Life-history strategy, resource dispersion and phylogenetic associations shape dispersal of a fig wasp community

Who holds the reins? Context-dependent resource allocation in the mutualism between fig trees and their fig wasp pollinators

Mutualisms are consumer-resource interactions, in which goods and services are exchanged. Biological market theory states that exchanges should be regulated by both partners. However, most studies on mutualisms are one-sided, focusing on the control exercised by host organisms on their symbionts. In the brood-site pollination mutualism between fig trees and their symbiont wasp pollinators, galled flowers are development sites for pollinator larvae and are exchanged for pollination services. We determined if pollinator galls influenced resource allocation to fig inflorescences called syconia and considered feedbacks from the host tree. We experimentally produced syconia containing only seeds (S), only pollinator galls (G) or seeds and galls (SG) with varying number of introduced female pollinator wasps, i.e., foundress wasps. Biomass allocation to syconia was affected by foundress numbers and treatment groups; SG treatments received highest biomass allocation at low foundress numbers, and both G and SG treatments at high foundress numbers. Seeds are important determinants of allocation at low foundress numbers; galls are likely more influential at high foundress numbers. Most allocation in the G and SG treatment was to the syconium wall, likely as protection from parasitoids and temperature/humidity fluctuations. Dry mass of individual seeds and wasps (except at low foundress numbers) was unchanged between treatment groups, indicating seeds and wasps regulate resource flow into them, with lower flow into galls containing the smaller males compared to females commensurate with sexual dimorphism. We demonstrate the importance of considering the direct role of symbionts in accessing resources and controlling exchanges within mutualisms.

The ephemeral resource patch concept

Ephemeral resource patches (ERPs) - short lived resources including dung, carrion, temporary pools, rotting vegetation, decaying wood, and fungi - are found throughout every ecosystem. Their short-lived dynamics greatly enhance ecosystem heterogeneity and have shaped the evolutionary trajectories of a wide range of organisms - from bacteria to insects and amphibians. Despite this, there has been no attempt to distinguish ERPs clearly from other resource types, to identify their shared spatiotemporal characteristics, or to articulate their broad ecological and evolutionary influences on biotic communities. Here, we define ERPs as any distinct consumable resources which (i) are homogeneous (genetically, chemically, or structurally) relative to the surrounding matrix, (ii) host a discrete multitrophic community consisting of species that cannot replicate solely in any of the surrounding matrix, and (iii) cannot maintain a balance between depletion and renewal, which in turn, prevents multiple generations of consumers/users or reaching a community equilibrium. We outline the wide range of ERPs that fit these criteria, propose 12 spatiotemporal characteristics along which ERPs can vary, and synthesise a large body of literature that relates ERP dynamics to ecological and evolutionary theory. We draw this knowledge together and present a new unifying conceptual framework that incorporates how ERPs have shaped the adaptive trajectories of organisms, the structure of ecosystems, and how they can be integrated into biodiversity management and conservation. Future research should focus on how inter- and intra-resource variation occurs in nature - with a particular focus on resource × environment × genotype interactions. This will likely reveal novel adaptive strategies, aid the development of new eco-evolutionary theory, and greatly improve our understanding of the form and function of organisms and ecosystems.

Low host specificity and broad geographical ranges in a community of parasitic non-pollinating fig wasps (Sycoryctinae; Chalcidoidea)

Plants, phytophagous insects and their parasitoids form the most diverse assemblages of macroscopic organisms on earth. Enclosed assemblages in particular represent a tractable system for studying community assembly and diversification. Communities associated with widespread plant species are especially suitable as they facilitate a comparative approach. Pantropical fig-wasp communities represent a remarkably well-replicated system, ideal for studying these historical processes. We expect high dispersal ability in non-pollinating fig wasps to result in lower geographical turnover in comparison to pollinating fig wasps. The ability of non-pollinating wasps to utilise a number of hosts (low host specificity) is a key determinant of overall geographical range, with intraspecific competition becoming a constraining factor should diet breadth overlap among species. Finally, we expect conserved community structure throughout the host range. We aim to test these expectations, derived from population genetic and community studies, using the multi-trophic insect community associated with Ficus hirta throughout its 3,500 km range across continental and insular Asia. We collect molecular evidence from one coding mitochondrial gene, one non-coding nuclear gene and multiple microsatellites across 25 geographical sites. Using these data, we establish species boundaries, determine levels of host specificity among non-pollinating fig wasps and quantify geographical variation in community composition. We find low host specificity in two genera of non-pollinating fig wasps. Functional community structure is largely conserved across the range of the host fig, despite limited correspondence between the ranges of non-pollinator and pollinator species. While nine pollinators are associated with Ficus hirta, the two non-pollinator tribes developing in its figs each contained only four species. Contrary to predictions, we find stronger isolation by distance in non-pollinators than pollinators. Long-lived non-pollinators may disperse more gradually and be less reliant on infrequent long-distance dispersal by wind currents. Segregation among non-pollinating species across their range is suggestive of competitive exclusion and we propose that this may be a result of increased levels of local adaptation and moderate, but regular, rates of dispersal. Our findings provide one more example of lack of strict codiversification in the geographical diversification of plant-associated insect communities.

