The Effect of invasion Rate, Species Pool, and Size of Area on the Structure of the Diatom Community

The Effect of invasion Rate, Species Pool, and Size of Area on the Structure of the DiatomCommunityAuthor(s): Ruth PatrickSource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 58, No. 4 (Oct. 15, 1967), pp. 1335-1342Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/58291 .

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THE' FIFECT OF INVASION RATE, SPECIES POOL, AND SIZE OF AREA ON THE STRUCTURE OF THE DIATOMI COMMUNITY

BY RUTH PATRICK

ACADEMY OF NATURAL SCIENCES, PHILADELPHIA, PENNSYLVANIA

Communicated by G. E. Hutchinson, June 22, 1967

Many papers have been written concerning the diversity of communities of naturally occurring species. Various diversity indices (Fisher, 1943; Shannoni and Weaver, 1948) have been formulated, and various types of models (Preston, 1948; MacArthur, 1957) have been used to describe the structure of the community in terms of its diversity. l\More recently, 1\4ac.A,rthur and Wilson (1963) have em- phasized the importance of size of area and invasion rate in the maintenance of a diversified community.

In the present studies three series of experiments were performed. One was to show what effect the size of area and the number of species in the species pool which were c apable of invading an isolated area had on the numbers of species which composed the diatom community. The second series of experiments was to show what effect altering the invasion rate had on the structure of the community. The third series of experiments was a comparison of the structure of diatom communities in structurally and chemically similar streams on the island of Dominica and in the United States. The temperature of the water in the two rivers was similar when the studies were made.

Methods and Procedures.--In order to carry out the first series of experiments, duplicate smnall glass squares 9 mm2, 36 mm2, and 625 mm2 were each erected on a small plastic pedicel about 10 mm in length which was fastened onto a glass slide. These glass slides were placed in plastic boxes which were opened so that the current would pass across the glass squares, giving sirmilar conditions for the attachment of the diatoms across the surface. Having the current strike the slides would have created strong, uneven current patterns.

The slides were p]aced in clean boxes each day and the pedicels and slides care- fully cleaned. This was to minimize the chance that diatoms from sources other than the passing water might invade the glass squares on the ends of the pedicel. The experimental design was to simulate the invasion of species from distant sources onto islanids of varying size ranges. These slides were placed in the flowing water from a spring (Roxborough Spring) and in a eutrophic stream (Ridley Creek). Previous studies had shown that the total number of species in Roxborough Spring in the area studied was about 60 at any onie time, and in Ridley Creek about 250. It should be noted that the chemical characteristics of the water and the temperature were more variable in Ridley Creek than in Roxborough Spring. Since the spring has a deep source, the chemical characteristics and temperature of the water in it remain almost constant, over long perio(ds of time.

The seconid series of experiments was designed to simulate what might happen if the invasioni rate were lowered as hapiperis onl. an islanid fairly distanit from a highly diversified species pool, such as pools that exist on conrtinienits. An island of any signiificant size would have almost infinite size as far as the diatom com-

1335



1336 BO'T'ANY: R. PATRICK PROC. N. A. S.

mnuinity was cotncernied. Tllhe main factor limiting diversity would be the invasioni rate.

In this series of experiments, as conitrasted with the first series, the inlvading species in each case were from the same water and the same species pooi. was avai.l- able, but the rate of invasion was different.

Glass slides (3 in. X 1 in.) were placed in boxes and water flowed over their surfaces at 550-600 liters/hour. This rate of flow had previously been found to produce on such slides diatom communities which were typical of a natural stream. In another box the same rate of flow was maintained by recycling filtered stream water which furnished most of the flow. During the first three days diatoms were allowed to invade at a rate of 550-600 liters/hour. The reason. for allowing the full flow during the first few days was to develop some semblance of a diatom flora on the slides. It is usually about two days before the diatom flora starts to develop. After the first three days, the invasion rate of new diatoms was at 1.15 liters/hour. Two sets of these experiments were carried out in a eutrophic stream, one in late September to early October, and one in late October to early November.

The third series of experimeints was carried out under natural conditions. Di- atom communities were studied in similar oligotrophic streams on the island of Dominica and in the state of Maryland.

Discussion of Results.-The results of the first series of experiments are showil in Table 1A. These experiments show that size of area influences the number of species established in an area, that the number of species increases greatly at first aind is not very differenit betw-een four days and one week, and that subsequently the pattern of increase is irregular. On one of the 36-mm2 slides exposed in the fall in Roxborough Spring there seems to be a slight decrease of species at eight weeks. Other experiments carried out in a similar manner in Roxborough Spring in the summer, when growth is many times as rapid, showed this decrease more distinctly at the end of two weeks (Table iB). In each case the species which disappeared were those represented by very small populationis.

