The role of competition in extinction

Written discussion of Presidential Address

The Editor, 10 November 1977

Dear Sir,In his 1976 presidential address D. V. Ager wrote 'so the exit of the dinosaurs can probably beblamed partly on the climate and partly on a floral change which was itself a climatic cumevolutionary matter'. Ager goes on to relate periods of extraordinarily rapid change to thepossible simultaneous peaking of oscillations in a number of independent cyclical factors. I donot dispute the significance of these ideas but I hope a mere biologist may suggest an additionalingredient: competition. Competition is already implicit in a number of Ager's examples but isnever explicitly hailed as a likely major component of evolution .

It is my view that phyletic gradualism and punctuated equilibria are just the two extremes of acontinuum of modes of handing on a habitat down the generations. With respect to thestratigraphic column there are three ways in which succession could appear in a given area:

(i) The niche-boundaries change little and the niches are handed down to direct descendants.The rate of evolution depends on the degree of change of the niches . As long as the genetic andreproductive potential of the niche occupants is adequate to provide an evolutionary rate equalto the niche-change rate then this mode could go on forever. A good example is the lowerJurassic Gryphaea lineage now rescued by Hallam (1968). In general, though, periods ofniche-stability may have resulted in periods of little morphological change e.g. many of theLimid species revised by Cox (1943; 1944) have life-spans exceeding the length of the Bajocian.Detailed evolutionary work on groups of this kind is lacking, perhaps due to the lack ofstratigraphical usefulness of species with such long lifespans. Cox (1944) did mention thatPseudolimea duplicata was edentulous and had 24 ribs in the Upper Lias but small pseudo-taxodont teeth and only 21 ribs in the Cornbrash. For such species the nemesis of debunking willhave to wait until an evolutionary lineage has been established.

(ii) The niche-boundaries change little but the niches are taken over by invading species whooutcompete the descendants of the original occupants. As far as the occupation of stable-nichesgoes possession is worth even more than nine-tenths of the law. Occupiers of adjacent nichesproduce mainly offspring which are well-suited to the role of their parents; the few who couldsurvive across a broader part of the ecosytem will not be the best-suited to any part of it and willnot survive in the presence of competitors. There may, however be similar niches occupied bydifferent species in different areas; in this case if migration becomes possible the resultingcompetition may lead to rapid niche-expropriation . Ager's example of red versus grey squirrelsfits in here. Another example is the takeover of the South American mammal niches by invadingNorth American species when the land bridge was formed during the Pliocene (Hallam, 1973).If the invaders come from a small area or from an area with a poor fossil record it may be difficultto discover their ancestors; this problem is made doubly difficult if the invasion is followed by ashort burst of rapid evolution. The latter is not unlikely as the very act of niche-takeover changesthe biotic boundaries of other niches; when a whole community is taken over the evolutionaryjostling to fit the new niche-boundaries may be considerable. A small time-scale example is seenin the invasion of North America by European house sparrows (Passerdomesticus) . A little over

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a hundred years ago saw the deliberate release, by man, of a population drawn from a singlelocality; since that time they have spread all over North America, differentiating into differentraces to suit different physical conditions. Modern populations vary from larger birds in thenorth as compared with the south with darker individuals along the coast as against paler ones inthe arid interior. The differences are great enough for birds from some populations to bedistinguished with 100 per cent accuracy and they are also clearly adaptive being respectively inaccordance with Bergmann's and Gloger's ecological 'rules' (Johnston & Selander, 1971).

(iii) The niche-boundaries change relatively rapidly compared with the evolutionary rates ofthe niche-occupants. After a few generations the board is swept clean in the sense that all theniches are new; there are no niche-possessors as such and the ecosystem consists of a number ofunbounded niches and a plethora of competing organisms in an evolutionary free-for-all tobecome best adapted to some part of the mutual habitat. The problem for the locals isexacerbated if rivals who are already suited to the new conditions move in from outside.

It might be thought that the great depression in numbers oftaxa (House, 1971) at the end ofthe Cretaceous supported the idea of direct climatic extinction as such, unmitigated by competi-tion. Not so. During times of niche-stability ecological specialists are at an advantage and thisresults in the establishment of fewer, broader niches. In other words habitats with low niche-stability also have low species diversity (Slobodkin & Sanders, 1969).

The niche-takeover is completed when the rate of climatic change slows down to within therate of biotic evolution. When this occurs descendants of the generalists can specialise withintheir broad ancestral niches and a burst of adaptive radiation occurs. Increasing niche-stability isthus soon accompanied by increasing species diversity.

Phyletic gradualism, then, consists of successful interspecific niche-defence with intraspecificcompetition enabling the species to evolve enough to keep up with small niche changes.Punctuated equilibria occur when interspecific competition results in niche-expropriation. Thehanding on of niches within a habitat could occur by 100 per cent of one or 100 per cent of theother or by any proportion in between. A suitable example of the latter involves the Bajocianbivalves already referred to; these animals retained their niches over several zones whereasbrachiopods in the same rocks had theirs expropriated with great frequency (Ager, 1976). Thiscould have been due to the brachiopod niches being more prone to instability or to the morefrequent existence of potential invaders in neighbouring areas or to specialised brachiopodsbeing less able to keep up with small niche-changes or to any mixture of these. Whatever thecause the overall habitat picture is one of neither extreme conservatism nor absolute radicalism.

