Tritrophic interactions among plants, herbivores and plant mutualists

Plants form the first trophic level in terrestrial ecosystems and provide energy and nutrients to higher trophic levels. Herbivores, frugivores or fungal endophytes use plants directly, while predators consume plants indirectly by consuming herbivores. However, species are often simultaneously interacting with antagonistic and mutualistic partners at various trophic levels. For this reason, the outcomes of species interactions can indirectly affect other species in the community. The aim of my thesis was to study tritrophic interactions between plants, their antagonists, such as insect or avian herbivores, and mutualists such as insectivorous or frugivorous birds and symbiotic endophyte fungus. In Chapters I-III I concentrated on the interactions among plants, herbivores and protective plant mutualists. In the first two Chapters, I investigated whether birds use volatile organic compounds or changes in visual properties of leaves from herbivore-damaged trees as foraging cues. I found that trees respond to herbivore damage both locally and systemically, but the olfactory foraging cue hypothesis was not supported. Instead, herbivory affected visual properties of leaves viewed by birds, although these changes may be in the limit of detection to them. In addition, my results indicate that cryptically coloured herbivores may have slightly better camouflage when on herbivore-damaged trees, although the herbivores are discriminable to birds against the leaves of the host plant regardless of the treatment (Chapter II). In Chapter III I studied the relationship between plants and protective fungal symbiont by testing whether systemic endophyte fungi can protect grasses against wild avian grazers. In this study I used two grass species, red fescues and tall fescues, which differ in texture. Both species have naturally both endophytic and non-endophytic individuals. I found that softer red fescue was preferred over coarse tall fescue, regardless of the endophyte status. In Chapter IV I studied the interaction between plants, herbivores and seed-dispersing mutualists. I tested whether insect herbivory causes allocation cost to fleshy fruiting plants by affecting ripening or chemical composition of berries. I also investigated potential ecological cost of herbivory measured as probability for ripe berries to be removed by frugivorous birds. I found that berries in undamaged ramets neighbouring herbivore-damaged conspecifics had lower probability to be removed by frugivores, although herbivory did not affect ripening or chemical composition of berries. This indicates that in clonal plants, herbivore damage may cause priming effect on neighbouring ramets, which can affect plant mutualists. The results of this thesis extend current knowledge about plant responses to herbivory, and also how these responses affect plant mutualists. In addition, my thesis provides information about the foraging behaviour of herbivores and plant mutualists. This kind of knowledge is essential for biological control and agricultural procedures, as well as on the planning of urban grass areas.Kasvit muodostavat ensimmäisen ravintoketjun tason eli trofiatason maaekosysteemeissä, ja näin ollen toimivat ravinnon ja energianlähteinä ylemmille trofiatasoille. Kasvinsyöjät eli herbivorit, hedelmänsyöjät eli frugivorit ja symbionttiset endofyyttisienet ovat suoraan riippuvaisia kasveista, kun taas herbivoreja syövien petojen riippuvuus kasveista on epäsuoraa. Eri trofiatasoilla olevat lajit ovatkin jatkuvassa vuorovaikutuksessa keskenään, minkä vuoksi kahden lajin väliset vuorovaikutussuhteet voivat vaikuttaa myös muihin lajeihin. Väitöskirjassani olen tarkastellut usean trofiatason välisiä yhteyksiä kasvien ja niille haitallisten ja hyödyllisten eliöiden, eli antagonistien ja mutualistien, välillä. Esimerkkeinä antagonisteista käytin hyönteis- ja lintuherbivoreja, kun taas mutualisteina käytin hyönteissyöjälintuja, endofyyttisieniä ja siementenlevittäjälintuja. Tutkimuksissa I-II tutkin herbivorian aiheuttamien kasvista haihtuvien yhdisteiden, ja lehdissä tapahtuvien visuaalisten muutosten vaikutusta hyönteissyöjälintujen ravinnonhankintakäyttäytymiseen. Kasvit reagoivat sekä paikallisesti että kokonaisvaltaisesti herbivorian aiheuttamiin vaurioihin. En kuitenkaan löytänyt selvää tukea sille, että haihtuvat yhdisteet toimisivat hajuvihjeinä linnuille. Sen sijaan sain selville, että kasvinsyöjien aiheuttamat vauriot voivat vaikuttaa kasvin vahingoittumattomien lehtien ulkonäköön, joskin nämä muutokset voivat olla lintujen visuaalisen erotuskyvyn rajoilla. Herbivorian aiheuttamat muutokset kasvissa voivat lisäksi tehdä kryptisestä herbivorista vähemmän näkyvän linnuille, vaikka linnut todennäköisesti erottavatkin herbivorin lehtiä vasten riippumatta siitä onko kasvia vahingoitettu vai ei (II). Tutkimuksessa III tutkin voivatko endofyyttisienet suojella heiniä hanhien laidunnukselta. Tässä tutkimuksessa käytin kahta heinälajia, jotka eroat karkeudeltaan. Molemmilla lajeilla osa yksilöistä oli luontaisesti endofyytillisiä ja osa endofyytittömiä. Tulokseni osoittavat, että hanhet suosivat pehmeämpää ruoholajia karkean ruohon sijaan riippumatta endofyyttisienen läsnäolosta. Tutkimuksessa IV testasin aiheuttaako hyönteisherbivoria allokaatiokustannuksia kasville vaikuttamalla marjojen kypsymiseen tai biokemialliseen koostumukseen. Tutkin myös mahdollisia herbivorian aiheuttamia ekologisia kustannuksia kasville, joita mittasin marjojen todennäköisyytenä tulla siementenlevittäjien syömiksi. Tulokseni osoittavat, että marjoilla, jotka kasvoivat varvuissa lähellä herbivorien vaurioittamia varpuja, oli pienempi todennäköisyys tulla siementenlevittäjien syömiksi. Herbivoria ei vaikuttanut marjojen kypsymiseen tai biokemialliseen koostumukseen. Tämä viittaa siihen, että klonaalisilla kasveilla herbivoria voi aiheuttaa puolustusreaktion vahingoittumattomissa naapurikasveissa, mikä voi vaikuttaa myös kasvien mutualisteihin. Tämän väitöskirjan tutkimukset syventävät ymmärrystämme siitä, miten kasvit reagoivat herbivoriaan, ja miten nämä reaktiot vaikuttavat kasvien mutualisteihin. Tutkimukseni lisäävät myös tietoa herbivorien ja kasvien mutualistien ravinnonhankintakäyttäymisestä. Tällainen tieto on oleellista niin biologisen torjunnan kuin maataloudenkin kannalta, ja sitä voidaan soveltaa myös viheralueiden suunnittelussa.Siirretty Doriast

