure were limited to variation in habitat structure, seasonal and daily activity patterns, behaviour as well as size of target species since all traps were of the same size.
The three habitats sampled recorded a low Simpson’s Index and a Shannon’s index greater than 1.5 showing that there was high species diversity and the probability of randomly selecting two individuals from the same species in the sample was low. The ant mound showed the highest species richness due to abundance of resources and the grassland had the least due to limitation of resources in the open area. The grassland showed the highest evenness of species as their abundance in the habitat was relatively similar, thus the habitat could be described as even. Habitats under the tree showed relatively high evenness with the exception of bursting populations of three species that showed greater abundances than 10 individuals of the species (Table 3). The variation in abundance of species on the ant mounds suggests why it had the lowest evenness (Table 2).
The most abundant species in all three habitats were the ant spp 1 and 2, although their population is the highest in the ant mound (Table 2). This poses a bias to the sampling of all the other arthropod species as the target habitat was limited to most ant spp. populations thus the comparison of three different habitats for arthropod diversity was not fair. Species adapted to flight as a means of locomotion were limited to the fruit fly, honey bee, beetle and housefly spp, which does not begin to cover the arthropod species adapted to flight that inhabit all the habitats sampled. Thus, the pitfall trap was not effective in the fulfilment of the experimental objective of determination of “all” arthropod species diversity in the vlei. The trap was mostly limited to abundance of gressorial and cursorial ground dwelling arthropod species with the exception of a few species of flight.
The size of the trap was also a limiting factor as some larger arthropods which include locusts noted when setting the traps in the grassland and cockroaches expected under trees as they feed on decaying matter were not captured. Change in behaviour of the species may attribute to the type of species collected due to changes in micro-climate (Honek, 1988) and physical lay-out of habitat (Greensalde, 1964). An increase in temperatures during the diurnal time may result in increased movement of species and possibly increase in probability of capture than during the night. Physical lay-out of the habitat could have an effect on movement and consequently capture of the species as ground displacement and multiple bends or twists in pathways could determine capture (Crist & Wiens, 1995) by affecting speed of movement (Greenslade, 1964). Some studies show that speed of movement is essential is to ensure capture efficiency of species as ants that move faster are prone to capture in pitfall traps (Greenslade, 1973; Andersen, 1983). However, Halsall & Wratten (1988) argued that speed of movement had no effect on capture.
In conclusion, the pitfall trap can be used to determine the characteristics of species diversity in an ecosystem, although biases of the sampling technique should be taken into account.
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