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MODELLING LICHEN REINVASION FOR MONITORING AMELIORATING ENVIRONMENTS
847 State-of-the-Art in Ecological Modelling. Vol. 7.
MODELLING LICHEN REINVASION FOR MONITORING AMELIORATING ENVIRONMENTS A. Henderson-Sellers and M.R.D. Seaward
Introduction Plants and animals have been utilized as indicators of environmental conditions for many years.
Any successful use of such bio-
indicators must depend upon the rigorous definition of the scales monitored by the organism and, perhaps even more importantly, a complete analysis of all the environmental parameters which affect the organism's propogation, establishment and subsequent growth.
Use of bioindicators has the obvious advantage of per-
mitting long-term monitoring of environmental pollutant levels without widespread use and maintenance of costly and sophisticated equipment.
Recently, use has been made of lichens in sur-
veys of sulphur dioxide levels in the United Kingdom and elsewhere (Hawksworth & Rose, 1970 ).
Several different
techniques
can be applied including lichen transplantation of particular lichen rosettes which monitors the species performance within the new environmental regime· However, the most widely employed techniques are mapping of generalised zones based upon total number and diversity of species and spatial mapping over a number of years of a particular lichen species.
The former technique, al-
though widely used to date is less satisfactory with decreasing sulphur dioxide levels and a corresponding increase in importance of other environmental parameters in comparison with the local effective pollutant levels and consequently lichen colonisation and progation.
We consider that successful on-going monitoring
of ameliorating atmospheric pollution, especially in the urban environment, can best be pursued by the latter technique which involves detailed investigation of single lichen species· Single species monitoring Since the implementation of the Clean Air Acts (l956 and 1968) in the United Kingdom, sulphur dioxide concentrations in the large conurbations have decreased considerably.
At one time, use
of bioindicators often depended upon observation of marginal or complete necrosis as direct indication of increasingly inhospitable environmental conditions.
With the considerable araerio-
ration of the atmosphere of city environments, utilisation of
848 bioindicators is becoming less straightforward·
It is necessary,
first of all, to identify a direct relationship between the environmental conditions, especially the ambient pollution levels, and the success of the organism·
Extrapolation of sulphur diox-
ide levels from data collected for lichen species diversity would be of dubious use since reinvasion by lichens is highly speciesdependent.
We have monitored and modelled the reinvasion of the
West Yorkshire conurbation by the lichen Lecanora muralis, with the twofold intent of: (i) identification of all the environmental parameters of greatest importance to this particular species and (ii) possible usefulness of this and other lichen species in on-going pollutant monitoring· L. muralis is a particularly useful lichen for mapping in an urban environment since it adapts readily to man-made substrates, (especially in the British Isles) on asbestos cement and mortar, in contrast with its natural occurence on siliceous substrates. Recent work upon the ecological adaptions of L. muralis
(Seaward,
1976) seem to indicate that this species can tolerate higher atmospheric sulphur dioxide levels and/or more acidic rain than many other species by its utilisation of these strongly alkaline man-made substrates for colonisation.
Reinvasion of the West
Yorkshire conurbation by L. muralis has been fairly rapid, of the 2 order of 9 km per year, but the precise control mechanisms governing the spatial distribution of the lichen in succeeding time slots is not easily adduced. Modelling the reinvasion The West Yorkshire conurbation has been mapped on to a 4θ x kO km square grid aligned with the National Grid (see Figures 1 and 3).
Sulphur dioxide data for the sites within this unit have
been obtained from the Warren Spring Laboratory for the years 1962/3, 1964/5, 1966/7, 1968/9, 1970/71, 1972/3, from which estimates of the annual average levels of sulphur dioxide over the whole conurbation have been made (Figure l ) .
Surveys of the unit
in the years 1969, 1971, 1973, 1975, 1977 have permitted digital maps of the presence of L. muralis to be drawn.
These maps
(after Seaward, 1977 and others?) are shown in Figure 2.
849 A large number of computer simulations of the reinvasion of this lichen have been performed utilising a simple forward time-step numerical procedure and run-time graphical display·
Using these
we have attempted to establish the most important parameters affecting its colonisation and growth.
We have assumed that sub-
strates suitable for the lichen are available in all grid squares, and, for the first order simulation, that the substrate material is homogeneous.
