A considerable number of plants depend on structural support of other plants. To understand their diversity and ecology, it is essential to know how strongly potential host species differ in their suitability as hosts. This review focuses on vascular epiphytes, i.e. structurally dependent plants that do not parasitize their hosts. Despite a longstanding interest in the topic, our knowledge on the strength of their host specificity is still scanty. This is arguably due to conceptual confusion, but also because of the large complexity of the study system, which turns quantifying host specificity in the field into a challenge.

Local variation in the abundance and richness of vascular epiphytes is often attributed to environmental characteristics such as substrate and microclimate. Less is known, however, about the impacts of tree and branch turnover on epiphyte communities. To address this issue, we surveyed branches and epiphytes found on the forest floor in 96 transects in two forests (Atlantic rainforest in Brazil and Caribbean rainforest in Panama). In the Brazilian forest, we additionally distinguished between edge and core study sites. We quantified branch abundance, epiphyte abundance, richness and proportion of adults to investigate the trends of these variables over branch diameter. Branches <2 cm in diameter comprised >90% of all branches on the forest floor. Abundance and richness of fallen epiphytes per transect were highest in the Brazilian core transects and lowest in the Panamanian transects. The majority of epiphytes on the floor (c. 65%) were found attached to branches. At all three study sites, branch abundance and branch diameter were negatively correlated, whereas epiphyte abundance and richness per branch, as well as the proportion of adults were positively correlated with branch diameter. The relationship between branch diameter and absolute epiphyte abundance or richness differed between study sites, which might be explained by differences in forest structure and dynamics. In the Panamanian forest, epiphytes had been previously inventoried, allowing an evaluation of our surveying method by comparing canopy and forest floor samplings. Individuals found on the forest floor corresponded to 13% of all individuals on branches <10 cm in diameter (including crowns), with abundance, richness and composition trends on forest floor reflecting canopy trends. We argue that forest floor surveys provide useful floristic and, most notably, demographic information particularly on epiphytes occurring on the thinnest branches, which are least accessible. Here, branchfall acts as an important demographic filter structuring epiphyte communities.

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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.

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