Island Colonization by Carnivorous and Herbivorous Coleoptera

Island Colonization by Carnivorous and Herbivorous ColeopteraAuthor(s): Peter BeckerReviewed work(s):Source: Journal of Animal Ecology, Vol. 44, No. 3 (Oct., 1975), pp. 893-906Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/3726 .

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893

ISLAND COLONIZATION BY CARNIVOROUS AND HERBIVOROUS COLEOPTERA

BY PETER BECKER

Department of Zoology, University of Michigan, Ann Arbor, Michigan 48104, U.S.A.

INTRODUCTION

Several investigators have compared the trophic structure of island and mainland insect communities or discussed the relationship between different feeding habits and colonizing success. Janzen (1973) said that the relative species richness of predatory beetles in samples swept from secondary vegetation on Caribbean islands was greater than that in mainland (Costa Rican) samples. On the basis of species richness, Zimmerman (1948) believed that predator pressure in Hawaiian insect communities is lower than in mainland communities. Weiss (1925) concluded that all insect communities of areas (including islands) large enough to embrace different types of vegetation, possess certain fixed pro- portions of insect species having different types of feeding habits. Heatwole & Levins (1973) said that the trophic structure of arthropod communities on very small islands reflected the relative availability of plants and detritus, with the successful establishment and persistence of immigrants depending on their position in the trophic web. Dammer- man (1948) characterized the initial sequence of establishment of insects (and other animals) on newly-formed islands as scavengers and omnivores, followed by herbivores and, finally, carnivores. Lindroth (1957) found an advantage of polyphagy, but no disadvantage of phytophagy, in the introduction of beetles transported in ship ballast from Europe to North America.

Here, using published checklists and faunal descriptions, I compare the numbers of species occurring in selected herbivorous and carnivorous coleopteran families on various island groups with those of their probable mainland sources. In short, the proportion of carnivores on islands is greater than would be predicted if their fauna was merely a representative sample of mainland species. A similar comparison of heteropteran faunas provides partial confirmation of the biological reality of this pattern. These groups were chosen for analysis because they are among the better-known taxa including both herbivores and carnivores.

At least three major biological processes will affect the constitution of an insect fauna: (1) dispersal of propagules from the mainland (and stepping stone islands), (2) establishment and maintenance of the founding population (or its differentiated derivatives) and (3) adaptive radiation (speciation), usually following habitat changes or host shifts of the ancestral immigrant. The most readily supported hypothesis is that the generally greater trophic specialization of herbivores makes it more difficult for them to establish and maintain populations in new environments than for carnivores with polyphagous habits.



Island colonization by beetles

METHODS

Trophic categorization of beetle families I chose families for the comparison of island and mainland trophic structure before

tabulating the data. Blackwelder's (1944-57) checklist of the Coleoptera of Latin America and the West Indies includes 117 families, and I characterized their feeding habits by reference to Balduf (1935), Clausen (1940), Borror & DeLong (1971), Clark (1895), Mank (1923), Voris (1934) and D. H. Janzen (personal communication) assigning all to one of the six following categories.

(1) Adults and larvae mainly terrestrial predators (carnivores): Cicindelidae, Carabidae, Staphylinidae, Lampyridae, Cantharidae, Malachiidae, Dasytidae, Cleridae, Ostomati- dae (excluded from analysis due to small size of group), Cucujidae, Coccinellidae (except Epilachninae and Psylloborini), Pythidae.

(2) Possibly terrestrial predators (few records), or larvae or adults frequently feed on food other than animal prey, or many genera within family are chiefly non-predacious: Paussidae, Pselaphidae, Plastoceridae, Elateridae, Histeridae, Phengodidae, Drilidae, Nitidulidae, Colydiidae, Bothrideridae, Meloidae.

(3) Principally saprophagous, feeding on decaying organic matter or fungus: Ptiliidae, Silphidae, Passalidae, Lucanidae, Scarabaeidae, Lycidae, Dermestidae, Lyctidae, Biphy- Ilidae, Cryptophagidae, Endomychidae, Tenebrionidae, Platypodidae, Scolytidae, Coccinellidae (Psylloborini only).

(4) Adults and larvae feed principally on living, vascular plant material: Buprestidae, Bostrichidae, Languriidae, Byturidae (excluded from analysis due to small size of group), Cerambycidae, Chrysomelidae, Bruchidae, Curculionidae, Coccinellidae (Epilachninae only).

