Species Diversity in Aquatic Microecosystems

Species Diversity in Aquatic MicroecosystemsAuthor(s): Catherine ReedSource: Ecology, Vol. 59, No. 3 (Late Spring, 1978), pp. 481-488Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1936578 .

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Ecology, 59(3), 1978, pp. 481-488 ?D 1978 by the Ecological Society of America

SPECIES DIVERSITY IN AQUATIC MICROECOSYSTEMS1

CATHERINE REED Department of Biological Sciences, University of Northern Colorado, Greeley, Colorado 80639 USA2

Abstract. Aquatic microecosystems of 3 litres were established in replicate for the 8 combinations of high or low physical heterogeneity, high or low plant nutrient levels and stable or unstable temperature. Plankton succession from a pond water inoculum was observed for 5 mo. Microcrustacea, periphyton and benthic organisms were sampled only at the end of the study. Final species diversity (H') was higher for low-nutrient and for most high physical-heterogeneity systems. Stability of temperature did not affect species diversity. Numerous interactions of the 3 variables occurred, so that the diversity of the systems could not be predicted from the combined effects of single variables. The significance of this observation for theoretical analyses of species diversity is discussed.

Key words: aquatic; environmental heterogeneity; microcosms; microecosystems; productivity; species diversity; stability.

INTRODUCTION

Factors which influence species diversity include the stability, environmental heterogeneity, and pri- mary productivity of ecosystems and the predatory and competitive interactions among species.

Temporal heterogeneity, or environmental instability, is the departure from constancy of environmental factors. On an evolutionary time scale, well-adapted and highly diverse communities controlled by biotic interactions are more likely to develop in stable environments than in unstable ones (Sanders 1968, 1969). On an ecological time scale, high diversity may be observed in unstable environments as a result of the presence of many species, some just becoming established, some at their population peaks and others in decline. Instability can thus increase species diversity at any sampling time (Loucks 1970, Wilbur 1972, Moss 1973, Abele 1976, Porter 1977).

Complex or heterogeneous environments with a va- riety of physical features provide microhabitats. Such environments generally show higher species diversity than do simple ones (Hall et al. 1970, Harman 1972, Abele 1974). In laboratory cultures, physical complexity increases the probability of coexistence of competing species (Miller 1967, Smith 1972, Luckinbill 1973, 1974).

In established terrestrial communities, species diversity is usually correlated with productivity (Connell and Orias 1964, Karr 1971). Addition of plant nutrients to aquatic systems may increase species diversity (Noland and Gojdics 1967) or decrease diversity as eutrophication occurs (Patrick 1972, Hutchinson 1973).

The presence of carnivores increases diversity among animals of the rocky intertidal (Paine 1966, Dayton 1971) and in freshwater undergoing natural colonization (Maguire 1971). When prey species are

I Manuscript received 9 May 1977; accepted 28 November 1977.

2 Present address: 1588 Vincent St., St. Paul, Minnesota 55108 USA.

not competing with one another, the presence of predators does not increase species diversity (Addicott 1974). Interactions between phytoplankton and herbivores are important in determining the species diversity and composition of freshwater phytoplankton communities (Porter 1977).

Small artificial ecosystems have been used to identify the effects of many variables on species diversity and other ecosystem characteristics. These systems show many of the properties of natural systems, in- cluding diurnal patterns of photosynthesis and respi- ration, internal regulation and successional patterns (Cooke 1971). This paper reports the effects and interactions of plant nutrient level, temperature stability and physical complexity on the successional species diversity of aquatic microecosystems.

METHODS AND MATERIALS

A series of eight 3-litre open microecosystems was established in wide-mouth glass jars. The 8 combinations of 3 variables were tested: complex or simple physical configuration, stable or unstable temperature, and fertilizer addition or no addition. Each combina- tion was replicated 4 times.

The physically heterogeneous sytems were prepared by spreading a 3-centimetre layer of washed river sand and gravel on the bottom of the jar and adding (1) an upright 6.5-centimetre clay flowerpot, (2) a 50-millilitre beaker containing 8 g brown rice, and (3) an upright 100-millilitre beaker containing 16 pieces of 5- millimetre glass tubing 6 centimetres long. The physically simple systems received only the 8 g of brown rice, spread over the bottom. Three litres of pond water were added to every jar.

