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La dinmica de la captura de presas de la colonia de araas Metabus gravidus (Araneidae) en Monteverde, Costa Rica
The dynamics of prey capture of the colonial, orb-weaving spider Metabus gravidus (Araneidae) in Monteverde, Costa Rica
Prey capture dynamics of the colonial, orb-weaving spider Metabus gravidus (Araneidae)
are based on individuals. It is possible that prey caught in certain locations of the web
will stimulate more reactions, and that different types of prey will stimulate variable
reactions. Since the members of the colony live close to one another, it is also possible
that spiders exhibit competition over prey. Different insects were dropped into various
locations of individual orbs in order to observe reactions. The spiders did not seem to
prefer whether prey was dropped closer or further from the center of the web (Chisquared
goodness of fit, l2 = 0.684, P = 0.710, DF = 2); number of captures did not
depend on location either (Chi-squared goodness of fit, l2 = 0.651, P= 0.651, DF = 2).
Grasshoppers generated medium-speed reactions, but overall the spiders preferred moths
to grasshoppers or ants (chi-squared goodness of fit, l2 =14.88, P = 0.006, DF = 2). It
seems that variation in prey stimulates different behaviors in M. gravidus.
Las dinmicas de la captura de presas de la colonia de araas Metabus gravidus (Araneidae) se basa en los individuos. Es posible que las presas capturadas en determinados lugares de la telaraa provocaran ms reacciones. Ya que los miembros de la colonia viven cerca unos de otros, tambin es posible que las araas exhiban ms competencia sobre la presa.
Text in English.
Orb weavers--Costa Rica--Puntarenas--Monteverde Zone
Tejedores de orbe--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2008
Prey capture dynamics
Ecologa Tropical 2008
Dinmicas de captura de presas
t Monteverde Institute : Tropical Ecology
The Dynamics of Prey Capture of the Colonial, Orb Weaving Spider Metabus gravidus Araneidae in Monteverde, Costa Rica. Signe Spencer Department of Biology, Portland State University ABSTRACT Prey capture dynamics of the colonial, orb weaving spider Metabus gravidus Araneidae are based on individuals. It is possible that prey caught in certain locations of the web will stimulate more reactions, and that different types of prey will stimulate variable reactions. Since the members of the colony liv e close to one another, it is also possible that spiders exhibit competition over prey. Different insects were dropped into various locations of individual orbs in order to observe reactions. The spiders did not seem to prefer whether prey was dropped clo ser or further from the center of the web Chi squared goodness of fit, Ã°l 2 = 0.684, P = 0.710, DF = 2; number of captures did not depend on location either Chi squared goodness of fit, Ã°l 2 = 0.651, P= 0.651, DF = 2. Grasshoppers generated medium speed reactions, but overall the spiders preferred moths to grasshoppers or ants chi squared goodness of fit, Ã°l 2 =14.88, P = 0.006, DF = 2. It seems that variation in prey stimulates different behaviors in M. gravidus. RESUMEN Las dinÃ¡micas de captura d e presas en Metabus gravdius Araneidae depende de individuos, a unque vivan en colonias. Es pos ible que presas en diferentes lugares de la red provoquen reacciones diferentes . Debido a que los miembros viven en colonias, es posible que haya competencia p or las presas. Se colocaron insectos en diferentes lugares en las telaraÃ±as y se observaron sus reacciones. Las araÃ±as no tienen preferencia por la distancia de la presa al centro de la red Chi squared goodness of fit, Ã°l 2 = 0.684, P = 0.710, DF = 2; Las capturas no dependen de lugares Chi squared goodness of fit, Ã°l 2 = 0.651, P= 0.651, DF = 2. Los saltamontes ocasionaron reacciones de velocidad intermedia, pero las araÃ±as prefieren mÃ¡s las mariposas que los saltamontes o las hormigas chi squared goodn ess of fit, Ã°l 2 =14.88, P = 0.006, DF = 2. Parece que la variaciÃ³n en el tipo de presa estimula diferentes en comportamientos en M. Gravidus.
