The fast pace of the current anthropogenic-induced mass extinction has demanded conservation scientists to provide high-quality information that can guide policies aiming to minimize and mitigate the impacts of biodiversity loss. As no species lives alone in the environment, understanding how species loss can cascade across ecosystems through the networks formed by organisms and their interactions has become a major challenge. Empirical evidence shows that the loss of a single species may create a domino effect, resulting in the extinction of other species or even entire ecosystems. These cascading effects might be even more pervasive and severe in some ecological systems such as mutualisms, in which species exploit each other for mutual net benefits (Dunn et al. 2009). Ecologists believe that nearly every species is involved in at least one mutualistic interaction (Bronstein 2015). Consequently, this ubiquitous type of ecological interaction has important ecological and evolutionary implications for the maintenance of ecosystem diversity, functioning and stability.
Another complicating factor in studying cascading effects in ecological communities, is that organisms come in a complex array of forms, behaviors and functions, a dimension of biodiversity known as Functional diversity. It implies that the role an organism plays in an ecosystem is not determined by its taxonomic identity, but rather by its behavioral and morphological traits. Although every species differs from one another, they are not equally different, and some species may have disproportionate effects on ecosystems due to their unique set of traits. In an earlier contribution, we have shown that the loss of functional distinctiveness – a facet of functional diversity that warrants unique ecological interactions – takes a larger toll in ecological communities than the loss of taxonomic units alone. As species traits are largely a legacy of their evolutionary history, we might expect that the mode and tempo of trait evolution may play an important role in ecological dynamics and how ecological communities respond to perturbations. For instance, ecological communities in which species interactions have a strong phylogenetic component are more likely to suffer co-extinctions following primary extinction events (Rezende et al. 2007).
In a recent paper, we evaluated how evolutionary dynamics affects trait-based cascades in theoretical mutualistic communities, using minimal model systems of phylogenetically structured mutualistic networks. To do so, we simulated the evolutionary dynamics of bipartite mutualistic networks, formed by two sets of interacting species, in which species interactions are generated by a single-trait complementarity model. This modeling approach mimics ecological networks such as a pollination network formed by flowering plants and birds, in which species interactions is determined by flowers’ corolla length and birds’ bill length. Traits were simulated under distinct evolutionary dynamics:
- Traits following a random walk through evolutionary time (i.e., Brownian motion model of trait evolution);
- Recent diversification of traits with low phylogenetic signal (i.e., the tendency of related species to resemble each other more than species drawn at random from the same tree);
- Early diversification of traits with a strong phylogenetic signal.
We explored the effects of primary extinctions based on two specific dimensions of functional diversity expected to have strong consequences to ecological dynamics: size-related traits (a proxy of body size) and trait distinctiveness. We then estimated network robustness, a measure of the tolerance of a network to species loss based on ‘knockout extinction simulations’. The main idea behind robustness analyses is to evaluate co-extinction rates and their impacts on the integrity of a network (For more details in this kind of analysis see a post Frederico has recently written, and , a post I have written for a related paper here). In our case, we simulated three scenarios of primary species loss :
- Random extinctions, which serves as a baseline scenario;
- Loss of functional distinctiveness, in which species that are more different to the others in terms of their trait values are sequentially eliminated;
- Extinctions biased towards larger sizes, in which species with larger trait values are eliminated first.
In a nutshell, the results of our simulations suggest that mutualistic networks are especially vulnerable to extinctions based on trait distinctiveness, more so than size-based extinctions. This is an important result because it reinforces the notion that the role that functionally unique species perform in ecological systems, cannot be compensated for by the remaining species. Functionally distinct species are irreplaceable components of ecosystems, and yet, this facet of functional diversity still ignored in most conservation practices. This result stress a need to refocus conservation agenda towards species that play unique roles in ecosystems.
We also show that the mode of trait evolution affects how networks cope with trait-based cascades and that co-extinction cascades spread across phylogenetically related species, increasing the erosion of biodiversity. Networks in which phylogenetic signal in traits are strong are less robust than networks with weak phylogenetic signal, which means that mutualistic networks based upon current adaptations are more likely to cope with extinction dynamics than networks that are based upon conserved traits.
Despite its simplicity, our modeling approach reveals the importance of taking evolutionary dynamics into account in order to describe and predict the effects of extinctions cascades in ecological systems. To find out more, read our new paper “The role of evolutionary modes for trait-based cascades in mutualistic networks” just published in the journal Ecological Modelling.
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