Case Studies of Arthropod Diversity and Distribution

PART Case Studies of Arthropod 6 Diversity and Distribution Scott E. Miller, Vojtech Novotny, and Yves Basset Because insects and other ar- How Many Species Are There ? thropods exhibit such striking diversity in the tropics, these taxa have provided models for For most of the twentieth century, the total the development of many key topics in tropical number of insects in the world was assumed to biology. Some of these topics are emphasized be around 2 million. For example, Sharp (1895) elsewhere in this book, including parts 3, 5 and Frost (1942) gave estimates of 2.5 million (mimicry and pollination), 4 (plant-herbivore and 2 million, respectively. Sabrosky (1952) re- relations), and 10 (ecosystem ecology). This viewed knowledge of insect diversity at that part highlights three papers that helped fuel time and said "workers in the division of insect major discussions of both pattern and process identification of the [United States] Depart- in the diversity of tropical arthropods and other ment of Agriculture estimate that [by] 1948 organisms. Erwin (1982; paper 28) laid out a set approximately 686,000 different species of in- of testable hypotheses that led to major rein- sects" had been described in the world (1952, terpretation of the magnitude of tropical diver- 1). For the total including undescribed species, sity. The series of studies in Haddow, Corbet, Sabrosky (1952) stated "recent guesses vary and Gillett (1961; paper 27) were an early and from 2,500,000 to 10,000,000 different kinds" elegant view of vertical stratification of insects and "[t]he final roll call may be far short of 10 in tropical forests. Wilson (1958; paper 26) million but it seems sure to be somewhere in wrote the first in a series of papers based on ob- the millions" (1952, 2). servations of ants in Papua New Guinea that Some twenty years ago, in an ingenious and led to major discussions of heterogeneity, ant visionary paper, Erwin (1982) suggested that mosaics, and taxon cycles. The discussion below there may be as many as thirty million species will show that these are still important topics in of insects, instead of the previously estimated tropical biology. two or so million species. His conclusions were The authors thankfully acknowledge support from the U.S. National Science Foundation, which has enabled their collaborative research on arthropod diversity and distribution. 407 Remarkable beetles found at Simunjon, Borneo. From Wallace (1869) Arthropod Diversity and Distribution 409 based on beetles sampled in insecticidal fogging across broader assemblages of hosts. In a study of nineteen individuals of the tree Luehea see- of leaf-chewing insects on ten tree species in mannii in Panama (Erwin and Scott 1980; see Papua New Guinea, Basset et al. (1996) found Basset et al. 1996 for a table summarizing the 4.3 percent of the beetles were restricted to one assumptions and steps in the estimate). Subse- of the ten tree species. In a much larger study of quently, Erwin (1988) admitted that he was host specialization of leaf-chewing insects on surprised by the attention that his calculations sixty-two species of woody rain forest plants, received and the controversy they generated, Novotny, Basset, and Miller (in preparation) stating that he had simply advanced a hypothe- found that most herbivore species had wide sis that could be tested rigorously, and sug- host plant ranges within plant genera, but were gested that this "must begin by refining of our restricted to only a few genera. A similar pat- knowledge about host specificity of insects in tern has been found for tropical mites that feed tropical forests" (1988,123). on nectar in hummingbird-pollinated flowers Although many papers have debated Erwin's (Naskrecki and Colwell 1998). Because speciose conclusion, very few have actually attempted plant genera (such as Ticus and Psychotria) are to test his assumptions, in particular the pro- a characteristic feature of tropical floras, the portion of host-specific insect herbivores asso- large overlap among herbivore communities ciated with different species of tropical trees on congeneric plants means that the total num- (e.g.. Stork 1988; Erwin 1988; May 1988,1990, ber of herbivores is lower than might otherwise 2000; Thomas 1990a; Hammond 1992; Gaston be expected. Similar low host specificity at the 1991; Hodkinson and Casson 1991; Basset et al. plant species level was found by Wagner (1998) 1996). Erwin's paper raised many questions re- from canopy fogging in Uganda, and Tavakilian garding how to properly assess the number of et al. (1997) in rearing cerambycid beetles from host-specific arthropod species in general, and trees in French Guiana. in the tropics in particular. One issue is the prob- Moreover, tropical tree species may support lem of transient species that are dispersing very different ratios of specialist species, vary- from tree species not under direct study (Jan- ing with host plant, forest characteristics, and zen 1976; Stork 1993). Specialist species