N.-A. Nilsson
Institute of Freshwater Research
Drottningholm, Sweden
ABSTRACT
The niche concept has given rise to some confusion ever since it was first introduced by Gause (1934) and Elton (1946) among others. Elton's simple definition still stands well as a working paradigm: “The status of an organism in its environment”.
It soon became clear that the seemingly inflexible concept that “two or more species cannot live in the same niche” is obscure, mainly because of the obvious fact that niches very often do overlap, or may even temporarily seem identical. Hutchinson's definitions of “fundamental” versus “realized” niches, as well as his definition of “the N-dimensional hypervolume” as a handy conceptual tool to study niches, mathematically, has given rise to an ever-growing literature on “niche overlap”, “niche breadth”, etc. Related concepts of “interactive segregation” and “species dominance” have also attracted attention and many parallels have been drawn between insects (Brian, 1956), birds (Svardson, 1949) and fish (e.g., Nilsson, 1978; Svardson, 1976). Recent findings that exploitative competition by selective feeding forces fish species to segregate into their “realized niches” (literature compiled by Nilsson, 1978) has given us a possible means for monitoring introductions of “exotic species”, including subspecies, stocks, etc.
It is suggested that the introduction of “exotics” leads to any of the following results. The introduced stock:
is rejected, because there is no “vacant niche” or predators graze down the population at early stages;
hybridizes with very closely related stocks, formerly adapted to the ecosystem;
eradicates a stock that is either an “ecological homologue” or a very available prey;
finds a “vacant niche” within the community, which means that it adapts to resources that are not fully exploited by other species and finally makes it able to survive as a member of the community.
The four alternatives suggested are demonstrated in the paper by European and North American experiences.
RESUME
Le concept de niche a entraîné une certain confusion et a provoqué bien des querelles sémantiques depuis qu'il a été introduit, par exemple par Gause (1934) et Elton (1946). Par sa simplicité, la définition d'Elton peut toujours servir de modèle: “le statut d'un organisme dans son environnement”.
Beaucoup de spécialistes ont dénoncé l'idée selon laquelle “deux ou plusieurs espèces ne peuvent pas vivre dans la même niche”; il est en effet évident, entre autres choses, que les niches se recouvrent très souvent ou qu'elles semblent même identiques temporairement. Hutchinson et Ivlev nous ont beaucoup aidé à éclaircir nos idées. La définition par Hutchinson des niches “fondamentales” par opposition aux niches “effectives” ainsi que sa définition de “l'hypervolume de dimension N”, qui est un instrument commode pour étudier les niches du point de vue mathématique, ont donné naissance à une documentation de plus en plus abondante sur le “chevauchement des niches”, la “largeur des niches”, etc. Par ailleurs, le concept de “ségrégation interactive” a acquis une certaine importance; on a établi beaucoup de parallèles entre les insectes (Brian, 1956), les oiseaux (Svardson, 1949) et les poissons (par exemple, Nilsson, 1978; Svardson, 1976). En ce qui concerne le poisson, on a découvert récemment que la concurrence entraînée par l'alimentation sélective oblige les espèces à se séparer dans leurs niches “effectives” (documentation réunie par Nilsson en 1978), ce qui nous a donné un fil directeur pour suivre les introductions d'espèces exotiques, y compris les sous-espèces, les stocks, etc.
On estime que l'introduction de poissons exotiques aboutit à l'un des résultats suivants. Le stock introduit:
est rejeté parce qu'il n'y a pas de niche vacante ou que les prédateurs détruisent rapidement les populations;
s'hybride avec des stocks apparentés, déjà adaptés à l'écosystème;
détruit un stock qui est, soit un “homologue écologique” soit une proie facile;
trouve une niche vacante dans la communauté, ce qui signifie qu'il s'adapte à des ressources qui ne sont pas totalement exploitées par d'autres espèces et parvient finalement à survivre en tout que membre de la communauté.
Les expériences réalisées en Europe et en Amérique du Nord servent à illustrer ces quatre possibilités.
The niche concept has caused some confusion and ever since early trials to define the concept (Grinnel, 1904; Lotka, 1932; Gause, 1934; Hutchinson, 1957) Elton (1946) termed the phenomenon “the status of an organism in its community”, which still stands as a simple as well as a good definition.
Fisheries biologists, forced as they are to use theory as a foundation for those practical actions called fishery management, soon objected to the seemingly inflexible concept that “two or more species cannot exist in the same niche”, as they frequently observed that different fish species very often consume similar food or share other essential resources (e.g., Forbes, 1914; Hartley, 1948; Starrett, 1955; Nilsson, 1955; Larkin, 1956). Because of this, further concepts of “niche overlap”, “niche breadth”, etc., were stressed by scientists interested in species interaction (e.g., Hurlbert, 1971). The Swedish zoologist Lonnberg perhaps was the first to formulate this by pointing out what he names “det dukade bordets princip” translated by Johnson (1980) to “the smorgasbord principle”, which states that "in Nature - miscellaneous animals make use of one kind of food when it is available in plenty, even those to which that particular item is not the natural or common food (Lonnberg, op.cit., quoted by Nilsson, 1960).
