Community structure determines the predictability of population collapse

1. Early warning signals (EWS) are phenomenological tools that have been proposed as predictors of the collapse of biological systems. Although a growing body of work has shown the utility of EWS based on either statistics derived from abundance data or shifts in phenotypic traits such as body size, so far this work has largely focused on single species populations. 2. However, to predict reliably the future state of ecological systems, which inherently could consist of multiple species, understanding how reliable such signals are in a community context is critical. 3. Here, reconciling quantitative trait evolution and Lotka–Volterra equations, which allow us to track both abundance and mean traits, we simulate the collapse of populations embedded in mutualistic and multi‐trophic predator–prey communities. Using these simulations and warning signals derived from both population‐ and community‐level data, we showed the utility of abundance‐based EWS, as well as metrics derived from stability‐landscape theory (e.g. width and depth of the basin of attraction), were fundamentally linked. Thus, the depth and width of such stability‐landscape curves could be used to identify which species should exhibit the strongest EWS of collapse. 4. The probability a species displays both trait and abundance‐based EWS was dependent on its position in a community, with some species able to act as indicator species. In addition, our results also demonstrated that in general trait‐based EWS were less reliable in comparison with abundance‐based EWS in forecasting species collapses in our simulated communities. Furthermore, community‐level abundance‐based EWS were fairly reliable in comparison with their species‐level counterparts in forecasting species‐level collapses. 5. Our study suggests a holistic framework that combines abundance‐based EWS and metrics derived from stability‐landscape theory that may help in forecasting species loss in a community context

Effect of local climate anomalies on giraffe survival

With the rapid pace of global warming, there is an urgent need to understand survival responses to climate, particularly for large mammals that are already experiencing population declines associated with anthropogenic pressures such as poaching and habitat loss. We tested hypotheses about the interactive effects of local climatic anomalies (variations around a long-term mean) and proximity to edge of protected area boundaries on seasonal adult and juvenile survival in a population of 2,385 individually identified giraffes monitored over 8 years in the Tarangire Ecosystem of northern Tanzania. Temperature anomalies were positively correlated with seasonal survival of adult giraffes, suggesting these megaherbivores are adapted to hot conditions. Higher seasonal rainfall anomalies were negatively correlated with both juvenile and adult survival, and greater vegetation greenness was associated with lower adult survival. During seasons of anomalously high rainfall and vegetation greenness, higher parasite and disease abundance, poorer-quality nutrition in forage, and higher predation risk may all play a role in lowering giraffe survival. Furthermore, climate-associated reduction in survival was most pronounced during the short rainy season for adult giraffes living closer to the edge of protected areas, indicating that the influence of climate anomalies may be exacerbated by anthropogenic edge effects such as poaching or livestock keeping. Precipitation in East Africa is projected to increase substantially, with a greater proportion of rain falling during heavy events in the short rainy season, which may threaten persistence of giraffes in one of Earth’s most important landscapes for large mammals

Including trait-based early warning signals helps predict population collapse

Foreseeing population collapse is an on-going target in ecology, and this has led to the development of early warning signals based on expected changes in leading indicators before a bifurcation. Such signals have been sought for in abundance time-series data on a population of interest, with varying degrees of success. Here we move beyond these established methods by including parallel time-series data of abundance and fitness-related trait dynamics. Using data from a microcosm experiment, we show that including information on the dynamics of phenotypic traits such as body size into composite early warning indices can produce more accurate inferences of whether a population is approaching a critical transition than using abundance time-series alone. By including fitness-related trait information alongside traditional abundance-based early warning signals in a single metric of risk, our generalizable approach provides a powerful new way to assess what populations may be on the verge of collapse

