Download Speciation in the tropical rain forest and the concept of the niche

Bio1.J. Linn. SOC.,1, p p . 149-153
April 1969
Speciation in the tropical rain forest and the concept of
the niche
P. W. RICHARDS, F.L.S.
University College of North Wales,Bangor
Two statements about the tropical rain forest are constantly reiterated: (1) it is the richest
in species of all plant communities, (2) it is unusual among species-rich communities in the
frequency of series of closely related (or at least congeneric) apparently sympatric species (cf.
Fedorov, 1966).
The comparison of species diversity in plant communities of very different physiognomy,
e.g. forest and grassland, presents difficulties. On a specieslarea basis it is possible that some
non-forest communities, e.g. the fynbosch (sclerophyll scrub) of the Cape and sclerophyll
communities in Australia and New Caledonia, may be richer than the tropical rain forest, but
the latter is certainly much richer than any otherforest community.
The total species-richness of the rain-forest flora depends in part on the large number of
synusiae present, e.g. of lianes, epiphytes, ground herbs etc., but only trees will be discussed
here.
The species diversity of rain-forest trees varies regionally (e.g. richest in south-east Asia,
poorer than elsewhere in most parts of tropical Africa) and according to site conditions (e.g.
poorer on podsols and on sites with impeded drainage than on well drained latosols in the same
area).
The frequency of groups of closely related species varies in different regions and in different
storeys in the same region. The very large number of Shorea spp. and other nearly related
groups of Dipterocarps in the A storey in south-east Asia is very striking, but in Africa and
America, the numerous congeneric species in the B and C storeys is equally remarkable, e.g.
Diospyros and Rinorea in Africa, Miconia in tropical America. Some families tend to be represented in all storeys, but not by many species in any one, others in only one or two storeys, but
by many species in the same storey.
The common occurrence of groups of morphologically similar and nearly related tree
species in the rain forest seems in contradiction to Gause’s competitive exclusion principle.
These questions suggest certain reflections on the concept of the niche as applied to plants.
If it applies in any exact sense to rain forest trees it implies that species which occupy niches
must stand in a different relation to the resources (light, water, mineral nutrients) of the
environment and to other organisms of the ecosystem.
It is well known that even unrelated rain-forest trees of the same storey tend to be very
similar in such features as crown form, size and shape of leaves etc., and are often hard to
distinguish in the sterile condition. Characters of possible ecological importance in which they
differ include the following: usual mature height (determines the place of the tree in the
stratification), growth rate, life span, shade tolerance, type of dispersal mechanism, reproductive strategy, flowering season etc. These tend to be correlated together and not combined
at random, e.g. shade-intolerant species tend to be fast-growing and to have efficient (wind or
bird) dispersal mechanisms, shade-tolerant to be slow-growing and to be gravity-dispersed.
But even if allowance is made for all these known differences of possible ecological importance
it is difficult to believe that they are enough to fit as many species to occupy different niches as
seems to be required by the competitive exclusion principle. Other differences between species
probably exist: possibilities are in the way different species exploit the mineral resources of the
soil (root activity, differences in recycling of nutrients depending on different length of life of
leaves) and defence mechanisms against herbivores and pathogens. The latter may be biochemical or biological as in myrmecophytes.
We can only conclude that we know very little about the autecology of rain-forest trees.
Comparative studies of selected species from different storeys in the same stand may be very
valuable.
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P.W. RICHARDS
As I neither have the mathematical equipment of a population geneticist nor can
claim any special knowledge of the mechanics of evolution, my qualifications for
contributing to this symposium are somewhat slender; all I wish to do is to make some
general comments on the problem before us as it appears to a plant ecologist with a
fairly wide field experience of the tropical rain forest in the Old and New Worlds.
Two statements about the tropical rain forest are constantly reiterated. First, that
it is the richest in species as well as the most complex in structure of all plant communities. Secondly, it has frequently been asserted that it is unusual, perhaps unique,
among species-rich communities, in the large number of groups of closely related (or
at least congeneric), apparently sympatric species which it contains. The latter feature
was recently the main theme of an important paper by Fedorov who concluded that the
low population density of many species tends to lead to a lack of intraspecificcompetition
and hence creates ‘favourable conditions for the process of speciation in which the role
of genetic drift prevails over that of natural selection’ (Fedorov, 1966: 9).
Though the species-richness of tropical rain forests and the common occurrence of
groups of closely allied species cannot be denied, I think it possible that its uniqueness
in both respects has been exaggerated.
