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Suffolk Natural History, Vol. 38 THE ECOLOGY OF A CHANGING FLORA KEVIN WALKER

Introduction Over the last two centuries the most profound changes to the flora of the British Isles has been the decline of species associated with agricultural and semi-natural habitats and a marked increase in the abundance of vigorous weeds and established aliens (Rich & Woodruff, 1996). This polarisation of the British flora into “winners” and “losers” has occurred primarily as a result of increasing population pressures, the indirect effects of which have been unprecedented land-use and agricultural changes (McKinney & Lockwood, 1999; Thompson, 1994). One of the many ways to study these changes is to analyse the ecological characteristics of the most “successful” and “unsuccessful” species. Such studies have become increasingly possible in recent years with the publication of numerous county floras which include detailed historical assessments of change and lists of extinct species.

Furthermore, comprehensive ecological data are now available for most British species (eg. Fitter & Peat, 1994; Grime, Hodgson & Hunt, 1988; Preston & Hill, 1997; Hill et al., 1999). Such studies have a number of advantages: firstly, because they include a temporal element, they allow modern changes to be seen in an historical context; secondly, they help to identify the main causes of change; and finally, they can help to identify the types of species that are likely to succeed (or fail) within the future flora of the British Isles. As a result they can be of equal interest to botanists, ecologists and conservationists alike. The aim of this paper is to provide an overview of such approaches with respect to the extinction of species within Watsonian vice-counties. In particular, the extent to which these species share ecological, morphological and phytogeographical attributes is addressed and discussed in relation to other work on the ecology of change in the British flora.

Nomenclature follows Stace (1997).

Change in the British Flora National scale change Early county floras and maps in the 1962 Atlas (Perring & Walters, 1962) show that many species had suffered marked contractions in range by the early part of the 20th Century (e.g Drosera rotundifolia, Lycopodiella inundata, Myosurus minimus, Orchis ustulata, Torilis arvensis). Many of these declines were discussed in detail by Perring (1970) who was in no doubt that although “some may be due to subtle biological causes…the vast majority are due to man, and the evidence is that his destructive effect on the flora is accelerating” (pp. 130–131). In recent years a number of recording projects have showed the full nature and extent of these declines. Probably the most informative have been the results of the B.S.B.I. Monitoring Scheme which showed that 195 species had decreased significantly in England since fieldwork for the original atlas, the majority of which were typical species of calcareous, arable, acid and wetland habitats (Fig. 1; Rich & Woodruff, 1996).

Conversely, habitats with few declining species included stable semi-natural

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Water edge

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Figure 1. The number of species which have declined significantly in England since the 1960s in relation to broad habitat type (n = 192). Data adapted from Rich & Woodruff (1996). and natural habitats such as mountains and woods. Subsequent studies have shown the extent of these declines for some habitats; e.g. freshwaters (Preston & Croft, 1996), grazing marshes (Mountford, 1994) and cultivated ground (Sutcliffe & Kay, 2000).

Local scale extinction – one species lost every year?

An alternative approach is to look at change within more restricted geographical areas. This has been carried out for extinctions in Bedfordshire (Boon, 1999; Dony, 1977; Robertson, 1982), Hertfordshire (James, 1997), Cambridgeshire (Preston, 2000), Middlesex (Preston, 2000) and Huntingdonshire (Wells, 1989). In addition, county changes have also been described for Cheshire and Lancashire (Greenwood, 1999), the Howardian Hills in Yorkshire (Gulliver, 1990), the Cambridgeshire fens (Sheail & Wells, 1980, 1983) and Northamptonshire (McCollin, Moore & Sparks, 2000). The main advantage of these more spatially limited studies is that they can incorporate historical information that often predates modern recording. For example, in lowland counties the effects of parliamentary enclosure, which resulted in the wide-scale drainage and ploughing-up of grasslands, are likely to have had a major impact on floras prior to the 20th Century. In rural Cambridgeshire, for instance, this led to a period of heightened extinction

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Extinction rate per decade

12 10 8 6 4 2 0 1750- 1780- 1810- 1840- 1870- 1900- 1930- 19601779 1809 1839 1869 1899 1929 1959 1989

Figure 2. Decadal extinction rate in Cambridgeshire (v.c. 29) since 1750 plotted in 30 year time intervals. The earlier peak coincides with a major period of parliamentary enclosure in the county. Data adapted from Preston (2000). during the early part of the 19th Century, when 24 species disappeared in as many years (1810–1839) (Fig. 2; Preston, 2000). As a consequence environmental change was a central theme of Charles Babington’s flora of the county in 1860 in which he blamed this wave of extinction on “the rapacity of the modern agriculturalist” (Babington, 1860).

