William A. Foster

THE EVOLUTION OF SOLDIERS IN APHIDS

1. Defensive individuals, termed soldiers, have recently been discovered in aphids, Soldiers are typically early instar larvae, and in many species the soldiers are reproductively sterile and morphologically and behaviourally specialized. 2. Since aphids reproduce parthenogenetically, we might expect soldier production to be more widespread in aphids than it is. We suggest that a more useful way to think about these problems is to attempt to understand how a clone (rather than an individual) should invest in defence and reproduction. 3. Known soldiers are currently restricted to two families of aphids, the Pemphigidae and Hormaphididae, although they are distributed widely among genera within these families. We discuss the use of a phylogenetic perspective to aid comparative studies of soldier production and we demonstrate this approach using current estimates of phylogenetic affinities among aphids. We show that the distribution of soldier production requires a minimum of six to nine evolutionary origins plus at least one loss. 4. At least four main types of soldiers exist and we present and discuss this diversity of soldiers. 5. Most soldier-producing species produce soldiers within plant galls and we discuss the importance of galls for the evolution of soldiers. 6. We summarize the evidence on the interactions between soldiers and predators and between soldier-producing aphids and ants. 7. We present an optimality model for soldier investment strategies to help guide investigations of the ecological factors selecting for soldiers. 8. The proximate mechanisms of soldier production are currently very poorly understood and we suggest several avenues for further research.

Doubling the estimate of invertebrate biomass in a rainforest canopy

Forest canopies represent the functional interface between 90% of the Earths terrestrial biomass and the atmosphere and include some of the most threatened of all terrestrial ecosystems. However, we lack even a basic understanding of how the biomass of plants and animals is distributed throughout forest canopies, even though this information is vital for estimating energy flow, carbon cycling, resource use and the transfer of materials within this ecosystem. Here we measure the biomass of invertebrates living in a common rainforest epiphyte, describe a striking relationship between fern size and the biomass of animals within the ferns, and reveal that one large epiphyte may contain an invertebrate biomass similar to that found in the whole of the rest of the tree crown on which it is growing. Using these data, we show that including the fauna of these epiphytes—a neglected component in rainforest ecosystems—can more than double our estimate of the total invertebrate biomass in an entire rainforest canopy.

Group transmission of predator avoidance behaviour in a marine insect: The trafalgar effect

Abstract The approach of a predator model induces rapid changes in the locomotory behaviour of individuals at the periphery of flotillas of a marine insect, the ocean skater Halobates robustus Barber. There is an abrupt and substantial increase in velocity, frequency of turning, the number of apparently random encounters between individuals and also the frequency of changes in light reflection from the body surfaces. These behavioural responses are postulated to cause predator confusion and, less frequently, to initiate rapid, synchronous dispersal of the flotillas. Interactions between individuals in the flotillas mediate a rapid transmission of avoidance behaviour through the flotilla which, in our field experiments, greatly exceeded the speed of approach of a predator model. This behaviour (named the Trafalgar Effect) enables individuals to initiate avoidance behaviour before the approaching predator can be seen.

Establishing the evidence base for maintaining biodiversity and ecosystem function in the oil palm landscapes of South East Asia

The conversion of natural forest to oil palm plantation is a major current threat to the conservation of biodiversity in South East Asia. Most animal taxa decrease in both species richness and abundance on conversion of forest to oil palm, and there is usually a severe loss of forest species. The extent of loss varies significantly across both different taxa and different microhabitats within the oil palm habitat. The principal driver of this loss in diversity is probably the biological and physical simplification of the habitat, but there is little direct evidence for this. The conservation of forest species requires the preservation of large reserves of intact forest, but we must not lose sight of the importance of conserving biodiversity and ecosystem processes within the oil palm habitat itself. We urgently need to carry out research that will establish whether maintaining diversity supports economically and ecologically important processes. There is some evidence that both landscape and local complexity can have positive impacts on biodiversity in the oil palm habitat. By intelligent manipulation of habitat complexity, it could be possible to enhance not only the number of species that can live in oil palm plantations but also their contribution to the healthy functioning of this exceptionally important and widespread landscape.

Stochastic and deterministic processes jointly structure tropical arthropod communities

The question of whether ecological assemblages are structured by stochastic and deterministic (e.g. interspecific competition) processes is controversial, but it is difficult to design sampling regimes and experiments that can dissect the relative importance of stochastic and deterministic processes in natural assemblages. Using null models, we tested communities of arthropod decomposers in tropical epiphytes for patterns of species co-occurrence, while controlling for habitat gradients, seasonal variations and ecological succession. When environmental conditions were controlled, our analysis showed that the communities were structured stochastically. However, analysing mixed sets of communities that were deliberately created either from two distinct heights or two successional stages revealed that communities were structured deterministically. These results confirm that habitat gradients and dispersal/competition trade-offs are capable of generating non-random patterns within decomposer arthropod communities, but reveal that when such effects are accounted for, species co-occurrence is fundamentally random.

