Cyclic succession is a pattern of vegetation change in which in a small number of species tend to replace each other over time in the absence of large-scale

disturbance Disturbance and its variants may refer to: Math and science * Disturbance (ecology), a temporary change in average environmental conditions that causes a pronounced change in an ecosystem * Disturbance (geology), linear zone of faults and folds ...

. Observations of cyclic replacement have provided evidence against traditional Clementsian views of an end-state

climax community In scientific ecology, climax community or climatic climax community is a historic term for a community of plants, animals, and fungi which, through the process of ecological succession in the development of vegetation in an area over time, hav ...

with stable

species composition Species richness is the number of different species represented in an community (ecology), ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the Abundance (ecology), abunda ...

s. Cyclic succession is one of several kinds of

ecological succession Ecological succession is the process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire) or more or less. Bacteria allows for the cycling of nutrients such as ca ...

, a concept in

community ecology In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, ...

. When used narrowly, 'cyclic succession' refers to processes not initiated by wholesale

exogenous In a variety of contexts, exogeny or exogeneity () is the fact of an action or object originating externally. It contrasts with endogeneity or endogeny, the fact of being influenced within a system. Economics In an economic model, an exogeno ...

disturbances or long-term physical changes in the environment. However, broader cyclic processes can also be observed in cases of

secondary succession Secondary succession is the secondary ecological succession of a plant's life. As opposed to the first, primary succession, secondary succession is a process started by an event (e.g. forest fire, harvesting, hurricane, etc.) that reduces an alrea ...

in which regular disturbances such as insect outbreaks can 'reset' an entire community to a previous stage. CyclicSuccession

These examples differ from the classic cases of cyclic succession discussed below in that entire species groups are exchanged, as opposed to one species for another. On geologic time scales, climate cycles can result in cyclic vegetation changes by directly altering the physical environment.

History

The cyclic model of succession was proposed in 1947 by British ecologist

Alexander Watt Alexander Stuart Watt FRS(21 June 1892 – 2 March 1985) was a Scottish botanist and plant ecologist. Life Watt was born on an Aberdeenshire farm and went to school at Turriff Secondary School and Robert Gordon's College, Aberdeen. He gradua ...

. In a seminal paper on vegetation patterns in grass, heath, and bog communities, Watt describes the plant community is a regenerating entity consisting of a "space-time mosaic" of species, whose cyclic behavior can be characterized by

patch dynamics Patch dynamics is an ecological perspective that the structure, function, and dynamics of ecological systems can be understood through studying their interactive patches. Patch dynamics, as a term, may also refer to the spatiotemporal changes wi ...

. Based on the current composition and its corresponding stage of succession, he explains, a community can either be in an 'upgrade' phase toward late-successional shrubs or 'downgrade' degenerate phase toward grasses. These phases would occur in a predictable cycle. Watt's study has since become a classic example frequently cited in scientific ecology.

Modeling cyclic succession

The cyclic model of succession can be displayed in terms of a transition matrix. Based on the

Markov chain A Markov chain or Markov process is a stochastic model describing a sequence of possible events in which the probability of each event depends only on the state attained in the previous event. Informally, this may be thought of as, "What happe ...

, the matrix describes the likelihood of future states based on the milieu of present states. The three states in the simplest cyclic model are open substrate (usually a bare patch of land), Species A dominance, and Species B dominance. With respect to facilitation,

inhibition Inhibitor or inhibition may refer to: In biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotra ...

, and tolerance models of succession, the key feature of the cyclic model is that A and B are not autosuccessional – that is, they do not facilitate their own growth. Rather, A will either facilitate the succession of B or be eliminated (through mortality) such that the patch occupied becomes open substrate. Likewise, B will either facilitate the succession of A or be eliminated. Open substrate can remain open or become occupied by either A or B. This configuration results in a cyclic scheme of species dominance.

Mechanisms

Cyclic succession is a descriptive phenomenon that can be accounted for in several ways. In Watt's bog system, he suggested that factors

endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, es ...

to the plant species were at play. He writes, "Each patch in this space-time mosaic is dependent on its neighbours and develops under conditions partly imposed by them." In other words, species life history characteristics fluctuate cyclically under the influence of surrounding species. These periodic shifts in life history properties produce observable changes in community composition. In the system Watt observed, phasic development was specifically responsible for changes in growth and mortality rate. As a result of changes in survival and growth ability, the balance of species dominance shifts, thus marking discrete stages. If the milieu of interspecific relationships satisfies the conditions described in the model above, a cyclic pattern of succession is observed. Exogenous factors, such as depredation by herbivores, can also be indirect drivers for cyclic succession if they differentially modulate plant life history properties over time. Density-dependent root gnawing by rodents is proposed as one such mechanism in the Larrea-Opuntia system. Watt noted that cyclic fluctuations in mortality rate could also be produced through differential response to seasonal conditions like frost. It is important to note that patterns cyclic succession cannot be readily linked to any single species, as Watt's ''Calluna'' bushes have been observed in non-cyclic systems. Rather, it is the aggregate composition of species that gives rise to the cyclic process.

Additional empirical evidence

Strong empirical evidence for cyclic succession can be found in Watt's follow-up publication on the

bracken Bracken (''Pteridium'') is a genus of large, coarse ferns in the family Dennstaedtiaceae. Ferns (Pteridophyta) are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells (eggs ...

system in the Journal of Ecology. ''Calluna vulgaris'' and ''Pteridium aquilinum'' were found to replace each other. Another salient example of cyclic replacement occurs in a two-species plant community in the

Sonoran Desert The Sonoran Desert ( es, Desierto de Sonora) is a desert in North America and ecoregion that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the southwestern United States (in Arizona ...

. Even though water availability is limiting such that only one species would be predicted to survive, ''Larrea tridentata'' and ''Opuntia leptocaulis'' are observed to replace each other in the absence of environmental disturbance.Yeaton, Richard (1978).

History

Modeling cyclic succession

Mechanisms

Additional empirical evidence

Notes

References

Further reading