The comparison of species diversity in plant communities of very different physiognomy, e.g. forest and grassland or forest and scrub, presents difficulties which need not
be discussed here and we must distinguish between species-richness in large sample
areas (of the order of one to hundreds of square kilometres) and in small samples of
only one or a few hectares. In the former, factors such as the speed and efficiency of
dispersal and adaptation to slightly different climatic and edaphic factors play a much
larger part than in the latter. Though comparable figures are hard to come by, I think
that on a crude species/area basis some non-forest communities, e.g. the fynbosch
(sclerophyll scrub) of the Cape Peninsula and some sclerophyll communities in
Australia and New Caledonia, may well be richer than most types of rain forest. There
can, however, be no doubt that small samples of the latter have more tree species than
those of any other type offorest community; even the richest temperate forests such as
the ‘Mixed Mesophytic forest’ of the Appalachians, and probably the forests of Western
China, are considerably poorer. If figures were available for a meaningful comparison
of large areas I suspect that the difference between the tropical rain forest and all other
types would be even greater.
The coexistence of large numbers of morphologically very similar and apparently
closely related species is a feature of most rain forests, though more marked in some
than others, but there are many other plant communities in which there is a similar
state of affairs. Is it really more difficult to explain the presence of 15 Eugenias and 12
Shoreas in the same small area of rain forest (Poore, 1968 :191,194)than the coexistence
of numerous very similar grasses in some tropical savannas, or even of numerous
similar Cyperaceae in some temperate wetland communities ? Or again, what about
the multitude of similar and apparently sympatric rhododendrons in the mountains of
Asia ?
The total richness in plant species of the rain forest depends in part on the large
number of physiognomically and ecologically distinct synusiae present, e.g. trees,
lianes, epiphytes and ground herbs. Members of these different groups are obviously
SPECIATION
IN THE TROPICAL
RAIN FOREST
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playing different roles in the ecosystem : they have adopted quite different ‘strategies’
in the struggle for survival. It will focus the problem I wish to discuss more sharply
if from now on I restrict my remarks to trees. In rain forests these range from a few
metres to 60 m or more in height: without begging the much debated question whether
storeys (strata) exist objectively or not, it is convenient to regard the trees as belonging
to three storeys, designated, from above downwards, as A, B and C.
The species diversity of rain-forest trees (defined as the number of species on a unit
area attaining some arbitrary lower limit of girth or diameter) in both large and small
samples varies from one geographical region to another. There can be little doubt
that the rain forest of south-east Asia, particularly the lowland forest of the Malay
Peninsula and Borneo, is richer than any other. T o give a few figures-Poore (1968)
has recently reported 375 tree species ‘estimated to reach the upper canopy’ (in practice
trees over about 90 cm girth at breast height) on 23 ha of lowland forest which, though
it included some swamp forest, was selected for its uniformity. As the total number of
trees in the sample was 2773, the average number of individuals per species was 7.4.
In other localities in Malaya, Cousens (1951) had previously found over 200 species
of 10 cm diameter (approximately 30 cm girth) on 1 ha (individual/species ratio about
1.6). Some parts of the Amazonian forest approach this degree of species diversity, but
the African rain forest is everywhere poorer in species and in some areas considerably
so.
Though differences in the present climates of the three great rain-forest regions may
be of some significance (the two richer regions, south-east Asia and South America
having larger areas of ‘ever-wet’ climate than tropical Africa, most of which has a more
or less marked dry season), it seems likely that historical factors such as climatic
changes in the past are chiefly responsible for these regional differences in speciesrichness.
If small samples (1-2 ha) in the same region are compared the effect of environment
differences becomes apparent. Forests on swampy ground are always poorer in species
than those on terrafirme and those on sandy podsolic soils (‘white sands’) are poorer
than those on latosols in the same area. As I have often pointed out, forests on sites
which are in any way unfavourable tend to have fewer species than those growing under
more optimal conditions. Often the smaller number of species on the non-optimal
sites is associated with a tendency towards dominance by one or two species.
Groups of morphologically similar congeneric or closely related species are found
in all geographic regions of the rain forest and both among the large trees of the ‘A’
storey and the smaller trees of the ‘B’ and ‘C’ storeys-but not with equal frequency.
In general the size and frequency of such groups seems to be proportional to speciesrichness. In Malaysia the very large number of species of Shorea and other groups of
nearly related Dipterocarps is most striking, but similar species groups are found
among the smaller trees, e.g. Annonaceae, Eugenia and others. In Africa, with its
less rich flora, species groups are most marked perhaps in the ‘B’ and ‘C’ storeys with
their numerous species of Diospyros, Rinorea, Cola and other genera. In tropical
America as in south-east Asia, there are many examples of species groups both among
the larger trees (Lecythidaceae, Licania) and among the smaller (Protiurn,Miconia and
others). Some tropical families, e.g. Apocynaceae, Rubiaceae, tend to be represented
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P. W. RICHARDS
by some species in all storeys, but not by many in any one: other families, e.g. Dipterocarpaceae, Annonaceae, Ebenaceae, are represented in one or at most two storeys, but
by many species in the same storey.
How can the existence of these groups, involving in some cases ten or more very
similar tree species, be reconciled with the competitive exclusion principle ? There
seem to be two possibilities. If there are (say) 100 tree species in 1-5 hectares of forest
are they all occupying different niches ? Or is it possible that the number of niches is
much less than 100 and that whether species A,B,C . . . occupies a given niche when
it becomes vacant is determined to a large extent by chance ?
