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Spatial relationships between spiders and their host vascular epiphytes within shade trees in a Mexican coffee plantation

Citation

Quijano Cuervo, Luis Guillermo et al. (2021), Spatial relationships between spiders and their host vascular epiphytes within shade trees in a Mexican coffee plantation, Dryad, Dataset, https://doi.org/10.5061/dryad.r2280gbc7

Abstract

Within tree canopies, vascular epiphytes create habitats for other taxa, and their heterogeneous spatial distribution could affect the distribution of organisms associated with them, such as spiders. This study was performed in shade trees of a rustic coffee plantation located within a Tropical Cloud Forest region of Mexico. We used a spatially explicit approach to examine (1) if the richness and abundance of epiphyte-dwelling spiders have a positive association with epiphyte abundance within trees and (2) if spiders (richness and abundance) show the same patterns of spatial distribution as the epiphytic habitat. We found that spiders were distributed on gradients of the same type as their host epiphytes. These gradients were: a decrease from the center toward the edges of the tree and a decrease from the base of the trunk toward the canopy. Spiders also had aggregation patches with similar dimensions to those exhibited by their host epiphytes. Those spiders' patches were fully explained when their epiphytic habitat was also aggregated within trees. Spatial models suggested that epiphytes and spiders were also spatially structured at scales larger than a tree and smaller than an epiphyte mat. Our findings demonstrate that the spatial distribution of epiphyte-dwelling spider communities, may partly be explained by the distribution patterns of their host plants. However, other environmental and biotic factors, not associated with epiphyte communities, are likely to be responsible for the remaining spatial patterns of spider distribution.

Methods

Data collection

We used traditional single rope-climbing to manually extract the majority of vascular epiphytes present in the trees (i.e., bromeliads, orchids, cacti, Piperaceae, ferns, aroids, and others [hemiepiphytes]). Only the epiphytes present at the very ends of the branches were not collected, due to the unsafe climbing conditions. The removal of the epiphytes was carried out in a period of approximately six weeks per tree, between October 2013 and March 2014. Prior to extraction, the position of each epiphyte on its host tree was mapped in three dimensions. Mapping was done by establishing a Cartesian coordinate plane (X, Y) on each tree using measuring tapes attached to the main trunk. The center of the tree (i.e., the trunk) was considered as the origin and the axes had a South-North (Y) and a West-East (X) direction. The X-Y position was measured perpendicular to the trunk and parallel to the ground, independently from the origin of the branch. The third axis (Z) corresponded to the height and was measured drawing the measuring tape from each epiphyte, perpendicularly to the ground.

Once removed, epiphytes were placed in properly labelled plastic bags and subsequently dismantled in the laboratory to collect all spiders present on each individual plant. The spiders were collected manually while alive and preserved in 80% ethanol. The adult individuals were sorted into morphospecies according to their morphological traits (somatic or sexual). When possible, individuals were assigned to species using taxonomic keys.

Variables

For each observation unit, we quantified the richness of the vascular epiphyte groups (bromeliads, orchids, cacti, Piperaceae, ferns, aroids, and others [hemiepiphytes]), biomass (measured as dry weight) and abundance (number of individuals per mat). An epiphyte with an independent root system or several epiphytes of the same taxonomic group sharing the same rhizosphere were considered as one individual. For spiders, we recorded species richness (number of species per mat) and abundance (number of individuals per mat).

Data analysis

Spatial modeling

To model the spatial distribution of epiphytes and their hosted spider, we performed a two-stage analysis for each plane. In the first stage, we modeled gradients (i.e., monotonic changes in some direction) using simple linear models. In the second stage, we modeled aggregation patches (i.e., aggregation zones where the values of a variable are more similar to each other than expected by chance) using variography, as a way to estimate the degree of spatial dependence.

Average and spatial relationship

Once we analyzed the spatial distribution of spiders and epiphytes, spatialized linear models (Generalized Least Square- GLS) were used to determine the explanatory power of epiphyte abundance on the variation in spider richness and abundance and their spatial distribution in the trees.