A N N A L E S D E L A S O C I É T É G É O L O G I Q U E D E P O L O G N E
V o l. L — 1: 161—172 K r a k ó w 1980
Antoni H o f f m a n *
SYSTEM-ANALYTIC CONCEPTUAL FRAMEWORK FOR COMMUNITY PALEOECOLOGY
Analiza systemów a paleosynekologia
A b s t r a c t . A conceptual fram ew ork derived from th e general system theory is here applied to com m unity paleoecology. S ix distinct categories of com m unity developm ent through tim e are defined in term s of system regulation, adaptation, and loss of structural identity. Three of these (ecological stability, ecological resi
lience, and com m unity replacem ent) deal wiith ecological tim e. T hree others (com
m u n ity perm anence, com m unity evolution, and com m unity reorganization) deal w ith evolutionary time. When considering evolu tionary tim e, one deals how ever not as much w ith sin gle com m unities as w ith com m unity types w h ich term is here m eant as a group of com m unities resem bling each other very closely in their taxonom ic com position and ecological structure, and lim ited by th e sam e environ
m ental factors. Some actual exam ples of com m unity developm ent through evolu tionary tim e are interpreted in th e system -analytic terms.
INTRODUCTION
In a rec en t paper, G ould (1977) proposes th a t since th e v e ry incep
tio n of ev o lu tio n ary paleontology th re e m ain questions organized pale
ontological research . One m ay claim th a t th e sam e q u estio n s are also th e m ost p erv asiv e topics of m o dern com m unity paleoecology. In th e la tte r field, one deals h o w e v er w ith a n o th e r o rganizational lev el of or
ganic life and hence, th e q uestions are u su ally ex p ressed in a n o th e r th e o re tic a l language. Since the p io n eer papers b y V alen tin e (1968) an d B re tsk y (1969) appeared, com m unity paleoecologists ask w h e th e r the h isto ry of ecological organization of th e biosphere is u n id ire c tio n a l o r ste ad y -sta tic ; does it d epend m ostly upon e n v iro n m en ta l or biological factors; is it g rad u a l or spasm odic? A ll th ese problem s a re to be ap proached th ro u g h carefu l em p irical studies.
T he aim of th is p a p er is to p resen t a conceptual fram e w o rk fo r such em p irical com m unity-paleoecological investigations. In fact, I feel th a t a fte r the e arly vigor a dozen y ears ago, f u r th e r d ev elo p m en ts in the field of com m unity paleoecology hav e becom e slow. D espite conti
* 00-490 W arszawa, ul. W iejska 14 m. 8.
11 — R o c z n i k P T G 50/1
nuous effo rts by S c o tt (1972, 1974, 1976, 1978) to provide sta n d ard s fo r paleocom m unity descrip tio n (see also H offm an 1978a, H offm an &
al. 1978), th e d ata p re se n te d b y d iffe re n t a u th o rs a re u su ally incom patible. The in v estig atio n s a re com m only organized as p u re ly descrip
tiv e w orks w ith o u t an y refe ren ce to the th eo re tic al background; or at b est, as m ere illu stra tio n s of ecology-derived p rin cip les (th ere are ob
viously a few o u tstan d in g exceptions). The theories, concepts, and term s h av e becom e so vague and cloudy th a t th ey can n o t even p rovide th e paradigm needed b y a no rm al science to develop. I believe th a t system m ethodology m ay prove u seful a t least in clarify in g th e ideas, p roviding precise term in o lo g y and m ore or less unequivocal criteria, an d o rg a n izing th e scope of a fu tu re com m unity-paleoecological research .
