Species richness in Madeiran land snails, and its causes

Journal of Biogeography (J. Biogeogr.) (2008) 35, 647–653 

ORIGINAL 

ARTICLE 

The Manchester Museum, Oxford Road, 

Manchester M13 9PL, UK 

*Correspondence: Laurence Cook, The 

Manchester Museum, Oxford Road, Manchester 

M13 9PL, UK. 

E-mail: lcook@manchester.ac.uk 

INTRODUCTION 

Species richness in Madeiran land snails, 

and its causes 

Laurence M. Cook* 

In the Madeiran archipelago, there are over 250 land mollusc 

taxa, of which over 70% are endemic (Waldén, 1983). Extant 

and fossil faunas have been studied on all the islands in the 

group (Cook et al., 1990; Goodfriend et al., 1996; Cameron & 

Cook, 2001; Cameron et al., 2006 and elsewhere). Using this 

information, it is possible to examine how species proliferation 

came about and, in particular, the importance of competition 

and selection, as opposed to accident, in generating species 

richness. 

The islands and molluscan fauna 

ABSTRACT 

The Madeiran archipelago is 800 km west of Morocco (Fig. 1). 

Madeira is 58 km at its broadest and rises to 1860 m. Porto 

Santo is 40 km north-east, and 12 km in breadth. It rises to 

520 m with several small offshore islets. Between them, the 

ocean bed drops to 2600 m. The three Deserta Islands are 

10 km south-east of Madeira and linked to it by a submarine 

Aim To unravel the reasons why a group of islands of 800 km 2 area should 

support over 200 extant land snail species. 

Location The temperate Atlantic Madeiran archipelago. 

Methods Distributional surveys and model testing. 

Results Repeated volcanic events have changed the topography of the islands 

and created periodic further isolates. There has been climatic fluctuation and 

varying sea level. Individual locations are not species-rich, but there is 

replacement from one subregion to another. Evidence of competitive 

interactions is lacking. 

Main conclusions High diversity results from the fortuitous coincidence of 

rates of geological and climatic change and isolation on the one hand, and 

migration and genetic divergence on the other. Hubbell’s neutral model of 

biodiversity explains species richness if a factor is added describing structural 

instability or periodicity, here called the geodetic rate. This geodetic, biological 

interaction can explain why some archipelagos are species-rich. 

Keywords 

Competition, geodetic change, island biogeography, land snails, Madeiran 

archipelago, neutral model, niche, speciation. 

ridge. The total land area of all islands is about 800 km 2 . 

Madeira and the Desertas may once have been connected, but 

Porto Santo was always separate. Fossil snails are present on all 

three groups. 

There were at least four main volcanic phases on Madeira. 

Its eastern peninsula is younger than the central mass of the 

island, as are the Desertas. Porto Santo is ancient, without 

more recent overlying volcanic deposits. Sea mounts running 

towards Portugal (Geldmacher et al., 2001) were probably 

once islands. Vulcanism, erosion, sea-level change and changes 

in the ocean floor (Nascimento Prada & Serralheira, 2000) led 

to variation in island size. Intense erosion has taken place, with 

the deposition of mud flows. In some places younger lava 

occupies pre-existing valleys. Space for plants and animals has 

therefore repeatedly changed. Nascimento Prada & Serralheira 

(2000) give ages for Madeira from over 5.2 Myr, with the latest 

volcanic events as little as 2500 years ago and the last volcanic 

activity on Porto Santo at about 8 Ma. Other authors reduce 

the age of Madeira to 4.6 Myr and extend that of Porto Santo 

to 14 Myr (Geldmacher et al., 2000; Brehm et al., 2003). 

ª 2007 The Author www.blackwellpublishing.com/jbi 647 

Journal compilation ª 2007 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2007.01801.x

L. M. Cook 

Figure 1 The Madeiran archipelago. 

