Plant–herbivore interactions: silicon concentration in tussock sedges and population dynamics of root voles

Abstract

Summary1. It has been hypothesized that the induction of silicon (Si)-based plant defence in response to herbivore damage may engender rodent population cycles. Many studies have also considered accumulation of Si as a process controlled by geo-hydrological factors. 2. To test these ideas, we investigated the relationship between concentration of Si in fibrous tussock sedge (Carex appropinquata) and the population density of a major sedge consumer, the root vole (Microtus oeconomus), in field enclosures in natural habitat under a variety of natural water regimes and weather conditions. 3. We found that a high density of voles at the end of summer resulted in the immediate accumulation of Si by rhizomes, followed by accumulation of Si in leaves with a 1-year lag time. The level of river flooding in the same year had an additional impact on Si concentration in rhizomes but did not affect silicification of leaves. 4. Overwinter changes in concentration of Si in sedges were influenced by fluctuations in ambient temperature and the depth of snow cover (multiple freeze-thaw cycles), thus affecting the quality of winter food available for voles. 5. Smaller voles had lower mortality during early winter than large voles, which seemed to be connected with changes in the quality of the autumn rather than the winter food base. Winter survival of voles was not associated with Si concentration in their faeces, however. 6. Our results suggest that changes in Si concentration in fibrous tussock sedge can be induced by changes in vole population density and are also additionally affected by the amount of flooding and weather conditions.

Silicon-based plant defences, tooth wear and voles

“…This textural signal is consistent with the expected seasonal variation in the dietary phytolith content. Studies conducted in the Arctic also show that in spring, M. oeconomus is forced to feed on silicon-rich sedges (genus Carex, mean Si=1.31% of dry matter mass; Hodson et al, 2005; although there is a wide inter-annual variation in the Biebrza Marshes; Wieczorek et al, 2015b) because other food sources are scarce. In summer, the dietary spectrum is wider and includes less abrasive dicotyledons (mean Si=0.43%; Hodson et al, 2005) as well (Tast, 1974;Batzli and Henttonen, 1990;Soininen et al, 2009Soininen et al, , 2013.…”

“…Although the sites are very similar, Barwik is located closer to the mineral edges of a river valley and is characterized by a higher influx of silica, whereas Olszowa Droga is separated from the mineral part of the valley by alder forests on organic soils, and is thus most probably characterized by a lower Si concentration (Struyf et al, 2009). These differences in soil and water Si concentrations may lead to different Si uptake by plants McNaughton et al, 1985;Quigley and Anderson, 2014;Wieczorek et al, 2015b). More soil Si may also imply more grit that would wear the teeth (e.g.…”

“…Plants respond to grazing intensity of the previous year by adjusting the Si content (by 0.6-1 percentage point; Wieczorek et al, 2015b) in their leaves (see also Massey et al, 2008;Reynolds et al, 2012), and voles are in turn negatively affected by increased phytolith levels (Massey and Hartley, 2006;Massey et al, 2008). But how plant phytoliths affect voles is still unclear.…”

“…S1C). Repeated and significant damage to leaves (at least 20% of leaves eaten) is required to induce a response in plants Massey et al, 2007a;Wieczorek et al, 2015b). So, vole density in the increase phase is therefore probably not high enough to induce a significant increase in leaf Si that would result in more abrasive foods during the peak year.…”

“…Very recently, Wieczorek et al (2015b) demonstrated in the first field-based study that (1) silicon concentrations in leaves are determined by the vole population density, and thus by the grazing intensity, of the previous year, and (2) overwintering success in voles is correlated with leaf silicon levels in autumn. However, one question remains: 'how does silicon have its effect on herbivore performance?'…”