Hutchinson (1957, 1967) defined the term “niche” as an “N-dimensional hypervolume” designating “the requirements of an organism abstracted from the specially extended habitat. The habitat of two species may overlap completely; it is empirically probable that at equilibrium, their niches never do”. Thus he distinguished between “the fundamental niche”, which means the virtue of a species to make use of available resources through its physiological capabilities and the “realized niche”, which is that portion of the hyper-space that is actually occupied, the difference being due to exclusion from certain parts of the niche by other species in the community.
This philosophy agrees well with the theories of “dominance-subordinance” and “interactive segregation”, which posits that interactions between species of subpopulations of species are fundamentally variable creating “realized niches” sensu Hutchinson (Nilsson, 1978). In other words, species occupying the same water body are forced by interaction to make the most of their assets, when resources are at a minimum.
Svardson's (1976) theory of “dominance-subordinance” points to the fact that the standing crops of fish in lakes are hierarchical in nature, which means that the reduction or elimination of the dominant species tends to lead to drastic changes in the lower ranked species. On the whole species with pelagic capabilities generally are dominant over littoral species (Svardson, 1976; Skud, 1982; Ryder and Kerr, 1983).
Entomologists (Brian, 1956; Park, 1954; Ross, 1957), ornithologists (Svardson, 1949; Cody, 1968; MacArthur, 1958) and fish ecologists (Nilsson, 1967, 1978; Svardson, 1976) have arrived at very similar conclusions which eventually could be of help in judging whether or not “exotics” should be introduced in a stablized ecosystem.
The study of competition is, of course, closely related to these problems. Park (1948), Brian (1956) and others distinguished between two components in interspecific competition: interference and exploitation. Interference means direct damage to one or both species, for instance, by aggressive behaviour such as fighting for territories, etc. Exploitation on the other hand means an interaction that develops whenever one species is more efficient in using available resources more easily and quickly than their competitors.
To turn from theory to practice, it seems to me that when introducing a new species (population, subspecies, etc.) into a new community, it may face any of the following fates:
An exotic species:
is rejected because there is no “vacant niche” or predators graze down the population at early stages, or gets harmfully infected by native diseases, or abiotic factors like temperature, pH, etc., do not fulfil the needs of the species at crucial circumstances;
hybridizes with very closely related stocks, formerly adapted to the ecosystem;
eliminates (completely or partly) a species that is either an “ecological homologue” or a very available prey, or is sensitive to foreign diseases and parasites, carried by the exotic species;
or finds a “vacant niche” in the community, which means that it adapts to food, space, spawning sites, etc., that are not fully exploited by other species or stocks. It also means that because of competition, niche overlap, etc., the species within the community have to make the most of their individual advantages which results in their restriction to “realized niches” through interactive segregation.
Fig. 1 is an attempt at modelling a Scandinavian example as regards the “fundamental” and “realized” niches of three salmonine species. The hypervolumes of the niches are hypothetical in the graph, whereas the indications of the zooplankton communities based on quantitative information (Nilsson and Pejler, 1973).
Immigrants into Europe and America enthusiastically tried to introduce species from one continent to another over a very long time, in attempts to “improve” the native fauna. European starling, house sparrow, European carp and brown trout are well known examples of introductions into North America. Many of these attempts either failed or proved disastrous. As Ryder and Kerr (1983) have reviewed these in some detail, I will comment on some European experiences, using the introductory scheme (1–4) above.
Theoretically, this should be the most likely outcome, as the indigenous fauna should a priori be best adapted to the ecosystem in question, and thereby should not incorporate an “intruder”. However, several experiences, such as those in Australia as well as plant introductions worldwide, have provided terrifying lessons (cf. Harlan, 1981). Fish species on the whole, however, are less likely to be disastrous as they are introduced into aquatic systems that are more closed than terrestrial ones.
