Aspects of foliage reveal the fact that stress, particularly drought, is a powerful selective force on plants anchored in tree crowns. Whereas phorophytes are usually characterized by mesomorphic leaves and C3 photosynthesis, their epiphytes tend to xeromorphism and unusual mechanisms for procuring, as well as greater capacity for storing, water. Therefore, if accommodations to aridity can be identified, so will many of the epiphytes' most distinguishing features. In this chapter, that challenge is met by first laying some groundwork and then examining the nature of water balance mechanisms and their influence on overall epiphyte biology.

Water use and conservation: defense against drought

All land plants must expend water in order to create biomass. As stomata open for CO2 influx, water vapor exits at a much higher rate. Xerophytic forms manage this unavoidable trade-off with suprising success: Their transpiration ratios (TRs) are in the neighborhood of 100:1, and exceptional performers do considerably better (Table 3.1). But water economy always has its price; productivity is slowed as leaf conductance falls – the lower the TR, the slower it is. In contrast, species native to humid habitats or those arid-land dwellers whose active phase coincides with the rainy season expend as much as 1000 g of water for each gram of dry matter they create. These drought-sensitive taxa serve notice that parsimonious water use is not always the best mechanism and can be decidedly disadvantageous when moisture is plentiful.

Introduction

The numbers of ecological field experiments have grown at an increasing rate over the past two decades, and because the theme of this book is that experimentation is necessary for progress in ecology, it is fair to ask what the experiments have revealed about ecological processes. Obviously, observations of field situations and spontaneous occurrences have provided insight throughout the history of ecology, but the present discussion is restricted to intentional manipulations and their contributions to understanding ecological processes.

The most important conclusion to be drawn from the whole group of experiments is that each kind of environment should be considered separately, because there are few, if any, specific statements about ecological processes that will be true across all environments. As stated in the Preface, that is the reason for arranging the experiments by kind of environment in which they were conducted, instead of by kind of interaction, for example. Thus, simultaneous consideration of all experiments on competition, or predation, would not permit as clear statements about ecological processes as does the arrangement adopted.

Forests

The ecological phenomena that have been revealed or confirmed for forests can be assembled into a more coherent analysis than can the results in any other kind of habitat. The experiments in each trophic level have given internally consistent results, and the relationships between trophic levels are consistent with those within each.

Introduction

Deserts and semideserts are characterized by a striking difference from nearly all other terrestrial environments. Except for brief periods following rare rains, most of the ground is bare, shrubs and some perennial grasses being the only obvious vegetation. This has important implications not only within the producer trophic level but also for the interactions between the producers and primary consumers. Annual plants, which form a nearly negligible fraction of the vegetation in undisturbed parts of better-watered regions, assume a major importance in those deserts that have been investigated ecologically. The reason for the prominence of annuals lies in the resistance of their seeds to the harsh conditions that prevail in deserts most of the time. Thus, the reservoir of seeds left from past brief periods of favorable weather is vital to the maintenance of that ecosystem, and the magnitude of the reservior determines the abundance of the seed-eating animals that are so characteristic of many deserts.

The shrubs and perennials in such areas have deep roots, which spread far beyond the spans of their canopies, and frequently the roots of neighboring shrubs actually touch, even though there is much bare ground between the branches. The distribution usually is attributed to the potential for obtaining as much water as is possible, and competition for water is commonly claimed for arid-land plants as reviewed by Fowler (1986). Thus, at shallow depths of soil, there is space available for occupancy by the shallow-rooted annuals at any time when conditions permit the germination of their seeds.

Introduction

Unlike experiments undertaken in terrestrial situations, or in lakes or ponds, the logical orientation in marine locations deals less with trophic position than with exposure to wave action and the nature of the substrate. Herbivory and predation do indeed play important roles in some marine locations, but producers, filter feeders, and sessile predators have been found to compete for space in various combinations, making the classical trophic level organization less basic to understanding the ecology of marine shores, where nearly all of these experiments have taken place.

The nature of the substrate has provided the most convenient basis for dividing experiments into groups, rocky shores and areas with soft sediments (either sand or mud) being the two principal types. All experiments on soft bottoms have taken place in protected bays or marshes, because it is not feasible to conduct experiments on wave-swept beaches.

