Niche, habitat, and related ecological concepts

Acta Biotheoretica XXIV (3-4) : IOO-lO7 (I975) NICHE, HABITAT, AND RELATED ECOLOGICAL CONCEPTS by M. REJMANEK and J. JENIK Department of Botany, Caroline University, Praha Botanical Institute, Czechoslovak Academy of Sciences, Prfihonice (Received 27-VIII-I974; revised 2I-I-I975) SUMMARY DARWIN'S phrase "place in natural economy", and SPENCER'Sterm "correspond- ence" can be regarded as first attempts to express the organism-environment relationships. The same concept has more recently been approached from the point of view of (I) life-form, (2) external activities, and (3) habitat. Though all these points are interlocking, they have been stressed differently in the writings of American and European ecologists. It is proposed that the term "niche" would be most useful and rational if applied to the total of relationships between a living organism (population, species) and its complete environment, both biotic and abiotic. INTRODUCTION T h e concepts of " n i c h e " and " h a b i t a t " occur v e r y f r e q u e n t l y in current ecological literature, and h a v e become an i m p o r t a n t tool in the develop- m e n t of ecological theory. N u m e r o u s a t t e m p t s to classify and re-define t h e m h a v e been made, particularly in American journals. The recent paper b y WHITTAKER, LEVlN & ROOT (1973) was a m a j o r contribution in exploring this problem, and the present p a p e r is in p a r t a response to some of the ideas expressed in t h a t work. Many E u r o p e a n ecologists now realise t h a t " n i c h e " , after all, is a useful t e r m in the ecological vocabulary, although E u r o p e a n ecology m a n a g e d to do w i t h o u t it until v e r y recently. HISTORICAL NOTES The history of the concept and definition of the t e r m " n i c h e " has been described b y SAVACE (I958), UDVARDY (I959), WOENSCHER (I969), VANDER•EER (1972) and others. Some points in the old E u r o p e a n litera- t u r e are particularly interesting in the d e v e l o p m e n t of the idea of organ- i s m - e n v i r o n m e n t links. F r o m the v e r y beginning this idea of relationships between a living organism and its surroundings, was coupled with the basic biological concept of "species". DARWIN (1859) refers to species "filling nearly the same place in the n a t u r a l e c o n o m y of the l a n d " (ot). cit.,  SOME ECOLOGICAL CONCEPTS IOl p. 173; see also p. 76 and 81) and to "a place in the natural polity of the country" (off. cit., p. 177-178 ). It seems that DARWIN followed LINNAEUS' term "Oeconomia Naturae" which was commonly used, in the pre- Haeckelian period, for a category of interrelationships in nature. Taking into account the context in which these phrases occur, we m a y assume that DARWIN'S "place in natural economy" contained in essence the features later united with the term "niche" as variously defined b y GRINNEL (I917), ELrON (1927), HUTCHINSON (1958) and other proponents of these views. Tile need for a precise term expressing close organism-environment relationships can be traced back to the synthetic philosophy of SPENCER (1898, ed. 1915). This book contains a separate chapter on "The Cor- respondence Between Life and Its Circumstances" (op. cit., p. 91-1oo). For the system of mutual relations between living organisms and their environment, SPENCER proposed the term "correspondence". He was aware of verbal imperfection in defining the interrelationships (op. cit., P. 97), and wrote as follows: " . . . we have no word sufficiently general to comprehend all forms of this relation between the organism and its medium, and yet sufficiently specific to convey an adequate idea of the relation; and the word correspondence seems the least objectionable" (o25. cit., p. 98). A study of the relevant chapter suggests that SPENCER'S "correspondence" involved tile complexity and dynamics attributed to the modern concept of "niche". THREE APPROACHES A complete understanding of organism-environment link has so far proved extremely difficult. It can be looked at and analyzed from many points of view, which are affected considerably by the w a y conceptual systems have developed in various countries; b y differences between particular biological sciences (especially in zoology and botany); and partly b y the goal of the research. Three aspects have appeared most frequently: (I) that of the morphological features of the organism (or species) itself 1) ; (2) from the point of view of the external activities of the organism (species) itself; and lastly (3) from a consideration of opera- tional environment. Some comments on these three approaches will be made. 1) H an organism is considered, we have in mind factual data on a single pheno- type; in case of the population or species, average data are concerned referring to more or less statistically defined distribution of actual data in relation to the mean representative. The same distinction can be applied if environment of an individual, population or species is concerned (see MAELZEI~1965). 