Fertilized systems were prepared by adding 3 ml liquid fish emulsion fertilizer (Orthog; Chevron Chem- ical Company) to provide 6 mg/l N-NO3 and 2 mg/l P- P04. Each fertilized system received 0.25 ml fish emulsion per week throughout the study period, but the unfertilized systems received no fertilizer.

Unstable systems were produced by packing the



482 CATHERINE REED Ecology, Vol. 59. No. 3

systems in ice until the temperature was 12'C. They returned to ambient temperature within 12 h. The cold treatment was applied at random intervals of 1 to 5 days based on a random number table. All the systems were maintained in a growth chamber at 260C with 12 h of fluorescent and incandescent light per day at an intensity of 3.2 kilolux. Glass-distilled H2O was added weekly to maintain water volume.

Each month 125 ml were taken from each system. This was mixed with 4 litres fresh pond water and 250 ml of the mixture was returned to each system. This procedure allowed species exchange among all the systems.

The microecosystems were maintained for 20 wk. Plankton samples were counted initially and every 2 wk thereafter. Samples were obtained by lowering a 125-ml flask under the water surface. A 1-ml aliquot was sampled using a Sedgwick-Rafter counting cell (Taras et al. 1971). Ten fields were counted at 100x magnification. In a few cases, the plankton was too small and numerous to count, so diversity values were not obtained. Statistical comparison of within-jar and between-jar samples indicated that 1 aliquot per jar was adequate. Initially, the aliquots from the 4 replicates were combined and treated as 1. After the 12th week, each jar was treated separately.

Microcrustacea, periphyton and micro- and macrobenthos were sampled at termination. Microcrustacea were counted under 20x magnification after straining from the water using No. 20 silk bolting cloth. Periph- yton was scraped from the sides of each jar with a tongue depressor, suspended in ~25 cm:' of distilled H20 and homogenized in a Waring? blender at low speed for 20 s. The material was diluted and 10 fields of a 1-ml aliquot counted using the Sedgwick-Rafter cell. The sediment from each jar was decanted and sampled with a 10-ml pipet. The sample was diluted and 10 fields counted for microbenthos at 100x magnification. Macrobenthos was counted in the remain- der of the sediment at 20x magnification.

The pH of the water was determined with a pH me- ter after 6 h of light. Nitrate and nitrite concentration were measured using the cadmium reduction method and orthophosphate was measured using the Stannav- er method (Taras et al. 1971).

Final species diversity was calculated for each sampling fraction using the Shannon-Weiner diversity in- dex (Pielou 1969). A 1-, 2-, and 3-way analysis of variance was performed to identify interactions of the variables. Evenness was calculated according to Pie- lou's J' formula (Pielou 1974). The association of in- dividual species with the different microecosystems was analyzed using a chi-square test.

Mean diversity values for systems which differed in the level of I variable but were otherwise identical were listed for each sampling date. One value was subtracted from the other to give a diversity difference, and the differences were ranked in order of size.

The number of rank position changes ("flips") re- quired to match the diversity difference trend (increasing or decreasing) with the chronological order of sampling dates was determined. The sign test for large samples, 2-tailed version (Siegel 1956) was applied to test whether the number of rank position changes was less than that expected to order 2 sets of data which fluctuated randomly.

RESULTS

After 20 wk, the water in the fertilized systems appeared grass-green whereas the water of the unfertilized systems appeared light yellow-brown. All the jars had a heavy growth of dark-green algae on their walls. Portions of this algal layer flaked off irregularly, resulting in the accumulation of a 2-cm layer of loose sediment. The rice in the small beakers of the complex systems was partly decomposed, forming a foul-smell- ing reddish-brown slimy material.

Mean pH values in all systems ranged from 8.2 to 9.4, an increase from the initial pH of 7.8. The pH of the simple microecosystems, 9.0, was significantly higher than that of the complex microecosystems, 8.3 (chi-square test, P < .05). Differences in pH between the fertilized and unfertilized and between the stable and unstable systems were not significant.

