On the processes generating latitudinal richness gradients: identifying diagnostic patterns and predictions

Macroecological patterns of planktonic unicellular eukaryotes richness in the Southeast Pacific Ocean

In recent years, studies focusing on microbial biogeography have been developed, but macroecological processes in marine microorganisms remain unclear, especially in seemingly continuous environments such as the Southeast Pacific Ocean (SPO), where information on microbial distribution patterns is limited, and they may vary depending on the habitat and lifestyle. We used unicellular planktonic eukaryotes as model organisms to determine their biogeographic patterns in the SPO, identify the underlying ecological and historical-evolutionary processes and compare with other microorganism groups. Our analyses were based on the Niche Theory to model species diversity distribution using large open-access ecological and physical-biogeochemical databases based on Bayesian approaches, an integrated nested Laplace approximation (INLA), and Generalized Additive Models (GAM). As a result, two richness hotspots were observed, which are associated with coastal and offshore regions in the central southern areas of SPO. The richness hotspots were associated mainly with nutrients (N/Si ratio) and Mixed Layer Depth (MLD), which could be explained by highly productive upwelling events in the SPO. In contrast, the negative correlation of predicted richness with low pH is strongly related to the effect of calcareous shells (tests), as lower pH levels hinder the formation and stability of calcium carbonate shells in protists like foraminifera and radiolaria, thereby affecting overall unicellular planktonic eukaryote diversity. Our results support the role of ecological processes related to productivity, energy dynamics, and ecological limits in shaping broad-scale diversity patterns of unicellular planktonic eukaryotes in the SPO. The results show colonization and extinction dynamics through species replacement (i.e. High Turnover) along the Chilean and Equatorial coasts associated mainly with the Hotspots of their biodiversity, but also a gradual species loss (i.e. High Nestedness) along the Peruvian Coast associated mainly with the Coldspots of their biodiversity; highlighting how local environmental fluctuations can shape these planktonic microorganisms' behavior, ecology and distribution. The distribution patterns of planktonic unicellular eukaryotes show little evidence of the effects of historical and evolutionary processes. This is because the high dispersal capacity of planktonic microbes probably dilutes the influence of these processes in environments lacking clear barriers to species dispersal. Additionally, the effect of historical events could be highlighted in specific taxonomic groups at the kingdom, phylum level or habitat type and addressing gaps about latitudinal richness in the SPO. This provides insight into the spatial distribution of marine microbes and contributes to conservation efforts, as these organisms are an essential foundation of the upper levels of the food web.

Explanations for latitudinal diversity gradients must invoke rate variation

The latitudinal diversity gradient (LDG) describes the pattern of increasing numbers of species from the poles to the equator. Although recognized for over 200 years, the mechanisms responsible for the largest-scale and longest-known pattern in macroecology are still actively debated. I argue here that any explanation for the LDG must invoke differential rates of speciation, extinction, extirpation, or dispersal. These processes themselves may be governed by numerous abiotic or biotic factors. Hypotheses that claim not to invoke differential rates, such as 'age and area' or 'time for diversification', eschew focus from rate variation that is assumed by these explanations. There is still significant uncertainty in how rates of speciation, extinction, extirpation, and dispersal have varied regionally over Earth history. However, to better understand the development of LDGs, we need to better constrain this variation. Only then will the drivers of such rate variation - be they abiotic or biotic in nature - become clearer.

Temperature-dependent evolutionary speed shapes the evolution of biodiversity patterns across tetrapod radiations

Biodiversity varies predictably with environmental energy around the globe, but the underlaying mechanisms remain incompletely understood. The evolutionary speed hypothesis predicts that environmental kinetic energy shapes variation in speciation rates through temperature- or life history-dependent rates of evolution. To test whether variation in evolutionary speed can explain the relationship between energy and biodiversity in birds, mammals, amphibians, and reptiles, we simulated diversification over 65 million years of geological and climatic change with a spatially explicit eco-evolutionary simulation model. We modelled four distinct evolutionary scenarios in which speciation-completion rates were dependent on temperature (M1), life history (M2), temperature and life history (M3), or were independent of temperature and life-history (M0). To assess the agreement between simulated and empirical data, we performed model selection by fitting supervised machine learning models to multidimensional biodiversity patterns. We show that a model with temperature-dependent rates of speciation (M1) consistently had the strongest support. In contrast to statistical inferences, which showed no general relationships between temperature and speciation rates in tetrapods, we demonstrate how process-based modelling can disentangle the causes behind empirical biodiversity patterns. Our study highlights how environmental energy has played a fundamental role in the evolution of biodiversity over deep time.

