Stability of CFT communities
Longevity of species and community fluctuation
Reproductive modes and recruitment patterns
Stochastic factors in community development
Longevity of species and community fluctuation
For most CFT species there is little information under this heading,
but for some of the more prominent ones studies have been made. Some of these have
relatively long life spans ranging from 6 - 100 years.
The soft coral, Alcyonium digitatum, is a very prominent member
of the CFT community. Observations of marked colonies have shown that colonies of 10-15 cm
in height are between five and ten years old (Hartnoll, unpublished). The life span
certainly exceeds 20 years - colonies have been followed for 28 years in marked plots
(Lundälv, pers. comm.). The sea urchin, Echinus esculentus is the most
prominent grazer in the CFT community, and a species which has suffered from commercial
collecting (see section V.C). Specimens of 10 cm diameter are about 6 years old (Comely
& Ansell, 1988). Gage (1992) determined an age of eight years for larger specimens,
and suggested they may live to 16 or more years in Scottish waters. Nichols et al. (1985)
give a life span of up to 12 years off Plymouth. The sea fan, Eunicella verrucosa is
another species which has suffered from collecting. This is notoriously slow growing (5 cm
height after 5 years, Keith Hiscock cited in Eno et al., 1996), colonies increasing in
length by only about 10 mm per year at Lundy (Fowler & Pilley, 1992), and by 6 mm or
less per year at Skomer where Bullimore (1986) suggested that the largest colonies may be
over 100 years old. However, analysis of the detailed Skomer monitoring data (Gilbert, in
prep.) suggests that smaller colonies grow faster, and that a colony of under 20 cm is
probably no more than 5 years old. The various cup corals are described as slow growing
and long lived on the basis of photographic monitoring (Fowler & Pilley, 1992). The
starfish, Asterias rubens, is a common predator within many CFT biotopes. It has a
life span of 7-8 years (Schafer, 1972). Some ascidians are long lived, 3-4 years in Boltenia
echinata, 5-8 years in Ascidia mentula, and probably over 20 years in Pyura
tesselata (Svane & Lundälv, 1981, 1982a, 1982b).
In contrast there are other CFT species which are short lived,
essentially annual in fact. These include the ascidians Ciona intestinalis, Clavellina
lepadiformis, Ascidiella aspersa and A. scabra (Costelloe et al., 1986; Dybern,
1965; Svane, 1983).
Information is restricted, but it is clear that a number of the more
prominent members of the CFT community are relatively long lived, and fairly slow growing.
It may be concluded that because of this communities which they dominate will be
relatively stable with time, but that when they are damaged recovery to their original
complexity may be slow. This recovery will be hindered by the fact that for a number of
species recruitment was observed to be very spasmodic, particularly for species near the
limits of their geographical range (Fowler & Pilley, 1992; Hiscock, 1998a). On the
other hand communities dominated by the annual species will exhibit marked seasonal and
year to year fluctuations - for example Ciona intestinalis (Costelloe et al.,
1986). In general it appears that longevities, and community stability, increase with
increasing depth, though hard data to support this are limited (Lundälv, 1985).
Detailed information on changes with time in circalittoral communities
derive only from studies where fixed quadrats have been monitored (normally
photographically) over a period of years. In Britain such studies have been carried out
Lundy and the Scillies over the period from 1983 (see Fowler & Pilley, 1992 for
summary), and at Skomer from 1982 (Bullimore, 1987). Various changes were detected,
particularly in the abundance of species of cup corals.
- Thus in the Scilly Isles both Leptopsammia pruvoti and Caryophyllia smithii declined
between 1984 and 1991.
- A number of the trends observed were similar at the Scilly, Lundy and Skomer sites,
probably a response of southern species to climatic changes.
- Although various changes were observed, the general conclusions for studies at all three
sites was that there was considerable stability both seasonally and from year to year,
with conspicuous species represented by specimens of considerable age (Fowler &
If this were generally true for CFT biotopes there would be encouraging
implications for management strategies, and for the development of monitoring programmes
to detect unusual levels of change. However, other studies discussed below show that in at
least some CFT biotopes change on a seasonal and year-to-year basis are the norm rather
than the exception.
The only other comprehensive north European studies are those on the
Swedish west coast by Lundälv and his co-workers. Changes in abundance are described for
a variety of ascidians and for the anthozoan Protanthea (for summary of work see
Lundälv, 1985). Annual seasonal fluctuations were clear, and year-to-year variations also
occurred, as did longer term trends: some changes were common to a series of sites,
suggesting a common cause. The composition of the community at a site could change
completely. One area was dominated by ascidians from 1970-81, but these were replaced by
the tube worm Pomatoceros from 1982-93, with ascidians only beginning to reappear
in the mid nineties (Lundälv, 1996). Lundälvs work generally tend to confirm that
stability of communities increases in more stable environments - those which are deeper
and more sheltered.
