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Oikos 000: 000000, 2009 doi: 10.1111/j.1600-0706.2009.17245.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Eric Seabloom. Accepted 9 March 2009 How human disturbance of tropical rainforest can influence avian fruit removal Kara L. Lefevre and F. Helen Rodd K. L. Lefevre ([email protected]) and F. Helen Rodd, Dept of Ecology and Evolutionary Biology, Univ. of Toronto, 25 Harbord Street, Toronto, Ontario, M5S 3G5, Canada. Present address for KLL: Dépt des sciences du bois et de la forêt, Univ. Laval, 2405 rue de la Terrasse, Québec, QC, G1V 0A6, Canada. Fruit consumption by birds is an important ecological interaction that contributes to seed dispersal in tropical rainforests. In this field experiment, we asked whether moderate human disturbance alters patterns of avian frugivory: we measured fruit removal by birds in the lower montane rainforest of Tobago, West Indies, using artificial infructescences made with natural fruits from two common woody plants of the forest understory (Psychotria spp., Rubiaceae). Displays were mounted simultaneously in three forest habitats chosen to represent a gradient of increasing habitat disturbance (primary, intermediate and disturbed), caused by subsistence land use adjacent to a protected forest reserve. We measured the numbers of fruits removed and the effect of fruit position on the likelihood of removal, along with the abundances of all fruits and fruit-eating birds at the study sites. Fruit removal was highly variable and there was not a significant difference in removal rate among forest habitats; however, the trend was for higher rates of removal from displays in primary forest. Canopy cover, natural fruit availability, and frugivore abundance were not good predictors of fruit removal. Birds preferred more accessible fruits (those proximal to the perch) in all habitats, but in disturbed forest, there was a tendency for distal fruits to be chosen more frequently than in the other forest types. One possible explanation for this pattern is that birds in disturbed forests were larger than those in other habitats, and hence were better able to reach the distal fruits. Coupled with differences in bird community composition among the forest types, this suggests that different suites of birds were removing fruit in primary versus disturbed forest. As frugivore species have different effectiveness as seed dispersers, the among-habitat differences in fruit removal patterns that we observed could have important implications for plant species experiencing disturbance; these possible implications include altered amounts of seed deposition and seedling recruitment in Tobago’s tropical rainforest. The effects of human disturbance on population dynamics are relatively well-studied, but less attention has been paid to potential impacts on species interactions and the basic functioning of ecosystems (Orians et al. 1996, Kremen 2005). The concern is that, by altering community structure, anthropogenic disturbances could disrupt complex biotic interactions that maintain ecosystem integrity, causing a cascade of ecological impacts (Bond 1994). For example, there is evidence that habitat disturbance can cause declines in pollination (Kremen et al. 2002), and changes in the visitation rates of animal fruit-consumers to rainforest trees (Luck and Daily 2003). Empirical studies demonstrate that these anthropogenic disruptions can have important effects on plant populations including declines in fruit and seed set (Aizen and Feinsinger 1994), increased inbreeding (Fuchs et al. 2003), altered seed dispersal patterns (Cordeiro and Howe 2003, Levey et al. 2005), reduced germination (Bruna 1999), and lower seedling recruitment (Chapman and Chapman 1995). It has been hypothesized that potential outcomes of disrupted seed dispersal could thus include changes in plant species distributions, community composition, and even species extinction (Bond 1994, Loiselle and Blake 2002). Altered patterns of frugivory might have a smaller measurable effect on plants than altered pollination, because the relationships between species are not as tightly coupled in seed dispersal. Instead, plants usually rely on suites of vertebrates for dispersal (Wheelwright and Orians 1982, Levey et al. 1994), which may be why there is little evidence for the role of animal dispersers in the diversification of angiosperms (Herrera 1989). However, disturbance impacts on many seed dispersers simultaneously could interfere with plant regeneration (Willson and Traveset 2000). Concomitantly, animal dispersers can play a critical role in maintaining forest plant communities following habitat loss (Montoya et al. 2008). Thus, studies of patterns of frugivory in disturbed areas are important for determining the potential consequences for plant populations and forest ecosystems. We investigated how anthropogenic, tropical rainforest disturbance influences avian frugivory the consumption of fruit by birds. Species of fruit consumers differ in their Early View (EV): 1-EV effectiveness as seed dispersal agents (Schupp 1993), due to variations in body size, diet composition, feeding behaviour, digestive physiology, and movement patterns (Levey et al. 1994, Jordano et al. 2007). Frugivore species therefore generate distinctive seed shadows, resulting in different effects on plant fitness (Willson and Traveset 2000, Loiselle and Blake 2002). Changes in avian frugivory could thus alter plant reproduction, distributions, and forest composition (Levey et al. 1994). In these ways, birdfruit interactions contribute to the structure and biodiversity of tropical rainforest communities (Snow 1971, Morton 1973, Howe and Smallwood 1982, Howe 1986, Stutchbury and Morton 2001). Studies of fruit consumption by birds show that plant distributions and spatial