Clustered versus catastrophic global vertebrate declines.

Recent analyses have reported catastrophic global declines in vertebrate populations1,2. However, the distillation of many trends into a global mean index obscures the variation that can inform conservation measures and can be sensitive to analytical decisions. For example, previous analyses have estimated a mean vertebrate decline of more than 50% since 1970 (Living Planet Index2). Here we show, however, that this estimate is driven by less than 3% of vertebrate populations; if these extremely declining populations are excluded, the global trend switches to an increase. The sensitivity of global mean trends to outliers suggests that more informative indices are needed. We propose an alternative approach, which identifies clusters of extreme decline (or increase) that differ statistically from the majority of population trends. We show that, of taxonomic-geographic systems in the Living Planet Index, 16 systems contain clusters of extreme decline (comprising around 1% of populations; these extreme declines occur disproportionately in larger animals) and 7 contain extreme increases (around 0.4% of populations). The remaining 98.6% of populations across all systems showed no mean global trend. However, when analysed separately, three systems were declining strongly with high certainty (all in the Indo-Pacific region) and seven were declining strongly but with less certainty (mostly reptile and amphibian groups). Accounting for extreme clusters fundamentally alters the interpretation of global vertebrate trends and should be used to help to prioritize conservation efforts.

Among-species variation in six decades of changing migration timings explained through ecology, life-history and local migratory abundance.

Species exploiting seasonal environments must alter timings of key life-history events in response to large-scale climatic changes in order to maintain trophic synchrony with required resources. Yet, substantial among-species variation in long-term phenological changes has been observed. Advancing from simply describing such variation towards predicting future phenological responses requires studies that rigorously quantify and explain variation in the direction and magnitude of changing timings across diverse species in relation to key ecological and life-history variables. Accordingly, we fitted multi-quantile regressions to 59 years of multi-species data on spring and autumn bird migration timings through northern Scotland. We demonstrate substantial variation in changes in timings among 72 species, and tested whether such variation can be explained by species ecology, life-history and changes in local abundance. Consistent with predictions, species that advanced their migration timing in one or both seasons had more seasonally restricted diet types, fewer suitable breeding habitat types, shorter generation lengths and capability to produce multiple offspring broods per year. In contrast, species with less seasonally restricted diet types and that produce single annual offspring broods, showed no change. Meanwhile, contrary to prediction, long-distance and short-distance migrants advanced migration timings similarly. Changes in migration timing also varied with changes in local migratory abundance, such that species with increasing seasonal abundance apparently altered their migration timing, whilst species with decreasing abundance did not. Such patterns broadly concur with expectation given adaptive changes in migration timing. However, we demonstrate that similar patterns can be generated by numerical sampling given changing local abundances. Any apparent phenology-abundance relationships should, therefore, be carefully validated and interpreted. Overall, our results show that migrant bird species with differing ecologies and life-histories showed systematically differing phenological changes over six decades contextualised by large-scale environmental changes, potentially facilitating future predictions and altering temporal dynamics of seasonal species co-occurrences.

Comparing temporal dynamics of compositional reorganization in long-term studies of birds and fish.

The composition of ecological assemblages has changed rapidly over the past century. Compositional reorganization rates are high relative to rates of alpha diversity change, creating an urgent need to understand how this compositional reorganization is progressing. We developed a quantitative framework for comparing temporal trajectories of compositional reorganization and applied it to two long-term bird and marine fish datasets. We then evaluated how the number and magnitude of short-term changes relate to overall rates of change. We found varied trajectories of turnover across birds and fish, with linear directional change predominating in birds and non-directional change more common in fish. The number of changes away from the baseline was a more consistent correlate of the overall rate of change than the magnitude of such changes, but large unreversed changes were found in both fish and birds, as were time series with accelerating compositional change. Compositional reorganization is progressing through a complex mix of temporal trajectories, including both threshold-like behaviour and the accumulation of repeated, linear change.

A word on habitat complexity.