Design of SNP markers for Aldabra giant tortoises using low coverage ddRAD-seq

The Aldabra giant tortoise (Aldabrachelys gigantea) is one of only two remaining giant tortoise species worldwide. Captive-bred A. gigantea are being used in rewilding projects in the Western Indian Ocean to functionally replace the extinct endemic giant tortoise species and restore degraded island ecosystems. Furthermore, large-scale translocations may become necessary as rising sea levels threaten the only wild population on the low-lying Aldabra Atoll. Critical management decisions would be greatly facilitated by insights on the genetic structure of breeding populations. We used a double-digest restriction-associated DNA sequencing (ddRAD-seq) approach to identify single nucleotide polymorphisms (SNP) among the wild population and two additional captive populations of A. gigantea. A total of 1674 unlinked, putatively neutral genome-wide SNPs were identified. The values of expected heterozygosity ranged from 0.33 to 0.5, whereas the minor allele frequency ranged from 0.20 to 0.5. These novel SNP markers will serve as useful tools for informing the conservation of A. gigantea

Spatial heterogeneity in temporal dynamics of Alpine bird communities along an elevational gradient

Aim: Mountains are biodiversity hotspots and are among the most sensitive ecosystems to ongoing global change being thus of conservation concern. Under this scenario, assessing how biological communities vary over time along elevational gradients and the relative effects of niche-based deterministic processes and stochastic events in structuring assemblages is essential. Here, we examined how the temporal trends of bird communities vary with elevation over a 20 year-period (1999-2018). We also tested for differences in temporal dynamics among habitat types (among-community variability) and functional groups (within-community variability). Taxon: 97 species of common breeding birds. Location: Swiss Alps. Methods: We used abundance data from the Swiss breeding bird survey to compute different temporal dynamics metrics (temporal turnover, synchrony, rate of community change, and community-level of covariance among species). We also examined the relative contribution of deterministic and stochastic processes in community assembly using the Raup-Crick method and the normalized stochasticity ratio. Results: We found that, with greater elevation, temporal species turnover increased while the rate of overall community change over successive years decreased, suggesting that high-elevation communities display more erratic dynamics with no clear trend. Despite this, we found a more deterministic assembly of alpine communities in comparison to those located at lower elevations. Deterministic processes had greater influence than stochastic processes on community assembly along the entire elevational gradient (80% of communities). Forest communities exhibited higher synchrony in comparison to the remaining habitats likely because they consisted of species with greater functional redundancy, whereas alpine communities were the least stable as a result of their low taxonomic richness (‘portfolio’ effect). Main conclusions: Community-level synchrony was overall positive supporting the idea that compensatory mechanisms are rare in natural biological communities. Our results suggest that rather than competition, the existence of differences in the ecological strategies of species may have a stabilizing effect on bird communities by weakening the concordance of species responses to fluctuations in environmental conditions (i.e., enhanced interspecific temporal asynchrony). This study provides evidence that, although species turnover in metacommunities is frequent, a high temporal turnover does not necessarily imply the overriding importance of stochastic processes.info:eu-repo/semantics/acceptedVersio

Temporal homogenization of functional and beta diversity in bird communities of the Swiss Alps

Aim Describing the spatio‐temporal dynamics of biotic communities is critical for understanding how environmental change can affect biodiversity. Mountains are especially susceptible to such changes (e.g., climate change) and, consequently, have been identified as ecosystems of conservation concern. With their sharp physical and ecological transitions, altitudinal gradients allow examining the influence of different climatic conditions and land use types on species assemblages across small spatial extents, and thus, they constitute natural laboratories to study diversity–environment relationships. Location Switzerland. Methods We take advantage of long‐term (20 years) monitoring data and an extensive trait dataset (100 traits) to examine spatial patterns, temporal trends, and spatio‐temporal dynamics in functional and beta diversity of bird communities in the Swiss Alps. Results Functional diversity indices showed a congruent pattern over time and across space; most indices decreased over the study period and were strongly correlated with altitude. In agreement with studies from the tropics, we found that communities in the lowlands were functionally over‐dispersed, whereas communities at higher elevations were functionally clustered. High‐altitude communities exhibited high functional originality, low levels of niche differentiation and a high turnover rate. Beta diversity declined over the study period. Conclusions Our findings suggest that pastoral abandonment does not result in an increase in avian functional diversity as most species colonizing woody‐encroached grasslands are functionally redundant, whereas alpine meadows are inhabited by species exhibiting a high degree of habitat specialization and unique functional traits. Hence, the tree line constitutes a boundary between two well‐differentiated functional groups: one representing a functional continuum from lowlands dominated by agricultural landscape to high‐mountain forests, and the other one composed of alpine communities. Overall, this study reveals a process of biotic homogenization (i.e., increasing functional similarity) across the last two decades in the Swiss Alps, coinciding with the recently reported increases in the abundance of generalist species