102 M. REJMXNEK AND J. JENfK (1) A marked feature of the organism-environment link is the morpho- logical features of the organism. In the majority of cases, morphologicM characteristics are the only information available on the biology of the species being studied. Many of these show obvious adaptive and functional significance, which make them a suitable basis for the classification of life-forms, such as that introduced by WARMING (1884) and RAUNKIAER (1905) into botany, and by REMA~E (1943) into zoology. In a compre- hensive study of life-forms, GAMS (1918) first expressed clearly the idea that life-forms of both plants and animals can be the key to comprehen- sion of ecological relationships; this can be seen in his chapter "Life-forms as an expression of entire ecology" (@. ciZ., p. 311). It was GAMS' work which led BALOGH (1958, p. 79) to claim the terms "niche" and "life form" are synonymous, and to point out the far-reaching parallels in terminology within American and European ecological literature. (2) Understandably enough, zoologists have mainly emphasized organ- ism-environment relationships from the point of view of the activities and responses of the organism, and this has led to the notion of "niche" as defined by ELTON (1927, p. 64), CLARKE (1954, p. 468), SAVAGE (1958) and 0DUM (1964, p. 27). The last author has defined the niche as the status (profession or role) of an organism in its community or ecosystem, which Call be estimated according to various criteria. A frequent result of this approach is the classification of organisms according to their food requirements (herbivores, carnivores, necrophages, omnivores, etc.). The given two approaches, viz. through the life-forms and external activities, can be considered as special case of "form-function problem" (@ JEUI~EN 1958). (3) The third approach towards organism-environment interaction is that from a consideration of the surrounding matter and energy, i. e. operational environment in the sense of MASON & LANGENttEIM (1957) and SPONER (1973). The older terms of "habitat", "Standort", "Lebens- ort", "MeCTOO6~TaH~e" etc. frequently contained ideas pertaining to the concept of the organism-environment complex. The newer term "niche" has naturally often overlapped the older definitions. This in- terpretation is already apparent in the work of GRItqNEL (1924, p. 227). The elaborate notion of HVTCt~INSO~ (1958) presenting "niche" as n-dimensional hypervolume of environmental variables, is also close to the habitat concept. Consequently, most European ecologists would regard the use of the term "niche" in this sense as superfluous. However, in a later work HUTCHI?;SON (e. g. 1965, p. 32) also includes in his hyperspace "a relevant variable in the life of a species of organism", and gives the  SOME ECOLOGICAL CONCEPTS lO3 life-form of two African rhinoceros species as an example of niche diversi- fication (@. cit., p. 33-35) 2). LEVlNS (1965, p. 326), following HUTCmN- SON'S idea, defines niche as "a fitness measure on an environmental space". COMPLEXITY AND D E F I N I T I O N OF NICHE Though the three approaches already mentioned m a y appear very different, they are in fact related and can often be deduced from each other, as can be seen from the following diagram: Morphological f e a t u r e s / Operational environment (life-form) A c t i v i t i e s and r e s p o n s e s < (habitat) > (behaviour) For example, knowing that the food of a certain animal is found only in the soil (a type of habitat), we can deduce that this animal will gain its food by digging (its behaviour); in order to dig, the animal must be adequately adapted morphologically (its life-form). Another example: if a solid rock provides a suitable substratum for growth of a lichen (feature of the environment), the lichen must possess substances which can desinte- grate the substratum (external activities), and also develop a cortical thallus enabling maximum contact with the substratum (life-form). These deductions can also be made in the reverse direction and m a n y examples of this type of reasoning can be found in comprehensive aute- cological monographs. These deductions, however, need not be always unambiguous. In order to comprehend the totality of organism-environ- ment relationships, simultaneous utilisation of all three approaches seems to be desirable. As seen above the concept of "organism-environment linkage" is a o) This w a y of niche definition appears to be less ambiguous for A m e r i c a n authors who s p o n t a n e o u s l y assume t h a t " h a b i t a t refers to some particular c o m b i n a t i o n of e n v i r o n m e n t a l characteristics, t a k i n g into account their n o r m a l t e m p o r a l fluctua- tions, as t h e y are found in a p a r t i c u l a r geographical l o c a t i o n " (MAGuIRE 1973). M a n y E u r o p e a n authors t e n d to use the " h a b i t a t " t e r m as general t e r m for ecologi- cal conditions of a species, s y n o n y m o u s to the G e r m a n " S t a n d o r t " which f r e q u e n t l y express t h e t o t a l of soil, ar and o t h e r ecological conditions, w i t h o u t p a r t i c u l a