Gleason (1962) opposed Clement's successional theory but agreed with Egler's
 second model of succession. Gleason (1962) said that entirely different vegetation
 associations might occupy climatically identical sites. Basically, he theorized that
 species are organized along environmental gradients and that species existing together do
 so because they arrive together and endure the site conditions.

 E. P. Odum (1969, 1971) restated classical successional theory as an orderly,
 reasonably directional, foreseeable process that results in alteration of the physical
 environment by the community and ending in a constant or climax ecosystem. He
 described succession as community-controlled, where the physical environment decides
 the pattern and rate of change within the ecosystem. He suggested that succession is not
 a simple clear-cut idea, but that multiple successional pathways are possible for any given
 ecosystem. He described succession as involving an interacting development of
 processes that often offset one another and physical limitations that can set restrictions on
 system development. At the next larger or regional scale, the system selects and supports
 those species that feed back energies and materials that maximize flow (Odum 1994).

 Horn (1971, 1974) proposed that early successional species create an environment
 where later successional species are competitively superior. In essence, pioneer species
 prepare the way for the more complex structure of late succession (Odum and others
 1997; Odum 1994; Rushton 1983). The organization of an ecosystem changes through
 time. Succession is an ever-changing procedure, developing structure and processing
 energy (Richardson 1988).

 In its broadest sense, succession deals with the initial approach or return to a
 climax or steady state condition of a system from some non-climax state. Margalef
 (1968) termed this stage maturity. He defined maturity as relating only to those
 situations that start with low quantities of resource materials and species where time
 causes increases in these quantities in reaching a steady pattern. These resource materials
 appear in many forms including the accumulation of high quality matter and structure by
 ecosystems on land in the form of soil and partly in the vegetation structure of above
 ground biomass (trunks, branches, leaves), below ground biomass (roots, tubers),
 microbes, and animals.

 Generally speaking, succession involves possible choices based on positive
feedbacks that work toward maximizing power and useful transformation (H. T. Odum
1994; E. P. Odum 1971). Odum (1994) offers a bioenergetics view of ecological
succession and self-organization that focuses on energy inputs and energy use by the
developing system. This definition of succession emphasizes "useful" power resulting
from the physical structure being built. This feedback energy acts as reinforcement and
intensifies efficiencies and energy flows into the structure of the system. The efficiency
of the maximum power principle at each successional stage emphasizes the advancement
of natural succession. Rushton (1983) hypothesized that when choices are limited,
ecosystem development decreases, and stress on the ecosystem delays the timely
evolution of new components when inflowing energy sources are limiting.