PREY CHOICE AND SPIDER FITNESS

Abstract

Although spiders in general are polyphagous, indiscriminate feeding is not advantageous because prey vary enormously in quality due to toxicity or nutrient deficiency. Active prey selection serves to find the optimal compromise between three ‘‘nutritional goals’’: maximize energy intake, balance nutrient composition of the body, and minimize toxin consumption. Consumption of toxic prey is reduced by more or less specific induced aversions, probably associated with both prey taste and behavior. Spiders’ ability to avoid toxic prey seems limited because aversions are short-lasting and some toxic prey do not induce an aversion. Such prey may be lethal. Toxic prey in a mixed diet may inhibit feeding on and utilization of good prey. Induced tolerance to toxic prey may be possible, however. Nutritional balance may be obtained through consumption of high-quality prey or through mixing of prey types. It is argued that nutrient balance is more important than maximization of energy intake for fitness. Foraging decisions influence individual fitness in a variety of ways. Choice of foraging habitat (patch) has been recognized as being of primary importance through its effect on feeding rates, with derived benefits to growth (size) and reproduction (Riechert 1981; Morse & Stephens 1996). Once in a feeding patch, the spider is confronted with an array of potential prey species. Prey selection is the consumption of prey relative to the composition of prey available in the spider’s microhabitat. So defined, selectivity has two aspects (cf. Pastorok 1981; Sih & Moore 1990): 1) passive selection or capture success, and 2) active selection (‘‘choice’’), i.e., acceptance/rejection of the prey. Both are clearly important for determining the diet composition in the field. The capture success reflects the co-evolutionary balance between prey and predator in a broad sense (Malcolm 1992) and is largely outside the individual spider’s control. Active selection reflects the spider’s decisions (choices) whether positive (acceptance) or negative (rejection). Spiders are selective if they choose differentially between the prey species according to their preferences. Such preferences by individuals may change over short time spans and, contrary to expectations, they are not necessarily related in a simple way to the nutritional value of the prey (see later). Active and passive selection can be distinguished if prey is presented with and without possibilities for escape (Onkonbury & Formanowicz 1997; Lang & Gsodl unpubl. data). This review is concerned only with active selection or ‘‘prey choice,’’ and in our own experiments we have confined spiders and prey to small containers. Additionally, I have largely neglected the phenomenon of size selectivity and focus on choice between prey species. Finally, my treatment is biased towards wolf spiders (Lycosidae) and money spiders (Linyphiidae). These two families account for most of the species and individuals of spiders found in agricultural fields of Northern and Central Europe (Sunderland 1987; Nyffeler & Benz 1988a, b; Toft 1989), and most of the studies reviewed were completed with the specified goal of analyzing potentially important trophic links of the agroecosystem. The experimental prey types were likewise selected to represent the most important prey groups as revealed by studies of European cereal fields, i.e., Collembola, Diptera and aphids (Nyffeler & Benz 1988a, b; Sunderland et al. 1986, 1987; Alderweireldt 1994). Food contains three main components: energy, nutrients, and toxins. Viewed in isolation, feeding behavior should aim at the following three ‘‘nutritional goals’’: maximize energy intake, balance body nutrient composition, and minimize toxin consumption. They cannot all be realized at the same time. Prey choice is expected to achieve the optimal compromise that maximizes the fitness of the spider. As spiders are generalist predators, we might a priori expect that energy gain would be maximized by accepting all kinds of prey, 302 THE JOURNAL OF ARACHNOLOGY i.e., by being non-selective. In line with this, the broad polyphagy of spiders is considered to follow from the prevailing sit-and-wait strategy and a temporally varying food supply (Riechert & Luczak 1982; Riechert & Harp 1987). In the notion of polyphagy it is usually understood that most kinds of potential prey are of approximately equal value to the predator and may substitute each other in the predator’s diet (cf. Slansky & Scriber 1985; Waldbauer & Friedman 1991; Wise 1993). Rejection of toxic, dangerous or difficult prey (Riechert & Luczak 1982; Nentwig 1987) as well as novel (unfamiliar) prey (Turnbull 1960; Riechert & Luczak 1982) has, however, been recognized. Prey selection aimed at obtaining a specific composition of amino acids was indicated by Greenstone (1979); but, generally, nutrient balance is considered to be achieved through a mixed diet (Riechert & Harp 1987; Uetz et al. 1992). In conclusion, though the existence of active choice has been acknowledged, most reviews have considered it to be of limited importance. The questions raised here are: 1) Does active prey choice improve fitness? 2) What mechanisms influence prey choice? 3) Are the three nutritional goals equally important? 4) Do the answers to these questions confirm the inferior role of active selection for spider nutrition?