Locomotor activity and zonation of upper shore arthropods in a sandy beach of north central Chile

The tenebrionid beetle Phalerisida maculata Kulzer, the talitrid amphipod Orchestoidea tuberculata Nicolet and the oniscid isopod Tylos spinulosus Dana are semi-terrestrial burrowing species, which coexist on sandy beaches of north central Chile (28–30 S). During the night, these scavengers emerge to make downshore migrations. Given the similarity in niches of these three species (all are known to include macroalgal detritus in their diet) and their relatively high abundance on that beaches, there is the potential for some degree of interaction, both inter- and intraspecific. Field studies were carried out to examine zonation of these burrowing organisms and eventual time and/or space partitioning of locomotor activity during night hours. Locomotor activity on the beach surface was analyzed over 12 h periods during spring and neap tides of September and December 2000, and March 2001. Scavengers moving over the beach surface were captured using pitfall traps buried with their rims flush with the beach surface along a transect extended from the foot of the dunes to the highest levels reached by the swashes. Every 1 h the captured animals in the traps were collected. Locomotor activity was also studied in the laboratory with chambers equipped with infrared recording systems (actographs). Data downloaded from the actographs were graphed to obtain a display of locomotor activity per 15 min interval during the course of the 7 day experiments. Results show space partitioning of burrowed organisms and time partitioning in the locomotor activity of O. tuberculata, T. spinulosus and P. maculata over the beach surface. Circular statistics showed that usually the activity peaks of O. tuberculata were more different from those of P. maculata and T. spinulosus than those of the last two species when compared with each other. Intraspecific differences were also found in the surface locomotor activity, primarily between juveniles and adults of O. tuberculata. Interseasonal comparisons of capture figures show that the highest locomotor activity occurred during early summer (December 2000). Moon phases apparently affect locomotor activity (i.e. T. spinulosus and P. maculata had higher locomotor activity during neap tides as compared with that observed during spring tide samplings carried out with full moon). Periodograms resulting from the locomotor activity of adults of O. tuberculata, T. spinulosus and P. maculata studied with actographs and total darkness show evidence of a circadian endogenous component close to 23–25 h. Activity peaks close to 11–14 h were also found that probably represents a circatidal component in the locomotor activity. Results of actograph experiments under constant light show that the circadian rhythm of locomotor activity of O. tuberculata was the only one maintained throughout the experiment and phased with the subjective night. Analyses of contour distributional maps and mean hourly zonations show that the locomotor activity of the studied species also differed, specially that of O. tuberculata versus that of T. spinulosus and P. maculata. Results of coexistence experiments showed no evidence of intraspecific interactions. Similar experiments evidentiated interspecific interactions: those species with similarities in locomotor activity (that is T. spinulosus and P. maculata) showed no interactions between them, while both of them had negative interactions with O. tuberculata, the species which separated more in time and hourly zonation of locomotor activity. Thus, differences in time/space partitioning of surface locomotor activity can be interpreted as a means to avoid detrimental interactions in this guild of scavengers. That partitioning would allow coexistence of interacting scavenger species and provides evidence that biological interactions are indeed important in community structure of sandy beach macroinfauna.