The time-scale for bursts of adaptive radiation is short by geological standards: the age of theGalapagos Islands is about 2 Ma (Open University, 1971) and within this time soil formationand floral establishment were necessary before animals could colonise. This leaves a time-scaleof the order of a million years as the maximum for an original population of finches to reach andspread across the islands and to diversify into today's thirteen species.

In one very pronounced way, the mammals versus the reptiles is not a particularly outstandingexample of competition having a major role in extinction, as mammals have an undoubtedphysiological advantage in cooler climates. In this respect the brachiopod-pelecypod dual is abetter example as the pelecypods' success, owed to the achievement of mantle-fusion (Stanley,1968) and better osmo-regulatory powers, cannot be directly related to a change of climate.

Physiologically or morphologically disadvantaged groups have, however, done well in theabsence of competitors: the Marsupials in Australasia, Sphenodon in New Zealand, theLemuridae in Malagasy, the finches et at. of the Galapagos, the red squirrels before the greysand, most significantly, the one group of large reptiles that did survive, the crocodiles. Why did

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they survive when other large reptiles did not? The answer could be a lack of appropriatecompetitors. If this is so then we must consider it a possibility that direct descendants of thedinosaurs could have evolved to occupy Cainozoic niches if only the mammals had not got therefirst.

REFERENCESAGER, D. V. 1976 . The nature of the fossil record.

Proc. Geol. Ass., 87, 131-{j0.COX, L. R. 1943 . The Engli sh Upper Lias and Inferior

Oolite species of Lima. Proc. Maloc. Soc., 25, 151-87.COX , L. R. 1944. OnPseudolimea Arkell. Proc. Maloc.

Soc., 26, 74-88.HALLAM, A. 1968. Morphology, palaeoecology and

evolution of the genus Gryphaea in the British Lias.Phil. Trans. Roy. Soc., B. 254,91-128.

HALLAM, A. 1973. Distributional patterns incont emporary terrestrial and marine mammals .Palaeont. Ass. : Special papers in Palaeontology, 12,93-105 .

HOUSE, M. R. 1971 . Evolution and the fossil record .In: l. G. Ga ss, P. J . Smith & R. C. L. Wilson (edits.) :Understanding the Earth. Artemis Press, Sussex.

JOHNSTON, R. F. & R. K. SELANDER, 1971 . Evolu-tion in the house sparrow. II. Differentiation in NorthAmerican populations. Evolution, 25, 1-28.

OPEN UNIVERSITY. 1971. Unity and diversity. (Sci-ence Foundation Course Unit 21). Open UniversityPress, Milton Keynes.

SLOBODKIN, L. B. & H. L. SANDERS. 1969. On thecontribution of environmental predictability tospecies diversity. Brookhaven Symp. in Bio., 22,82-95.

STANLEY, S. M. 1968. Post-palaeozoic adaptive radia-tion of infaunal bivalve molluscs-a consequence ofmantle fusion and siphon formation. J. Palaeont., 42,214-29.

J. P. A. AngseesingSI. Paul' s College,Cheltenham,GL504AZ.

The Editor,

Reply by the author

18 November 1977

Dear Sir,I agree with almost everything Dr. Angseesing says, we differ chiefly in emphasis. Competitionis implicit in every ' take-over bid' that I postulated in my address. Allopatric speciation-ofisolated populations-produces new forms which are more successful than the old ones incompetition for the same ecological niches. If they were not more successful in competition wewould probably know nothing about them. But biologist friends have told me that competition isa very difficult process to prove, even at the present day, so I was all the more hesitant aboutpostulating it in the palaeontological past. No one has, so far as I know, ever observed a greysquirrel grabbing an acorn from the paws of a red squirrel let alone a bivalve out-sucking abrachiopod for organic particles in the sea! I am particularly interested in the way bivalvesevidently 'took over' from the brachiopods, as mentioned by Dr. Angseesing, and the wholeprocess has been considered in detail by Dr. H. M. Steele-Petrovic who has kindly let me sec hermanuscript in advance of publ ication. However, I can think of no evidence of direct competitionbetween one and the other. Thus in late Palaeozoic times most benthonic habitats were stilldominated by brachiopods, whereas in the early Mesozoic certain habitats (notably muddybottoms) were mainly occupied by bivalves, whilst the brachiopods still kept their hold oncarbonate floors. But I cannot think of any bed I have ever seen where two groups were in'competition' . Gryphaea did not (in my view) compete with Liostrea, it occupied a differentniche.

The writer mentions the interesting case of Pseudolimea duplicata having 24 ribs in the UpperLias and only 21 in the Cornbrash. If it could be proved statistically that this species had 23 in the