Conserving the abundance of nonthreatened species.

Human modification of the environment is driving declines in population size and distributional extent of much of the world's biota. These declines extend to many of the most abundant and widespread species, for which proportionally small declines can result in the loss of vast numbers of individuals, biomass, and interactions. These losses could have major localized effects on ecological and cultural processes and services without elevating a species' global extinction risk. Although most conservation effort is directed at species threatened with extinction in the very near term, the value of retaining abundance regardless of global extinction risk is justifiable based on many biodiversity or ecosystem service metrics, including cultural services, at scales from local to global. The challenges of identifying conservation priorities for widespread and abundant species include quantifying the effects of species' abundance on services and understanding how these effects are realized as populations decline. Negative effects of population declines may be disconnected from the threat processes driving declines because of species movements and environment flows (e.g., hydrology). Conservation prioritization for these species shares greater similarity with invasive species risk assessments than extinction risk assessments because of the importance of local context and per capita effects of abundance on other species. Because conservation priorities usually focus on preventing the extinction of threatened species, the rationale and objectives for incorporating declines of nonthreatened species must be clearly articulated, going beyond extinction risk to encompass the range of likely harmful effects (e.g., secondary extinctions, loss of ecosystem services) if declines persist or are not reversed. Research should focus on characterizing the effects of local declines in species that are not threatened globally across a range of ecosystem services and quantifying the spatial distribution of these effects through the distribution of abundance. The case for conserving abundance in nonthreatened species can be made most powerfully when the costs of losing this abundance are better understood.

Conservación de la Abundancia de Especies No Amenazadas Resumen La modificación del ambiente causada por los humanos está resultando en la declinación del tamaño poblacional y de la extensión de la distribución de la mayor parte de la biota mundial. Estas declinaciones llegan hasta muchas de las especies más abundantes y con mayor distribución, para las cuales una declinación proporcionalmente pequeña puede resultar en la pérdida de un número extenso de individuos, biomasa e interacciones. Estas pérdidas podrían tener mayores efectos localizados sobre los procesos y servicios ecológicos y culturales sin elevar el riesgo de extinción mundial de la especie. Aunque casi todos los esfuerzos de conservación están dirigidos hacia especies bajo amenaza de extinción a corto plazo, el valor de mantener la abundancia sin importar el riesgo de extinción mundial es justificable con base en muchas medidas de biodiversidad o de servicios ambientales, incluyendo los servicios culturales, a escalas desde lo local hasta lo global. El reto de identificar prioridades de conservación para especies abundantes y de distribución extensa incluye la cuantificación de los efectos que la abundancia de la especie tiene sobre los servicios y el entendimiento de cómo estos efectos ocurren conforme las poblaciones declinan. Los efectos negativos de la declinación poblacional pueden estar desconectados del proceso que ocasiona la declinación por causa del movimiento de las especies y los flujos ambientales (p. ej.: la hidrología). La priorización de la conservación de estas especies comparte muchas más similitudes con la evaluación de riesgo de las especies invasoras que las evaluaciones de extinción de riesgo debido a la importancia del contexto local y los efectos per cápita de la abundancia sobre otras especies. Ya que los esfuerzos de conservación generalmente se enfocan en la prevención de la extinción de las especies amenazadas, la lógica y los objetivos detrás de la incorporación de las declinaciones de las especies no amenazadas deben estar articulados claramente, llegando más allá del riesgo de extinción para englobar la gama de efectos dañinos probables (p. ej.: extinciones secundarias, pérdida de servicios ambientales) en el caso de que las declinaciones persistan o no sean revertidas. La investigación debería enfocarse en la caracterización de los efectos de las declinaciones locales de especies que no estén amenazadas mundialmente a lo largo de una gama de servicios ambientales y en la cuantificación de la distribución espacial de estos efectos por medio de la distribución de la abundancia. Se puede argumentar de manera más poderosa el caso para la conservación de la abundancia de especies no amenazadas cuando se entienden mejor los costos de la pérdida de esta abundancia.

A global database and "state of the field" review of research into ecosystem engineering by land animals.

Ecosystem engineers have been widely studied for terrestrial systems, but global trends in research encompassing the range of taxa and functions have not previously been synthesised. We reviewed contemporary understanding of engineer fauna in terrestrial habitats and assessed the methods used to document patterns and processes, asking: (a) which species act as ecosystem engineers and with whom do they interact? (b) What are the impacts of ecosystem engineers in terrestrial habitats and how are they distributed? (c) What are the primary methods used to examine engineer effects and how have these developed over time? We considered the strengths, weaknesses and gaps in knowledge related to each of these questions and suggested a conceptual framework to delineate "significant impacts" of engineering interactions for all terrestrial animals. We collected peer-reviewed publications examining ecosystem engineer impacts and created a database of engineer species to assess experimental approaches and any additional covariates that influenced the magnitude of engineer impacts. One hundred and twenty-two species from 28 orders were identified as ecosystem engineers, performing five ecological functions. Burrowing mammals were the most researched group (27%). Half of all studies occurred in dry/arid habitats. Mensurative studies comparing sites with and without engineers (80%) were more common than manipulative studies (20%). These provided a broad framework for predicting engineer impacts upon abundance and species diversity. However, the roles of confounding factors, processes driving these patterns and the consequences of experimentally adjusting variables, such as engineer density, have been neglected. True spatial and temporal replication has also been limited, particularly for emerging studies of engineer reintroductions. Climate change and habitat modification will challenge the roles that engineers play in regulating ecosystems, and these will become important avenues for future research. We recommend future studies include simulation of engineer effects and experimental manipulation of engineer densities to determine the potential for ecological cascades through trophic and engineering pathways due to functional decline. We also recommend improving knowledge of long-term engineering effects and replication of engineer reintroductions across landscapes to better understand how large-scale ecological gradients alter the magnitude of engineering impacts.