In th is paper, I assum e th a t com m unities a re open system s (sensu B e rta la n ffy 1968). In o th e r w ords, I re g a rd co m m u n ity s tru c tu re s as m ore or les in te g ra te d and hom eostatic w ebs of biological in teractio n s, im posing some c o n strain ts upon b o th ecological an d e v o lu tio n ary b e
h a v io r of the c o n stitu en t populations. This point is in h o t d isp u te am ong ecologists. One m ay a c tu ally suppose th a t th e above assum ption is valid only fo r those com m unities se p ara ted clearly fro m o th e r com m unities by sh a rp e n v iro n m en ta l g rad ien ts. In fact, th e v e ry concept of ecolo
gical and e v o lu tio n ary e q u ilib ria deriv ed fro m th e th e o ry of island brageography (M acA rtur & W ilson, 1967; W ilson, 1969) recalls th e no- .tio n of open system s.
W hen co m m u n ities are tre a te d in term s of open system s, th e ir b e h av io r m u st be m ad e clearly d istin c t from th a t of th e ir c o n stitu en t populations. T his d istin ctio n does not im ply th a t one m ay a ttrib u te a selection value to com m unities. N e ith e r com plexity, stab ility , nor even efficiency of com m unities is selected for in ecological tim e. In co n trast, such “re w a rd s ” m ay h in d e r a com m unity fro m p ersistin g H olling, 1973). This is also th e case in ev o lu tio n ary tim e (H offm an,
1978b). T h ere is no ecological or ev o lu tio n ary process o p e ratin g d ire c t
ly upon com m unities. C om m unities a p p ear a c tu a lly as m ere epipheno- m en a of species evolution (H offm an, 1979a) b u t n ev erth eless, th e ir eco
logical and ev o lu tio n ary b eh av io r m ay be irred u cib le a n a ly tic ally to species-level processes.
SYSTEM BEHAVIOR
W einberg (1972) h as re c e n tly developed a conceptual fram ew o rk fo r analysis of sy ste m b eh av io r in tim e,
O w ing to the hom eostatic p ro p erties of its stru c tu re , an open sys
te m p erfo rm in g its n o rm al fu n ctio n m ain tain s a ste ad y state. S y stem fu n ctio n reflects th e sy stem ch aracteristics w hich is h e re m e a n t as a set of ru les d ete rm in in g th e w ay th e sy stem responds to a d v a n
tageous events in its su rro u n d in g s. As a firs t ap p ro x im atio n , com
m u n ity c h aracteristics can be described in te rm s of en erg y exchange w ith th e im m ediate en v iro n m en t an d en erg y flow th ro u g h th e tro p h ic s tru c tu re .
A sy stem responds also to d isad v an tag eo u s events. T he reactio n is calle d re g u la tio n if a d istu rb e d sy stem does n o t change its c h a ra c te r
istics w h en ten d in g to an e q u ilib riu m state. T hen, the sy stem re tu rn s to its original stead y state.
D isadvantageous events m ay h o w ev er d istu rb a sy stem so sev erely t h a t a reg u la tiv e response ap p ears in su fficien t to achieve a n e q u ilib riu m . T he reactio n is called a d a p tatio n if a d istu rb ed sy stem does change
its ch aracteristics in o rd er to endure.
V ery severe d istu rb an ces req u ire of a sy stem v e ry fa r-re a c h in g ad ap tatio n s. This m a y re s u lt in a loss of sy stem id en tity . In fact, th e re a re no unequivocal c riteria to d istin g u ish clearly b e tw een a p o st-ad ap ta tio n system sta te and a new system .