Concentration of P-P04 averaged 7.9 mg/l in the fertilized systems and 1.9 mg/l in the unfertilized systems compared to .9 mg/l in the original pond water. The increase in the unfertilized systems was caused by cross-inoculation and decomposition of the rice. The N-NO3 concentration averaged 5.8 mg/l in the fertilized systems and 5.0 mg/l in the unfertilized systems. The difference was not significant.

A total of 56 kinds of organisms were observed; from 16 to 32 were present in any microecosystem. Most organisms were identified to genus; nematodes were not identified beyond the class. Small, fast-mov- ing, colorless forms were grouped as unidentified flagellates and minute green algae as Chlorella-like.

Nitzschia, Oscillatoria, Ankistrodesmus, Chlorella, Cyclops, Cypridopsis and unidentified flagellates and ciliates were found in all the systems. Nav'icula, An- abaena, Phormidiumn, Scenedesinus, Euglena, Cryp- tomonas, Paramecium caudatuin, Philodina and nematodes were found in 19 or more of the systems. With few exceptions flagellates were the most numerous group in the plankton fraction, and CVpridopsis in the microcrustacea. Chlorella dominated the microbenthos but rotifers, Cyclops, CYpridopsis and nematodes were about equally common in the macrobenthos. Chlorella, Ankistrodesinus, Oscillatoria and Nitzschia comprised the algal layer on the walls of the jars, or periphyton (Table 1).

The 16 fertilized systems showed a mean species number (total kinds of organisms) of 22.0, the unfertilized, a mean of 26.6 (Table 2). The difference was significant at the .01 level. The simple systems showed



Late Spring 1978 MICROSYSTEM SPECIES DIVERSITY 483

TABLE 1. Organisms which represented >5% of the total individuals counted in each fraction. Numbers are percentages of total individuals counted. C complex; Si = simple

Microecosystems containing the organism

Fertilized Unfertilized

Stable Unstable Stable Unstable Fraction and

organism C Si C Si C Si C Si

Plankton Ankistrodesmus 40.3 38.8 9.1 4.6 7.6 9.1 24.2 2.6 Unidentified flagellates 53.6 43.1 70.4 63.0 81.2 88.8 30.1 93.5 Oscillatoria ... * . . . 13.0 ... ... ... 6.2 ... Chlorella-like ... ... ... 29.1 ... ... ... ... Cryptomonas ... ... . .. ... ... ... 17.2 ... Scenedesmus . . . 13.7 ... ... ... ... . . Phormidium ... ... ... ... ... ... 5.6 ...

Microcrustacea Cyclops ... 50.0 26.0 52.7 9.3 13.3 12.0 48.6 Cypridopsis 94.9 49.8 67.3 47.3 54.4 31.9 72.4 16.3 Ceriodaphnia ... ... 6.7 ... 27.0 7.6 ... 34.7 Dero ... ... ... ... 9.2 47.2 9.5 ...

Periphyton Chlorella-like 55.2 30.5 30.8 65.6 57.9 57.6 74.4 69.0 Ankistrodesmus 5.3 30.9 ... ... ... ... Nitzschia 7.8 12.1 8.9 5.6 .. . 6.8 . . . 9.6 Oscillatoria ... 9.6 12.5 ... ... 21.5 ... . Scenedesmus ... 9.1 13.3 6.0 ... ... ... ... Phormidium 27.8 6.8 22.4 19.1 34.8 10.1 19.6 10.7

Microbenthos Chlorella-like 62.4 49.5 87.1 70.1 41.6 58.2 25.5 56.2 Ankistrodesmus 25.8 37.9 ... ... ... 8.1 ... 7.8 Unidentified flagellates 9.4 ... ... ... 52.4 32.5 68.0 32.2 Scenedesmus ... 9.5 5.6 20.7 ... ... ...

Macrobenthos Philodina 26.6 49.1 32.9 27.2 7.4 16.6 12.0 11.6 Stentor ... ... ... ... ... 25.2 ... ... Ceriodaphnia ... ... ... .. . ... 10.9 . .. 12.4 Cyclops ... 14.7 20.4 50.0 27.3 10.7 8.0 65.3 Cypridopsis 8.9 . . . 35.5 9.5 48.7 22.6 63.7 7.0 Nematodes 60.8 31.2 7.8 11.7 ... ... 5.8 ... Dero ... ... ... ... 7.7 8.1 ... ...

