University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Supervised Undergraduate Student Research Chancellor’s Honors Program Projects and Creative Work Summer 8-1997 Tests for superfluous killing in five species of web-building spiders Jennifer Lyn Maupin University of Tennessee - Knoxville Follow this and additional works at: https://trace.tennessee.edu/utk_chanhonoproj Recommended Citation Maupin, Jennifer Lyn, "Tests for superfluous killing in five species of web-building spiders" (1997). Chancellor’s Honors Program Projects. https://trace.tennessee.edu/utk_chanhonoproj/269 This is brought to you for free and open access by the Supervised Undergraduate Student Research and Creative Work at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Chancellor’s Honors Program Projects by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. Appendix D - UNIVERSITY HONORS PROGRAM SENIOR PROJECT - APPROVAL Name: ;;r{--'J-~J~£~£--M-~ll.f-~(t------------------------- College: flLt~_-t_,,£~_t(fltJ:S. Department:Ec2_1.9--31_:t_[iQJ"rh~fl£lf-{ fSio/Oj: F acul ty Men tor: __D!~_~~~~~_~~_~.fL~{_t ___________________ _ PROJE C T TI TLE: __ 71~f;!i_fQc __SM-f-eLfJ~~J<5,E _________ _ -- __kijjL0.()-_!fLl11L'-_l#fYiS'- ~-d--(,,~L(~::bJd:,-ilC!!ilJ- - ___ ___~pj_if(~ ____ __________________________ _ I have reviewed this completed senior honors thesis with this student and certify that it is a project commensurate with honors level undergraduate research in this field. 'I: Signed: b_~_.f:_i&.:~~ ______________ , Faculty Mentor Date: ---Q~~J!J-j-~l--- Comments (Optional): ~(-€CL+-,~6 l 27 Appendix C - UNIVERSITY HONORS PROGRAM SENIOR PROJECT - PROSPECTUS Name: __;[~D_0J_~iJC __ 1Y1~~~LtL _________________________ _ College: .f1Lt:$__ L~C:iitl~( S; Department: &?J.:?-1~-~~£l(i'-L0:-h .of\. lJ.-l '1 t 610 lQ1j F ac ul ty Men tor: __ Qc~_~~:~_.r~!.}_K~f_c:::b_t.'~~t _________________ _ P R OJ E CT TITLE: -_11:_.$_Lt?._££c_.s.1Af-~L£lJ6:_<?'_l0_2;______ _ _______ ){jjjj4j_-'_~ __£~~I{L_'N~9_~_b~~Lc!_'~-,$"-f-JdtL~ Date: ----q::.~-!L'l----- Return this completed form to The University Honors Program, F10l Melrose Hall, 974-7875, not later than the end of your 3rd year in residence. 26 ABSTRACT Several studies on spiders report that when faced with high levels of prey encounter, individuals appear to kill more than they are able to consume. This behavior, variously referred to as wasteful killing, overkill, or superfluous killing, may have important implications for biological pest control and the development of predator-prey models of population growth. The existence of superfluous killing has been challenged on the grounds that the hypothesis has not been subjected to quantitative study and that it predicts a behavior that is maladaptive. This study tested for superfluous killing by five species of web-building spiders having divergent web types. In lab and field tests, 25 spiders of each species were presented with sequential prey items until they ceased to capture prey. A measure of the mass of prey captured was then compared with the average mass of prey consumed by spiders fed to satiation in the lab (optimal consumption). Lab tests were more useful than those conducted in the field because of the inability to obtain accurate weights of both the spiders tested and prey encountered in field trials. For each species investigated, mean mass captured was significantly greater than the mean optimal consumption level for that species. In four of the five species, some proportion of the individuals tested actively captured far more prey than they were able to consume. The results indicate that superfluous killing is likely to occur when flushes in prey numbers are encountered. Also tested was one possible adaptive explanation for superfluous killing: that spiders can more easily extract nutrients from many partially consumed prey items than from one fully consumed item. This explanation was found to be plausible in only one of the species tested. Table of Contents Section Page Introduction 1 Density-Dependent Predation 2 Superfluous Killing 5 Possible Explanations for Superfluous Killing 7 Partial Consumption of Prey 9 Goals of the Present Study 1 0 Methods 1 1 Study Organisms 1 1 Natural Observations of Prey Encounter 1 1 Supplemented Field Observations 1 1 Laboratory Supplements 12 Full-time Feeds on Single Prey 1 3 Half-time Feeds on Single Prey 13 Results 14 Natural Observations 14 Superfluous Killing 14 Supplemented Field Observations 16 Laboratory Supplements 16 Single Prey Tests for Feed Rate Over Time 21 Discussion 21 Natural Observations 21 Supplemented Field Observations 22 Laboratory Supplements 23 Single Prey Tests for Feed Rate Over Time 24 Literature Cited 26 List of Tables Table Page 1. F statistics results for hypotheses tested. 17 List of Figures Figure Page 1. Holling's (1959) functional response curves 4 2. Mean number of prey items encountered 1 5 during natural observations. 