1. T H E STUDY O F F R E S H W A T E R A N I M A L S A N D P L A N T S AS A N INTRODUCTION TO E C O L O G Y by P. E. Hyde and W. F. Ponder S U M M A R Y This article is an introduction to fresh-water ecological studies. It is especially directed at pupils and teachers of 6th form biology classes and junior university students, and can be used as a basis for any field work in conjunction with the whole series of articles which follow. Outlines are given of the study of the living organism in its environment; analysis of the environment; microhabitats; the necessary equipment; field and laboratory procedure, and the interrelationships of organisms and environmental factors are described, together with a synthesis of all aspects to give the reader a complete idea of the concept of ecology. INTRODUCTION Can you answer these questions? How do may-fly larvae cling to the stones in fast streams? Why do bloodworms live in black, stagnant mud at the bottom of ponds? How do they survive there? How do spiders walk on the surface film of the water? How do air-breathing insects breathe while under water? Such questions are based on observations which are within the capabilities of 6th form classes. Often even simple observations are not made in the cursory glances characteristic of many unenthusias tic 6th form biology pupils on field trips. How then can we study such animals and their habitats with only limited time, and still get the maximum return for our effort? We suggest that a study of a few living animals, brought back to the laboratory would yield more information under careful observation than any other method in the time available. The study of the living animal is both interesting and rewarding, but previously has been a neglected aspect of biological studies. Fresh-water animals and plants are easily kept alive in the laboratory in simple aquaria, and they present a diversity of structural adaptations, all easy to observe e. g. the breathing mechanisms of many fresh-water anthropods. Direct contact may be made to the air above the surface of the water by siphons, or a bubble of air may be trapped on the surface of the body and forms a physical lung while the animal swims through the water. The usual course of a field investigation is that of a broad overall appreciation of the habitat, with a rapid glance and a nodding acquaintance gained with as many
2. species as possible in an afternoon's ramble. Little or nothing is learnt of how the animals live, and why they live in particular habitats. To survey a lake or stream broadly can be likened to conducting an investigation of the ocean, in its entirety. In fact, only the minor habitats are within our capabilities. By choosing one or two small habitats, the animals and plants in these can be looked at in some detail. Before we begin the practical side of this article though we will look more closely at the terms "environment" and "habitat". The Concept of an Environment The environment of an animal or plant is all those things that influence that organism's chances of surviving and reproducing. These factors are outlined by Browning (1963), and a modified version of this is as follows: - 1. Weather: This incorporates temperature, light, evaporation, humidity and current. This is the immediate weather effect on the organism on a very small scale (= microclimate), rather than the overall weather of a district. 2. Resources: These may be present as food, shelter, building materials, chemical constituents, etc. Shortage of any one of these limits the extent of the population. 3. Members of the same species: When abundant, overpopulation puts a heavy demand on the available food resources, and competition results. If scarcity occurs, then there is less chance of individuals meeting and mating and maintaining the population. 4 Members of other species: Here are included predators, parasites, and other competitors for space and food. Food organisms are not included in this category. 5. Hazards: These are large or small-scale accidents affecting the organisms e. g. the tumbling of boulders or the altering of a water course in a flood: a larger animal such as a fish dislodging small, clinging animals as it swims about. The above plan is only a broad outline and the categories are very plastic. It is merely a guide in the study of any organism's physical environment, and it has been given as a theoretical concept only, not as part of the practical investigations outlined in this article. Analysis of the Habitat There are a number of ways of listing the types of freshwater macrohabitats.. Here is one of them. 1. Stillwater a) small-ponds b)large-lakes 2. Running water a) slow-ditches b) fast and compact-streams c) large-rivers
Estuaries and swamps are more specialised habitats and are not included here Animals from such areas show tolerance to stagnant conditions, and in estuaries, to an increase in salinity. In any macrohabitat, there is present a variety of smaller microhabitats each with its own selection of animals and plants, and it is these, we suggest, that deserve the closer study Here are some examples of microhabitats which are typical of many fresh-water situations, though all of them may not be represented at any one locality. A Still or slow moving water 1 The Surface Film: Here are animals which can walk on the surface of the water e g pond skaters (Microvelia) and whirligig beetles They feed on terrestrial animals which fall onto the surface of the water, and are trapped by the surface tension 2 In Open Water: This includes the suspended plankton species, which are fewer than those of the sea, e. g. Daphnia, and the actively swimming fish. 3. In Well-lit waters: This is not strictly a macrohabitat but a large number of species are found in shallow, sheltered creeks and pools with muddy bottoms, there being plenty of food and oxygen. There are few chemical and physical problems, but life is not easy, competition is severe and predators common. Water beetles and boatmen pursue their prey: Hydra snares it with tentacles, and dragon-fly nymphs lurk concealed awaiting prey. The animals have various adaptations for protection such as tubes, shells, or protective coloration. 4. On water Weeds: In shallower waters, the lighted zone accommodates many plant species and their associated herbivorous animal species. Because of readily available food and shelter requirements, there is an abundant flora and fauna on the weeds, with severe competition developing between some of them. This is probably one of the richest microhabitats that is likely to be encountered in a fresh-water environment. 