The main early author of Madeiran mollusc taxonomy was 

Lowe (1854 and earlier), and there has been a thorough review 

by Waldén (1983). Cameron et al. (2006) refer to other recent 

taxonomic work, which pre-dates the revisions of Bank et al. 

(2002). The taxa recognized are morphospecies, usually based 

on shell characters, sometimes with anatomical descriptions. 

The 19th century descriptions have been supported by later 

work, and the species-level systematics is relatively stable. 

Wollaston (1878) was the first to discuss distribution on the 

islands in detail, and this was later examined by Waldén 

(1984). 

We recorded 117 extant endemic and 36 non-endemic 

species in our surveys. On Madeira, 24 endemics were 

restricted to the west, south and east coasts, 23 to upland 

forest. On Porto Santo, nine endemic species were found only 

in the west, 15 only in the east, and 20 were distributed more 

broadly. There are 25 endemic species on the Desertas, 10 

restricted to that group and 11 on only one of the islands. 

Species encountered only as fossils bring the totals to 141 

endemics and 37 non-endemics, 178 in total (see Appendix S1 

in Supplementary Material). Almost all non-endemic species 

are European, introduced since human colonization in the 

15th century, and mostly on Madeira. 

The number of species collected in different parts of the 

archipelago is shown in Table 1. Small islets all have reduced 

faunas. Numbers of endemics in other sections vary from 12 in 

the sandy northern part of Porto Santo to 27 in Madeiran 

section D, where forest comes down to the coast and upland 

species feed into the fauna coming from coastal regions to east 

and west. More homogeneous sections have between 14 and 23 

species. Section B is the eastern peninsula. It is small in area, 

quite low-lying and dry, but with some forest species, probably 

relicts from an earlier, damper phase (Goodfriend et al., 1996). 

It is impossible to know whether any rare species have been 

overlooked, but the surveys have been thorough enough to 

provide substantially correct lists for the different sections. 

Despite the high count for the whole island group, the average 

number of species in individual regions is not large compared 

Table 1 Numbers of land snail species in identified subsections of 

the Madeiran islands. 

Location 

Total 

species 

Mean per 

sample 

Total 

endemics 

Madeira 

A 32 18.8 19 5 

B 22 12.3 14 3 

I 38 13.7 19 11 

H 39 15.1 26 9 

C 30 13.1 17 9 

D 41 25.7 27 3 

E disturbed wood 36 12.2 21 12 

F wood > 600 m 29 14.3 19 8 

G wood > 900 m 35 10.1 23 14 

Deserta Islands 

Ilhéu Chão 9 8.2 9 4 

Deserta Grande 25 7.2 14 10 

Bugio 22 8.0 11 4 

Porto Santo 

Ilhéu de Ferro 15 13.0 13 2 

Ilhéu de Baixo 15 12.5 13 2 

West 32 10.1 28 15 

North 16 8.3 12 7 

East 40 13.1 32 34 

Ilhéu de Cima 15 11.3 13 3 

Ilhéu das Cenouras 9 9.0 7 1 

Ilhéu de Fora 9 9.0 8 1 

No. 

samples 

Madeira: letters refer to divisions in Fig. 3 and tables in Cameron 

& Cook, 2001;. Desertas: Cameron & Cook (1999a) 19th century 

totals. Porto Santo: Cameron et al. (1996b). More samples are 

available for many of these areas; those shown were collected under 

comparable conditions. 

with other islands or continental areas, including western 

Europe (Solem, 1984; De Winter & Gittenberger, 1998; 

Cameron et al., 2003; Nekola, 2005). 

The faunas of the island groups are distinct. In our surveys, 

Madeira (with 62) and Porto Santo (with 47) have only three 

species in common. Vertiginidae and Vitrinidae are today only 

on Madeira. Three helicid genera are only on Porto Santo. 