habitat variables can influence avian fruit choice and removal rates (Denslow 1987, Sargent 1990, Levey et al. 1994, McCarty et al. 2002, Saracco et al. 2005). The position of fruit on plants also influences frugivory so that, given a choice, birds usually select fruits closest to a perch because they are more accessible (Denslow and Moermond 1982, Moermond and Denslow 1983, Levey et al. 1994). Because human alteration of forests can influence habitat structure and plant and bird community composition (reviewed in Lefevre 2008), important variables that can influence fruit selection by birds, we postulated that disturbance might also influence patterns of avian fruit removal. In this study, we asked whether anthropogenic forest disturbance affects patterns of fruit removal by birds; specifically, we asked whether disturbance affected the numbers of fruit taken and the positions on plants from which fruits were removed. To do so, we used concurrent experiments in protected rainforest and in neighbouring, disturbed rainforest, in Tobago. We consider the subsistence human activity in the disturbed area as moderate, compared to the more severe, well-studied disturbances caused by outright habitat loss or fragmentation. Fruit removal was quantified using standardized displays of natural rainforest fruits. Despite considerable variation in fruit removal rates documented in the literature, in general, previous studies have shown that more fruits are typically removed near forest edges and in gaps than under closed canopy (Thompson and Willson 1978, Denslow and Moermond 1982, Moore and Willson 1982, Restrepo et al. 1999); this may occur because fruits are discovered more easily in sunlit patches (Thompson and Willson 1978), or because frugivores generally forage more in gaps than in forest interior (Willson et al. 1982, Levey 1988b). We therefore predicted that fruit removal rates would be higher in our disturbed sites, because the partial clearing in these areas might be similar to natural forest openings. We also considered the effect of natural fruit and frugivore abundances on fruit removal rates. Methods Study site and design Fruit removal experiments were conducted on Tobago, West Indies (11817?N, 60837?W, Trinidad and Tobago), a small continental island approximately 120 km from 2-EV Venezuela. We worked in and near the Main Ridge Forest Reserve, an area of 3500 ha protected rainforest along the island’s mountainous backbone, which has been protected since 1765 and is administered by Tobago’s Dept of Natural Resources and the Environment (DNRE). The forest reserve and adjacent Crown Lands were likely never altered by significant amounts of human interference because of their inaccessibility, and they do not have a history of cultivation (Beard 1944). The vegetation is lower montane rainforest (Beard 1944; premontane wet forest according to Holdridge et al. 1971), which covers the Main Ridge from approximately 250 m to 575 m, its highest elevation. It is characterized by a 1530 m canopy depending on exposure, with abundant lianas and epiphytes, and was described extensively in past surveys (Beard 1944, Keeler-Wolf 1982). Seasonality is driven by rainfall: the dry season occurs roughly from JanuaryMay and the wet season from JuneDecember. This study took place from March to May 2004 (experiment dates listed in Appendix 1), when fruiting diversity peaks in the regions (Snow 1965). Experiments were conducted in three types of forest habitat, chosen to represent an increasing gradient of habitat disturbance: primary, intermediate and disturbed. Primary forest Primary control plots were located in undisturbed forest in the reserve’s interior. To our knowledge, the only notable human disturbance is guided nature walks for tourists, and some wildlife poaching (DNRE pers. comm.). Principal species in the canopy include Byrsonima spicata (Malpighiaceae), Eschweilera decolorans (Lecythidaceae), Licania biglandulosa (Chrysobalanaceae), Simarouba amara (Simaroubaceae), Sloanea laurifolia (Eleaocarpaceae), and several Palmae. The forest understory consists of shrubs and small trees including numerous members of the Melastomataceae, Palmae and Rubiaceae. Disturbed forest Disturbed forest represented the other end of the disturbance gradient. This habitat was located beyond the reserve within 1 km of its boundary. To the best of our knowledge, the area is Crown Land (Beard 1944) with pockets of private holdings, including small-scale, abandoned cacao plantings that were established in rainforest by local people, during the first half of the 20th century (H. Jack pers. comm.). The disturbed forest has significantly less canopy cover than primary forest (Lefevre 2008); the vegetation is now a mosaic of some large trees, secondary forest, and openings dominated by pioneer plant species including Cecropia peltata (Cecropiaceae), Heliconia spp. (Heliconiaceae) and Piper spp. (Piperaceae). While no longer actively cultivated, these areas experience subsistence use at a low intensity by local people, for harvesting fruits (banana, papaya), grazing goats and cows, and for travel by foot. For comparison with other studies, we suggest that this constitutes ‘moderate’ habitat disturbance, a term that has been applied to activities such as selective logging or shifting agriculture (Gray et al. 2007). Intermediate forest Intermediate forest plots were located in rainforest at the reserve boundary, directly abutting disturbed areas. Many large, possibly old-growth trees remained intact in this habitat. These areas have not had the same small-scale cultivation as disturbed areas (D. Henry pers. comm.), but they are not legally protected like the primary forest. An assessment of plant community composition in the three habitats showed that intermediate forest includes plant species characteristic