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Host spatial structure and disperser activity determine mistletoe infection patterns

Citation

Sasal, Yamila (2020), Host spatial structure and disperser activity determine mistletoe infection patterns, Dryad, Dataset, https://doi.org/10.5061/dryad.4j0zpc89d

Abstract

What processes and factors are responsible for species distribution are long-standing questions in ecology and a key element for conservation and management. Mistletoes provide the opportunity to study a forest species whose occurrence is expected to be constrained by multiple factors as a consequence of their life form. We studied the mistletoe Tristerix corymbosus (Loranthaceae) on its most common hosts species in northwest Patagonia. The seeds of this mistletoe are almost exclusively dispersed by the small arboreal and endemic marsupial Dromiciops gliroides (Microbiotheridae). We assessed the underlying causes of plant spatial patterns through point pattern analysis and we used different variables that characterize the neighborhood around each host to analyze the relative effect of host availability, potential for disperser movement, and canopy light conditions. We found that potential hosts were strongly aggregated and that the three most common host species were distributed independent of each other. Considering all host species together, infected and non-infected host were individually aggregated but segregated from each other. The aggregated pattern of infected hosts could be explained in part by the template of potential hosts distribution, but was subsequently modulated by the activity of the mistletoe disperser. Potential for disperser movement,  the proximity to reproductive mistletoes and habitat complexity, increased mistletoe infection probability. However, neighboring host availability decreased mistletoe infection probability, and tree DBH (used as surrogate for light conditions) had no detectable effect. Our results suggested that the distribution of mistletoe infection was determined by the structure of potential host populations and by the marsupial disperser activity. Compared to bird dispersed mistletoes, the scale of the infection was smaller and the proximity to reproductive mistletoes and habitat complexity were important for seed arrival and infection. The interplay between landscape structure and disperser activity determine the spatial structure of mistletoe future generations.

Methods

Data were collected on a plot of 0.9 ha (80 x 100 m) where we mapped all potential hosts, trees, and habitat complexity in X-Y coordinates. We recorded all potential hosts in species and status as infected with mistletoe or non-infected. For each mistletoe, we recorded the reproductive status (juvenile or reproductive) by the presence of flowers or fruits. Habitat complexity was measure as understory vertical structure within the plot, we defined interceptions points separated by 2 m (total of 2104 points). At each point, we recorded the number of branches (minimum diameter 1 mm) that intercept a 3 m vertical pole. Branches with diameter greater than 1 mm allow the marsupial to move through the understory. To derive a proxy for light conditions, we recorded X-Y coordinates for canopy trees species with diameter at breast height (DBH) greater than 5 cm. We define this minimum DBH because canopy trees species of this size or larger affect light conditions in the understory. We estimate the characteristics of neighborhoods around focal hosts within a fixed radius of 5 m from the focal host. All spatial point-pattern analyses were performed using the Programita software and with R software version 3.6.1 (R Development Core Team 2015).

Funding

Agencia Nacional de Promoción Científica y Tecnológica, Award: PICT joven 2011-1899