Leiostyla (Pupillidae) has 20 species on Madeira, seven on 

Porto Santo and two on the Desertas. In the helicid genus 

Discula, there are two species on Madeira, 15 (including 

extinct) on Porto Santo, and three on the Desertas. Among 

fossil endemics, Craspedopoma mucronatum (Mencke), 

Heterostoma paupercula (Lowe) and Caseolus punctulatus 

(Sowerby) are common to all three groups. Craspedopoma 

mucronatum is now limited to Madeira. Caseolus punctulatus is 

no longer on Madeira, where H. paupercula is largely restricted 

to the eastern parts in conditions like those widespread on 

Porto Santo. The high forests in Madeira are the centres for 

Vertiginidae, Vitrinidae and Pupillidae. On Porto Santo, there 

are eastern and western faunal groupings that show up 

when data from sample sites are clustered (Fig. 2). On 

Madeira, clustering indicates at least four coastal regions with 

different faunal compositions (Fig. 3), some species showing 

648 Journal of Biogeography 35, 647–653 

ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd

(a) 

(b) 

Faunal similarity 

Endemic species 

Figure 2 (a) Porto Santo, showing regional localization of 

taxa in two subgenera of Discula. The central part of the island 

is lower and sandier than the east, which has a series of peaks, or 

the west, which also rises but to a lower altitude. (b) Faunal 

similarity, based on the determinant of presence–absence data 

for each pairwise comparison using endemic species only. Data 

from Cameron et al. (1996b). Divisions: W, western higher land; 

N, sandy north-central; WIs, western islets; EIs, eastern islets; and 

north-east, south-east and central sections of the eastern part of 

the island. 

geographical replacement. There have been species radiations 

on the islands – can they be said to be adaptive? 

Adaptive and non-adaptive radiation 

A radiation is the differentiation of a single ancestor into an 

array of species. It may be non-adaptive, occurring simply 

because the habitat is fragmented and without different 

adaptive peaks in the localities the new species come to 

occupy (Gittenberger, 1991). It is adaptive when the new 

species occur in different environments and differ in traits used 

to exploit them (Schluter, 2000). In that case, the speciation 

process may be mediated by competition, resulting in disruptive 

selection. This is not necessarily so, however, and we 

should look critically for evidence of competition among the 

molluscs. 

(a) 

(b) 

Faunal similarity 

Endemic species 

Madeiran land snails 

Figure 3 (a) Madeira, showing regions found to have differing 

faunas. Regions E, F and G are endemic laurel forest at successively 

higher altitudes. (b) Faunal similarity, based on the determinant 

of presence–absence data for each pairwise comparison using 

endemic species only. Modified from Cameron & Cook (2001). 

Direct evidence of competition is not easy to establish, but 

we have tried two indirect routes. First, the Madeiran endemic 

fauna is often associated with non-endemics introduced after 

human colonization. On Porto Santo, endemic species are 

much more abundant on rocky volcanic sites than on sandy 

ones, while the reverse is true for introduced species (Cameron 

et al., 1996b). It is likely that, having evolved in southern 

Europe, the introduced species are pre-adapted to sandy 

conditions that the local fauna has yet to penetrate. At drier 

sites from Madeira and the Desertas, introduced species add 

to, rather than replace, the endemic species, so as to produce 

richer associations (Cook, 1984; Cameron & Cook, 2001). In 

the Madeiran forest, numbers of introduced species are 

positively correlated with numbers of endemics (Cameron & 

Cook, 1997). This correlation could indicate that good sites 

favour non-endemics and endemics alike, or that high species 

number is connected with greater microhabitat heterogeneity; 

it does not indicate replacement. The most convincing 

evidence for replacement is loss of the fossil Caseolus 

bowdichianus (Férussac) at about the time of introduction of 

Theba pisana (Müller), a species similar in size and shape 

(Goodfriend et al., 1996). Even here, however, the period of 

human colonization was one of massive ecological change that 

could have exterminated the one and independently favoured 

the other. 

