When can we distinguish between neutral and non-neutral processes in community dynamics under ecological drift?

If two species live on a single resource, the one with a slight advantage will out-compete the other: complete competitors cannot coexist. This is known as the competitive exclusion principle. If no extinction occurs, it is because evolutionary adaptation to slightly different niches takes place. Therefore, it is widely accepted that ecological communities are assembled by evolutionary differentiation and progressive adaptation of species to different niches. However, some ecologists have recently challenged this classic paradigm highlighting the importance of chance and stochasticity. Using a synthetic framework for community dynamics, here we show that, while deterministic descriptors predict coexistence, species similarity is limited in a more restrictive way in the presence of stochasticity. We analyse the stochastic extinction phenomenon, showing that extinction occurs as competitive overlap increases above a certain threshold well below its deterministic counterpart. We also prove that the extinction threshold cannot be ascribed only to demographic fluctuations around small population sizes. The more restrictive limit to species similarity is, therefore, a consequence of the complex interplay between competitive interactions and ecological drift. As a practical implication, we show that the existence of a stochastic limit to similarity has important consequences in the recovery of fragmented habitats.

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4. Local Community Dynamics under Ecological Drift

The Unified Neutral Theory of Biodiversity and Biogeography (MPB-32) ◽

10.1515/9781400837526.76 ◽

2011 ◽

pp. 76-112

Keyword(s):

Community Dynamics ◽

Local Community ◽

Ecological Drift

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The effect of periodic disturbances and carrying capacity on the significance of selection and drift in complex bacterial communities

10.1101/2021.07.18.452870 ◽

2021 ◽

Author(s):

Madeleine Stenshorne Gundersen ◽

Ian Morelan ◽

Tom Andersen ◽

Ingrid Bakke ◽

Olav Vadstein

Keyword(s):

Community Assembly ◽

Carrying Capacity ◽

Community Dynamics ◽

Temporal Dynamics ◽

Rate Of Change ◽

Stochastic Effects ◽

Periodic Disturbances ◽

Period 2 ◽

Biomass Loss ◽

Ecological Drift

Understanding how periodical disturbances affect the community assembly processes is vital for predicting temporal dynamics in microbial communities. The effect of dilutions as disturbances are poorly understood. We used a marine bacterial community to investigate the effect of disturbance (+/-) and carrying capacity (high/low) over 50 days in a dispersal-limited 2x2 factorial crossover study in triplicates. The community's disturbance regime was crossed halfway. We modelled the rate of change in community composition between replicates and used this rate to quantify selection and ecological drift. The disturbed communities increased in Bray-Curtis similarity with 0.011+/-0.0045 (Period 1) and 0.0092+/-0.0080 day-1 (Period 2), indicating that selection dominated community assembly. The undisturbed communities decreased in similarity at a rate of -0.015+/-0.0038 day-1 in Period 1 and were stable in Period 2 at 0.00050+/-0.0040 day-1, suggesting drift structured community assembly. Interestingly, carrying capacity had minor effects on community dynamics. This study is the first to show that stochastic effects are suppressed by periodical disturbances resulting in exponential growth periods due to density-independent biomass loss and resource input. The increased contribution of selection as a response to disturbances implies that ecosystem prediction is achievable.

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Community size affects the signals of ecological drift and niche selection on biodiversity

10.1101/515098 ◽

2019 ◽

Cited By ~ 2

Author(s):

Tadeu Siqueira ◽

Victor S. Saito ◽

Luis M. Bini ◽

Adriano S. Melo ◽

Danielle K. Petsch ◽

...

Keyword(s):