Foraging Preferences of Barnacle Geese on Endophytic Tall and Red Fescues

Many grasses (Poaceae) have symbiotic fungal endophytes, which affect livestock by producing unpalatable or harmful secondary compounds. Less is known about the repelling effects of fungal endophytes on avian grazers despite potential wildlife management implications. Herbivorous goose (Branta spp.) species may become a nuisance in recreational use areas via fecal littering. Planting these areas with grasses that avian grazers avoid may help mitigate this damage. In 2016, we studied the foraging preference of the barnacle geese (B. leucopsis) with endophytic (E+) or endophyte-free (E-) red fescue (Festuca rubra) and/or tall fescue (Schedonorus phoenix) in 2 sites in Finland that had a history of nuisance geese damage. In the high grazing pressure site, we planted both grass species, while in the low grazing pressure site only tall fescue was used. Geese preference was measured as the percentage of the area grazed, the height of the residual grass grazed, and the number of fecal droppings in the grass plots. Geese foraging did not differ between E- and E+ grasses, but red fescues were preferred over tall fescues. This supports previous findings that tall fescues or other coarse species could reduce the attractiveness of recreational areas to geese

Differences in incubation behaviour and niche separation of two competing flycatcher species

Food availability sets the stage for incubation behaviour of a female bird and thereby indirectly determines the nest temperature, which in turn affects development and metabolism of avian embryos. Changes in development and metabolism in turn are known to influence offspring's ability to adjust to environmental changes later in life. However, few studies have investigated the role of interspecific differences in incubation behaviour in relation to niche separation between competing sibling species. We studied the effects of habitat quality (in terms of caterpillar availability) on incubation behaviour of two ecologically similar and closely related species, collared and pied flycatchers (Ficedula albicollisandF. hypoleuca), in their hybrid zone on the island of oland, Sweden. Even though both species prefer caterpillar-rich deciduous forests as nesting sites, collared flycatchers, whose nestlings have higher energetic demands, are able to nest only in deciduous forests, whereas pied flycatchers have more flexible habitat requirements. Overall, higher food availability was associated with increased nest attendance, higher incubation temperature and a lower number of foraging trips across species. In addition, collared flycatchers had more frequent and shorter foraging trips across habitat types, allocated more heat to eggs and therefore maintained higher nest temperatures compared to pied flycatchers. We argue that the higher heat allocation or the need to maintain a higher nest temperature for embryo development may constrain collared flycatchers to focus on relatively more profitable prey. Our results highlight the importance of considering incubation behaviour in the context of understanding species differences in niche use. Significance statement Niche separation plays an important role in mitigating effects of competition between closely related species. Whether species differences in incubation behaviour relate to differences in niche use remains unknown. We compared incubation behaviour of two sympatric flycatcher species that differ in sensitivity to food availability. The competitively more dominant and larger species, the collared flycatcher, whose nestlings are more sensitive to food shortages, made more frequent foraging trips but allocated more heat to eggs, leading to higher nest temperature despite lower nest attendance, compared to pied flycatchers. These interspecific differences may be a result of differences in embryo sensitivity or female physiology and contribute to the niche separation between the species, which in turn can facilitate coexistence.Peer reviewe

