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Agroforestry has been increasingly recognized as a key example of agroecological praxis contributing to the sustainable intensification of food production while providing a number of additional benefits to society. However, a quantitative synthesis of the impact of agroforestry on soil health and associated ecosystem services in the humid and sub-humid tropics is still lacking. The objective of this study was to quantify the contribution of agroforestry practices to soil-mediated ecosystem services, specifically, regulation of soil erosion, storage of soil organic carbon (SOC) and nitrogen (N), availability of soil N and phosphorus (P) to crops, and alleviation of soil acidity across the humid and sub-humid tropics. The analysis demonstrated that agroforestry can reduce soil erosion rates by 50 % compared to crop monocultures. This finding is supported by higher infiltration rates, lower runoff, higher proportion of soil macroggregates, and greater stability of soil structure under agroforestry. SOC increased by 21 %, N storage increased by 13 %, available N by 46 % and available P by 11 % while soil pH increased by 2% under agroforestry compared to crop monocultures. We conclude that agroforestry can make significant contributions to provision of soil-mediated ecosystem services in the humid and sub-humid tropics.

The top-down and indirect effects of insects on plant communities depend on patterns of host use, which are often poorly documented, particularly in species-rich tropical forests. At Barro Colorado Island, Panama, we compiled the first food web quantifying trophic interactions between the majority of co-occurring woody plant species and their internally-feeding insect seed predators. Our study is based on more than 200,000 fruits representing 478 plant species, associated with 369 insect species. Insect host-specificity was remarkably high: only 20% of seed predator species were associated with more than one plant species, while each tree species experienced seed predation from a median of two insect species. Phylogeny, but not plant traits, explained patterns of seed predator attack. These data suggest that seed predators are unlikely to mediate indirect interactions such as apparent competition between plant species, but are consistent with their proposed contribution to maintaining plant diversity via the Janzen-Connell mechanism.

We propose a new classification of rainforest plants into eight fruit syndromes, based on fruit morphology and other traits relevant to fruit-feeding insects. This classification is compared with other systems based on plant morphology or traits relevant to vertebrate fruit dispersers. Our syndromes are based on fruits sampled from 1,192 plant species at three Forest Global Earth Observatory plots: Barro Colorado Island (Panama), Khao Chong (Thailand) and Wanang (Papua New Guinea). The three plots differed widely in fruit syndrome composition. Plant species with fleshy, indehiscent fruits containing multiple seeds were important at all three sites. However, in Panama a high proportion of species had dry fruits, while in New Guinea and Thailand, species with fleshy drupes and thin mesocarps were dominant. Species with dry, winged seeds that do not develop as capsules were important in Thailand, reflecting the local importance of Dipterocarpaceae. These differences can also determine differences among frugivorous insect communities. Fruit syndromes and colours were phylogenetically flexible traits at the scale studied, as only three of the eight seed syndromes, and one of the 10 colours, showed significant phylogenetic clustering at either genus or family levels. Plant phylogeny was, however, the most important factor explaining differences in overall fruit syndrome composition among individual plant families or genera across the three study sites.

Natural enemies of plants such as insect herbivores can contribute to structuring and maintaining plant diversity in tropical forests. Most research in this area has focused on the role of specialized enemies and the extent to which herbivory on individual plant species is density‐dependent. Relatively few insect herbivores specialize on a single host plant species. Insect herbivores that feed on more than one plant species may link the regeneration dynamics of their host species through “apparent competition” or “apparent mutualism.” We investigated herbivory and survival of seedlings of two tropical tree species (Cordia alliodora and Cordia bicolor) in the forests of Barro Colorado Island (Panama). We used experiments and observations to assess seedling fate in relation to the presence of conspecifics and heterospecifics across a range of spatial scales. Herbivory significantly increased seedling mortality and was highest at high local densities of C. alliodora seedlings. There was also evidence that high local densities of C. alliodora increased herbivory on co‐occurring C. bicolor seedlings. Synthesis. The elevated rates of seedling herbivory at high densities of conspecifics documented in our study are consistent with the predictions of the Janzen–Connell hypothesis, which explains how so many plant species can coexist in tropical forests. Our data also highlight the possibility that herbivore‐mediated density‐dependence, facilitated by herbivores that feed on multiple plant species, can also occur across plant species. Enemy‐mediated indirect effects of this sort have the potential to structure plant communities.