In their recent synopsis, Loke and Chisholm (Ecology Letters, 25, 2269-2288, 2022) present an overview of habitat complexity metrics for ecologists. They provide a review and some sound advice. However, we found several of their analyses and opinions misleading. This technical note provides a different perspective on the complexity metrics assessed.

Disturbance-Induced Changes in Population Size Structure Promote Coral Biodiversity.

AbstractReef-building coral assemblages are typically species rich, yet the processes maintaining high biodiversity remain poorly understood. Disturbance has long been thought to promote coral species coexistence by reducing the strength of competition (i.e., the intermediate disturbance hypothesis [IDH]). However, such disturbance-induced effects are insufficient to inhibit competitive exclusion. Nevertheless, there are other mechanisms by which disturbance and, more generally, environmental variation can favor coexistence. Here, we develop a size-structured, stochastic coral competition model calibrated with field data from two common colony morphologies to investigate the effects of hydrodynamic disturbance on community dynamics. We show that fluctuations in wave action can promote coral species coexistence but that this occurs via interspecific differences in size-dependent mortality rather than solely via stochastic fluctuations in competition (i.e., free space availability). While this mechanism differs from that originally envisioned in the IDH, it is nonetheless a mechanism by which intermediate levels of disturbance do promote coexistence. Given the sensitivity of coexistence to disturbance frequency and intensity, anthropogenic changes in disturbance regimes are likely to affect coral assemblages in ways that are not predictable from single-population models.

Elevated compositional change in plant assemblages linked to invasion.

Alien species are widely linked to biodiversity change, but the extent to which they are associated with the reshaping of ecological communities is not well understood. One possible mechanism is that assemblages where alien species are found exhibit elevated temporal turnover. To test this, we identified assemblages of vascular plants in the BioTIME database for those assemblages in which alien species are either present or absent and used the Jaccard measure to compute compositional dissimilarity between consecutive censuses. We found that, although alien species are typically rare in invaded assemblages, their presence is associated with an increase in the average rate of compositional change. These differences in compositional change between invaded and uninvaded assemblages are not linked to differences in species richness but rather to species replacement (turnover). Rapid compositional restructuring of assemblages is a major contributor to biodiversity change, and as such, our results suggest a role for alien species in bringing this about.

Biodiversity change under adaptive community dynamics.

Compositional change is a ubiquitous response of ecological communities to environmental drivers of global change, but is often regarded as evidence of declining "biotic integrity" relative to historical baselines. Adaptive compositional change, however, is a foundational idea in evolutionary biology, whereby changes in gene frequencies within species boost population-level fitness, allowing populations to persist as the environment changes. Here, we present an analogous idea for ecological communities based on core concepts of fitness and selection. Changes in community composition (i.e., frequencies of genetic differences among species) in response to environmental change should normally increase the average fitnessof community members. We refer to compositional changes that improve the functional match, or "fit," between organisms' traits and their environment as adaptive community dynamics. Environmental change (e.g., land-use change) commonly reduces the fit between antecedent communities and new environments. Subsequent change in community composition in response to environmental changes, however, should normally increase community-level fit, as the success of at least some constituent species increases. We argue that adaptive community dynamics are likely to improve or maintain ecosystem function (e.g., by maintaining productivity). Adaptive community responses may simultaneously produce some changes that are considered societally desirable (e.g., increased carbon storage) and others that are undesirable (e.g., declines of certain species), just as evolutionary responses within species may be deemed desirable (e.g., evolutionary rescue of an endangered species) or undesirable (e.g., enhanced virulence of an agricultural pest). When assessing possible management interventions, it is important to distinguish between drivers of environmental change (e.g., undesired climate warming) and adaptive community responses, which may generate some desirable outcomes. Efforts to facilitate, accept, or resist ecological change require separate consideration of drivers and responses, and may highlight the need to reconsider preferences for historical baseline communities over communities that are better adapted to the new conditions.

A millennium of increasing diversity of ecosystems until the mid-20th century.

