the problems of ecology

7/29/2019 The Problems of Ecology

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The Problems of Ecology. 191Fig. 9. Germ -tube of ureduspoi-c of Piiccinia sp. from Cardiiss formingswelling in stomn of C. paliistris (2nd day).Fig . 10. Similar to Fig . 9 (2nd day).Fig. 11. Germ -tube of aecidiospore of P. i?ojnf having formed a swelling instoma of R. Ficaria has put forth two hyphae (3rd day).Fig . 12. Stom a of C. palitstris in which germ-tube of uredospore of P. Poarumhas formed a swelling from which it has put forth a curious hypha(3rd day ),Fig . 13. Som ewh at similar to Fig. 12, stoma being that of R. Ficaria andinfecting fungus being U. Chrysanthemi (4th da y).Fig. 14. Stom a of C. paliistris shewing germ-tube of uredospore of P.glninaruin growing straight through without forming any swelling (2nd daj).Fig. 15. Germ-tube of uredospore of U. Chrysantliemi growing straightthrough stoma of R. Ficaria and forming swe lling deep in the leaf (4th day).Fig . 16. Som ew hat sim ilar to last, but swe lling at a less depth and is alreadydead (4th day).Fig. 17. Stoma of Tropaoliim with germ-tube of P. rnra.rnci growing throughit, internal hypha much swollen (2nd day).Fig. 18. Sh ew s a similar hypha of U. Chrysanthemi which has penetrated theleaf of R. Ficaria to some depth (4th day).Fig . 19. Germ -tube of Piiccinia sp. from Carditss which has formed a some-what deep-seated swelling in the mesophyll of C. palitstris and has thenemitted a hypha from it (2nd day).Fig . 20. Germ-tube of (/. Geranii entering C. paliistris ; the part ou tside is stillalive, th at w ithin the leaf is dead (4th day).Figs. 21 and 22 shew the different sta ge of development arrived at by two

gei'm-tubes of P. Cciitaurcac entering Valcriaiia (2ncl da y).Fig. 23. Germ -tube of uredospore of P. McntUac entering stoma of Valeriana(2nd day).Fig . 24. Germ-term of uredospore of P. pitlveritlenta entering stoma ofValeriana (3rd day).Fig . 25. Germ-tube of uredospore of P. gramiiiis entering stoma of C.paliistris (2nd day).Fig . 26. Germ-tube of uredospore of C. Soitclii entering stoma of Vahriaua(2nd day).Fig . 27. Germ-tube of aecidiospore of P. Poariim growing through stoma ofC. paliistris and forming swelling against a cell (3rd day).

THE PROBLEMS OF ECOLOGY,B Y A. G. TANSLEV.'

CO LOGY may now be considered almost a fasbionable study,but tbere are not wanting botanists wbo tacitly distrust,


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192 A. G. Tansley.the pape rs of some of i ts vo tarie s; on the othe r band the hostili tyalluded to is not seldom due to misapprehension, and it thereforeap pe ars useful to consider th e subjec t from a genera l point of view inthe hope of pointing out some of its dangers and of clearing upsome of the misapprehension.In what follows the term " Ecology " is used in asomewhat nar-rower sense tban is often tbe c ase. Instead of taking it as syno nym ouswith "B ion om ics" or "N atu ra l History," to include all the vi talrelations of plan ts, as living and com petitive orga nism s, with tbe irsurroundings and with one anoth er, it will be restricted to w bat maybe called tbe topog rapbical aspe ct of these relation s, or, in oth erw ords, to tbose relations wbich depen d directly upon differencesof ha bitat amo ng pla nts . In this sense it is practically equivalentto the subject matter of Warming's book, a work tbat may betruly described by tba t often abused term " epocb-making,"CEco logische Pfianzengeographie, or to tb a t o f S cb im pe r ' s encyc lo-paedic work Pflanzengeographii aitf physiologischci^ G rnndlage.