5. In Mud in Deeper Pools: With lack of light at depth, there are no plants. The mud is formed from organic waste falling from above. The animals include freshwater mussels, worms, and midge larvae, and are either burrowing forms or suspension feeders. Most of them have well-developed methods of carrying out respiration under difficult conditions. Whilst the density is great, the diversity in the habitat is poor. 6. Rock faces and Rocky Shores: Sheltered rocks may be almost bare, with a thin covering of algal film, or may be covered with mosses. The animal species are few. B. Fast flowing water e. g. Rapids. 7. The Surfaces of Pebbles and Boulders: Here, the animals are exposed to the strong currents, but have the advantage of being in a well-oxygenated environment. There are no silting-up or sanitory problems. The species are well-adapted for clinging, although there is a thin layer of water close to the surface of the stones, where the current
4. is appreciably slowed by friction, and this is utilised by these animals. 8. In the Lee of Pebbles and Boulders: In such places, the animals which are without special adaptations for escaping from predators and current, find sufficient shelter. Thus, each stone or boulder represents more than one macrohabitat; the exposed upperside, and the sheltered downstream side. One must remember that each of the microhabitats listed above can be highly Variable and unstable in the environment that it presents. This, together with the partial isolation occurring between populations of fresh-water animals, causes the great variability seen in most species. P R O C E D U R E Most of the necessary equipment is inexpensive, and should be readily obtainable. A working list is given below, but this is of course, only a guide to the items needed and could be enlarged. a) in the field: small pipette or eye-dropper small hand net collecting jars and/or dishes sieve tubes or small bottles forceps notebook pencil handlens b) in the laboratory: watchglasses small aquarium tank microscope or dissecting lens fine forceps and needles small aerating mechanism e. g. pump (advisable if obtainable) Field Procedure: If possible, visit the locality at least twice. The first trip is to obtain a general idea of the area; for each student to pick a macrohabitat; and to collect specimens from this to study in the laboratory. The second is to try and relate the total information, derived from the study of the specimens in the individual microhabitats, to an appreciation of life in the habitat on a larger scale. The following plan is suggested: - 1. Each member of the party should choose as small a macrohabitat as possible. 2. Note where the macrohabitat fits into the structure of the pond or stream (i. e. macrohabitat).
3. Note the distinctive features of the macrohabitat, and any other information which may be considered relevant. 4. Collect all the organisms in a given area of the particular macrohabitat. This should be of such a size that the number of animals and plants is sufficiently small to encourage accurate observation and identification of them all. 5. Note any obvious interrelationships of different species of animals and plants in the macrohabitat (e. g. snails mostly on the undersides of leaves of waterweeds). 6. As a group pool the data gained in the laboratory studies outlined below, together with your field data, and form possible hypotheses as to the overall pattern of life. 7. Revisit the locality, and check the hypothetical data where possible. (NOTE: It is only with the "Visit - Study - Visit" programme that the full benefit of such an investigation may be obtained). Laboratory Procedure: Set up an aquarium with as close a resemblance to natural conditions as possible (Richardson, 1953), but even small dishes will suffice. Remember that each container may house a number of microhabitats. Each student should deal with the species from one macrohabitat only, observing the animals in the community directly in the tank, or the individuals in a watchglass under a lens or microscope. The tank should be disturbed as little as possible. The points to note are outlined as follows: - In all cases, note which organs perform the function, and how it does this 1. feeding - how and what? 2. respiration - any modifications? 3. habits - locomotion: posture 4 reproductive activity - mating, nests, eggs. 5. growth - different stages representing different age groups in the population. Do these suggest a limited or continuous breeding season? 6. stimuli responses - light, gravity, temperature, current, food. 7. other aspects - parasites, predators, camouflage. The facts that are derived from these points then serve to suggest how the animal manages to remain in, or maintain, an optimum environment. THIS IS T H E C R U X O F T H E STUDY.
6. A SYNTHESIS The correlation of habits, habitats of animals and plants, and their physical environment is called ecology. A simple definition of ecology could be "The study of animals and plants in their natural habitats, and the investigation of the relationships of them and their dependences on their environment" The eventual study of the whole population on this basis is important in ecology. However, rather than become too deeply involved in ecological principles, we have tried to outline a course of study that is easy to follow, and which should prove to be of value to beginners in biology The former part of this text was to present the principles of such studies, giving a perspective of the whole, once the preliminary investigations have been satisfactorily concluded. We hope to have shown that by careful observation of all the previously mentioned aspects of the life of an animal or plant, one can get an idea of how it actually lives If, as we suggest, each pupil carries out a study on a macrohabitat, then an overall appreciation of the habitat may be obtained, together with hypotheses as to possible food chains and other relationships within the community. Similar investigations involving a series of field trips would reveal any patterns in the life of the community that vary with time such as seasonal changes involving reproductive cycles. With this introduction to any particular area, the major principles of fresh-water ecology will have been covered, and the student will be better prepared for more intensive ecological field work.