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.
•We show how correlational niche models extrapolate when projected into novel areas.•Extrapolation was visualized using novel single-variable and multi-variable plots.•We amend the Multivariate ...Environmental Similarity Surface metric currently used.
Correlational models of species’ ecological niches are commonly used to transfer model rules onto other sets of conditions to evaluate species’ distributional potential. As with any model fitting exercise, however, interpretation of model predictions outside the range of the independent variables on which models were calibrated is perilous, herein denoted as strict extrapolation to distinguish from extrapolation onto novel combinations of variables. We use novel visualization techniques to characterize model response surfaces for several niche modeling algorithms for a virtual species (wherein the truth is known) and for two transfer-based studies published by one of our group. All modeling algorithms for each species showed strict extrapolation, such that biologically unrealistic response surfaces were reconstructed. We discuss the implications of these results for calibration and interpretation of niche models and analysis of ecological niche evolution. We present Mobility-Oriented Parity (MOP), a modification and extension of the Multivariate Environmental Similarity Surface (MESS) metric currently in use, as a means of both quantifying environmental similarity between calibration and transfer regions and highlighting regions in geographic space where strict extrapolation occurs.
Aims
Climate change is expected to have profound effects on species' distributions into the future. Freshwater fishes, an important component of freshwater ecosystems, are no exception. Here, we ...project shifts in suitable conditions for Australian freshwater fishes under different climate change scenarios to identify species that may experience significant declines in habitat suitability.
Location
Australia.
Methods
We use MAXENT bioclimatic models to estimate the effect of climate change on the suitable conditions for 154 species of Australian freshwater fishes, of which 109 are endemic and 29 are threatened with extinction. Suitable conditions for freshwater fish species are modelled using three different Earth System climate models (ESMs) under two different emission scenarios to the year 2100. For each species, we examine potential geographic shifts in the distribution of suitable conditions from the present day to 2100 and quantify how habitat suitability may change at currently occupied sites by the end of this century.
Results
Broadscale poleward shifts in suitable conditions are projected for Australian freshwater fishes by an average of up to 0.38° (~180 km) across all species, depending on the emission scenario. Considerable loss of suitable conditions is forecast to occur within currently recognized distributional extents by 2100, with a mean projected loss of up to 17.5% across species. Predicted geographic range shifts and declines are larger under a high‐emission scenario. Threatened species are projected to be more adversely affected than nonthreatened species.
Main Conclusions
Our models identify species and geographic regions that may be vulnerable to climate change, enabling freshwater fish conservation into the future.
Anthropogenic activity is changing Earth's climate and ecosystems in ways that are potentially dangerous and disruptive to humans. Greenhouse gas concentrations in the atmosphere continue to rise, ...ensuring that these changes will be felt for centuries beyond 2100, the current benchmark for projection. Estimating the effects of past, current, and potential future emissions to only 2100 is therefore short‐sighted. Critical problems for food production and climate‐forced human migration are projected to arise well before 2100, raising questions regarding the habitability of some regions of the Earth after the turn of the century. To highlight the need for more distant horizon scanning, we model climate change to 2500 under a suite of emission scenarios and quantify associated projections of crop viability and heat stress. Together, our projections show global climate impacts increase significantly after 2100 without rapid mitigation. As a result, we argue that projections of climate and its effects on human well‐being and associated governance and policy must be framed beyond 2100.
We argue that projections of climate and its effects on human well‐being and associated governance and policy must be framed beyond 2100. To highlight the need for more distant horizon scanning, we model climate change to 2500 under a suite of emission scenarios and quantify associated projections of crop viability and heat stress. Together, our projections show global climate impacts increase significantly after 2100 without rapid mitigation.
The latitudinal diversity gradient (LDG) is a prevalent feature of modern ecosystems across diverse clades
. Recognized for well over a century, the causal mechanisms for LDGs remain disputed, in ...part because numerous putative drivers simultaneously covary with latitude
. The past provides the opportunity to disentangle LDG mechanisms because the relationships among biodiversity, latitude and possible causal factors have varied over time
. Here we quantify the emergence of the LDG in planktonic foraminifera at high spatiotemporal resolution over the past 40 million years, finding that a modern-style gradient arose only 15 million years ago. Spatial and temporal models suggest that LDGs for planktonic foraminifera may be controlled by the physical structure of the water column. Steepening of the latitudinal temperature gradient over 15 million years ago, associated with an increased vertical temperature gradient at low latitudes, may have enhanced niche partitioning and provided more opportunities for speciation at low latitudes. Supporting this hypothesis, we find that higher rates of low-latitude speciation steepened the diversity gradient, consistent with spatiotemporal patterns of depth partitioning by planktonic foraminifera. Extirpation of species from high latitudes also strengthened the LDG, but this effect tended to be weaker than speciation. Our results provide a step change in understanding the evolution of marine LDGs over long timescales.
