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

Abstract

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 13 years. 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.

Understorey succession after burial by tephra from Mount St. Helens

“…Our data support a more dynamic understanding of secondary succession, where both resilience models and alternative steady‐state models can be true in the same vegetation zone. As such, our data contribute to a growing body of literature supporting a more dynamic understanding of the complexities of succession generally (Christensen, ; Marteinsdóttir et al, ; Meiners, Cadotte, Fridley, Pickett, & Walker, ), and secondary succession more specifically (Donato, Campbell, & Franklin, ; Måren et al, ).…”

Alternative transient states and slow plant community responses after changed flooding regimes

“…In many zoned ecosystems, such as riparian and tidal ecosystems, the lower boundary of a species is hypothesized to be determined by stress tolerance, while the upper border is formed by competition (Bertness & Callaway, ). We therefore assume that for turfs that were moved to more frequently flooded locations, vegetation changes could be driven by (sudden) plant death opening space for invasion, while when moving to the higher, drier locations, where mortality is lower, competition could be the driving force (Maren et al., ). Our observations support this idea, with vegetation responses to moving downward in elevation proceeding more rapidly compared to moving to higher elevations, and the correlation between rate of vegetation change and our proxies for mortality being steeper than with our proxies for invasion.…”

“…Recent work, however, suggests that stochastic factors, like historic contingency, can interfere with the straightforward replacement of sensitive with tolerant species and can result in alternative transitional stages, or alternative stable states (Fukami & Nakajima, 2011;Maren, Kapfer, Aarrestad, Grytnes, & Vandvik, 2018;Sarneel, Kardol, & Nilsson, 2016;Stuble, Fick, & Young, 2017;Vannette & Fukami, 2014). For instance, in heathlands, replicate turfs followed different routes before converging to a similar vegetation type 7 years after fire disturbance (Maren et al, 2018). The rapidly developing theoretical framework behind alternative transitional stages suggests that the duration of the transient period could be correlated with mortality (Fukami & Nakajima, 2011).…”

Scale sensitivity of the relationship between alpha and gamma diversity along an alpine elevation gradient in central Nepal

“…A long-standing debate concerns whether the assembly and spatiotemporal dynamics of communities are governed by deterministic or stochastic processes (e.g., Hubbell 2001;Tilman 2004;Chase & Myers 2011;Bhaskar et al 2014) or by a combination of both (e.g., Gravel et al 2006;Leibold & McPeek 2006;Adler et al 2007;Caruso et al 2011;Måren et al 2018). A recent consensus is that both deterministic and stochastic processes operate simultaneously in the assembly of local communities (Leibold & McPeek 2006;Chase 2010;Caruso et al 2011;Chase & Myers 2011), and that the relative importance of stochastic (neutral) versus deterministic (niche-based) processes of temporal changes is a matter of scale (Fig.…”