The study of interspecific competition has long been one of ecology's most fashionable pursuits. Stimulated in part by a simple theory (Lotka 1932; Volterra 1926; Gause 1934; Hutchinson 1959; MacArthur and Levins 1967), ecologists gathered numerous data on the apparent ways species competitively coexist or exclude one another (reviews in Schoener 1974b, 1983). As is typical in science, most of the early data were observational, and the few experimental studies were mostly performed in the laboratory rather than in the field. Though never lacking its doubters, the belief in the natural importance of interspecific competition is now being severely questioned (review in Schoener 1982). Many of the putatively supportive observations have been challenged as being statistically indistinguishable from random contrivance. Most such attacks have been rebutted, but not without some modification of original conclusions (e.g., papers in Strong et al. 1983). New observations have been gathered for certain systems, suggesting a lack of competitively caused patterns and catalyzing the variable environment view of Wiens (1977) in which competition is seen as a temporally sporadic, often impotent, interaction. Other critics have charged that the lack of experimental field evidence for competition would preclude its acceptance regardless of the quality of observational data. Indeed, results of some of the earlier field experiments are in part responsible for competition's presently beleaguered state. Connell (1975), after reviewing the field experiments known to him through 1973, concluded that predation, rather than competition, appears to be the predominant ecological interaction and should be given "conceptual priority." Shortly afterward, Schroder and Rosenzweig (1975) showed experimentally that two species of desert rodents overlapping substantially in habitat did not appear to affect one another's abundances. This result was interpreted as contradicting a crucial assumption of competition theory, almost its linchpin: the greater the resource overlap between species, the greater the competition coefficient, a measure of the intensity of interspecific competition (relative to intraspecific competition; MacArthur and Levins 1967; review in Roughgarden 1979).

Field experiments on interspecific competition

Interactions between species are usually categorized as either competition (-), predation/parasitism (+ -), mutualism (+ +), commensalism (+ 0), or amensalism (-0). Intraguild predation (IGP) is a combination of the first two, that is, the killing and eating of species that use similar, often limiting, resources and are thus potential competitors. Intraguild predation is distinguished from traditional concepts of competition by the immediate energetic gains for one participant (the predator). It differs from classical predation because the act reduces potential exploitation competition. Thus, its impact on population dynamics is more complex than either competition or predation alone (see below). Our purpose is to document the ubiquity and importance of intraguild predation and to establish a theoretical framework for its analysis. A taxonomically widespread interaction, IGP significantly affects the distribution, abundance, and evolution of many species. One intriguing conclusion is that IGP promotes the occurrence of alternative stable states in many circumstances. Although sometimes recognized by theoreticians and empirically oriented biologists, this is the first synthesis of IGP into a general work.

https://www.jstor.org/stable/info/2097094

THE ECOLOGY AND EVOLUTION OF INTRAGUILD PREDATION: Potential Competitors That Eat Each Other

Null models in ecology

In a dynamic world, mechanisms allowing prediction of future situations can provide a selective advantage. We suggest that memory systems differ in the degree of flexibility they offer for anticipatory behavior and put forward a corresponding taxonomy of prospection. The adaptive advantage of any memory system can only lie in what it contributes for future survival. The most flexible is episodic memory, which we suggest is part of a more general faculty of mental time travel that allows us not only to go back in time, but also to foresee, plan, and shape virtually any specific future event. We review comparative studies and find that, in spite of increased research in the area, there is as yet no convincing evidence for mental time travel in nonhuman animals. We submit that mental time travel is not an encapsulated cognitive system, but instead comprises several subsidiary mechanisms. A theater metaphor serves as an analogy for the kind of mechanisms required for effective mental time travel. We propose that future research should consider these mechanisms in addition to direct evidence of future-directed action. We maintain that the emergence of mental time travel in evolution was a crucial step towards our current success.

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The evolution of foresight: What is mental time travel, and is it unique to humans?

The role of herbivores in controlling plant species richness is a critical issue in the conservation and management of grassland biodiversity. Numerous field experiments in grassland plant communities show that herbivores often, but not always, increase plant diversity. Recent work suggests that the mechanisms of these effects involve alteration of local colonization of species from regional species pools or local extinction of species, and recent syntheses and models suggest that herbivore effects on plant diversity should vary across environmental gradients of soil fertility and precipitation.

/pdf/effects-of-herbivores-on-grassland-plant-diversity-528tzj21w0.pdf

Effects of herbivores on grassland plant diversity.