of both protected and disturbed forest, and that this habitat has a medium amount of canopy cover (mean 53%, range 1085), compared to the more intact primary forest (mean cover66%, range 2590) and the more open disturbed forest (mean cover 43%, range 585; 92.1 SE, n 36 per habitat) (Lefevre 2008). We emphasize that these levels of disturbance were moderate, compared to the severe disturbance that might occur in the case of canopy clearing and other forms of habitat loss or conversion. Four study plots (0.75 ha) were established in each of the three forest types (see Lefevre 2008 for more details). Locations were chosen based on the availability and accessibility of suitable habitat. We selected three study trees in each plot as replicate units for measuring fruit removal (n 12 trees per forest type). Trees were chosen depending on the presence of thin horizontal branches ideal for attaching fruit displays, and the trees selected in each plot were spaced at least 10 m apart. We chose this minimum inter-tree distance because field experiments have shown that fruit displays at least 6.4 m apart are treated separately by some Neotropical frugivores (Levey et al. 1984). Fruit removal experiment To control for fruit display size and spatial arrangements, we used artificial infructescences made with natural rainforest fruits that occur in the study area. Medium-gauge wire (2 mm diameter) was used to create V-shaped ‘twigs’ 15 cm long, with 10 small loops to which a single fruit was attached (Fig. 1). Fruits were collected and sewn onto loops the day before an experiment began. To create a fruit Distal fruits (1, 6) 6 1 7 2 8 3 9 4 Proximal fruits (5, 10) 5 10 Tree branch Figure 1. Schematic drawing of one artificial infructescence display (‘twig’). Each arm of the twig is 15 cm long. Circles represent loops made by twisting the wire every 3 cm, to which fruits were sewn. Numbers indicate positions of fruits, representing five different distances from the main tree branch. The branch is a potential bird perch. display, we attached three twigs to each experimental tree, using duct tape to affix wires upright on a horizontal branch. Each twig was placed on a different branch, within 50 cm of each other, and 1.52 m above the ground. There were 30 fruits presented on each tree, for a total of 1080 fruits per replicate experiment (10 fruits per twig3 twigs per tree36 trees). Displays were mounted at sunrise. We revisited at sunset to record the number of fruits removed, and the twig positions from which fruits were removed. Fruits were presented for 10 to 11 hours, depending on the time required to hike in to study plots. We assumed that removals were due largely to birds because they are mostly diurnal, as opposed to mammals, which are mostly nocturnal. We frequently observed squirrels Sciurus granatensis active in the day, but observed them feeding only on large fruits, never on berries similar to those used in this experiment. Each replicate experiment took two days to run, with half of the study trees tested on each day (i.e. 18 trees per day, six from each forest type). We repeated the experiment ten times to maximize the chance that birds would respond, because fruit crops of understory plants may go undetected by frugivores when crop sizes are relatively small (Murray 1987), search time between patches may be relatively high (Howe 1986), and frugivorous birds may not visit new food sources immediately (Restrepo et al. 1999). We chose two Psychotria species for use in experiments because members of the family Rubiaceae are generally bird-dispersed and are relatively common fruiting plants in the Neotropical rainforest understory (Levey et al. 1994, Smith et al. 2004). Their ripe fruits were available in large numbers in the Main Ridge during the dry season. Psychotria muscosa is a 13 m tall shrub that produces terminal clusters of 37 white to purple fleshy berries, 0.5 1 cm in size. Psychotria tobagensis is a small tree 37 m tall, with panicles bearing scores of 0.5 cm oval-shaped orangered berries with a very thin pulp layer. Fruits of both species contain two seeds. We did five replicates with each species, running subsequent experiments when a large enough crop of either species was ripe and accessible for collection. The average time between replicates was 6 days (range 214 days; Appendix 1). Fruits were gathered in the Reserve away from study plots, and experiments were not conducted during heavy rain. In addition, we attempted one replicate with a fruit species more common to disturbed/edge forest (Ficus tobagensis), and one with a fruit that does not grow in the study area (dried cranberries, Vaccinium macrocarpon), but as birds did not respond to the displays, these data are not included here. We also attempted to collect direct observations of fruit removal by conducting focal watches at natural P. muscosa and P. tobagensis plants. Removal was not observed during these periods. This is not surprising since birds removing fruits in the understory tend to be wary (Howe 1979) and generally avoid feeding near observers (Murray 1987), and the fruit crops of these plants were small, compared to large fruiting trees such as Ficus. However, based on casual observations and on fecal samples we collected, we know that P. muscosa was eaten by blue-backed manakins Chiroxiphia pareola and P. tobagensis was eaten by yellowlegged Platycichla flavipes and white-necked thrushes Turdus albicollis. Psychotria fruits in Panama are eaten by migrant 3-EV Empidonax flycatchers (E. S. Morton pers. comm.) and several other species of the thrush, manakin, flycatcher, trogon and tanager families (Poulin et al. 1999), so we infer that these fruits could also be eaten by other frugivores in our study site. Environmental variables We collected data on three environmental variables that we