Animals originated in the oceans and evolved there for hundreds of millions of years before adapting to terrestrial environments. Today, oceans cover more than two-thirds of Earth and generate as ...much primary production as land. The path from the first macrobiota to modern marine biodiversity involved parallel increases in terrestrial nutrient input, marine primary production, species’ abundance, metabolic rates, ecotypic diversity and taxonomic diversity. Bottom-up theories of ecosystem cascades arrange these changes in a causal sequence. At the base of marine food webs, nutrient fluxes and atmosphere–ocean chemistry interact with phytoplankton to regulate production. First-order consumers (e.g., zooplankton) might propagate changes in quantity and quality of phytoplankton to changes in abundance and diversity of larger predators (e.g., nekton). However, many uncertainties remain about the mechanisms and effect size of bottom-up control, particularly in oceans across the entire history of animal life. Here, we review modern and fossil evidence for hypothesized bottom-up pathways, and we assess the ramifications of these processes for four key intervals in marine ecosystems: the Ediacaran–Cambrian (635–485 million years ago), the Ordovician (485–444 million years ago), the Devonian (419–359 million years ago) and the Mesozoic (252–66 million years ago). We advocate for a clear articulation of bottom-up hypotheses to better understand causal relationships and proposed effects, combined with additional ecological experiments, paleontological documentation, isotope geochemistry and geophysical reconstructions. How small-scale ecological change transitions into large-scale evolutionary change remains an outstanding question for empirical and theoretical research.
Gwen Antell and Erin Saupe provide a synthetic framework for the bottom-up mechanisms proposed as causes of major ecological revolutions, from the origin of animals to the present day.
Fundamental ecological and evolutionary theories, such as community saturation and diversity-dependent diversification, assume that biotic competition restricts resource use, and thus limits realized ...niche breadth and geographic range size 1–3. This principle is called competitive exclusion. The corollary (ecological release) posits that, after competitors disappear from a region, species that were previously excluded will invade. Hundreds of field experiments have demonstrated ecological release in living populations. However, few of these studies were conducted in marine environments, and almost no work extended beyond 10 years and 1,000 km2 4, 5. In limited investigation of marine taxa at larger spatiotemporal scales, macroecologists and paleobiologists have observed little evidence of competitive exclusion 6–9. Here, we quantified spatial trends in the rich and densely sampled fossil history of brachiopods and bivalves, while accounting for inconsistent sampling coverage through time using a new method of spatial standardization. The number of potential competitors in a region did not explain the geographic distribution of constituent species or genera. Furthermore, although ecological release predicts species to expand after extinction events, survivors of intervals with net species loss expanded as rarely as species in other intervals. Regression model estimates indicated different spatial responses of brachiopods and bivalves, and of habitat specialists and generalists, but no effect from changes in number of potential competitors. Biotic competition may control the distribution of populations, but, on larger spatiotemporal scales, non-competitive factors may have driven biogeographic patterns of brachiopods and bivalves.
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•Geographic range sizes vary independently of the number of species in assemblages•Species that survive mass extinctions do not invade the space of fallen competitors•Competition affects populations but appears weak on larger spatiotemporal scales
Long-standing ecological and evolutionary theories assume that competition between individuals controls the cumulative resource use and geographic ranges of species. Antell et al. demonstrate that spatial distributions of marine invertebrates are incongruent with this expectation throughout the last 485 million years.
Many higher level avian clades are restricted to Earth’s lower latitudes, leading to historical biogeographic reconstructions favoring a Gondwanan origin of crown birds and numerous deep subclades. ...However, several such “tropical-restricted” clades (TRCs) are represented by stem-lineage fossils well outside the ranges of their closest living relatives, often on northern continents. To assess the drivers of these geographic disjunctions, we combined ecological niche modeling, paleoclimate models, and the early Cenozoic fossil record to examine the influence of climatic change on avian geographic distributions over the last ∼56 million years. By modeling the distribution of suitable habitable area through time, we illustrate that most Paleogene fossil-bearing localities would have been suitable for occupancy by extant TRC representatives when their stem-lineage fossils were deposited. Potentially suitable habitat for these TRCs is inferred to have become progressively restricted toward the tropics throughout the Cenozoic, culminating in relatively narrow circumtropical distributions in the present day. Our results are consistent with coarse-scale niche conservatism at the clade level and support a scenario whereby climate change over geological timescales has largely dictated the geographic distributions of many major avian clades. The distinctive modern bias toward high avian diversity at tropical latitudes for most hierarchical taxonomic levels may therefore represent a relatively recent phenomenon, overprinting a complex biogeographic history of dramatic geographic range shifts driven by Earth’s changing climate, variable persistence, and intercontinental dispersal. Earth’s current climatic trajectory portends a return to a megathermal state, which may dramatically influence the geographic distributions of many range-restricted extant clades.
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