Nā Pōhaku o Hauwahine




What is Ecological Succession?

Succession is an important concept in ecology and must be understood in order to fully grasp the purpose and philosophy of our planting efforts at Nā Pōhaku o Hauwahine. Succession is the natural change (the Odum's would say "orderly" change) in the composition of an ecosystem from pioneering stages through climax<1>. Allow me to clarify by example. After a serious disturbance—let us imagine something both relevant and effective, like cutting down all of the existing non-native vegetation—new plants appear on this disturbed landscape. These plants are, for the most part, different from those that occupied the area previously. They appear from seeds or spores, some left in the area and representing the removed or nearby vegetation, and others brought in by birds, wind, and hikers (accidently on shoes). Many plants are very good at broadly distributing their seeds or spores, so any effective removal of vegetation is followed fairly quickly by a replacement vegetation, at least once the rains appear. So certain is this process that only extremely bad soils (or environments extreme in some other respect) see little or no regrowth of anything.

The composition of the "new" vegetation is not going to be a replication of what was removed, unless that removed vegetation was the result of some other recent disturbance. If a forest is removed, replacement of the trees might take years, perhaps decades. Instead, the plants that appear are species that can do well in the new "open" (no longer shaded) environment, and may be none of the previous inhabitants but for scattered seedlings of the harvested trees. Shrubs and herbs previously growing in the forest will do poorly after the removal of shade. Species best adapted to disturbed conditions are known as pioneer species. We also call these ruderal weeds: the sort of plants that take over an empty lot after it is cleared. Although the ruderal species we now generally encounter are non-natives, native pioneer species do exist. The plants that come in not long after a lava flow has burned out and buried the previous vegetation are native pioneers uniquely adapted to taking hold on bare rock and cinder<2><3>.

If left undisturbed, these pioneer species will eventually themselves be replaced in a process called succession (that is, one group of plant species succeeds another). The very first plants alter the original harsh environment, gradually making it more suitable for other plants. Eventually, conditions encourage the return of the forest, or at least a forest. The constant introduction of new species to the Islands greatly complicates orderly succession, introducing new possibilities of plant species dominating any particular landscape to the exclusion of the natives that would have succeeded there.

Consider all this in light of the past history of these isolated islands. The Polynesians arrived and cut down the lowland forests to use the land for villages and agriculture. Ruderal "weeds" moved in, being mostly plants accidently introduced by the immigrants from elsewhere in the Pacific. Much of the lowland landscape was utilized in support of species—the so-called "canoe plants"—useful to these first inhabitants. As effective as this transformation of the landscape proved to be, lacking the modern-day global transportation network very much limited the number of plant species introduced to new places as Polynesians emigrated across the Pacific.

Modern history of disturbance of the land has had a far different outcome. More effective land clearing, introduction of "landscaping" with ornamentals from afar, and opening of routes of introduction for seeds and spores from all corners of the globe set up scenarios that greatly disadvantaged the native flora of the Islands. The introduction of these alien plant species has and continues to alter the multiple courses of succession that had previously led to stable, native ecosystems (the climax plant communities). Every disturbance of a native ecosystem accelerates the process of invasion by restarting a process of succession with many new "players" involved. Still, succession is a process and processes can be interfered with. If we can mess with the natural order to have an undesirable outcome, it should be possible to mess with the "unnatural" order and guide it towards a desirable outcome. And we do this by messing with the seed bank.

Basic principals: Seed Bank CLICK HERE


ma'o hau hele - Hawai'i state flower

Hibiscus brackenridgei A. Gray




Note <1> The "climax" community being an assemblage of species that will dominate the location more or less without change until a new disturbnance occurs. Climax is essentially the end of ecosystem succession, for as long as the environment remains stable.
Note <2>The Overstory agroforestry ejournal has a relevant (Hawai‘i) article (#22 Pioneering Difficult Sites) on this subject for those wishing further reading.
Note <3> –Closer to home (if you reside on O‘ahu), consider the fate of mass-wasting landslides on the upper slopes of the Ko‘olau such as can be observed from the H-3 on the Honolulu side of the Tetsuo Harano tunnels. These steep, bare cuts are exposures of extremely poor "soil" with minimal stability. However, the native uluhe fern (Dicranopteris linearis) is adapted by virtue of its rambling growth form to cover the scars and begin a process of succession resulting in soil improvement and physical stability, eventually permitting forest plants to recolonize the slope.

Page last modified: June 14, 2011 by Webmaster (webguy@aecos.com). © 2011. Eric B. Guinther ~ All rights reserved.