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

Abstract

Understanding plant community responses to combinations of biotic and abiotic factors is critical for predicting ecosystem response to environmental change. However, studies of plant community regulation have seldom considered how responses to such factors vary with the different phases of the plant growth cycle. To address this deficit we studied an aquatic plant community in an ecosystem subject to gradients in mute swan (Cygnus olor) herbivory, riparian shading, water temperature and distance downstream of the river source. We quantified abundance, species richness, evenness, flowering and dominance in relation to biotic and abiotic factors during the growth-, peak-, and recession-phases of the plant growth cycle. We show that the relative importance of biotic and abiotic factors varied between plant community properties and between different phases of the plant growth cycle. Herbivory became more important during the later phases of peak abundance and recession due to an influx of swans from adjacent pasture fields. Shading by riparian vegetation also had a greater depressing effect on biomass in later seasons, probably due to increased leaf abundance reducing light intensity reaching the aquatic plants. The effect of temperature on community diversity varied between upstream and downstream sites by altering the relative competitiveness of species at these sites. These results highlight the importance of seasonal patterns in the regulation of plant community structure and function by multiple factors.

Herbivore regulation of plant abundance in aquatic ecosystems

“…The observed result that bird herbivory was associated with smaller changes in plant abundance relative to most other taxa may be linked to inequalities in the movement ability of these taxa. Birds are highly mobile consumers which can, and frequently do, move between foraging sites in response to changes in site profitability (Nolet et al, 2001;Gyimesi et al, 2012;Wood et al, 2013). At foraging sites where plant abundance becomes substantially depleted birds, unlike the other taxa in our study, can abandon the site and fly elsewhere.…”

“…herbivory), bottom-up (e.g. nutrient availability), and competitive processes on plant abundance and ecosystem primary productivity (Hughes et al, 2004;Gayet et al, 2011;Wood et al, 2012c;Sarneel et al, 2014). Currently, our limited knowledge of multifactorial regulation of primary producers hinders efforts to manage aquatic ecosystems effectively (Chambers et al, 1999;Bakker et al, 2013).…”

“…Previous studies have suggested that plant abundance appears to be negatively related to herbivore biomass density due to greater grazing pressure at high herbivore densities (Crivelli, 1983;Wood et al, 2012b;Atkins et al, 2015). Indeed, Wood et al (2012c) reported a significant linear relationship between mute swan (Cygnus olor) biomass density and the abundance of aquatic plants in shallow rivers. Therefore our first hypothesis was that greater negative changes in plant abundance would be associated with higher herbivore biomass densities.…”

The role of season and social grouping on habitat use by Mute Swans (Cygnus olor) in a lowland river catchment

“…EU Habitats Directive 92/43/EEC). However, grazing conflicts with this protected plant community have been reported for non-territorial flocks of swans in summer, including reductions in abundance and flowering (O'Hare et al 2007, Wood et al 2012b. The exploitation of river habitat at other times of year, and by territorial swans, has not yet been addressed.…”

“…In this study we used data from repeated surveys over two years to quantify the numbers of territorial and nonterritorial swans and their habitat preferences in a chalk river catchment which has had recent reports of grazing damage to aquatic plants (O'Hare et al 2007, Wood et al 2012b. We tested three hypotheses regarding Mute Swan habitat use within our study system.…”