Final ENFAs (with ‘best’ variables from each of the groups)

Final ENFAs for all of the species were run without producing an eigenvalue warning, except for P. punctata, which even after removal of all the highly correlated variables, still produced a ‗very large eigenvalue‘ warning. Low sample sizes (P. punctata had the second lowest sample size) are more prone to getting very large or negative eigenvalues because of the correlation among EGVs, owing to low variance among species sites (Hirzel, 2008). However, all of the highly correlated variables had been removed. Negative or low eigenvalues may also be caused by the EGV maps not being continuous enough (Hirzel, 2008), although this was not a problem for the other species. Therefore, the HS map was computed anyway. A list of the variables included in the final ENFAs for each species can be found in Appendix 9.

2.3.2.2.1.1. Score matrices for final ENFA’s

The output ENFA score matrices for each species can be found in Appendix 10 and were used to determine the most important ecological variables for each species (see section 2.3.2.2.1.2 below). Overall marginality, specialisation and tolerance values (see Table 1 below) were also provided with the ENFA score matrix for each species, and allow for among-species comparisons within a given area (Hirzel et al., 2002). The global marginality takes into account all the EGVs and gives a summary of how much the species habitat differs from the available conditions (Hirzel, 2008). The global specialisation is the inverse of global tolerance, but as it varies between 1 and infinity, it is less easy to interpret (Hirzel, 2008).

Species Marginality Specialisation Tolerance

C. nobile 0.74 1.08 0.926 G. constrictum 1.38 2.15 0.464 G. illyricus 2.57 3.16 0.317 H. semele 1.62 1.48 0.674 N. sylvestris 0.982 3.35 0.299 P. argus 1.66 1.46 0.684 P. globulifera 1.53 2.20 0.454 P. punctata 1.14 3570000 0.000

Table 1. Overall marginality, specialisation and tolerance values to accompany the score matrices obtained by ENFA analysis using Biomapper. For explanations of each of these, see text.

None of the species had low (close to 0) marginality values, suggesting that all of the species tend to live in particular habitats relative to the mean (i.e. the available

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conditions in the study area) (Sattler et al., 2007). The lowest marginality value (at 0.744) was for C. nobile, which of all the species is probably one of the most widespread in different habitats within the New Forest, and the highest marginality value was for G. illyricus (2.565), which occurs in a fairly particular bracken- dominated habitat.

All of the tolerance values were below 1, indicating that the species all have some form of specialisation (a randomly chosen set of cells is expected to have a tolerance of 1 (Sattler et al., 2007)). However, C. nobile had the highest tolerance value, closest to 1 (0.926), so is the least specialised/more generalist in its living environment (Hirzel, 2008). This fits with it being the least marginal as well. H.

semele (0.674) and P. argus (0.684) also had fairly high specialisation values,

although both had high marginality values (1.620 and 1.664 respectively). The lowest tolerance value was for P. punctata (0), indicating that it is a very specialist species living in a very narrow range of conditions (Hirzel, 2008). However, P.

punctata did not have the highest marginality value, although it was still high.

2.3.2.2.1.2. Interpretation of species’ ecological requirements from score matrix Further information about the species‘ relationships with the environmental variables can be obtained from the score matrix by examining the marginality values (column 1) and the specialisation values (subsequent columns) (see Appendix 10 for the score matrices). The most important variables (EGVs) in terms of the highest (absolute) marginality values and specialisation values for each species from the score matrixes are shown in Tables 2 to 9 below. (Note that positive marginality values mean the species prefers locations with higher values on the corresponding EGV than the mean location in the New Forest study area, whereas negative values on this factor mean the species prefers locations with lower values. Signs of coefficient have no meaning on the specialisation values, where a higher absolute value means the range of the species is more restricted on that variable).

47 C. nobile EGV Marginality value Highest specialisation value

Edge density of dry heath/acid grassland mosaic (HL3)

0.640 0.189

Edge density of dense scrub (ST1) 0.345 0.064

% cover of soil type 64303 0.305 0.503

Euclidean distance to HL3 -0.293 0.719

Slope -0.287 -0.643

Euclidean distance ponds >0.5 ha (AQ6)

-0.198 0.556

Edge density of dry heath (HL1) 0.195 0.470

Table 2. Marginality and specialisation values for C. nobile resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