anticipated would be most likely to influence fruit removal rates: canopy cover, fruit abundance, and abundance of fruit-eating birds. To collect data on these variables, we established three 10 10 m quadrats in each 0.75 ha plot, with quadrats spaced evenly through the plot (one at each end and one in the middle). We measured canopy cover because it influences the level of sunlight that penetrates into the forest interior and hence fruit abundance. Based on the spherical densiometer method (Engelbrecht and Herz 2001), two observers made independent estimates of percent canopy cover to the nearest 10% at three locations in each quadrat, using a hand-held, 117 cm mirror marked with a grid. We took the average of the two estimates for each location. We measured natural fruit abundance, because this influences fruit consumption by animals (Howe and Smallwood 1982). We censused all fruiting plants in each quadrat once per month. For every plant bearing fruit, we recorded the species and estimated the number of ripe and unripe fruits present. Only ripe fruits were included in the estimate of fruit available to avian frugivores, because birds tend to eat ripe fruits except during periods of scarcity (Moermond and Denslow 1983). We also quantified the abundance of fruit-eating birds because temporal variation in their numbers is likely to affect fruit removal rates (Levey et al. 1994). We used point counts, a standard method to census birds by sight and vocal display (Karr 1981): 10 min, unlimited radius counts at two points per plot, 150 m apart, one morning each month between sunrise and 11 a.m. Then, we calculated mean bird abundance (number of individuals) for the pair of points in each plot, because we were not certain that points were far enough apart to be considered independent samples in the more open habitat of disturbed forest. Both frugivores and fruit-eating omnivores were included in this calculation, to account for all bird species that regularly included fruit in their diet. Feeding guild designations were based on our own field observations and literature reports of avian feeding ecology in the Neotropics (point count data along with the references used for guild classification are detailed in Lefevre (2008)). Data analysis First, we asked whether fruit removal rates varied among habitat types. Removal was calculated as the rate of fruit removed per twig per hour; we used this standardized measure because fruits were presented for slightly different durations in each plot, due to differing times required to reach locations on foot. Removal rates were not normally distributed even after transformation. Also, we wanted to express removal as a rate per twig in order to examine 4-EV variation within experimental replicates (i.e. trees). Thus, we calculated the mean removal rate per twig at each replicate tree species (Hurlbert 1984), across the five experiments with each fruit type, and performed a separate one-way, non-parametric analysis of variance (Kruskal Wallis test) for each Psychotria species, with forest habitat as the dependent variable. Then we repeated the analyses, after excluding trees from which no fruit was removed; again rates were not normally distributed. This second test accounted for the likelihood of frugivore visitation, an important component of seed dispersal (Murray 1987). Next, we wanted to compare fruit removal among habitats within each replicate experiment, to account for temporal variation in removal. We did this by analyzing total fruit removal at the study plot level. Within each replicate experiment, we compared the total amount of fruit removed from all interior forest plots to the mean of total fruit removed from intermediate and disturbed forest plots (i.e. (intermediatedisturbed)/2)). This enabled a paired comparison of fruit removed from the forest reserve versus fruit removed from both habitats experiencing some degree of disturbance. We transformed the total amount of fruit removed (square root (x3/8)) to obtain a normal distribution and conducted a paired t-test. We could not perform a multiple regression with our environmental data because fruit removal rates were not normally distributed. Therefore, we used Spearman rank correlations to test for associations between fruit removal rates and canopy cover, fruit abundance, and frugivore abundance. We also asked whether Psychotria removal rates were higher in plots where the same species was fruiting naturally, compared to plots where it was absent or not fruiting, using a Wilcoxon rank sum test. To determine whether study plots near to each other had similar patterns of fruit removal, we used hierarchical clustering. Plots were clustered based on removal rates of both Psychotria species (complete data sets, including instances of zero removal), using an unweighted average distance algorithm (UPGMA). We visually inspected the resulting dendrogram to see if trees were clustered more by habitat type or spatial location. To analyze the effect of fruit position on likelihood of removal, we conducted a categorical analysis considering only twigs from which fruit was removed. We used G-tests to determine whether there was uniform fruit removal among four possible categories (fruit positions shown in Fig. 1): distal fruit removed (positions 1 or 6), proximal fruit removed (position 5 or 10), both terminuses removed (position 1 and 6, or 5 and 10), or no terminal fruits removed (i.e. only central positions 24 or 79 removed). Separate G-tests were conducted for each replicate because consecutive replicates at the same tree were not independent. This analysis was conducted for P. muscosa experiments only because frequencies of P. tobagensis removal were too low to allow for valid tests. To ascertain whether the likelihood of removal increased at each tree over time, as birds became