<P> Tropical freshwater swamp forests, or "flooded forests", are found in Amazon basin (the Várzea) and elsewhere . </P> <P> Rainforests are divided into different strata, or layers, with vegetation organized into a vertical pattern from the top of the soil to the canopy . Each layer is a unique biotic community containing different plants and animals adapted for life in that particular strata . Only the emergent layer is unique to tropical rainforests, while the others are also found in temperate rainforests . </P> <P> The forest floor, the bottom-most layer, receives only 2% of the sunlight . Only plants adapted to low light can grow in this region . Away from riverbanks, swamps and clearings, where dense undergrowth is found, the forest floor is relatively clear of vegetation because of the low sunlight penetration . This more open quality permits the easy movement of larger animals such as: ungulates like the okapi (Okapia johnstoni), tapir (Tapirus sp .), Sumatran rhinoceros (Dicerorhinus sumatrensis), and apes like the western lowland gorilla (Gorilla gorilla), as well as many species of reptiles, amphibians, and insects . The forest floor also contains decaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay . Many forms of fungi growing here help decay the animal and plant waste . </P> <P> The understory layer lies between the canopy and the forest floor . The understory is home to a number of birds, small mammals, insects, reptiles, and predators . Examples include leopard (Panthera pardus), poison dart frogs (Dendrobates sp .), ring - tailed coati (Nasua nasua), boa constrictor (Boa constrictor), and many species of Coleoptera . The vegetation at this layer generally consists of shade - tolerant shrubs, herbs, small trees, and large woody vines which climb into the trees to capture sunlight . Only about 5% of sunlight breaches the canopy to arrive at the understory causing true understory plants to seldom grow to 3 m (10 feet). As an adaptation to these low light levels, understory plants have often evolved much larger leaves . Many seedlings that will grow to the canopy level are in the understory . </P>

Which of the following is most likely to be a limited resource on the floor of a tropical rainforest