Yoram Ayal

Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.

The use of kairomones for foraging decisions by an aphid parasitoid in small host aggregations.

1. The wasp Diaeretiella rapae uses honeydew emitted by its host, the cabbage aphid Brevicoryne brassica, as a kairomone (chemicals emitted by an organism as part of its activity and used by its natural enemies to their advantage). The role of the kairomone in foraging decisions by the parasitic wasp was explored by manipulating the amount of honeydew and the number of aphids in a colony independently. The count‐down patch‐exploitation mechanism (Iwasa et al., 1981) was employed to predict the results of these manipulations and contrast them with the predictions of Waages (1979) model.

Optimal Egg Distribution Among Host Patches for Parasitoids Subject to Attack by Hyperparasitoids

Using an aphid-parasitoid-hyperparasitoid system as an example, we study the optimal oviposition strategy of a parasitoid whose offspring are subject to attack by hyperparasitoids. We assume that hyperparasitoids, which search aphid colonies for aphids that have been infected by parasitoids, decide to leave a colony when they have searched some fixed number of aphids consecutively without finding an infected one. We use a simulation model to investigate how many hosts the parasitoid should infect per colony to maximize the long-term average rate of producing eclosing offspring. We consider three different variables and deal with them one at a time: (1) N = the number of aphids in a colony, (2) H = the average number of perparasitoids visiting each colony, and (3) τ = the parasitoid travel time between colonies. The optimal number of aphids to infect in a colony is sometimes much less than the total number of aphids available. The optimal number of aphids to infect within a colony decreases with a decrease in the colony size, with an increase in the average number of perparasitoid visits, and with a decrease in travel time between aphid colonies.

Seasonal changes in darkling beetle communities (Coleoptera: Tenebrionidae) in the Ramon erosion cirque, Negev Highlands, Israel

Darkling beetles (Tenebrionidae) were studied using pitfall traps in the Ramon erosion cirque in Israel for 2 years. The species showed similar phenologies in different habitats. Only one species ( Erodins edomitus ) was a temporal specialist. Most species exhibited 7–10 month activity periods with one or two peaks of abundance. Diversity of the beetle communities was maximal in summer. The communities were most equitable in winter and summer and least equitable in spring and autumn. Multivariate analysis showed four temporal phases of community composition in the course of a year; (a) February–April, (b) May–July, (c) August–November and (d) December–January. The tenebrionid communities of these temporal phases differed more in species relative abundances than in species composition.

r-CURVES AND THE COST OF THE PLANKTONIC STAGE

When it is impossible to calculate the intrinsic rate of increase of a population because survivorship probabilities of prerecruitment stages are not available, it may be instructive to study the behavior of r as a function of prerecruitment survivorship probabilities along a reasonable range of such hypothetical probability values. When such functions, called r-curves, are constructed for benthic organisms with planktonic larvae, the true value ofr for a given larval survivorship probability lies within a range bounded from below by a value generated by Laughlins equation for r of a cohort, and from above by a value generated by Lotkas equation for r of a population with overlapping generations and a stable age distribution. The r-curves not only enable a meaningful comparison of closely related species with regard to their capacity for increase, but also enable comparisons to be made between populations within species, with regard to the quality of their habitats. They also provide a powerful tool for investigating the adaptive significance of the planktonic larva versus the cost of direct development; their analysis in species of Cerithium snails provides evidence that the significance of the planktonic larva lies in its potential for maximizing dispersal.

Time-lags in insect response to plant productivity: significance for plant-insect interactions in deserts

Abstract. 1 The interactions between the univoltine mirid bug Cupsodes infuscatus and its food plant, the geophyte Asphodelus ramosus, were studied in the Negev desert for a 5 year period. The bug feeds mainly on Asphodelus inflorescence meristems, flowers and fruits, and in some years may destroy more than 95% of the plant population expected fruit production. 2 Asphodelus expected fruit production fluctuated widely during the study period, but was not related to precipitation. Cupsodes density was related to the plant expected fruit production, but with a 1 year time lag. In years of high inflorescence production, a high per‐capita reproduction of the bug resulted in a dense bug population in the following year. This dense population then decimated the plant fruit production, became food limited and had a low per‐capita reproduction. 3 This kind of time lag is expected to be common among desert insect herbivores that specialize in using ephemeral resources. The rare years of high plant production are in general preceded and followed by years of low plant production. Hence, in years which contribute most to plant reserves (seed, underground storage organs), insect herbivores are relatively rare as a result of food limitation in preceding low production years. But the insect populations which build up during years of high plant production decimate their food resources and become food limited in subsequent years with low plant production. 4 Thus, herbivorous insects seem to have a limited ability to affect plant population dynamics in desert ecosystems. In contrast, the potential appears to be much greater for herbivorous insects to be regulated by their food plants.

