Foundercontrolled communities

In the dominance-controlled communities discussed in Section 16.7.1 there was the familiar r and K selection dichotomy in which colonizing ability and competitive status are inversely related. In founder-controlled communities, on the other hand, all species are both good colonists and essentially equal competitors; thus, within a patch opened by disturbance, a competitive lottery rather than a predictable succession is to

... and gap shape colonization of gaps... ... in mussel beds,...

... and in mangrove forest founder-controlled communities: a competitive lottery not a predictable succession

Chthamalus bisinuatus

Small

Medium

Large

Small

Medium

Large

Colonizing Species Ecology

Collisella subrugosa

Collisella subrugosa

Brachidontes solisianus

e 40

Brachidontes solisianus

Brachidontes darwinianus

Brachidontes darwinianus

0I- i^fc^fcqfc

Sep Nov 1994

May July Sep 1995

0-49 50-99 100-400

Gap size (cm2)

ls 15

Chthamalus bisinuatus ls 15

Center Periphery

0.4

Brachidontes recruits

0.3

0.2

-

0.1

- y,Oil

0

- ? i ?

Collisella subrugosa

Collisella subrugosa

Colonizing Species Ecology

Sep Nov 1994

May July 1995

Sep Nov 1994

May July 1995

Year

Year

Figure 16.19 (a) The four shapes used in patch-shape experiments: square, circle, rectangle and 'sector' (see Table 16.5). (b) Size distribution of natural gaps in the mussel beds. (c) Mean abundances (±SE) of four colonizing species in experimentally cleared small, medium and large square gaps. (d) Recruitment of three species at the periphery (within 5 cm of the gap edge) and in the center of 400 cm2 square gaps. (After Tanaka & Magalhaes, 2002.)

Forest Gap Competitive Lottery

Figure 16.21 Hypothetical competitive lottery: occupancy of gaps which periodically become available. Each of species A-E is equally likely to fill a gap, regardless of the identity of its previous occupant. Species richness remains high and relatively constant.

Figure 16.20 Frequency distribution of gaps created by lightning in a tropical mangrove forest in the Dominican Republic. (After Sherman et al., 2000.)

Figure 16.21 Hypothetical competitive lottery: occupancy of gaps which periodically become available. Each of species A-E is equally likely to fill a gap, regardless of the identity of its previous occupant. Species richness remains high and relatively constant.

be expected. If a large number of species are approximately equivalent in their ability to invade gaps, are equally tolerant of the abiotic conditions and can hold the gaps against all comers during their lifetime, then the probability of competitive exclusion may be much reduced in an environment where gaps are appearing continually and randomly. A further condition for coexistence is that the number of young that invade and occupy the gaps should not be consistently greater for parent populations that produce more offspring, otherwise the most productive species would come to monopolize space even in a continuously disturbed environment.

If these idealized conditions are met, it is possible to envisage how the occupancy of a series of gaps will change through time (Figure 16.21). On each occasion that an organism dies (or is killed) the gap is reopened for invasion. All conceivable replacements are possible and species richness will be maintained at a high level. Some tropical reef communities of fish may conform to this model (Sale, 1977, 1979). They are extremely diverse. For example, the number of species of fish on the Great Barrier Reef ranges from 900 in the south to 1500 in the north, and more than 50 resident species may be recorded on a single patch of reef 3 m in diameter. Only a proportion of this diversity is likely to be attributable to resource partitioning of food and space - indeed, the diets of many of the coexisting species are very similar. In this community, vacant living space seems to be a crucial limiting factor, and it is generated unpredictably in space and time when a resident dies or is killed. The lifestyles of the species match this state of affairs. They breed often, sometimes year-round, and produce numerous clutches of dispersive eggs or larvae. It can be argued that the species compete in a lottery for living space in which larvae are the tickets, and the first arrival at the vacant space wins the site, matures quickly and holds the space for its lifetime.

Three species of herbivorous pomacentrid fish co-occur on the upper slope of Heron Reef, part of the Great Barrier Reef off eastern Australia. Within rubble patches, the available space is occupied by a series of contiguous and usually nonoverlapping territories, each up to 2 m2 in area, held by individuals of Eupo-macentrus apicalis, Plectroglyphidodon lacrymatus and Pomacentrus wardi. Individuals hold territories throughout their juvenile and adult life and defend them against a broad range of chiefly herbivorous species, including conspecifics. There seems to be no particular tendency for space initially held by one species to be fish coexisting on coral reefs

Table 16.6 Initial size, and growth and mortality rates over a 1-year period of saplings of three mangrove species in lightning-induced gaps and under intact forest canopy. (After Sherman et al., 2000.)

Initial sapling diameter (cm ± SE)

Growth rate-diameter increment (cm ± SE)

Mortality (%)

Gaps

Canopy

Gaps

Canopy

Gaps

Canopy

Rhizophora mangle

1.9 ± 0.06

2.3 ± 0.06

0.58 ± 0.03

0.09 ± 0.01

9

16

Laguncularia racemosa

1.7 ± 0.11

1.8 ± 0.84

0.46 ± 0.04

0.11 ± 0.06

32

40

Avicennia germinans

1.3 ± 0.25

1.7 ± 0.45

0.51 ± 0.04

-

56

88

Table 16.7 Numbers of individuals of each species observed occupying sites, or parts of sites, that had been vacated during the immediately prior interperiod between censuses through the loss of residents of each species. The sites vacated through loss of 120 residents have been reoccupied by 131 fish; the species of the new occupant is not dependent on the species of the previous resident.

Table 16.7 Numbers of individuals of each species observed occupying sites, or parts of sites, that had been vacated during the immediately prior interperiod between censuses through the loss of residents of each species. The sites vacated through loss of 120 residents have been reoccupied by 131 fish; the species of the new occupant is not dependent on the species of the previous resident.

Reoccupied by:

Resident lost

E. apicalis

P. lacrymatus

P. wardi

Eupomacentrus apicalis

9

3

19

Plectroglyphidodon lacrymatus

12

5

9

Pomacentrus wardi

27

18

29

taken up, following mortality, by the same species. Nor is any successional sequence of ownership evident (Table 16.7). P. wardi both recruited and lost individuals at a higher rate than the other two species, but all three species appear to have recruited at a sufficient level to balance their rates of loss and maintain a resident population of breeding individuals.

Thus, the maintenance of high reef diversity depends, at least in part, on the unpredictability of the supply of living space; and as long as all species win some of the time and in some places, they will continue to put larvae into the plankton, and hence, into the lottery for new sites. An analogous situation has been postulated for the highly diverse chalk grasslands of Great Britain (Grubb, 1977) and even for trees in temperate and tropical forest gaps (Busing & Brokaw, 2002). Any small gap that appears is rapidly exploited, by a seed in grassland and very often by a sapling in a forest gap. In these cases, the tickets in the lottery are saplings or seeds (either in the act of dispersal or as components of a persistent seed bank in the soil). Which seeds or saplings develop to established plants, and therefore which species comes to occupy the gap, may depend on a strong random element since many species overlap in their requirements for successful growth. The successful plant rapidly establishes itself and retains the patch for its lifetime, in a similar way to the reef fish described above.

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