As has already been discussed by Seaward
(1976),
the effects of local unavailability of substrate, climate,acidity of rainfall and altitude should be included in analysis of the environmental parameters effecting the lichen, but these are usually considered to be of secondary importance in vitiated atmospheres. The ambient SO
sulphur dioxide was expected to be the most
important forcing parameter but we also attempted to identify the importance of the time lag between pollution levels dropping below an identifiable threshold and reinvasion and colonisation. Obviously no effective use can be made of lichen maps for predicting S0„ concentrations if this time-lag has not been credibly established.
The initial computer simulations were thus used
to compare the rates of reinvasion under different time sequence -3 S0p.
regimes whilst holding a constant threshold of 120 μg m
Table 1 shows the results of three simulations compared with the observed field data.
From this table it is obvious that a time-
lag of approximately five years seems to be the most realistic. Hence our further simulations have used this time-lag and considered only variations in SO
threshold and colonisation cri-
teria. Figure 3 provides an outline map of the extent of Lecanora muralis within the West Yorkshire conurbation (cf. digitised data in Figure 2 ) . It should be noted that the invasion hardly affects the northwest corner of the area and also that the total outline retains approximately its original dimensions with much invasion expanding from central regions.
Three sets of results of our computa-
tional model are shown in Figures 4, 5 and 6 for comparison with the field data.
850 Figure k illustrates the simulated invasion under the condition -3 of a threshold of 70 μ^ m whilst Figure 5 shows the results, _3 for the same h years, with a threshold of 120 μ^ m · Obviously the invasion is much too extensive by 1977 utilising the latter model·
The final simulation illustrated here (Figure 6) is in-
teresting since we have this time allowed the lichen to be wiped out if SO
levels rise above the threshold (in this case 70 μg m ~jl
It should be noted that only this run correctly simulates the re-invasion failure of the lichen in the north-west corner which is seen in the real data.
The first model has the advantage of
retaining approximately the correct gross outline but fails to initiate any central expansion of lichen colonisation whilst the second model (Figure 5) has this certral development but permits much too rapid encroachment in the fringe areas, Conclusions Certainly the simplistic method of extrapolation of pollutant levels from lichen maps is seen to be premature,
All interpre-
tations of lichen distribution must consider the fact that even a lichen as well adapted to the urban environment as L. muralis requires a considerable number of years (probably about five) before responding to ameliorating conditions.
Against this it
appears that any sudden increase in pollution seems to produce a much more rapid effect.
Further, it is very difficult to esta-
blish a simple threshold level below which lichen colonisation is certain and growth may be predicted.
From the results we have
presented here, it seems that the usefulness of L, muralis (or any other bioindicator) in the future will depend critically upon the sucessful completion of muchneeded laboratory work, Obviously our simulations become less successful by the year 1977· We consider that these results underline the fact that sulphur dioxide levels alone are no longer the major factor affecting colonisation within the West Yorkshire (and probably many other) urban area,
851 Table 1 Rate of re-invasion of Lecanora muralis (square km per two year period) Field data 7 year lag; 120 pg m m
5 year lag; 120 μg
_3
"
3
3 year lag; 120 μg m
1971 47
1973 4θ
1975 49
threshold
17
26
15
threshold
54
60
66
146
128
117
109
threshold
I977 33
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e West Yorkshire conurbation under investigation.
Dots show the location of sites at which measurements of sulphur dioxide levels have been made.
852
Fig. 2 Digitized field data showing the (coloured) areas which Lecanora Muralis has re-invaded.
Fig. 3 Outline map of the extent of re-colonisation for two particular years (1969 and 197^) - after Seaward.
853 1971 1971
1973 1973
1975 1975
1977 1977
F i g . 4.
(M Fig.
5.
Fig. k Results of eight years simulation under the condition of o
a five-year time-lag and a threshold value of 70 μgm/m of SO. Fig. 5 As figure 4, but for 120 μgm/π
1971
1973
1975
1977
Cê°
o
Fig. 6 As figure h with a threshold of 70 μgm/m
but with the
additonal condition that established lichen dies in areas which later experience an increased SO« level above the threshold. REFERENCES Hawkswoth, D.L. & F. Rose (ΐ97θ). Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature, 227» 145-1^8. Seaward, M.R.D. (1976). Performance of Lecanora Muralis in an urban environment; p. 323-357 in lichenology: Progress and Problems, Brown,D.H.,Hawksworth, D.L. & Bailey, R.H. (eds.) Academy Press.