(5) Habitat of one or more life stages aquatic or semi-aquatic: Haliplidae, Dytis- cidae, Gyrinidae, Limnebiidae, Hydrophilidae, Cyphonidae, Heteroceridae, Elmidae, Dryopidae, Psephenidae, Georyssidae.

(6) Feeding habits unknown or else so varied between taxa as to make it difficult to assign the group to one of the first four categories: most families.

Only those families in (1) and (4) were tabulated and considered in the comparison of island and mainland faunas. Although they represent a small proportion of the total number of families, some of them are very large groups and together account for approximately half the total number of recorded species in either mainland or island coleopteran faunas. The checklists used somewhat different classifications, but I followed Black- welder's terminology for convenience. Questions of familial synonymy were resolved by consulting Crowson (1955) and Schenkling (1940).

Limitations of analysis There are problems with the above categorization. For example, the diets of certain

carnivorous carabids and coccinellids include pollen and fungal material (Webster 1880; Forbes 1880, 1883). Some carabid genera are principally herbivorous (Davies 1953). Many of the Cerambycidae and Bostrichidae, especially those from islands, feed in dead wood although Zimmerman (1948) indicates that they can be remarkably host- specific. I believe that these problems do not significantly affect the analysis presented later because herbivores' and carnivores' colonizing ability probably depends more on their dietary specificity than on the nature of their food.

My analysis does not require a complete enumeration of predatory and herbivorous

894



species provided there is no bias in the choice of taxa. Bias resulting from differential collection or description is difficult to evaluate as the original compilers rarely discussed this specifically. With one exception I used the faunal lists as originally prepared. The most satisfactory solution is to compare several island groups with nearby mainland areas, trusting that consistent patterns emerging from independent studies are likely to have biological explanations.

I did not prepare absolutely accurate compilations in Tables 1 and 2 (see Results) when working with enormous, unsummarized checklists. I did, however, replicate enough counts to convince myself that any error in the figures does not affect the conclusions drawn here.

Description of the islands and their faunistic affinities The Greater Antilles, situated in the Caribbean with the largest island, Cuba, 200 km

from Yucatan and 224 km from Florida, have a coleopterous fauna allied to that of America (Blackwelder 1943; Darlington 1970; Leng & Mutchler 1914). The Galapagos Islands are on the Equator about 960 km from the west coast of South America to which their fauna is related. Their Coleoptera are derived especially from Peru and Ecuador (Van Dyke 1953). The Atlantic Madeira-Canary group is 560 km off the north-west coast of Africa, and its Coleoptera have a chiefly Mediterranean origin (Wollaston 1865). The Seychelles, just off the Equator, are 2400 km south-west of India and about 920 km north of Madagascar, which is 400 km off the eastern coast of Africa. Their Coleoptera show Indo-Australian, Madagascar-Mascareigne, African, Indo-Malayan and Oriental relationships with great differences between families existing (Scott 1933). The Mas- careignes lie 720 km east of Madagascar; I do not have information on the faunistic affinities of their Coleoptera. For a description of the islands' climate, geology and vegetation refer to the above literature, except for the Greater Antilles (see Anon. 1973) and for the Mascareignes (see Toussaint 1972).

Since comparative figures from a mainland source are unavailable for the Seychelles and Mascareignes, data from the island of Madagascar are substituted. This seems justi- fiable because apparently very large islands have a proportion of predatory Coleoptera similar to that of the mainland: New Zealand, with an area of 2-66 x 105 km2 and 1920 km south-east of Australia, has 330 predators compared to 340 for Australia (calculated from data in Tillyard (1926); the figure for New Zealand is affected in an unknown, but slight, fashion by the inclusion of species peculiar to the Chatham, Ker- madec and sub-Antarctic islands). Madagascar, with 340 predators, is remarkably close to these values.

Only the Madeiras and Canaries are outside tropical latitudes, and all the islands except the Seychelles are of volcanic origin. The Greater Antilles are considered oceanic (Carlquist 1965); the nature of the remainder is still disputed. All of these groups possess some mountainous islands, and whatever remains of native forest occurs in the high- lands. Human settlement of the islands, with subsequent destruction of lowland forests, happened several hundred years before any extensive, published faunistic surveys were made. Usually this has meant the replacement of the native flora, to varying degrees, by plants associated with human commerce.

