Vertical Stratification and Caloric Content of the Standing Fruit Crop in a Tropical Lowland Forest¹

Abstract

Fruit abundance in tropical forests thus far has been studied in relation to consumer populations. Area–based surveys that focus on the quantity and quality of the standing fruit crop of an entire plant community, however, are lacking. This paper presents the results of a four–month study on the seasonality and vertical distribution of the standing fruit crop within a tropical forest during the dry season in southern Venezuela. Fruit numbers ranged from 262,000 to 424,000 fruits/ha. The standing crop was between 39 and 92 kg/ha, yielding 67,000 to 126,000 kj. Fruits were not evenly distributed in different forest layers. They were most abundant in the canopy above 16 m and scarce between 4 and 12 m above the ground. Despite a scarcity of fruits in the mid–strata, the caloric value of the total fruit supply peaked within a narrow layer of subcanopy at 12 to 16 m above ground due to a high amount of energy per fruit. Palms fruited mostly in this layer, their fruits contributing 59 percent of the energy supplied by all fruits. Above 12 m, we found a broader range of fruit sizes than in the forest below 12m. Small–sized fruits were distributed in the understory and in the canopy but were nearly absent from die mid–story. Corresponding to the abundance of small–sized fruits in the canopy, small frugivorous species, such as members of the Thraupidae, limited their foraging to the canopy strata whereas larger avian frugivores foraged mainly in die mid–story.

Functionally specialised birds respond flexibly to seasonal changes in fruit availability

“…This is in line with other studies on fruiting phenology in tropical rain forests (e.g. Schaefer, Schmidt & Wesenberg ). Despite this variability in fruit resources, we found little seasonal variation in the functional niche breadth and functional originality of frugivorous bird species.…”

Morphological trait matching shapes plant–frugivore networks across the Andes

“…The shape of a bird's wing is related to its movement capacity and maneuverability (Rayner 1988). Wing shape therefore directly influences where a bird species encounters fruits while foraging (Schaefer et al 2002). Wing shape therefore directly influences where a bird species encounters fruits while foraging (Schaefer et al 2002).…”

Specialization and interaction strength in a tropical plant–frugivore network differ among forest strata

“…Low specialization in plantfrugivore relationships is traditionally expected in tropical ecosystems because the diversity and abundance of plants with fleshy fruits and the year-round dependence of frugivores on fruits are high in tropical forests (Jordano 2000, Kissling et al 2009). Canopy plants contributed most to the overall fruit crop (Schaefer et al 2002), therefore attracting a wide spectrum of obligate frugivores that foraged on many different canopy species with fleshy fruits. A very interesting finding of our study is that the degree of generalization in the plant-frugivore network was determined by the vertical stratification of the forest, resulting in generalized plant-frugivore relationships in the canopy and more specialized associations in the mid-and understory.…”

“…We expected that obligate frugivores feed on more fruit sources than opportunistic frugivores to meet their nutritional requirements (see Whelan et al 1998), and that they forage mostly where fruit availability is high (Shanahan andCompton 2001, Saracco et al 2004), such as in the forest canopy (Schaefer et al 2002). We recorded fruit removal from plants in different forest strata (canopy, midstory, and understory) and habitats (primary and secondary forest), and classified frugivore visitors into guilds according to their niche breadth in terms of diet and habitat specialization.…”