In recent years there has been increasing interest in the study of floristic changes at the county scale. In particular, Peter Marren’s study on county extinction provides a startling account of the decline of species in England over the last century (Marren, 2000, 2001). Using lists of extinct species published in county floras and historical studies he calculated an average extinction rate of 0·7 species per year since 1900, with figures ranging from 0·3 species per year in Norfolk (v.c. 27 & 28) to 1·4 species a year in Northamptonshire (v.c. 32). His “league table of extinctions” (Fig. 3) also gave an indication of the uneven nature of decline: southern and eastern counties have suffered the worst, with an annual rate of 0·7 species per year, whereas northern and western counties have lost around 0·6 species per year.

However, subsequent research has shown that Marren overestimates the rate of extinction in half of these counties. In particular, the rate of extinction in Northamptonshire, which tops the league table, is overly high because Marren included pre-1930 extinctions within his figures for 1930–1995. Similarly he overestimated the rates for Surrey and Lincolnshire (inclusion of 19th Century extinctions), Durham and Gloucestershire (inclusion of non-native or doubtful

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Figure 3. Marren’s “league table” of county extinctions since 1900. Rates for counties shown in grey have been revised by the author and with the exception of Norfolk are lower than those given by Marren (2000, 2001). species), Westmorland and Cumberland (inclusion of hybrids), Cheshire (inclusion of recently discovered species) and Suffolk (inclusion of aliens and charophytes). Norfolk is the only county for which he underestimated the extinction rate due to the exclusion of four species which were last recorded before 1914. When these revised figures are taken into account they suggest an average loss of around 0·5 species a year at the vice-county level during the 20th century.

The Ecology of Extinct and Declining Species The results of a number of comparative studies on both national and local datasets suggest extinct (or declining) species tend to share a number of ecological attributes. The most frequently cited of these are habitat requirement, competitive and dispersal ability, reproductive strategy and position within range (Table 1).

Habitat requirements Not surprisingly extinct species tend to be associated with habitats that have suffered the most as a result of land-use change. In the lowland south east these include acid, freshwater and cultivated habitats, all of which have lost around 20% of their typical species. For example, in Hertfordshire 19% of acid, 17% of arable and 21% of marsh species have been lost since the 1700s (James, 1997). In Cambridgeshire, Northamptonshire and Bedfordshire these figures are slightly higher for acid, marsh and freshwater habitats (Fig. 4) and as a result 12 of the 16 species lost from all three counties are typically

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Table 1. The ecological attributes of “unsuccessful� species and theoretical cause of decline.

Attribute 1. Habitat Requirements Habitat specificity (niche breadth) Requirements for main plant resources (light, nutrients, acidity, moisture) 2. Competitive Ability Plant height Life-history Life-form Lateral spread

Character state and theoretical cause of decline Narrow niche breadth; more susceptible to habitat loss and modification Specialised ecological requirements; more susceptible to habitat loss and modification

Small stature; inability to compete under fertile conditions Short lifecycle; inability to withstand unfavourable episodes Short lifecycle and small stature; as above Non-clonal; inability to compete under fertile conditions