INTRODUCTION The colonial, orb weaving spider Metabus gravidus Araneidae can be found in riparian environ ments in the Central American tropics. These spiders suspend their orbs over flowing river water forming a community network of webs. Despite their intricate web system, they are not truly social as the individuals are merely tolerant of one another Downe s 1995. Metabus gravidus communities are usually found near gentle slopes or ledges and over a steady current. These diurnal spiders feed daily on a variety of insects including moths, flies, dragonflies and bees Buskirk 1975b. The communal web buildi ng of M. gravidus allows them to access a unique niche by bridging rivers. This niche is subject to a vast amount of insects that travel near the water, and though the spidersÂ€ webs are connected, prey capture is dependent upon the individual orb of each spider Uetz 1986. Metabus gravidus usually sit in the center of the web while awaiting prey and it is reasonable to predict that insects landing in different parts of the web would stimulate different reactions. Previous studies have observed prey captu re dynamics in regards to the dynamics of the colony as a whole Buskirk 1975b, yet little has been documented about reactions to prey on an individual level. I hypothesized that spiders would respond differently depending on where is prey is located wit hin the web and what type of prey is caught in the web. I predicted that the spiders would respond quickest to prey dropped close to the center of the web and would prefer moths to grasshoppers or ants. Since M. gravidus live in colonies, I also expected to see some cases of competition over prey. MATERIALS AND METHODS The study site was along La Quebrada MÃ¡quina in the Monteverde Cloud forest of Costa Rica. The surrounding environment was dense forest with occasional light gaps and the river was narro w with a moderate flow. Ten different M. gravidus colonies were found along this river and labeled accordingly. The number of individuals per colony varied between one and twelve. The spiders were always found around congregations of logs and/or rocks t hat they used for anchoring their webs. Data collection with observations consisted of a three day period, beginning each day at 9:00 and ending at 13:00. Before experimentation could begin, three different types of prey were collected and held in their respective jars: ants, grasshoppers and moths. These insects would serve as prey variables. The ants had to be large enough to stick in the webs, while the grasshoppers and moths had to be small enough to avoid destroying the webs. Size of prey varied d epending on the size of the spiders small, medium or large, as the goal was to facilitate capture. Experimentation began immediately upon encountering a new spider colony. Each colony was first labeled, and then dusted with baby powder for visibilit y purposes so that individuals could be identified and counted. Then, an individual was randomly chosen from within the colony to have an ant dropped into the web. Location of drop was relative to the center of the web, where the spider waits for prey, an d included: close to center, middle from center, or far from center. A second random individual was then chosen to have a grasshopper dropped close, medium and far from the center of the web.
A third, and final, random individual was then chosen and the s ame process was used for the moth. Each type of prey, ant, grasshopper, and moth were dropped at each location for a total of three drops/insect/colony. A new prey was dropped into the webs every 15 25 minutes to allow a break between captures. Prey ty pe, distance of prey form center of web, and reaction of spider was recorded for each drop. Reactions included: no reaction, slow speed reaction, medium speed reaction, fast speed reaction and retreating reaction speeds based on relativity. Once these data were collected the materials were gathered and I continued to move down the river to the next colony; the same procedure was carried out for each. Upon returning to the station, data were analyzed using chi squared contingency tables and goodness of fit tests statistics. The statistics compared location of prey drop and spider reaction, location of prey drop and successful captures, and type of prey and spider reaction. Type of prey and successful captures were also compared. RESULTS There was no trend in number of reactions compared to prey location within the web Chi squared goodness of fit, Ã°l 2 = 0.684, P = 0.710, DF = 2. There was also no trend in number of successful captures compared to prey location within the web Chi squared goodness of fit, Ã°l 2 = 0.651, P = 0.651, DF = 2. Only one significant trend was found for prey type compared to spider reaction Table 1. The only behavior that differs between types of prey was medium speed reaction Table 1. The spiders usually reacted at medium speed when a grasshopper was dropped into the web Figure 1. The final analysis compared prey type versus successful captures. Spiders