Susan E. Riechert

Prey Control by an Assemblage of Generalist Predators: Spiders in Garden Test Systems

The hypothesis that generalist predators limit associated prey populations was tested using spiders, a group classically thought to be an insignificant component of the agroecosystem. Two habitat manipulations (addition of mulch and flowers) in separate and combined treatments were utilized to enhance spider numbers in a mixed vegetable system. Compared to control plots, significantly higher spider densities were observed in the plots to which mulch alone or both mulch and flowers had been added. Insect damage to the plants was significantly lower in the plots to which mulch had been added as well, which correlates with the lower numbers of pest insects in plots containing mulch. That spiders were the probable cause of the effect was demonstrated in plots containing both mulch and flowers but from which spiders had been systematically removed. Removing spiders from plots with mulch and flowers removed the effect of this treatment on pest numbers and plant damage. In addition, 84% of the predators observed foraging in the study areas during timed watches were spiders and 98% of the predation events observed were by spiders. In a separate experiment, spiders were added to a subset of individually bagged broccoli plants that had been infested previously with known numbers of herbivorous pests. Plant damage in bags lacking spiders averaged 93.3%, whereas damage averaged 31.8% in bags to which spiders had been added.

Thermal Balance and Prey Availability: Bases for a Model Relating Web‐Site Characteristics to Spider Reproductive Success

Analyses of the movements and web-site characteristics of the desert spider Agelenopsis aperta (Gertsch) demonstrate that web locations offering the following habitat features are actively selected: shrubs, depressions, litter, and flowering herbs. A model of the effects of the thermal environment and prey avalability on the reproductive success of spiders occupying various web-site types is developed. The estimated productivity for an excel- lent web site (grassland depression with attractants) is 13X that determined for a poor site (lava surface). Model results suggest that more energy is to be obtained from selection of a favorable thermal environment (eight-fold difference) than from a site offering greater num- bers of prey (two-fold difference). The presence of flowers at web sites increases the prob- ability of receiving an occasional high prey density, whereas litter and habitat features pro- viding shade (shrubs and depressions) allow increased spider activity through limitation of body temperature.

Games spiders play: Behavioral variability in territorial disputes

Summary1.The agonistic behavior of the funnel-web building spider, Agelenopsis aperta (Gertsch), was studied using induced encounters between adult females at natural web sites. All behavior exhibited by either individual during the course of an encounter was recorded. The results were analyzed through the use of transition matrices and the following multivariate treatments: factor analysis, ordination, and multiple regression. These latter methods were used to provide insight into possible sources of variability and their underlying causes.2.Sequence outcome is primarily determined by the relative weight of the two contestants engaged in a territorial dispute. If the size difference is large, the larger of the two individuals wins in a significant number of cases. Home bias is evidenced in cases where body weights are close.3.A stereotypy measure is devised that reflects the percent similarity of each sequence of events to an expected sequence (average). The frequency distribution, presence-absence or duration of all behavior patterns observed during the course of the disputes are utilized in measuring stereotypy. The territorial disputes of A. aperta exhibit low stereotypy, averaging 43% on a scale from 0 to 100%.4.Low stereotypy is, in part, related to the utilization of 33 different action patterns by spiders in these disputes. Factor analysis is used to express these action patterns in terms of five functional groups including locating behavior, signaling behavior, threat behavior, contact behavior, and a multiple function category. The order of these categories represents increasing cost based on relative estimates of the energy expenditure necessary to complete a specific behavior pattern and the potential for injury through the use of it.5.A Bray and Curtis ordination shows the important sources of between sequence variability to be the total energetic cost of the dispute and the complexity of behavior exhibited in it. The factors, in turn, depend on the resident-visitor status of the losing spider and on the relative size of the two contestants. The energetic cost of a dispute is markedly higher in those disputes in which the resident loses her territory. The number of action patterns and total frequency of acts observed are also greater in encounters in which the resident is the losing spider. Behavioral complexity is higher as well in cases where the weights of the two contestants are close.6.The variation in the pathways through which the sequences progress is shown to reflect the operation of assessment strategies by A. aperta. Initial assessment of the relative weights of the opponent is made through movements on the web at a distance (locating behavior). Subsequent activities depend on the results of this assessment, the predominant strategy being ‘retaliator’ (Maynard Smith and Price, 1973) in which an individual responds to escalation with further escalation. Spiders with a large weight advantage over the opponent tend to escalate directly to threat and contact behavior (‘hawk’ strategy). The corresponding strategy for a much smaller visiting spider is immediate retreat (‘mouse’). A much smaller resident spider, however, will exhibit the ‘retaliator’ strategy to the ‘hawk’ rather than the more conservative ‘mouse’ strategy. The particular stategy exhibited, then, also depends on the energetic investment a particular individual has in the contended resource.7.Within functional group variability is shown to significantly affect the outcome of territorial disputes. Winning spiders exhibit an average of 20% less stereotypy than losing spiders. Unpredictable behavior possibly confuses the opponent, causing it to make inaccurate assessments of the weights of the opponent relative to it. Retreat follows. This behavior is linked to the ‘protean displays’ exhibited in defense against predators (Humphries and Driver, 1967).

