Neutral theory reveals the challenge of bending the curve for the post-2020 Global Biodiversity Framework.

In October, nations of the world will begin negotiations for the post-2020 Global Biodiversity Framework under the Convention on Biological Diversity. An influential ambition is "bending the curve of biodiversity loss," which aims to reverse the decline of global biodiversity indicators. A second relevant, yet less prominent, milestone is the 20th anniversary of the publication of The Unified Neutral Theory of Biodiversity and Biogeography. Here, I apply neutral theory to show how global biodiversity indicators for population size (Living Planet Index) and extinction threat (Red List Index) decline under neutral ecological drift. This demonstrates that declining indicators are not solely caused by deterministic species-specific or geographical patterns of biodiversity loss. Instead, indicators are sensitive to nondirectional stochasticity. Thus, "bending the curve" could be assessed relative to a counterfactual based on neutral theory, rather than static baselines. If used correctly, the 20-year legacy of neutral theory can be extended to make a valuable contribution to the post-2020 Global Biodiversity Framework.

An empirical confirmation of diversified bet hedging as a survival strategy in unpredictably varying environments.

Environmental change jeopardizes the survival of species from variable environments by making the occurrence of favorable conditions less predictable. For organisms with long-lived propagules (e.g., spores, eggs, or seeds), the theory of diversified bet hedging (DBH) predicts that delayed hatching over different growing seasons can help populations avoid extinction. Empirical observations in different organisms are consistent with DBH, but integrated tests that simultaneously validate the main theoretical assumptions and predictions are lacking. In this study, we combine field and multi-generational lab experiments to provide a complete test of DBH. Consistent with DBH predictions, resting egg clutches of the fairy shrimp Branchipodopsis wolfi, which inhabits rain-fed temporary rock pool environments with unpredictable inundations, hatched partially over a succession of inundations with identical hatching cues. Bet hedging was more common in populations from more unpredictable habitats where hatching fractions were lower. This differentiation in hatching strategies was preserved after two generations under common garden conditions, which implies intrinsic (epi-)genetic control of hatching. Finally, a demographic model confirmed that lower hatching fractions increase long-term population growth in unpredictable habitats. With this paper we propose a method to calculate probabilities of successful recruitment for organisms that use imperfect cues and show that this drives selection for variation in life history strategies as part of a DBH strategy.

Random population fluctuations bias the Living Planet Index.

The Living Planet Index (LPI) is a standardized indicator for tracking population trends through time. Due to its ability to aggregate many time series in a single metric, the LPI has been proposed as an indicator for the Convention on Biological Diversity's post-2020 Global Biodiversity Strategy. However, here we show that random population fluctuations introduce biases when calculating the LPI. By combining simulated and empirical data, we show how random fluctuations lead to a declining LPI even when overall population trends are stable and imprecise estimates of the LPI when populations increase or decrease nonlinearly. We applied randomization null models that demonstrate how random fluctuations exaggerate declines in the global LPI by 9.6%. Our results confirm substantial declines in the LPI but highlight sources of uncertainty in quantitative estimates. Randomization null models are useful for presenting uncertainty around indicators of progress towards international biodiversity targets.

Reduced ecological resilience jeopardizes zero loss of biodiversity using the mitigation hierarchy.

One proposal for the Convention on Biological Diversity's post-2020 strategic plan is 'zero loss' of natural habitats. However, the feasibility of zero loss was questioned during the Trondheim Conference for Biodiversity, and it was suggested that biodiversity losses are instead balanced by compensatory efforts (that is, 'no net loss'). The focus on net outcomes is echoed by separate calls for a global mitigation hierarchy to deliver no net biodiversity loss and accommodate both conservation and development goals. Here we show that 'no net loss' is not the same as 'zero loss'. We use a delayed differential model of nonlinear habitat dynamics to demonstrate how applying the mitigation hierarchy for net biodiversity outcomes will lead to biodiversity declines by midcentury. Delayed compensation of human impacts reduces ecological resilience and causes prolonged biodiversity losses. These effects are greatest when impacts are large and compensation delays are long. Our results support the use of fixed targets, rather than net outcomes, as part of the post-2020 biodiversity framework.

Ecosystem services and ecological degradation of communal wetlands in a South African biodiversity hotspot.

Wetlands provide important ecosystem services to rural communities. However, wetlands are often on communal land, so they may become degraded when individual users act to maximize their personal benefit from ecosystem services without bearing the full environmental costs of their actions. Although it is possible to manage communal resources sustainably, this depends on the dynamics of the socio-ecological system. In this study, we used a structured questionnaire to examine whether demographic characteristics of a rural community and the propensity for partaking in damage-causing activities affected the benefits obtained from the wetlands. Responses from 50 households in the rural Hlabathi administrative area within the Maputo-Albany-Pondoland Biodiversity Hotspot, South Africa, indicated that the entire community obtains some benefits from wetlands; most notably regulating ecosystem services. However, males were more likely to benefit from wetlands, which highlights a potential power imbalance. Respondents were more likely to blame others for wetland degradation, although there was no link between the damage-causing activities and benefits from wetlands. The high dependence on ecosystem services by community members, when combined with gender-based power imbalances and the propensity to blame others, could jeopardize the sustainable use of communal wetlands. Therefore, we describe how strong leadership could nurture a sustainable social-ecological system by integrating ecological information and social empowerment into a multi-level governance system.