Some of the most challenging questions of contemporary biology concern the diversity and stability of ecological communities. In most natural habi­ tats many coexisting species of plants, animals, and microbes interact as competitors, mutualists, predators, and prey. These biotic processes interact with the physical environment to determine the structure and function of ecological systems. The numerous theoretical and empirical studies of com­ petition and predation have left much to learn about how these processes contribute to the structure and stability of natural communities. Some of the questions may be answered by empirical investigations of systems com­ plex enough to contain diverse species that interact in different ways yet simple enough to permit detailed comparative analyses and experimental manipulations. The seeds and seed-eaters of desert habitats provide a system of two trophic levels that offers many advantages for community studies. Some investigators have suggested that the struggle to exist in the harsh physical

Granivory in Desert Ecosystems

We limit our discussion of food storage, or caching, to the movement of potential food items from one location to another for eating at some later time. This activity occurs exclusively in those animals that bring food to their offspring, and not in those that bring their offspring to food (i.e. that lay their eggs near food in a favorable microhabitat). Provisioning offspring is limited taxonomically to mammals, most birds, and some Hymenoptera. Moving food to a favorable microhabitat was apparently the first transitional step in the evolution of more complex systems of provisioning offspring in hymenopterans (72). Although all species that cache food provision their offspring, the converse is not true. Relatively few of the animals that provision their young with food also cache food, and caching food has no obvious connection to the glandular secretion of milk, which probably initiated offspring provisioning in the evolutionary history of mammals. Repeated traveling from a foraging area to dependent young seems to precondition animals for caching food. One of the goals of our review will be to determine what other conditions among species of birds and mammals and their food favor the evolution of caching. We limit our discussion to birds and nonhuman mammals because of our own backgrounds and because experimental studies have recently been done on these vertebrates (4, 14, 20, 48a, 62, 10la, 103-105, 116, 117, 128), although earlier investigations started with wasps (121). A corollary of provisioning offspring with food is a stable, relatively high metabolic rate (even in the Hymenoptera), which ensures that parents can maintain a steady flow of food to the relatively few, "expensive" offspring

The Evolution of Food Caching by Birds and Mammals

This study was designed to determine the diets of four species of Sonoran Desert rodents ( Dipodomys merriami, Perognathus ampins, Perognathus baileyi , and Perognathus intermedins ) and compare the diets to available food resources. Items ingested and their relative frequencies were determined by microscopic examination of stomach contents. Contents of cheek pouches of these heteromyid rodents also were analyzed and the relative density of each item was calculated. Seeds, especially those of forbs were the primary food of all species, but insects were important in diets of D. merriami and P. intermedins . Green vegetation was eaten infrequently. Soil samples taken at the capture sites of the rodents were analyzed for seeds to determine their relative density. It was found that availability of resources generally determined the pattern of food utilization, but that the preferences of the various species determined the exact quantities used. Cheek pouch contents were found to be consistently different from stomach contents and poor indicators of how much and when specific items were ingested.

Relation of Desert Rodent Diets to Available Resources

Reciprocal increases in rodent and ant densities on 0.1 ha plots from which the other taxon had been excluded demonstrate that these distantly related desert granivores compete for seeds. Relative to unmanipulated control plots, numbers of ant colonies increased 71% on plots where rodents were excluded; rodents increased 20% in numbers of individuals and 29% in biomass in the absence of ants. Comparisons of seed levels in the soil and of annual plant densities on experimental and control plots provide evidence that the rodent and ant populations are limited by and compete for food. Greater numbers of seeds and annuals occurred on plots where rodents and ants had been excluded than on plots where both taxa were present. Particular species of annuals were reduced in density by foraging of rodents. Ants increased species diversity by differentially harvesting seeds of the most common species. Results of these and other recent studies suggest that competition among distantly related organisms plays a major role in the organization of ecological communities.

An Experimental Study of Competition Between Seed-eating Desert Rodents and Ants

Fossorial herbivores are common in Eurasia, North and South America, and Africa, particularly in arid and semiarid regions. There is ample evidence, although often indirect and semiquantitative, that they have local and broad-scale effects on vegetation. These effects arise from feeding, harvesting, and caching, local soil disturbances, and longer-term changes in soil structure and topography, many aspects of which are similar among the diversity of fossorial herbivores. Often, the net effect of these animals' activities is alteration of composition of plant species and increased plant diversity. Standing crop of plants often is reduced, and long- and short-term changes in productivity can occur. Better understanding of the rules that govern diet selection and burrow size, dynamics, and geometry are needed to predict effects of these animals under specific environmental conditions.

O. J. Reichman

Papers

Granivory in Desert Ecosystems

The Evolution of Food Caching by Birds and Mammals

Relation of Desert Rodent Diets to Available Resources

An Experimental Study of Competition Between Seed-eating Desert Rodents and Ants

Effects of Subterranean Mammalian Herbivores on Vegetation