Journal of Biogeography 35, 647–653 649 


L. M. Cook 

A second approach concerns the size and shape of shelled 

molluscs. The relation of shell height to diameter for modern 

gastropods falls into two groups: high-spired, or globular to 

disc shaped (Cain, 1977). Divergence into one of two distinct 

modes suggests an adaptive cause, possibly related to the way 

species use their habitat. High-spired species tend to be active 

on vertical surfaces, while low-spired species use horizontal 

substrates, both in the European (Cain & Cowie, 1978; 

Cameron, 1978) and Madeiran faunas (Cameron & Cook, 

1989). Differences in shape, as well as in size, are associated 

with different microhabitats, and animals similar in size and 

shape are more likely to compete. If competitive exclusion 

limits overlap, there will be a minimum distance apart in the 

height/diameter space. The mean distance should be greater for 

species associating together than for arbitrarily pooled groups 

of such associations. A test of this expectation showed no 

reduction for pooled subsets of the Madeiran fauna (Cook, 

1984) or for regional components of the Turkish molluscan 

fauna (Cook, 1997). We could look harder for competition in 

experimental situations, but the results may not mirror field 

conditions, and there are many theoretical reasons to doubt 

that competition directly determines presence or absence. 

SPECIES FORMATION 

All speciation processes may involve isolation, divergent 

selection and reinforcement. Sympatric and parapatric speciation 

both require divergent selection and reinforcement, but 

are aided by anything that helps to isolate diverging subgroups. 

If allopatric, isolation could lead to speciation on its own, but 

there may be divergent selection while groups are apart and 

reinforcement when they meet. Intergradations are sometimes 

encountered in the molluscan fauna. On the Madeiran eastern 

peninsula, Discula polymorpha (Lowe) changes from one shell 

form to another over a short distance (Cook et al., 1990), while 

on Porto Santo, similar taxa do not intergrade and are 

recognized as good species (Fig. 2a). There is sufficient 

character variation in Heterostoma to have led to three species 

names (Waldén, 1983), although the morphological characters 

do not associate consistently (Lace, 1992; Cameron et al., 

1996a), and there are slight ecological differences between 

forms on Porto Santo (Craze & Lace, 2000). Both these 

examples could indicate parapatric evolution. The majority of 

Madeiran taxa are well defined, however, and the general 

relationship of species richness to geographical location points 

to isolation as an essential feature, with divergent selection in a 

subordinate role. Taking all lines of evidence together, species 

proliferation might typically proceed as follows. 

Porto Santo is the oldest island, available to colonizers for 

perhaps 10 Myr without major volcanic activity. Colonizers 

arrived by island-hopping from Europe through the chain now 

remaining as sea mounts. There may be 300 endemic taxa in 

total on all the islands, including extinct forms, derived from at 

least 20 colonizing founders (Cameron & Cook, 1992). If the 

colonizers arrived 10–5 Ma, then the doubling time for 

number of species (Turner, 1999) is 1.25–2.5 Myr. If there 

were more colonizing species, or more time was available, 

these times are lengthened, and they are shortened if the 

islands are younger. This is not rapid species formation. 

Hawaiian Drosophila and Baikal amphipods evolved more 

slowly; Hawaiian crickets and Malawian cichlids much faster 

(Turner, 1999). After a molluscan fauna was established, other 

islands became available. If migration to these was high 

compared with the speciation rate, little or no divergence 

would occur; if very low, new territory would remain empty. 

Between these limits speciation would be likely, especially 

where the environment differed from that of the donor 

population. Some of the new species would recolonize the 

original island, possibly to diverge from their parental 

populations, while extinctions would simplify the faunas at 

rates likely to be inversely proportional to area. The topography 

and habitability of all the islands altered periodically. 

Sea-level change joined islets and exposed new territory or 

reduced continuous land to isolates. On Madeira there were 

volcanic eruptions. Climate change may have resulted in 

merged or fragmented habitat types. The islands rise steeply 

from the sea bed and there is evidence of slumping in several 

places. One such event may have modified the fauna in both 

eastern Madeira and western Porto Santo (Cameron et al., 

2006). 