* <5% of the total individuals counted for that system and fraction.

a mean species number of 23, but the complex systems had a mean of 25.6 species per system, significant at the .05 level. There was no significant difference in species number between the stable (x - 24.1) and the unstable systems (. - 24.4).

A chi-square test of association between microecosystem treatment and organism presence was calculated for those organisms present in from 6 to 23 of the 32 microecosystems. Twenty-three of the 56 species were in this group. Seven of these showed no significant association with any of the treatments. An- abaena, Coleps, Spirostomum, Stentor, Tetrahymena, Dero, Cocconeis and Fragilaria were found significantly more often in unfertilized than fertilized systems. Nematodes, desmids, Paramecium aurelia and Halteria were found more often in the fertilized systems. Coleps, Spirostomum, Tetrahymena, Rotaria, Cocconeis and Fragilaria were found more often in complex than in simple systems but only P. aurelia

"'preferred" the simple systems. Euplotes, Spirosto- mum and Stentor were found more often in the stable systems, but P. aurelia was more frequently found in unstable systems.

The plankton species diversity fluctuated (Table 3). Diversity values dropped initially, increased to peak between the 2nd and 3rd mo, then continued to drop during the remaining weeks. The sign test indicated that 6 of the 12 pairs of diversity values tended to become significantly more different from each other as the experiment progressed: 2 pairs differed in that I member of each was simple and the other complex, 2 differed in fertility levels, and 2 differed in stability. The members of I pair became more similar in their diversity and the remaining 5 pairs did not converge or diverge significantly.

The plankton species number showed a rapid decrease from the inoculum, then an increase (Table 3). No equilibrium species number was reached. With I



484 CATHERINE REED Ecology, Vol. 59, No. 3

TABLE 2. Species diversity (H'), evenness (J'), and species number after 20-wk incubation. Values based on mean of 4 replicates

Total species present

(excluding Micro- Micro- Peri- Macro- duplica-

System Parameter crustacea benthos Plankton phyton benthos tions)

Fertilized Stable Complex H' mean .33 .64 .81 .98 .98

H' SD .33 .38 .29 .23 .34 it .25 .17 .23 .32 .53 Species no. 2.25 14.00 12.00 7.75 3.75 23.0

Simple H' mean .75 1.05 1.55 1.84 1.22 H' SD .51 .22 .14 .35 .26 if .64 .29 .41 .65 .64 Species no. 1.75 13.00 14.00 7.00 3.75 20.7

Fertilized Unstable Complex H' mean .94 .76 1.21 1.90 1.66

H' SD .31 .23 .23 .20 .23 it .67 .18 .34 .60 .77 Species no. 2.75 17.50 12.00 9.00 4.50 24.2

Simple H' mean .67 1.05 1.11 1.52 1.51 H' SD .29 .12 .44 .37 .50 it .67 .29 .23 .53 .84 Species no. 2.00 12.75 11.75 7.50 3.75 20.2

Unfertilized Stable Complex H' mean 1.22 1.32 1.06 1.10 1.71

H' SD .41 .52 .34 .29 .38 1' .75 .32 .29 .37 .70 Species no. 3.75 16.50 12.75 8.00 5.50 27.2

Simple H' mean 1.36 1.00 .73 1.64 2.07 H' SD .32 .36 .17 .52 .12 it .88 .26 .22 .47 .76 Species no. 3.00 14.75 10.50 11.25 6.75 25.7

Unfertilized Unstable Complex H' mean 1.03 1.25 2.34 1.13 1.62

H' SD .49 .78 .55 .62 .31 it .58 .30 .67 .35 .62 Species no. 3.50 17.25 11.75 10.00 6.25 28.0

Simple H' mean 1.05 1.20 .55 1.50 1.08 H' SD .28 .53 .08 .42 .30 if .74 .37 .15 .43 .59 Species no. 2.75 14.50 12.25 11.50 4.00 25.2

exception, the number of plankton genera decreased for the first 8 wk. By 10 wk, more genera were found, and after this no pattern was observed. In every system, there were more genera present at 20 wk than at 10 wk.