MODELLING LICHEN REINVASION FOR MONITORING AMELIORATING ENVIRONMENTS A. Henderson-Sellers and M.R.D. Seaward
Introduction Plants and animals have been utilized as indicators of environmental conditions for many years.
Any successful use of such bio-
indicators must depend upon the rigorous definition of the scales monitored by the organism and, perhaps even more importantly, a complete analysis of all the environmental parameters which affect the organism's propogation, establishment and subsequent growth.
Use of bioindicators has the obvious advantage of per-
mitting long-term monitoring of environmental pollutant levels without widespread use and maintenance of costly and sophisticated equipment.
Recently, use has been made of lichens in sur-
veys of sulphur dioxide levels in the United Kingdom and elsewhere (Hawksworth & Rose, 1970 ).
Several different
techniques
can be applied including lichen transplantation of particular lichen rosettes which monitors the species performance within the new environmental regime· However, the most widely employed techniques are mapping of generalised zones based upon total number and diversity of species and spatial mapping over a number of years of a particular lichen species.
The former technique, al-
though widely used to date is less satisfactory with decreasing sulphur dioxide levels and a corresponding increase in importance of other environmental parameters in comparison with the local effective pollutant levels and consequently lichen colonisation and progation.
We consider that successful on-going monitoring
of ameliorating atmospheric pollution, especially in the urban environment, can best be pursued by the latter technique which involves detailed investigation of single lichen species· Single species monitoring Since the implementation of the Clean Air Acts (l956 and 1968) in the United Kingdom, sulphur dioxide concentrations in the large conurbations have decreased considerably.
At one time, use
of bioindicators often depended upon observation of marginal or complete necrosis as direct indication of increasingly inhospitable environmental conditions.
With the considerable araerio-
ration of the atmosphere of city environments, utilisation of
848 bioindicators is becoming less straightforward·
It is necessary,
first of all, to identify a direct relationship between the environmental conditions, especially the ambient pollution levels, and the success of the organism·
Extrapolation of sulphur diox-
ide levels from data collected for lichen species diversity would be of dubious use since reinvasion by lichens is highly speciesdependent.
We have monitored and modelled the reinvasion of the
West Yorkshire conurbation by the lichen Lecanora muralis, with the twofold intent of: (i) identification of all the environmental parameters of greatest importance to this particular species and (ii) possible usefulness of this and other lichen species in on-going pollutant monitoring· L. muralis is a particularly useful lichen for mapping in an urban environment since it adapts readily to man-made substrates, (especially in the British Isles) on asbestos cement and mortar, in contrast with its natural occurence on siliceous substrates. Recent work upon the ecological adaptions of L. muralis
(Seaward,
1976) seem to indicate that this species can tolerate higher atmospheric sulphur dioxide levels and/or more acidic rain than many other species by its utilisation of these strongly alkaline man-made substrates for colonisation.
Reinvasion of the West
Yorkshire conurbation by L. muralis has been fairly rapid, of the 2 order of 9 km per year, but the precise control mechanisms governing the spatial distribution of the lichen in succeeding time slots is not easily adduced. Modelling the reinvasion The West Yorkshire conurbation has been mapped on to a 4θ x kO km square grid aligned with the National Grid (see Figures 1 and 3).
Sulphur dioxide data for the sites within this unit have
been obtained from the Warren Spring Laboratory for the years 1962/3, 1964/5, 1966/7, 1968/9, 1970/71, 1972/3, from which estimates of the annual average levels of sulphur dioxide over the whole conurbation have been made (Figure l ) .
Surveys of the unit
in the years 1969, 1971, 1973, 1975, 1977 have permitted digital maps of the presence of L. muralis to be drawn.
These maps
(after Seaward, 1977 and others?) are shown in Figure 2.
849 A large number of computer simulations of the reinvasion of this lichen have been performed utilising a simple forward time-step numerical procedure and run-time graphical display·
Using these
we have attempted to establish the most important parameters affecting its colonisation and growth.
We have assumed that sub-
strates suitable for the lichen are available in all grid squares, and, for the first order simulation, that the substrate material is homogeneous.
As has already been discussed by Seaward
(1976),
the effects of local unavailability of substrate, climate,acidity of rainfall and altitude should be included in analysis of the environmental parameters effecting the lichen, but these are usually considered to be of secondary importance in vitiated atmospheres. The ambient SO
sulphur dioxide was expected to be the most
important forcing parameter but we also attempted to identify the importance of the time lag between pollution levels dropping below an identifiable threshold and reinvasion and colonisation. Obviously no effective use can be made of lichen maps for predicting S0„ concentrations if this time-lag has not been credibly established.