Given these structural changes, many isolates must come 

and go. Migration allowed Porto Santan endemics to reach 

Madeira, but was rare enough for unique species to develop on 

small islets. Examples are Actinella laciniosa (Lowe) on Ilhéu 

Chão, Discula turricula (Lowe) on Ilhéu de Cima, and the 

sporadic distribution of Discus guerinianus/calathoides (Lowe) 

(Cameron & Cook, 1999b). Different animal and plant groups 

in the Madeiras show levels of endemism inversely related to 

their migration expectations (Cook, 1996). It is the threefold 

interaction of production of isolates, migration rate and 

speciation rate that has been the critical determinant of species 

richness. Zonitid molluscs of the Azores (Frias Martins, 2005) 

and clausiliids in Crete (Gittenberger, 1991) show similar 

diversification patterns and, at a general level, the European 

molluscan fauna tells the same story. The British Isles have no 

endemics and show almost no geographical variation, while 

there is high diversity in the much smaller area of the Alps. The 

British fauna reflects the ability to migrate and to occupy 

suitable territory, while the Alps are highly dissected with 

isolated habitable pockets. There, and in Mediterranean 

refugia, divergence has been promoted by isolation (Bouchet, 

2006). 

DISCUSSION 

The patterns on the Madeiras fit clearly into one of Solem’s 

(1984) categories of land snail diversity. Local (sympatric, 

alpha) diversity is low and species richness in the island 

group relates to microgeographical (allopatric, beta) diversity 

in conditions described by Solem as a mosaic. Species are 

undoubtedly adapted to the environments in which they 

live. That is most obvious when we compare forest species, 



which often have reduced or delicate shells, with robust dryfacies 

species. Original colonizers survive in a given habitat, 

and after establishment may become better adapted. As a 

result, the mosaic of habitat types is occupied. Although not 

ruled out, evidence of faunal assembly with competition is 

lacking. 

The idea that species richness could be understood in terms 

of introductions and losses is at least as old as MacArthur and 

Wilson’s equilibrium model of insular biogeography. More 

recently, Hubbell (2001) discussed a model including intraspecific 

but not interspecific competition and the possibility of 

species formation. Species arise, enter new territory and 

become locally or globally extinct, giving rise to equilibrium 

communities. Empirical data from ecology, biogeography and 

phylogenetics can be explained using formulations that include 

a quantity h =2Jm, described by Hubbell (2001) as the 

fundamental biodiversity number, where m is the speciation 

rate and J the size of the metacommunity. Extinction rate, 

obviously an essential feature, is not an independent parameter 

but varies inversely with population. The metacommunity is 

the total number of individuals of all species that are connected 

by migration to the community studied, including, for an 

island group, continental populations that provide colonists. If 

colonization consists of only a few non-recurrent events, the 

metacommunity is the total population of the archipelago. 

Species richness depends on, and increases more or less linearly 

with, h. 

Much variation in species number can be put down to 

variation in J, while m could remain constant. However, species 

richness is often enhanced on archipelagos, pointing to an 

effect of physical conditions. These generate a dynamic balance 

involving archipelago J, rate of migration and rate of species 

formation. Hubbell treats the latter in a manner analogous to 

mutation, with one or a few individuals giving rise to new 

species. An equilibrium number of species arises because 

abundant species are more likely than rare ones to give rise to 

new species, while those that are rare are more likely to become 

extinct. Abundant species survive for longer than rare ones, so 

there is a wide range of residence time. If speciation results 

from partitioning of an extensive range, then species richness 

and evenness are increased, essentially because extinction rate 

is reduced. 

In genetic terms, speciation usually depends on divergence 

at several to numerous loci, where allelic substitutions 

accumulate (Nei, 1976). The number of substitutions required 

and the probability that they result in incompatibility are 

therefore important variables. Speciation rate differs between 

groups due to intrinsic genetic differences, but for a homogeneous 

group it varies inversely with mutation rate if selection 

is absent (Nei, 1976). Evolution is related to selection, when it 

is present, and local adaptation and selection for reinforcement 

can dramatically increase speciation rate (Gavrilets, 2003). The 

genetic conditions can be accommodated in Hubbell’s system 

by addressing the environmental factors that influence them. 