The Scent of Life: Phoretic Nematodes Use Wasp Volatiles and Carbon Dioxide to Choose Functional Vehicles for Dispersal

Hitchhikers (phoretic organisms) need vehicles to disperse out of unsuitable habitats. Therefore, finding vehicles with the right functional attributes is essential for phoretic organisms. To locate these vehicles, phoretic organisms employ cues within modalities, ranging from visual to chemical senses. However, how hitchhikers discriminate between individual vehicles has rarely been investigated. Using a phoretic nematode community associated with an obligate fig-fig wasp pollination mutualism, we had earlier established that hitchhiking nematodes make decisions based on vehicle species identity and number of conspecific hitchhikers already present on the vehicle. Here we investigate if hitchhikers can differentiate between physiological states of vehicles. We asked whether phoretic nematodes choose between live or dead vehicles present in a chemically crowded environment and we investigated the basis for any discrimination. We conducted two-choice and single-choice behavioral assays using single nematodes and found that plant- and animal-parasitic nematodes preferred live over dead vehicles and used volatiles as a sensory cue to make this decision. However, in single-choice assays, animal-parasitic nematodes were also attracted towards naturally dead or freeze-killed wasps. The volatile profile of the wasps was dominated by terpenes and spiroketals. We examined the volatile blend emitted by the different wasp physiological states and determined a set of volatiles that the phoretic nematodes might use to discriminate between these states which is likely coupled with respired CO₂. We determined that CO₂ levels emitted by single wasps are sufficient to attract nematodes, demonstrating the high sensitivity of nematodes to this metabolic product.

Staying in the club: Exploring criteria governing metacommunity membership for obligate symbionts under host-symbiont feedback

Metacommunity membership is influenced by habitat availability and trophic requirements. However, for multitrophic horizontally transmitted symbiont communities that are closely associated with hosts, symbiont-host interactions may affect membership criteria in novel ways. For example, failure of beneficial services from symbionts could influence the host, and in turn, the entire community. Understanding such host-symbiont feedback effects on symbiont community membership, symbiont community structure, and function is important for understanding if host-symbiont communities are fundamentally different from more traditional ecological communities. We investigate the membership criteria for a multitrophic insect symbiont community that colonizes host inflorescences at specific developmental stages termed colonization windows. The inflorescences serve as microcosm habitats. Symbionts exhibit a range of interactions from mutualism to parasitism. Hosts exhibit feedback by aborting inflorescences not pollinated by mutualistic symbionts. Habitats are consequently lost for all other symbiont species in such host-derived organs whose development is mutualist-dependent. Using empirical measurements to characterize inflorescence development, we simulate symbiont dispersal colonization across hosts. We vary host densities and lengths of symbiont colonization windows, and track the persistence of each symbiont species in the metacommunity based on its trophic requirements and resource availability within the microcosm. Since the persistence of the microcosm habitat is dictated by pollination performed by the mutualist, the mutualist fared better than all other symbionts. The length of symbiont colonization windows was positively related with colonization success and symbiont persistence. The cumulative length of the colonization windows of prey dictated predator success; diet breadth or prey colonization success did not influence predator persistence. Predators also had a greater host-plant density requirement than prey for persistence in the community. These results offer valuable insights into host densities required for maintaining symbionts, and have implications for multitrophic symbiont community stability. Special constraints can govern symbiont community membership, function and structure and symbiont persistence when host-symbiont feedback impacts host microcosm development.

Resource dispersion influences dispersal evolution of highly insulated insect communities

Communities in which species are obligately associated with a single host are ideal to test adaptive responses of community traits to host-imposed selection because such communities are often highly insulated. Fig species provide oviposition resources to co-evolved fig-wasp communities. Dispersing fig-wasp communities move from one host plant to another for oviposition. We compared the spatial dispersion of two fig species and the dispersal capacities of their multitrophic wasp communities. Dispersal capacities were assessed by measuring vital dispersal correlates, namely tethered flight durations, somatic lipid contents and resting metabolic rates. We suggest that dispersal-trait distributions of congeneric wasp species across the communities are an adaptive response to host plant dispersion. Larger dispersal capacities of the entire multitrophic community are related to more widely dispersed resources. Our results provide evidence and a novel perspective for understanding the potential role of adaptation in whole-community dispersal-trait distributions.