The initial computer simulations were thus used
to compare the rates of reinvasion under different time sequence -3 S0p.
regimes whilst holding a constant threshold of 120 μg m
Table 1 shows the results of three simulations compared with the observed field data.
From this table it is obvious that a time-
lag of approximately five years seems to be the most realistic. Hence our further simulations have used this time-lag and considered only variations in SO
threshold and colonisation cri-
teria. Figure 3 provides an outline map of the extent of Lecanora muralis within the West Yorkshire conurbation (cf. digitised data in Figure 2 ) . It should be noted that the invasion hardly affects the northwest corner of the area and also that the total outline retains approximately its original dimensions with much invasion expanding from central regions.
Three sets of results of our computa-
tional model are shown in Figures 4, 5 and 6 for comparison with the field data.
850 Figure k illustrates the simulated invasion under the condition -3 of a threshold of 70 μ^ m whilst Figure 5 shows the results, _3 for the same h years, with a threshold of 120 μ^ m · Obviously the invasion is much too extensive by 1977 utilising the latter model·
The final simulation illustrated here (Figure 6) is in-
teresting since we have this time allowed the lichen to be wiped out if SO
levels rise above the threshold (in this case 70 μg m ~jl
It should be noted that only this run correctly simulates the re-invasion failure of the lichen in the north-west corner which is seen in the real data.
The first model has the advantage of
retaining approximately the correct gross outline but fails to initiate any central expansion of lichen colonisation whilst the second model (Figure 5) has this certral development but permits much too rapid encroachment in the fringe areas, Conclusions Certainly the simplistic method of extrapolation of pollutant levels from lichen maps is seen to be premature,
All interpre-
tations of lichen distribution must consider the fact that even a lichen as well adapted to the urban environment as L. muralis requires a considerable number of years (probably about five) before responding to ameliorating conditions.
Against this it
appears that any sudden increase in pollution seems to produce a much more rapid effect.
Further, it is very difficult to esta-
blish a simple threshold level below which lichen colonisation is certain and growth may be predicted.
From the results we have
presented here, it seems that the usefulness of L, muralis (or any other bioindicator) in the future will depend critically upon the sucessful completion of muchneeded laboratory work, Obviously our simulations become less successful by the year 1977· We consider that these results underline the fact that sulphur dioxide levels alone are no longer the major factor affecting colonisation within the West Yorkshire (and probably many other) urban area,
851 Table 1 Rate of re-invasion of Lecanora muralis (square km per two year period) Field data 7 year lag; 120 pg m m
5 year lag; 120 μg
_3
"
3
3 year lag; 120 μg m
1971 47
1973 4θ
1975 49
threshold
17
26
15
threshold
54
60
66
146
128
117
109
threshold
I977 33
DU
•
I1 Ι·Ι· 1 H ' '1
Λ0 **u
LLLI
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M 1 Ul 1 II 1 II Ul L 11 1 1 1 1 11 1 1 Ul 1h ri
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UO
50
e West Yorkshire conurbation under investigation.
Dots show the location of sites at which measurements of sulphur dioxide levels have been made.
852
Fig. 2 Digitized field data showing the (coloured) areas which Lecanora Muralis has re-invaded.
Fig. 3 Outline map of the extent of re-colonisation for two particular years (1969 and 197^) - after Seaward.
853 1971 1971
1973 1973
1975 1975
1977 1977
F i g . 4.
(M Fig.
5.
Fig. k Results of eight years simulation under the condition of o
a five-year time-lag and a threshold value of 70 μgm/m of SO. Fig. 5 As figure 4, but for 120 μgm/π
1971
1973
1975
1977
Cê°
o
Fig. 6 As figure h with a threshold of 70 μgm/m
but with the
additonal condition that established lichen dies in areas which later experience an increased SO« level above the threshold. REFERENCES Hawkswoth, D.L. & F. Rose (ΐ97θ). Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature, 227» 145-1^8. Seaward, M.R.D. (1976). Performance of Lecanora Muralis in an urban environment; p. 323-357 in lichenology: Progress and Problems, Brown,D.H.,Hawksworth, D.L. & Bailey, R.H. (eds.) Academy Press.