The functional extinction of Andean megafauna.

Controversy exists over the cause and timing of the extinction of the Pleistocene megafauna. In the tropical Andes, deglaciation and associated rapid climate change began ~8,000 years before human arrival, providing an opportunity to separate the effects of climate change from human hunting on megafaunal extinction. We present a paleoecological record spanning the last 25,000 years from Lake Pacucha, Peru (3,100 m elevation). Fossil pollen, charcoal, diatoms, and the dung fungus Sporormiella, chronicle a two-stage megaherbivore population collapse. Sporormiella abundance, the proxy for megafaunal presence, fell sharply at ~21,000 years ago, but rebounded prior to a permanent decline between ~16,800 and 15,800 years ago. This two-stage decline in megaherbivores resulted in a functional extinction by ~15,800 years ago, 3,000 years earlier than known human occupation of the high Andes. Declining megaherbivore populations coincided with warm, wet intervals. Climatic instability and megafaunal population collapse triggered an ecological cascade that resulted in novel floral assemblages, and increases in woody species, fire frequency, and plant species that were sensitive to trampling. Our data revealed that Andean megafaunal populations collapsed due to positive feedbacks between habitat quality and climate change rather than human activity.

Extinction of one of the world's largest freshwater fishes: Lessons for conserving the endangered Yangtze fauna.

The mega river ecosystem of the Yangtze River was once home to diverse aquatic megafauna but is increasingly affected by various anthropogenic stressors that have resulted in continuous loss of biodiversity, such as the probable extinction of Yangtze River Dolphin. The Chinese paddlefish, Psephurus gladius, was one of only two extant members of a relict lineage that was most diverse and widespread 34-75 million years ago. It is also one of the largest freshwater fish species, reaching up to 7 m in length. The Chinese paddlefish was once common in the Yangtze River, with c.25 t being harvested per annum during the 1970s. Populations have, however, declined drastically since the late 1970s as a result of overfishing and habitat fragmentation. Here, a basin-wide capture survey during 2017-2018 found 332 fish species, but did not find a single specimen of Chinese paddlefish. Furthermore, 140 historically reported fish species have not been found and most of them are considered highly endangered. Based on 210 sightings of Chinese paddlefish during the period 1981-2003, we estimated the timing of extinction to be by 2005, and no later than by 2010. In addition, the paddlefish probably became functionally extinct (i.e. it was unable to reproduce) by 1993, before it went extinct. It is likely that the lack of reproduction was among the major causes of extinction. As no individuals exist in captivity, and no living tissues are conserved for potential resurrection, the fish should be considered extinct according to the IUCN Red List criteria. The delayed extinction of Chinese paddlefish resulted from multiple threats, suggesting that optimizing conservation efforts on endangered Yangtze fauna is urgently needed.

Cryptic function loss in animal populations.

The essential functional roles performed by animal species are lost when they become locally extinct, and ecosystems are critically threatened by this decline in functional diversity. Theory that links function, diversity, and ecosystem stability exists but fails to assess function loss that occurs in species with persistent populations. The entire functional role of a species, or a critical component of it, can be lost following large population declines (functional extinction), following population increase, or after behavioural adaptations to changes in the population, community, habitat, or climate. Here, we provide a framework that identifies the scenarios under which 'cryptic' function loss can occur in persistent populations. Cryptic function loss is potentially widespread and critically threatens ecosystem stability across the globe.

Low redundancy in seed dispersal within an island frugivore community.

The low species diversity that often characterizes island ecosystems could result in low functional redundancy within communities. Flying foxes (large fruit bats) are important seed dispersers of large-seeded species, but their redundancy within island communities has never been explicitly tested. In a Pacific archipelago, we found that flying foxes were the sole effective disperser of 57 % of the plant species whose fruits they consume. They were essential for the dispersal of these species either because they handled >90 % of consumed fruit, or were the only animal depositing seeds away from the parent canopy, or both. Flying foxes were especially important for larger-seeded fruit (>13 mm wide), with 76 % of consumed species dependent on them for dispersal, compared with 31 % of small-seeded species. As flying foxes decrease in abundance, they cease to function as dispersers long before they become rare. We compared the seed dispersal effectiveness (measured as the proportion of diaspores dispersed beyond parent crowns) of all frugivores for four plant species in sites where flying foxes were, and were not, functionally extinct. At both low and high abundance, flying foxes consumed most available fruit of these species, but the proportion of handled diaspores dispersed away from parent crowns (quality) was significantly reduced at low abundance. Since alternative consumers (birds, rodents and land crabs) were unable to compensate as dispersers when flying foxes were functionally extinct, we conclude that there is almost no redundancy in the seed dispersal function of flying foxes in this island system, and potentially on other islands where they occur. Given that oceanic island communities are often simpler than continental communities, evaluating the extent of redundancy across different ecological functions on islands is extremely important.