3. Dispersal Ability Seed size Large seeds; poor spatial dispersal ability Structures on the dispersule Lack of structures on the dispersule; poor spatial dispersal ability Persistence in the seedbank Lack of persistent seed bank; poor temporal dispersal ability 4. Breeding System Pollination strategy Self-incompatabilty; inability to find a mate 5. Position in Range Phytogeographical element Edge of range; change in distribution due to climate change associated with acidic conditions (Anagallis minima, Carex binervis, C. dioica, Drosera rotundifolia, Erica tetralix, Hypochaeris glabra, Lycopodium clavatum, Moenchia erecta, Myriophyllum alterniflorum, Oreopteris limbosperma, Potentilla palustris and Utricularia minor). In contrast woodland and wood-edge species appear to have remained relatively stable both at the national and local scale despite significant post-war changes in woodland management (coppicing, ride management, etc.) and increased deposition of atmospheric pollutants. This may indicate a flora which is largely resistant, or responding only very slowly, to current land-use changes.

Although most analyses have been based on typical habitat associations, most species occur across a range of conditions. Theoretically this means that a species with a large niche breadth can survive in more places and hence over a larger area, and conversely one with a very narrow niche breadth is more likely to be rare (Brown, 1984). Unfortunately, there is very little information

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Scrub/wood edge Neutral grass Wood Disturbed ground Wide amplitude Rock, stony sites etc.

Calcareous grass Acid grass / heath Freshwater / mire 0

5

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25

30

% species within each habitat which are extinct Figure 4. Summary of extinct species in Cambridgeshire (v.c. 29), Northamptonshire (v.c. 32) and Bedfordshire (v.c. 30) in relation to typical vegetation type. Extinctions are expressed as a percentage of all the species within each habitat type in the three counties. Habitat associations are taken from Ellenberg (1988). For ease of interpretation species within Ellenberg’s “conifer woods and allied heaths” and “saltwater and seacoasts” have been excluded. The total number of extinctions in each county are as follows (total in parentheses): Cambridgeshire, 111 (855); Northamptonshire, 109 (813); Bedfordshire 94 (838). on the niche breadth of many British species. However, a detailed study of the ecology of plants within the Sheffield region showed that species which occurred in a range of habitats also had a large range size in the UK as a whole (Thompson, Hodgson & Gaston, 1998, 1999). In addition, rare species in Northamptonshire tend to be those with a high habitat specificity (McCollin et al., 2000).

An alternative approach is to classify the realised niche of each species in relation to the major environmental factors that determine its distribution (and thus habitat). This was originally carried out by the German plant ecologist Ellenberg who defined a set of indicator values (Zeigerwerte) for all central European vascular plants based on their tolerance to the major plant resources; namely light (L–value), temperature (K–value), salinity (S–value), acidity (R– value), moisture (F–value) and fertility (N–value) (Ellenberg, 1979, 1988).

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Figure 5. The proportion of extinct species in Middlesex (v.c. 21) and Cambridgeshire (v.c. 29) in relation to Ellenberg values for light (L), moisture (M), acidity (R) and nitrogen (N). Original data taken from Preston (2000) but re-analysed using the British Ellenberg values (Hill et al., 1999).

Within this system species were assigned a score between 1 and 9 (1 and 12 for moisture) in relation to the range of conditions encountered in central Europe. These original values have recently been recalibrated (or estimated) for all British native species (Hill et al., 1999) in order to account for the differences in ecological requirements of some species at the western edge of their European range (Pigott & Walters, 1954).

In Cambridgeshire and Middlesex the highest proportion of species which have become extinct are those associated with open habitats (L = 7–9), such as grasslands, rather than those associated with woodland or partial shade (Fig. 5). The results from other lowland vice-counties are broadly similar. For example, 28 of the 34 species lost from both Northamptonshire and Bedfordshire are indicative of well-lit places (L = 7–9). Similarly both Cambridgeshire and Middlesex have lost a disproportionate number of species indicative of extreme acidity (R = 1–2; Fig. 5) and extreme infertility (N = 1; Fig 5). These results are very similar to those from Northamptonshire and Bedfordshire where 50% (4 species) and 70% (9 species) of the indicators of sites poor in fertility (N = 1) have become extinct (respectively), compared to just 12% (3 species) and 4% (1 species) of species characteristic of eutrophic

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sites (N = 8). Interestingly the results of the B.S.B.I. Monitoring Scheme are remarkably similar: species which have declined the most in England are disproportionately represented in the infertile categories (N = 2–4) whereas those that have increased occur within the rich to eutrophic categories (N = 7– 9). The same pattern is true for rare species: 55 (70%) of the 79 British species with an N value of 1 are classified as Red Data or Nationally Scarce. In contrast only 23 (15%) of 148 British species with an N value of 9 are rare.

