Frequency of gap formation

The influence that disturbances have on a community depends strongly on the frequency with which gaps are opened up. In this context, the intermediate disturbance hypothesis (Connell, 1978; see also the earlier account by Horn, 1975) proposes that the highest diversity is maintained at intermediate levels of disturbance. Soon after a severe disturbance, propagules of a few pioneer species arrive in the open space. If further disturbances occur frequently, gaps will not progress beyond the pioneer stage in Figure 16.16, and the diversity of the community as a whole will be low. As the interval between disturbances increases, the diversity will also increase because time is available for the invasion of more species. This is the situation at an intermediate frequency of disturbance. At very low frequencies of disturbance, most of the community for most of the time will reach and remain at the climax, with competitive exclusion having reduced diversity. This is shown diagrammatically in Figure 16.17, which plots the pattern of species richness to be expected as a result of unphased high, intermediate and low frequencies of gap formation, in separate patches and for the community as a whole.

The influence of the frequency of gap formation was studied in southern California by Sousa (1979a, 1979b), in an intertidal algal community associated with boulders of various sizes. Wave action disturbs small boulders more often than large ones. Using a sequence of photographs, Sousa estimated the probability that a given boulder would be moved during the course of 1 month. A class of mainly small boulders (which required a force of less than 49 Newtons to move them) had a monthly probability of movement of 42%. An intermediate class (which required a force of 50-294 N) had a much smaller monthly probability of movement,

Connell's 'intermediate disturbance hypothesis'

boulders on a rocky shore that vary in disturbability...

Frequent

Gap 1

Gap 2

Gap 3

Whole community

Rate of disturbance Intermediate

Rare

Figure 16.17 Diagrammatic representation of the time course of species richness in three gaps, and in the community as a whole, at three frequencies of disturbance. The disturbance is unphased. Dashed lines indicate the phase of competitive exclusion as the climax is approached.

9%. Finally, the class of mainly large boulders (which required a force >294 N) moved with a probability of only 0.1% per month. The 'disturbability' of the boulders had to be assessed in terms of the force required to move them, rather than simply in terms of top surface area, because some rocks which appeared to be small were actually stable portions of larger, buried boulders, and a few large boulders with irregular shapes moved when a relatively small force was applied. The three classes of boulder (<49, 50-294 and >294 N) can be viewed as patches exposed to a decreasing frequency of disturbance when waves caused by winter storms overturn them.

Species richness increased during early stages of succession through a process of colonization by the pioneer green alga Ulva spp. and various other algae, but declined again at the climax because of competitive exclusion by the perennial red alga Gigartina canaliculata. It is important to note that the same succession occurred on small boulders that had been artificially made stable. Thus, variations in the communities associated with the surfaces of boulders of different size were not simply an effect of size, but rather of differences in the frequency with which they were disturbed.

Communities on unmanipulated boulders in each of the three size/ disturbability classes were assessed on four occasions. Table 16.4 shows that the percentage of bare space decreased from small to large boulders, indicating the effects of the greater frequency of disturbance of small boulders. Mean species richness was lowest on the regularly disturbed small boulders. These were dominated most commonly by Ulva spp. (and barnacles, Chthamalus fissus). The highest levels of species richness were consistently recorded on the intermediate boulder class. Most held mixtures of three to five abundant species from all successional stages. The largest boulders had a lower mean species richness than the intermediate class, although a monoculture was achieved on only a few boulders. G. canaliculata covered most of the rock surfaces.

These results offer strong support for the intermediate disturbance hypothesis as far as frequency of appearance of gaps is concerned. However, we must be careful not to lose sight of the fact that this is a highly stochastic process. By chance, some small boulders were not overturned during the period of study. These few were dominated by the climax species G. canaliculata. Conversely, two large boulders in the May census had been overturned, and these became dominated by the pioneer Ulva. On average, however, species richness and species composition followed the predicted pattern.

This study deals with a single community conveniently composed of identifiable patches (boulders) that become gaps (when overturned by waves) at short, intermediate or long intervals. Recolonization occurs mainly from propagules derived from other patches in the community. Because of the pattern of disturbance, this mixed boulder community is more diverse than would be one with only large boulders.

Disturbances in small streams often take the form of bed movements during periods of high discharge. Because of differences in flow regimes and in the substrates of stream beds, some stream communities are disturbed more frequently and to a larger extent than others. This variation was assessed in 54 stream sites in the Taieri River in New Zealand (Townsend et al., 1997) by recording the frequency at which at least 40% (chosen arbitrarily) of the bed moved and the average percentage that

... provide support for the hypothesis further support from a study of streams

Census date

Species richness

Boulder class

Percentage bare space

Mean

Standard error

Range

Census date

Mean

Standard error

November 1975

< 49

78.0

1.7

0.18

1-4

50-294

26.5

3.7

0.28

2-7

> 294

11.4

2.5

0.25

1-6

May 1976

< 49

66.5

1.9

0.19

1-5

50-294

35.9

4.3

0.34

2-6

> 294

4.7

3.5

0.26

1-6

October 1976

< 49

67.7

1.9

0.14

1-4

50-294

32.2

3.4

0.40

2-7

> 294

14.5

2.3

0.18

1-6

May 1977

< 49

49.9

1.4

0.16

1-4

50-294

34.2

3.6

0.20

2-5

> 294

6.1

3.2

0.21

1-5

Table 16.4 Seasonal patterns in bare space and species richness on boulders in each of three classes, categorized according to the force (in Newtons) required to move them. (After Sousa, 1979b.)

Figure 16.18 Relationship between invertebrate species richness and: (a) frequency of disturbance - assessed as the number of occasions in 1 year when more than 40% of the bed moved (analysis of variance significant at P < 0.0001), and (b) intensity of disturbance - average percentage of the bed that moved (polynomial regression fitted, relationship significant at P < 0.001) assessed at 54 stream sites in the Taieri River, New Zealand. The patterns are essentially the same; intensity and frequency of disturbance are strongly correlated. (After Townsend et al., 1997.)

Figure 16.18 Relationship between invertebrate species richness and: (a) frequency of disturbance - assessed as the number of occasions in 1 year when more than 40% of the bed moved (analysis of variance significant at P < 0.0001), and (b) intensity of disturbance - average percentage of the bed that moved (polynomial regression fitted, relationship significant at P < 0.001) assessed at 54 stream sites in the Taieri River, New Zealand. The patterns are essentially the same; intensity and frequency of disturbance are strongly correlated. (After Townsend et al., 1997.)

moved (assessed on five occasions during 1 year, using painted particles of sizes characteristic of the stream bed in question). The pattern of richness of insect species conformed to the intermediate disturbance hypothesis (Figure 16.18). It is likely that low richness at high frequencies and intensities of disturbance reflects the inability of many species to persist in such situations. Whether low richness at low frequencies and intensities of disturbance is due to competitive exclusion, as proposed in the intermediate disturbance hypothesis, remains to be tested.

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