A theory of pulse dynamics and disturbance in ecology

We propose four postulates as the minimum set of logical propositions necessary for a theory of pulse dynamics and disturbance in ecosystems: (1) resource dynamics characterizes the magnitude, rate, and duration of resource change caused by pulse events, including the continuing changes in resources that are the result of abiotic and biotic processes; (2) energy flux characterizes the energy flow that controls the variation in the rates of resource assimilation across ecosystems; (3) patch dynamics characterizes the distribution of resource patches over space and time, and the resulting patterns of biotic diversity, ecosystem structure, and cross-scale feedbacks of pulses processes; and (4) biotic trait diversity characterizes the evolutionary responses to pulse dynamics and, in turn, the way trait diversity affects ecosystem dynamics during and after pulse events. We apply the four postulates to an important class of pulse events, biomass-altering disturbances, and derive seven generalizations that predict disturbance magnitude, resource trajectory, rate of resource change, disturbance probability, biotic trait diversification at evolutionary scales, biotic diversity at ecological scales, and functional resilience. Ultimately, theory must define the variable combinations that result in dynamic stability, comprising resistance, recovery, and adaptation.

The Latitudinal Diversity Gradient: Novel Understanding through Mechanistic Eco-evolutionary Models

The latitudinal diversity gradient (LDG) is one of the most widely studied patterns in ecology, yet no consensus has been reached about its underlying causes. We argue that the reasons for this are the verbal nature of existing hypotheses, the failure to mechanistically link interacting ecological and evolutionary processes to the LDG, and the fact that empirical patterns are often consistent with multiple explanations. To address this issue, we synthesize current LDG hypotheses, uncovering their eco-evolutionary mechanisms, hidden assumptions, and commonalities. Furthermore, we propose mechanistic eco-evolutionary modeling and an inferential approach that makes use of geographic, phylogenetic, and trait-based patterns to assess the relative importance of different processes for generating the LDG.

The influence of ecological and geographic limits on the evolution of species distributions and diversity

The role of ecological limits in regulating the distribution and diversification of species remains controversial. Although such limits must ultimately arise from constraints on local species coexistence, this spatial context is missing from most macroevolutionary models. Here, we develop a stochastic, spatially explicit model of species diversification to explore the phylogenetic and biogeographic patterns expected when local diversity is bounded. We show how local ecological limits, by regulating opportunities for range expansion and thus rates of speciation and extinction, lead to temporal slowdowns in diversification and predictable differences in equilibrium diversity between regions. However, our models also show that even when regions have identical diversity limits, the dynamics of diversification and total number of species supported at equilibrium can vary dramatically depending on the relative size of geographic and local ecological niche space. Our model predicts that small regions with higher local ecological limits support a higher standing diversity and more balanced phylogenetic trees than large geographic areas with more stringent constraints on local coexistence. Our findings highlight how considering the spatial context of diversification can provide new insights into the role of ecological limits in driving variation in biodiversity across space, time, and clades.

The more-individuals hypothesis revisited: the role of community abundance in species richness regulation and the productivity-diversity relationship

Species richness increases with energy availability, yet there is little consensus as to the exact processes driving this species-energy relationship. The most straightforward explanation is the more-individuals hypothesis (MIH). It states that higher energy availability promotes a higher total number of individuals in a community, which consequently increases species richness by allowing for a greater number of species with viable populations. Empirical support for the MIH is mixed, partially due to the lack of proper formalisation of the MIH and consequent confusion as to its exact predictions. Here, we review the evidence of the MIH and evaluate the reliability of various predictions that have been tested. There is only limited evidence that spatial variation in species richness is driven by variation in the total number of individuals. There are also problems with measures of energy availability, with scale-dependence, and with the direction of causality, as the total number of individuals may sometimes itself be driven by the number of species. However, even in such a case the total number of individuals may be involved in diversity regulation. We propose a formal theory that encompasses these processes, clarifying how the different factors affecting diversity dynamics can be disentangled.

Uncovering Higher-Taxon Diversification Dynamics from Clade Age and Species-Richness Data

The relationship between clade age and species richness has been increasingly used in macroevolutionary studies as evidence for ecologically versus time-dependent diversification processes. However, theory suggests that phylogenetic structure, age type (crown or stem age), and taxonomic delimitation can affect estimates of the age-richness correlation (ARC) considerably. We currently lack an integrative understanding of how these different factors affect ARCs, which in turn, obscures further interpretations. To assess its informative breadth, we characterize ARC behavior with simulated and empirical phylogenies, considering phylogenetic structure and both crown and stem ages. First, we develop a two-state birth-death model to simulate phylogenies including the origin of higher taxa and a hierarchical taxonomy to determine ARC expectations under ecologically and time-dependent diversification processes. Then, we estimate ARCs across various taxonomic ranks of extant amphibians, squamate reptiles, mammals, birds, and flowering plants. We find that our model reproduces the general ARC trends of a wide range of biological systems despite the particularities of taxonomic practice within each, suggesting that the model is adequate to establish a framework of ARC null expectations for different diversification processes when taxa are defined with a hierarchical taxonomy. ARCs estimated with crown ages were positive in all the scenarios we studied, including ecologically dependent processes. Negative ARCs were only found at less inclusive taxonomic ranks, when considering stem age, and when rates varied among clades. This was the case both in ecologically and time-dependent processes. Together, our results warn against direct interpretations of single ARC estimates and advocate for a more integrative use of ARCs across age types and taxonomic ranks in diversification studies. [Birth-Death models; crown age; diversity dependence; extinction; phylogenetic structure; speciation; stem age; taxonomy; time dependence; tree simulations.].

