The topology of ecological interaction webs holds important information for theories of coevolution, biodiversity, and ecosystem stability . However, most previous network analyses solely counted the number of links and ignored variation in link strength. Because of this crude resolution, results vary with scale and sampling intensity, thus hampering a comparison of network patterns at different levels . We applied a recently developed quantitative and scale-independent analysis based on information theory to 51 mutualistic plant-animal networks, with interaction frequency as measure of link strength. Most networks were highly structured, deviating significantly from random associations. The degree of specialization was independent of network size. Pollination webs were significantly more specialized than seed-dispersal webs, and obligate symbiotic ant-plant mutualisms were more specialized than nectar-mediated facultative ones. Across networks, the average specialization of animal and plants was correlated, but is constrained by the ratio of plant to animal species involved. In pollination webs, rarely visited plants were on average more specialized than frequently attended ones, whereas specialization of pollinators was positively correlated with their interaction frequency. We conclude that quantitative specialization in ecological communities mirrors evolutionary trade-offs and constraints of web architecture. This approach can be easily expanded to other types of biological interactions.

The results point to a medium-effect size relationship between AJ and physical activity. Interventions that change AJ are scarce despite their potential for changing physical activity. Future experimental work designed to evaluate the causal impact ofAJ on physical activity is required.

The most conspicuous and species-rich ant-plant mutualism in the Malesian region is found in the important pioneer tree genus Macurungu, yet little is known about the identities or community ecology of the species involved. Our studies have revealed a far more complex system than previously thought. This paper presents the first extensive investigation in the whole distribution area of myrmecophytic Mucarungu. All ant-inhabited species were restricted to the moister parts of SE Asia: Peninsular Malaysia, South and East Thailand, Sumatra and Borneo. We found a rather strict and similar altitudinal zonation of myrmecophytic Macaranga species in all regions. Here we focus on the majority of the 19 Mucaranga species obligatorily associated with ants of the genus Crematogater. We identified a total of 2163 ant queens which belonged to at least eight (morpho)species of the small subgenus Decacraa as well as to one non-Decacrau @robably from Atopogyne). The ant species were not randomly distributed among the Mucuranga species but distinct patterns of associations emerged. Despite common sympatric distribution of Mucurangu species, in most cases a surprisingly high specificity of ant colonization was maintained which was, however, often not species-specific but groups of certain plant species with identical ant partners could be found. These colonization patterns usually but not always mirror existing taxonomic sections within the genus Macaranga. Possible mechanisms of specificity are discussed. The results are compared with other ant-plant mutualisms. 0 1999 The Linnran Socirtv of Ixlndon ADDITIONAL KEY WORDS:-biogeography ~ co-evolution -Decucremuhost specificity~

Plant species in at least 66 families produce extrafloral nectar (EFN) on their leaves or shoots and therewith attract predators and parasitoids, such as ants and wasps, which in turn defend them against herbivores. We investigated whether EFN secretion is induced by herbivory and͞or artificial damage, and thus can be regarded as an induced defensive response. In addition, we studied the underlying signaling pathway. EFN secretion by field-grown Macaranga tanarius increased after herbivory, artificial leaf damage, and exogenous jasmonic acid (JA) application. Artificial damage strongly enhanced endogenous JA concentrations. The response in EFN production to artificial damage was much less pronounced in those leaves that were treated with phenidone to inhibit endogenous JA synthesis. Quantitative dose-response relations were found between the increase in nectar production and both the intensity of leaf damage and the amounts of exogenously applied JA. The amount of endogenously produced JA was positively correlated with the intensity of leaf damage. Increased numbers of defending insects and decreased numbers of herbivores were observed on leaves after inducing EFN production by exogenous JA treatment. Over 6 weeks, repeatedly applied JA or artificial damage resulted in a ten-fold reduction in herbivory. These results demonstrate that EFN production represents an alternative mechanism for induced, indirect defensive plant responses that are mediated via the octadecanoid signal transduction cascade. E xtrafloral nectaries are nectar-secreting glands that are not involved in pollination (1). They are known in at least 66 plant families (2). Many studies have shown that extrafloral nectar (EFN) can play an important role in a plant's defense against herbivores (for reviews, see ref. 3 and 4). Given a choice, ants forage preferentially on plants with extrafloral nectaries (5-7), and several ant and wasp species directly defend the nectary-bearing plant parts against other insects (8). Both insect groups reduce numbers of herbivorous insects on plants possessing extrafloral nectaries and thus lessen damage by herbivores (5,6,(8)(9)(10)(11)(12).Several studies have already indicated that EFN secretion or amino acid concentrations in EFN may increase in response to herbivory (10,(13)(14)(15) and that this reaction does not require herbivore-specific elicitors (16,17). However, most of these studies have suffered from methodological problems (discussed in ref. 17). Because of the lack of specific elicitors, it has been discussed whether EFN can actually be considered as an induced defense (18). Moreover, nothing is known about the underlying signaling pathway, and no study has focused on the effects of induced EFN production on nectary-visiting insects and herbivores.In this study, we used saplings of the southeast Asian pioneer tree Macaranga tanarius (L.) Muell. Arg. (Euphorbiaceae) to address the questions of (i) whether EFN production can be induced by natural or artificial damage; (ii) whether the plant hormone jasm...