Adriana Salvo

Habitat Fragmentation and Species Loss across Three Interacting Trophic Levels: Effects of Life-History and Food-Web Traits

Not all species are likely to be equally affected by habitat fragmentation; thus, we evaluated the effects of size of forest remnants on trophically linked communities of plants, leaf-mining insects, and their parasitoids. We explored the possibility of differential vulnerability to habitat area reduction in relation to species-specific and food-web traits by comparing species-area regression slopes. Moreover, we searched for a synergistic effect of these traits and of trophic level. We collected mined leaves and recorded plant, leaf miner, and parasitoid species interactions in five 100-m2 transects in 19 Chaco Serrano woodland remnants in central Argentina. Species were classified into extreme categories according to body size, natural abundance, trophic breadth, and trophic level. Species-area slopes differed between groups with extreme values of natural abundance or trophic specialization. Nevertheless, synergistic effects of life-history and food-web traits were only found for trophic level and trophic breadth: area-related species loss was highest for specialist parasitoids. It has been suggested that species position within interaction webs could determine their vulnerability to extinction. Our results provide evidence that food-web parameters, such as trophic level and trophic breadth, affect species sensitivity to habitat fragmentation.

Network topology: patterns and mechanisms in plant‐herbivore and host‐parasitoid food webs

1. Biological communities are organized in complex interaction networks such as food webs, which topology appears to be non-random. Gradients, compartments, nested subsets and even combinations of these structures have been shown in bipartite networks. However, in most studies only one pattern is tested against randomness and mechanistic hypotheses are generally lacking. 2. Here we examined the topology of regional, coexisting plant-herbivore and host-parasitoid food webs to discriminate between the mentioned network patterns. We also evaluated the role of species body size, local abundance, regional frequency and phylogeny as determinants of network topology. 3. We found both food webs to be compartmented, with interaction range boundaries imposed by host phylogeny. Species degree within compartments was mostly related to their regional frequency and local abundance. Only one compartment showed an internal nested structure in the distribution of interactions between species, but species position within this compartment was unrelated to species size or abundance. 4. These results suggest that compartmentalization may be more common than previously considered, and that network structure is a result of multiple, hierarchical, non-exclusive processes.

Quantitative food webs of dipteran leafminers and their parasitoids in Argentina

The quantitative structure of two host–parasitoid communities based on leaf-mining flies (Diptera, Agromyzidae) in Argentina is described. The two communities consisted of 29 and 27 hosts, 46 and 40 parasitoids, and 193 and 179 recorded host–parasitoid associations. Also, food webs were constructed for one community based solely on samples taken in the wet and dry seasons. Data were expressed as quantitative food webs, and the manner in which food web properties, such as connectance and compartmentalization, were influenced by sampling intensity was explored. The potential importance of indirect effects between hosts mediated by parasitoids (e.g. apparent competition) was assessed using quantitative parasitoid overlap diagrams. The study’s results suggest that indirect effects are likely to be important in these highly connected communities. The limitations of the study’s analysis, and how the conclusions can be tested experimentally, are discussed.

A tritrophic analysis of host preference and performance in a polyphagous leafminer

The optimal oviposition theory predicts that oviposition preferences of phytophagous insects should correlate with host suitability for their offspring. As plant host suitability depends not only on its quality as food, but also on its provision of enemy‐free space, we examined the relationship between adult host preference and offspring performance for the leafminer Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae) on various host plants, considering also the interaction with natural enemies. Preference and offspring performance were assessed through observational field data and laboratory experiments in central Argentina. Field data suggested a positive host preference – performance linkage, as the leafminer attained larger body size on the crops where it was more abundant. Laboratory trials supported these results: Vicia faba L. (Fabaceae) was the preferred host in the laboratory as well as in the field, performance of L. huidobrensis being also best on this host, with highest survival rates and shortest development time. The actively feeding larval stage showed the largest plant‐related effects. Higher overall parasitism rates were found on plants from which smaller leafminers were reared, reinforcing the preference–performance linkage. On the other hand, the main parasitoid Phaedrotoma scabriventris Nixon (Hymenoptera: Braconidae) reached larger body size, and caused higher mortality rates on crops where the leafminer was larger. Changes in abundance of particular parasitoid species could thus modify overall parasitism trends.