Rocky shores

The favorite environment in which experiments have been conducted has been the rocky coast. For readers not familiar with rocky shores, such as those living south of New York on the Atlantic Coast of the United States, there are many books describing the habitats, such as Rickets and Calvin (1968) and the beautiful volume by Wertheim (1985). Here, there are vertical levels that may determine the possible array of species. At the highest level, the rock may become wet only because of breaking waves – not actually covered by the tide at all.

Introduction

Much of the area that was once forested, especially in the North Temperate Zone, is now either under cultivation or undergoing some stage of secondary succession. Such areas are more convenient for experimentation than forests themselves, because the vegetation is low enough to be accessible. A perusal of the table of information amassed by Schoener (1983) on field experiments reveals the large number of experiments that have been conducted in fields, meadows, or pastures – sixty-six species or groups of plants involved, compared with thirty-eight in forests – and it should be recognized that the experiments in pine forests reported by Korstian and Coile (1938) (Chapter 4) were in a late stage of succession.

The distinction between mature and successional stages is not trivial, as a comparison of the results reported by Schoener (1983) reveals: Of the 38 species in forests, 33 showed competition “always,” 2 “sometimes,” and only 3 “never.” In contrast, of the 66 species in fields, meadows, or pastures, less than half (29) showed competition “always,” 25 “sometimes,” and 12 “never.” The difference is highly significant statistically (p < .001). Despite the lower frequency of interspecific competition among plants in successional locations, the descriptions of experiments follow the same order as in Chapter 4, because that procedure will permit comparisons of the interactions causing the differences between forest communities and successional communities. Excellent experiments have been conducted in deserts and other arid areas; they are deferred to Chapter 6, because many of the interspecific relationships are unique to such habitats.

Introduction

It is conventional to classify freshwater habitats as lakes, ponds, and streams. That will serve well here, because many of the specific problems being attacked are quite different, and many of the techniques used in conducting experiments in these three kinds of habitats are different. The separate conditions have resulted in experimental designs specific to the habitat, even when the problems have been much the same in principle.

In the case of bodies of water large enough to be considered lakes, most ecological thinking has concentrated on the plankton. This is the general term designating drifting algae and small animals whose vertical movements may be important, but whose horizontal movements are insignificant from the standpoint of their ecology. Although fish predation on zooplankton is important in many lakes, the ecology of the plankton frequently has been studied independent of the fish and other large animals in the lake, and a number of experiments were carried out on the assumption that the important external influence was the supply of nutrients to the system. The edges and bottoms of lakes have entered limnological thought largely through the bacterial mineralization of nutrients that arrive at the bottom in the form of dead organisms. These nutrients are recycled when the lake becomes isothermal and the water can be mixed by the wind. Mixing is largely prevented in summer because the upper level of water becomes warmer and lighter than the deeper water and tends to be circulated by the wind as the mostly independent epilimnion.

The presidential address of G. C. Varley to the British Ecological Society in 1957 was entitled “Ecology as an Experimental Science.” Though delivered in January, it was published in November and thus was not known to me in June, when I made a plea for ecological field experiments at the Cold Spring Harbor Symposium on Quantitative Biology. It is an irony of history that Hutchinson's “Concluding Remarks” at the symposium set off an extremely popular movement in ecology that was almost the antithesis of experimentation. Despite the widely acclaimed experiments of Connell and Paine, the method attracted few practitioners for at least fifteen years, while the ecological world stood bemused by mathematical theory as a way of explaining observations made in the field. The wave of enthusiasm was most pronounced in the United States, but it affected, and still affects, ecologists in all parts of the world, as a perusal of the journals will show.

Some of the observations did not confirm the theories, but were nevertheless claimed to do so (Roth 1981; Simberloff & Boecklen 1981). During that period, few authors considered alternative explanations for their observations, and in addition there was the danger that the confirmatory evidence was known in advance of the construction of theory. Such a sequence involves circularity, with its obvious flaws as a method of scientific procedure. Explaining data, whether verbally or through mathematics, is a legitimate method of stating hypotheses, but they remain just that until put to an a priori test.