COMMUNITY DEVELOPMENT THROUGH ECOLOGICAL TIME
A com m unity d istu rb ed by som e en v iro n m en ta l facto rs is degraded, th a t is som e of its c o n stitu en t populations a p p ea r u n ab le to re sist th e stre s s an d becom e e x te rm in a te d . W hen th e prev io u s e n v iro n m en tal
•conditions are re-estab lish ed , a deg rad ed co m m u n ity m ay r e tu r n to its o rig in a l e q u ilib riu m state . T he com m unity s tru c tu re refle c tin g sy stem ch a rac te ristics rem a in s th e n u n ch an g ed and hence, th e co m m u n ity r e sponse can be tre a te d in te rm s of sy stem reg u latio n . T his ab ility of co m m u n ities to re tu rn to th e ir original e q u ilib riu m sta te a fte r a tem po
r a r y d istu rb an ce was te rm e d by H olling (1973) ecological stab ility . A n e n v iro n m en tal d istu rb an ce m ay also be so severe as to induce sig n ifican t changes in th e s tru c tu re of a degraded co m m u n ity tending to an e q u ilib riu m state. T he c o n stitu en t p o pulations and th e ir in te r actio n s m ay rem a in the sam e as th ey w ere b u t th e p o p u latio n sizes and hence, th e p a tte rn of en erg y flow th ro u g h th e ecosystem m ay becom e sh a rp ly d iffe re n t due to som e u n iq u e h isto rical events. In fact, n a tu ra l com m unities do often display m u ltip le e q u ilib riu m poin ts (S u th erlan d , 1974; H orn, 1976). One m ay th e n claim th a t th e com m unity c h a ra c te r
istics h as changed. N evertheless, th e id en tity of ecological sy stem is m a in ta in e d p e rfe c tly an d hence, th e co m m u n ity response is to be tre a te d in term s of sy stem ad ap tatio n . This ab ility of com m unities to absorb change an d d istu rb an c e an d still m a in ta in th e sam e w eb of bio
logical in te ra c tio n s w as te rm e d by H olling (1973) ecological resilience.
In response to v e ry sev ere and lo n g -lastin g e n v iro n m en ta l d is tu r
bances, a com m unity m ay receive ecologically new species. T his re s u lts fin ally in a loss of th e com m unity id en tity . Such process of a fu n d a -
u "
m en tal change in com m unity com position due to a change in en v iro n m en ta l conditions m a y be te rm e d com m unity rep la ce m e n t (cf. H off
m an & N arkiew icz, 1977). A ccording to R. G. Jo h n so n (1972), th is p ro cess includes tw o d istin c t ph ases in te rg rad m g one into th e other;
nam ely, ecological d e g rad atio n of th e original .com m unity and. in tro d u c tion an d su b se q u e n t dev elo p m en t of th e new com m unity.
T h ere is a re a l co n tin u u m of ecological e v en ts b e tw een the ex
trem es of a co m m u n ity resilien t response and co m m u n ity rep lace m e n t.
The b o u n d ary m ay be tra c e d b u t a rb itra rily . M ore or less precise id en tity of com m unity taxonom ic com position is h e re proposed fo r th e criterio n of ecological resilience.
COMMUNITY DEVELOPMENT THROUGH EVOLUTIONARY TIME
Scaling m ay hav e its u su al effects on co m m u n ity d ev elo p m en t a n d th erefo re, ecological tim e m u st be m ade clearly d istin ct fro m evolu
tio n a ry tim e. E v o lu tio n ary tim e is com patible w ith specific longev
ities. A ctually, changes occurring in ecological tim e im p e rc e p ta b ly in te rg rad e into changes occu rrin g in ev o lu tio n ary tim e. H ow ever, th e processes can be d istin g u ish ed a t least conceptually.
W hen considering ev o lu tio n ary tim e, one deals n o tasm u ch w ith sin
gle com m unities as w ith com m unity types, since com m unities th e m selves disapear as soon as th e ir p a rtic u la r biotopes v an ish o r undergo m ajo r changes; re c a ll the Q u a te rn a ry h isto ry of In d o -F acific coral ree f ecosystem s (C happell, 1974; T aylor, 1978). T he te rm co m m u n ity ty p e is h e re m ean t as a group of com m unities resem b lin g each o th er v e ry closely in th e ir taxonom ic com position an d ecological s tru c tu re , and lim ited by th e sam e en v iro n m en tal factors. T he taxonom ic com position m ay v a ry am ong p a rtic u la r com m unities b u t th e rep la ce m e n t species are v e ry close ecologically to each other. D isp ersal of th e larv a e and m ig ratio n of th e a d u lt in d iv id u als p e rm it a continuous exchange of species b etw een com m unities assigned to a single co m m u n ity type.