Forest fragmentation reduces parasitism via species loss at multiple trophic levels

Although there is accumulating evidence from artificially assembled communities that reductions of species diversity result in diminished ecosystem functioning, it is not yet clear how real-world changes in diversity affect the flow of energy between trophic levels in multi-trophic contexts. In central Argentina, forest fragmentation has led to species loss of plants, herbivore and parasitoid insects, decline in trophic processes (herbivory and parasitism), and food web contraction. Here we examine if and how loss of parasitoid species following fragmentation causes decreased parasitism rates, by analyzing food webs of leaf miners and parasitoids from 19 forest fragments of decreasing size. We asked three questions: Do reductions in parasitoid richness following fragmentation directly or indirectly affect parasitism rate? Are changes in community parasitism rate driven by changes in the parasitism rate of individual leaf miner species, or changes in leaf miner composition, or both? Which traits of species determine the effects of food web change on parasitism rates? We found that habitat loss initiated a bottom-up cascade of extinctions from plants to leaf miners to parasitoids, with reductions in parasitoid richness ultimately driving decreases in parasitism rates. This relationship between parasitoid richness and parasitism depended on changes in the relative abundance (but not occurrence) of leaf miners such that parasitoid-rich fragments were dominated by leaf miner species that supported high rates of parasitism. Surprisingly, we found that only a small subset of species in the food web could account for much of the increase in parasitism with parasitoid richness: lepidopteran miners that attained exceptionally high densities in some fragments and their largely specialist parasitoids. How specialized a parasitoid is, and the relative abundance of leaf miners, had important effects on the diversity-parasitism rate relationship, but not other leaf miner traits including trophic breadth, body size, and mine shape. Our results show that a full understanding of the functional consequences of perturbations and species loss requires both a multi-trophic perspective and a trait-based approach, which together capture some of the biological complexity of natural systems.

Parasitoides de minadores de hojas y manejo de plagas

Los minadores de hojas son insectos cuyas larvas viven y se alimentan dentro de las hojas, consumiendo el mesofi lo sin danar la epidermis foliar. Varias especies son consideradas serias plagas de cultivos intensivos, horticolas y ornamentales. Entre las fuentes de mortalidad mas importantes para este gremio de fi tofagos se citan a los enemigos naturales, de los que se destacan los parasitoides como el grupo mas efectivo y mejor representado. Este articulo proporciona un resumen actualizado de la informacion disponible sobre parasitoides de minadores de hojas en relacion al manejo de plagas. Por ser generalistas, los parasitoides de minadores de hojas pueden incluir rapidamente en su rango alimenticio a especies introducidas, muchas veces lograndose un control efectivo luego de unos pocos anos de establecida la plaga. Control biologico clasico y aumentativo son estrategias ampliamente usadas para regular las poblaciones de minadores de hojas plaga. Numerosos estudios abordan la compatibilidad del uso de parasitoides con control quimico y cultural. Si bien la mayoria de los insecticidas convencionales poseen efectos adversos para los parasitoides, otros serian compatibles con el control biologico. Se conoce que la combinacion de diversas estrategias de control en programas de manejo integrado de plagas ha resultado efectivo contra minador de hojas plaga. Sin embargo, los efectos de practicas culturales que podrian favorecer las poblaciones de parasitoides han sido escasamente estudiados. Palabras clave: Control biologico, control cultural, control quimico, minadores de hojas, parasitoides

Sharing enemies: evidence of forest contribution to natural enemy communities in crops, at different spatial scales

