Increased resistance towards generalist herbivory in the new range of a habitat‐forming seaweed

Abstract

Abstract. Interactions between plants and their biotic environment can drastically change during rangeexpansion and result in rapid adaptive evolution of plant traits. According to the influential evolution of increased competitive ability (EICA) hypothesis escape from specialist natural enemies will lead to a reduction in defense levels, but the way in which generalist consumers in the new ranges affect the evolution of plant defenses remains poorly understood. We conducted a four month controlled environment experiment to examine if the high densities of the generalist herbivore Idotea baltica in the Baltic Sea have selected for increased grazer-resistance in Fucus vesiculosus, a North Atlantic seaweed that has expanded into large parts of the brackish Baltic Sea. Genetic analysis using microsatellites showed that the sampled populations are genetically distinct, which strongly suggests that traits under divergent selection may readily diverge and populations evolve local adaptations. Feeding trials and measurements of defense metabolites, i.e., phlorotannins, showed that F. vesiculosus from the Baltic Sea was least preferred and contained more than 50% higher constitutive levels of phlorotannins than conspecifics in the North Sea (Skagerrak), while algae from the Ö resund, one of the sounds connecting the Baltic Sea to the Skagerrak, had intermediate resistance levels both in terms of grazer preference and phlorotannin levels. These results suggest that the higher grazing pressure on F. vesiculosus expanded into the Baltic Sea has resulted in the evolution of increased resistance towards generalist grazing.

The chemical defences of the invasive alga Sargassum muticum (Yendo) Fensholt correlate to mesoherbivore diversity, but not to time-since-invasion

“…Although phlorotannins are found in many fucoids, the abundances observed in S. muticum were particularly high; comparable to those found in the highly grazer-resistant Ascophyllum nodosum Kurr & Davies, 2017b). Pronounced invasion success has been attributed to chemical defences in other species of algae (Forslund et al, 2010;Enge et al, 2012;Nylund et al, 2012;Svensson et al, 2013) and plants (Van Kleunen & Schmid, 2003;Leger & Forister, 2005;Huang et al, 2010;Lind & Parker, 2010), and may also explain the success of S. muticum outside of its native range (Engelen et al, 2015). Although a large body of evidence suggests that a high diversity of native plants often affords better resistance to invasions by controphics (see Theoharides & Dukes, 2007), surprisingly few studies have investigated the diversity of native enemies on the topdown-pressure experienced by invasive species (Maron & Vilá, 2001;Agrawal & Kotanen, 2003).…”

Evolutionary responses to climate change in a range expanding plant

“…In range expanding algae, resistance towards generalist herbivores was lower in core populations to populations in the extended range (Nylund et al 2012). In contrast, resistance against a generalist caterpillar was higher in core populations of the range expanding plant Bunias orientalis (Fortuna et al 2014).…”

Mapping present and future potential distribution patterns for a meso‐grazer guild in the Baltic Sea

“…The overlapping habitat ranges of Idotea spp. and F. vesiculosus have forced a selection for high grazing-resistance in F. vesiculosus during colonization of the BS (Nylund et al ., 2012). This seems not to be the case for F. radicans .…”

“…The combination of serious changes in nutrient load, oxygen concentration, SSS and SST predicted for the Baltic Proper, and a northern/north-eastern shift of species, may result in genetic separation of local populations. Phylogeographical studies have indicated that Baltic populations in general (Johannesson & Andr e, 2006), and populations of Idotea spp., F. vesiculosus and G. aculeatus in particular, have lost genetic variation in contrast to populations from the Atlantic (Tatarenkov et al, 2007;Nylund et al, 2012;DeFaveri et al, 2013;Leidenberger, 2013). A large population size with a high level of genetic variation can increase the capacity to adapt to environmental changes, in comparison with small isolated populations with low genetic diversity (Johannesson et al, 2011).…”

“…In the Baltic Proper, grazing by Idotea can lead to a dramatic decline of Fucus populations (Nilsson et al, 2004). The high grazing pressure of Idotea has driven selection for increased grazer resistance in Baltic populations of F. vesiculosus compared with populations outside the BS (Nylund et al, 2012). Idotea is also suggested to limit the southern distribution range of F. radicans in the BS through intense grazing effects (Gunnarsson & Berglund, 2012).…”