TABLE 1

(A) EXPERIMENTS IN SEPTEMBER-OCTOBER 1964 -- _~ IRoxborough Spring Ridley Creek

Size of slide: 625 mm2 36 mm2 9 mm2 36 mm2 Number of Species No. of species,

Box I Box 2 Box 3 Box 4 13ox 5 Box 6 box 7

4 days 46 37 23 23 1 3 1 week 40 32 28 24 7 2 weeks 54 35 22 1t) 10- 8 weeks - - 29 14 19 14 160

(B) EXPERIMENTS IN 1{OXBORO-UGH SPRING DURING SUMMER 1964 1 Week, 2 Weeks, 1 Week, 2 Weeks,

144-mm2 slide 144-mm2 slide 625-mm2 slide 625-mm2 slide Box I Box 2 Box 3 Box 4 Box 5 Box 6 Box 7 Box 8

No. of species 32 28 23 22 47 44 29 28

When we compare slides of the same size (36 mm2) from Roxborough Spring and from an area in Ridley Creek which had a much larger species pool and a more rapid invasion rate because of sw-ifter current, we see that at the end of eight weeks 160 species were established on the Ridley Creek slides whereas 14-29 species were established O11 th1e Roxborough Sprinig slides (Table lA.).

The second series of experiments in which the invasion rate was reduced but the



VTOL. 58, 1967 BOTANY: R. PATRICK 1337

area was large produced the results shown in Figures 1-4. The number of species composing the community was reduced and the sizes of the populations were more variable than in the communiities with a high invasion rate. This is shown by the reduction in the height of the mode, the reduction in the observed and calculated species in the community, the increase in a-2, and the number of intervals covered by the curve. The diversity index (Shannion and Weaver, 1948) is also less. This decrease in the diversity inidex, particularly in the October-November study, is due to the fact that two species, Nitzschia paleca and Navicula luzonensis, are ex- cessively commoni in the reduced-flow-rate conmmunity. TIn the September-October reduced-flow study, the commonest species is represented by about one seventh the number of specimens as in the October-November reduced-flow experiments, anld more species are represented by fairly large populations. Therefore, the diversity index is not as greatly reduced. This difference in sizes of populations of species is probably due to the fact that conditions for growth were better for more species in the September-October period than in the October-November period.

When one compares the total biomass of the September-October community that had a lower invasion rate and lower niumber of species with the biomass of the community that had a higher invasion rate and larger number of species, it is only 17 per cent greater in the community with fewer species (21.2 mg as compared to 18 mg). This is probably within the range of natural variation, and the biomasses of the two communities are not really different. It would appear that the numbers of species composing the communities does not significantly affect the total biomass of the community.

30

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INTERVALS -0 1 2 3 4 5 6 7 8 9 i0 11 i2 13 14 15 16

FIG. 1.-Invasion rate 550-600 liters/hr, October-November, 1964. Height of mode, 22.4 species; observed species, 123; ?a, 6.2; int;ervals covered by the curve, 9; diversity index, 3.805.



1338 BOTANY: R. PATRICK PROC. N. A. S.

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INTERVALS -0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

FIG. 2.-Invasion rate 1.5 liters/hr, October-November, 1964. Height of mode, 15.3 species; observed species, 97; a-', 12; intervals covered by the curve, 15; diversity index, 0.972.

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INDIVIDUALS 1-2 2-4 4-8 8-16 16-32 32-64 64- 128- 256- 512- 1024- 2048- 4096- 8192- 16384- 32768- 128 256 512 1024 2048 4096 8192 16384 32768 65536

INTERVALS -0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

FIG. 3.-Invasion rate 550-600 liters/hr, September-October, 1964. -Height of mode, 22.5 species; observed species, 129; 2, 6.9; intervals covered by the ctirve, 9; diversity index, 3.713.



VOL. 58, 1967 BOTANY: R. PATRICK 1339

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128 256 512 1024 2048 4096 8192 16384 32768 65536 INTERVALS =0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

FIG. 4.-Invasion rate 1.5 liters/hr, September-October, 1964. Height of mode, 13.9 species; observed species, 100; S2, 12.6; intervals covered by the curve, 12; diversity index, 2.522.

The results of the third series of studies are set forth in Figures 5-7. It will be seen that the number of species in the two Dominica diatom communities is con- siderably smaller than in the diatom community in the United States. The sizes of the populations of the most common species in all three communities fell in the same interval of the truncated normal curve and were represented by 14,400 specimens in the Layou River, 9,525 specimens in Check Hall River, and 15,975 specimens in Hunting Creek. The main difference in the population sizes of the various species in the streams was that although there were fewer species in the Dominica streams, more of them had fairly large populations than those in Hunting Creek. Alost of the species in Hunting Creek had moderate to very small populations. Therefore a2 is larger in the Dominica streams than in Hunting Creek. Likewise the Shannon-Weaver diversity index is larger in the Dominica streams. However, Fisher's a (1943), which effectively indicates species numbers even if the populations are small, is much larger in Hunting Creek (25.99) than in the Dominica streams (bayou River, 5.594; Check Hall River, 5.667). This latter index seems more clearly to indicate the differences in species numbers in these two types of communities, while the Shannlon-Weaver index more clearly indicates the uneven- ness of the distribution of individuals in the various species populations.