In contrast, the relation between Ellenberg values for moisture and extinction are less clear. In Cambridgeshire and Middlesex there is an overall tendency for a higher proportion of species within the “wetter” categories to have gone extinct, although this is much less apparent in Middlesex (Fig. 5).

Competitive ability A number of studies have shown that changes in land-use practices over the last fifty years have meant that fertile, man-made habitats are becoming more widespread whereas semi-natural, undisturbed grasslands are becoming increasingly scarce. In ecological terms this suggests that taller, more competitive (and often ruderal) species will be the most “successful” species in the future flora of the British Isles, whereas small habitat specialists will continue to decline.

This is clearly displayed by studies which have investigated floristic changes in relation to plant height (Preston, 2000; Thompson, 1994). This is often used as a surrogate measure for competitive ability because taller, longlived species will be more effective in competing for available resources. In addition, such species are likely to persist longer in the landscape once populations become non-viable. Plant height data (maximum summer) is now available for all British species (excluding submerged and floating aquatic species) (Grime et al., 1988) and has recently been analysed in relation to extinction in Cambridgeshire and Middlesex (Preston, 2000). In this study the greatest proportions of species which had gone extinct were in the shortest categories whereas none of the taller species had been lost (Fig. 6). In contrast the height of the winter buds (i.e. the Raunkiaer life-form) did not appear to follow this pattern with all the categories having a similar proportion of extinctions (Fig. 6).

Another measure of competitive ability is to classify species in terms of their life-history, or more specifically their longevity (annuals and biennals versus perennials). In studies of change the assumption is often made that shorter-lived species (e.g. summer annuals) are more susceptible to change because they cannot persist unfavourable seasons or episodes of disturbance (unless they can form a persistent seedbank). Thus a high proportion of extinct, declining or rare species should be shorter-lived. However, despite the appeal of this classification it does not appear to explain the decline or rarity of species within England (Quinn et al., 1994; Thompson, 1994) or the extinction of species in vice-counties (Preston, 2000). For example, annual species (therophytes) in Cambridgeshire and Middlesex have had a much lower proportion of extinctions than the other life-form categories (Fig. 6).

The failure of this attribute (plant longevity) to explain decline may be related

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Ar Bo ct icre m oon ar ct ta ic ne m o W nta id e- ne Bo bo re re al al Bo m o re o- nta ne te m W pe id ete rate m pe So Te rat e ut he mp e rn -te rate m M ed per ite ate rra ne an

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Major Biome category

Figure 6. The proportion of extinct species in Middlesex (v.c. 21) and Cambridgeshire (v.c. 29) in relation to typical maximum plant height categories, Raunkiaer life-form, Eastern Limit category and Major Biome category. Data adapted from Preston (2000). to the ability of some shorter lived-species to overcome the limitations of a rapid lifecycle. Indeed many therophytes avoid localised extinction by dispersing widely to new sites or by persisting for long periods within seedbanks.

Finally species which can spread clonally are assumed to be more successful because they are less susceptible to population declines as a result of seed or pollination failure (Bond, 1995; Fischer & StĂścklin, 1997).

However, with the exception of McCollin’s (2000) study in Northamptonshire, there is very little evidence to support this theory. For example, this attribute was unimportant in explaining the decline of species in England and the Netherlands (Thompson, 1994), or the change in abundance of 373 species in Auckland, New Zealand, between 1871 and 1985 (Duncan & Young, 2000).

Dispersal ability It has been suggested that there is a positive relationship between dispersal ability and range size as species with poor dispersal abilities cannot attain distant suitable sites. Thus the majority of rare or declining species should have attributes associated with immobility and be increasingly confined to

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small, fragmented populations. In contrast long-distance dispersers are likely to be more widespread because they can “rescue” populations isolated by habitat fragmentation or colonise unoccupied habitat fragments. However, current research suggest that, unlike bryophytes (Longton, 1992), the relationship between seed dispersal and range size in seed plants is very weak.