M oreover, such com m unities m ay also sh are a se t of h o m eo static m e
chanism s developed th ro u g h co-evolution am ong th e species. Thus, com m unity ty p e can be reg ard ed as a biological u n it, eventhoug'h h id den in a b ack g ro u n d of th e ecological org an izatio n of th e biosphere.
M etaphorically speaking, th e relatio n sh ip b etw een com m unities and com m unity ty p es ap p ears analogous to th a t b etw een local p o p u latio n s and species. In fact, com m unity ty p e can be reg a rd e d as a pool of sp e
cies or a ran g e of taxonom ic v a ria tio n w ith in th e co n stan t fra m e w o rk of an ecological stru c tu re . T he concept of com m unity ty p e m ay resem ble T h o rso n ’s (1957) p a ra lle l com m unities b u t I believe th e fo rm e r one to be m u ch less vague. A ctually, m u ch precise ecological w o rk is
needed in o rd er to p e rm it th e u ltim a te assig n m en t of som e com m unities, or paleocom m unities to a single com m unity type.
L et us consider a com m unity type. U n d er sig n ifican tly changing e n v iro n m e n ta l conditions, it has to resp o n d if it is to e n d u re because a t least some c o n stitu en t ta x a are affected by th e stress. T he affected ta x a m ay ad ap t p e rfe c tly to th e new en v iro n m en t, e v en th o u g h p erh ap s u n d e r the form of descen d an t species. T hen, th e co m m u n ity ty p e p e r
sists th ro u g h ev o lu tio n ary tim e owing to su b seq u en t co -ad ap tatio n (if needed) of o th e r com m unity m em bers. T he ecological s tru c tu re reflec
tin g sy stem ch aracteristics rem ain s co n stan t provided th a t com m uni
ties of d iffe re n t geological ages display a taxonom ic id e n tity a t th e generic level, whiich m ay be reg a rd ed as a c riterio n fo r sy stem reg u la tion. I propose to re s tric t th e use of th e te rm com m unity p e rm an en ce to th is p a rtic u la r category of c o m m u n ity -ty p e developm ent.
T ax a u nable to evolve into descen d an ts a d ap ted to th e new en v i
ro n m e n ta l conditions m ay be replaced b y som e o th e r ta x a u n ad ap ted to th e original biotope. D espite th is in tro d u c tio n of new com m unity m em bers, th e we!b of biological in teractio n s m ay rem a in m ore or less co n stan t due to th e ecological equivalence of p a rtic u la r rep lacem en t taxa. C om m unities of d iffe re n t geological ages ap p ea r th e n hom eom or- phic in th e ir ecological s tru c tu re despite th e ir considerable taxonom ic v ariatio n . N evertheless, th e p a tte rn s of en erg y flow and hence, th e sys
tem ch aracteristics a p p e a r u su ally q u ite d iffe re n t, w hich show s th a t one deals h ere w ith sy stem ad ap tatio n . I propose to re s tric t th e use of the term co m m u n ity evolution to th is p a rtic u la r categ o ry of com
m u n ity -ty p e response in e v o lu tio n ary tim e. In fact, th is d efinition seem s to be less vague th a n used th u s fa r in com m unity paleoecology (cf. V alen tin e, 1968; B retsk y , 1969; Boucot, 1975, W atk in s & Boucot, 1975). It is ad v an tag eo u s because th e c riterio n of s tru c tu ra l hom eo- m o rp h y am ong com m unities can be tre a te d a t least s e m i-q u a n tita ti- v ely (H offm an & al. 1978).