Land-use change is widely regarded as a simplifying and homogenising force in nature. In contrast, analysing global land-use reconstructions from the 10th to 20th centuries, we found progressive increases in the number, evenness, and diversity of ecosystems (including human-modified land-use types) present across most of the Earth's land surface. Ecosystem diversity increased more rapidly after ~1700 CE, then slowed or slightly declined (depending on the metric) following the mid-20th century acceleration of human impacts. The results also reveal increasing spatial differentiation, rather than homogenisation, in both the presence-absence and area-coverage of different ecosystem types at sub-global scales-at least, prior to the mid-20th century. Nonetheless, geographic homogenization was revealed for a subset of analyses at a global scale, reflecting the now-global presence of certain human-modified ecosystem types. Our results suggest that, while human land-use changes have caused declines in relatively undisturbed or "primary" ecosystem types, they have also driven increases in ecosystem diversity over the last millennium.

Long-term changes in temperate marine fish assemblages are driven by a small subset of species.

The species composition of plant and animal assemblages across the globe has changed substantially over the past century. How do the dynamics of individual species cause this change? We classified species into seven unique categories of temporal dynamics based on the ordered sequence of presences and absences that each species contributes to an assemblage time series. We applied this framework to 14,434 species trajectories comprising 280 assemblages of temperate marine fishes surveyed annually for 20 or more years. Although 90% of the assemblages diverged in species composition from the baseline year, this compositional change was largely driven by only 8% of the species' trajectories. Quantifying the reorganization of assemblages based on species shared temporal dynamics should facilitate the task of monitoring and restoring biodiversity. We suggest ways in which our framework could provide informative measures of compositional change, as well as leverage future research on pattern and process in ecological systems.

Climate change transforms the functional identity of Mediterranean coralligenous assemblages.

Quantifying changes in functional community structure driven by disturbance is critical to anticipate potential shifts in ecosystem functioning. However, how marine heatwaves (MHWs) affect the functional structure of temperate coral-dominated communities is poorly understood. Here, we used five long-term (> 10 years) records of Mediterranean coralligenous assemblages in a multi-taxa, trait-based analysis to investigate MHW-driven changes in functional structure. We show that, despite stability in functional richness (i.e. the range of species functional traits), MHW-impacted assemblages experienced long-term directional changes in functional identity (i.e. their dominant trait values). Declining traits included large sizes, long lifespans, arborescent morphologies, filter-feeding strategies or calcified skeletons. These traits, which were mostly supported by few sensitive and irreplaceable species from a single functional group (habitat-forming octocorals), disproportionally influence certain ecosystem functions (e.g. 3D-habitat provision). Hence, MHWs are leading to assemblages that are deficient in key functional traits, with likely consequences for the ecosystem functioning.

Divergent biodiversity change within ecosystems.

The Earth's ecosystems are under unprecedented pressure, yet the nature of contemporary biodiversity change is not well understood. Growing evidence that community size is regulated highlights the need for improved understanding of community dynamics. As stability in community size could be underpinned by marked temporal turnover, a key question is the extent to which changes in both biodiversity dimensions (temporal α- and temporal ß-diversity) covary within and among the assemblages that comprise natural communities. Here, we draw on a multiassemblage dataset (encompassing vertebrates, invertebrates, and unicellular plants) from a tropical freshwater ecosystem and employ a cyclic shift randomization to assess whether any directional change in temporal α-diversity and temporal ß-diversity exceeds baseline levels. In the majority of cases, α-diversity remains stable over the 5-y time frame of our analysis, with little evidence for systematic change at the community level. In contrast, temporal ß-diversity changes are more prevalent, and the two diversity dimensions are decoupled at both the within- and among-assemblage level. Consequently, a pressing research challenge is to establish how turnover supports regulation and when elevated temporal ß-diversity jeopardizes community integrity.

Climate-driven shift in coral morphological structure predicts decline of juvenile reef fishes.