RESULTS

Representation of beetle families on mainlands and islands It is relevant to my analysis whether the insular representation of particular, large

PETER BECKER 895



Tat

Latin America* Mexico (excl. Baja) Br. Honduras Guatemala Nicaragua Costa Rica Panama Colombia Venezuela

Greater Antilles* Cuba Hispaniola Jamaica Puerto Rico

W. coast S. America* Ecuador Peru Chile

Galapagos Is.t All islands Albemarle Indefatigable James Chatham Charles

Iberian Peninsulat Spain and Portugal

Madeira-Canaries? All islands Madeira Porto Santo Ilheo Chao Deserta Grande Teneriffe Fuerteventura Grand Canary Lanzarote Palma Gomera Hierro Bugio

Madagascar? Mascareignes?

All islands Reunion Maurice Rodrigue

Seychelles* * All islands

00

ON )le 1. Numbers of species in predacious coleopteran families on islands and nearby mainland Cicindelidae Carabidae Staphylinidae Lampyridae Cantharidae Malachiidae Dasytidae Cleridae Cucujidae Coccinellidae Pythidae

73 807 701 3 39 24

13 319 734 17 127 135 16 82 169 17 243 494 32 406 383 25 141 185

12 163 194 7 148 129 1 80 142 5 68 101

50 74 21 44 125 172

4 210 197

2 23 2 1 7 1 0 2 0 0 5 0 0 7 1 0 4 0

7 282 111

0 187 205 0 73 106 0 25 28 0 4 2 0 15 5 0 65 101 0 24 39 0 37 55 0 33 55 0 15 51 0 33 78 0 11 37 0 8 5

65 314 112

3 22 41 1 16 24 2 11 29 0 1 2

1 9 81

64 146 8 4

56 88 27 16 34 28 44 72 84 100 14 9

34 8 15 4 43 4 10 3

32 39 54 88 14 47

0 0 0 0 0 0 0 0 0 0 0 0

2 65

0 3 0 1 0 1 0 0 0 0 0 1 0 1 0 2 0 1 0 1 0 1 0 1 0 0

21 9

0 0 0 0 0 0 0 0

1 1

74 52 196 74 148 9 1 0 5 12 16 2

26 17 91 102 80 9 6 0 28 14 17 1 1 1 52 17 25 1

15 6 74 47 80 6 13 40 63 42 106 6 8 21 17 21 31 1

2 0 16 24 24 1 1 0 3 10 7 1 0 0 1 8 9 0 0 1 6 18 24 0

0 17 6 0 9 1 4 34 48 2 51 2 2 68 81 10 42 19

1 2 1 4 0 0 0 1 0 1 0 0 0 0 1 3 0

1 1 0 0 0 0 0 1 0 0 0 0 0 1 0 2 0

48 87 8 4 7 0

27 12 5 21 20 1 3 4 2 14 14 1 2 2 0 0 4 0 0 2 0 0 1 0 1 3 0 1 2 0 8 4 3 10 9 0 6 2 3 3 6 0 6 4 2 6 6 0 7 2 3 5 5 0 5 3 1 7 7 0 6 3 2 6 7 0 1 2 1 6 5 0 2 2 0 0 0 0 5 6 142 23 26 6

0 10 5 12 4 1 0 4 2 5 3 1 0 9 1 8 1 0 0 1 0 1 0 0

3 7 17 10 0 * Compiled from Blackwelder (1944-57) except for Greater Antillean Carabidae which are from Darlington (1943), t Van Dyke (1953), $ Winkler (1924-32), ? Wollaston

(1865), ?1 Alluaud (1900), ** Scott (1933).

c4,

cll C)

Z7,

'It 2

CW



taxa within trophic groups differs greatly from expectations based on the mainland faunas. The numbers of species occurring in the selected predacious and herbivorous families and reported for the islands and nearby mainlands are compiled in Tables 1 and 2. The figures give some notion of the relative species richness of the various taxa and demonstrate that a few families (Carabidae, Staphylinidae, Cerambycidae, Chrysomelidae and Curculioni- dae) account for the majority of the coleopteran fauna as discussed here. Broadly speak- ing, these families, which are the largest on the mainland, are also well represented on the islands.