accustomed to the fruit displays during subsequent replicate experiments, we used one-tailed runs tests (Zar 1984) to test for contagion in the distribution of fruit removals. All analyses were performed using the statistical software JMP version 5.0.1a (SAS Inst.), other than runs tests, which were calculated by hand. 2.0 a 1.5 Forest Primary Intermed 1.0 Results Fruit removal rates and quantity Mean frequency (/10 replicates) Removal rates were highly variable for both fruit species, and their distributions were positively skewed due to the prevalence of zeros in the data sets. Birds removed more P. muscosa than P. tobagensis fruits (Appendix 1): at least one fruit was removed from 35% of P. muscosa displays, versus 23% of P. tobagensis displays. No fruit was removed from a study tree 38.4% of the time. When fruit was removed from a plot during a replicate experiment, it was taken typically from just one tree (62.2% of trees), sometimes from two trees (33.8%), and rarely from all three trees in a plot (4.1%; Fig. 2a). At the twig level, most often no fruit was taken from a display (70.8% of the time). When fruit was removed from a tree, it was often taken from one (48.6%), two (24.8%), or all three twigs (26.7%) within the tree (Fig. 2b). These removal patterns suggest that birds were most often feeding at just one tree within a plot, but from multiple twigs on a tree and, therefore, were treating trees, rather than twigs, as independent fruit patches. We detected no significant difference among forest types in the rates of fruit removal per display when data for all replicates at all trees were included in the analysis (Fig. 3a). This was true for both species: removal rates did not depend on habitat for P. muscosa (KruskalWallis test, x2 0.32, n 36 trees, DF 2, p0.85) or P. tobagensis (Kruskal Wallis test, x2 0.41, n 36 trees, DF 2, p0.81). Considering only instances when at least one fruit was taken from a tree during a replicate experiment (Fig. 3b), removal did not differ with forest type for either P. muscosa 10 a 10 8 8 6 6 4 4 2 2 b 0 0 0 1 2 3 No. of trees with fruit removed 0 1 2 3 No. of twigs with fruit removed Figure 2. Pattern of Psychotria fruit removal (mean9SE) from trees and twigs, for ten replicate experiments (all experiments in all habitats, combined). (a) Number of trees with at least one fruit removed, of three trees in a plot (n 12 plots). When removed, fruit was typically taken from only one tree, or sometimes two trees. (b) Mean number of twigs per tree with at least one fruit removed, of three twigs on a tree (n 36 trees). When removed, fruit was taken from one, two, or three twigs on a tree. These plots show that frequently, no fruits were removed from a twig, or even an entire tree. When fruits were removed, birds were mainly feeding at one tree within a plot but from multiple twigs on that tree. Mean fruits removed / twig Disturbed 0.5 0 6 b 4 2 0 Psychotria muscosa Psychotria tobagensis Figure 3. Amount of fruit removal for two Psychotria species in three rainforest habitats. (a) Number of fruit removed per display (‘twig’) (maximum 10 fruits) from each experimental tree (n 12 trees per forest type, for five replicates with each fruit species). (b) Number of fruit removed per twig, after twigs from which no fruit was removed were excluded from the analysis, to account for the likelihood of frugivore visitation. Bars show actual numbers of fruit removed (mean9SE) during a 10 h period, to show standardized results for the minimum amount of time that all experiments were run. Analyses were conducted on hourly removal rates to account for small differences in the time fruit was available to birds. (KruskalWallis test, x2 1.57, n 34 trees, DF 2, p 0.46) or P. tobagensis (KruskalWallis test, x2 0.58, n 26 trees, DF 2, p 0.75). However the plot-level analysis, comparing the total amount of fruit removed within each replicate experiment, showed that fruit removal from primary forest was greater than from the other two habitats (paired t-test: t 2.4, n 10 replicates, p 0.04). No habitat variable was a good predictor of fruit removal neither canopy cover, ripe fruit abundance, nor frugivore abundance was clearly related to removal rates (Table 1). Moreover, experimental fruit removal was not correlated with the natural abundance of the same Psychotria fruit in study plots. Nor was fruit removal simply dependent on whether or not the species used in the experiment was fruiting naturally in the same study plot, for either P. muscosa (Wilcoxon rank sum test, Z 0.73, n 9,27, p0.46) or P. tobagensis (Z 0.23, n 3,33, p0.82). The clustering analysis of trees, based on removal rates of both fruit species (i.e. the complete data set including instances of no removal), suggests that removal patterns were driven by a combination of forest type and local spatial effects (Fig. 4). Our study plots did not cluster clearly according to habitat or spatial location alone. Rather, clusters on the dendrogram include trees from a mixture of habitat types and locations. The likelihood of fruit removal from each tree did not increase significantly over time. Data for each of 32 trees were tested separately. Runs tests were not significant in 31 cases (p 0.05). One test was significant for a tree in interior forest (m2, m0.05(1)3,7 2, pB0.025), where 5-EV Table 1. Associations between removal rate of two species of Psychotria fruits (mean of five replicates at each tree) and habitat variables (Spearman rank correlations, n 36 trees). Habitat variables (measured in each study plot) Canopy cover (%) Fruit abundance (all ripe fruits) Psychotria abundance (only the species being tested) Frugivore abundance removal was clustered toward the end of the 10 replicates. There were too few instances of fruit removal to conduct a test for the other five trees: fruit was never removed at two trees, and was removed in only one replicate at three trees. P1-1 P1-2 D4-3 P2-2 P2-3 I3-1 P2-1 P4-1 I2-1 I3-2 I3-1 I1-3 P4-2 I1-2 D2-1 I1-1 D2-3 D4-1 P3-3 D1-3 I2-2 I4-1 I4-2 D3-2 D1-1 D2-2 P3-2 I2-3 D3-3 D4-2 P4-3 I3-3 I4-3 D1-2 P1-3 D3-1 Figure 4. Dendrogram from hierarchical clustering (algorithm method average, or UPGMA) of 36 experimental trees, based on similarities in avian fruit removal of two species of Psychotria. Alpha-numeric codes identify each tree, for comparison of locations and habitats: letters denote the forest habitat (P primary, I intermediate, Ddisturbed), the first number is the replicate study plot in that habitat, and the second number is the individual tree in the plot. Trees sharing a symbol are in the same cluster of the diagram. 