The Role of Gulls in Spreading Olive Seeds Across Balearic Islands

The spread of the olive tree through­out the Balearic Islands is more closely linked to the thriv­ing local gull pop­u­la­tion than pre­vi­ously thought, a new study indi­cates.

Researchers found that these birds sig­nif­i­cantly con­tribute to the dis­per­sal of olive seeds, which are found inside the pit, across the Mediterranean arch­i­pel­ago, with some seeds trav­el­ing con­sid­er­able dis­tances.

Birds that feed pri­mar­ily on fruit, known as fru­gi­vores, play a cru­cial role in the dis­tri­b­u­tion and ger­mi­na­tion of sev­eral plant species. However, these birds are rel­a­tively rare on the Spanish islands.

Traditionally not con­sid­ered fru­giv­o­rous, gulls usu­ally eat insects, fish, marine inver­te­brates and small mam­mals.

Still, an inter­na­tional team of sci­en­tists inves­ti­gat­ing the gulls’ role in seed dis­per­sal found that the birds con­tributed to the long-dis­tance spread of local olive seeds, poten­tially facil­i­tat­ing the col­o­niza­tion and expan­sion of the vari­ety.

In a recent study pub­lished in the Journal of Biogeography, the researchers exam­ined the behav­ior of the yel­low-legged gull (Larus micha­hellis) and two types of olives: the domes­tic Olea europaea and the wild Olea europaea sylvestris.

“The two fleshy fruit eco­types exhibit dif­fer­ent fruit sizes and spa­tial dis­tri­b­u­tions,” the researchers wrote. ​“Large domes­tic olive trees are pri­mar­ily found in the olive fields of human-inhab­ited areas, whereas the smaller wild olive tree, a dom­i­nant species in the Mediterranean basin, is widely dis­trib­uted in the wild areas (maquis) of the Balearic Islands.”

The team researched all four major arch­i­pel­ago islands and vis­ited sur­round­ing small islets.

They crafted new spa­tial data mod­els com­bin­ing GPS track­ing data, gut pas­sage time and seed via­bil­ity. That led to the devel­op­ment of mod­els for seed dis­per­sal.

Monitored gulls ingested olives on one island, then moved to another loca­tion and excreted the seeds there.

Through mon­i­tor­ing gull behav­ior, the researchers observed that domes­tic olive seeds were dis­persed up to a max­i­mum dis­tance of 12.57 kilo­me­ters, while wild seeds reached up to 7.67 kilo­me­ters. Some olive seeds were dropped into the sea.

Gulls tended to trans­port domes­tic olive seeds from larger to smaller islands, where gull colonies reside, whereas wild olives were dis­persed in more var­ied direc­tions.

The Balearic Islands host sev­eral olive cul­ti­vars, and olive cul­ti­va­tion has a long-stand­ing tra­di­tion in the arch­i­pel­ago, prob­a­bly dat­ing back to Roman rule in the 2^nd cen­tury BCE. Millennia-old olive trees can be found in sev­eral loca­tions, mainly in Mallorca.

The researchers high­lighted that seabirds are cru­cial com­po­nents of island ecosys­tems, and their daily move­ments can affect plant com­mu­ni­ties by increas­ing nitro­gen avail­abil­ity in the soil through food trans­port or seabird guano.

While gulls’ feed­ing habits largely depend on resource dis­tri­b­u­tion, there was pre­vi­ously no data on their effec­tive­ness or the dis­tances they cover as seed dis­per­sal agents.

According to the researchers, the study under­scores the impor­tance of gulls as vec­tors for the long-dis­tance dis­per­sal of olives and other pit­ted fruits in island ecosys­tems, where spe­cial­ized large fru­gi­vores are absent.