Above- and below-ground vertebrate herbivory may each favour a different subordinate species in an aquatic plant community

Abstract

At least two distinct trade-oVs are thought to facilitate higher diversity in productive plant communities under herbivory. Higher investment in defence and enhanced colonization potential may both correlate with decreased competitive ability in plants. Herbivory may thus promote coexistence of plant species exhibiting divergent life history strategies. How diVerent seasonally tied herbivore assemblages simultaneously aVect plant community composition and diversity is, however, largely unknown. Two contrasting types of herbivory can be distinguished in the aquatic vegetation of the shallow lake Lauwersmeer. In summer, predominantly above-ground tissues are eaten, whereas in winter, waterfowl forage on below-ground plant propagules. In a 4-year exclosure study we experimentally separated above-ground herbivory by waterfowl and large Wsh in summer from below-ground herbivory by Bewick's swans in winter. We measured the individual and combined eVects of both herbivory periods on the composition of the three-species aquatic plant community. Herbivory eVect sizes varied considerably from year to year. In 2 years herbivore exclusion in summer reinforced dominance of Potamogeton pectinatus with a concomitant decrease in Potamogeton pusillus, whereas no strong, unequivocal eVect was observed in the other 2 years. Winter exclusion, on the other hand, had a negative eVect on Zannichellia palustris, but the eVect size diVered considerably between years. We suggest that the colonization ability of Z. palustris may have enabled this species to be more abundant after reduction of P. pectinatus tuber densities by swans. Evenness decreased due to herbivore exclusion in summer. We conclude that seasonally tied above-and below-ground herbivory may each stimulate diVerent components of a macrophyte community as they each favoured a diVerent subordinate plant species.

Restoring macrophyte diversity in shallow temperate lakes: biotic versus abiotic constraints

“…For example, in Lake Zwemlust in the Netherlands, the macrophyte vegetation that had developed after the lake's restoration by biomanipulation, was markedly grazed down by coots and rudd, shifting the dominance of Elodea nutallii to codominance by Ceratophyllum demersum and Potamogeton berchtholdii (Van Donk & Otte, 1996). Waterfowl has been documented to graze selectively on P. pectinatus: in Matsalu bay in Estonia, herbivores selectively removed P. pectinatus plants in favour of the charophytes (Hidding et al, 2010a), whereas in the Lauwersmeer in The Netherlands, waterfowl suppressed dominance of P. pectinatus in favour of subordinate Zannichellia palustris and Potamogeton pusillus (Hidding et al, 2010b).…”

“…However, at high macrophyte productivity, moderate levels of disturbance may actually increase macrophyte diversity. Herbivores that graze on the dominant plant species and create moderate sediment disturbance create recruitment opportunities for subordinate plants, thereby improving macrophyte diversity (Sandsten & Klaassen, 2008;Hidding et al, 2010b). Similarly, water level fluctuations can enhance macrophyte diversity (Rorslett, 1991): drawdowns particularly have been shown to improve richness of submerged macrophytes (Van Geest et al, 2005).…”

Ecosystem services provided by waterbirds

“…The elimination of the Aleutian cackling goose Branta hutchinsii leucopareia by introduced foxes is thought to have contributed to the low diversity of plants in the Aleutian Islands (Maron et al, 2006), owing to release from grazing pressure. Grazing by waterfowl can also increase the diversity of submerged macrophytes (Hidding et al, 2010), as well as their productivity (Nolet, 2004). Grazing on seagrasses by geese and wigeon Anas penelope can have a positive long-term effect on seagrass beds due to reduced mud accretion in grazed beds (Nacken & Reise, 2000).…”

Understanding Plant Community Responses to Combinations of Biotic and Abiotic Factors in Different Phases of the Plant Growth Cycle

“…Simultaneously, the growth rate of aquatic plants in temperate rivers declines after the spring period of growth [12], [22]; thus plants experienced the highest grazing pressures when they were senescing and thus their capacity for compensatory growth was low [12], [22]. This led to substantial reductions in plant abundance as have been reported for other aquatic ecosystems [50], [51], [52]. The positive relationship between swan biomass density and species evenness in the peak-phase, suggested grazing of the more naturally-abundant species [29].…”