Ecological s tru c tu re of a com m unity ty p e m ay also u n d erg o m ajo r changes due to th e lack of ecalogical eq uivalence b etw een p a rtic u la r rep lacem en t taxa. T hen, th e ecological sy stem does n o t only change its c h aracteristics b u t also loses its s tru c tu ra l id en tity . T he sy stem id en tity can be tre a te d m e re ly in fu n ctio n al term s in such a case. The fu n ctio n of a com m unity ty p e w ith in th e w hole biosphere is here m ea n t as th e position of th e involved com m unities in a specified set of p h y sical en v iro n m en ts. One m ig h t suppose th a t s tru c tu ra l id e n tity is a n e ce ssa ry p rere q u isite to the fu n ctio n al id e n tity of a system . In fact, w ere th e biosphere co n stan t th ro u g h o u t ev o lu tio n ary tim e, a specified' se t of e n v iro n m en ta l conditions w ould stric tly d e te rm in e th e stru c tu re of resp ectiv e com m unity type. A nd y e t th e b iosphere does change. T h ere fore, constancy of co m m u n ity -ty p e fu n ctio n m ay or m ay not be re
lated to the constancy of ecological stru c tu re . I propose to use the term com m unity reo rg an izatio n fo r such a response of co m m u n ity types in ev o lu tio n ary tim e w hich leads to the sy stem b rea k -d o w n and loss of its s tr u c tu r a l id e n tity .
T h ere is a re a l continuum of evolutionary-ecological ev en ts b e
tw e e n th e e x tre m e s of co m m u n ity evo lu tio n an d com m unity reo rg a n iza tion. One m ay claim th a t w h en a considerable p ro p o rtio n of ecologi
cally new tax a (rep re se n ta tiv e of p rev io u sly a b se n t fam ilies) ap p ea r w ith in a com m unity type, th e process is to be assigned to th e la tte r
•category.
R eorganized s tru c tu re of a com m unity ty p e m ay also becom e q u ite sim ila r to the ascen d an t one. Such a co m m u n ity congruence ap p ears e x tre m e ly close to th e effects of com m unity evo lu tio n as defined in the p resen t paper. N ev erth eless, one can h a rd ly ex p ect th a t com m uni
ties larg ely d iffe re n t in taxonom ic com position w ill ex h ib it identical w ebs of biological in terac tio n s and id en tical p a tte rn s of en erg y flow;
unless th e v e ry concept of com m unity s tr u c tu r a l id e n tity is reduced to m ere resem blance of organism -ibiotope relatio n sh ip s.
ADEQUACY OF THE CONCEPTUAL FRAMEWORK
A dequacy of th is conceptual fram ew o rk to th e actu al d evelopm ent of com m unities th ro u g h ecological tim e can be fin a lly teste d only by ne о ecologists. As a paleoecologist, I can m ere ly assum e its a p p ro p ri
ateness to v e ry precise resea rch on sh o rt-te rm s tra tig ra p h ie sequences like those stu d ied by W alker & A l'berstadt (1975), H offm an (1977,
1979b), or M. E. Johnson (1977).
W hen dealing w ith ev o lu tio n ary tim e, adequacy of th e sy stem m od
el and term in o lo g y to th e actu al d ev elo p m en t of co m m u n ity ty p es can be tested solely in th e fossil record. Then, all the lim itatio n s and biases in h e re n t in each paleoecological in te rp re ta tio n m u st be alw ays k ep t firm ly in m ind (cf. S tan to n , 1976; S tan to n & Dodd, 1976; F ü rsich , 1978; H offm an, 1979a, b). T h u s far, v e ry few ex am p les h a v e been described p recisely enough to p e rm it th e ir un eq u iv o cal assig n m en t to th e above-defined categories of c o m m u n ity -ty p e developm ent th ro u g h ev o lu tio n ary tim e.
C om m unity perm an en ce m ay a p p e r som ew hat a b stra c t or even u n rea listic . N evertheless, som e fa irly clear exam ples have a lre a d y been
docum ented. B en tb ic com m unities dom inated b y tu rrite llid gastropods are w ell know n to occur in alm ost all tro p ic al to cool R ecent seas.