Though it may be open to question whether tbis use is strictlyjustifiable on logical or on etym ological gro un ds, tber e can be nodoubt that we want a single term for the subject matter so defined,as is sufficiently evidenced by the limitation of tb e subject m att erof Schimper and Warming, and this is one which is simple andready to ban d. A single critical instan ce of the distinction to bedrawn will make this limitation of subject m atte r perfectly c lear.The study of individual pollination-mechanisms or of pollination-mechanisms in general would not be included as part of tbe subject-matter of ecology, since they do not directly depend upon anytopographical fa cto r; but, on tb e o ther hand, the general resultarrived at by Hermann Muller in his Alpenhlnmen, that Alpineflowers are especially and pre-eminently adapted to pollination byLe pido ptera, a s opposed to the bee- and fly-pollination prevalentin tbe lowlands, is an integral part of tbe ecology of Alpinepla nts, since a topographical factor clearly e nte rs into this adap -tatio n, the interrelation of th e flowers and insects of tbe Alps asa ba bita t exbibiting ch ara cteristic life-conditions shewing a


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The Problems of Ecology. 193fact that meets us is its diversity. This diversity, however, is anordered dive rsity. It is due in th e first place , of co urse , to th eeno rm ou s num ber of different species of plan ts th at exist, butthese species are not mixed haphazard upon the surface of theearththey are associated in aggregates which are the result ofperfectly definite and ascertainable, though often immenselycomplex, cau ses. Som e of thes e cause s may be called phylogenetic.Certain stock s or families of pla nts are confined to ce rtain regionsof the earth because their ancestors were evolved in those regionsand have not succeeded in spreading beyond them. Th us manygreat families are exclusively tropical and subtropical e.g. theMyrtacete, the Melastomacea, the Scitauiinece. Others are mainly orexclusively tem pe rat e, e.g. Ranuncidacea , Papa veracete, Cruciferce, &c.Th ere is no reason to assum e th at there is anything inherent inthe con stitution of th e m em bers of many of thes e families whichprevents their spreading into those regions of the earth where theyare no t in fact found. W e m ust rath er suppose th a t having firstappe ared and obtained a footing in a certain region they haveeithe r not as yet had tim e to spread out of those regions, or (andthis is probably the explanation in the majority of cases) they havefailed to widen the lim its of the ir distribu tion owing to the compe-tition of the occ upa nts of th e oth er regions of the ea rth . In somecases, however, of highly specialised families, of which we may takethe Podostemacea and tbe Balanophoracece as extreme instances, themembers of the family have become so highly specialised toparticular circumstances of life as to preclude the possibility ofthe ir spreading beyond the are a of prevalence of the se particu larenvironmental conditions.

On th e oth er hand m any families are cosm opolitan. Amongthe lower forms of plant life, as for instan ce the Algas and Fung i,not only is this tru e of the imm ense majority of the families, it istru e also of many gen era and species. Th e generic and even th especific iden tity of th e algse inha biting the freshw aters of theTrop ics w ith those of an Eng lish pond or stream is a rem arkablefact, in striking contrast with the extreme rarity of generic identity


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194 A. G. Tansley.the wide prevalence of tbe conditions under which they are able toexist. Tb e sam e explana tion holds in tbe cas e of tbe most widelyspread genera of bigber plan ts. W ate r plants are p erbap s tbem ost striking exam ple. Tb e c ond itions of life in w ate r are farm ore uniform tbro ug bo ut tb e world tba n tbe cond itions of life onland, while tbe ease w ith which aq ua tic plan ts are propaga tedvegetatively is well know n, and Ibese no doubt are tbe deter -m ining factors in the w orld-wide d istribution of so man y of tbebigber water plants.

Tbrougb tbese considerations we are naturally brought fromtbe contem plation of w bat we m ay c.\[ geoginphical aggregates \iotbose otbe r aggrega tes wbicb we may distinguisb as topographical,and witb wbicb ecology in tbe sense we bave defined, is mainlyconcerned.

In any region of tbe eartb, in any country, in any limited area,we find tbe flora com posed as a wbole partly of m em bers of tbefamilies cbara cteristic of tbe region, and partly of tbe m em bers ofcosm opo litan families naturally falling into m ore or less definite"plant-associat ions" according to habitat. We find the plants oftbe mo untain differing from those of tb e valley, tbe pla nts ofmarsby land differing from tbose of tbe dry plain, tbe plants of tbecoast differing from tbose of tbe interior, tbe plants of tbe sar:d-ilunes differing from tbose of tbe salt-m arsb es. Tbe se differencesar e due to differences in tb e na ture of tbe soil dep en de nt largelyupon tbe geological formations underlying tbe various areas; upondifferences of wa ter-c on ten t of tbe soil depe nd en t pa rtly upon tbenature of tbe soil and partly upon tbe neigbbourbood of largepermanent bodies of water; upon differences of rainfall and of tbebum idity and mo vem ent of tb e a ir ; upon differences of tbe salinityof tbe soil; and upon differences of temperature and illumination.