Does a Suitable Habitat Guarantee Successful Colonization

Among nine rocky intertidal cerithiid species of the Sinai coast of the Red Sea, only Cerithium scabridum has managed to traverse the length of the Suez Canal, as well as to colonize the Mediterranean. By comparing the degree of overlap, between the colonizer and each of the non-colonizing species, in the ranges of habitat space along the axes of several abiotic environmental gradients relevant to cerithiids, it was found that one of the non-colonizing species, C. caeruleum, is definitely capable of both immigrating successfully through the Canal, and of finding a suitable habitat in the Mediterranean. Yet, although C. caeruleum did invade parts of the Suez Canal, it did not colonize the Mediterranean. Hence, the availability of a suitable habitat in an area open for colonization need not necessarily guarantee its successful colonization.

The effect of the mirid bug Capsodes infuscatus on fruit production of the geophyte Asphodelus ramosus in a desert habitat

The effect of feeding of the mirid bug Capsodes infuscatus on fruit production of the geophyte Asphodelus ramosus was studied in a desert area in Israel. Plant and bug densities and percent loss of fruit production were measured in a relatively dry lower colluvial slope, an intermediate upper colluvial slope, and a relatively mesic wadi. Overall damage levels were very high, with 100% loss of fruit production in many plants. Within each habitat, the number of nymphs per plant clone was positively correlated with the number of ramets per clone and percent damage was positively correlated with number of nymphs per clone. However, percent damage was not correlated with number of plants per clone in any habitat. Although damage did significantly increase with plant density in the slope habitats, mean damage to fruit production per clone was lowest (50%) in the wadi where Asphodelus density was highest. As new ramets are tightly interwoven with their mother plants, occupation of new microsites depends on establishment of new clones from seeds. Therefore, the strong and density-dependent reduction in fruit production inflicted by Capsodes on the Asphodelus population on the slope has the potential to regulate the plant density in this habitat.

A simple Markov model for the assessment of host patch quality by foraging parasitoids

Abstract Insect parasitoids search for their hosts using a method that may be broken into three parts. First, they locate plants which may harbor their hosts, then they assess the quality of these plants to decide whether to search them further for hosts and, finally, if they decide to accept a plant for further search, they exploit the plant by searching for hosts and attacking them when they are found. We study the way that parasitoids assess plant quality by developing a mathematical model based on behavioral observations of foraging parasitoids that attack aphids which infest crucifers. Assessment of plants is based on the concentration of cues produced by hosts that inhabit them. Parasitoids are more likely to exploit plants on which more host cues are detected, and the willingness of a parasitoid to exploit a given plant depends on the quality of other plants that have been visited recently. Plants whose quality exceeds a certain threshold will be accepted for exploitation. The threshold for plant acceptance will change with the experience of the parasitoid, increasing when plants heavily infested with hosts are encountered, decreasing when uninfested plants are encountered. We analyze several rules that might describe how the acceptance threshold changes with parasitoid experience, and for each rule we show how the number of parasitoids willing to accept plants with various levels of infestation depends on the number of plants with various levels of infestation. We then consider different rules for exploitation of hosts on plants and find how the proportion of hosts attacked depends on host density.

Primary Consumer Body Size and Food-Chain Length in Terrestrial Communities

Using 21 community food webs, we tested Eltons two hypotheses regarding the main factors limiting food-chain length in terrestrial communities, namely, energy (energy limitation hypothesis—ELH) and body size (size limitation hypothesis—SLH). As predators tend to be larger than their prey, food-chains are size-structured: animal size increases with trophic position. We found a negative correlation between the size of the primary consumer and the length of the chain. Food-chains based on small primary consumers are longer than those based on large primary consumers, and size rather than energetic efficiency is the main contributing factor. We found no correlation between habitat productivity and mean food-chain length. All these findings support the SLH over the ELH. Our results suggest that, as in aquatic communities, a single factor—a predator/prey size-ratio greater than 1—governs the structure of terrestrial communities.