Climate change jeopardizes the persistence of freshwater zooplankton by reducing both habitat suitability and demographic resilience.

BACKGROUND: Higher temperatures and increased environmental variability under climate change could jeopardize the persistence of species. Organisms that rely on short windows of rainfall to complete their life-cycles, like desert annual plants or temporary pool animals, may be particularly at risk. Although some could tolerate environmental changes by building-up banks of propagules (seeds or eggs) that buffer against catastrophes, climate change will threaten this resilience mechanism if higher temperatures reduce propagule survival. Using a crustacean model species from temporary waters, we quantified experimentally the survival and dormancy of propagules under anticipated climate change and used these demographic parameters to simulate long term population dynamics. RESULTS: By exposing propagules to present-day and projected daily temperature cycles in an 8 month laboratory experiment, we showed how increased temperatures reduce survival rates in the propagule bank. Integrating these reduced survival rates into population models demonstrated the inability of the bank to maintain populations; thereby exacerbating extinction risk caused by shortened growing seasons. CONCLUSIONS: Overall, our study demonstrates that climate change could threaten the persistence of populations by both reducing habitat suitability and eroding life-history strategies that support demographic resilience.

Modelling the sensitivity of life history traits to climate change in a temporary pool crustacean.

Temporary pool inhabitants face altered inundation regimes under climate change. While their exposure to these changes has received considerable attention, few studies have investigated their sensitivity or adaptability. Here, we use zooplankton as a model to explore how decreasing hydroperiods affect extinction risks and assess whether changes in life history traits could promote persistence. For this, we construct a three-stage matrix population model parameterised with realistic life-history values for the fairy shrimp Branchipodopsis wolfi from pools with varying hydroperiods. Our results suggest that extinction risks increase drastically once the median hydroperiod drops below a critical threshold. Although changes in life-history parameters could potentially compensate for this risk, the relative importance of each trait for population growth depends on the median hydroperiod. For example, survival of dormant eggs seemed to be most important when hydroperiods were short while the survival of freshly laid eggs and adult individuals were more important in longer-lived pools. Overall, this study demonstrates that zooplankton species are sensitive to climate change and that the adaptive capacity of organisms from temporary pools with dissimilar hydrology hinges on selection of different life history traits.

Adding energy gradients and long-distance dispersal to a neutral model improves predictions of Madagascan bird diversity.

Macroecological patterns are likely the result of both stochastically neutral mechanisms and deterministic differences between species. In Madagascar, the simplest stochastically neutral hypothesis - the mid-domain effects (MDE) hypothesis - has already been rejected. However, rejecting the MDE hypothesis does not necessarily refute the existence of all other neutral mechanisms. Here, we test whether adding complexity to a basic neutral model improves predictions of biodiversity patterns. The simplest MDE model assumes that: (1) species' ranges are continuous and unfragmented, (2) are randomly located throughout the landscape, and (3) can be stacked independently and indefinitely. We designed a simulation based on neutral theory that allowed us to weaken each of these assumptions incrementally by adjusting the habitat capacity as well as the likelihood of short- and long-distance dispersal. Simulated outputs were compared to four empirical patterns of bird diversity: the frequency distributions of species richness and range size, the within-island latitudinal diversity gradient, and the distance-decay of species compositional similarity. Neutral models emulated empirical diversity patterns for Madagascan birds accurately. The frequency distribution of range size, latitudinal diversity gradient, and the distance-decay of species compositional similarity could be attributed to stochastic long-distance migration events and zero-sum population dynamics. However, heterogenous environmental gradients improved predictions of the frequency distribution of species richness. Patterns of bird diversity in Madagascar can broadly be attributed to stochastic long-distance migration events and zero-sum population dynamics. This implies that rejecting simple hypotheses, such as MDE, does not serve as evidence against stochastic processes in general. However, environmental gradients were necessary to explain patterns of species richness and deterministic differences between species are probably important for explaining the distributions of narrow-range and endemic species.

Disturbance regime alters the impact of dispersal on alpha and beta diversity in a natural metacommunity.

Disturbance and dispersal are two fundamental ecological processes that shape diversity patterns, yet their interaction and the underlying mechanisms are still poorly understood, and evidence from natural systems is particularly lacking. Using an invertebrate rock pool metacommunity in South Africa as a natural model system, we studied potential interactive effects of disturbance regime and patch isolation on diversity patterns of species with contrasting dispersal modes (passive vs. active dispersal). Isolation and disturbance regime had negative synergistic effects on alpha diversity: both directly, by excluding late-successional species from isolated patches; and indirectly, by modulating establishment success of generalist predators in well-connected patches. Unimodal relationships between isolation and alpha diversity, as predicted by mass effects, were only detected for passive dispersers in frequently disturbed patches and not in active dispersers. For passive dispersers, indications for a positive effect of isolation and a negative effect of disturbance on beta diversity were found, presumably due to differences in deterministic succession and stochastic colonization-extinction dynamics among different patch types. Our findings illustrate that interactions between dispersal rates and disturbance regime are important when explaining species diversity patterns in metacommunities and support the idea that diversity in frequently disturbed habitats is more sensitive to effects of dispersal-based processes.