The highest marginality value for C. nobile was edge density of dry heath/acid grassland mosaic, demonstrating that C. nobile is found in locations with higher edge density HL3 values than the mean HL3 edge density of the whole national park. This means that it tends to be found in sites (pixels) containing dry heath/acid grassland mosaic with neighbouring pixels that also contain this habitat type, which is consistent with it being a species of moderately acidic grassland (see Appendix 2.1). However, the highest specialisation value was not particularly high, but was still greater than 0, indicating that C. nobile was found to occupy a narrower range of HL3 edge density values that the available range across the whole of the national park study area. Low Euclidean distance (i.e. closer) to this habitat type was also important. The next highest marginality value was for edge density of dense scrub, although the specialisation value for this was very low. Soil type 64303 is associated with lowland heath habitats, so it is not surprising that high marginality and specialisation values were obtained for C. nobile for this variable. C. nobile was also associated with lower slope values (i.e. flatter), which are perhaps more likely to be wetter, particularly in winter, which C. nobile favours (see Appendix 2.1). This association with wetter areas of habitats was also suggested by the preference for lower (closer) Euclidean distance values to larger ponds (>0.5 hectares).

None of the factors accounted for a large proportion of the specialisation (see Appendix 10), with a low value for the first (marginality) factor of 11%. This species

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also had relatively low global marginality and high tolerance values (see section 2.3.2.2.1.1), so appears to occur in a more generalist range of habitats.

G. constrictum

EGV Marginality

value

Highest specialisation value

% cover of soil type 84102 0.366 0.069

% cover of ponds <0.5 ha (AQ5) 0.319 0.014 Edge density of dry heath/acid

grassland mosaic (HL3)

0.301 -0.059

Patch area of HL3 0.294 0.085

% cover of HL3 0.271 -0.257

Slope -0.234 0.346

Euclidean distance to dry heath (HL1) -0.228 -0.244

Edge density of HL1 0.219 0.067

Annual mean temperature (Bio1) 0.215 0.365

Elevation -0.214 0.531

Euclidean distance to unimproved acid grassland (GL11)

-0.212 0.391

Euclidean distance to HL3 -0.211 0.403

Table 3. Marginality and specialisation values for G. constrictum resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

The results showed that G. constrictum tends to occur on sites with higher percentage cover of soil type 84102 than the global mean. This is a seasonally wet deep loam (see Appendix 6) associated with permanent grassland and deciduous woodland, so is consistent with G. constrictum being a plant of wet habitats (see Appendix 2.2), as is the association with ponds less than 0.5 ha in size, suggesting that smaller ponds are preferable to larger (>0.5 ha – AQ6) ponds. Edge density, patch area and percentage cover of dry heath/acid grassland mosaic were also important, which although it is a dry habitat, may contain ponds or other wetland features. The specialisation values for all these variables were fairly low. Euclidean distance to this habitat type was also important, but had a lower marginality value but a higher specialisation value. Euclidean distance to unimproved acid grassland and to dry heath also featured in this table of most important variables, so the cover of Calluna (see Appendix 5) does not seem to be too relevant, just the presence of the acidic habitat, consistent with the literature (see Appendix 2.2). The association

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with lower values for slope (i.e. flatter) and elevation also indicate sites where ponds or other wetland habitats are more likely to occur. The first two factors (marginality and the first specialisation factor) accounted for 42% of the specialisation (see Appendix 10). G. illyricus EGV Marginality value Highest specialisation value Patch area of continuous bracken cover

(GL8)

0.540 -0.006

% cover of GL8 0.444 0.016

Patch compactness of GL8 0.349 -0.012

Edge density of GL8 0.299 -0.042

Patch compactness of dry heath/acid grassland mosaic (HL3)

0.196 -0.025

Edge density of merged dry heath and dry heath/acid grassland mosaic (HL1_HL3)

0.189 -0.025

% cover of ponds <0.5 ha (AQ5) 0.178 0.004 Euclidean distance to acid fen/flush/valley

mire in heathland situations (AQ1)

-0.167 -0.666

Euclidean distance wet heath (HL2) -0.164 -0.807

% cover HL3 0.147 0.038

Table 4. Marginality and specialisation values for G. illyricus resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

Consistent with the literature (see Appendix 2.3), G. illyricus showed a strong preference for continuous bracken habitats (GL8), although the size of the patch of that habitat type was the most important, with the highest marginality value (0.540), indicating a preference for larger patches than the average. Association with dry heath and dry heath/acid grassland mosaic can also be seen from the table (patch compactness, edge density and Euclidean distance), which is again consistent with the suggestion in the literature of its occurrence on moderately acid soils. Bracken is also often found in these habitats. The association with small ponds may also confirm the findings of Stokes (1987) (see Appendix 2.3) that most New Forest G. illyricus sites were within 100 m of water. The preference of closer Euclidean distances to the valley mire (AQ1) and wet heath (HL2) habitats also suggest this association with water or wetter areas. Interestingly, Euclidean distance to

50

continuous bracken (GL8) and Euclidean distance to coniferous woodland edge were highly correlated, but the latter, although it made the final ENFA selection, did not show a high marginality value.