Tb e m orpbological and pbysiological cb ara cte rs of tbe plantswhicb make up tb e different associations found in tbese differentenvironments are, as is well known, tbe result of adaptation to tbecon ditions of life. Broad ly speak ing it may be said tb a t only thosespecies or individuals wbicb possess, or are capable of developing


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The Problems of Ecology. 195Now it is the study of sucb plant-associations, tbe species and

individuals composing them, in their relations to one another and totheir common environment whicb constitutes the main subjectmatter of ecology.

I think the re is no general fact conn ected w ith plant-distributionwhich ha s struck me personally with gre ater force tha n the veryclose similarity of the plan t-assoc iations living in similar ha bita tsin different par ts of the globe. W ith ce rtain no table excep tionsthe vegetation of the river-side, of the sea-shore, of the alpinesum m its of th e tropics possesses exactly the same cha racteristicsas tha t of a similar locality at home in W est ern E urop e. In spiteof the enormous differences of climate, i.e., of temperature, rain-fall and air humidity, the compelling effect of the topographicalfactor ill forcing, so to speak, the vegetation found in similar typesof habitat along parallel lines, into parallel associations, is every-where seen.

It is obvious th a t th e study of sucb associations falls u nde rtwo heads, correspond ing to th e two stages of procedure inevitablein natu ral science. First, the mainly descriptive stage. Th e plant-associations which are the objects of our study must first bech arac terised, enum erated , and de scribed. Th e different speciesmaking up an association m ust be catalogued, their obvious relationsdescribed, the physical conditions under which they exist taken noteof. T he varia tions of each association m ust be recorded, as wellas the trans itions from o ne to an othe r. Th is is the b usiness ofecological survey. Without it, our knowledge, regarded merely fromthe descriptive standpoint, of the flora of a country or region of theea rth , is entirely one-sided and inade qua te. W e cannot contentourselves merely with the knowledge that certain species of plantsoccu r. W e m ust know exactly how they occu r, in w hat situations,associated with what other plants, before we can have a completem ental picture of the vegetation. Such knowledge is as theoreticallyjustifiable and n ecessa ry a s, and, in fact, is in every respect com parab lew ith, th at gained by a geological survey and the con struction ofgeological m ap s. T he philosoph ers tell us th a t the first object of


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196 A. G. Tansley.description of new species should be the f irst object of the botanicalexplorer. Even then som ething wa s done by the more intel ligentco llecto rs in the w ay of description of the vege tation a s a wh ole, ofthe associat ions in which the spe cies were found. Bu t now, wh enthe vast m ajority of sp ec ies of flowering plan ts at lea st hav e beendescribed, our attent ion m ust be more and more turned towardsacquiring a know ledge of the asso ciatio ns or com bina tions in wh ichplants occur. Even in the coun tries of W ester n Euro pe and America,th e hom e of tbe races by whom the study of plan ts is mainly culti-vated, we are st il l very far from an approxim ately com plete know-ledg e of our plan t-associa tions; thoug h th e work of W arm ing inDenm ark, Flab ault and his pupils in Fr an ce, G raebn er and other sin Germ any, many botan ists in Am erica, and quite recently severalin G reat Britain, espe cially in the north of E nglan d and in Sco tland,has done and is doing som ething to take away this reproach.Schimper's great work to which I have already alluded is amagnificent encyclopaedia of what was known at the date of itspublication of the plan t-associations thr ough out the world, know-ledg e of whicb his own investigation s on tropical floras contributedno small part.Bu t we can not, of cou rse, stop at this first stage . Th ephilosoph ers go on to tell us that the secon d and high est goal ofscien tific cur iosity is to unravel th e causes of phenomena, to gobeyond the intuition of the gross app earan ces of wh at we se earound us, to enquire as to their cau sation, or, to speak moreprecisely, to attempt tbe determination ofthe simple invariable t ime-sequences which exist in nature.W he n w e conside r th e ap plication of this principle to ourecology or topographical physiology, we see that it involves anattempt to determine why the plants which live together on adefinite area with definite environm ental cond itions com e to be thu sassociated how they com e to be and how they maintain them selveswh ere they are, how they com e to exh ibit th e morp hological andphysiological features the y do exhibit , and wh at are tbeir detailedfunctional relations to one ano ther and to their inorganic sur-


