The Two Factor Hypothesis

The advent in the mid-twentieth century of field experiments, conducted as manipulations of the densities consumers or potential competitors, demonstrated that species interactions can affect lower boundaries. For practical reasons, most experimental studies focused on manipulations of a single large or abundant species, which was sometimes coupled with natural or experimentally induced variation of a physical factor. The findings gave rise to the two-factor hypothesis, in which the upper boundary of a species is set by physical stress (extreme temperature or desiccation) and the lower boundary is set by biotic interactions, primarily competition or predation. Thus, tidal dynamics do play a role in this view, but its influences on patterns of zonation are both direct, by imposing physical stress at the upper boundaries, and indirect, by moderating the intensity of species interactions at the lower boundary. For example, the up-shore movement of aquatic consumers is constrained by tidal emersion, favoring the growth of dense prey populations beyond the predator's reach.

Several lines of evidence support the proposition that physical stresses set upper boundaries. On wave-exposed shores of the British Isles, spat (newly settled juveniles) of the barnacle Semibalanus balaniodes above the zone of adults die when prolonged periods of emergence of neap tidal series coincide with hot weather. In sheltered areas, low tides and episodes of hot weather in summer sometimes coincide with little up-shore wave wash. During these episodes, the plants comprising the upper margins of Pelvetia sp. and Fucus sp. die, and the boundaries descend. Upper boundaries of sedentary animals and plants shift upward in the runoff of artificially constructed high shore pools, or of drip bottles set above the reach of the waves. Thus, it appears that the upper limit of many species is set by desiccation or other physical stresses associated with emersion.

An effect of biotic factors, (primarily competition or predation) on the lower boundaries was established with population manipulations. When fragments of rock bearing juvenile barnacles Chthamalus spp. were transplanted from the high intertidal zone ofthe adults to a lower shore level covered by the barnacle S. balanoides, the Chthamalus suffered high mortality rates as it was overgrown and undercut by juvenile Semibalanus settling among them. Semibalanus showed faster seasonally varying growth rates, and periods of its most rapid growth coincided with the highest mortality rates of the slower-growing Chthamalus. Thus, competition for attachment space appears to set the lower limit of Chthamalus. Removal of predacious sea stars Pisaster ochraceus from of wave-beaten shores of the Pacific Northwest caused the downward expansion of the zone of sea mussels, Mytilus californianus, into the zone of algae and other sedentary invertebrates below. These early experiments were corroborated by others producing downward expansions of zones in response to removal of predators of competitors.

The experimental evidence demonstrates that physical and biotic factors play a role in boundary formation. It does not imply that physical and biotic factors always act independently of each other at either boundary. Critical examination of experimental findings for some systems reveals complex dynamics. For example, the eulittoral zone on some wave-exposed shores may be subdivided into an upper zone with abundant limpet populations and sparse covers of erect algae, and a lower zone characterized by high covers of relatively large algae and few if any adult limpets. Experiments show that the long periods of emersion in the upper half of the eulittoral zone limit the growth of the erect algae, and the small, stunted algal fronds are easily removed by the limpets, setting an upper boundary of the algae. Below, the algae grow rapidly into thick stands of large individuals that are well defended against limpet grazing, and the massive algal holdfasts and moving stipes dislodge the limpets, an instance of competition for space. Tidal emersion in this case regulates algal growth rates, and the resulting vertical cline in growth rates causes the shift in the relative degrees of grazing and competition. Thus, physical factors act only indirectly in the formation of the upper boundary of the algae, by moderating biotic processes.

One implication of these findings is that the tidal emersion gradient presents organisms with a spatial cline in resources (nutrients and opportunity for settlement) in addition to a cline in physical stress. The role of this compound gradient is illustrated by the following examples. On sheltered shores of the British Isles, the upper boundary of alga F. vesiculosus shifts upward following experimental or natural clearings of the band of F. spiralis above. The upward expansion remains for years, until an episode of hot weather kills the highest plants, driving the boundary back to lower levels. Thus, it appears that competition is a persistent factor in the mechanism setting the upper boundary F. vesiculosus, and mechanisms have not evolved in this species conferring tolerance to the episodic physical stresses that occur above the level of the boundary set by competition. Similarly, the mussel M. californianus displays indeterminate growth: initial growth rates, and the maximum size it eventually attains, depend directly on nutrient availability. Individuals comprising the upper boundary feed for shorter periods, grow slowly, and attain sizes only slightly larger than the minimum reproductive size. Episodic desiccation or freezes are observed to readily kill juvenile mussels at and above the level of the upper boundary of adults, but rates settlement there are a small fraction of those on lower shore levels. Thus, while physical stresses may be the proximal cause of the loss of the sparse cohorts settling near the upper boundary, organism responses to the spatial cline in resources appear to be the ultimate determinant of the boundary.

The preceding examples suggest a final limitation of the two-factor hypothesis. Zonation is explained as the result of post-settlement mortality, and therefore, the hypothesis does not apply to cases in which vertical distribution is fixed by the processes of settlement. In an example drawn from a harbor on the Atlantic seaboard of North America, larvae of the acorn barnacle S. glandula occur in the water column near the surface, while larvae of the barnacles Balanus crenatus occur in a deeper stratum just below S. glandula. The stratification transfers directly to vertical settlement patterns on the shore, and thence, to the zonation of the adults. Post-settlement competition and predation do not appear to play a role. The two-factor hypothesis is the most widely disseminated explanation of the mechanisms of zonation. However, it is not universally applicable.

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