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Life on a leaf: species interactions and community dynamics in epiphyll communities

Periodic Reporting for period 2 - EPIDYN (Life on a leaf: species interactions and community dynamics in epiphyll communities)

Periodo di rendicontazione: 2018-09-01 al 2019-08-31

Global climate change (GCC) is among the greatest threats to planet Earth. GCC influences all ecosystems, initiates negative and irreversible processes, and causes biodiversity loss. Biodiversity in particular plays an important role in ecosystem function over time. Furthermore, it plays a critical and widely recognized role in human wellbeing in terms of biological resources, recreation and cultural value. The failure of clear predictions of the consequences of biodiversity loss is due in part to the lack of a theoretical and empirical foundation incorporating the aspect of species interaction. Species dynamics are key processes in the structuring of ecological communities.
The highest biodiversity worldwide has been found in tropical forests, where bryophytes and lichens represent a significant amount of diversity and cover. In spite of this, due to their relatively small size and challenging identification, these organisms have been largely neglected in many ecological studies.
Tree and shrub leaves that harbour epiphyll communities (bryophytes, lichens, algae and fungi) in miniature ecosystems are an interesting model system in which to study ecological processes, precisely because epiphyll community dynamics happen quickly, during the life-span of the leaf, ensuring economy both in terms of time and resources. Due to the ephemeral character of their substrate and the small scale and relatively fast dynamics, epiphyllous communities offer an ideal system to study dynamics of primary succession and mechanisms explaining biodiversity maintenance (BM) in relation to environmental variables.
Taking a closer look, epiphyll communities can be extremely diverse, with up to 80 species of lichens on a single leaf, plus numerous species of bryophytes (mostly foliose liverworts), algae, fungi and bacteria. As in other tropical biotic communities, it is unknown why these epiphyll communities are so diverse or how biodiversity is maintained. Theoretical models of BM range from ‘neutral theories,’ where random processes explain the high degree of coexistence without counting on different species responses to the environment, to niche-based models, where interactions between species and the environment play a more central role in BM. When a new area is first colonized by plants—for example in primary succession—the course of succession may be a chance process depending on what species happen to arrive first (the ‘priority effect’); or at the other extreme, it may be highly deterministic with very specific roles (e.g. first colonizer, late-successional) or niches for each species. In all these models of BM, species interaction plays an important role, but the predicted outcomes of the interactions differ. While the priority effect implies that the first species to arrive will dominate, models including species-specific niches allow for a range of interactions, both negative (competition) and positive (facilitation). In this project, we aim to determine which are the most appropriate theoretical models for describing species interactions and successional dynamics in epiphyll communities (liverworts, lichens, algae and fungi) and how the relative role of each model depends on environmental conditions.
The project involves both a scientific and social dimension. The first phase started with an introductory lecture about the project at the University of Panama. Afterwards, field work planning and preparation at Smithsonian Tropical Research Institute (STRI) continued. The preliminary data were collected on Barro Colorado Island (BCI) in Panama and data collection methods were improved. Seven closed forest sites and seven gap sites were selected to study epiphyll communities (lichens, liverworts, fungi and algae) in relation to environmental factors (humidity, temperature, light and leaf texture), and to follow epiphyll communities through time to study their community dynamics. Digital photographs of leaves with epiphylls were taken and digitized to study patterns in diversity and cover (first manuscript) and the spatial point patterns(SPPs) and dynamics (second manuscript) of epiphyll cover on these leaves. Epiphyll richness and cover were then analyzed using multivariate statistics and generalized linear mixed models of dependence on environmental factors.
Species composition differed between closed forest and gap sites, and specialist species could be identified. Higher epiphyll species richness and cover were found on older leaves, which was due to an accumulation of species rather than a turnover. Lichen cover did not differ between closed forest and gaps, liverwort cover was higher in gaps. These data are compiled in the first manuscript, accepted by the Journal of Vegetation Science and published in open access. Data used in this article will be soon freely available in Dryad digital repository.
The second manuscript on epiphyll SPPs is under preparation. We analyzed epiphyll communities in four epiphyll functional groups (liverworts, lichens, algae and fungi) and found that epiphyll SPP change over time, but not between microsites (gaps vs. closed forest). Within epiphyll functional groups, we found two SPPs, aggregated and random, while between functional group pairs, in addition to aggregated and random, we also found segregated SPPs.
Part of the research results were presented in a Bambi talk on BCI, at the University of Chiriqui, at the Technological University of Panama, during bryophyte course at STRI and at Marburg University.The researcher also presented the project results in five International conferences and the researcher's oral presentation has been accepted at the British Ecological Society Annual meeting 2019 in Belfast.
The research results also received attention from the general public (school children, students, tourists) during field trips on BCI and in Punta Culebra and at the Marburg University Bryophyte course for students. A wider audience was reached with information about the project through press releases and project websites which were picked up and elaborated upon on websites and newspapers in Germany and Panama.
The present results are contributing new knowledge about successional patterns and environmental requirements of epiphylls from evolutionary and morphologically very different taxonomic groups growing together in a small-scale microhabitat. The first part of our study thereby adds importantly to our understanding of epiphyll community ecology and environmental distributions. In the second analysis of the collected data, we focused on community dynamics by following the development of spatial patterns in epiphylls on leaves through time. This analysis, following single leaves, allowed a more detailed analysis of successional processes and interactions between epiphyll functional groups. By following community dynamics on a highly replicated set of miniature communities, we add not only to our understanding of epiphyll communities, but more generally of community ecology of sessile organisms.
Epiphyll communities on a leaf fragment
Epiphyllous liverwort Leptolejeunea elliptica
Leaf with epiphylls