At a finer level of resolution, a comparison of the major taxa's colonizing success is not straightforward. This is because considerable variation in their proportional representation in the mainland faunas makes it difficult to define expected species richness for the corresponding island faunas. For example, in the Latin American locales, staphylinid species-richness ranges from less than to twice that of carabids. Nearly the same variation occurs between these groups in the Greater Antilles.

Proportion of predators on islands and mainland Table 3 gives the total numbers of predacious and herbivorous species and the pro-

portion that are predators for the taxa and locales listed in Tables I and 2. While the variation within and between island groups is considerable, the proportion of predator species is greater on the islands than on their presumed mainland sources with only two exceptions (Chile and Rodrigue). The calculations in which Staphylinidae have been excluded offer a partial test of the sensitivity of this result to the influence of particular taxa. The basic pattern is hardly affected in the comparisons involving the Greater Antilles, Galapagos, and the Madeira-Canary groups. The percentage of predators in the Seychelles and Mascareignes, however, is thereby reduced to a value lower than that obtained for Madagascar. This result for the Seychelles is chiefly due to their depauperate carabid fauna, which is puzzling considering the consistently strong representation of this group on other islands (Scott 1933).

Supporting evidence from Heteroptera Additional evidence consistent with the pattern shown by predatory beetles comes from

an analysis of insular heteropteran faunas (Table 4). The Azoran fauna has originated mainly from Portugal (Leston 1957). The proportion of predators is more than two-fold greater on the Azores than in their mainland source area. The fauna of Puerto Rico is not so readily characterized, but many of its genera may also be found in Florida. Florida, rather than a Central American locale, is compared because of its more complete heteropteran list and its proximity to Puerto Rico. The direction of the comparison is favourable, but the difference is less marked than with the Azores. The agricultural pests of Puerto Rico are especially well-studied (Wolcott 1948) and the phytophagous fauna may be relatively better known.

Effects of area and isolation on proportion of predators The percentage predators decreases with increasing area for Jamaica, Hispaniola and

Cuba (Table 3), which supports the earlier suggestion that very large islands like New Zealand and Madagascar have a proportion of predators similar to mainland values. The Puerto Rican value contradicts this trend in the Greater Antilles (see the remarks on Puerto Rican Heteroptera). This relationship between area and percentage of predators is not detectable among the islands in the Galapagos, Madeira-Canary, or Mascareigne

PETER BECKER 897



00

00

Table 2. Numbers of species in herbivorous coleopteranfamilies on islands and nearby mainland (data sources as in Table 1) Latin America

Mexico (excl. Baja) Br. Honduras Guatemala Nicaragua Costa Rica Panama Colombia Venezuela

Greater Antilles Cuba Hispaniola Jamaica Puerto Rico

W. coast S. America Ecuador Peru Chile

Galapagos Is. All islands Albemarle Indefatigable James Chatham Charles

Iberian Peninsula Spain and Portugal

Buprestidae 513 18

142 50

126 140 115 56

47 25 14 15

21 78

117

3 1 1 1 1 0

Bostrichidae 26

1 15 3 1 7 6 4

10 4 6 6

0 4

10

3 1 2 0 1 0

Languriidae 42 12 25 13 3

17 7 2

0 0 0 0

0 5 0

1 0 0 0 1 0

O

O

Coccinellidae 17 2 8 2 8 8

29 11

1 0 0 0

17 47 0

0 0 0 0 0 0

Cerambycidae 816 70

408 365 125 369 251 201

175 89 60 77

117 149 153

17 7 5 9 4 2

Chrysomelidae 1641 155 848 309 362 712

1114 289

214 95 33

112

394 820 106

6 2 2 0 2 2

Bruchidae 69 11 73 12 8

50 23 13

23 226 6 87 4 30

16 83

4 166 13 418 27 282

2 0 0 0 0 0

0 1 55 193

Curculionidae 1641 155

1157 405 421

1154 629 349

t:;

ZL.