r reference to geographical location (see WALTER 1960). Io4 M. REJMfiNEK AND J. JENiK very complex phenomenon which definitely requires a distinctive and unambiguous term. The word "niche" could be used in this connection, but the problem remains of defining its limits. For example, WHITTAKER (1970, 1972 ) and WmTTAKER et al. (1973) recommend calling our No. 2 approach the niche, and limit its meaning to "intracommunity factors" in contrast to the "intercommunity factors" referring to habitat. Regard- less to these suggestions, perhaps the most useful and rational definition in the setting of ecological theory would be to describe the niche as the total of relationships between a living organism (~@ulation, species) and its complete environment, both biotic and abiotic. As suggested by HUTCI~IINSON, one can distinguish "a realised niche" (referring to a specific individual organism or population in one habitat or succession of habitats), and "a fundamental niche" (referring to the total number of potential niches, i. e. all possible relationships in which representatives of the population (or species) can exist). This broad definition is in agreement with BOCK & WAI~LERT (I965) , who introduced a useful concept of the "synerg" for a single component of the "niche"; in the majority of cases, niche was illustrated in two- dimensional diagrams only by two synergs (WARBURG 1965, Fig. I; etc.). A synerg has been defined as a link between the organism and its environ- ment formed by one single selection force of the operational environ- ment and one biological role of an organism. Then the niche of an organ- ism can be defined as the sum of all its synergs (BOCK & WAItLERT 1965, p. 281-2). Other recent definitions also emphasize the complementary significance of both the environment and the response of the organism, e.g. WUENSCI~ER (1969), MAYR (1970, p. 42I), MAGUIRE (1973) and E?~ILEN (1973, p. 210-14). It is clear that this broad meaning of "niche" is also in agreement with the ideas of DARWIN and SPENCER quoted above. There is an urgent need for quantification of ecological concepts, including that of "niche". VAXDER~IEER (1972, p. 108) remarks: " . . . quantification is what niche theory is all about". The mentioned authors has used the competition model as a starting point. LEVlNS (1968) points out that any satisfactory theory of the niche must allow: (I) a measure of niche breadth, (2) a measure of niche dimension, (3) a measure of niche overlap between species, and (4) a method of using the niche model to explain species diversity. SHUGART & PATTEN (1972) suggested the general method for niche quantification from the point of view of habitat selection. Other papers streamlined to the quantification of certain parts of niche were presented by COLWELL & FUTUYMA (I97I), GALLOPIN (I972), GREEN (I97I), PIANKA (I969), and PIELOIJ (1972).  SOME E C O L O G I C A L CONCEPTS IO5 However, a comprehensive analysis of the total biological mealr of niche should precede this quantification, and this paper is an attempt to contribute towards this goal. CONCLUDING REMARKS In the sense defined here, "niche" may serve as a fundamental ecologi- cal concept of similar importance to "ecosystem". One might even define ecology as the "science of niches and ecosystems". For this purpose, however, we need to know the mutual relationships of these terms. Ac- cepting the broad definition of DALE (1970 , p. 3) that "an ecosystem is a system open for at least one property, in which at least one of the entities is classed as living", and assuming that the ecosystem concept is concerned with function and not merely components, we can take the niche of an individual organism as the most elementary ecosystem. But in m a n y cases, "niche" will be connected with the concept of "species". At this level, "niche" represents a quite different concept from "ecosystem", since the space occupied by a species population is often discontinuous, and individual members of the population frequently migrate within various ecosystems. On the other hand all complex ecosystems can be considered as systems of realised population niches and/or realised niches of individual organisms. The proposed broad definition of niche seems to be appropriate be- cause of its possible restriction to one or several synergs (se~su BOCK & WAHLERT 1965) including the "intracommunity aspect" of WHITTAKER et al. (1973). Thus it is always possible to specify the type of niche by describing various attributes, e. g. food niche, habitat niche, time niche, etc. (see ALLEE et al. 1949; PIANKA 1969; etc.). Finally, a comment should be made on the term "ecotope" proposed by WHITTAKER et al. (1973) for Nos. 2 and 3 aspects of niche (see above) which cover both the role of a species in a given community and its habitat. In the European literature there are two prevailing meanings of this term: "landscape element" (Sc~IMITH/3SEN 1961, p. 78; TROLl. 1968, p. 15) and "abiotic part of the environment" (LAVRENKO1959, p. 35; SCHENNIKOV 1964, p. 27; ZLATNIKet al. 1973, p. 40). 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