Rainbow trout (Salmo gairdneri), a western North-American salmonid was originally native to lakes and streams from Alaska to Mexico. It has many migratory and resident stocks and subspecies, which have been spread all over North America and later over most continents: New Zealand, Australia, Tasmania, South America, Africa, Japan, southern Asia, Hawaii and many parts of Europe (McCrimmon, 1971). In Europe S. gairdneri has, on the whole, been used in “put and take” fisheries. or cultured for direct consumption. However, as far as natural reproduction is concerned, it has not been suited to European habitats. For instance, Wheeler and Maitland (1973) stated that in the British Isles “in spite of such widespread introductions the species has appeared in relatively few places”. In fact, Worthington (1941) listed only about 14 waters in the south of England and one in Ireland where the species is found regularly. The same is true in Scandinavia, where many thousands of introductions have been made since the turn of the century but only two or possibly three reproducing stocks have been recorded. There has been much speculation on why these introductions have failed, the presence of strong competitors or predators, and the genetic inappropriateness of the strains of the species adapted as they are to the American west coast, rich in lime as a buffering substance, and with very few competing or predatory species present (cf. Nilsson and Northcote, 1979) have both been advanced as reasons. In Scandinavia the very disastrous acidification problem should imply a severe threat to the limited stocks of species that have become established.
The kokanee (Oncorhyncus nerka) native to the American west coast as a landlocked variety was introduced in Sweden in 1959 in some ten lakes and also in the Baltic with discouraging results. Although the stocking of kokanee fry in some lakes reclaimed with rotenone proved to be successful. For instance, in one case, one third of the introduced fish were recaptured in excellent condition. This experience has led to the idea that kokanee might be a possibility for fish farming. There is some evidence for natural reproduction which has hitherto had very little significance. Of course, the increasing acidification problem in Scandinavia forces a lessened interest in trying to introduce species from less acid environments, for instance, the American west coast into the acid-tressed environments.
The brook trout (Salvelinus fontinalis) was introduced into Scandinavia at about the same time as the rainbow trout. Although it appeared to be more successful than rainbow in establishing breeding populations it is now confined to cold head waters of small streams, where apparently it can compete with the native brown trout. This is consistent with the introductions of brown trout at the American east coast which forced the brook trout to inhabit head water refugia of small streams (e.g., Brynildson et al., 1964).
The Danube salmon (Hucho hucho) was imported in 1963 to Sweden from Yugoslavia, with the general idea that, in addition to being an excellent game fish it could possibly use habitats which differed from those of its ecological homologue, the Northern pike. The introductions, however, completely failed. No Danube salmon has been recaptured in Sweden, although the places of release were chosen very carefully.
Similar results have characterized many other introductions of “game fish” in Europe - mainly centrarchids such as rock bass (Ambloplites rupestris), pumpkinseed (Lepomis gibbosus) - and above all largemouth bass (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu). As to the two last mentioned species, Wheeler and Maitland (1973) have stated that no population has been established in the British Isles. The same is true for Scandinavla (Svardson, pers.comm.), in spite of early attempts at introduction. Failure to acclimatize has been attributed to temperatures which are not correct for hatching (Svardson, pers.comm.).
Similarly, introductions of grass carps (mainly Ctenopharyngodon idellus) into temperate European areas have mainly failed to produce self-reproducing populations where the water temperature at stages crucial for breeding is unsatisfactory.
Mayr (1942) stressed the significance of hybridization between closely related species or subspecies, often leading to a “regressive subspeciation” (Svardson, 1970). Sibling species such as coregonids and Salvelinus may hybridize (Svardson, 1970; Nyman et al., 1981). Rainbow and cut-throat trout, cyprinids and many other taxa also hybridize, as well as “stocks” introduced to improve fisheries. Hybridization (cross breeding) can be divided into two categories: intraspecific crosses between strains “stocks”, “races”, etc., or interspecific crosses between species. Hybridization can achieve either of two favourable outcomes: viz (i) heterosis or hybrid vigour or (ii) non-heterotic effects, “the improved performance of the progeny as the result of simple combination of parental genotypes” (FAO/UNEP, 1981).
The elimination of organisms by introduced species has been one of the most delicate areas of debate since the early and uninformed attempts of the pioneers to improve natural resources.
Experience from introductions of terrestrial animals or plants have taught us that from an anthropocentic view an exotic can eliminate either (a) a less esteemed species, (b) a more esteemed species, by direct competition or predation or less directly by introducing diseases or parasites to which the native species are more succeptible.
Few cases of really favourable introductions where exotics have eliminated native species can be found. Nevertheless some successful introductions of carp, grass carp or brown trout have been recorded from areas where these species are looked upon as more favourable than native species. Brown trout, however, has been shown to interact to the detriment of the equally esteemed brook trout where it has been introduced on the American east coast.
More severe have been deliberate or accidental introductions of species directly harmful to the native fish community, which have been documented by North American workers (Courtenay and Taylor, 1983; Ryder and Kerr, 1983).
Two cases of disastrous introductions of exotics illustrate this effect in European waters.