2:

17 3 3 2 2 1

40 1 17 589



Madeira-Canaries All islands Madeira Porto Santo Ilheo Chao Deserta Grande Teneriffe Fuerteventura Grand Canary Lanzarote Palma Gomera Hierro Bugio

Madagascar Mascareignes


Seychelles

7 1 0 0 0 2 1 4 0 1 0 1 0

220

13 3

11 0 5

6 3 0 0 0 1 0 0 0 2 2 0 0

24

11 3 7 0 7

0 0 0 0 0 0 0 0 0 0 0 0 0 1

0 0 0 0 0

3 1 0 0 0 1 1 1 0 1 1 0 0

21

0 0 0 0 0

20 10 0 0 2 9 3 1 2 5 5 4 1

348

28 14 11 3

22

61 26 8 1 5

28 12 16 11 17 14 15 0

371

10 1 8 1

17

7 234 3 89 1 29 1 3 1 18 5 80 2 34 4 45 2 28 4 36 2 42 2 41 0 5

16 386

13 67 2 38

11 24 0 6 1 125

TIt

tTi 0)

00 ,No



0 0

Table 3. Total numbers of herbivorous andpredaceous species in selected coleopteran families and the proportion of predators on islands and nearby mainland (data sources as in Table 1)

Herbivores Predators Herbivores and predators %o Predators Area (km2) % Predators* Latin America

Mexico (excl. Baja) Br. Honduras Guatemala Nicaragua Costa Rica Panama Colombia Venezuela

Greater Antilles Cuba Hispaniola Jamaica Puerto Rico

W. coast S. America Ecuador Peru Chile

Galapagos Is. All islands Albemarle Indefatigable James Chatham Charles

Iberian Peninsula Spain and Portugal

4765 424

2676 1159 1054 2457 2174

925

2344 114

1535 388 426

1098 1277 471

696 478 306 325 147 288 309 236

719 1534 695

249 624 694

49 35 14 11 13 6 12 7 11 9

5 7

7109 538

421 - 1547 1480 3555 3451 1396

1174 631 435 545

968 2158 t389

84 25 19 19 20 12

33 1-82 x 106 21 2-27 x 104 37 1-16x 105 25 1-32 x 105 29 5-89 x 104 31 8-29 x 104 37 1-13x 106 34 101x 106

41 1-13x 105 52 7-56 x 104 66 1-13 x 104 43 8-70 x 103

26 2-56 x 105 29 1-23 x 106 50 7-30 x 105

42 44 32 37 45 58

4-22 x 103 9-96x 102 5-20x 102 4-99 x 102 1-64x 102

621 1517 41 5-89x 105

26 18 23 18 20 20 29 24

It rr

;(3 11

N. Z 29 39 50 30

24 23 42

40 42 32 37 42 58

896 36



Madeira-Canaries All islands Madeira Porto Santo Ilheo Chao Deserta Grande Teneriffe Fuerteventura Grand Canary Lanzarote Palma Gomera Hierro ,Bugio

Madagascar Mascareignes


Seychelles

338 133 38 5

26 126 53 71 43 66 66 63 6

1387

481 218 62 9

27 201

84 118 111 90

136 64 17

729

142 98 61 56 72 61 10 5

59 62 62 64 51 61 61 62 72 58 67 50 74

819 351 100 14 53

327 137 189 154 156 202 127 23

2116

240 117 133 15

177 130 307 * Calculated after excluding Staphylinidae.

7*81 x 102 4-4 x 101

2-04x 103 1-71 x 103 1-52x 103 8-27x 102 7-17 x 102 3-74 x 102 2-74x 102

34 5-80x 105

41 48 2-47 x 103 46 1-8x 103 33 1-Ox 102 42 2-74x 102

45 46 47 58 46 44 46 47 57 37 47 30 67 31

'IO H tT

rtl

PIN ii'

29 34 31 23 22

0




Table 4. Total numbers of herbivorous and predacious species in terrestrial heteropteran taxa* and the proportion of predators on islands and nearby

mainland

Herbivores Predators Herbivores and predators % Predators

Portugalt 350 57 407 14 Azores' 25 12 37 32** Florida? 317 106 423 25 Puerto Rico? 125 52 177 29

* Feeding habits characterized by reference to Borror & Delong (1971) and Clausen (1940). Herbivores: Thaumastocoridae, Miridae, Tingidae, Piesmatidae, Lygaeidae (except Geocorinae), Berytidae, Largidae, Pyrrhocoridae, Coreidae (Coreinae and Rhopalinae only), Pentatomidae (Scutellerinae and Pentatominae only). Predators: Anthocoridae, Miridae (Cyrtorhinus and Deraeocoris only), Nabidae, Enicocephalidae, Reduviidae, Phymatidae, Lygaeidae (Geocorinae only), Pentatomidae (Acanthoso- matinae and Asopinae only).

