CR2025_19 Seed predation by insects and its implications for tropical plant diversity
Lead Supervisor: Sofia Gripenburg, Department of Ecology and Evolutionary Biology, University of Reading
Email: s.gripenburg@reading.ac.uk
Co-supervisors: Kalsum Yusah, Royal Botanical Gardens, Kew; Camilo Zalamea, Department of Integrative Biology, University of South Florida; Owen Lewis, Department of Biology, University of Oxford.
Factors affecting seed production and survival can leave long-lasting imprints in plant populations. Once seeds have dispersed from the mother plant, they can be killed by enemies such as fungal pathogens and insect seed predators (e.g. certain ant and beetle taxa that forage on the forest floor or in the soil). These types of interactions have received particular attention in the context of tropical forests, where they have been hypothesised to be responsible for maintaining the astonishingly high tree diversity. If enemies tend to have a disproportionately negative effect on plant individuals surrounded by members of their own species, this could create a negative feedback mechanism that suppresses locally abundant species and favours rare species. This phenomenon, known as conspecific negative density-dependence (CNDD), is a key mechanism of the Janzen-Connell hypothesis – a leading explanation for the long-standing question of what allows so many species to coexist in tropical forest tree communities.
While CNDD is widely documented in tropical forest plant communities and other contexts, the scientific literature is dominated by studies assessing patterns of mortality of plants that have already survived the seed stage (e.g. seedlings, saplings), and with the causal agent(s) of CNDD typically unknown. Enemy-mediated negative feedback mechanisms have been documented and described in some species, but there are still important gaps in our understanding of the spatial, temporal, and phylogenetic scales across which they operate at different stages of the plant life cycle.
In this project, the student will conduct field-based manipulative experiments and observational studies to investigate the spatial and temporal dynamics of interactions between seeds and insect seed predators and the consequences of insect-mediated seed mortality for the diversity of seedlings. A particular focus will be on seed survival and seedling recruitment in small-seeded ‘pioneer’ tree species. While our previous work shows that many of these species tend to escape attack from internally feeding seed predators that commonly feed on larger-seeded tree species, small-seeded tree species are more likely to be consumed by externally feeding insect seed predators such as seed-eating ants. We envisage that the bulk of the field work will be conducted on Barro Colorado Island in Panama, where it can be linked to other ongoing research activities by some of the supervisors. As an example, a current research project by some members of the supervisory team is yielding distribution maps of reproductive individuals of selected tree species (Fig. 1) at the wider landscape scale (the entirety of Barro Colorado Island), allowing the student to investigate enemy responses to seed densities at spatial scales that have not previously been possible. Depending on the availability and/or interests of the student, the core questions in this project can also be addressed in temperate settings (UK) using small-seeded weed species and their insect seed predators (e.g. seed-feeding carabid beetles) as a model system.
A detailed research plan will be developed together with the student, but the project is likely to target the following questions:
Q1. What seed traits make some plant species more susceptible to post-dispersal seed predation by insects than other?
Q2. Is post-dispersal seed predation by insects on individual plant species density-dependent, and if so, at what spatial scale(s)?
Q3. Do patterns of insect-inflicted seed mortality affect the species composition and diversity of emerging seedlings?
Q4. Do seed-feeding insects structure the composition of species through density-dependence across species (a phenomenon known as ‘apparent competition’)?
Depending on the interests of the student, further subprojects related to the interests and expertise of the supervisors may be possible, for example studies addressing similar questions but with a focus on seed-pathogen interactions (co-supervisor Zalamea), studies investigating the sensitivity of seed-insect interactions to environmental change (co-supervisor Lewis), and studies investigating the density-dependence of mutualistic interactions between seeds and seed-dispersing ants (co-supervisor Yusah).
Training opportunities:
The student will receive training in multiple ecological subdisciplines (plant ecology, insect ecology, seed ecology, population and community ecology) and research skills including experimental design, ecological field techniques, and statistical analysis. During part of the thesis work, the student will conduct field work in Panama where they will be affiliated with the Smithsonian Tropical Research Institute (STRI). The student will have access to the specialist and generic skills and training activities made available to PhD students by the Reading Researcher Development Programme (RRDP) including statistical methods, conference presentation and writing skills.
Student profile:
The project is suitable for a student with a degree (undergraduate or MSc) in biological or environmental sciences or a related discipline. We are looking for a student with good oral and written communication skills who is keen to engage in field-based research. Ideally, the candidate will have strong numerical literacy. Experience of ecological field research involving plants and/or insects would be an advantage.
References (relevant background reading):
- Janzen, D. H. (1970) https://doi.org/10.1086/282687
- Bagchi, R. et al. (2014) https://doi.org/10.1038/nature12911
- Gripenberg et al. (2019) https://doi/10.1111/ele.13359
- Comita & Stump (2020) https://doi.org/10.3417/2020591