Stress or help, how does the grey heron Ardea cinerea L. modify the vegetation structure of the forest floor?

Abstract

The grey heron (<em>Ardea cinerea</em>) is a good example of an engineering species that forms nesting colonies (called heronries) composed of up to a few hundred nests during the breeding season. It exerts strong pressure on surrounding vegetation, mainly because of the heavy input of organic matter and high eutrophication. The birds also alter light conditions through direct tree damage. We aimed to examine the influence of a grey heron breeding colony on the soil properties and functional composition of the herb layer in two different forest communities: a suboceanic pine forest and a riparian mixed forest. We also wanted to establish whether these changes would make the forest ecosystem more vulnerable to colonization by nonforest species with higher light and trophic demands. Small-seeded, light demanding eutrophic species showed a tendency to be more abundant under the nests in both forest types. The calculated odds ratio (OR) showed that the probability of the presence of nonforest species under the nests was 29.5 times higher than that in the control plots (56 plants species were analyzed). Additionally, the nonforest species appeared to have high light and trophic demand (OR of 12.3 for light demand; OR of 7.0 for trophic demand), which explains the species turnover observed in the bird-affected microhabitats.

How Does the Engineering Species Affect Forest Ecosystems? Changes in Germination and Development Rate of Non-Forest Species Under the Nests of Ardea cinerea L.

“…The percent share of forest taxa (both pine and riparian forest) decreased in the soil of the breeding sites, which was accompanied by an increase in the share of ruderal species. What is important for the observed forest floor changes (described in Hryń et al, 2019), the presence of ruderal species seeds was noted in the soil from the control plots, but their participation was lower than that in the breeding sites.…”

“…Two microhabitats were examined in each of the two forest stands: the heronry (hereafter called the breeding site) and the control (area without nests located in the same forest in the vicinity of the breeding site). The habitat properties and vegetation of these sites were described in Hryń et al, 2019. The most striking difference in the soil properties in both forest types was the significant increase in the N-NO 3 content in the soil from microhabitats influenced by bird activity.…”

“…The aim of our study was to compare the structure of the soil seed bank in control and nesting sites of two different forest communities: an oligotrophic pine forest and a mesotrophic riparian mixed forest. Our previous study showed that non-forest species were characterized by 29.5-fold higher probability of occurrence in the nesting sites in those communities than in the control sites, but the adaptation to long-distance dispersal proved to be insignificant in terms of increasing the probability of colonization (Hryń et al, 2019). This strongly suggests that the grey heron modifies the herb layer structure not by enhancing invasion from the outside of the ecosystem but by activating the existing seed reserves in the soil.…”

“…The engineering activity of A. cinerea provides the ecosystem with matter and energy from the outside (the bird forages in water ecosystems located away from the colony and can travel up to 40 km to find food; Marion, 1989). Dropped and regurgitated food and feces fall upon the soil and cause rapid eutrophication, mainly due to an increase in the nitrogen amount (Mun, 1997;Hryń et al, 2019). This, in turn, affects the vegetation of the site; in the case of forest ecosystems, withdrawal of native forest species accompanied by increased abundance of ruderal species is observed (Mun, 1997;Żółkoś, Meissner, 2008;Hryń et al, 2019).…”

“…One of the factors that may contribute to the activation of the seed pool of ruderal plants might be the higher concentration of nitrogen in the nesting sites than in the control sites (Hryń et al, 2019). In order to test its relevance, we also examined its influence upon the seed germination and seedling growth rate, which may enhance seedling survival.…”