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The Problems of Ecology. 197descriptive and the experimental, need not be and should not becompletely separated in practice, though they may be logicallydis tinc t. So far as possible the desc riptive study of plant-asso ciation s should go hand -in-hand w ith e nquiries into the causationof the phenomena involved, but it must be admitted that this idealunion is not alw ays a practicable one . In the case of the b otanicalexploration of a new country, it is clearly out of the question forthe traveller to do much m ore than collect and record the asso-ciatio ns he m eets with. He is fortu na te if he is able to niaUe a fewhasty observations on tem pe ratu re, atm ospheric humidity and thelike, while it is obvious th at no system atic experimental work canbe u nd ertake n. Yet h is collections and reco rds, if intellifjentlym ade , ar e of the gr ea te st use in filling up gaps in our knowledge ofwo rld-vegetation. And the same is tru e 1 think even of our owncou ntry. H ere , of cou rse, th e resident recorder of the broadfeatures of the vegetation has great advantages over the travellingreco rder. He ha s time to prep are vegetation-map s of any degreeof acc urac y and de tail th a t seem s called for, and he can visit hisareas over and over again, noting not only seasonal, but progressivesecular chan ges in the flora. He has leisure to make system aticobservations on soil, temperature and rainfall, and to determinetheir relations with the distribution of vegetation. Botanicalsurvey work of this kind, such as has been extensively carriedout on the practically untouched mountain and moorland plant-associations of Scotland and the north of England during the lastfew ye ars, is of great value, qu ite ap ar t from th at expe rimen talinvestigation which everyone will admit is required to elucidatecom pletely th e ecological relatio ns of plan t life. Broad problem sarise which can be solved only by extensive com parative work inth e field, and until we go into the field and work out the distri-bution of vegetation systematically, we may not even suspect theirexiste nc e. T he experim ental investigator confining himself to alimited flora will necessarily miss altogether unsuspected combi-nations of plants which the surveyor and cartographer of largeareas meets with.


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19'" A. G. Tansley.je t ill its infancy, and th e pioneer work of Drud e, Fla ha ult and th eSm iths has only begun to shew us its po ssibilities. T he variousdifficulties tha t surrou nd it can no t be en tered upon he re, but onepoint of im portan ce may be m entioned. Th e vegetation of an areacan not be presen ted once for all on a single m ap. Different m apsar e required for bringing out different asp ec ts of the flora, and m apson different scales are absolutely es sen tial. A scale suitable forexhibiting the main fea tures of the vegetation of a given regionoften requires supp lem enting by large r scale m aps of portions ofthe region to shew m ore detail. Again a scale ad eq ua te to thedisplay of the leading vegetation-features of one region may shewnothing worth shewing in another part of the country.

For these reason s I should like to see a cen tral com m itteeformed for the system atic survey and m apping of the British Isles.Th e time necessarily taken by the travelling and field work m akesit most desirable that such work should be planned on a compre-hensive scale and the proper co -ordination of th e resu lts obtainedin different parts of the country makes co-operation, frequentconsu ltation of different w orkers, and I think, in th e case of theless experienced, some kind of cen tral direction, of g rea t im portan ce.A further point which such a centra l com m ittee m ight hope to dealwith, and one which if dealt with successfully would do much toremove what I have always thought is one of the most cryingexam ples of the w aste of good work and sound knowledge in thefield of m odern bo tany, is th e utilisation of the work of the localbo tanist a nd the local field club. Sc attere d up and down thecou ntry are scores of men whose hobby is botany and whoseacq ua intan ce with the ir !ocal floras is absolutely une qualled. Toooften they carry with them to their graves knowledge which wouldbe of th e gr ea tes t value in help ing to build up a pictu re of tlievegetation of the cou ntry as a whole. Con vince them of theint ere st of ecological survey work, and you would secu re the irco-operation in working out and mapping local floras from thatpoint of view, which with the requ isite gen eral knowledge of methodsand a certain amount of help and direction, they would do a