Herbivorous insects comprise a major part of terrestrial biodiversity, and their interactions with their host plants and natural enemies are of vast ecological importance. A large body of research demonstrates that the ecology and evolution of these insects may be affected by trophic interactions, by abiotic influences, and by intraspecific processes, but so far research on these individual aspects has rarely been combined. This thesis uses the leaf-mining moth Tischeria ekebladella and the pedunculate oak (Quercus robur) as a case study to assess how spatial variation in trophic interactions and the physical distribution of host trees jointly affect the distribution, dynamics and evolution of a host-specific herbivore. With respect to habitat quality, Tischeria ekebladella experiences abundant variation at several spatial scales. Most of this variation occurs at small scales notably among leaves and shoots within individual trees. While hypothetically this could cause moths to evolve an ability to select leaves and shoots of high quality, I did not find any coupling between female preference and offspring performance. Based on my studies on temporal variation in resource quality I therefore propose that unpredictable temporal changes in the relative rankings of individual resource units may render it difficult for females to predict the fate of their developing offspring. With respect to intraspecific processes, my results suggest that limited moth dispersal in relation to the spatial distribution of oak trees plays a key role in determining the regional distribution of Tischeria ekebladella. The distribution of the moth is aggregated at the landscape level, where local leaf miner populations are less likely to be present where oaks are scarce. A modelling exercise based on empirical dispersal estimates revealed that the moth population on Wattkast an island in south-western Finland is spatially structured overall, but that the relative importance of local and regional processes on tree-specific moth dynamics varies drastically across the landscape. To conclude, my work in the oak-Tischeria ekebladella system demonstrates that the local abundance and regional distribution of a herbivore may be more strongly influenced by the spatial location of host trees than by their relative quality. Hence, it reveals the importance of considering spatial context in the study of herbivorous insects, and forms a bridge between the classical fields of plant-insect interactions and spatial ecology. De växtätande insekterna utgör en artrik grupp av stor ekologisk och ekonomisk betydelse. Utbredningen av växtätare har omväxlande ansetts avspegla rumslig variation i värdväxtkvalitet, utbredning av naturliga fiender eller yttre miljöfaktorer. Under senare år har ekologisk forskning dock visat att en art kan uppträda på vissa lokaler och saknas på andra utan nämnvärd variation i ovannämnda faktorer detta p.g.a. begränsningar i artens spridningsförmåga och begränsad livslängd hos lokala bestånd. Trots detta har den relativa betydelsen av livsmiljöns kvalitet och rumsliga utbredning sällan beaktats i studier av växtätande insekter. I min avhandling undersöker jag hur värdväxtens kvalitet och utbredning påverkar utbredning, beståndsfluktuationer och evolution hos den större ekpannluggmalen (Tischeria ekebladella), en art som är specialiserad på ek, Quercus robur. I min avhandling påvisar jag att enskilda ekar ur ekpannluggmalens perspektiv ter sig som mosaiker av blad och skott av varierande kvalitet, medan skillnaderna i bladkvalitet mellan olika trädindivider är förhållandevis små. Trots att malarnas resursval kan ha viktiga konsekvenser för avkommans livsduglighet förmår äggläggande malhonor inte urskilja de blad och skott som är bäst för larvernas överlevnad. En möjlig förklaring är att ett adaptivt resursval är omöjligt i en situation där den relativa kvaliteten hos olika resursenheter varierar oförutsägbart i tid. Mina resultat tyder på att rumsliga mönster i ekpannluggmalens utbredning till stor del beror på malarnas begränsade spridningsförmåga. Arten förekommer med en lägre frekvens på isolerade ekar än på träd i större ekbestånd. En populationsmodell för malbeståndet på ön Wattkast i Korpo visar att graden av växelverkan mellan malar på olika träd kraftigt varierar med ekarnas avstånd från varandra. Sammanfattningsvis bygger mitt arbete med ekpannluggmalen en bro mellan tidigare forskningstraditioner som å ena sidan fokuserat på den lokala växelverkan mellan värdträd och växtätare och å andra sidan på rumsliga processer. Mina resultat visar att utbredningen av växtätande insekter inte nödvändigtvis avspeglar rumsliga mönster i livsmiljöns kvalitet, och att värdväxtens utbredning har viktiga konsekvenser för insekternas rumsliga populationsstruktur. Var värdväxten finns påverkar därigenom det ekologiska och evolutiva samspelet mellan växtätande insekter, värdväxter och naturliga fiender.