6-EV P. muscosa P. tobagensis rs p rs p 0.08 0.05 0.18 0.24 0.67 0.79 0.30 0.17 0.07 0.08 0.04 0.09 0.74 0.71 0.80 0.67 Fruit removal position Fruit position on the twig had a significant influence on the likelihood of fruit removal in four of five P. muscosa replicates, with fruits closest to the branch (where birds would likely perch) being chosen more than distal fruits. This pattern varied significantly with habitat type in two of the replicates (G-tests, Table 2). In primary forest, proximal fruits were the most likely to be removed from displays. Proximal fruits were also removed more often in intermediate and disturbed forest, but the pattern was less pronounced in these habitats (Fig. 5). This result could be due to differences in the feeding ecology of birds that predominate in each forest type, because bird community composition differed among our forest habitats (Appendix 2). For example, experiments with captive frugivores show that birds with strong legs (e.g. Thraupinae and Emberizinae: thrushes and finches) tend to feed from a perched position and select more accessible fruits, while birds with weaker legs (e.g. Pipridae, Trogonidae and Tyrannidae: manakins, trogons and flycatchers) tend to feed on the wing while hovering and are less restricted by fruit position (Moermond and Denslow 1983, Moermond et al. 1986). To determine whether such differences in foraging behaviour might explain our observed fruit removal pattern, we categorized birds as either ‘perchers’ or ‘hoverers’, and compared the abundance of individuals in each of these feeding groups, across forest types. Based on our observed fruit removal pattern, we would expect perchers to be most common in primary plots, where fruit removal was most affected by accessibility. In contrast, we found that perchers were most common in disturbed plots (ANOVA, F2,9 4.8, p0.04) while hoverers were most common in interior plots (ANOVA, F2,9 31.5, pB0.001). Thus, these differences in feeding behaviour alone cannot explain the removal pattern. Another possible explanation for the pattern of fruit removal from displays is frugivore size. For example, a large percher might easily be able to reach the fruits at the terminus of our 15 cm infructescences despite its inability to hover. Thus, to determine if differences in body size were consistent with fruit removal patterns, we obtained the body length of frugivores in our data set from the literature (ffrench 1991), assuming that length determines how far birds can reach to feed. Based on the observed fruit removal pattern, we expected to find larger frugivores in disturbed plots, because relatively more fruits were taken from the distal position in that habitat. Indeed, mean body size of frugivores (weighted by the abundance of each species) was greatest in plots with the least canopy cover Table 2. Categorical analysis of the relationship between Psychotria muscosa fruit position versus removal frequency. Separate G-tests were conducted for each repeated experiment. Replicate Does fruit position influence removal? (DF3) G p Depends on habitat type? (DF6) G p yes yes yes yes no 22.7 10.2 21.3 7.95 6.22 B0.0001 0.017 B0.0001 0.047 0.10 yes yes no no no 19.2 23.4 9.76 4.3 3.14 B0.01 B0.01 0.14 0.63 0.79 1 2 3 4 5 (Fig. 6), indicating that larger birds were more common in disturbed plots (r2 0.37, n 4, p0.035). Discussion Patterns of fruit removal by birds in Tobago’s rainforest were characterized by a high amount of variation, across all three forest habitats. Removal rates varied dramatically in both space and time, as has been found in other studies of 40 30 proximal 20 both distal neither 10 P I D 1.2 b closer to branch Mean fruits removed 1.0 0.8 40 • Primary 0.6 0.4 0.2 0.0 P I D Forest habitat Figure 5. Influence of fruit position on likelihood of Psychotria muscosa removal by birds, in primary (P), intermediate (I), and disturbed (D) forest. (a) Summarized results for all experiments, illustrating whether fruit was removed close to (proximal) or furthest from (distal) the tree branch to which infructescences were affixed. Each count represents one infructescence (twig) including only those from which fruit was removed. (b) Mean fruit removal at each of five distances from the tree branch (shown in Fig. 3a), including instances of zero removal. Within each habitat, the left bar represents the most distal fruit, and bars to the right are proximal fruits (i.e. closest to the main branch to which displays were attached). Mean frugivore body length (cm) Frequency of removal a frugivory (McCarty et al. 2002). Local spatial effects alone were not driving fruit removal patterns, as our clustering analysis rarely grouped trees from the same plot. Also, we found that the likelihood of fruit removal did not increase over time, suggesting that birds were not simply responding more to known fruit locations. We had expected that the abundance of fruit-eating birds and natural fruit availability within a plot would be important influences on removal rates. Other studies have shown that the abundance of fruit