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The Problems of Ecology. 199vege tation of an are a as we actua lly see it, gives no doubt am pleopportunity for the waste of time over recording details of triflingim po rtanc e, and this dang er is all tbe gre ate r because tbe bigherscientific development of tbe subject is still in sucb a rudimentarycond ition t ba t we ar e frequently qu ite in tbe dark as to wbat /,s ofimp ortanc e and w ba t is no t. Tb e danger I have tbu s brieflytoucbed upon bas not been without its exemplification in some of tbework tb at ba s been publisbed, and ha s been respo nsible for mucb ofthe distrust of ecological work to which reference was made at theoutset .

Another type of criticism wbicb bas been made, tbougb super-ficially plausible, is I venture to tbink largely due to misapprehension.It bas been o bjected tba t tbe straight-forward mapping of tbevegetation of an are a w ithout the p revious formulation of definiteproblem s for solution is a m istaken and unscientific procedure. Ibave already attem pted to shew tb at it is, on tbe con trary, alegitimate and necessary part of our attempt to become completelyacquainted witb tbe vegetation of the world, and tbat if we neglectit and co nten t ourselves with tbe fioristic system we obtain a one-sided and misleading knowledge. Till a certain number of accuratelyobserved facts reveal tbemselves we cannot possibly formulateproblem s. T be re can be no problems in com parative anatom y tillwe know tbe structure of a certain number of organisms, nor instratigraphical geology till we know tbe distribution and successionot a certa in num ber of rocks. And similarly tbe re canno t beproblems in tbe study of plant-associations till we know tbe natureand distribution of tbe associations in que stion. Tb e problemsarise, and are arising, fast enougb as tbe facts are asce rtained .And tba t m ust be so in all bran cbe s of science W e m ust rem em bertb at tbe system atic study of ecology is no tnore advanced thanplant-anatomy was in tbe time of Von Mobl, or geology beforeLyell, Sedgwick and M urc bis on ; thoug h no do ub t it differs fromthe se sub jects in being a kind of syntb esis of sev eral bran che s ofscience, rather than a new brancb having to develop from thegroun d. It is possible, of co urse , to go on aimlessly piling up facts


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2OO A Second Experiment in Eco logical Surveying.con stantly revealing themselves during the ordinary work ofsurveying, but it is not everyone th at has th e op po rtunity or theskill to und ertake the m . A good deal of sca ttered work in this fieldha s of course been done by various bo tan ists, whom it would besuperfluous to name. One of the mo st im po rtant m eans ofprogress in these higher branches of ecology is undoubtedly theestablishment of laboratories in regions exhibiting specialised typesof veg etation. Th e most rece nt and one of th e m ost no tableexamples of such a laboratory is the Desert Laboratory of theCarneg ie Institution , established in Novem ber, 1903, nea r Tu cson,Arizona, under the d irection of D r. Coville, who se rese arch es onDesert vegetation, published in the Report of the Death ValleyE xpedition, are already well-known to ecologists. Th e Tucsonlabora tory site was chosen after a careful tou r throu gh the No rthAm erican de ser ts by D rs. MacD ougall and Coville. No t only theDirector but visiting botan ists able to make a stay, will work at theecological relations of the desert flora.

W e have no de serts in the British Isles, but we have a fairrange of climatic conditions and a very interesting set of" edaphic " plant formations. In the future we may pe rha ps hopeto see various local sta tions established w here exp erime ntal workmay be carried on.A S E C O N D E X P E R I M E N T IN E C O L O G I C A L SU R V E Y IN G .

ASHORT account was given in this journal (Vol. II., p. 167) of

an atte m pt m ade in July , 1903 to study the vegetation of agiven are a by an expedition from the Botanical D ep artm en t ofUn iversity C ollege, Lo ndo n, composed of the staff and a num berof advanced students.

T ha t expedition was to the "B ro a d " district of E ast Norfolk,and was devoted partly to th e m agnificent fresh-water m arsh-vegetation cha racteristic of the region and partly to the sand-dunesfringing the co ast. Many difficulties were m et w ith. Th e tim eavailable was scarcely long enough to map even a small typical area


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the problems of ecology

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