Trends in Ecology & Evolution
OpinionUniting niche differentiation and dispersal limitation predicts tropical forest succession
Section snippets
Classic theories of succession do not incorporate seed dispersers
In tropical forests, where 65–95% of tree species depend on animals for seed dispersal [1,2], the assembly of plant and animal communities is intrinsically linked during the first decades of succession [3]. However, classical theories of tropical succession are founded in temperate ecology and focus on plant community assembly while neglecting biotic drivers that modulate species turnover (e.g., seed dispersal) [4., 5., 6.]. Contemporary succession theories emphasize plant functional traits and
Uniting niche-based models with dispersal limitation
We propose that by combining niche-based models with concepts of dispersal limitation we can provide a better theoretical explanation of tropical forest succession [9]. Niche differentiation explains how plant species exploit shifting resource availability over succession via tradeoffs [5,13]. However, community reassembly may be thwarted at the onset or during succession because seeds are unable to reach a site and establish [9,13,14]. Thus, dispersal limitation may be a stronger determinant
Components of dispersal limitation: seed and dispersal vector
Dispersal limitation can be divided into two core components: the reproductive unit that is dispersed (seed or diaspore) and the vector of dispersal (animal or wind). Typically, seed limitation is assessed as source limitation and dispersal limitation [16,19]. We suggest a nuanced way to address the same phenomena by focusing on ‘source limitation’ and ‘disperser limitation’ because it is the interaction between the seed and the vector that results in the movement and eventual establishment of
Source limitation
Source limitation proposes that insufficient seeds are produced to saturate available recruitment sites [19]. Seed availability in secondary forests is dictated by the abundance, location, and species composition of adult trees in the surrounding landscapes [3]. Seed source limitation is higher in landscapes with lower tree cover, increased distance to old-growth forest, and higher levels of ongoing disturbance [13,14]. Farmed tropical landscapes often retain considerable tree cover; a recent
Disperser limitation
Seed dispersal initiates secondary forest succession in deforested tropical landscapes, with dispersers transporting seeds from sources and depositing them in areas where seed banks are depauperate [33]. Wind, frugivorous bats, and small birds are the key seed dispersers in early succession (Figure 1) because they transport many seeds to regeneration sites [1,24,31]. In degraded landscapes, wind-dispersed species tend to dominate seed rain in pastures and can be important to initiate
Establishment limitation
Seedling establishment may limit forest regeneration if the site is unsuitable for seed germination and early seedling survival [64]. Establishment is dictated by the biotic and abiotic conditions of the microsite and the seed and seedling traits of tree species [19]. For example, we see selection for fast-growing species dispersed into open pastures because they are more likely than slower-growing species to outcompete grasses and shrubs for available light [29,52]. The high light and low
Moving forward using the Source-Disperser Limitation Framework
Given that tropical forest succession hinges on overcoming limitations in seed sources and dispersers, we propose the Source-Disperser Limitation Framework (SDLF) to better understand succession in tropical agricultural landscapes. The central tenet of the SDLF is that the interaction between seed sources and dispersers controls seed movement to sites starting or undergoing regeneration (Figure 2A). Seed movement from sources to microsites by dispersers is the primary limitation, since seedling
Concluding remarks
The unification of niche differentiation and dispersal limitation can provide a mechanistic explanation of succession in the tropics. The study of seed dispersal has a long history in the context of tropical secondary forests [1,2,16,29,31]; here we are proposing an integration of seed dispersal limitation with niche differentiation to better predict tropical forest succession. Specifically, we need a mechanistic approach to understand what limits seed arrival (seed sources and dispersers) and
Acknowledgments
We thank Luke Browne, Liza Comita, Meg Crofoot, Dina Dechmann, Ana Cristina Palma, and Pablo Stevenson for comments on earlier drafts of this article. We also thank three anonymous reviewers and Andrea Stephens; their comments helped us improve the ideas proposed in this opinion article. D.D. was supported by a Resesearch Fellowship from the Alexander von Humboldt Foundation, Germany.
Declaration of interests
The authors have no interests to declare.
Glossary
- Disperser limitation
- insufficient numbers or types of dispersers that preclude the arrival of seeds at recruitment sites. Disperser limitation can also occur due to changes in behavior (e.g., animals actively avoid areas with low forest cover).
- Endozoochory
- seed dispersal via ingestion by vertebrate animals.
- Extirpations
- local extinctions of dispersers, potentially due to predation, hunting, or habitat loss, among other factors.
- Metabarcoding
- using short sequences of DNA from specified genes of
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