and frugivores typically increases in naturally disturbed and second-growth forest (Levey 1988a). However, we found no significant influence of fruit abundance on fruit removal rates; rates were not correlated with either Psychotria abundance or the total amount of fruit available in study plots. We also found no significant influence of fruit-eating bird abundance on removal rates. This suggests that there was neither increased frugivory nor increased competition among plants for seed dispersers in the disturbed areas that we studied. Indeed, others have found that fruit removal rates are difficult to predict (McCarty et al. 2002), because they are influenced by many complex, interrelated factors in addition to fruit and frugivore abundance. A number of studies found little or no evidence for a positive relationship between fruit crop size, frugivore visitation, and the amount of fruit removal (Moore and Willson 1982, Davidar and Morton 1986, Denslow 1987, Murray 1987, Laska and Stiles 1994, Korine et al. 2000). However, Ortiz-Pulido and Rico-Gray (2000) found that all of these measures were x Intermed 35 o Disturbed 30 25 20 30 40 50 60 70 80 Percent canopy cover Figure 6. Relationship between body size of frugivorous birds and percent canopy cover (r2 0.37, p0.035). Every bird observed in a study plot was included in the analysis; body size data were obtained from the literature (Methods). Each symbol represents the mean for one forest plot (n 4 plots/treatment). 7-EV positively correlated, and suggested that previous results may be unclear because spatio-temporal variation in fruit production tends to be overlooked. Also, frugivores are selective in their choice of fruit (Moermond et al. 1986) because, in addition to fruit abundance, a vast array of fruit traits including size, nutritional content, and digestibility influences the attractiveness of fruit to birds (Denslow 1987, Levey et al. 1994). Extrinsic factors such as habitat characteristics, neighbourhood effects related to plant densities and spatial configurations (Sargent 1990, McCarty et al. 2002, Saracco et al. 2005), predation risk (Howe 1979), and direct or indirect edge effects (Restrepo et al. 1999) also play an important role in avian fruit choice. Any of these factors could have thus been influencing fruit removal in our study site, making it more difficult to detect any underlying patterns among habitats. We observed no significant difference in the rate of fruits removed per display, among forest types. In terms of total fruits removed in each experiment, however, we found that birds removed significantly more Psychotria fruits from primary forest, compared to intermediate and disturbed forest. One explanation could be that birds in primary forest were more likely to respond to our displays because fruiting Psychotria plants are more common in that habitat, making birds there more familiar with the fruits. However, there was no correlation between fruit removal and the abundance of the same Psychotria species in study plots. Alternatively, we suggest that this occurred because disturbance in our study site is altering bird community composition, including frugivore abundances, which in turn could influence fruit consumption. Perhaps this is why we did not find higher rates of fruit removal with disturbance as some other studies have detected in forest gaps and near forest edges (Thompson and Willson 1978, Denslow and Moermond 1982, Moore and Willson 1982), including at edges created by human activity (Restrepo et al. 1999, Galetti et al. 2003). However, we note that Galetti et al. (2003) also found that the probability of fruit consumption declined in smaller forest fragments. In our study area, a possible implication of lower fruit removal following disturbance could be a decline in the dispersal of shade-tolerant plant species and, correspondingly, a lack of seed input from primary forest plants into disturbed areas. For example, blue-backed manakins, which frequently eat P. muscosa, and white-necked and yellowlegged thrushes, which eat P. tobagensis, were most common in our primary forest study plots, and less abundant or absent from disturbed plots. Given that fewer fruits were removed from disturbed forest, it seems that other frugivore species common to disturbed forest (e.g. blue-gray tanagers) are not simply interchangeable with primary forest frugivores, in terms of fruit consumption and dispersal services provided to Psychotria plants. Therefore, if disturbance affects bird species composition (as it has done here), there can be important implications for plant species that are dispersed by birds that are sensitive to disturbance. We did detect some noteworthy patterns in the positions from which fruits were removed from displays. Fruits closest to the branch, where some birds presumably perched, were chosen most frequently. This impact of accessibility on fruit choice has been demonstrated in other feeding experiments. Birds that feed while perching prefer 8-EV fruits that are closest to their perch (Denslow and Moermond 1982, Moermond and Denslow 1983). However, we found that this preference for proximal fruits was strongest in primary forest; in disturbed forest, where birds were generally larger, distal fruits were chosen at a frequency similar to proximal fruits. This pattern suggests that differences in bird species composition, due to disturbance, could be influencing the accessibility of fruits, and hence fruit selection by birds in this ecosystem. While we cannot comment on the precise implications in Tobago’s rainforest, because we did not make observations of fruit consumption and dispersal, other studies have found that species of animal consumers contribute to seed dispersal in very different ways. For example, Jordano et al. (2007) found that larger birds dispersed