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Remov­ing exotic inva­sive plants makes pol­li­na­tion more efficient

pub­lished 05 Feb­ru­ary 2017 | mod­i­fied 05 Feb­ru­ary 2017

Restora­tion of native ecosys­tems strength­ens the resilience and func­tion of pol­li­na­tion networks

Remov­ing exotic plant species has a much greater impact on ecosys­tems than pre­vi­ously thought. Pol­li­na­tion processes become more effi­cient, and the pol­li­na­tion net­work soon becomes more resilient. These are the find­ings of a major field study car­ried out on the Sey­chelles, and are pub­lished online on 30 Jan­u­ary in the sci­en­tific jour­nal Nature.

Pitcher plantThe car­niv­o­rous Pitcher Plant needs ani­mals as pol­li­na­tors and as food.
Image credit: Christo­pher Kaiser-​Bunbury /​TU Darmstadt

Ecosys­tem restora­tion often focuses on plant species, explains Dr Christo­pher Kaiser-​Bunbury, mem­ber of the Eco­log­i­cal Net­works Study Group of the TU Darm­stadt Biol­ogy Depart­ment, and lead author of the arti­cle “Ecosys­tem restora­tion strength­ens pol­li­na­tion net­work resilience and func­tion”. An estab­lished method was used to remove all non-​native plants that were becom­ing too dom­i­nant and inva­sive. “Up until this point, how­ever, we did not know whether inter­fer­ing with the veg­e­ta­tion in this way would have any effect on pol­li­na­tors and, in turn, on impor­tant processes within the ecosystem.”

This very ques­tion was the focus of biol­o­gists in a research project car­ried out on eight insel­bergs on the Sey­chelles’ largest island, Mahé. In a defined area on four insel­bergs, all exotic flora – such as cin­na­mon, euca­lyp­tus and Prune de France – was removed, leav­ing behind the orig­i­nal, native veg­e­ta­tion. For com­par­i­son, the veg­e­ta­tion of four other insel­bergs was left untouched.

The study shows that dam­age within an ecosys­tem is reversible, at least to a cer­tain extent
Dr. Christo­pher Kaiser-​Bunbury, lead author, Eco­log­i­cal Net­works, Depart­ment of Biol­ogy, TU Darm­stadt, Germany »

Kaiser-​Bunbury and his team observed the plants, and counted and cat­a­logued the pol­li­nat­ing ani­mals – bees, wasps, flies, bee­tles, moths, but­ter­flies, birds and geckos – over a period of eight months from Sep­tem­ber 2012 to May 2013. The result­ing data reveals the sig­nif­i­cant impact of veg­e­ta­tion restora­tion. On the restored insel­bergs, the team noted up to 22% more pol­li­na­tor species, with pol­li­na­tors vis­it­ing plants 23% more fre­quently. Plants pro­duced around 17% more flow­ers, which cor­re­lated with a sig­nif­i­cantly higher fruit yield.

Gen­er­alised pol­li­na­tors visit a wider vari­ety of plant species
These observed find­ings allow researches not only to draw con­clu­sions from the direct species pat­terns, but they also pro­vide an insight into the under­ly­ing inter­ac­tions within the ecosys­tem. Accord­ing to Kaiser-​Bunbury, restora­tion improved the qual­ity of pol­li­na­tion. Plants needed fewer vis­its from pol­li­na­tors to pro­duce a higher pro­por­tion of fruit to develop. The inter­ac­tion between plants and pol­li­na­tors also became more com­plex: pol­li­na­tor species in the restored sys­tems were more gen­er­alised than those in the non-​restored insel­bergs. Gen­er­ally speak­ing, pol­li­na­tors were con­sid­er­ably less selec­tive and vis­ited more plant species in the restored com­mu­ni­ties. “Our exper­i­ments sug­gest that restora­tion results in func­tion­ally diverse and stronger plant and ani­mal net­works,” explains Kaiser-​Bunbury. “The study shows that dam­age within an ecosys­tem is reversible, at least to a cer­tain extent.” Pat­terns to sup­port this con­clu­sion were observed by Kaiser-​Bunbury and his team just a few months after restora­tion work had been car­ried out.

Eco­log­i­cal mem­ory and restor­ing the bal­ance
One of the fac­tors that enable the sys­tem to respond rapidly to restora­tion could be the con­cept of “eco­log­i­cal mem­ory”, as Kaiser-​Bunbury explains. The high pol­li­na­tion rate may be due to inter­ac­tions between dif­fer­ent pol­li­na­tor species that show func­tional sim­i­lar­i­ties to the orig­i­nal com­mu­nity and processes. This explains how they can quickly and effi­ciently react to any dis­tur­bance. It is also pos­si­ble that pol­li­na­tors from the sur­round­ing forests are able to return to insel­bergs once they have been restored, to visit native flow­ers. Because the over­growth of exotic plants has been removed, pol­li­na­tors can find and approach native flow­ers more eas­ily. The thinned veg­e­ta­tion also means that pol­li­na­tors lose less pollen as they move from one plant to another.

When restora­tion involves the removal of exotic species it allows native plants to get more light, water and nutri­ents, the lat­ter of which are scarce on the Sey­chelles. This study goes beyond these direct effects of restora­tion. The authors show that there is a clear rela­tion­ship between the com­po­si­tion of the eco­log­i­cal net­work and its over­all func­tions. Accord­ing to Kaiser-​Bunbury, restora­tion does not aim to return a degraded area to its orig­i­nal state. “We demon­strate that restora­tion plays a role in reac­ti­vat­ing nat­ural processes. In other words, it’s a help­ing hand to restore bal­ance to the system.”

(Source: Tech­nis­che Uni­ver­sität Darm­stadt press release, 31.01.2017)

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