Epiphytes and orchids are well suited to be indicators of the health and biodiversity of the rainforest, not only because they are an important source of nutrients for other flora and fauna, but because they are very sensitive to shifts in microclimate and they have slow growth. The performance, survival, and distribution of epiphytes is dependent on stand density, microclimate, distance from seed source, tree size and species, type and history of disturbance, population dynamics of epiphytes and trees, and epiphyte physiology (Hietz 1999).

Epiphytes are far more vulnerable to deforestation than  other flora. For example, 26% of  vacular plant species present in 1900 are now exinct, but 62% of  epiphyte species are exinct. Epiphytes are completely dependent on their host plants, so if a tree is cut down, all of the epiphytes residing on that tree will die. In addition, they have very specific zoning constraints, so secondary vegetation might not have all of the nessassary microsites for different epiphyte species.
 
 

Table 1 (Hietz 1999) illustrates the loss of species and biodiversity in a plantation as compared with oldgrowth forest. While the number of species for the two groups is not very different, there is considerable loss of biodiversity, because only epiphytes residing in some of the locations on a tree are present.

    TABLE 1. Epiphyte Richness and Occurrence of Ecological Groups in an Oldgrowth Forest and an Alnus Plantation.
    Figures in parentheses for the plantation are fertile species, all species in the oldgrowth forest were found with fertile individuals.
    Epiphytic Species Living on:                     Oldgrowth                                         Plantation
    stem base                                                         9                                                        0 (0)
    stem and thick branches                                 14                                                       7 (3)
    thick and thin branches                                   11                                                      11 (7)
    preference unclear                                            5                                                         3 (1)
    total                                                                 39                                                       21 (11)
 

In addition ot complete deforestation, epiphytes are hurt by fragmentation. Their wellbeing depends on the distance from their fragment to closed forest, and the shape of the fragment (these effect seed dispersal).
They are also affected more adversly by increased CO2 levels in the atmosphere. Raised CO2 levels shift climate zones, forcing flora and fauna to migrate. Although epiphytes could migrate more easily than trees, many models of the effect of fluctuating CO2 levels predict increased seasonality of precipitation, and thus a reduction of the perhumid area containing the hightest epiphyte diversity (Hietz 1999).

Specific species or groups have been identified as good bioindicators.

• the hemiepiphytic Ficus is a keystone species because of its "numerical abundance, intra-crown synchrony of fruit ripening, relatively short intervals between fruiting, large crop sizes and intrapopulation fruiting asynchrony" and the fact that it is often available as a food source for birds when other fruits are scarce (Lambert and Marshall 1991).
• Lichens are often used as an indicator of air quality: Tillandsia, for example, is paticularly sensitive to lead pollution.

Epiphyte N concentration:

Epiphytes obtain their Nitrogen either from canopy soil or from nutrients in rainwater. Nitrogen-15 concentration is much higher in ground-rooted plants than in epiphytes with access to canopy soil, pointing to a much richer source of Nitrogen in terrestial soil versus canopy soil. In addition, N-15 concentration is much higher for those epiphytes in canopy soil than those on smaller branches, indicating that epiphytes on smaller branches have to rely almost exclusively on rainwater as a source of Nitrogen. This means that these epiphytes (on small branches) are much more susceptible to drought and thus would be better bioindicators.
 

Sources:

Hietz, Peter. "Diversity and Conservation of Epiphytes in a Changing Environment." International Union of Pure and Applied Chemistry (IUPAC). 1998. Volume 70: Issue 11. Available at: http://www.iupac.org/symposia/proceedings/phuket97/hietz.html

Turner, T.H.; Tan, H.T. W.; Wee, Y.C.; Ibrahim, Ali Bin; Chew, P.T.; Corlett, R.T. "A Study of Plant Species Extinction in Singapore: Lessons for the Conservation of Tropical Biodiversity." Conservation Biology. September 1994. Volume 8: Issue3, pgs. 705-12.

Lambert, Frank R. and Marshall, Adrian G. "Keystone characteristics of Bird-dispersed Ficus in a Malaysian lowland Rain Forest." Journal of Ecology. 1991. Volume 79, pgs. 793-809.

Hietz, Peter; Wanck, Wolfgang; Wania, Rita; Nadkarni, Nalim M. "Nitrogen-15 natural abundance in a montane cloud forest canopy as an indicator of nitrogen cycling and epiphyte nutrition." Oecologia. 2002. Volume 131, pgs. 350-355.