Ow ing to th e ir greg ario u s h a b it an d hig h com petitive p o te n tia l for space at and ju s t below th e se d im e n t-w a te r in terface, these larg ely .sessile gastropods u su a lly m ake th e in fa u n a l h a b ita t inaccessible fo r
m ost o th er m acro b en th ic in v erteb ra te s. P re d a to ry n aticid gastropods are h o w ev er an exception. Thus, th e ecological s tru c tu re c h arac teristic of th is com m unity ty p e ap p ears v e ry sim ple. T u rrite llid -d o m in ated , h ig h -d en sity , lo w -d iv ersity paleocom m unities have been com m only re corded a t least since th e P aleocene (e. g. H ecker & al. 1063; M enesini, 1976; H offm an, 1977). The lon g -lastin g h isto ry of th is c o m m u n ity ty p e m ay provide an exam ple of com m unity perm anence.
D evelopm ent of coribulid-dom inated, h ig h -d en sity , lo w -d iv ersity com m unity ty p e p e rsiste n t u n d e r th e sam e fo rm an d in th e sam e en
v iro n m e n t at least since th e Paleocene to M iocene (e.g. H ecker & al.
1963; B oekschoten, 1963; B âldi, 1973; H offm an 1977) m ay also be a ttr ib u te d to com m unity perm anence.
B oth these com m unity types possess som e C retaceous c o u n te rp a rts (Scott, 1974; Sohl, 1977). H ow ever, in b o th th e cases th e C retaceous com m unities a p p ea r to have been m ore species-diverse an d less d en sely packed th an th e ir Cenozoic descendants. T his difference- m ay be a ttrib u te d to com m unity evolution consisting in in tro d u c tio n of th e n aticid p red a to rs to these a n cien t co m m u n ity types an d th e ir effect upon th e tax a show ing a low er rep ro d u ctiv e p o ten tial th a n th e tu r r i- tellid s and c'oa foul ids.
D evelopm ent of tro p ical seagrass-associated com m unity ty p e a t least since th e M iocene th ro u g h R ecent p ro b ab ly re p re se n ts a n o th e r exam ple of ccm m u n ity evolution. In fact, th e ecological s tru c tu re of sem e Miocene paleocom m unities has re c e n tly been d e m o n stra te d to r e sem ble v e ry closely th e p re se n t-d a y seagrass-associated m acrobenthic com m unities (H offm an 1977). N evertheless, som e ta x a ra re or" even a b sen t fro m those M iocene paleocom m unities occur r a th e r com m only in th e ir R ecent c o u n te rp a rts (e. g. tellinid, p in n id , and ep ib y ssate arcid bivalves); w hile some fo rm e rly com m on ta x a have becom e v e ry ra re or ab sen t from the m o d ern seagrass beds (e. g. nucuiloid bivalves and scaphopods). Such an assig n m en t of th e developm ent of th e tro p ic al seagrass-associated com m unity ty p e is h o w ev er a m ere supposition for th e m om ent since th e in te rp re ta tio n is based upon fa irly v ague q u ali
ta tiv e criteria. M uch w ork on bo th an cien t an d m o d ern com m unities of th is ty p e is needed before th e use of m ore rigorous c rite ria w ill be allowed.
The M iddle Paleozoic developm ent of carbonate in te rtid a l and sh al- lo w -su b tid al com m unities leading u ltim a te ly to th e s tru c tu ra l congru
ence of paleocom m unities w idely se p ara ted in e v o lu tio n ary tim e (W al
k e r & L aporte, 1970) provides a clear exam ple of com m unity re o rg a n ization.
CONCLUSIONS
One m ay conclude th a t th e sy ste m -an a ly tic term inological fram ew o rk (Table 1) ap p ears applicable to n a tu ra l com m unities and com m unity types. It is ad v an tag eo u s because its th eo re tic al b ack g ro u n d is v e ry clear, th e te rm s a re p recisely defined, and se m i-q u a n tita tiv e c rite ria can be easily applied (cf. th e m ethodology of paleo co m m u n ity descrip tio n in H offm an & al. 1978). T hen, th e ab o v e-o u tlin ed fram ew o rk m ay su b s ta n tia lly fac ilitate conceptualization of th e m ajo r field of change in ecolo
gical organization of the biosphere th ro u g h tim e.
