Acquisition of Resources Solid Substratum

Structures vary greatly in their suitability as a surface to grasp or stick to, with solidity, shape, inclination, aspect, texture, and sediment- and water-holding ability among the important characteristics for epibiota.

Light

In dense forests light at ground level is <2% of that falling on the canopy, so epiphytism is the only way that small plants can access strong light for photosynthesis. However, not all terrestrial epiphytes require full sun and some are tolerant of deep shade. Light levels are also greatly reduced under dense aquatic macrophyte beds, but for epiphytes there tends to be less vertical light stratification within hosts than that experienced by their terrestrial counterparts, due to the flexibility of macrophytes subject to water motion.

Water

In the canopies of many forests, precipitation is sporadic and exposure to the wind and sun creates drier conditions than on the forest floor, so water availability strongly affects the growth and survivorship of epiphytes lacking a root connection to the ground. Epiphytes maximize water uptake and minimize loss by (1) rapidly absorbing rain, dew or mist using specialized leaf and root structures,

(2) storing water when it is plentiful (in their own tissues, root-mass humus, or 'tanks' in the case of bromeliads), and

(3) using CAM photosynthesis to reduce losses through transpiration. Many can tolerate severe desiccation. Water availability is obviously not a factor for aquatic epiphytes, except those exposed by low tide or drought.

Nutrients/Food

Nutrients are often in short supply for terrestrial epiphytes, since most have no root connection to the ground and must instead obtain their nutrients from solutes in rain and stemflow water, wind-borne particles, litterfall, carnivory, or animal excrement (especially the waste products of ants living in the root mass or within specialized plant organs). For aquatic organisms, living on surfaces can increase their access to nutrients/food in a number of ways. (1) Organisms obtaining nutrients from the water column, either as ions in solution (epiphytes) or suspended particles (many epifauna), should profit from the increased access to water flow achieved by living on other organisms that project into the water column, or in the case of epifauna living directly on the seafloor, inhabiting the surface of sediments or rocks rather than living in burrows. Aquatic epiphytes can take up nutrients from the surrounding water across their entire surfaces, so they generally have better access to nutrients than do terrestrial epiphytes. They usually have higher surface:area volume ratios than their host macrophytes, enabling them to take up nutrients more rapidly, but like their hosts they are still potentially limited by C, N, or P. Other food sources available to epifauna are (2) periph-yton, for which host surface area is important, (3) trapped detritus, for which host structure is important, and (4) host macrophyte tissue, for which intrinsic host properties such as nutritional value and chemical defenses are important.

Refuge from Consumers

Living in a tree or aquatic macrophyte reduces the threat to epibiota from ground- or seafloor-dwelling consumers. Predation is intense in many aquatic habitats, but epibiota can acquire critical refugia on hosts possessing (1) structural complexity, which reduces the foraging efficiency of predators, and/or (2) chemical defenses against consumers, which provide epibiota with 'enemy-free space' where they are safe from incidental consumption or destruction of their host (an 'associational defense'). For herbivorous seaweed epifauna, shelter from predators often appears to outweigh food quality as a factor influencing host plant selection or subsequent survivorship.

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