The specialisation values were all low except for Euclidean distance to AQ1 and to HL2, for which G. illyricus occupied a narrower range of values than available, confirming the message of the high marginality values, demonstrating that it tends to occur at sites close to these habitat types (i.e. low Euclidean distance values). However, 50% of the specialisation was accounted for by the first (marginality factor) and 23% percent by the second factor, with 88% of the specialisation accounted for by the first four factors (see Appendix 10).

H. semele

EGV Marginality

value

Highest

specialisation value Patch area of merged dry heath and dry

heath/acid grassland mosaic (HL1_HL3)

0.414 0

% cover of soil type 64303 0.334 0

Edge density of dry heath (HL1) 0.316 0

% cover of HL1 0.296 0

Euclidean distance to HL1 -0.24 0

Euclidean distance to wet heath (HL2) -0.233 0 Euclidean distance to acid fen/flush/valley

mire in heathland situations (AQ1)

-0.222 0

Patch compactness of HL1 0.216 0

Edge density of dense scrub (ST1) 0.213 0

Patch compactness of ST1 0.197 0

Table 5. Marginality and specialisation values for H. semele resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

The preference of H. semele for larger patches of dry heath or dry heath/acid grassland mosaic is consistent with the literature describing this butterfly as being associated with arid acidic grassy habitats (see Appendix 2.4). These habitats are also associated with soil type 64303, on which the species mean for H. semele is higher than the global mean. H. semele also shows a preference for sites with lower (i.e closer) Euclidean distance values to wet heath and valley mire (AQ1), which is not consistent with the literature. However, Euclidean distances to all the heathland

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habitats (HL1, HL2, HL3 and AQ1) were all highly correlated as they tend to occur close to each other, often in a mosaic, so occurrence near one type is likely to be close to another. In addition, H. semele is a fairly mobile species so could be recorded as it moves through them.

H. semele also showed higher than the global mean edge density and patch compactness values of dense scrub. This may not seem like preferable habitat, but this association could be because H. semele is often found in areas where heathers are regenerating after burns (see Appendix 2.4) and areas of dense scrub are often cleared as part of heathland management. As the habitat layer is based on the habitat types at a fixed point in time and the species observations span several years, the dense scrub habitat type in particular is likely to have changed over that time.

All the specialisation values were 0 for the variables included in Table 5, indicating that H. semele can occur across a wide range of values on those variables, but (as shown by the marginality values) is most likely to occur on sites that are closer to the Euclidean distance variables, or sites that have a higher than the global average value for the other variables. None of the factors individually accounted for a large proportion of the specialisation (see Appendix 10).

N. sylvestris

EGV Marginality

value

Highest specialisation value

Patch area of deciduous woodland (W1_W2)

0.430 -0.085

% cover of deciduous woodland (W1_W2)

0.373 -0.212

Euclidean distance to mixed woodland (W7_W8)

-0.290 0.190

Edge density of mixed woodland (W7_W8)

0.259 -0.042

Edge density of dry heath/acid grassland mosaic (HL3)

0.204 0.059

Patch compactness W7_W8 0.203 0.144

Euclidean distance W1_W2 -0.198 -0.229

Table 6. Marginality and specialisation values for N. sylvestris resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

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The highest marginality value for N. sylvestris was for patch area of deciduous woodland, which fits with its requirements for deciduous leaf litter, and confirming the conclusions of Brouwers (2008), that the likelihood of N. sylvestris presence increased with patch size (see Appendix 2.5). The high marginality value for percentage cover of that habitat type within the 100 m x 100 m pixel also suggests that relationship. However, it was Euclidean distance to mixed (deciduous and coniferous) woodland that had the next highest marginality value (ahead of Euclidean distance to broadleaved woodland), with a preference for sites closer to mixed woodland. Higher edge density and patch compactness values of mixed woodland were also favoured. This is particularly interesting because mixed woodland has a lower proportion of deciduous trees, which are favoured by N. sylvestris. However, as long as some deciduous trees, and the resulting leaf litter, are present it seems suitable. It may just be that the records happened to be more from those types of woodlands.