Up to date, th e re is no su fficien t em pirical evidence to recognize w h e th e r th e ecological organization of the b io sp h ere evolves a t a slow b u t noticeable ra te , or w h e th e r m ost co m m u n ity types u n d erg o b u t m ajo r ibreaks-dow n an d reorganizations. Possibly, b o th th ese processes are equally com m on. As a m a tte r of fact, even th e v e ry m echanism s of com m unity developm ent th ro u g h e v o lu tio n ary tim e, involving su p posedly co-evolution am ong sev eral taxa, rem ain unclear. O ne m ay only claim th a t th ey a re to be an aly sed w ith in lo n g -lastin g h a b ita ts w here th e m isleading effects of species im m igration and local ex te rm in a tio n can be easily recognized; o therw ise, co m m u n ity rep la ce m e n t m ig h t be m isin te rp re te d as com m unity evolution or reo rg an izatio n .
As to the facto rs c o n tro llin g co m m u n ity -ty p e developm ent th ro u g h e v o lu tio n ary tim e, one m ay only sta te th a t th e re is no single m ajo r fe a tu re of com m unity types responsible fo r th e ir a b ility to develop or persist. In fact, th is ab ility a p p e a rs re la te d to b o th the v irtu a l e u ry - to p y of co n stitu e n t species and th e sim plicity of ecological s tru c tu re (H offm an, 1978b). S im plicity of a com m unity s tru c tu re (and its oppo
site, com plexity) is h e re m ea n t as a m easu re of in terd ep en d en c e am ong th e com m unity m em bers. A co m m u n ity com posed exclusively of m u tu a lly in d ep en d en t p o pulations is to b e re g a rd ed as an e x tre m e ly sim p le one. W hen an ecological s tru c tu re is v e ry sim ple and v irtu a l e u ry -
to p y .of th e species v e ry large, c o m m u n ity p e rm a n en c e can b e expected.
W hen an ecological s tru c tu re is v e ry com plex an d v irtu a l e u ry to p y of th e species v e ry sm all, even a slig h t change in en v iro n m en ta l condi
tions m ay re s u lt in com m unity reorganization. B e tw een these tw o e x trem es. th e re is a co n tin u u m of evolu tio n ary -eco lo g ical ev en ts reflected in m ore or less d ram a tic changes in com m unity com position and stru c ture.
To stu d y th e freq u e n c y d is trib u tio n of p a rtic u la r categories of com m u n ity -ty p e d ev elo p m en t th ro u g h e v o lu tio n ary tim e an d its dependence upon v ario u s e n v iro n m en ta l as w ell as (biological facto rs is th e m eans of recognizing th e mode <of ecological evolution of th e biosphere.
M anuscript rece ive d II 1979 accepted V 1979
A c k n o w l e d g e m e n t s . I g re a tly b e n efitte d from discussions w ith P e te r B retsk y , A ndrzej Gecow, A dam Łom nicki, and M arek N arkiew icz.
I am also g re a tly in d eb ted to Jam es V alen tin e and A r th u r B oucot fo r th e ir criticism of an e a rlie r d ra ft of th is paper.
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STRESZCZEN IE
P rzedm iotem p racy je s t próba stw o rzen ia sy stem u pojęciow ego od
pow iedniego do rozw ażań nad rozw ojem biocenoz zarów no w czasie eko
logicznym , jak i ew olucyjnym . P odstaw ow ym założeniem całej p racy jest, że biocenozy m ożna trak to w ać jako sy stem y o tw arte, że — innym i słow y — s tru k tu ra biocenozy to m niej lub b ard ziej zin teg ro w an a i ho- m eosta tyczna sieć in te ra k c ji biologicznych n arzu cająca ograniczenia rozw ojow i poszczególnych populacji. Założenie to nie pociąga jed n ak za sc-bą p rzy p isan ia /biocenozom jak iejś w arto ści selek cy jn ej. Nie m a ża
dnego procesu ekologicznego czy ew olucyjnego działającego bezpośre
dnio na biocenozy, a sam e 'biocenozy to nic innego jak p rze ja w ew olucji gatunków . Ich rozw oju w .czasie ekologicznym i ew o lu cy jn y m m oże się jed n ak nie dać sprow adzić an ality czn ie do sam ych tylko w łasności ga
tunków .
