Formation and filling of gaps

Gaps of different sizes may influence community structure in different ways because of contrasting mechanisms of recolonization. The centers of very large gaps are most likely to be colonized by species producing propagules that travel relatively great distances. Such mobility is less important in small gaps, since most recolonizing propagules will be produced by adjacent established individuals. The smallest gaps of all may be filled simply by lateral movements of individuals around the periphery.

Intertidal beds of mussels provide excellent opportunities to study the processes of formation and filling-in of gaps. In the absence of disturbance, mussel beds may persist as extensive monocultures. More often, they are an ever-changing mosaic of many species that inhabit gaps formed by the action of waves. Gaps can appear virtually anywhere, and may exist for years as islands in a sea of mussels. The size of these gaps at the time of formation ranges from the dimensions of a single mussel to hundreds of square meters. In general, a mussel or group of mussels becomes infirm or damaged through disease, predation, old age or, most often, the effects of storm waves or battering by logs. Gaps begin to fill as soon as they are formed.

influence of gap size...

Table 16.5 Measures of area, perimeter and perimeter : area ratio for the experimental gaps created in two experiments on semiexposed shores in southeast Brazil. (From Tanaka & Magalhaes, 2002.)

Area (cm2) Perimeter (cm) Perimeter : area ratio

Table 16.5 Measures of area, perimeter and perimeter : area ratio for the experimental gaps created in two experiments on semiexposed shores in southeast Brazil. (From Tanaka & Magalhaes, 2002.)

Area (cm2) Perimeter (cm) Perimeter : area ratio

Patch size effects

Square

25

20

0.8

Square

100

40

0.2

Square

400

80

0.2

Patch shape effects

Square

100.0

40.0

0.4

Circle

78.5

31.4

0.4

Rectangle

112.5

45.0

0.4

Sector

190.1

78.6

0.4

In their experimental study of mussel beds of Brachidontes solisianus and B. darwinius in Brazil, Tanaka and Magalhaes (2002) aimed to determine the differential effects of patch size and perimeter : area ratio on the dynamics of succession. In an experiment on one moderately exposed shoreline, they created square gaps with different areas (because of identical shapes, the bigger squares had smaller perimeter : area ratios) (Table 16.5). On a nearby and physically very similar shore, they created patches of four different shapes and chose areas for each that produced identical perimeter : area ratios (Figure 16.19a). Note that a circle has the most perimeter per unit area of any shape. The gap sizes were within the range observed for natural gaps, which did not differ on the two shores (Figure 16.19b).

Higher densities of the herbivorous limpet Collisella subrugosa occurred in the small gaps in the first 6 months after gap formation (Figure 16.19c). Small gaps, compared to medium and large gaps, were also most quickly colonized by lateral migration of the two mussel species, but with B. darwinius predominating. The larger gaps had higher densities of the barnacle Chthamalus bisinuatus and sheltered more limpets at their edges, while central areas had more Brachiodontes recruited from larvae after 6 months (Figure 16.19d). The gaps with identical perimeter : area ratios showed very similar patterns of colonization despite their different sizes, emphasizing that colonization dynamics are mainly determined by distance from adjacent sources of colonists.

The limpet is probably associated with patch edges because here they are less vulnerable to visually hunting predators. The negative relationship between distributions of the limpet and the barnacle may be due to the former dislodging the latter from the substrate. Tanaka and Magalhaes conclude that the mussel

B. darwinius is a more effective colonist of disturbed patches than B. solisianus, and suggest that B. darwinius would gradually come to dominate the whole of the shoreline if it were not for occasional massive recruitment events of B. solisianus.

The pattern of colonization of gaps in mussel beds is repeated in almost ... in grassland ... every detail in the colonization of gaps in grassland caused by burrowing animals or patches killed by urine. Initially, leaves lean into the gap from plants outside it. Then colonization begins by clonal spread from the edges, and a very small gap may close up quickly. In larger gaps, new colonists may enter as dispersed seed, or germinate from the seed bank in the soil. Over 2-3 years the vegetation begins to acquire the character that it had before the gap was formed.

The gaps produced in forests vary greatly in size. Lightning-induced gaps in mangrove forest in the Dominican Republic, for example, range from 200 to 1600 m2 or more (Figure 16.20). Lightning almost always kills groups of trees in a 20 -30 m circle, and the trees remain as standing dead for several years. In a forest dominated by red mangrove Rhizophora mangle and white mangrove Laguncularia racemosa, and with some black mangrove Avicennia germinans, Sherman et al. (2000) compared the performance of the three species in lightning gaps and under forest canopy. Seedling density did not differ in gaps and intact forest, but sapling density and the growth rates of all three species were much higher in the gaps (Table 16.6). However, gap regeneration was dominated by R. mangle because its mortality rate was much lower in gaps than was the case for the other species. Sherman et al. (2000) note that the peat mat on the forest floor usually collapses after lightning damage, resulting in increased levels of standing water. They suggest that the success of R. mangle in gaps is due to their higher tolerance of flooding conditions.

Organisms other than plants can also be overrepresented in gaps. In a study of tropical rainforest in Costa Rica, Levey (1988) found that nectarivorous and frugivorous birds were much more abundant in treefall gaps, reflecting the fact that understory plants in gaps tend to produce more fruit over a longer period than conspecifics fruiting under a closed canopy.

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