Behavioural syndromes and their fitness consequences in a socially polymorphic spider, Anelosimus studiosus

The temperate comb-footed spider, Anelosimus studiosus, shows a social behaviour polymorphism: individuals may defend asocial nests against intrusion by conspecifics or cooperate with them in multifemale nests. A suite of behavioural traits, including response to predators and prey, degree of superfluous killing, exploratory behaviour and general level of activity, was examined in laboratory trials to: (1) establish the extent to which these traits are correlated with social phenotype and (2) explore the potential adaptive value of this trait suite to the respective asocial and social phenotypes. Populations from the two latitudes studied, 26° and 36°, showed similar suites of correlated traits or syndromes. The individuals tested generally fell into two groups in the behavioural trials, although the asocial individuals from the mixed-phenotype population at 36° latitude were intermediate with respect to some of the measures. Individuals scored as ‘social’ generally showed less aggressive behaviour towards predatory cues, were less responsive to prey encountering their webs, showed little evidence of superfluous killing of prey and showed lower levels of activity than did most individuals scored as ‘asocial’. These results suggest linkage or pleiotropy between social behaviour and these traits. The behaviour of individuals in staged, mixed-phenotype feeding pairs indicated that social individuals may suffer fitness consequences in polymorphic populations. These results are discussed with respect to the stability and dynamics of the respective phenotypes in polymorphic populations.

The Consequences of Being Territorial: Spiders, a Case Study

The potential for energy-based territoriality to limit population numbers is demonstrated here for a population of the desert spider Agelenopsis aperta (Gertsch). Enclosure experiments run on groups of spiders subjected to different feeding histories and representing two different populations demonstrate that territory size is a fixed characteristic of specific populations which is not influenced by temporal changes in prey abundance or in the numbers of competitors present. Data are presented which further show that territory size is adjusted over evolutionary time to lows in prey availability (times of greatest stringency). The inflexible nature of territorial behavior resulted in the saturation of one habitat offering a limited number of adequate web sites. Removal experiments showed that approximately 35% of the population occupying this habitat in 1978 existed as floaters. Floaters suffered marked daily losses in fitness relative to the gains exhibited by territory holders (measured as changes in body wt). It is concluded that where population growth has not been checked by external factors (e.g., flooding, drought), the poorer competitors fail to obtain territories and as a consequence die. Surplus individuals are thus eliminated and population numbers will not increase beyond a fixed upper limit, set by the number of available territories. The system described for one spider species is extended to others and its consequences to their patterns of speciation, community structure, and predatory role are discussed.

How within-group behavioural variation and task efficiency enhance fitness in a social group

How task specialization, individual task performance and within-group behavioural variation affects fitness is a longstanding and unresolved problem in our understanding of animal societies. In the temperate social spider, Anelosimus studiosus, colony members exhibit a behavioural polymorphism; females either exhibit an aggressive ‘asocial’ or docile ‘social’ phenotype. We assessed individual prey-capture success for both phenotypes, and the role of phenotypic composition on group-level prey-capture success for three prey size classes. We then estimated the effect of group phenotypic composition on fitness in a common garden, as inferred from individual egg-case masses. On average, asocial females were more successful than social females at capturing large prey, and colony-level prey-capture success was positively associated with the frequency of the asocial phenotype. Asocial colony members were also more likely to engage in prey-capture behaviour in group-foraging situations. Interestingly, our fitness estimates indicate females of both phenotypes experience increased fitness when occupying colonies containing unlike individuals. These results imply a reciprocal fitness benefit of within-colony behavioural variation, and perhaps division of labour in a spider society.