Introduction

Humans are terrestrial. Our experience is almost exclusively with that environment, and we should therefore have an intuitive understanding of terrestrial situations that is superior to our grasp of aquatic situations, whether freshwater or marine. For that reason, I have begun this description and criticism of ecological experiments with those performed on land. One of the themes of this book is that many generalizations in ecology are unlikely to hold across environments that differ in major ways. A consideration of the results of observations and experiments suggests that on land, interactions among the elements of the biota will be distinctive for at least the following categories: forest, prairie, desert, and tundra. Further subdivisions may be necessary.

A conventional and logical arrangement of the experiments to be described is to consider them in the context of the trophic level on which they were conducted, starting with the decomposers. It is recognized that it is not always possible to make the assignment cleanly. For example, in taking prey, most predators do not discriminate between herbivores and smaller predators, but for a considerable majority of the experiments, the assignments are reasonable. No attempt is made to describe all of the experiments on any trophic level. Those experiments selected either illustrate the interactions of greatest interest, or are examples of problems that face experimental ecologists, or represent good or bad experimental design.

Introduction

Ecological experiments come in many forms, and at all levels of sophistication. At the simplest, they involve an ecologist thinking that something interesting is happening, and deciding to change the system to find out more about possible causes. If the change is followed by a dramatic “result,” the curious ecologist may decide that the case is proved, and write the experiment up for publication. The change in the system would be classified as an experiment, but there would be problems in accepting the conclusion, unless certain precautions had been observed. There is the question whether or not a rare or even unique event has been observed. Can the result be repeated? Very few ecological field experiments have been repeated, but the use of a satisfactory experimental design can remove at least some of the uncertainty. Our ecologist is faced with a choice, either to carry out a simple manipulation to satisfy a perhaps uncritical curiosity or to invest more work and conduct an experiment that will convince the array of scientific colleagues.

It might seem superfluous to describe the requirements of experimental design, but they have been violated regularly enough in published works that all should be warned of errors. It is possible to do anything badly, even something as desirable as an experimental approach to ecology. One of the features that distinguish science from other approaches to understanding the world is the set of rules by which scientists operate.

Introduction

Much of ecology consists in making observations and then devising plausible explanations for the observations. Because alternative explanations of the observed phenomena frequently are available, the process by which the conclusions have been reached is known as “weak inference.” It is not that the conclusions are necessarily wrong; the problem is that there is little assurance that they are right, and the widespread use of the approach has led to severe criticism, from both outside and inside the field.

In principle, manipulative experiments provide a preferable alternative, because their planning requires at least an implied prediction of the outcome, and making predictions is an integral part of science. The after-the-fact explanations mentioned earlier are sometimes called “predictions” by their promulgators, but there is no assurance that the information was not available before the prediction was made. There is no implication of dishonesty. The danger is as follows: All workers in ecology have a lot of factual information about the real world, and assuming that they are interested in understanding nature, they think about how things work. It is virtually impossible to separate known facts from the thought process. The knowledge is, perhaps unconsciously, taken into account in formulating hypotheses about how nature works. Thus, when a “test” is proposed, the prior knowledge is used, and the outcome is known in advance. Such a confirmation is, of course, spurious. I have selected an example from one of my own publications (Hairston 1964).

Introduction

Ecological experimentation has some characteristics in common with mathematical modeling in ecology, which has been stated by Levins (1968) to face the impossibility of simultaneously maximizing precision, realism, and generality [see a discussion of this problem by Hunt & Doyle (1984)]. Ecologists conducting experiments face analogous choices, in that providing confidence in a relevant ecological process may preclude its general application, or the requirements of a sophisticated experimental design may severely decrease realism, or the use of an elaborate design in the field may put the necessary amount of replication beyond the resources of the investigator. In each case, the ecologist faces a difficult choice. All of the choices should be made deliberately, because to let them be made by default can lead to a misinterpretation or to such an unfortunate effect as replication insufficient to yield a convincing (that is, statistically significant) result.

Generality versus confidence

Conclusions reached from ecological studies increase in value as they can be extended to more and more situations. Therefore, any factors reducing this generality should also incorporate sufficient benefits to offset the loss. Experience over a wide range of habitats might be offset by a thorough knowledge of the ecological processes taking place in one or a few. Anyone undertaking ecological experiments is faced with this dilemma. The experiments are needed to test hypotheses about the processes, but they necessarily limit the results of the tests to one or a few localities.