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Variability in the physical, chemical, and bioloqical environment
Variability in the environment plays a key role in the development of islands and their
charaeteristie reef systems. On the whole, islande may be considered as being unstable
ecosystems. Factors in the physical and chemical environrnent are discussed by McLean
(f980b). These are divided into factors of the land, the sea, and the atmosphere. In the first
category falls the immense vaniety of island types already discussed. Also considered here is
the plate tectonie setting of rnost islands in the tropical Pacific which is responsible for
long-term instabiliiy in the form of vertical and horizontal movemenls, and present
perturbations zuch as earthquakes and vulcanism.
Of the factors associated with the sea, perhaps the most severe effect exerted on
island ecosysLems has been that of the rise in sea level in recent geologic time. This
phenomenon has resulted in changes in island distnibution, size, and type. Small oceanic
islands were particularly vulnerable, being drastically altered or disappearing allogether.
Reefs either grew proportionately, or wene drowned (McLean, 1980a). Present evidence
suggests that sea level is continuing to rise (McLean, f980b). A second factor lhat has
shaped much of coastal morphology and that is responsible for the formation of a number of
islands are ocean waves. Of partieularly great. impact over Lhe shorl term are tsunamis, storm
surges, and hurricane waves. Effects of hurricane waves on reef structures in the Pacific
have been documented by Baines et a! (1974), Baines and McLean Q97O, and Randall and
Eldredge (1977). Fortunately, reefs afford a measure of protection for island shores againsl
the destructive effects of the sea. Altogether, it should be emphasized that there exists an
intimate linkage between reefs, marine processes, and the zubaerial coastal land in the
building and alteration of coastal features (McLean, 1980a). The latter is known to have
occurred rapidly throughout prehistoric times.
Factors of the atmosphere are wind, rainfall, and storms. These are closely linked with
currents and the oceanographic processes described above, as well as relevent chemical
parameters such as salinity and rutrient loading.
Perhaps the most significant biological factor affecting Pacific reefs on a large scale,
and hence worth mentioning here, is predation by the crown-of-thorns starfish Acanthaster
planci (Cheney, 1974; Nishihira and Yamazato, 1974). Repeated population explosEiilFlFiE
organism have occurred in a number of localities and are believed to be cyclic, though
probably aggravated by human interference in the environnrent. These have been reported to
eeuse extrenre damage to reefs, as much as 90% in a single area. Serious concern has thus
been generated (e.9. Chesher, 1969), including speculations on severe economie loss and the
possible extinction of scleractinian corals in some erees. Additional studies indicate, however,
that this corel-eating starfish may be a normal component, of tropical Pacific eoral reef
communities, and lhat the publicized population explosions in areas such as Guam end Pelau
may be isolated, local infeetations of an unknown cause (Weber and Woodhead, 1970). At
present, it does not appear to pose a serious threat in any part of the region (Marsh and
Tsuda, 1971; Alcala, 1976).
Fraoilitv of gmall ieland ecosvetems
As a result of physical and biological instability generated by the fectors discussed
above, and becauee of lhe significant role of isolation (Desmann, 1971; Grigq, 1979), island
ecosystems have evolved to be extrennly fragile and vulnerable to disturtance. This
vulnerability extends even to disruptions of locsl flora and fauna by the introduction of new
speciea because of the high degrees of specializstion that the forrner have achieved through
evolutionery time. Other sources of vulnerability are rcstricted land and reef areas, and
limited carrying capacities for hurnan settlement (Meson, 1979).
Small islands tend to be more densely populeted than Iarger ones, and hence epproach
rmre closely the limits of the environrnent to arpport the population (Bayliss-5mith, f978). A
more extended discugsion of the camying capacities of islends with r€8pect to human
populations is provided by Bayliee-Smith (1980),