Beta Diversity ◽

Community Dynamics ◽

Habitat Destruction ◽

Community Size ◽

Insect Communities ◽

Environmental Pressures ◽

Small Communities ◽

Β Diversity ◽

High Dispersal ◽

Ecological Drift

AbstractEcological drift can override the effects of deterministic niche selection on small populations and drive the assembly of small communities. We tested the hypothesis that smaller local communities are more dissimilar among each other because of ecological drift than larger communities, which are mainly structured by niche selection. We used a unique, comprehensive dataset on insect communities sampled identically in a total of 200 streams in climatically different regions (Brazil and Finland) that differ in community size by fivefold. Null models allowed us to estimate the magnitude to which beta diversity deviates from the expectation under a random assembly process while taking differences in species richness and relative abundance into account, i.e., beta deviation. Beta diversity of small tropical communities was consistently higher but closer to null expectations than β-diversity of large communities. However, although β-deviation and community size were strongly related in both regions, the direction of the relationship varied according to dissimilarity metrics. While incidence-based β-diversity was lower than expected (communities were less dissimilar than null expectations) and negatively related to community size in Brazil, abundance-based β-diversity was higher than expected (communities were more dissimilar than null expectations) and positively related to community size in both regions. We suggest that ecological drift drives variation in small communities by increasing the chances of species with low abundance and narrow distribution to occur within the metacommunity. Also, while weak niche selection and high dispersal rates likely reduced variation in community structure among large tropical streams, niche selection was likely sufficient to cause non-random variations in the relative abundances of genera among large communities in both regions. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities. These environmental pressures will make smaller communities more vulnerable to novel conditions and render community dynamics more unpredictable, as random demographic processes should prevail under these conditions. Incorporation of community size into ecological models should provide conceptual, empirical and applied insights into a better understanding of the processes driving changes in biodiversity.

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Testing ecological release as a compensating mechanism for mass mortality in a keystone predator

Marine Ecology Progress Series ◽

10.3354/meps13227 ◽

2020 ◽

Vol 637 ◽

pp. 59-69 ◽

Cited By ~ 1

Author(s):

J Sullivan-Stack ◽

BA Menge

Keyword(s):

Community Dynamics ◽

Limiting Factor ◽

Sea Star ◽

Top Predator ◽

Oregon Coast ◽

Ecological Release ◽

Keystone Predator ◽

Pisaster Ochraceus ◽

Wasting Syndrome

Top predator decline has been ubiquitous across systems over the past decades and centuries, and predicting changes in resultant community dynamics is a major challenge for ecologists and managers. Ecological release predicts that loss of a limiting factor, such as a dominant competitor or predator, can release a species from control, thus allowing increases in its size, density, and/or distribution. The 2014 sea star wasting syndrome (SSWS) outbreak decimated populations of the keystone predator Pisaster ochraceus along the Oregon coast, USA. This event provided an opportunity to test the predictions of ecological release across a broad spatial scale and determine the role of competitive dynamics in top predator recovery. We hypothesized that after P. ochraceus loss, populations of the subordinate sea star Leptasterias sp. would grow larger, more abundant, and move downshore. We based these predictions on prior research in Washington State showing that Leptasterias sp. competed with P. ochraceus for food. Further, we predicted that ecological release of Leptasterias sp. could provide a bottleneck to P. ochraceus recovery. Using field surveys, we found no clear change in density or distribution in Leptasterias sp. populations post-SSWS, and decreases in body size. In a field experiment, we found no evidence of competition between similar-sized Leptasterias sp. and P. ochraceus. Thus, the mechanisms underlying our predictions were not in effect along the Oregon coast, which we attribute to differences in habitat overlap and food availability between the 2 regions. Our results suggest that response to the loss of a dominant competitor can be unpredictable even when based in theory and previous research.

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The Township, the Pregnant Girl and the Church: Community Dynamics, Gender and Social Control in Early Nineteenth-century Scotland

Northern Scotland ◽

10.3366/nor.2019.0168 ◽

2019 ◽

Vol 10 (1) ◽

pp. 41-67

Author(s):

Elizabeth Ritchie

Keyword(s):

Nineteenth Century ◽

Rural Communities ◽

Social Control ◽

Community Dynamics ◽

Church Discipline ◽

Sexual Offences ◽

Early Nineteenth Century ◽

Female Community ◽

The Church ◽

Unmarried Girl

In 1814 in a small Highland township an unmarried girl, ostracised by her neighbours, gave birth. The baby died. The legal precognition permits a forensic, gendered examination of the internal dynamics of rural communities and how they responded to threats to social cohesion. In the Scottish ‘parish state’ disciplining sexual offences was a matter for church discipline. This case is situated in the early nineteenth-century Gàidhealtachd where and when church institutions were less powerful than in the post-Reformation Lowlands, the focus of most previous research. The article shows that the formal social control of kirk discipline was only part of a complex of behavioural controls, most of which were deployed within and by communities. Indeed, Scottish communities and churches were deeply entwined in terms of personnel; shared sexual prohibitions; and in the use of shaming as a primary method of social control. While there was something of a ‘female community’, this was not unconditionally supportive of all women nor was it ranged against men or patriarchal structures.

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