Testing the roles of competition, facilitation and stochasticity on community structure in a species‐rich assemblage

Abstract

Summary1. There is an ongoing debate about whether communities are closely integrated and bound together via interactions such as competition or facilitation, or are disintegrated and dominated by chance. We still lack community-wide data on the intensities of interactions and randomness, and measurements of their impacts on community structure. 2. Using a long-term data set, we sought to measure the effects of interactions and stochasticity in structuring a highly diverse (>100 species) semi-arid grassland plant community, testing for positive and negative interactions at different stages of population growth of all species. 3. During the colonization of new patches, most species were facilitated or inhibited by several others. These opposite effects can potentially have a large effect on species abundances, but they were correlated and cancelled out at the community level. Nevertheless, competition during colonization was strong enough to cause poor competitors to have small population sizes. 4. The subsequent phase of population growth (increase and subsequent change in numbers within occupied patches) was mainly driven by intraspecific density dependence, and we found little evidence for interspecific interactions. 5. Model results showed that stochasticity and recurrent colonization of transient, favourable patches maintained diversity, keeping poor competitors from becoming extinct. 6. Synthesis: Our results, taken together with recent studies on tropical forests, suggest that weak interactions among established plants may be a general phenomenon, but that local interactions during colonization are important drivers of community composition. Most of the variance in species abundance in our community was explained by intraspecific competition and stochasticity, with interspecific interactions playing a minor role due to their overall weakness, interaction changes over ontogeny, and the cancellation of opposite-sign interactions when all the species in the community are considered. Despite this, some species were rare seemingly because they cannot withstand interspecific competition. Thus, to untangle the effects of interactions on community structure, future research should focus on interactions occurring at different phases of population growth and on whole communities.

Effectiveness of ecological rescue for altered soil microbial communities and functions

“…This was concomitant with a decrease in the relative abundance of certain OTUs between D 1 and D 3 along with greater (Figure 3). Likewise, community composition analyses suggest that dilution followed by interspecific interactions during the colonization process (Martorell and Freckleton, 2014) resulted in two alternative community compositional states (Figure 3 and 4a, Supplementary Table S2). The first state, observed in D 1 , D 2 and D 3, consisted of a gradually impoverished community with decreasing phylogenetic diversity and species richness levels (Figure 2b and Supplementary Table S2).…”

A new modeling approach estimates the relative importance of different community assembly processes

“…Although other approaches of estimating the relative contribution of different processes in shaping communities have recently been developed (e.g., Laughlin et al 2012, Shipley et al 2012, Martorell andFreckleton 2014), our STEPCAM approach has two main advantages. Firstly, while other approaches are biased toward the detection of filtering processes (Laughlin and Laughlin 2013), our approach explicitly incorporates limiting similarity processes, and as demonstrated, it is able to distinguish between these processes.…”

Species‐level and community‐level responses to disturbance: a cross‐community analysis

“…Species populations and community-level diversity metrics could show different responses to disturbance if they are at least partially influenced by different processes. Within a community, the presence and abundance of species is often thought to reflect nichebased processes operating primarily at the local scale (Chase and Leibold 2003, Tilman 2004, Martorell and Freckleton 2014. As such, the identity and abundance of individual species often exhibit high spatial and temporal sensitivity to environmental change (Cottenie 2005, Ernest et al 2008, Donohue et al 2013, as fit between the environment and the specific niche parameters of individual species change.…”