Trends in Ecology & Evolution
ReviewMigratory Birds as Global Dispersal Vectors
Section snippets
The Need to Quantify Long-Distance Dispersal
Long-distance dispersal (LDD; see Glossary) allows organisms to cross population boundaries, move among habitat patches, and colonize remote areas, thus having important ecological, biogeographical and evolutionary consequences 1, 2, 3. Its study and quantification have been, however, hindered by the low frequency of LDD events, the difficulty of tracking propagules over large geographic scales, and the unpredictable nature of LDD vectors operating at such scales (such as ocean currents,
Overlooked Vectoring Potential of Migratory Birds
Birds are probably the most abundant and competent vertebrate vectors [23]. They can disperse propagules both internally, following voluntary or involuntary ingestion of propagules (endozoochory), and externally, following attachment of propagules to feathers or legs (epizoochory). Birds also transport entire organisms, including pathogens and parasites, in both ways 24, 25 (Box 1).
Among birds, migratory species can be key LDD vectors because (i) they move seasonally over broad spatial scales
A Framework for the Study of LDD by Migratory Birds
Propagule dispersal comprises three consecutive phases: initiation (propagule uptake by the vector), transport (propagule movement along with the vector), and deposition (propagule retrieval following transport) [4]. To understand the various determinants of each of these three phases, it is particularly useful to consider the movement ecology framework proposed by Nathan et al. (2008) [38], which comprises four basic components: internal state, motion capacity, navigation capacity, and
Effectiveness of LDD
The realization of dispersal depends on its effectiveness, that is, on the combination of successful transportation and deposition of viable propagules, plus their successful establishment and reproduction. Such effectiveness is critically related to the gains and costs involved in reaching distant habitat patches through LDD (e.g., [49]), and ultimately depends on the constraints posed by a combination of abiotic and biotic filtering of arriving propagules. The expected establishment
Ecological Consequences of LDD
Migratory birds can promote the movement and connectivity of many taxa over extremely large spatial scales, with important ecological consequences. They can promote large-scale connectivity in anthropogenic (e.g., forest–pasture mosaics) and naturally isolated (e.g., lakes and wetlands, mountain tops) landscapes 59, 60, as well as the colonization of distant habitat patches, including those in different continents 24, 61 or hemispheres 16, 17 and on oceanic islands 29, 62, and thus contributing
Hypothesis Testing and Model Validation
LDD predictions might be tested using a combination of direct observations and analysis of their ecological consequences. Direct observations of LDD (e.g., [29]) might be achieved by examining birds arriving from long-distance flights, such as those killed while on active migration by predators, human hunters, or collision with man-made structures (e.g., lighthouses or wind turbines). The origin of collected propagules might then be traced using stable isotopes or genetic markers (see [69] for
Concluding Remarks and Future Directions
A wide range of organisms uses the LDD services provided by birds; hence, more accurate LDD estimations might be achieved by incorporating the birds’ vectoring potential, and thus the full dispersal potential of vectored organisms. Studies of diaspore (e.g., seed) dispersal and pathogen dispersal have traditionally been studied in parallel research lines, but studying the common and distinct processes underlying their dispersal might contribute to and cross-fertilize both research lines. The
Acknowledgments
We thank Ran Nathan and one anonymous reviewer for useful discussions. This study was supported by project CGL2015-65055-P from Ministerio de Economía y Competitividad (Spain) and RECUPERA 2020, Hito 1.1.1, cofinanced by the European Regional Development Fund (FEDER).
Glossary
- Endozoochory
- dispersal of propagules inside an animal vector.
- Epizoochory
- dispersal of propagules attached to an animal vector.
- Disjunct distribution
- species showing large discontinuities in their distribution (e.g., transoceanic and bipolar distributions).
- Dispersal kernel
- a probability distribution of dispersal distances and the associated spatial distribution of dispersal units.
- Dispersal vector
- any agent transporting propagules (e.g., birds or wind).
- Long-distance dispersal
- dispersal acting beyond
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