Compiled from t Seabra (1941), Leston & Carthy (1957), ?Blatchley (1926), ? Wolcott (1948).

** Range for individual islands: 21-53%.

groups in which insular sizes and their range are smaller. There are several reasons why very large islands may have a smaller proportion of predatory beetles than small islands. Greater area should correlate with a more diverse flora, increasing the probability that colonizing herbivores will find an acceptable host. The probability of local catastrophes eliminating a plant species from an island, and thereby causing the extinction of its associated herbivores, should decrease with increasing area.

Differences between these island groups, in terms of distance from their colonizing sources, have no consistent, detectable effect on the proportion of predators in the coleopteran fauna. By this criterion the Galapagos, Madeira-Canaries and Seychelles are all more isolated than the Greater Antilles, but only the Madeira-Canary group has a greater proportion of predators. However, distance is a crude measure of isolation; also, these island groups differ considerably in the climates and resource bases that they offer to insects.

Colonization of new islands

Some information on the proportion of carnivorous coleoptera in newly created environments comes from surveys of Memmert's and Krakatau's faunas. Memmert is a very small (c. 8 km2) East Friesian island lying at the mouth of the Ems River about 16 km from the coast of Germany. Until about 1890, Memmert was merely a sandbank. By the early 1900s there were 8-m-high dunes and the island laid above the highest tides (Fiige 1918-19; Alfken 1924). In 1883, the island of Krakatau was partially destroyed by a volcanic eruption, which covered it and the nearby island of Verlaten with ash 30-60 m deep. Situated only 41 km from the coast of west Java, these islands were partially re- forested before even fifty years elapsed (Dammerman 1948). Using the data reported by Fiige (1918-19) for Memmert and by Dammerman (1948) for Krakatau and Verlaten, I calculated the proportion of predatory beetles on the islands although many of the smaller families in my previous analyses were not recorded during the initial period of colonization.

Approximately twenty-five years after Memmert rose above its beginnings as an inundated sandbar, 85% of its 136 carnivorous and herbivorous species were predators. Fiige doubted the residency of some of the carabids and staphylinids since many were

902



collected from shoreline refuse, especially after storms. He collected early in the growing season during May, and believed that more chrysomelids could be found later in the year.

On Krakatau the proportion of predatory beetles (followed by the total number of herbivore and carnivore species in parentheses) was 12% (16) in 1908, 52% (60) in 1921 and 56% (52) in 1933, and on Verlaten the figures for the same years are 00 (2), 47/

(30) and 58% (31). Compared with the samples in 1921 and 1933, those in 1908 were collected in a short time and did not include forest litter (Dammerman 1948). So the low proportion of predators in the earliest stage of colonization is not of particular sig- nificance. Dammerman (1948) regarded the later appearance of particular predatory families such as the Cicindelidae and Coccinellidae as evidence for his general proposal that colonizing herbivores become established before carnivores, but this fails to take into account the relative availability of potential immigrants from the source areas.

While comparative data from source areas are not presented, the proportion of predators on Memmert, Krakatau and Verlaten is high in comparison with the mainland values in Table 3. I doubt that low floristic richness is mainly responsible for the relative paucity of herbivores in these new environments. By 1912, 200 plant species were 'unquestion- ably established' on Memmert (Fiige 1918-19), and 210 species of vascular plants with fifty-nine fern species occurred on Krakatau fifty years after its eruption (Dammerman 1948). In comparison with the islands previously discussed, these islands, because of their very short distance from source areas, probably had higher immigration rates of insects and plants, which should give colonizing herbivores a greater probability of finding their mainland hosts. Perhaps this was prevented by rapid floristic succession on Memmert, Krakatau and Verlaten.

Exceptions to the rule

There may be exceptions to the tendency for islands to have a greater proportion of predacious species of coleoptera than mainlands. Those I am familiar with are very small, such as Columbrete Grande (Compte 1969), or extremely isolated, such as the Tristan da Cunha-Gough Island group (Holdgate 1960), St Helena (Wollaston 1877, cited in Wallace 1880, who enumerates only the endemic species) and Hawaii (Zimmerman 1948 and personal communication). It is difficult to compare these with the examples discussed previously as their faunas are very small or disharmonic. Thus, caution should be expressed in generalizing from the patterns described here.