The increase in cultivated lands has led to ecosystem and biodiversity loss. Arthropod natural enemies, involved in the ecosystem service of biological control, benefit from non‐crop habitat and may be affected by its proximity and amount in the landscape. We have evaluated natural enemy richness and composition in relation to forest cover in the landscape and distance from the forest, for a Chaco Serrano forest‐soybean crop system. Forest contribution to natural enemies on soybean was also investigated, by assessing similarities between forest and crop assemblages, and examining body size of shared enemies in relation to distance from the forest. In nine landscape circles, yellow pan traps were placed in forest and soybean crops (5, 25, 50 and 100 m from forest edge), which collected 8041 specimens representing 290 species. Species richness of natural enemies was positively related to forest cover and declined from the forest to the furthermost soybean locations, with both forest cover and its proximity affecting community composition. Also, similarity of forest and crop assemblages increased with forest cover and proximity. Finally, forest‐crop shared assemblages showed larger average body size at greater distances from the forest, indicating dispersal limitations on forest contribution to the crop. Our results suggest that forest species are important components of natural enemy communities on soybean, and that both the amount of natural habitat and its proximity may influence agroecosystems. We emphasise the importance of non‐crop habitat to help maintain natural enemy communities and ensure the ecosystem service of pest control.

Plant patch structure modifies parasitoid assemblage richness of a specialist herbivore

1. The spatial structure of plant patches has been shown to affect host–parasitoid interactions, but its influence on parasitoid diversity remains largely ignored. Here we tested the prediction that parasitoid species richness of the specialist leafminer Liriomyza commelinae increases in larger and less isolated patches of its host plant Commelina erecta. We also explored whether parasitoid abundance and body size affected the occurrence of parasitoid species in local assemblages.

Community dynamics of leafminers (Diptera: Agromyzidae) and their parasitoids (Hymenoptera) in a natural habitat from Central Argentina

We examined the temporal community dynamics of leafminers (Agromyzidae: Diptera) and their mainly polyphagous parasitoids in a natural habitat from Central Argentina. Changes in community composition are shown by changes in dominant species and similarity coefficients. Abundance of leafminers was highest in winter, and was not related to their species richness. Abundance and species richness were correlated in the parasitoid community. Temporal variation in parasitoid diversity and abundance were positively correlated with species number and density in the host community. Apparent parasitism was greater when parasitoid species richness was high and parasitoid community dominance was low.

Not all in the same boat: trends and mechanisms in herbivory responses to forest fragmentation differ among insect guilds

Habitat fragmentation can alter fundamental ecological interactions such as insect herbivory. Few studies of habitat fragmentation effects on herbivory have examined the mechanisms involved, and differences among insect guilds have been largely ignored. Here, we studied area and edge effects on herbivory by three guilds of phytophagous insects in a fragmented Chaco Serrano forest. We estimated herbivory levels on native Croton lachnostachyus plants and assessed plant availability (distance to nearest conspecific) and quality indicators (leaf water, carbon and nitrogen content), as well as richness and abundance of the associated insect community, in order to explore mechanisms underlying herbivory changes. Herbivory by chewing and sap-sucking insects decreased, and herbivory by leaf miners increased in plants growing at the forest edge, compared with those at the interior. Forest area effects were detected only in interaction with edge effects on chewing, leaf mining and total herbivory. Lower herbivory at the edge appeared to be mediated by changes in leaf water and nitrogen content for sap-sucking herbivory, and linked to strong direct effects for chewing damage and total herbivory. Instead, higher damage levels by leaf miners at the forest edge seemed to be driven by increased plant availability and lower water content. Further studies are needed to unravel the factors involved in the strong direct effects detected here for all herbivory types. These results emphasize the necessity to consider differential responses from diverse phytophagous insect guilds, and factors operating at multiple levels, in order to disentangle, and ultimately understand, forest fragmentation effects on herbivory.