Demographic cost and mechanisms of adaptation to environmental stress in resurrected Daphnia

A characteristic feature of the Daphnia (Crustacea: Cladocera) life cycle are the so-called ephippia, which are fertilised eggs that need to undergo diapause. When they are shed by the female, they sink to the lake bottom, where they may become embedded in the sediment and may remain viable for decades. Extracting and hatching ephippia in the laboratory and subjecting resurrected lineages to conditions representative of historic lake environments allows retrospective investigation of life-history responses to environmental change. Here we reanalyse data from such a resurrection experiment (Piscia et al., 2015: Bull. Environ. Contam. Toxicol. 94:46-51). Contemporary and past lineages of Daphnia galeata Sars 1863 were obtained from Lake Orta (Italy), a deep, subalpine lake with a well-documented history of industrial copper pollution. Experimental Daphnia were subjected to three copper treatments representative of two levels of historic as well as to current (i.e., unpolluted) lake conditions, and life-table data were collected. With these data at hand, we first estimated vital rates (survival, maturation, and reproduction) and used these rates to project the asymptotic population growth rates (?) for each population-by-treatment combination. Next, we performed life-table response experiments (LTRE) to estimate the contributions of the vital rates to observed differences in ?. Finally, we used elasticity analysis to explore the functional relationship between ? and each of the vital rates. We found that survival rates were only compromised at elevated copper levels. Moreover, past, resurrected Daphnia had a higher ? at low copper concentrations compared to unpolluted conditions, but a lower ? when exposed to high copper levels. Contemporary Daphnia, on the other hand, only reproduced successfully in unpolluted water. Under these conditions, however, they had a higher population growth rate than the past Daphnia, suggesting a cost of copper tolerance in the latter. This cost was mainly due to a lower probability of reproduction and reduced fecundity, whereas survival remained largely unaffected. Finally, we found higher elasticity values of ? to survival than to reproductive rates. This suggests that any change in the environment that will affect survival rather than reproductive parameters will have a much larger impact on Lake Orta\u27s current Daphnia population

Effect of time series length and resolution on abundance‐ and trait‐based early warning signals of population declines

Seasonal environmental conditions shape the behavior and life history of virtually all organisms. Climate change is modifying these seasonal environmental conditions, which threatens to disrupt population dynamics. It is conceivable that climatic changes may be beneficial in one season but result in detrimental conditions in another because life-history strategies vary between these time periods. We analyzed the temporal trends in seasonal survival of yellow-bellied marmots (Marmota flaviventer) and explored the environmental drivers using a 40-y dataset from the Colorado Rocky Mountains (USA). Trends in survival revealed divergent seasonal patterns, which were similar across age-classes. Marmot survival declined during winter but generally increased during summer. Interestingly, different environmental factors appeared to drive survival trends across age-classes. Winter survival was largely driven by conditions during the preceding summer and the effect of continued climate change was likely to be mainly negative, whereas the likely outcome of continued climate change on summer survival was generally positive. This study illustrates that seasonal demographic responses need disentangling to accurately forecast the impacts of climate change on animal population dynamics