Age Structure Component of Niche Width and Intraspecific Resource Partitioning: Can Age Groups Function as Ecological Species?

Many species pass through several distinct age classes (instars or year classes) as they grow. The variety of resources used by age classes effectively expands niche width. This "age-specific component" of niche width is analyzed for a desert scorpion, Paruroctonus mesaensis, whose populations exhibit both instars and discrete year classes. Another purpose of this paper is to evaluate if instars or year classes function as "ecological species" by showing the same differences in morphology and resource use as exists between biological species. The similarity of the growth ratio (Dyar's constant) to Hutchinson's Santa Rosalia ratio has promoted the speculation that the size divergence between age groups has the same ecological basis as that between competing species. Empirical differences between species for a variety of resource-related parameters were determined from a survey of the literature. These interspecific differences are compared to differences between age classes of the same population. Intraspecific differences in body size and resource use were analyzed statistically, and then quantified by using the following similarity indices: the Santa Rosalia ratio for predator length, overlap in prey taxa and prey size, and total overlap (including prey taxa, prey size, and phenology). Successive instars exhibited growth ratios greater than the Santa Rosalia ratio and were significantly different in the size of prey eaten. Differences in actual prey eaten, however, were less than the differences that exist between most species. The three year-classes are fairly distinct in patterns of prey size and use: differences are well within the range of interspecific differences. Young and adults are less similar in all parameters of size and resource use than are the majority of species pairs. The differences between young and intermediate year-classes are typical of the differences observed between many species. Although intermediates and adults show moderately high overlap in resource use, they still differ more than 16%-80% of all species for the various resource parameters. During certain periods of the year, intermediate-aged scorpions experience moderate to high overlap with large populations of both young and adult year-classes. By foraging primarily during colder periods of the year when the other two age classes are relatively inactive, intermediate animals tend to minimize co-occurrence on the surface and use different resources. The evolution and significance of the "age structure component" of niche width is discussed. Under conditions of reduced interspecific competition, there is selective pressure for age groups to diverge in resource use. This appears to be the case with P. mesaensis because there is little competition by other scorpion species, in part, as a result of frequent intraguild predation by adult P. mesaensis on smaller scorpion species. The age structure component of niche width may have significant interspecific influence on both competition and guild structure. Species that develop through a wide size range during their lifetime will use a broader range of resources and affect other species differently than those species that have an effectively more narrow size range during their lifetime of activity.