A preference for higher edge density values of dry heath/acid grassland mosaic habitat is also interesting and unexpected, as N. sylvestris is a woodland species. However, perhaps this is something to do with woodland edge meeting heathland, as when N. sylvestris is recorded at a woodland edge, then it is a location where the woodland meets another habitat type which, in the New Forest, is frequently heathland. Therefore, within a 100 m x 100 m pixel containing woodland edge, there will be another habitat, which has a high likelihood of being heathland, present. Another explanation is that N. sylvestris has been recorded on heathland sites under bracken (S. Douglas, personal observation), and is known to venture several hundred metres from a woodland edge in hot weather (N. Brouwers, personal communication, January 9, 2009).

The specialisation values were generally low, with the highest values for Euclidean distance to broadleaved woodland (-0.229) and to mixed woodland (0.190), indicating, as for the marginality values, that N. sylvestris tends to occur on sites with a lower value for the Euclidean distance to these habitats. However, most of the specialisation (74%) was accounted for by the first (marginality) factor (see Appendix 10), indicating that these values are the most important. N. sylvestris also had the second lowest overall tolerance value (see section 2.3.2.2.1.1).

53 P. argus EGV Marginality value Highest specialisation value

Edge density of dry heath (HL1) 0.304 0.003

% cover of merged dry heath and dry heath/acid grassland mosaic (HL1_HL3)

0.302 -0.640

Patch area of HL1_HL3 0.298 -0.006

% cover of HL1 0.251 0.524

Patch compactness of HL1 0.236 0.014

Euclidean distance to HL1 -0.232 0.060

Euclidean distance to wet heath (HL2) -0.223 0.064 Euclidean distance to acid fen/flush/valley

mire in heathland situations (AQ1)

-0.215 0.073

Table 7. Marginality and specialisation values for P. argu s resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

As documented in the literature (see Appendix 2.6), P. argus had a strong association with dry heath (HL1) and dry heath/acid grassland mosaic (HL3), with high marginality values for edge density, percentage cover, patch area and Euclidean distance, as well as high specialisation values for percentage cover of dry heath and dry heath/acid grass mosaic. This indicates that P. argus occurs on a more restricted range of values (i.e. higher percentage cover values) than the range of values available for sites across the study area. P. argus is also known to use wet heath, which is demonstrated by the marginality values for Euclidean distance to wet heath (HL2) and valley mire (AQ1), indicting a preference for sites closer to these habitat types. None of the factors individually accounted for a large amount of the specialisation (with the first factor explaining 17%) and the total amount of specialisation accounted for by the 9 retained factors was only 68% (see Appendix 10).

54 P. globulifera EGV Marginality value Highest specialisation value

% cover of ponds (AQ5_AQ6) 0.358 0

Edge density of merged wet heath and acid fen/flush/ valley mire in heathland situations (HL2_AQ1)

0.234 0

Edge density of dry heath/acid grassland mosaic (HL3)

0.287 0

% cover of HL2_AQ1 0.234 0

% cover of HL3 0.233 0

Euclidean distance to wet heath (HL2) -0.232 0 Euclidean distance to acid fen/flush/valley

mire in heathland situations (AQ1)

-0.230 0

Patch area of HL2_AQ1 0.219 0

Euclidean distance to HL3 -0.212 0

Euclidean distance to dry heath (HL1) -0.208 0

Patch compactness of HL2_AQ1 0.205 0

Edge density of HL1 0.186 0

% cover of Aspect - Flat 0.179 -0.485

Table 8. Marginality and specialisation values for P. globulifera resulting from ENFA analysis. EGVs are sorted by decreasing absolute value of coefficients on the marginality factor. Highest specialisation value indicates the highest (absolute) specialisation value (signs of coefficient have no meaning on the specialisation values) for that variable from only the number of factors retained to calculate the habitat suitability map (i.e. the first few columns) (see Table 10, section 2.3.2.3).

The percentage cover of ponds had the highest marginality value for P. globulifera, known as a wetland plant (see Appendix 2.7). This was also reflected by the high marginality values for edge density, percentage cover, patch area and patch compactness of wet heath and valley mire habitats combined (HL2_AQ1), as well as occurrence on sites with lower Euclidean distances to these habitat types individually. However, P. globulifera also showed a preference for high percentage cover of dry heath/acid grassland mosaic (HL3) and low Euclidean distance to this habitat, but this could be because water bodies such as ponds can occur within this habitat. This may also explain the preference for higher edge density of dry heath (HL1). The association with sites with a higher percentage of flat ground may reflect the fact that ponds, rivers and other water bodies may be more likely to be found in areas with flat ground, rather than sloping, where the water is more likely to lie.

The specialisation values were all zero except for this last variable, suggesting that P. globulifera tends to occupy a narrower range of values (i.e. higher percentage of

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flat aspect) than the available range across the study area. The rest of the

In document Habitat suitablity modelling in the New Forest National Park (Page 67-101)