Effects of Insect Herbivory on Bilberry Production and Removal of Berries by Frugivores

The evolutionary purpose of a fleshy fruit is to attract seed dispersers and get the seeds dispersed by frugivorous animals. For this reason, fruits should be highly rewarding to these mutualists. However, insect herbivory can alter plant reproductive success e.g. by decreasing fruit yield or affecting the attractiveness of the fruits to mutualistic seed dispersers. Under natural conditions, we tested the effects of experimental larval-defoliation on berry ripening and consumption of a non-cultivated dwarf shrub, the bilberry (Vaccinium myrtillus L.), which produces animal-dispersed berries with high sugar and anthocyanin concentration. Bilberry ramets with high fruit yield were most likely to have their berries foraged, indicating that frugivores made foraging choices based on the abundance of berries. Moreover, the probability for berries being foraged was the lowest for non-defoliated ramets that grew adjacent to larval-defoliated ramets, even though larval-defoliation did not affect the biochemical composition (total concentrations of anthocyanins, sugars and organic acids) or the probability of ripening of berries. We hypothesise that the lower probability for berries being foraged in these ramets may be a consequence of rhizome- or volatile-mediated communication between ramets, resulting in a priming effect of the herbivore defence and lower attractiveness of the non-defoliated ramets.Peer reviewe

'Resistance Mixtures' Reduce Insect Herbivory in Strawberry (Fragaria vesca) Plantations

The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no "silver bullet " available among nonchemical tools. Integrating several plant protection approaches may thus be needed for efficient pest management. Recently, increasing the genetic diversity of plantations via cultivar mixing has been proposed as a possible method to reduce pest damage. However, previous studies have not addressed either the relative efficiency of exploiting cultivar mixing and intrinsic plant herbivore resistance or the potential utility of combining these approaches to increase cropping security. Here, using a full factorial experiment with 60 woodland strawberry plots, we tested for the relative and combined effect of cultivar mixing and intrinsic plant resistance on herbivore damage and yield. The experiment comprised two levels of diversity ( "high " with 10 varieties and "low " with two varieties) and three levels of resistance ( "resistant " comprising only varieties intrinsically resistant against strawberry leaf beetle Galerucella tenella; "susceptible " with susceptible varieties only; and "resistance mixtures " with 50:50 mixtures of resistant and susceptible varieties). The experiment was carried out over two growing seasons. Use of resistant varieties either alone or intermixed with susceptible varieties in "resistance mixtures " reduced insect herbivory. Interestingly, resistant varieties not only reduced the mean damage in "resistance mixtures " by themselves being less damaged, but also protected intermixed susceptible varieties via associational resistance. The effect of higher genetic diversity was less evident, reducing herbivory only at the highest level of herbivore damage. In general, herbivory was lowest in plots with high diversity that included at least some resistant varieties and highest in low diversity plots consisting only of susceptible varieties. Despite this, no significant difference in yield (fruit biomass) was found, indicating that strawberry may be relatively tolerant. Our results demonstrate that combined use of high genetic diversity and resistant varieties can help reduce pest damage and provide a useful tool for sustainable food production. "Resistance mixtures " may be particularly useful for sensitive food crops where susceptible varieties are high yielding that could not be completely replaced by resistant ones.</p

‘Resistance Mixtures’ Reduce Insect Herbivory in Strawberry (Fragaria vesca) Plantations

The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no “silver bullet” available among nonchemical tools. Integrating several plant protection approaches may thus be needed for efficient pest management. Recently, increasing the genetic diversity of plantations via cultivar mixing has been proposed as a possible method to reduce pest damage. However, previous studies have not addressed either the relative efficiency of exploiting cultivar mixing and intrinsic plant herbivore resistance or the potential utility of combining these approaches to increase cropping security. Here, using a full factorial experiment with 60 woodland strawberry plots, we tested for the relative and combined effect of cultivar mixing and intrinsic plant resistance on herbivore damage and yield. The experiment comprised two levels of diversity (“high” with 10 varieties and “low” with two varieties) and three levels of resistance (“resistant” comprising only varieties intrinsically resistant against strawberry leaf beetle Galerucella tenella; “susceptible” with susceptible varieties only; and “resistance mixtures” with 50:50 mixtures of resistant and susceptible varieties). The experiment was carried out over two growing seasons. Use of resistant varieties either alone or intermixed with susceptible varieties in “resistance mixtures” reduced insect herbivory. Interestingly, resistant varieties not only reduced the mean damage in “resistance mixtures” by themselves being less damaged, but also protected intermixed susceptible varieties via associational resistance. The effect of higher genetic diversity was less evident, reducing herbivory only at the highest level of herbivore damage. In general, herbivory was lowest in plots with high diversity that included at least some resistant varieties and highest in low diversity plots consisting only of susceptible varieties. Despite this, no significant difference in yield (fruit biomass) was found, indicating that strawberry may be relatively tolerant. Our results demonstrate that combined use of high genetic diversity and resistant varieties can help reduce pest damage and provide a useful tool for sustainable food production. “Resistance mixtures” may be particularly useful for sensitive food crops where susceptible varieties are high yielding that could not be completely replaced by resistant ones