An analysis of variance of the final diversity values was performed based on the means of 4 replicates (Table 4). The fractions differed in their responses to the variables. No system was sensitive to the stability variable alone. The variables interacted in all possible combinations in the plankton fraction, whereas no interactions occurred in the microcrustacea and microbenthos. No triple interactions were observed.

Diversity values for each sampling fraction were grouped according to the 2 levels of each variable and a further analysis of variance performed. Directions of the diversity differences are given in Table 5 for significant differences only. Fertility decreased diver-

sity in complex systems. In simple systems, fertiliza- tion affected the fractions differently or had no effect. Fertility level had no consistent effect on stable microecosystems compared to unstable ones.

Environmental complexity increased diversity in unfertilized systems. Complexity increased diversity in unstable systems and decreased it in stable ones. In fertilized microecosystems, the effect of complexity was influenced by stability.

Stability increased diversity in unfertilized systems and in simple fertilized systems. Stability increased diversity in unfertilized simple systems, but it decreased diversity in complex systems.

In fertilized microecosystems, periphyton usually showed the highest diversity, followed by the macrobenthos, plankton, microbenthos and microcrustacea. In the unfertilized systems, the macrobenthos was usually the most diverse fraction, followed by periph-




TABLE 3. Species diversity (H'), evenness (J'), and number of genera of the plankton fraction' (mean of 4 replicates)

Sampling date

System Parameter 4 wk 8 wk2 10 wk 12 wk 16 wk 18 wk 20 wk

Fertilized Stable Complex H' mean 1.83 1.25 1.86 1.61 1.14 .98 .81

H' SD t ... ... ... .72 .60 .28 1' .50 .36 .50 .28 .40 .30 .23 No. genera 12.00 17.00 12.00 13.00 10.25 10.00 12.00

Simple H' mean 2.12 1.47 2.40 2.55 1.17 1.43 1.55 H' SD ... ... ... ... .45 .20 .14 if .57 .31 .72 .96 .33 .40 .41 No. genera 12.00 10.25 10.00 7.00 12.00 11.25 14.00

Fertilized Unstable Complex H' mean .94 . . . 1.62 1.59 1.12 2.18 1.21

H' SD ... ... ... ... .65 .66 .23 if .25 ... .44 .54 .30 .64 .34 No. genera 12.00 ... 12.00 14.00 13.00 10.50 12.00

Simple H' mean 1.24 .. . 1.67 2.01 .91 1.17 1.11 H' SD ... ... ... ... .53 .58 .44 it .39 ... .88 .52 .28 .37 .23 No. genera 9.00 . . . 5.00 12.00 12.00 13.00 11.75

Unfertilized Stable Complex H' mean 1.40 1.25 1.44 1.27 1.92 1.51 1.06

H' SD ... .46 ... ... 1.04 .49 .34 it '.37 .49 .62 .40 .55 .54 .29 No. genera 14.00 8.25 5.00 9.00 10.50 7.25 12.75

Simple H' mean 1.42 1.89 1.82 1.70 .84 .67 .73 H' SD ... .06 ... ... .67 .44 .17 if .34 .67 .91 .71 .24 .27 .22 No. genera 18.00 6.70 4.00 5.00 10.00 5.75 10.50

Unfertilized Unstable Complex H' mean .78 ... 2.04 1.72 1.29 2.17 2.34

H' SD ... ... ... ... .54 .83 .55 it .26 ... .53 .54 .19 .60 .67 No. genera 8.00 . . . 9.00 8.00 7.75 11.75 11.75

Simple H' mean .71 1.99 1.35 1.44 .62 .58 .55 H' SD ... ... ... ... .23 .28 .08 1' .22 .63 .58 .67 .40 .24 .15 No. genera 10.00 9.00 5.00 8.00 9.50 6.25 12.25

'Initial values: H' = 2.41; S DH' .78; J' .56; 20.0 genera present. 2 Three systems had too many plankton to count. t Where no SD is given, H' was calculated from a combined sample.

yton, microbenthos, plankton and microcrustacea. The environmental complexity and stability treatments did not influence this pattern. The treatments did not influence the diversity of the fractions to the same extent in all systems; in general, a poor correlation among the diversity values of the fractions of each system was observed.

