Biodiversity evaluation in agricultural landscapes: above-ground insects

Abstract

In agriculture, sustainability can be linked to ecological resilience. In view of present or imminent environmental changes in agricultural landscapes, the diversity of species and genotypes, particularly of potential beneficials and alternative prey, may become of increasing importance. However, the available methods and empirical data concerning species diversity of above-ground insects in agricultural landscapes do not yet allow comprehensive evaluation. Standardized inventory methods must be used more rigorously and over longer time periods to detect significant differences in space and in time. Indicator groups for biodiversity estimates must be defined.

Methods for optimizing the reliability and comparability of faunistic inventories are proposed, including rarefaction for reference functions and estimation of species numbers per unit area. Recommendations for optimum sampling periods and average empirical numbers for species diversity and abundance of major arthropod groups are given and compared to published data.

In general, organismal biodiversity is higher in less intensely cultivated habitats. Apart from the impact of biocides, variation in species diversity often depends on the biodiversity of the surroundings (mosaic landscape) rather than on differing management regimes. The focus in preserving or enhancing, but also in evaluating biodiversity in cultivated areas thus should clearly be on the landscape level. Structural biodiversity in agricultural areas appears to be correlated with functional and organismal biodiversity of the above-ground insect fauna.

Introduction

The debate on whether there is a basic causal relation between biodiversity and ecosystem stability has never ended since Pimm (1991) published his enlightning review on the topic of ‘the balance of nature’. Today there is even less general agreement on the relation between organismal biodiversity (diversity of species and higher taxa; see Harper and Hawksworth (1994) and sustainability. However, there are several examples in which biodiversity can easily be seen as a vital component of sustainability or sustainable development (Lovejoy, 1995). The numerous antagonists of pest organisms have made an obvious contribution of ecological and economical importance to agriculture and pest management. The more predatory and parasitoid species and/or genotypes present in a particular landscape, the higher will be the chances that the effects of sudden environmental changes can be absorbed by ecological resilience, i.e. the ecological communities have a higher capacity to return to equilibrium population densities (Pimm, 1991).

There are three good reasons why surface-dwelling arthropods (mainly carabids and spiders, sometimes staphylinid beetles) are most often used for faunistic inventories in agricultural areas: (1) Most of the species are polyphagous predators and thus the taxonomic groups as a whole are considered as beneficial organisms. (2) All three taxa are easily collected in pitfall traps and thus allow for standardized sampling and comparative interpretation. (3) The catches in most habitat types contain sufficiently high numbers of species and individuals to allow for standard statistical treatment. Moreover, pitfall catches in agricultural habitats rarely contain protected or threatened species.

On the other hand, there are also three good reasons why pitfall traps are not always used in biodiversity evaluation: (1) While epigeal predators are excellent indicators for habitat quality in terms of biological control of pest organisms, they make poor correlates for overall organismal biodiversity (Duelli and Obrist, 1998). (2) Biodiversity evaluation in most cases is primarily motivated by nature conservation, therefore, it tends to be focussed on rare, attractive and threatened species rather than on common and inconspicuous beneficials. (3) The efforts and costs for collecting, sorting and identifying epigeal arthropods is often too high compared to inventories of higher plants or birds. Only a reduction and strict standardization of effort and costs will render pitfall trapping more applicable and the results more comparable.

Above-ground arthropods in agricultural areas can be used for biodiversity evaluation in three ways:

• Nature conservation is mainly interested in rare, threatened or even protected species. Inventories must avoid destructive methods, and in many cases even the removal of specimens for reference collections is not possible. Under such circumstances, adequate sampling methods are visual or auditory counts by proven experts, mostly in the form of standardized transects for inventories of butterflies, grasshoppers or dragonflies. The assessment is based on regionally known and agreed conservation values for the identified species (red lists, protected species, faunistic singularities). It is probably realistic to assume that rare and threatened insect species are not playing a major trophic role in agricultural habitats and thus will only have a minimal influence on ecological stability or sustainability. Therefore, the present paper will not go into any further detail concerning this conservation-centered group of taxa.

• Agriculture, forestry and medicine focus on either harmful or beneficial insects. Inventory methods are usually optimized for collecting large numbers of individuals, especially when pest organisms are the target. In addition, in recent years biological control and ecological compensation measures have shifted their attention towards species numbers of potentially beneficial organisms, especially in view of a sustainable management in cultivated landscapes in times of imminent global change. Antagonists of pest organisms therefore, are highly relevant to the topic of this paper.

• In the last decade biodiversity per se has turned into a very general and basic conservation value (Gaston, 1996). Genetic, organismal and ecological diversity are seen as indicators for environmental quality, especially in areas heavily influenced by human activities such as intense cultivation. The very idea of sustainable development is inherently linked to the concept of biodiversity as advocated at the Rio-Convention in 1992 (Johnson, 1993).

Since arthropods comprise most of the organismal variability in practically all habitats, they are good candidates for a quantitative biodiversity evaluation. But not all taxonomic groups in agricultural habitats are equally well suited as unbiased correlates for overall organismal biodiversity at a given site. Considering both effort (costs for collecting and identification) and yield (quantitative correlation with regard to site-specific organismal diversity), three insect taxa seem to perform especially well as biodiversity indicators: the insect order Heteroptera (bugs), and within the order Hymenoptera the Symphyta and Aculeata (Duelli and Obrist, 1998). An excellent way to collect these taxa in a standardized manner is the use of flight traps such as window (interception) traps, Malaise tents and yellow water pans.

With sustainable development in view, biodiversity evaluation based on above-ground insects can concentrate on two of the above mentioned methods: polyphagous predatory arthopods collected in pitfall traps, and flight trap samples of biodiversity indicator groups such as the Heteroptera, Symphyta and/or the Aculeata, in combination with vegetational inventories.

A major problem in reviewing the present knowledge on insect biodiversity evaluation in agricultural areas is the fact that most of the many local inventories are not published in all the details necessary to assess them in a comparative way. Most of this information can only be found in the ‘grey’ literature of unpublished student theses or in more or less confidential reports to local, regional or federal funding authorities or private enterprises.

The aim of the present paper therefore, cannot be to give an account of all the innumerable empirical data on species numbers in habitats of agricultural landscapes, but rather to present a standardized method in order to make those data comparable.