Unifying latitudinal gradients in range size and richness across marine and terrestrial systems

Many marine and terrestrial clades show similar latitudinal gradients in species richness, but opposite gradients in range size-on land, ranges are the smallest in the tropics, whereas in the sea, ranges are the largest in the tropics. Therefore, richness gradients in marine and terrestrial systems do not arise from a shared latitudinal arrangement of species range sizes. Comparing terrestrial birds and marine bivalves, we find that gradients in range size are concordant at the level of genera. Here, both groups show a nested pattern in which narrow-ranging genera are confined to the tropics and broad-ranging genera extend across much of the gradient. We find that (i) genus range size and its variation with latitude is closely associated with per-genus species richness and (ii) broad-ranging genera contain more species both within and outside of the tropics when compared with tropical- or temperate-only genera. Within-genus species diversification thus promotes genus expansion to novel latitudes. Despite underlying differences in the species range-size gradients, species-rich genera are more likely to produce a descendant that extends its range relative to the ancestor's range. These results unify species richness gradients with those of genera, implying that birds and bivalves share similar latitudinal dynamics in net species diversification.

Patterns and Variation in Benthic Biodiversity in a Large Marine Ecosystem

While there is a persistent inverse relationship between latitude and species diversity across many taxa and ecosystems, deviations from this norm offer an opportunity to understand the conditions that contribute to large-scale diversity patterns. Marine systems, in particular, provide such an opportunity, as marine diversity does not always follow a strict latitudinal gradient, perhaps because several hypothesized drivers of the latitudinal diversity gradient are uncorrelated in marine systems. We used a large scale public monitoring dataset collected over an eight year period to examine benthic marine faunal biodiversity patterns for the continental shelf (55-183 m depth) and slope habitats (184-1280 m depth) off the US West Coast (47°20'N-32°40'N). We specifically asked whether marine biodiversity followed a strict latitudinal gradient, and if these latitudinal patterns varied across depth, in different benthic substrates, and over ecological time scales. Further, we subdivided our study area into three smaller regions to test whether coast-wide patterns of biodiversity held at regional scales, where local oceanographic processes tend to influence community structure and function. Overall, we found complex patterns of biodiversity on both the coast-wide and regional scales that differed by taxonomic group. Importantly, marine biodiversity was not always highest at low latitudes. We found that latitude, depth, substrate, and year were all important descriptors of fish and invertebrate diversity. Invertebrate richness and taxonomic diversity were highest at high latitudes and in deeper waters. Fish richness also increased with latitude, but exhibited a hump-shaped relationship with depth, increasing with depth up to the continental shelf break, ~200 m depth, and then decreasing in deeper waters. We found relationships between fish taxonomic and functional diversity and latitude, depth, substrate, and time at the regional scale, but not at the coast-wide scale, suggesting that coast-wide patterns can obscure important correlates at smaller scales. Our study provides insight into complex diversity patterns of the deep water soft substrate benthic ecosystems off the US West Coast.

Species richness at continental scales is dominated by ecological limits

Explaining variation in species richness among provinces and other large geographic regions remains one of the most challenging problems at the intersection of ecology and evolution. Here we argue that empirical evidence supports a model whereby ecological factors associated with resource availability regulate species richness at continental scales. Any large-scale predictive model for biological diversity must explain three robust patterns in the natural world. First, species richness for evolutionary biotas is highly correlated with resource-associated surrogate variables, including area, temperature, and productivity. Second, species richness across epochal timescales is largely stationary in time. Third, the dynamics of diversity exhibit clear and predictable responses to mass extinctions, key innovations, and other perturbations. Collectively, these patterns are readily explained by a model in which species richness is regulated by diversity-dependent feedback mechanisms. We argue that many purported tests of the ecological limits hypothesis, including branching patterns in molecular phylogenies, are inherently weak and distract from these three core patterns. We have much to learn about the complex hierarchy of processes by which local ecological interactions lead to diversity dependence at the continental scale, but the empirical evidence overwhelmingly suggests that they do.