Every fisherman knows that the northern pike (Esox lucius) has threatened the existence of many species sensitive to predation. For instance, Toner (1959) estimated that 2 594 pike in two lakes consumed approximately 112.5 t of brown trout in one year, and Went (1957) gave strong evidence that pike not native to Ireland, eliminated the salmonids in some very important salmonid waters after its introduction. In Scandinavia the presence of pike has made the introduction particularly of salmonids impossible. Further evidence for this has been provided by the success of such introductions where pike have been eliminated from “coarse fish waters” by rotenone.
The introduction of pike into salmonid waters, therefore, is a good example of unfortunate predation on esteemed species.
The elimination of ecological homologues, however, seems to pose a still more important problem and involves competition that is not only a matter of interactive segregation but a real elimination of a species that is unable to defend the niche it was once adapted to.
One example of this has been very carefully studied - the introduction of whitefish (Coregonus sp.) into brown trout-Arctic charr lakes in the north of Sweden. This is a very complicated story which has been elucidated through Gunnar Svardson's (1961, 1979) important studies of the ecology of Arctic charr-whitefish interactions. Ekman (1910) had already observed that introductions of coregonids tended to eliminate charr populations and Svardson's (1979) studies have shown that one species of whitefish especially, the “alvsik” (Coregonus lavaretus has been especially adverse to charr populations. Fig. 2 shows the elimination of Arctic charr after the introduction of “alvsik” in a northern Swedish lake. Recent investigations 'Nilsson and Pejler, 1973) have indicated “size biased” predation to be the fundamental factor leading to this effect.
The main philosophy regulating introductions of exotics should be to find organisms that fit into an ecosystem where there possibly is a niche that is not fully exploited by native species.
A Swedish example involving three aspects of the introduction of exotics is Lake Storsjon where two food organisms (Mysis relicts and smelt) and one predator (Salvelinus namaycush were introduced. Lake Storsjon is one of the large lakes of Sweden (surface area 456 km2, with a complex fish community, dammed for hydro-electric purposes), which is important both for professional and sport fishery, but in which the Arctic charr and brown trout fishery, mainly because of the environmental modifications arising from hydro-electric constructions. To improve the declining community American lake trout (Salvelinus namaycush) were introduced in order to convert the flesh of four species of whitefish into more valuable food as well as to provide sport fishing opportunities (cf. Gonczi and Nilsson, 1983).
Simultaneously the glacial relict Mysis relicta was introduced into the head waters of the lake and soon appeared as a very important member of the plankton community of the lake where it formed an important fish food organism.
The initial growth rate of the lake trout was very good (Fig. 3) but after some years the growth declined, possibly because of overgrazing of pelagic food organisms, such as stickleback (Pungitius pungitius) and stunted coregonids (Fig. 4). After considerable discussion, another food organism was introduced in 1974 - the smelt (Osmerus eperlanus). The population of smelt very rapidly increased and after only three or four years it became the most important food of both lake trout and Arctic charr (close to 100 per cent in 1980).
The history of Lake Storsjon provides a very interesting example of how three exotics (lake trout = predator; Mysis = invertebrate prey, and smelt = fish prey) interacted in a way that now looks satisfactory, although all interrelationships have to be carefully studied before we are ready to design an ultimate model how to manage a fish community similar to this one.
The above case histories, which are limited to the temperate/Alpine/sub-Arctic region serve to illustrate the potential dangers as well as the advantages of introductions of non-native species. The risks of hybridization with an elimination of native species makes it imperative that proper scientific evaluation of the risks and advantages of introductions be made prior to such action.
Governments which do not now have mechanisms to ensure that an objective analysis of risks precedes the introduction of an aquatic organism into national waters should take immediate steps to establish such mechanisms of should seek help should they lack the necessary means themselves to do so. Genetic, behavioural and ecological data, as well as potential for introduction of disease, should be included in the analysis of risks. Governments should be aware that the probability of escape of cultivated aquatic species (even those kept only for research purposes) is so high that intent to confine imported aquatic animals does not obviate the need for such risk assessment.
On the other hand, the introductions of exotics seem to have been favourable in some of the cases referred to above and on the whole the benefits of introduction are usually perceived to exceed the risks. Thus, there is still an imperative to introduce exotics.
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Fig. 1 Model of the “dimensions” of the niches of brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus) and whitefish (Coregonus sp.) in allopatry and sympatry, and the dominant species of zooplankton (after Nilsson and Pejler, 1973)
Fig. 2 Decrease in the gillnet catch of charr (Salvelinus alpinus) as the catch of the introduced whitefish (Coregonus sp.) increased, Lake Västansjö, north Sweden (from Nilsson, 1967)
Fig. 3 The food of lake trout in Lake Storsjön
Fig. 4 The growth of tagged lake trout in the Lakes Storsjön and Kallsjön