For example, no significant correlations have been found between dispersal ability (measured largely as “wind” or “not wind-dispersed”) and the distribution of Nationally Scarce species (Thompson & Hodgson, 1996; but see Quinn et al., 1994) or increasing and decreasing species in the UK (Thompson, 1994).

Species with heavier seeds are also assumed to have poor dispersal ability because they are unlikely to be wind-dispersed. However, seed weight has been shown to be unrelated to the distribution of Nationally Scarce species (Thompson et al., 1999; Quinn et al., 1994) or changes in abundance (Thompson, 1994). Furthermore, in Northamptonshire the opposite was true; heavier-seeded species have increased in abundance over the last 70 years (McCollin et al., 2000).

The lack of evidence to link poor dispersal ability with decline is may be due to the simplicity of the classifications used. The division of species into “wind” or “not-wind” dispersed is certainly far too simplistic, and ignores other potentially important mechanisms, such as dispersal by animals (zoochory). Furthermore, some species are far more effective in dispersing through time than space. For example, most arable weeds, which are now strongly dispersal-limited due to more efficient seed-cleaning techniques, are able to overcome this (in part) by forming persistent seed banks.

Breeding system It has been suggested that cross-pollinated plants are likely to have more localised distributions than self-fertilising species because they are limited by the distribution, and ecological requirements, of their associated pollinators. In addition, Baker’s Law (Baker, 1953) states that a self-compatible monoecious species will be a better coloniser than a dioecious plant because that it does not need a mate. As a result it is expected that dioecious (and possibly hermaphrodite) species will be more susceptible to the effects of land-use change, and in particular isolation, as the fragmentation of obligate outcrossing populations will reduce their chance of reproductive success (Kunin, 1992). Support for this theory has come from the work of Quinn et al. (1994) who showed that obligate out-crossing Nationally Scarce species had more strongly aggregated distributions than predominantly crossing, crossing and selfing and predominantly selfing species. Interestingly the opposite statements appears to be true for mosses with dioecious species tending to be more widely distributed in Britain, and with a lower proportion of species considered to be rare (Longton, 1992).

Position in range Species at the edge of their ranges are more likely to respond to the effects of land-use and climate change, because they are often rare and highly habitat specific. For example, an increase in mean temperatures may result in a poleward shift in the distribution of some species, as has been shown for

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Table 2. “Unsuccessful” species in lowland England (L = 8–9; N = 1–2; R = 1–3; plant height = < 60cm) with status in Cambridgeshire (v.c. 29), Bedfordshire (v.c. 30) and Northamptonshire (v.c. 32).

Carex curta Carex echinata Drosera intermedia Drosera longifolia Drosera rotundifolia Erica tetralix Festuca filiformis Genista anglica Hypericum elodes Narthecium ossifragum Pedicularis sylvatica Polygala serpyllifolia Rhynchospora alba Teesdalia nudicaulis Trichophorum cespitosum Vaccinium oxycoccos Number recorded in county % extinct

Status with date of last record v.c. 29 v.c. 30 v.c. 32 1853 Extant Not present 1954 Extant 1950 1820 Not present Not present 1840 1798 Not present 1913 1942 1822 1920 1880 1980 1964 Not present 1878 1932 1875 Extant 1930 Not present Not present 1860 Not present Not present 1912 1969 Extant 1954 1976 Extant 1860 1798 Not present 1954 Extant 1712 1820 1798 Not present 1859 1798 Not present 16 12 8 100 75 63

butterflies (Roy & Sparks, 2000), and possibly an eastward shift in the range of more continental species, as warming reduces the effects of oceanicity.

Preston and Hill’s (1997) new phytogeographical classification of the British flora, which assigns species into elements on the basis of latitudinal extent (Major Biome category) and eastern limits in Europe (Eastern Limit category), provides data on which to test this hypothesis. When applied to extinctions in Cambridgeshire and Middlesex this showed that there had been a proportionally greater loss of species at the southern edge of their ranges (Boreo-arctic Montane species) and Oceanic species (Fig. 6; Preston, 2000).