These questions suggest certain reflections on the concept of niche as applied to
plants. ‘Niche’, whether conceived by Elton as the role of the species in the ecosystem
-its profession, not its address-or by Hutchinson as a ‘hypervolume’, is primarily
a zoological concept, but it is generally assumed that in some modified form it can also
be applied to plants. If it can be applied in any exact sense to rain-forest trees it implies
that species of each niche stand in a different relation to the resources of the environment (light, water, mineral nutrients) from those of other niches, as well as to the other
organisms in the ecosystem.
It is well known that even rain forest trees belonging to different families, especially
those in the same stratum, may be very similar in size, shape and texture of their
leaves, branching system, crown form etc., so that they may be quite difficult to distinguish in the absence of flowers or fruit. I remember the late Major R. W. G.
Hingston, an excellent field zoologist, being genuinely surprised to learn that the
dominant trees at Moraballi Creek, Guyana (which belonged to 20 or 30 different
families) were not all ‘speciesof laurel’. But though unrelated rain-forest trees may look
very much alike superficially,they may differ in a large number of less obvious characters
which are almost certainly of great ecological importance. Examples are: the usual
mature height (which determines the place of the tree in the forest structure), the
growth rate, life span, shade tolerance, reproductive strategy, type of dispersal
mechanism, phenology of leafing, flowering and fruiting.
It would not be difficult to find biological differences of this kind even among members of large sympatric groups. For example, Snow (1968) has recently pointed out that
in the Northern Range of Trinidad there are more than 22 sympatric species of Miconia,
all of them rather similar-looking small trees or shrubs. Each species has a regular,
but different, flowering and fruiting season, staggered so that there is no time of
year when one or other species is not fruiting. Since dispersal in this genus depends
on frugivorous birds, this situation may well be of selective advantage to the species,
Snow suggests that the staggered fruiting seasons have evolved through interspecific
competition: any tendency towards fruiting at a time when as few other species as
possible were in fruit must carry a strong selective advantage. These Miconias admittedly are sera1 species of cleared ground, not trees of the climax forest, but a similar
situation may well exist in other species groups.
But biological characters of the kind just mentioned tend to be correlated together
and not combined at random, thus shade-tolerant species are usually slow growing
and often have heavy gravity-dispersed seeds or fruits, while intolerant species are
relatively fast growing and usually have efficient dispersal often by wind or animals.
SPECIATION
IN THE TROPICAL
RAINFORIBT
153
This is one reason why it seems rather difficult to believe that even if allowance is made
for all known or possible characters of ecological importance, there are enough differences between species to fit them to occupy as many different niches as seem to be
required by the competitive exclusion principle.
There may, of course, be differences other than those mentioned, about which we
can only guess : two possibilities are differences in the way different species exploit the
mineral resources of the soil (root activity, differences in recycling of nutrients depending on different length of life of leaves) and possibly defence mechanisms against herbivores. The latter may be biochemical in nature (cf. Ehrlich & Raven, 1964) or of a
quite different kind as in some myrmecophytes. It should also be added here that though
the ecological ranges of these species groups undoubtedly overlap, closer examination
may show that they are not identical. Such a conclusion can be drawn from recent
attempts to study the distribution of rain-forest tree species by ordination methods
(Ashton, 1964; Greig-Smith et al., 1967).
We can conclude that we know very little about the comparative autecology of rain
forest trees. T o study this for even a large proportion of the tree species of one area
would be an immense task, but a useful beginning would be to make a comparative study
of four or five selected species from different storeys in the same stand. Until we have
this sort of knowledge, my question how many niches 100 species occupy must remain
one of the 64-thousand-dollar questions of tropical ecology.
REFERENCES
ASHTON,P. S., 1964. Ecological studies in the Mixed Dipterocarp forests of Brunei State. Ox$ For.
Mem. No. 25.
COUSENS,
J. E., 1951. Some notes on the composition of lowland tropical rain forest in the Rengam Forest
Reserve, Johore. Malay. Forester, 14 (3): 131-139.
EHRLICH.
P. R. & RAVEN,
P. H., 1964. Butterflies and plants: a study in co-evolution. Evolution, Lancaster,
Pa. 8 (4): 586-608.
FEDOROV,
An.A., 1966. The structure of the tropical rain forest and speciation in the humid tropics. J .
E d . 54: 1-1 1.
GREIG-SMITH,
P., AUSTIN,M. P. and WHITMORE,
T. C., 1967. The application of quantitative methods
to vegetation survey. I. Association-analysisand principal component ordination of rain f0rest.J. Ecol.
55 (2) : 483-504.
POORE,
M. E. D., 1968. Studies in Malaysian rain forest. I. The forest on Triassic sediments in Jengka
Forest Reserve.J. Ecol. 56: 143-196.
SNOW,D. W., 1968. Fruiting seasons and bird breeding seasons in the New World tropics (Abstract).
J . Ecol. 56 (2): 5-6.