DISCUSSION

Interspecific differences in vagility exist among the Coleoptera, and contemporary zoo- geographers (e.g. Darlington 1970; Gressitt 1974; Holzapfel & Harrell 1968) have em- phaisized that aerial transport is probably the principal means of insect dispersal to distant islands. There is no suitable information for comparing the long-distance dispersal capacity of herbivorous and carnivorous beetles, but the possible importance of storms moving contrary to prevailing winds in transporting insects to islands (Darlington 1938; Gressitt et al. 1961) suggests that a general distinction is unlikely.

A more probable explanation for the preponderance of predators on the islands shown here is that their polyphagous habits facilitate population establishment and maintenance (cf. Janzen 1973). The im i ession that predatory beetles tend to be less restricted than herbivorous ones in the range of acceptable food species is exemplified by the work of Lindroth (1957, Table 6), who claimed that of the 167 species belonging to families

PETER BECKER 903




categorized in this paper as predatory, two are oligophagous and the remainder polyphagous. Of the forty-four species belonging to herbivorous families, seventeen are monophagous, twenty-one oligophagous and only five polyphagous.

In addition to the importance of dietary specificity, there are some reasons for believing that carnivory per se may enhance the colonizing success of predacious Coleoptera. Most Spermatophyta tissues contain lower concentrations of protein than those of insects and small cryptogams (Southwood 1972); along with the extra costs of processing chemical defensive compounds for herbivores, this suggests that carnivores may more easily procure nitrogen reserves for egg synthesis. Compensation for the lower energy content of plant material by high feeding rates may be difficult for foliage-consuming herbivores during the dry season, especially since reduced island floras are less likely to provide suitable alternate hosts. Forced to survive on its reserves during the non-growing season, a non-daipausing herbivore may face a greater probability of death by starvation than a predator. The latter may also be able to supplement its reserves by feeding on insects specialized at using dry season resources such as seeds. My preliminary observations on a few species suggest that predacious Coleoptera and Heteroptera are able to survive longer without food in the laboratory than phytophagous ones.

A further difficulty that invading herbivores might encounter would arise from the influence of changes in food plant quality on their diapause-controlling mechanisms (Kevan 1944; Norris 1964; Squire 1940a, b). A herbivore, which has evolved its physiological and developmental responses to mainland host plant cues, may respond inappropriately to the information presented by its taxonomically or phenologically different island host.

Differential rates of speciation provide another potential explanation of the over- representation of carnivorous species on islands. Leston (1957) asserted that none of the Azoran Heteroptera are endemic so that in this instance (Table 4) the effect of speciation may be ruled out. In the other examples there is, unfortunately, no evidence. A remaining factor which must be considered is the influence of man (see Janzen 1973; Allan et al. 1973). A common feature of the islands discussed above (except Memmert, Krakatau and Verlaten) is the partial destruction of native vegetation and the addition of foreign plants during human settlement. Perhaps the ability of predators to switch food sources at such times of change would make them more resistant to extinction than herbivorous species.

ACKNOWLEDGMENTS

I appreciate the substantial assistance of Betsy Cantine, Malcolm Coulter, Val Dryer, Bill Durham, Win Fairchild, Karl Guthe, Dan Janzen, Pamela Johns, John Lawrence, Marlene Palmer, Judy Rankin, Les Real, Edward Rothman, Vickie Sork, Jean Stout, Ava Weinberger, Ron Westman, David Wilson, David Wise and Marina Wong. The Department of Zoology of the University of Michigan provided a Teaching Assistantship and computer funds. This paper is dedicated to my family.

SUMMARY

(1) The species-richness of herbivorous and carnivorous families of beetles on the Greater Antilles, Galapagos, Madeira-Canaries, Mascareignes and Seychelles is compared with that of nearby mainland areas.

904



(2) Carnivores are relatively better represented than herbivores on these islands and a similar trend is shown by the Azoran and Puerto Rican Heteroptera.

(3) The likely explanation is that carnivores are trophic generalists and so find it easier to establish and persist than herbivores, and perhaps have greater physiological resilience to withstand unfavourable periods. Differences in the dispersal powers of these groups are probably less relevant.

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(Received 8 January 1975)



Island Colonization by Carnivorous and Herbivorous Coleoptera

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