seeds over greater distances and into more open microhabitats, affecting seed-mediated gene flow differently than smaller birds. The increase in avian body size that we noted in disturbed habitats may hence be having a direct influence on bird-mediated seed dispersal patterns. Certainly the exact nature of these relationships will differ depending on the plant species in question, the frugivores present, their degree of frugivory and individual fruit preferences. All of these factors are of interest for future studies of disturbance impacts on fruit consumption and seed dispersal. Despite the complexity of factors influencing fruit consumption, and the large variation in avian fruit removal that we found, our results suggest that moderate human disturbance could be affecting patterns of avian fruit consumption in Tobago. We found some evidence for less fruit removal and different patterns of removal from displays in disturbed forest, and we infer that these changes may result from the altered bird community composition in habitats affected by human activity in our study area. These differences in fruit consumption could indicate some rather serious, indirect impacts of forest disturbance on plants. That is, considering the essential role of avian frugivory in the seed dispersal of forest plants, and considering the influence that interspecific differences in bird feeding behaviour and dietary composition can have on seed shadows and the effectiveness of seed dispersal (Schupp 1993, Loiselle and Blake 2002), we hypothesize that changes in fruit removal could influence plant reproduction in our study site. For example, a change in bird community composition due to disturbance might decrease the likelihood of shade-tolerant plant species being dispersed to gaps for future regeneration, and might increase the likelihood of pioneer plant species penetrating into the periphery of primary forest. The differences in fruit removal patterns that we observed among rainforest habitats might therefore have long-term implications for plant populations experiencing disturbance. Acknowledgements Tobago’s Dept of Natural Resources and the Environment (DNRE) authorized use of the Main Ridge Reserve. We thank their staff for training and logistical support, especially J. Edwards, N. George, D. Henry, H. Jack and A. Ramsey. Fieldwork in Tobago was assisted by H. Robert, A. Stephens, A. Suley and H. Thorpe. J. D. Thomson encouraged us to pursue this experiment, and M. J. Fortin provided helpful statistical advice. We thank them, E. S. Morton, and T. C. Moermond for guidance and comments on the manuscript. D. J. Levey gave encouraging feedback, N. Cordeiro offered invaluable suggestions for field methods, and for other helpful advice we thank F. Hayes, L. Manne, L. Nagel, S. Peters, J. Thaler, K. Whitney, and U of T’s Evolutionary Ecology Group. D. 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Values are standardized as the number of fruits removed in a 10 h period, the minimum amount of time all displays were presented. Date Mar 26 Apr 3 Apr 17 Apr 21 Apr 26 Apr 29 May 1 May 4 May 11 May 20 Fruit species Total fruits removed from forest type t m m m t t t t m m Primary Intermediate Disturbed 1 39 54 49 15 15 9 32 74 26 1 55 24 44 5 4 12 13 41 6 3 29 11 31 20 7 14 16 75 6 Appendix 2. Presence () and absence (0) of fruit-eating birds censused in the study area on Tobago (Lefevre 2008), during the time period of fruit removal experiments (MarMay 2004). Letters in brackets denote feeding guilds (f frugivore, o omnivore). Numbers 15 indicate the five most abundant species in each habitat (duplicate numbers are ties) based on point count censuses. Mist netting data (Lefevre 2008) were also used to complete this species composition list. Frugivores were defined as species that regularly include fruit in their diet. Although they are among the most abundant frugivores in all three habitats, rufous-vented chachalaca and orange-winged parrot are not included in the rankings, because they tend to stay high in the canopy, and may be over-represented in counts due to their loud, frequent calls (Lefevre unpubl.). Avian frugivore species Common name Rufous-vented chachalaca (f) Pale-vented pigeon (f) White-tipped dove (f) Orange-winged parrot (f) Collared trogon (o) Blue-crowned motmot (o) Red-crowned woodpecker (o) Golden-olive woodpecker (o) Barred antshrike (o) Blue-backed manakin (f) Ochre-bellied flycatcher (f) Fuscous flycatcher (o) Yellow-bellied elaenia (o) Yellow-breasted flycatcher (o) Brown-crested flycatcher (o) Venezuelan flycatcher (o) Streaked flycatcher (o) Tropical kingbird (o) Yellow-legged thrush (f) Bare-eyed thrush (f) White-necked thrush (f) Chivi vireo (o) Scrub greenlet (o) Northern waterthrush (o) Bananaquit (o) Red-legged honeycreeper (o) Violaceous euphonia (f) Blue-gray tanager (f) Palm tanager (f) White-lined tanager (f) Black-faced grassquit (o) Carib grackle (o) Shiny cowbird (o) Crested oropendola (o) Smooth-billed ani (o) Rainforest habitats Latin name Primary Intermediate Disturbed Ortalis ruficauda Columba cayennensis Leptotila verreauxi Amazona amazonica Trogon collaris Momotus momota Melanerpes rubricapillus Piculus rubiginosus Thamnophilus doliatus Chiroxiphia pareola Mionectes oleagineus Cnemotriccus fuscatus Elaenia flavogaster Tolmomyias flaviventris Myiarchus tyrannulus Myiarchus venezuelensis Myiodynastes maculatus Tyrannus melancholicus Platycichla flavipes Turdus nudigenis Turdus albicollis Vireo chivi Hylophilus flavipes Seiurus noveboracensis Coereba flaveola Cyanerpes cyaneus Euphonia violacea Thraupis episcopus Thraupis palmarum Tachyphonus rufus Tiaris bicolor Quiscalus lugubris Molothrus bonariensis Psarocolius decumanus Crotophaga ani total no. species35 4 4 0 1 0 0 0 0 0 0 3 2 0 0 0 0 0 0 22 4 0 2 3 0 0 5 1 0 0 0 0 0 27 3 5 2 0 0 1 0 0 4 31 11-EV