Conclusions Although only a few species were lost from British flora as a whole during the 20th century it was an unprecedented period of extinction at the local scale.

Conservative estimates suggest a loss of around five species in every county per decade although is likely to be appreciably higher in south eastern counties where land-use changes have been more dramatic. These “unsuccessful” species appear to share a number of traits; they tend to grow in very open habitats, they are often associated with infertile soils which usually have low pH. In contrast, the attributes which are only weakly associated with decline include dispersal ability and breeding system. This suggests that within highly modified lowland landscapes declining species are primarily limited by habitat

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related factors and an inability to compete for resources under fertile conditions rather than a reduction in fecundity or an inability to colonise distant sites.

In the British flora as a whole 42 species share all four of the attributes mentioned above (L = 8–9; N = 1–2; R = 1–3; plant height = < 60 cm).

However, only 16 are fairly widespread in the lowlands of England.

Remarkably all of these have been lost from Cambridgeshire (Table 2) suggesting that this county is, as Preston (2000) feared, one of the worst affected in the country.

The main cause of these losses would appear to be increasing population density, the indirect effect of which has been unprecedented land-use and agricultural changes (Thompson & Jones, 1999). Although population density does not in itself account for many of these changes, it is an indicator of many aspects of land-use, such as road building, urbanisation and recreation. Indeed the population of England and Wales had almost doubled to around 60 million by the end of 20th Century. Coupled with a dramatic increase in the standard of living this produced a dramatic increase in both private car ownership, and consequently an increase in the extent of tarmac roads, and food production (Beebee, 2001). Crop yields in particular saw a dramatic increase from around 2–3 tonnes per hectare in the 1930s (the same as in 1800) to around 4 and 8 tonnes per hectare for barley and wheat respectively following the introduction of chemical farming methods in the 1950s (Beebee, 2001). These changes suggest a lowland landscape which has become increasingly fragmented and dominated by highly fertile man-made habitats during the latter part of the 20th Century.

However, the extent to which urbanisation or intensive agriculture has been the main cause of species extinction varies from county to county. In rural Cambridgeshire the main periods of extinction related to major periods of agricultural innovation between 1810–1830 and since 1930 (Fig. 2) when large areas of semi-natural habitat were enclosed and ploughed up for arable cultivation. Similarly, in adjacent Huntingdonshire, 25 of the 38 species which disappeared before 1900, became extinct following the drainage of Whittlesea Mere for agriculture use (Wells, 1989). However, in suburban Middlesex the peak period of extinction (> 7 species per decade) coincided with the spread of the London conurbation (1870+). This suggests that although intensive agriculture is often regarded as the main threat to wildlife in the UK urbanisation is a pervasive and often disregarded threat in some areas. For example, in a different study Thompson and Jones (1999) showed that current population density was directly linked to the localised (vice-county) extinction of nationally scarce plant species.

In both cases, the result has been the same: an increasingly impoverished native flora dominated by species associated with man-made habitats and “an increasing restriction of slow-growing plants of infertile, relatively undisturbed habitats to fragmented islands of suitable habitat, many of them in nature reserves, surrounded by a sea of unsuitable landscape” (Thompson, 1994). If we are to reverse this trend then our challenge is to make this “sea” a little less unsuitable for the many “unsuccessful” species in our flora.

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Acknowledgements I am very grateful to Chris Preston, Mark Hill and Brian Huntley for useful discussions on various aspects of this paper. In addition I would like to thank Chris Preston for allowing me to reproduce his data on extinction in Cambridgeshire and Middlesex and Chris Boon who supplied data on extinction in Bedfordshire. Much of this work has been carried out as part of a part-time doctoral study being carried out at CEH, Monks Wood in collaboration with the Department of Biological Sciences, Durham University.

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Kunin, W. (1992). Density and reproductive success in wild populations of Diplotaxis erucoides (Brassicaceae). Oecologia 55: 299–314.

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PE28 2LS

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