Forest fragmentation severs mutualism between seed dispersers and an endemic African tree

Abstract

Because bird species are lost when forests are fragmented into small parcels, trees that depend on fruit-eating birds for seed dispersal may fail to recruit seedlings if dispersal agents disappear. We tested this prediction in rainforest in the East Usambara Mountains of Tanzania, by using the endemic tree Leptonychia usambarensis (Sterculiaceae) and birds that disperse its seeds. We investigated bird abundance and Leptonychia dispersal ecology in fragments isolated for >70 yr, as compared with 3,500 ha of continuous forest. Birds that dispersed Leptonychia seeds in continuous forest were rare or absent in small fragments, where fewer seeds were removed from each tree, far fewer seedlings occurred >10 m from parent trees, and far more seedlings occurred in dense aggregations under parental crowns. Overall, our samples showed that fewer juvenile Leptonychia recruited in fragments than in continuous forest. We provide solid evidence that deficient dispersal due to habitat fragmentation seriously impacts the reproductive cycle of a tropical bird-dispersed tree.

Fig. 1.

Seed removal differs between trees in the continuous forest and small fragments (Mann–Whitney U test, U = 31.5, P < 0.01). This difference remains for 10 continuous forest trees clearly comparable in crop size to fragment trees (Mann–Whitney U test, U = 22, P < 0.05).

Fig. 2.

Summary of log-linear analyses of recruitment of Leptonychia in relation to fragment size, proximity to tree, and stage. (A) Fragment size interacted with proximity to tree because small fragments had more individuals near than far from trees, a pattern less striking in continuous forest. (B) Absolute numbers for each stage were lower in fragments than continuous forest. (C) Proximity to tree and stage had a significant interaction because disproportionate numbers of seedlings occurred <10 than 10–20 m from parent trees for all sites combined whereas juveniles showed no such differences. Arrows indicate direction of interaction between the operative and dependent variables.

Fig. 3.

Seedling and juvenile abundances under (<10 m) and away (10–20 m) from parent tree crowns in small fragments and continuous forest. Seedlings were equally common under parent trees in small fragments and continuous forest but were much less common 10–20 m away from parent trees in fragments than in continuous forest (χ² with a Yates correction = 20.83, df = 1, P < 0.00001). Far more juveniles occurred away from parental trees in continuous forest than in fragments (χ² with a Yates correction = 11.34, df = 1, P < 0.001).