How aphids lose their marbles

Insects provide examples of many cunning stratagems to cope with the challenges of living in a world dominated by surface forces. Despite being the current masters of the land environment, they are at constant risk of being entrapped in liquids, which they prevent by having waxy and hairy surfaces. The problem is particularly acute in an enclosed space, such as a plant gall. Using secreted wax to efficiently parcel and transport their own excrement, aphids were able to solve this problem 200 Myr ago. Here, we report on the physical and physiological significance of this ingenious solution. The secreted powdery wax has three distinct roles: (i) it is hydrophobic, (ii) it creates a microscopically rough inner gall surface made of weakly compacted wax needles making the gall ultra–hydrophobic, and (iii) it coats the honeydew droplets converting them into liquid marbles, that can be rapidly and efficiently moved.

The effects of group size on predator avoidance in a marine insect

Abstract The ocean skater, Halobates robustus (Hemiptera: Gerridae), is a flightless insect which forms loosely aggregated fluotillas of various sizes close to lava and mangrove edges in the Galapagos. Field observations showed a clear effect of group size on the distance at which an approaching predator model induced avoidance behaviour (rapid withdrawal and/or dispersal). It is suggested that in large, highly aggregated groups, the insects are maximally aroused as a result of frequent bodily encounters with adjacent members of the flotilla. An approaching object thus induces avoidance behaviour with minimal visual stimulation at maximal distance. With small groups and single individuals, which are less aroused, a closer approach of the object is required to produce visual stimulation sufficient to activate the escape response.

Experimental evidence for effective and altruistic colony defence against natural predators by soldiers of the gall-forming aphid Pemphigus spyrothecae (Hemiptera: Pemphigidae)

SummaryThe thick-legged first instar soldiers of the gall-forming aphid Pemphigus spyrothecae Pass. are able to protect the aphids in the gall from being eaten by a range of insect predators. In artificial galls, the soldier aphids were able to kill first instar ladybirds Adalia bipunctata (L.) (Coleoptera: Coccinellidae), early third instar hoverfly larvae Eupeodes (Metasyrphus) corollae (Fab.) (Diptera: Syrphidae), and first-third instar Anthocoris nemoralis (Fab.) (Hemiptera: Cimicidae). Almost all the aphids that attacked the predators were themselves killed. The soldiers were also able to kill predators introduced into natural galls. Experiments were devised in which individuals of Anthocoris minki, which is the most important insect predator of the gall generations of Pemphigus at the study site, were free to enter and leave the gall: the soldiers were effective both in preventing the predators access to the gall and in killing those predators that did manage to get in. In galls with experimentally manipulated numbers of soldiers and non-soldiers, it was clearly shown that it is the soldiers alone that kill the predators (Anthocoris minki and 1st instar Adalia bipunctata) (see Table 3). Even though many soldiers may die during these encounters, the selective advantage of killing the predators is high, since observations show that individual A. minki can pass through more than one instar inside a gall and kill all the aphids therein. The aphids were not observed to attack conspecific aphids from other galls or the cohabiting aphid Chaitophorus leucomelas Koch. The primary role of the soldier caste is therefore the defence of the aphid colony against predators.

The impact of forest conversion to oil palm on arthropod abundance and biomass in Sabah, Malaysia

Deforestation rates in South-East Asia are among the highest of any tropical region, with expansion of oil palm being one important factor. Despite this, few studies have investigated the impact of oil palm expansion on the arthropod fauna. We report here the first study on the impact of forest conversion to oil palm on overall arthropod abundance, biomass and composition. We compared arthropod abundance and biomass, collected from epiphytic birds nest ferns, the canopy, and leaf litter between primary forest, logged forest and oil palm plantation. Epiphytes, canopy and litter all contained a lower abundance (epiphytes: 67.2%, canopy: 2.3% and litter: 77.1% reduction) and biomass (epiphytes: 87.5%, canopy: 37.9% and litter: 72.4% reduction) of arthropods in oil palm compared with primary forest. However, not all orders of arthropods showed the same level of decline, with some groups having higher abundance and biomass in oil palm, resulting in an altered community composition in the epiphytes and canopy in oil palm compared with forest. Our results show that forest conversion to oil palm impacts detrimentally on invertebrates in all compartments of the forest ecosystem.

Group size and anti-predator strategies in a marine insect

Abstract Predator avoidance is a primary function of the flotillas of the marine insect Halobates robustus (Hemiptera: Gerridae). With fish predators whose approach is unseen by the insects, Halobates that live in groups acquire a double protection: a lower probability of attack, as a result of a simple dilution effect, and a higher probability of escape if attacked. With visible predators, such as surfacefeeding fish and birds, this protection is augmented by predator-induced avoidance behaviour, the nature of which is related to the size of the group. In large groups, the highly aroused individuals respond quickly during a predators approach and do not disperse, but maintain a high level of confusion behaviour within the aggregation. In small groups, the individuals are not highly aroused and respond only when the predator approaches very closely, whereupon they rapidly and synchronously disperse. Predator avoidance behaviour is postulated to result from a balance between the advantages of suppressing conspicuous escape movements until the last possible moment (the small group strategy) and the advantages of early warning and conspicuous confusion behaviour (the large group strategy).