The correlation of species diversity with evenness and with species number is shown in Table 6. The plankton, periphyton and microbenthos diversity values were very closely related to evenness values, whereas species number was unimportant in determining species diversity. In the macrobenthos and microcrustacea, species diversity was related to both species number and evenness.

DISCUSSION

The fertilized microecosystems were, in general, less diverse than the unfertilized systems, and some

species were absent from the fertilized systems. This corresponded to observations of eutrophic lakes where species diversity declined and the number of individuals increased. The microecosystems, unlike eutrophic lakes, were not dominated by blue-green algae, but by green algae and flagellates. The high nitrate concentration in the unfertilized systems may have been due to nitrogen fixation by Anabaena (Round 1973). The fish emulsion fertilizer was a source both of nutrient salts and of more complex organic compounds, so that both heterotrophs and autotrophs found a favorable environment.

Decreased species diversity in eutrophic systems may occur in this way: under normal conditions, the populations of algae are limited by various nutrients. Under eutrophic conditions, some nutrients are present in excess, so species formerly limited by them increase in population. Their increase causes other species to be deprived of some nutrients and they are




TABLE 4. Probabilities based on 1-, 2-, and 3-way analyses of variance of complete fractions

Total signifi-

cant Micro- Micro- Peri- Macro- differ-

Variables crustacea benthos Plankton phyton benthos ences

One-way analysis I. Fertilized vs. unfertilized .001 .050 .502 .155 .022 3

II. Stable vs. unstable .941 .714 .069 .650 .791 0 III. Simple vs. complex .582 .603 .000 .017 .849 2

Two-way analysis I x II .063 1.000 .003 .196 .000 2 I x III .988 .093 .000 .520 .572 1

II x III .144 .815 .000 .016 .010 3

Triple interaction .289 .555 .597 .061 .259 0

eliminated. Predation does not control the dominant species because they are relatively inedible (Petersen 1975). Because some algal species are missing, some herbivore populations also become extinct, resulting in lower species diversity of carnivores as well.

The complex microecosystems had more species than did the simple ones in all pairs. Physical complexity increased diversity in unfertilized systems and decreased it in fertilized systems. In addition, 2 sampling fractions of the stable, simple microecosystems

were more diverse than the corresponding fractions of the stable, complex microecosystems, regardless of their fertility.

Environmental complexity is believed to increase diversity in 2 ways: by providing microhabitats giving protection from unfavorable weather and by providing prey species with refuges from their predators. Either of these mechanisms could result in a decrease in evenness if the temperature fluctuations or predation were preventing some species from becoming domi-

TABLE 5. Directions of the diversity differences significant analysis of variance results

Fraction showing a

Microeco- significant Presence of systems difference More diverse system interaction*

Fertilized Plankton ** + Periphyton Unstable if complex, +

stable if simple Microbenthos Simple ... Macrobenthos Stable ...

Unfertilized Plankton Complex, stable + Macrobenthos Stable +

Simple Plankton Stable, fertile + Microcrustacea Unfertilized Macrobenthos Unstable if fertilized, +

stable if unfertilized

Complex Plankton Unstable, unfertilized + Microcrustacea Unfertilized ... Periphyton Unstable + Microbenthos Unfertilized ... Macrobenthos ... +

Stable Plankton Simple if fertilized, + complex if unfertilized

Microcrustacea Simple ... Periphyton Unfertilized if complex, .

fertilized if simple Stable Macrobenthos Simple ...

Unstable Plankton Complex; fertilized if simple, + unfertilized if complex

Macrobenthos ... +

* + means that the action of the variable in the left column varied in its action depending on the level of another variable affecting the same microecosystem.