In a defined archipelago, speciation requires a level of 

migration high enough for new territory to be colonized but 

not so high as to limit divergence. Speciation is speeded by 

local adaptation so that changing environmental conditions 

can be advantageous; it is modified by breeding population 

size so that there is an optimum area of occupancy for new 

isolates. 

To describe general conditions for evolution, we therefore 

need a further factor that takes into account the geological, 

geographical and climatic dynamics of the territory occupied. 

For want of a more comprehensive term, this could be 

referred to as geodetic, relating to the layout of the Earth. 

Species richness is proportional to Jmg, g being the rate of 

geodetic change. Geodetic change has to be concordant with 

m. If too slow, no opportunities for divergence occur; if too 

fast, there is ecological disturbance, leading at worst to 

catastrophe and decrease in J. Similarly, a moderate amount 

of habitat change promotes divergence, but there are limits to 

rates of adaptation. For these reasons, g is likely to be about 

zero for short-term disturbance, rising to above 1 at some 

intermediate level, followed by a decline towards or below 1 

for increasingly long periods of structural and ecological 

stability. Maximum species richness requires large J and the 

right mix of m and g. 

Hubbell’s theory is close to the neutral theory of genetic 

diversity (Kimura, 1983), with the same theoretical attractiveness 

and practical problems. Provided selection is left out, 

genetic diversity is predicted from the quantity 4Nu, analogous 

to Hubbell’s 2Jm, where N is effective population size (like J, a 

carefully defined but not intuitively obvious quantity) and u 

the mutation rate. It also provides time scaling, as the rate of 

divergence is determined directly by the mutation rate. 

However, mutation rate is difficult enough to determine, we 

can almost never get good estimates of effective population 

size, and selection is not necessarily absent. Studies at any level 

show the primacy of selection as an explanatory factor in 

individual genetic studies. Similarly, interspecific competition 

does occur in communities, speciation is probably rarely a 

mutation-like process, and the neutral theory of biodiversity 

becomes less tidy if a quantity like g is included. With both 

theories, however, the scale at which the problem is viewed 

may make these types of explanation appropriate. Molecular 

clock assumptions can provide a valid measure of the rate of 

genetic divergence despite selection on parts of the system; this 

is the assumption on which standard time-scale estimation is 

based. In the same way, a neutral explanation of the ensemble 

distributions of species abundance can accommodate the 

manifest evidence of adaptedness and competition. All the 

phenomena of evolution and ecology are then explained in 

terms of numbers and the rate of genetic change, which is 

bound up with geodetic history. 

ACKNOWLEDGEMENTS 


I thank Robert Cameron for collaboration, expertise and 

discussion of these problems over many years, and Alison 

Bates for the illustrations. The paper was developed at the 

Payamino Field Station, Orellana Province, Ecuador. 



L. M. Cook 

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SUPPLEMENTARY MATERIAL 

The following supplementary material is available for this 

article online: 

Appendix S1 Species of land snails recorded in surveys of the 

Madeiran islands. 

This material is available as part of the online article from: 

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365- 

2699.2007.01801.x 

Please note: Blackwell Publishing is not responsible for the 

content or functionality of any supplementary materials 

supplied by the authors. Any queries (other than missing 

material) should be directed to the corresponding author for 

the article. 

BIOSKETCH 

Laurence Cook is Visiting Professor at Manchester Metropolitan 

University and Honorary Associate at The Manchester 

Museum. His research has concentrated on investigating 

maintenance of visible polymorphisms in populations and 

species diversity in communities, mostly in molluscs and 

Lepidoptera. 

Editor: Mark Bush

Species richness in Madeiran land snails, and its causes

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