▪ Abstract In this review, we discuss the ecological and evolutionary consequences of plant-herbivore interactions in tropical forests. We note first that herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding. Leaves in dry tropical forests also suffer higher rates of damage than in wet forests, and damage is greater in the understory than in the canopy. Insect herbivores, which typically have a narrow host range in the tropics, cause most of the damage to leaves and have selected for a wide variety of chemical, developmental, and phenological defenses in plants. Pathogens are less studied but cause considerable damage and, along with insect herbivores, may contribute to the maintenance of tree diversity. Folivorous mammals do less damage than insects or pathogens but have evolved to cope with the high levels of plant defenses. Leaves in tropical forests are ...

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Herbivory and plant defenses in tropical forests

The latitudinal gradient in species richness is paralleled by a latitudinal gradient in geographical-range size called Rapoport's rule. It is suggested that the greater annual range of climatic con...

The Latitudinal Gradient in Geographical Range: How so Many Species Coexist in the Tropics

Hypotheses that attempt to explain latitudinal gradients in species diversity are reviewed. Some hypotheses are circular, i.e. they are based on the assumption that some taxa have greater diversity in the tropics. These include explanations assuming different degrees of competition, mutualism, predation, epiphyte load, epidemics, biotic spatial heterogeneity, host diversity, population size, niche width, population growth rate, environmental harshness, and patchiness at different latitudes

Latitudinal gradients in species diversity: the search for the primary cause

Intraspecific predation, the process of both killing and eating an individual of the same species, is a significant and widespread process that until recently has not received the attention it merits. It is a major factor in the biology of many species and may influence population structure, life history, competition for mates and resources, and behavior. It is commonly observed among many animals: For this review I read over 900 papers describing such predation in about 1300 species. Laurel Fox's 1975 review of cannibalism (54) should be read for additional information. Here I attempt to present new or different data, references, and ideas. One of my purposes is to establish a theoretical framework for the organization and interpretation of the numerous observations of intraspecific predation. Certain topics included here are speculation intended to spark interest and indicate important problems that need further research. Because of page limitations many appropriate and important citations are not cited here but may be found elsewhere as indicated by "& incl. refs."

The Evolution and Dynamics of Intraspecific Predation

Insects are by far the most diverse group of organisms on Earth; estimates of their current diversity range as high as 30 million species (58). A large fraction of these species feed on plants (154, 214), and Mitter et al (134) have shown that plant-feeding clades are consistently much more diverse than their nonphytophagous sister groups. By feeding at the base of the food chain, these insects have access to a potentially enormous supply of resources; these are extensively subdivided among species, as most phytophagous insects are highly host specific (55, 56, 133, 154). Investigations of the causes of host specialization in insects could therefore contribute substantially to our understanding of the origin and maintenance of diversity in this group. The evolution of plant-insect associations has been guided to a large extent by plant chemistry in some, but not all, insect groups (26, 55, 57, 153). For instance, related species of butterflies often use plants that are chemically similar, even if taxonomically distant (23, 57, 188, 190). Cladistic analyses of papilionid butterflies and Ophraella leaf beetles and their host plants have revealed that host shifts are most likely to occur among chemically similar plants, even though cladograms of these insects and their host plants are not congruent (68, 131, 132). In addition, introduced plants are generally colonized by insects that feed on chemically similar plants; those with impoverished faunas are biochemically or taxonomically unusual (214, 250). Finally, within a defined set of hosts, insect herbivores have been found to

Host Specialization in Phytophagous Insects

Many groups of animals show a gradient of increasing species richness from polar to equatorial regions1,2. Theory3,4 suggests that species should be more specialised ecologically in the tropics. Most of the evidence for this comes from studies on birds5–10. Birds appear to subdivide the habitat more finely and specialise more in their food in the tropics. It has been suggested11 that tropical Papilionidae (Lepidoptera) are more specialised in their hosts than temperate species, but host specificity was considered only at the level of the plant family, and more detailed studies of a wide range of butterflies12 suggest that they are little (if at all) more specialised in their food plants in the tropics. Similarly, monogenean trematode parasites of fish show no change in the degree of host specificity at different latitudes13. Here it is shown that one group of insects, the bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae), are less host-specific in the tropics than in temperate regions, even though there are considerably more species present in the tropics. The ‘reverse’ latitudinal gradient in host specificity can be related to the greater proportion of relatively non-specific xylomycetophagous species (ambrosia beetles) in the tropics and to the greater heterogeneity of tropical forests which will make host specialisation more difficult.

Host specificity of temperate and tropical animals

This paper is concerned with three species of bark beetle Scolytus scolytus (F.), S. multistriatus (Marsham) and Tomicus piniperda (L.). The first two species breed in elm trees (Ulmus spp.) and are well known as vectors of the Dutch elm disease. Scolytus scolytus is the larger of the two and usually breeds in stems with thicker bark. Tomicus piniperda breeds largely in pine (Pinus spp.), but also attacks other conifers (Abies, Larix, Picea). It too is of economic importance. The newly emerged adults feed in living pine shoots to mature the gonads and may also attack and kill trees previously weakened by other pests (e.g. Lekander 1955). All three species breed and make gallery systems in the phloem of unhealthy, dying or recently felled trees. The biology of the Scolytus species has been described by Fisher (1931), Readio (1935), Collins et al. (1936), Beaver (1967a) and others, and that of Tomicus piniperda by Hanson (1939, 1940), Chararas (1962) and others. In all three species, the parental gallery is of the simple longitudinal type (Schedl 1958). The eggs are laid in separate niches along both sides of this gallery. The larvae cut separate galleries through the phloem. They usually pupate in the outer bark, but sometimes in the phloem or, in the case of Scolytus scolytus, in the sapwood. The adults emerge from individual exit holes cut through the bark. There are two periods in the life cycle during which competition may occur. Firstly, there may be competition between the adults for space in which to construct the parental galleries and to lay eggs. Secondly, there may be competition between the larvae for food for growth and development. Only the latter period is dealt with in detail here and the study has been confined to intraspecific competition. The paper is concerned with the factors affecting the occurrence and intensity of larval competition, and its effects on the number and size of the adults that emerge. Nuorteva (1954, 1964) and Eidmann & Nuorteva (1968) have studied intraspecific competition in Tomicus piniperda, but there appear to have been no detailed studies of competition in Scolytus scolytus and S. multistriatus.

Intraspecific Competition among Bark Beetle Larvae (Coleoptera: Scolytidae)

Hypothenemus curtipennis (Schedl) is the first true ambrosia beetle known in the scolytid tribe Cryphalini. The male of the nominate subspecies is described from Fiji. Cryphalomorphus samoanus Browne is transferred to Hypothenemus, and considered a subspecies of H. curtipennis confined to Samoa. The life cycle, host trees, gallery system and causes of mortality of H. c. curtipennis are described. The species shows inbreeding polygyny.

The taxonomy, mycangia and biology of Hypothenemus curtipennis (Schedl), the first known cryphaline ambrosia beetle (Coleoptera: Scolytidae)

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R. A. Beaver

Papers

Host specificity of temperate and tropical animals

Intraspecific Competition among Bark Beetle Larvae (Coleoptera: Scolytidae)

The taxonomy, mycangia and biology of Hypothenemus curtipennis (Schedl), the first known cryphaline ambrosia beetle (Coleoptera: Scolytidae)

Analysis of host specificity of temperate and tropical animals (reply)