original text of the thesis:
Population dynamics of the Gyrinid beetle Gyrinus marinus Gyll (Coleoptera)
With special reference to its dispersal activities (1987)
CHAPTER III INTRODUCTION
1. STATING THE PROBLEM
1.1. Starting with the publications of Andrewartha and Birch (1954), Southwood (1962), MacArthur and Wilson (1967) Wynne-Edwards (1962), den Boer (1968), and Johnson (1969) serious attention was paid to the role of dispersal in population dynamics. Since then a growing number of publications contribute to discussions about the factors that introduce dispersal behaviour, about the genetic consequences of dispersal, and about the influence of dispersal on the variation of population size and thus on the chances of survival and (re)founding of populations (e.g. Gadgil 1971, Simberloff 1974, Diamond 1975, den Boer 1977).
1.2. Most contradictions concern the question why some individual should show dispersal activity. Other problems around dispersal may be converted to this central question. The fenomenon of dispersal can be approached from two sides.
(1). From the point of view of natural selection dispersal behaviour should only be shown when the chance to get progeny should at least not be reduced by the dispersal activities. As it is generally assumed that dispersal will introduce an extra risk not to encounter a mate, many authors suppose that dispersal behaviour will only occur when the risk not to reproduce in the present habitat has increased (e.g. Elton 1927, Southwood 1962).
(2). From the point of view of population dynamics there is evidence that populations of a number of different species become extinct and are refounded rather frequently (e.g. Simberloff 1974, Diamond 1975, den Boer 1985), as well as evidence that dispersal may lead to a rapid colonization of new habitats (e.g. Mook 1971, Lindroth 1971). The principal of 'spreading of risk', formulated by den Boer (1968, and e.g. 1977, 1981; Andrewartha and Birch 1984), emphasizes the con-tribution of exchange of individuals between (sub)populations to the survival of (sub)populations concerned. Moreover, such an exchange between populations with asynchronous changes of numbers may increase, just for mathematical reasons, net reproduction and thus mean population size (Kuno 1981, Metz et al 1983, Klinkhamer et al 1983).
1.3. This difference in the evaluation of dispersal, (1) in relation to the progeny of the dispersing individuals, and (2) in relation to the variation in population size and survival of populations, lead to differences in opinion about the causes of dispersal behaviour. We may bring these different appoaches under the same denominator by trying to answer the question: Under what conditions do the long-term advantages of exchange between populations counterbalance the short-term risk of dying without progeny of the dispersing individuals ?
This principal question can be divided in a number of subquestions that each can be answered by studies and experiments in field and laboratory:
| (a). | What is the frequency of dispersal activity, what kind of dispersal activity is shown, under what circumstances does dispersal occur? |
| (b.) | Which factors do influence the dispersal behaviour (weather, hunger, lack of reproduction, etc.)? |
| (c). | What is the exchange rate of individuals between populations, what is the chance to survive dispersal activities? |
1.4. Because the effects of dispersal on population dynamics will have to be traced, not only data on dispersal have to be collected, but also data on reproduction and survival.
2. FIELD STUDY
2.1. The possibilities to collect in the field data about dispersal, about exchange of individuals between (sub)populations and about reproduction and survival, are limited. However, we found a suitable object in the water beetle Gyrinus marinus Gyll. (whirligig beetles; Fig. III-1). These beetles live on the water surface of ditches and pools, mostly in groups at traditional places near the banks. The beetles hibernate until about mid-April below the water surface. Reproduction follows promptly hibernation. Until about mid-August females are frequently laying eggs under water. The larvae grow up below the water surface, pupate outside the water, and beetles of the new generation emerge from the end of June onwards. This summer generation also reproduces, resulting in an autumn generation that emerges in September and October. End of October the beetles of this generation start hibernation. During the whole active period between April and November the beetles can show dispersal activities, either by flight or by swimming. The year cycle of Gyrinus marinus is summarized in Fig III-2.
2.2. A suitable study area was found in the northern part of the Netherlands near Groningen. It consists of 15 pools, which are partly interconnected by ditches (Fig. III-3). In this area about 10 (sub)populations of Gyrinus marinus can be distinguised.
2.3. By marking the beetles their mobility as well as the contribution to exchange between different pools and ditches, could be estimated by regularly recapturing the beetles. From the same capture-recapture data the numbers of beetles, the survival rates and the sizes of progeny could be estimated.
2.4. In addition to the field study a number of laboratory experiments were carried out, for example to induce flight activity under different conditions and for different kinds of beetles (females, males, etc.).
3. EVALUATION OF THE DATA BY SIMULATION MODELS
3.1. The evaluation of the collected data occurred by developing simulation models, which made it possible to investigate the role of dispersal in the population dynamics of Gyrinus marinus, and to ascertain under what conditions the advantages of exchange between populations at the population level may counterbalance the risk of dispersal at the individual level. As the model is feeded with field data, it gives information about the data that have still to be collected, i.e. what kind of experiments have to be done to complete the study.
3.2. The difference between long term and short term effects of dispersal is schematically pictured in Fig III-4. Suppose there are X habitats in an area with at the start 1000 individuals in total, distributed over all habitats.
Suppose further, such a survival rate and such a reproduction rate that on the average the total population size increases.
Suppose, such a chance of extinction, that in each time period (e.g. a year) a number of populations (e.g. 10 %) become extinct.
In case dispersal occurs, suppose finally, that there is some chance to survive emigration (i.e. the chance to immigrate into another habitat, e.g. 70 %)
The short term effect of dispersal activity will be: on the average less individuals per habitat in comparison with the situation without dispersal activity, whereas the short term effect of no dispersal will be: a more steep decrease of the number of populations in the area than in case dispersal occurs.
The long term effect of no dispersal will be: a few populations with very high numbers, finally leading, however, to complete extinction of all populations in the area, whereas with dispersal a number of populations of moderate sizes will continue to exist.
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