** Although interaction occurred, the diversity values of the members of the pair were not significantly different.




TABLE 6. Correlation coefficients (r) for species diversity (H') with evenness (J') and species number for each sampling fraction

H' with Sampling H' species fraction with J' number

Plankton .97 .23 Microbenthos .92 .15 Periphyton .94 .07 Macrobenthos .69 .81 MNicrocrustacea .87 .71

nant. The glass tubing and other materials of the complex systems may have protected these species from cold or from their predators and allowed them to increase and become dominant, decreasing evenness and diversity.

The unfertilized microecosystems, with more species than the fertilized, responded in the expected manner, with species diversity increasing with complexity. Possibly some prey species were protected from extinction when refuges were provided; however, complex interactions of the added materials with the water chemistry of the systems cannot be ruled out.

The stability-instability variable gave no significant results in the analysis of variance, and no significant differences in species number between stable and unstable microecosystems were observed. When the 2 levels of each variable were analyzed separately, the results were inconsistent. Unstable systems were usually more diverse if also complex, and stable microecosystems were more diverse if also simple, but some exceptions occurred. Despite the lack of single effects. this variable showed a number of interactions with the other 2. and was effective in causing 2 of 4 otherwise similar systems to diverge in their diversity as shown by the sign test. Probably the magnitude and duration of the temperature change was insufficient to cause the extinction of any species. The cold treatment may have affected the numbers of individuals in some species, accounting for the observed interactions of this variable with the others.

Chi-square tests of association between certain species and niicroecosystem treatments indicated that more species tended to occur in the unfertilized, complex or stable systems than tended to occur in the fertilized, simple or unstable systems. The absence of these species was, in part, responsible for the lower species number of the fertilized and of the simple microecosystems. This reduction in species number was inadequate to explain the reduction in species diversity. because the species diversity in all the fractions sampled. except the microcrustacea. was more closely related to evenness than to species number. None of the species showing a preference was numerous enough to influence the evenness of the systems in which it was found. Coleps, Spirostoinuwn and Tetra-

hymena, absent from many fertilized systems, and Rotaria, absent from many simple systems, may be important in controlling the populations of smaller organisms. Possibly it was their absence that allowed Ankistrodesmus, Oscillatoria, Chlorella -like organisms, Nitzschia, and the small flagellates to increase to tremendous numbers, giving a low evenness and a low species diversity. The first 2 organisms may be inedible to some herbivores (Porter 1977); interactions among bacteria, algae and the large ciliates and rotifers have not been well studied.

Other interspecific interactions may have occurred in cases where the association of diversity values with environmental variables was inconsistent. Extracel- lular products of bacteria and other organisms, includ- ing waste products, vitamins and growth factors probably contributed to the temporal changes in species composition and population size. Differences in bac- terial populations and in soluble organic compounds were probably greatest between the fertilized and unfertilized microecosystems.

Changes in species composition during the experiment were expected, based on observations of the organisms involved under natural conditions; the phytoplankton do not comprise an integrated community, but a loose association of species with minimal biotic interactions (Porter 1977). In addition, change in species number and composition until nutrients become limiting is a common feature of microecosystems (Taub 1969). In Taub's (1969) systems, populations eventually stabilized at high levels and nutrient turn- over rates were assumed to be low. The decrease in plankton species number after inoculation of the microecosystems. followed by an increase in species number in the present study supports the hypothesis of Cairns et al. (1971) that new habitats are condi- tioned by pioneer species. After this has occurred, more species are able to become established.

In some real-life situations, alterations of a single environmental or biotic factor may have a major effect on the species diversity of a certain community or habitat. However, to formulate a theory of species diversity with general predictive value, a single-factor approach is inadequate. Even when predictions can be made about the actions of certain factors individually, the total ecosystem response based on the interactions of factors may not be what is expected. A predictive study of species diversity in any ecosystem, whether undertaken for an environmental impact statement or in the hope of determining underlying ecological principles, must involve the manipulation of several variables simultaneously.

ACKNOWLEDGMENTS

I thank Bert 0. Thomas, John K. Gapter, James P. Fitz- gerald, Deborah J. Dunn, John M. Emlen and Paul Whitson for help with the project and writing. This paper is based on a dissertation submitted for the degree of Doctor of Arts in Biology, University of Northern Colorado.




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Species Diversity in Aquatic Microecosystems