Temporal Patterns Of Resource Limitation In Natural Populations Of Rotifers

Ecology, Jan, 2000 by Jodie L. Merriman, Kevin L. Kirk

KEVIN L. KIRK [1]

Department of Biology, New Mexico Tech, Socorro, New Mexico 87801 USA

Abstract. The availability of resources may limit consumer populations, and temporal variation in resources may facilitate the coexistence of competitors. Accurate estimates of resource availability often cannot be obtained by counting potential food items, because of difficulties in determining resource quality. Instead, resource supplementation experiments must be conducted, but few experimental studies have examined the temporal patterns of resource limitation. To determine the occurrence, intensity, and temporal variation of resource limitation in natural populations of planktonic rotifers, experiments were conducted using two species of herbivorous rotifers from a eutrophic pond, Keratella cochlearis and Synchaeta sp. The difference in population growth rates in food-supplemented treatments relative to controls ([delta]r) was used as a measure of the intensity of food limitation. Rotifers were usually food limited, with Keratella and Synchaeta showing significant food limitation ([delta]r[greater than] 0) in 63% and 87% of the experiments, respectively. There was temporal variability in [delta]r on time scales of one to a few weeks, indicating rapid changes in resource availability. Synchaeta population dynamics were more closely controlled by resources than was the case for Keratella, and the intensity of food limitation was higher for Synchaeta than for Keratella, possibly due to the more specialized feeding habits of Synchaeta.

Key words: cladocerans; competition; effect size; egg ratio; food limitation, resource limitation; rotifers; temporal variation; zooplankton.

INTRODUCTION

The concepts of resource limitation and resource competition are central to much of population and community ecology (Lack 1954, Hairston et al. 1960, Oksanen et al. 1981, Tilman 1982, Grover 1997). Resource limitation is a necessary condition for the existence of resource competition, but resource limitation cannot be assumed to be universal and constant. Indeed, some have suggested that populations may be only rarely limited by resources (Wiens 1977). A population is resource limited if an increase in resource availability causes an increase in population growth rate, so resource supplementation experiments are a logical choice as a method to determine the presence and intensity of resource limitation.

Resource supplementation experiments have been conducted using many types of organisms living in many habitats, including terrestrial birds, reptiles and mammals (Newton 1980, Martin 1987, Boutin 1990), spiders (Wise 1993), and aquatic invertebrates (e.g., Tessier and Goulden 1982, Osenberg 1989, Williamson et al. 1996). Many of these experiments have found that reproduction, survival, or population growth are often, but not always, resource limited.

In order to understand the nature of the interaction between consumers and their resources, we need accurate estimates of resource availability. Simply measuring the abundance of potential resources in the field may often give inaccurate estimates of resource availability, because not all potential resources are edible or of high nutritional quality (Wiens 1984). In contrast, knowledge of the intensity of resource limitation can provide a biologically meaningful index of resource availability. If the addition of supplemental resources results in a large increase in consumer population growth rate, then the availability of natural resources is low. If the consumer shows no response to supplemental resources, then resource availability is high and likely growth saturating. Thus, experimental studies of resource limitation are a way of estimating resource availability by examining the responses of the consumers themselves. Simple resource supplementation experiments do not, however, allow the effects of resourc e abundance and quality to be separated. Thus, in this context, resource availability is a function of resource abundance and all aspects of resource quality, including edibility, digestibility, and nutritional content.

The interaction between freshwater herbivorous zooplankton and phytoplankton provides an interesting example of the difficulties of understanding consumer-resource interactions. Attempts to quantify the availability of phytoplankton resources by measuring total chlorophyll concentration or total phytoplankton biomass may give inaccurate estimates of resource availability because phytoplankton taxa differ greatly in their edibility, digestibility, and nutritional quality, and because these factors affect different zooplankton consumers in different ways. For example, some cyanobacteria are inedible or toxic to some zooplankton species (e.g., Kirk and Gilbert 1992), while some flagellates such as cryptomonads are edible and nutritious for many zooplankton (Stewart and Wetzel 1986). Even when phytoplankton taxa known to be inedible, toxic, or of low nutritional quality are excluded from consideration, an accurate measure of resource availability may still not be obtained due to intraspecific variation in nutrit ional quality. Recent studies have shown that the elemental and biochemical composition of phytoplankton can vary depending on the degree and type of nutrient limitation of the phytoplankton, and these differences in resource quality can have large effects on zooplankton consumers (Scott 1980, Giani 1991, Mitchell et al. 1992, Sterner et al. 1993, Muller-Navarra 1995).