Zdolność biocenoz do po w ro tu do sw ego p ierw o tn eg o sta n u rów no
w agi po chw ilow ym w ah n ięciu w a ru n k ó w ekologicznych określa się jako stabilność ekologiczną. O dpow iada ona reg u la cji system u. G d y s tru k tu ra ekologiczna w y trąco n ej ze sta n u rów now agi biocenozy osiąga now y stan rów now agi, zachow ując jed n ak w szy stk ie p o p u lacje sk ład o we, mówić m ożna o a d a p ta c ji sy tem u . Tę zdolność biocenoz określa się jako elastyczność ekologiczną. G dy n ato m iast w sk u tek zm iany w a ru n ków ekologicznych zm ienia się skład biocenozy, a zatem tra c i ona sw oją tożsam ość, mówić m ożna o następstw ie- biocenoz. W k a te g o ria c h analizy system ów nie sposób p rzeprow adzić jednoznaczną granicę pom iędzy a d ap tac ją sy stem u a k o m p letn ą jego przebudow ą. G ranicę m iędzy ela
sty czn ą reak cją biocenozy a n astęp stw em biocenoz m ożna w ięc p rz e prow adzić tylko a rb itra ln ie . A dekw atność ty c h trzech podstaw ow ych k ateg o rii ekologicznych do rozw oju biocenoz w czasie ekologicznym spraw dzić m ożna jed y n ie na m ateria le w spółczesnym .
K iedy m ow a o czasie ew olucyjnym , rozw ażać trzeb a nie ty le po
jedyncze biocenozy, ale ich typy. T yp biocenoz rozum ie się tu ta j jako g ru p ę biocenoz bardzo do siebie zbliżonych pod w zględem sk ład u tak so nom icznego i s tr u k tu r y ekologicznej i zam ieszkujących tak ie sam e śro dow iska. G dy takie sam e środow iska zam ieszkane b y ły w dw óch m o
m en tach czasu ew olucyjnego przez biocenozy identyczne pod w zględem składu taksonom icznego (na poziom ie rodzaju), m ówić m ożna o p rze trw a n iu biocenozy. O dpow iada to reg u lacji system u. G dy tak ie bioce
nozy różnią się pow ażnie składem taksonom icznym , ale są hom eom or- ficzne s tru k tu ra ln ie , m ówić m ożna o ew olucji biocenozy, oo odpow iada ad ap tac ji system u. K ied y jed n ak u traco n a zostanie n a w e t tożsam ość s tru k tu ra ln a i jed y n ą w spólną cechą rozw ażanych biocenoz będzie ten sam c h a ra k te r ich środow iska fizycznego, m am y do czynienia ze św ia
dectw em ‘ reo rg an izacji ekologicznej. I znów — granicę m iędzy ew o lu -
с ją biocenozy a reo rg an izacją ekologiczną przeprow adzić m ożna je d y nie a rb itraln ie .
Ż eby spraw dzić adekw atność ty ch k ateg o rii do ro zw o ju ty p ó w bio
cenoz w czasie ew olucyjnym , zastosow ano je do k ilk u p rzy k ła d ó w zna
n y ch z lite ra tu ry . O kazuje się, że w dziejach b io sfe ry rzeczyw iście n a potkać m ożna zarów no p rze trw an ie i ew olucję biocenoz, jak re o rg a n i
zację ekologiczną. Z aproponow any tu sy stem pojęciow y nadać się w ięc może do ro zstrzygnięcia k w e stii, w jak i sposób zm ienia się organizacja ekologiczna biosfery. J a k dotychczas 'bowiem za m ało udało się zebrać p o ró w n y w aln y ch d an y ch paleosynekologicznych, by m ożna 'było na to p y tan ie odpowiedzieć.