'Resistance Mixtures' Reduce Insect Herbivory in Strawberry (Fragaria vesca) Plantations

The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no "silver bullet " available among nonchemical tools. Integrating several plant protection approaches may thus be needed for efficient pest management. Recently, increasing the genetic diversity of plantations via cultivar mixing has been proposed as a possible method to reduce pest damage. However, previous studies have not addressed either the relative efficiency of exploiting cultivar mixing and intrinsic plant herbivore resistance or the potential utility of combining these approaches to increase cropping security. Here, using a full factorial experiment with 60 woodland strawberry plots, we tested for the relative and combined effect of cultivar mixing and intrinsic plant resistance on herbivore damage and yield. The experiment comprised two levels of diversity ( "high " with 10 varieties and "low " with two varieties) and three levels of resistance ( "resistant " comprising only varieties intrinsically resistant against strawberry leaf beetle Galerucella tenella; "susceptible " with susceptible varieties only; and "resistance mixtures " with 50:50 mixtures of resistant and susceptible varieties). The experiment was carried out over two growing seasons. Use of resistant varieties either alone or intermixed with susceptible varieties in "resistance mixtures " reduced insect herbivory. Interestingly, resistant varieties not only reduced the mean damage in "resistance mixtures " by themselves being less damaged, but also protected intermixed susceptible varieties via associational resistance. The effect of higher genetic diversity was less evident, reducing herbivory only at the highest level of herbivore damage. In general, herbivory was lowest in plots with high diversity that included at least some resistant varieties and highest in low diversity plots consisting only of susceptible varieties. Despite this, no significant difference in yield (fruit biomass) was found, indicating that strawberry may be relatively tolerant. Our results demonstrate that combined use of high genetic diversity and resistant varieties can help reduce pest damage and provide a useful tool for sustainable food production. "Resistance mixtures " may be particularly useful for sensitive food crops where susceptible varieties are high yielding that could not be completely replaced by resistant ones

"Resistance Mixtures" Reduce Insect Herbivory in Strawberry (<i>Fragaria vesca</i>) Plantings

The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no “silver bullet” available among nonchemical tools. Integrating several plant protection approaches may thus be needed for efficient pest management. Recently, increasing the genetic diversity of plantations via cultivar mixing has been proposed as a possible method to reduce pest damage. However, previous studies have not addressed either the relative efficiency of exploiting cultivar mixing and intrinsic plant herbivore resistance or the potential utility of combining these approaches to increase cropping security. Here, using a full factorial experiment with 60 woodland strawberry plots, we tested for the relative and combined effect of cultivar mixing and intrinsic plant resistance on herbivore damage and yield. The experiment comprised two levels of diversity (“high” with 10 varieties and “low” with two varieties) and three levels of resistance (“resistant” comprising only varieties intrinsically resistant against strawberry leaf beetle Galerucella tenella; “susceptible” with susceptible varieties only; and “resistance mixtures” with 50:50 mixtures of resistant and susceptible varieties). The experiment was carried out over two growing seasons. Use of resistant varieties either alone or intermixed with susceptible varieties in “resistance mixtures” reduced insect herbivory. Interestingly, resistant varieties not only reduced the mean damage in “resistance mixtures” by themselves being less damaged, but also protected intermixed susceptible varieties via associational resistance. The effect of higher genetic diversity was less evident, reducing herbivory only at the highest level of herbivore damage. In general, herbivory was lowest in plots with high diversity that included at least some resistant varieties and highest in low diversity plots consisting only of susceptible varieties. Despite this, no significant difference in yield (fruit biomass) was found, indicating that strawberry may be relatively tolerant. Our results demonstrate that combined use of high genetic diversity and resistant varieties can help reduce pest damage and provide a useful tool for sustainable food production. “Resistance mixtures” may be particularly useful for sensitive food crops where susceptible varieties are high yielding that could not be completely replaced by resistant ones