Concepts and problems - Nonequilibrium Ecology - page 18

Crawley distinguished three categories of plant populations. Only one of
these, comprising a relatively small number of species, shows ''typical''
population dynamics at a given point in space, following more or less
predictable trajectories described by the N t þ 1 ¼ f (N t ) model. The second
category includes a large number of short-lived plants with ephemeral,
pulsed dynamics lasting a single generation, recruitment almost entirely
determined by germination biology, and frequency and intensity of dis-
turbances. The third category includes some long-lived plants for which
population patterns cannot be established because of the long life spans.
Furthermore, most plant species are successional, whose recruitment
depends on the death (either resulting from senescence or disturbances)
of dominant plants.
An example discussed by Crawley with strong evidence for the effect of
disturbance but no evidence for equilibrium is a study by Hubbell and
Foster of plant species on a permanent plot on Barro Coloradi island. They
compared the flora in 1980 and 1985. In 40% of the plant species, abun-
dance had changed by more than 10%. There had been ''catastrophic''
mortality during a drought year, and rare species declined more strongly
and common species increased more strongly than expected by chance
alone. No evidence was found that the plant assemblage was in
equilibrium.
A discussion of tropical rainforests in Chapter 8 addresses the question
of the relative significance of nonequilibrium and equilibrium in plant
assemblages in greater detail.
Concerning the effects of disturbances on metapopulations, Hanski
( 1999 , references therein) reviews work on the Glanville fritillary butter-
fly, Melitaia cinxia, on southwestern Finnish islands. The work is one of
the few thorough long-term studies and is therefore discussed in some-
what greater detail. Habitat reduction led to nonequilibrium conditions
and projections showed that a new equilibrium would only be established
after many years.
The distribution of the butterfly in Europe has markedly declined over
the last decades, and it became extinct on the Finnish mainland in the late
1970s. It still survives on the ˚ land Islands, southwestern Finland. Hanski
and Kuussaari ( 1995 ) and Hanski ( 1999 , references therein) describe in
great detail the metapopulation dynamics of the butterfly on the ˚ land
Islands with about 1600 dry meadows suitable for colonization, beginning
in 1991. Examined were the four conditions necessary for persistence at
the metapopulation level. These conditions are: (1) the species has local
breeding populations in relatively discrete habitat patches, with some