Another series of long term observations on fixed circalittoral sites
was carried out on the east coast of the U.S.A. in Massachusetts (see Sebens, 1985a, for
summary). Year to year variability occurred in the percentage cover of component species,
but there was never a change in overall character. However the duration of study was six
years, and it was seen in Lundälvs work that sites could be stable for that
duration, yet still subsequently undergo major changes.
Reproductive modes and recruitment patterns.
We have seen that the majority of CFT species are sessile so how do
they get to new areas? The answer is that whereas the adults are indeed fixed in one
place, the larval stages are generally highly mobile. The majority of CFT species (well
over 90%) have planktonic larvae which float or swim in the water column, are carried by
the currents, and dispersed to new locations. Thus the common soft coral Alcyonium
digitatum has an actively swimming larva with a large store of energy-yielding yolk
(Hartnoll, 1977). In captivity some larvae were still actively swimming fourteen weeks
after hatching (Hartnoll, 1975). That duration is probably unnaturally long, but with a 1
knot drift (a fairly average rate) a floating larva can be carried 100 km in a mere two
Dispersal is one thing, but reaching a suitable habitat at the end is
another matter. Obviously there must be a great loss of larvae which never reach the right
place to settle. However, settlement is not a random process - larvae do have limited
powers of swimming, and they show adaptive behaviour patterns to help find a place where
they can survive (Crisp, 1974). A combination of responses to light and gravity, for
example, can ensure that settlement occurs only on steep or downwardly facing surfaces. A
preference to settle near to adults of the same species means that the environment must be
favourable for survival. Clearly, despite the risks, pelagic larvae are an effective
reproductive strategy. Most common CFT species produce such larvae, and experimental
deployment of settlement plates shows rapid recruitment by a variety of species (Hextall,
Nevertheless there are some CFT species which lack pelagic larvae, and
others whose pelagic larvae normally settle very quickly. In the soft coral, Parerythropodium
coralloides, the eggs are brooded and the larvae crawl away from the parent to settle
nearby (Hartnoll, 1975). The species tends to occur in large patches (successful
settlement will be almost guaranteed), but not very commonly, and it is not known how it
disperses over distances. The conspicuous plumose anemone, Metridium senile, is
interesting in that it has pelagic larvae produced by sexual reproduction, and also buds
off daughter anemones asexually from its base (Kaplan 1983). This might seem an ideal way
to hedge ones reproductive bets, but few species have such flexibility. Other anemones
reproduce asexually such as Gonactinia prolifera and Protanthea simplex. The
jewel anemone Corynactis viridis also reproduces asexually to produce patches of a
single colour. Swimming larvae which often settle within minutes of release occur
in various ascidians (Olson, 1983) and bryozoans (Young & Chia, 1981).
The relevance of reproductive strategy to SAC management is that any
species lacking a planktonic dispersal phase, or with otherwise constrained dispersal
power, must be regarded as more vulnerable to locally adverse conditions. Once removed, it
may not easily reappear, and management strategies should take account of this.
Stochastic factors in community development.
Previously ecologists have tended to emphasise the predictability of
community composition in relation to the physico-chemical environment. More recently
though, stochastic factors, centred upon the availability of larvae and the creation of
vacant space, have come to the fore. This is the topical 'supply side' hypothesis
(Underwood & Fairweather, 1989).
Figure - Alternative locally stable states of
vertical rock wall community in Massachusetts.
One of the features of CFT communities is their fine-scale spatial
variation - they tend to be very patchy. Whilst the infralittoral tends to be more
predictable, circalittoral rock tends to be a mosaic of different species patches. The
same is seen in some parts of the rocky intertidal, where it has been attributed to the
effects of biological interactions and unpredictable recruitment (Hartnoll & Hawkins,
1985). Similarly in the subtidal, the different assemblages may represent 'alternate
stable states' (Sutherland, 1974; Sebens, 1985a,b). In most CFT biotopes substratum space
is very fully occupied, and the availability of space is a controlling resource for the
settlement and growth of species. According to when free space is made available, and on
which species are recruiting at that time, different assemblages of species may develop
under the same physico-chemical conditions. Once established, often following a
successional sequence (Hextall, 1994), these assemblages are stable for long periods (we
have seen the long life span of many CFT species), and different assemblages may co-exist
in close proximity. In Massachusetts Sebens (1985a) described four alternative locally
stable states, dominated by the anemone Metridium senile, the soft coral Alcyonium
siderium, the tunicate Aplidium pallidum and crustose corraline algae
respectively. Once established each state is maintained by a different positive feedback
loop (Figure 8).
The practical implications of this are that it makes the objective
classification of communities, and the correlation of community composition with
environmental variables, much more difficult - more discrete 'communities' will be
described than in fact exist. If such areas are being monitored, community change may be
considered an indication of environmental change, whilst it may be only part of a natural
biological cycle involving a switch between stable states.