Conclusions.-From these studies, which were controlled, semilaboratory experiments as well as actual field studies, it is evident that size of area, number of species in the species pool which are capable of invading the area, and the rate of invasion by the species greatly influence the numbers of species and the diversity of the community.

A reduced invasion rate (size of area and number of species in the species pool remaining the same) reduced the total number of species in the community, par-



1340 BOTANY: 1U. PATRICK Piioc. N. A. S.

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FIG. 5.-Hunting Creek, Maryland; truncated curve for a diatom communlity. Height of mode, .12 species; observed species, 79; o-2, 9.1; initervals covered by the cuirve, 14; diversity index, 0.789.

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INDIVIDUALS -1-2 2-4 4-8 8-16 16-32 S264 64- 128- 256- 512- 1024- 2048- 4096- 8192- 16384- 32768- 128 256 512 1Q24 2048 4096 8192 6(384 32768 65536 INTERVALSO -0 I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

FIG. 6.-hc Hunin Criver, isrland,o oiia truncated curve for a diatom community. Eegto Hihofmode, 52 species; observed species, 46; ~2, 21.6; inltervals covered by the curve, 14;diest divdersit indx,1.919

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INDIVIDUALS -1-2- 2-4 4-8 8-16 16-32 32-64 64- 12-8- 256- 512- 1024- 2048- 4096- 8192- 16384-32768- 128 256 512 1024 2048 4096 8192 16384 32768 65536

INTERVALS =0 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16

FIG. 6.-Check Hall River, island of Dominica; l;runcated curve for a diatom commutnitv. Height of mode, 5 speeies; oh.served speci.es, 46; a', 21..6; intervals covered by the Clurve, 14; diversi.ty in.dex, .1.9~19.



VOL. 58, 1967 BOTANY: R. PATRICK 1341

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INTERVALS =0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

FIG. 7.-Layoul River, island of Dominica; truinicated curve for a diatom community. hIeight of the mode, 5 species; observed species, 49; -2, 26; intervals covered by the curve, 14; diversity index, 2.028.

ticularly those species with small populatiorLs which are typically part of a natural continental community. There is an increase in species with fairly large populations although the total number of species decreases.

Similar results were found when diatom communities on the island of Dominica were compared with a diatom comnmunity in a similar type of stream in the state of AMlaryland. The island communities had much smaller inumbers of species with larger populations than the continenital community. They also had fewer species with small populations.

One of the main results of a high invasion rate is to maintain in a community a number of species with relatively small populations. The value of these species to the community may be similar to the presence of relatively rare genes in a gene pool. They may function to preserve the diversity of the community under variable environmental conditions. Such rare species at any one point of time under chaniged environmental conditions might be better adapted than the presently common species and might be able to increase rapidly.

The size of the species pool which may potentially invade an area also has a great effect on the number of species composing the community, as seen in the comparison of the species numbers in the communities in the Roxborough Spring studies and the Ridley Creek studies. The size of the area to be invaded also affects the diversity of the comrmunity. Once the area becomes filled, the first species to be eliminated were those represented by very small populations.

Tie auithor wishes to express her gratituide to Dr. G. . Hlutchinsoii avid 1)i. Robert MacArthur for their helpful advice and criticism, and to Miss Noma Ann Rltoberts, Mr. RtaymoInd Cuimmins, Mr. Roger 1)aum, and MViss Lee Townsend, who have beenl most helpful in the carrying ouit of



It342 BOTANY: R. PATRICK PROC. N. A. S.

these studies. The author also wishes to thank the U.S. Public Health Service (WP-00475) foI their financial support for part of this work.

Fisher, It. A., A. S. Corbet, and C. B. Williams, "The relation between the ntumber of species and the niumber of individuLals in a random sample of an animal populationi," J. Animal Ecol., 12, 42-59 (1943).

MacArthur, R., "On the relative abuindance of bird species," these PROCEEDINGS, 43, 293-295 (1957).

MacArthur, R., and E. 0. Wilson, "An equilibrium theory of insuilar zoogeography," Evolution, 17, 373-387 (1963).

Preston, F. W., "The commonness and rarity of species," Ecology, 29, 254-283 (1948). Shaimion, C. E., and W. Weaver, The Mathematical Theory of Communication (Urbana: -Uni-

versity of Illinois Press, 1948), pp. 3-91.



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The Effect of invasion Rate, Species Pool, and Size of Area on the Structure of the Diatom Community