Biology subjects

Purves, D. W.

Publications and source records attributed to Purves, D. W..

4 recordsLinked to original sources

bioRxiv2020

Ecological theory predicts ecosystem stressor interactions in freshwater ecosystems, but highlights the strengths and weaknesses of the additive null model

Understanding and predicting how multiple co-occurring environmental stressors combine to affect biodiversity and ecosystem services is an on-going grand challenge for ecology. So far progress has been made through accumulating large numbers of smaller-scale individual studies that are then investigated by meta-analyses to look for general patterns. In particular there has been an interest in checking for so-called ecological surprises where stressors interact in a synergistic manner. Recent reviews suggest that such synergisms do not dominate, but few other generalities have emerged. This lack of general prediction and understanding may be due in part to a dearth of ecological theory that can generate clear hypotheses and predictions to tested against empirical data. Here we close this gap by analysing food web models based upon classical ecological theory and comparing their predictions to a large (546 interactions) dataset for the effects of pairs of stressors on freshwater communities, using trophic- and population-level metrics of abundance, density, and biomass as responses. We find excellent overall agreement between the stochastic version of our models and the experimental data, and both conclude additive stressor interactions are the most frequent, but that meta-analyses report antagonistic summary interaction classes. Additionally, we show that the statistical tests used to classify the interactions are very sensitive to sampling variation. It is therefore likely that current weak sampling and low sample sizes are masking many non-additive stressor interactions, which our theory predicts to dominate when sampling variation is removed. This leads us to suspect ecological surprises may be more common than currently reported. Our results highlight the value of developing theory in tandem with empirical tests, and the need to examine the robustness of statistical machinery, especially the widely-used null models, before we can draw strong conclusions about how environmental drivers combine.

ecology↗

bioRxiv2020

Modelling parameter uncertainty reveals bushmeat yield versus survival trade-offs in heavily-hunted duiker Cephalophus spp.

Reliably predicting sustainable exploitation levels for many tropical species subject to hunting remains a difficult task, largely because of the inherent uncertainty associated with estimating parameters related to both population dynamics and hunting pressure. Here, we investigate a modelling approach to support decisions in bushmeat management which explicitly considers parameter uncertainty. We apply the approach to duiker Cephalophus spp., assuming either a constant quota-based, or a constant proportional harvesting, strategy. Within each strategy, we evaluate different hunting levels in terms of both average yield and survival probability, over different time horizons. Under quota-based harvesting, considering uncertainty revealed a trade-off between yield and extinction probability that was not evident when ignoring uncertainty. The highest yield was returned by a quota that implied a 40% extinction risk, whereas limiting extinction risk to 10% reduced yield by 50%-70%. By contrast, under proportional harvesting, there was no trade-off between yield and extinction probability. The maximum proportion returned a yield comparable with the maximum possible under quota-based harvesting, but with extinction risk below 10%. However, proportional harvesting can be harder to implement in practice because it depends on an estimate of population size. In both harvesting approaches, predicted yields were highly right-skewed with median yields differing from mean yields, implying that decision outcomes depend on attitude to risk. The analysis shows how an explicit consideration of all available information, including uncertainty, can, as part of a wider process involving multiple stakeholders, help inform harvesting policies.

ecology↗

bioRxiv2019

Modelling variation in bushmeat harvesting among seven African ecosystems using the Madingley Model: yield, survival and ecosystem impacts

In principle, both the maximum sustainable yields of bushmeat, and the ecosystem impacts of extracting those yields, are likely to vary among ecosystems due to differences in the structure and function of ecosystems, but the data necessary to estimate this variation is lacking. Here, we compare seven different ecosystems on a North-South latitudinal gradient in Central Africa in terms of their trophic structure and capacity to support yields from bushmeat harvesting, using the Madingley General Ecosystem Model. The only factor that varies across simulations of these ecosystems is the climate which drives differences in vegetation structure and function, leading in turn to differences in the structure of the ecological community that emerge from the model. In a series of experiments (n=30), we simulate constant proportional harvesting of small and medium-sized warm-bloodied heterotrophs (1-23kg) over 30 years, recording expected bushmeat yields, and impacts on ecosystem structure, including trophic structure. Predictions for animal densities and trophic structures in the pristine (no harvesting) case varied among the ecosystems, with implications for bushmeat harvesting. For example, wooded savannah ecosystems stood out as having the greatest pristine densities in the target groups (11000-12000 animals per kilometre squared), greatest yields (100% higher than the tropical forest and 1000% higher than the desert ecosystem), and were the most resilient to harvesting. By contrast, small and medium-sized endothermic heterotrophs contributed only a small proportion of heterotrophs in the desert ecosystem, and thus the potential for bushmeat harvesting here was low. In all ecosystems, harvesting at the rate that maximised yield (55-65% population per year, except for the southern desert ecosystem) had strong impacts, causing drastic reductions in target functional groups, coupled with increases in smaller- and larger-bodied animals. Forest and desert ecosystems were particularly sensitive. Overall, the results suggest that, even for similar functional groups, bushmeat harvesting policies will need to vary substantially among ecosystems - and show that general ecosystem models could be a useful tool in helping to guide these policies.

ecology↗

bioRxiv2018

Non-linear changes in modelled terrestrial ecosystems subjected to perturbations

When perturbed ecosystems undergo rapid and non-linear changes, this can result in regime shifts to an entirely different ecological state. The need to understand the extent, nature, magnitude and reversibility of these changes is urgent given the profound effects that humans are having on the natural world. It remains very challenging to empirically document non-linear changes and regime shifts within complex, real ecological communities, or even to demonstrate such shifts in simplified experimental systems. General ecosystem models, which simulate the dynamics of entire ecological communities based on a mechanistic representation of ecological processes, provide an alternative and novel way to project ecosystem changes across all scales and trophic levels and to forecast impact thresholds beyond which dramatic or irreversible changes may occur. We model non-linear changes in four terrestrial biomes subjected to human removal of plant biomass, such as occurs through agricultural land-use change. We find that irreversible and non-linear responses are predicted to be common where removal of vegetation exceeds 80% (a level that occurs across nearly 10% of the terrestrial surface), especially for organisms at higher trophic levels and in less productive ecosystems such as drylands. Very large, irreversible changes to the entire ecosystem structure are expected at levels of vegetation removal akin to those in the most intensively used real-world ecosystems. Our results suggest that the projected 21st century rapid increases in agricultural land conversion to feed an expanding human population, may lead to widespread trophic cascades and in some cases irreversible changes to the structure of ecological communities.

ecology↗