The importance of neutral over niche processes in structuring Ediacaran early animal communities

Abstract

35The relative influence of niche versus neutral processes in ecosystem dynamics is a 36 fundamental question in community ecology, but the extent to which they structured early 37 animal communities is unknown. The oldest known metazoan-dominated paleocommunities 38 occur in Ediacaran age (~565 million years old) strata in Newfoundland, Canada and 39 Charnwood Forest, UK. These comprise large and diverse in-situ populations of sessile 40 organisms that are amenable to spatial point process analyses, enabling inference of the most 41 likely underlying niche or neutral processes governing their community structure. We 42 conducted comprehensive spatial mapping of seven of the largest Ediacaran 43 paleocommunities using LiDAR, photogrammetry and a laser-line probe. We find neutral 44 processes to dominate these paleocommunities with limited influence of niche processes. 45Our results differ from the niche-dominated dynamics of modern marine ecosystems, 46 revealing that the dynamics of environmental interactions prompted very different ecosystem 47 structuring for these early animal communities. 48 49 50Two opposing theories lie at the heart of debate regarding the fundamental mechanisms that 51 govern ecosystem structure and biodiversity: niche and neutral. Niche theory is a central 52 tenet of classical ecological theory, whereby species avoid competitive exclusion by 53 occupying different niches within the ecosystem (1). Smaller niche overlaps result in less 54 competition between taxa, permitting numerous taxa to exist in an area without excluding 55 each other. Species are able to co-exist because they have different requirements. Niche 56 models describe selection-dominated ecosystems, whereby species dynamics operate 57 deterministically as a series of inter-specific interactions, which act as stabilizing mechanisms 58 for the ecosystem (2). 59 60 Neutral processes are often referred to as the 'null model' of niche processes: instead of 61 species differences enabling co-existence, it is their similarities that drive high diversity (3). 62Within neutral models, species fitness is similar across a community, and so different taxa 63 can co-exist because no single taxon has a significant competitive advantage over any other. 64Despite this seemingly unrealistic assumption, neutral theories have been able to accurately 65 reproduce certain species-area-distributions (3) and beta diversity patterns (4, 5), sometimes 66 better than niche theories (1). 67 68In recent years, unified or continuous theories have emerged, whereby niche and neutral 69 processes combine to enable species coexistence (2, 6). In these combined models, species 70 can exhibit strong differences and strong stabilizations (niche-type), or similar fitness and 71 weak stabilizations (neutral-type), with the classic niche and neutral models forming the 72 extreme end-members of this continuum model. However, it is often not possible to select 73 5 the best-fit niche or neutral model, making it difficult to disentangle the relative influence of ...

Spatial point pattern analysis of traces (SPPAT): An approach for visualizing and quantifying site-selectivity patterns of drilling predators

“…The recognition that predation traces on prey skeletons are spatially explicit and can be mapped enables the development of a proxy for site selectivity in drilling predation based on spatial point process modeling (Baddeley et al 2016). Here, we introduce the spatial point pattern analysis of traces (SPPAT), an approach for visualizing and quantifying the distribution of predation traces on shelled invertebrate prey, which includes improved collection of spatial information inherent to drillhole location, improved visualization of spatial trends, and distance-based statistics for hypothesis testing (see also Clapham et al [2003], Mitchell and Butterfield [2018], and Mitchell et al [2019] for examples that use a similar spatially explicit approach to describe the distribution of wellpreserved Ediacaran fossils). We illustrate the SPPAT approach through case studies on museum samples of the Plio-Pleistocene venerid bivalve Lirophora latilirata (Conrad, 1841) from the Atlantic coastal plain of the United States, which has been previously used in studies on drilling predation (Hattori et al 2014;Klompmaker and Kelley 2015); drilling data from laboratory-based feeding trials of the tropical eastern Pacific naticid Notocochlis unifasciata (Lamarck, 1822) preying upon the venerid bivalve Iliochione subrugosa (W. Wood, 1828); and modern beach-collected samples of I. subrugosa from Central America.…”

Reconstructing the ecology of a Jurassic pseudoplanktonic raft colony

“…This result confirming existing observations was also made in an exceptionally preserved example from Holzmaden [ 8 ] and supports the hypothesis that our largest colony was structured as a floating body, not a benthic structure. Spatial distribution is notable in that it differs from all other previous palaeocommunities SPPA analyses of Ediacaran benthic communities across a range of different environmental settings [ 16 , 18 , 20 , 22 , 26 , 28 ]. There are no benthic palaeocommunities which show this linear structure.…”

“…The spatial positions of Seirocrinus on one of the largest and best-preserved Early Jurassic floating wood examples known, the large ‘Hauff Specimen’ from Holzmaden, Germany [ 15 ] were mapped following Mitchell et al . [ 16 ] ( figure 1 ; electronic supplementary material, figures S1–S3). The spatial patterns of benthic organisms depend on the dispersal of larvae [ 17 , 18 ], the environmental conditions in which they settled [ 16 ] and whether the conditions were favourable for them to grow to adulthood [ 19 , 20 ].…”

“…[ 16 ] ( figure 1 ; electronic supplementary material, figures S1–S3). The spatial patterns of benthic organisms depend on the dispersal of larvae [ 17 , 18 ], the environmental conditions in which they settled [ 16 ] and whether the conditions were favourable for them to grow to adulthood [ 19 , 20 ]. Therefore, the spatial patterns of the crinoids can be used to try to ‘reverse engineer’ the conditions in which they settled in order to deduce the environmental conditions (what part of the water column the log was in) when colonized (cf.…”

Deterministic Selection Dominates Microbial Community Assembly in Termite Mounds Across a Large Spatial Area

“…Deterministic (niche-based theory) and stochastic (neutral theory) processes are commonly used to explain the assembly of species into communities [3,5]. The fundamental premise of niche-based theories is that ecological traits differ among species within a community (e.g., different nutrition requirements), which allows them to occupy different niches and to differentiate limited resources within the ecosystem [6,7]. Neutral theories assume that all individuals are ecologically equivalent with the same demographic rates [8], and dispersal limitation and stochasticity are the key processes that drive the community assembly [9].…”