Rapid intensification of environmental disturbances has sparked widespread decline and compositional shifts in foundation species in ecosystems worldwide. Now, an emergent challenge is to understand the consequences of shifts and losses in such habitat-forming species for associated communities and ecosystem processes. Recently, consecutive coral bleaching events shifted the morphological makeup of habitat-forming coral assemblages on the Great Barrier Reef (GBR). Considering the disparity of coral morphological growth forms in shelter provision for reef fishes, we investigated how shifts in the morphological structure of coral assemblages affect the abundance of juvenile and adult reef fishes. We used a temporal dataset from shallow reefs in the northern GBR to estimate coral convexity (a fine-scale quantitative morphological trait) and two widely used coral habitat descriptors (coral cover and reef rugosity) for disentangling the effects of coral morphology on reef fish assemblages. Changes in coral convexity, rather than live coral cover or reef rugosity, disproportionately affected juvenile reef fishes when compared to adults, and explained more than 20% of juvenile decline. The magnitude of this effect varied by fish body size with juveniles of small-bodied species showing higher vulnerability to changes in coral morphology. Our findings suggest that continued large-scale shifts in the relative abundance of morphological groups within coral assemblages are likely to affect population replenishment and dynamics of future reef fish communities. The different responses of juvenile and adult fishes according to habitat descriptors indicate that focusing on coarse-scale metrics alone may mask fine-scale ecological responses that are key to understand ecosystem functioning and resilience. Nonetheless, quantifying coral morphological traits may contribute to forecasting the structure of reef fish communities on novel reef ecosystems shaped by climate change.

Partitioning colony size variation into growth and partial mortality.

Body size is a trait that broadly influences the demography and ecology of organisms. In unitary organisms, body size tends to increase with age. In modular organisms, body size can either increase or decrease with age, with size changes being the net difference between modules added through growth and modules lost through partial mortality. Rates of colony extension are independent of body size, but net growth is allometric, suggesting a significant role of size-dependent mortality. In this study, we develop a generalizable model of partitioned growth and partial mortality and apply it to data from 11 species of reef-building coral. We show that corals generally grow at constant radial increments that are size independent, and that partial mortality acts more strongly on small colonies. We also show a clear life-history trade-off between growth and partial mortality that is governed by growth form. This decomposition of net growth can provide mechanistic insights into the relative demographic effects of the intrinsic factors (e.g. acquisition of food and life-history strategy), which tend to affect growth, and extrinsic factors (e.g. physical damage, and predation), which tend to affect mortality.

More is less: net gain in species richness, but biotic homogenization over 140 years.

While biodiversity loss continues globally, assessments of regional and local change over time have been equivocal. Here, we assess changes in plant species richness and beta diversity over 140 years at the level of regions within a country. Using 19th-century flora censuses for 14 Danish regions as a baseline, we overcome previous criticisms concerning short time series and neglect of completely altered habitats. We find that species composition has changed dramatically and directionally across all regions. Substantial species losses were more than offset by large gains, resulting in a net increase in species richness in all regions. The occupancy of initially widespread species increased, while initially rare species lost terrain. These changes were accompanied by strong biotic homogenization; i.e. regions are more similar now than they were 140 years ago. Species declining in Denmark were found to be in similar decline all over Northern Europe.

A balance of winners and losers in the Anthropocene.

Scientists disagree about the nature of biodiversity change. While there is evidence for widespread declines from population surveys, assemblage surveys reveal a mix of declines and increases. These conflicting conclusions may be caused by the use of different metrics: assemblage metrics may average out drastic changes in individual populations. Alternatively, differences may arise from data sources: populations monitored individually, versus whole-assemblage monitoring. To test these hypotheses, we estimated population change metrics using assemblage data. For a set of 23 241 populations, 16 009 species, in 158 assemblages, we detected significantly accelerating extinction and colonisation rates, with both rates being approximately balanced. Most populations (85%) did not show significant trends in abundance, and those that did were balanced between winners (8%) and losers (7%). Thus, population metrics estimated with assemblage data are commensurate with assemblage metrics and reveal sustained and increasing species turnover.