may be geographic locations (Basset 1992, 1999). For collected on other tree species besides their example, chrysomelid beetle samples obtained normal hosts. With greater diversity of plant in Panama (Erwin and Scott 1980) and the Pe- species within the study site, more transient ruvian Amazon (Farrell and Erwin 1988) were species are likely to be collected on the foliage dominated by Alticinae, whereas in chryso- of study trees (Basset 1999; Novotny and Bas- melid samples from Papua New Guinea (Basset set 2000). and Samuelson 1996) and Borneo (Stork 1991) Another issue is the varying degree of host Eumolpinae dominated. Since Alticinae are of- specificity observed among taxonomic groups. ten more host specific than Eumolpinae (e.g., For example, wood-eating guilds of beetles Jolivet 1988), the overall proportions of special- rarely feed on living tissues and are therefore ists on particular tree species in the Neotropics often much less host specific in the tropics than may be higher than these in the Old World leaf-feeding beetles (Basset 1992). Studies of tropics. herbivorous insects associated with single plant Aside from differences in taxonomic com- genera in the Neotropics have recognized 30 position of the overall fauna, tropical regions percent monophagous (single-host-feeding) also differ in overall diversity. Although data species on Passiflom (Thomas 1990b) and 10 on regional diversity are too poor for compar- percent monophagous geometrid moths and 26 isons among most beetle taxa, it is well known percent monophagous species of weevils on that butterfly faunas differ markedly across re- Piper (Marquis 1991). But several recent stud- gions. Within equal-sized areas, species rich- ies have found much lower host specificity ness of butterflies in the Oriental region is about 410 Part Six half that in the Neotropics (Robbins 1993). Fi- ultimately conservationists. But sadly, this dis- nally, the relative diversity of canopy versus cussion also points out two other features of soil faunas remains a subject of conjecture tropical insect research•the low availability of (Stork 1988; Hammond 1992; André, Lebrun, data and how slowly the gaps are being filled. and Noti 1992). Much more work is needed on soil biota (see also the discussion of Fittkau and Vertical Patterns of Species Khnge 1973 [paper 45]). Distribution As illustrated by the papers cited above, vig- orous discussion of how to predict the global A large body of literature focuses on arthropod number of arthropods continues. Current evi- samples obtained from the forest "canopy," dence from the major museum collections of usually referring to samples obtained fifteen sorted and labeled insect species, whether de- meters or more above the ground, with vari- scribed or undescribed, does not support the ous methods (reviewed in Basset 2001). Most larger estimates, and insect taxonomists broadly entomological studies, either with insecticidal concur from this that although there may be up fogging (e.g., Erwin 1995), with light traps (e.g., to five million species of insects in the world, Sutton, Ash, and Grundy 1983; Wolda, O'Brien, there are probably less than ten million (Niel- and Stockwell 1998), or by felling trees (Ame- sen and Mound 2000). In a recent review. May degnato 1997; Basset, Charles, and Novotny (2000) settled on a "best guess" of four million 1999), cannot sample the upper canopy selec- species. tively. Recently, entomologists have been able Although the magnitude of undescribed in- to sample selectively the upper canopy either sects in the Andean region may be extremely with fixed canopy cranes (Wright and Colley high, this does not appear to be the case in other 1994) or a mobile canopy raft and sledge (Hallé regions. Scholtz and Chown (1995) analyzed and Blanc 1990), offering the promise of deeper taxonomic knowledge of insects in southern biological understanding. These studies have Africa and considered "a doubling in [species] generally found significant differences in com- numbers to be the upper limit of the increase in position and abundance of arthropods at differ- species richness" in the region (125). In a de- ent vertical levels in the canopy, but questions tailed reviewof present knowledge of ants (Hy- of what is actually being sampled and lack of menoptera; Formicidae) of tropical Africa, Rob- understanding of the biology of the organisms ertson (2000) suggests that about half the has limited the conclusions that can be made. species of ants have been described•a figure Some studies find greater abundance at higher that concurs approximately with these recent levels (e.g., Basset, Aberlenc, and Delvare 1992), world insect diversity estimates. while others find greater abundance at lower lev- Several factors contributed to the influence els (e.g., Wolda, O'Brien, and Stockwell 1998) of Erwin's paper. First, his study suggested that or no significant differences are observed (e.g., there are many more insect species than previ- Intachat and HoUoway 2000).

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