Mangrove Ecosystems: Definitions, Distribution, Zonation, Forest Structure, Trophic Structure, and Ecological Significance
By Karen L. McKee

Zonation
Spatial variation in species occurrence and abundance is frequently observed across environmental gradients in many types of ecosystems (Davis 1940; Smith 1992; Mendelssohn & McKee 2000). Zonation of plant communities in intertidal habitats is particularly striking and often results in monospecific bands of vegetation occurring parallel to the shoreline. Although zonation patterns are usually depicted in a manner that suggests a rigid sequence proceeding from the shoreline to upland regions, many patterns resemble a mosaic with vegetational patterns occurring repeatedly where the land mass is interrupted by watercourses or other variations in topography.

Mangroves exhibit zonation patterns in a number of different geographic regions (Davis 1940; Smith 1992; Mendelssohn & McKee 2000). The large variation in floristic composition of mangrove communities means that patterns of species distribution across the intertidal zone will vary substantially among geographic regions. For example, patterns for Florida and the Caribbean often show R. mangle (red mangrove) occupying the seaward zone, followed by A. germinans (black mangrove), and L. racemosa (white mangrove) in the most landward position. That pattern may be contrasted with a profile for northeastern Australia (Queensland), which is not only more complex due to a higher number of species, but the relative position of congeneric species is reversed from that in Florida (e.g., Avicennia spp. in the seaward position and Rhizophora spp. in the landward position).
Zonation patterns in mangrove forests may also vary on a local scale. Occurrence of species may differ across an estuary, apparently in response to differences in freshwater input. For example, species found at the seaward end of the estuary may be absent from the headwaters. Although zonation typically refers to patterns created by segregation of different species, differences in stature and productivity of plants across environmental gradients may also result in readily discernible patterns.
Zones may be comprised of different architectural forms that represent variations in height and vigor.

Several hypotheses have been proposed to explain species zonation patterns in mangroves:
1) zonation reflects land building and plant succession (Davis 1940);
2) geomorphological processes cause vegetation zonation (Thom 1967);
3) differential dispersal of propagules across a gradient results from a physical “sorting out” of species by tidal action (Rabinowitz 1978);
4) differential predation of propagules across the intertidal zone eliminates some species from certain zones (Smith 1987; Smith et al.1989, but see McKee 1995);
5) physiological specialization limits distribution of species to certain portions of the gradient where physicochemical conditions differ (Ball 1988; McKee 1993, 1995); and
6) interspecific competition (Ball 1980).

Succession due to land building is not considered to be a viable explanation for zonation by many mangrove ecologists, since evidence shows that mangroves respond to, rather than cause, coastal propagation (Thom 1967). However, it’s clear that some mangrove systems in sediment-poor environments have built vertically through deposition of organic matter (mangrove peat) (Woodroffe 1983; McKee & Faulkner 2000). Mangroves are probably best viewed as steady-state cyclical systems migrating toward or away from the sea depending on sea-level rise or fall, sedimentation rates, topography and tidal energy (Lugo 1980). Coastal geomorphology is important in determining physical and chemical conditions for mangrove development and may explain regional differences in zonation patterns. Geomorphology as an explanation of intertidal zonation patterns is unsatisfactory, however, because it provides no insight as to how the interaction of geomorphological processes with vegetation causes a segregation of species. The remaining four hypotheses—dispersal dynamics, seed predation, physiological tolerance, and interspecific competition—offer clear explanations for mangrove zonation. The relative importance of these processes is currently uncertain, but probably varies among geographic regions.

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