Test for predation effects of single versus multiple species of generalist predators: Spiders and their insect prey

The prediction that single spider species (as exemplary generalist predators) limit associated prey populations to the same extent that species assemblages do was tested in a well controlled and replicated old field experiment involving the following treatments: (1) the natural spider assemblage (2) a numerically prominent web building spider, (3) a numerically prominent wandering spider, (4) a biomass prominent web‐builder, and (5) a biomass prominent wandering spider. Pest insect numbers were significantly higher in spider removal controls than in any spider treament over the four month period of the study, both in terms of total numbers and per spider effects. The individual spider species, in general, showed reduced prey limitation effects relative to that of the spider assemblage, though the magnitudes of these differences were small when compared to those exhibited between the various treatments and the spider removal control. When insect numbers were partitioned according to taxa, no treatment was found to have limited the predaceous insects nor the phytophagous hemipterans. All treatments, however, showed significant limiting effects on the phytophagous homopterans, coleopterans, and dipterans in the old field system, and other taxa were significantly reduced in at least one treatment in addition to the spider assemblage as a whole.

Levels of predation and genetically based anti-predator behaviour in the spider, Agelenopsis aperta.

Abstract Two populations of the spider, Agelenopsis aperta, experience markedly different levels of avian predation. Although there is no measurable level of avian predation on this species inhabiting a grassland habitat, estimated bird predation on A. aperta in a riparian (woodland) habitat rangesfrom 15 to 86% of the local population per week. Population variability in anti-predator behaviour was tested in an experiment using large amplitude vibrations in the web to simulate a predator. Individuals from the respective populations were tested at their web-sites in the field and second-generation individuals reared in the laboratory were tested in the laboratory for their responses to this stimulus. In both the field and laboratory, significant population differences were observed in the latency to return to a foraging mode following retreat into the protection of the web-funnel. The population exposed to predation pressure by birds exhibited the longer latency to return. Within this population, moreover, latency to return to a foraging mode covaried at an individual level with latency to respond to prey encountering the web-sheet.

Thoughts on the Ecological Significance of Spiders

the funnel-web building spider, Agelenopsis aperta (Gertsch) (Araneae: Agelenidae) in desert habitats. The versatility shown by Agelenopsis in its food relations (Riechert 1973) suggests a need for reassessment of the importance of spiders as invertebrate predators. The following is a summary of some existing information and thoughts on spider predation placed with my ideas into the context of predator-prey models. Numerous agricultural reports on pest species discuss the potential of spiders to control certain insect populations. There is no consensus as to their

Investigation of potential gene flow limitation of behavioral adaptation in an aridlands spider

SummaryThis study investigates the possibility that gene flow underlies the apparently maladaptive behavior of a riparian woodland population of the desert spider Agelenopsis aperta with respect to territorial, foraging, and antipredatory behaviors. I found that other local populations of A. aperta in the vicinity of the riparian woodland habitat are prey-limited and exhibit an “aridlands” phenotype (high aggressiveness in competitive interactions over energy-based territories and a lack of discrimination among potential prey types). The riparian woodland population deviates from surrounding populations in the area in that prey are abundant and this population shows a mixture of “aridlands” and “riparian” (low aggressiveness towards conspecifics and discrimination of prey profitability) phenotypes. Electrophoretic analyses of population subdivision in the area indicate that significant levels of gene flow have occurred, at least, sometime in the past. Drift fence analyses of spider movement futher indicate that there is marked unidirectional movement of spiders each year from the more arid habitats into the riparian woodland. Experimental manipulation of gene flow and predation pressure demonstrates that gene flow restricts adaptation in this habitat: one generation of predation pressure in the absence of gene flow is sufficient to cause a marked shift in spider behavior towards the expected “riparian” phenotype.