Changing contributions of stochastic and deterministic processes in community assembly over a successional gradient.

Vigdis Vandvik,Per Arild Aarrestad,Inger Elisabeth Måren,John‐Arvid Grytnes,Jutta Kapfer

doi:10.1002/ecy.2052

Abstract

Successional dynamics in plant community assembly may result from both deterministic and stochastic ecological processes. The relative importance of different ecological processes is expected to vary over the successional sequence, between different plant functional groups, and with the disturbance levels and land-use management regimes of the successional systems. We evaluate the relative importance of stochastic and deterministic processes in bryophyte and vascular plant community assembly after fire in grazed and ungrazed anthropogenic coastal heathlands in Northern Europe. A replicated series of post-fire successions (n=12) were initiated under grazed and ungrazed conditions, and vegetation data were recorded in permanent plots over 13years. We used redundancy analysis (RDA) to test for deterministic successional patterns in species composition repeated across the replicate successional series and analyses of co-occurrence to evaluate to what extent species respond synchronously along the successional gradient. Change in species co-occurrences over succession indicates stochastic successional dynamics at the species level (i.e., species equivalence), whereas constancy in co-occurrence indicates deterministic dynamics (successional niche differentiation). The RDA shows high and deterministic vascular plant community compositional change, especially early in succession. Co-occurrence analyses indicate stochastic species-level dynamics the first two years, which then give way to more deterministic replacements. Grazed and ungrazed successions are similar, but the early stage stochasticity is higher in ungrazed areas. Bryophyte communities in ungrazed successions resemble vascular plant communities. In contrast, bryophytes in grazed successions showed consistently high stochasticity and low determinism in both community composition and species co-occurrence. In conclusion, stochastic and individualistic species responses early in succession give way to more niche-driven dynamics in later successional stages. Grazing reduces predictability in both successional trends and species-level dynamics, especially in plant functional groups that are not well adapted to disturbance.

Highlights

Ecological succession is an important process in natural as well as anthropogenic landscapes (Drake 1990, Glenn-Levin et al 1992, Pickett and Cadenasso 2005, Hobbs et al 2007, Pickett et al 2008)
We evaluate the relative importance of stochastic and deterministic processes in bryophyte and vascular plant community assembly after fire in grazed and ungrazed anthropogenic coastal heathlands in Northern Europe
Changes in species co-occurrence patterns over successional time indicate that both deterministic and stochastic processes play important roles in controlling the species assembly and succession of the post-fire heathland community, but their relative importance varies over time, between plant functional groups, and in response to other management practices, as in this case livestock grazing: Immediately after fire, we see large community turnover and high stochasticity in species co-occurrence due to the combined effects of strong selective pressures and unsaturated communities, and lower community turnover and more deterministic co-occurrences later in succession as selective pressures decrease and communities saturate

Summary

Introduction

Ecological succession is an important process in natural as well as anthropogenic landscapes (Drake 1990, Glenn-Levin et al 1992, Pickett and Cadenasso 2005, Hobbs et al 2007, Pickett et al 2008).

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