3. Ratio of mean mass captured to mean mass 1 8 consumed by each species in lab trials. 4. Percentage of observations in which prey 1 9 mass captured exceeds that eaten by spiders fed to satiation. 5. Ratio of average feeding rate during partial 20 consumption to that during full consumption of single prey. INTRODUCTION The importance of spiders as biological control agents has been long studied and pondered. A classic study conducted by Clarke and Grant (1968) revealed that spiders can play an important predatory role in natural systems. In this study, all spiders were removed from an enclosed area of a maple forest litter community. Clarke and Grant observed a considerable increase in the centipede and collembola densities in the removal plot compared with plots from which no spiders were removed. Because spiders are known to be predators of centipedes and collembola, the authors concluded that predation by spiders was an impediment to growth in centipede and collembola populations. The lack of replication of this experiment is unfortunate, as the results seem to clearly indicate spiders as important predators of centipedes and collembola. N yffeler and Benz (1987) reviewed studies conducted on spiders from all areas of the world and several different habitats in order to deduce the role of spiders in natural control of insect populations. They estimated that spiders can reach densities of up to 1000 individuals per square meter. They also cited Turnbull's (1973) calculation that the mean density of spiders in a sampling of various environments was 130.8 individuals per square meter (in Nyffeler and Benz 1987). When coupled with the fact that spiders are for the most part generalist predators that feed primarily on insects, these overwhelming estimates indicate that spiders are a valuable force in insect control. Nyffeler and Benz (1987), however, concluded that "the significance of [spiders] as natural control agents is still largely unknown". 2 Spiders have less of a predatory impact in agricultural systems because in these systems spider numbers and diversity tend to be low (Foelix 1996). While spiders may be important biological control agents of most insect populations, insect pests are more likely to escape the predatory impact of spiders. Insect pests are insects that inhabit agricultural areas and are known to damage crop plants. These insect pests are of great concern because a rapid exponential increase in insect numbers is more likely to occur in an agroecosystem than in stable natural communities (Riechert and Lockley 1984). It is such exponential increases that make the insect pest an economic threat to the agricultural systems it invades. A number of possible explanations exist for a noted scarcity of spiders in crop ecosystems. These include the absence of year-round vegetation in agricultural lots and the use of pesticides in these areas (Riechert and Lockley 1984). However, Riechert and Lockley (1984) concluded that, taken as an assemblage of various species, a community of spiders in an agricultural system could adequately control insect pest populations, and that "the application of spiders to the pest control effort should be actively pursued in at least some agroecosystems." Such control would limit the potential of insect pest populations to that seen in natural ecosystems, in which spider assemblages maintain insect numbers at levels low enough to prevent the threat of population explosions. As a basis for these conclusions, Riechert and Lockley point to aspects of the functional and numerical responses of spiders to fluctuating prey populations. Densi ty-Dependent Predation The functional response and the numerical response represent the two basic components of density-dependent predation (Holling 1966). The 3 functional response is the behavioral response (change in consumption rate) exhibited by a predator in response to an increase in either prey density or encounter rate with prey. The numerical response is the change in predator densities, caused by aggregation or reproduction, that corresponds to changes in prey densities (Solomon 1949). Because reproductive output is often a function of levels of food intake and nutrition, the functional response is of pnmary importance and greatly affects the numerical response (Holling 1966). The functional responses demonstrated by predators can be characterized by one of three response curves, as described by Holling (1959). The Type I functional response curve (Fig. 1a) is characterized by a linear increase in prey attack rates until a point of satiation is reached and the line levels off. This curve is demonstrated by a predator whose search pattern is random and whose search rate remains constant with prey density increases (Holling 1959). Filter feeders are an example of Type I predators. The Type II functional response curve (Fig. 1b) is one In which capture rates decrease with increasing prey encounter rates (Riechert and Harp 1987).