Islands in the trees: A biogeographic exploration of epiphyte‐dwelling spiders

Abstract

Aim Epiphytic plants are isolated from each other by nonhabitat canopy elements and are thus expected to act as islands, the biodiversity of their inhabitants (e.g., spiders) conforming to island biogeographic predictions of species‐richness patterns. Although it has been shown that arthropod diversity decreases with decreasing epiphyte size, the effects of isolation have not yet been addressed. We studied the joint effect of isolation, spatial position, and size of epiphytic plants (canopy islands) on species richness, relative rareness, and similarity of spider communities. Location A shade‐coffee plantation in central Veracruz, Mexico. Taxon Spiders (Araneae), vascular epiphytes (Bromeliaceae, Piperaceae, Orchidaceae, Araceae, Pteridophyta). Methods We collected all canopy islands occurring on three trees and recorded their three‐dimensional spatial position. In the laboratory, we disassembled the plants and collected all spiders present. We analysed the effects of island size, isolation, and spatial position on the species richness and the relative rareness of spider communities using generalized linear models (GLM), on the distribution of different spider guilds using a CCA, and on community composition using a permutational multivariate analysis of variance (Permanova). Isolation and spatial position were addressed using five distance measures representing isolation from different potential species sources. We tested how the similarity in spider community composition changed with the distance between islands using Mantel correlograms. Results As predicted by island biogeography theory, spider species richness increased with canopy‐island size and decreased with isolation. In comparison to island size, the effect of isolation was weak, though significant. Relative rareness was hardly affected by island size but more by isolation. Compositional similarity was affected by island size and decreased with increasing spatial distance up to ca. 4 m. Guilds separated along the main CCA axes, this ordination being driven by epiphyte size and position. Main conclusions Epiphytic plants behaved like canopy islands in that their size and isolation influenced the diversity and composition of spider communities. However, the effect of isolation was only a fraction of that of island size, perhaps because spatial relationships are taxon‐specific. This may be due to differences in hunting behaviour and dispersal capacities, for example, between guilds of hunting and web‐building spiders, which is a dimension deserving more attention. For a better understanding of biogeographic principles driving the diversity of canopy island inhabitants, further research on this topic should include position and isolation, at scales matching the mobility of different functional groups, as part of their explanatory variables.

A forest canopy as a living archipelago: Why phylogenetic isolation may increase and age decrease diversity

“…Host organisms can be understood as living islands upon which entire communities, or even meta communities, of colonizers can live or feed, surrounded by an unsuitable matrix of non-host organisms (Gripenberg & Roslin, 2005;Gossner, Chao, Bailey, & Prinzing, 2009;Méndez-Castro, Bader, Mendieta-Leiva, & Rao, 2018;Patiño, Gómez-Rodríguez, Pupo-Correia, Sequeira, & Vanderpoorten, 2018;Vialatte et al, 2010). Many of the patterns and processes that occur on true islands can be transposed, with certain limitations, to hosts (Santos et al, 2016).…”

Island‐biogeographic patterns of spider communities on epiphytes depend on differential space use among functional groups

“…The data used for this analysis are a subset of that used in Méndez-Castro et al (2018). We omitted 80 canopy islands located on secondary branches, for which we could not calculate the walking distance.…”

“…These canopy islands constitute microhabitats for many arthropods and other organisms (Rodgers & Kitching, 2011;Stuntz, Ziegler, Simon, & Zotz, 2002;Yanoviak, Nadkarni, & Solano J, 2006) and are arranged in 'archipelagos' in a three-dimensional non-habitat space (Bader, van Dunne, & Stuiver, 2000). In our previous work (Méndez-Castro et al, 2018) we showed how the isolation of such canopy islands had a significant, though small, effect on the spider diversity. Other studies on canopy islands have been unable to show such an effect (Richardson, 1999;Rodgers & Kitching, 2011;Wardle, 2003).…”

“…Isolation among the members of an archipelago can be addressed in two ways, as the distance from a single island to one or several nearest neighbours, or as the average distance of a given island to all the other islands within the same tree. Given that the nearest neighbour approach was not an informative isolation measure in our previous analyses (Méndez-Castro et al, 2018), here we use the second isolation criterion, that is mean pairwise distance among islands. According to the distance used for their calculation, we have two isolation variables: aerial isolation and walking isolation (Figure 1).…”

“…Therefore, the sampling scale in island-biogeography studies should be carefully chosen to make sure the interaction between the organisms and their habitat is captured (Barton, Evans, Foster, Cunningham, & Manning, 2017;De Mas, Chust, Pretus, & Ribera, 2009). If organisms that experience distance and habitat size in different ways are considered and analysed as one community, the mismatch between scales could mask specific spatial responses of the involved subgroups (De Mas et al, 2009;Larrivée & Buddle, 2010;Méndez-Castro, Bader, Mendieta-Leiva, & Rao, 2018). Every biotic community will contain a certain level of variability in the use of space between its components.…”