captured more moths than any other insect Figure 2. DISCUSSION Metabus gravidus show no preference for prey dropped in different parts of the web. The lack of preference for prey location could possibly be attributed to the quality of the webs and the spidersÂ€ sensitivity to vibrations throughout the entire web Foelix 1996. The data show that the spiders recognized the vibrations both close and far from the center of the web. Another observation, which also ran counter to my original prediction, is that prey dropped close to the center often generated a retreating reaction instead of a quick capture. S piders tended to act at medium speed when grasshoppers were dropped into the web. Perhaps because it is typical of spiders to spend a few moments tugging on the web to locate prey before they run out to catch it, a medium speed reaction would then indicate that the spider had ÂthoughtÂ€ about the prey before capture Foelix, 1996. After detecting vibrations, spiders must have decided that grasshoppers were a ÂgoodÂ€ prey. Concerning prey preference, the original prediction was supported as M. gravidus pre ferred moths to grasshoppers or ants Figure 2. Since M. gravidus build their webs over water, it was expected that prey with wings would be common prey within the webs and easily recognized by the spiders. Though the spiders often reacted quickly to ants they were rarely captured because the ants would readily defend themselves upon encounter. Also, the grasshoppers did not stick very easily in the webs and were often
able to break free. Though moths were captured more frequently than the other insects, M. gravidus usually attempted to capture whatever landed within the web, and location of drop did not seem to matter. Although subsequent studies of M. gravidus have discovered cases of fighting and web take over amongst members of a colony, there has b een no documentation of one spider stealing anotherÂ€s prey Buskirk 1975a. Throughout my observations, I did not see evidence of fighting, web take over, or stealing of prey. Further studies may want to observe the reactions of M. gravidus individual s to different sizes of prey or vary the type of prey used. Also, data may have yielded better results if more individuals per colony were tested. However, since prey capture is the reason that spiders build their elaborate webs, studies of prey capture dynamics are essential to understanding how they fit into the global ecosystem Riechert 1984. ACKNOWLEDGEMENTS Thanks to Karen Masters and the CIEE program for allowing me the opportunity to conduct this research and to the Biological Station of Monte verde for providing an area of private study within the cloud forest. Much appreciation goes to Tania Chavarria for assisting me with the development and statistical analysis of this project, and to Pablo Allen and Moncho CalderÃ³n for their helpful advice . Finally, thanks to all the students of this program for their support and enthusiasm! LITERATURE CITED Buskirk, R. E. 1975a. Aggressive display and orb defense in a colonial spider, Metabus gravidus . Animal Behavior 23: 560 567. Buskirk, R. E. 1975 b. Coloniality, activity patterns and feeding in a tropical orb weaving spider. Ecology 56: 1314 1328. DÃaz Fleischer, F. 2005. Predatory behaviour and prey capture decisionmaking by the web weaving spider Micrathena sagittata . Canadian Journal of Zoolog y 83: no2 268 73 F Downes, M.F. 1995. Australasian social spiders: what is meant by Â‚socialÂƒ?. Rec. West. Austral. Mus. Suppl.: 25 pages? Richert, S.E and T. Lockley. 1984. Spiders as biological control agents. Annual Review of Entomology 29: 299 320 Uetz, G. W. 1986. Web building and prey capture in communal spiders. Shear W.A Ed. Spiders: Webs, behavior, and evolution. Stanford University Press. Stanford, CA.: 226 231.
Table 1: Chi distribution and Probability Values for differ ent reactions in Metabus gravidus spiders. These reactions were compared against three different types of prey, but a medium speed reaction was the only on with a value that reveals a significant trend between prey type and reaction. Reaction Type Ã°l 2 P Value Overall 0.357 0.836 No reaction 0.230 0.883 Slow speed reaction 2.20 0.247 Medium speed reaction 5.20 0.074 Fast speed reaction 4.12 0.127 Scared 1.60 0.449 Figure 1. Chi distribution statistics reveal a trend bet ween prey type and spider Metabus gravidus reaction. Spiders tend to move at a medium speed toward grasshoppers more frequently than they will towards ants or moths Chi squared goodness of fit, Ã°l 2 = 5.20, P = 0.074, DF = 2. 0 2 4 6 8 10 12 Ant Grasshopper Moth Medium Reaction Prey Type
Figure 2. Chi distribution statistics show trend between prey type and capture frequency for Metabus gravidus spiders chi squared goodness of fit, Ã°l 2 = 14.88, P = 0.006, DF = 2. The spiders preferred moths 18 captures to grasshoppers 7 capture s or ants 2 captures. They preferred ants the least out of the three prey types. 0 2 4 6 8 10 12 14 16 18 20 Ant Grasshopper Moth Prey Capture Frequency Type of Prey