Coextinction Cascades Research Articles

Estimating co-extinction threats in terrestrial ecosystems.

The biosphere is changing rapidly due to human endeavour. Because ecological communities underlie networks of interacting species, changes that directly affect some species can have indirect effects on others. Accurate tools to predict these direct and indirect effects are therefore required to guide conservation strategies. However, most extinction-risk studies only consider the direct effects of global change-such as predicting which species will breach their thermal limits under different warming scenarios-with predictions of trophic cascades and co-extinction risks remaining mostly speculative. To predict the potential indirect effects of primary extinctions, data describing community interactions and network modelling can estimate how extinctions cascade through communities. While theoretical studies have demonstrated the usefulness of models in predicting how communities react to threats like climate change, few have applied such methods to real-world communities. This gap partly reflects challenges in constructing trophic network models of real-world food webs, highlighting the need to develop approaches for quantifying co-extinction risk more accurately. We propose a framework for constructing ecological network models representing real-world food webs in terrestrial ecosystems and subjecting these models to co-extinction scenarios triggered by probable future environmental perturbations. Adopting our framework will improve estimates of how environmental perturbations affect whole ecological communities. Identifying species at risk of co-extinction (or those that might trigger co-extinctions) will also guide conservation interventions aiming to reduce the probability of co-extinction cascades and additional species losses.

Novel plant–frugivore network on Mauritius is unlikely to compensate for the extinction of seed dispersers

Insular communities are particularly vulnerable to anthropogenic extinctions and introductions. Changes in composition of island frugivore communities may affect seed dispersal within the native plant community, risking ecological shifts and ultimately co-extinction cascades. Introduced species could potentially mitigate these risks by replacing ecological functions of extinct species, but conclusive evidence is lacking. Here, we investigate changes in plant–frugivore interactions involving frugivorous birds, mammals and reptiles in Mauritius, an oceanic island with an exceptionally well-specified frugivore community and well-described species introduction history. We demonstrate substantial losses of binary interaction partnerships (at the species level) resulting from native species extinctions, but also gains of equal numbers of novel interactions with introduced species, potentially supporting the idea that non-native species might compensate for lost seed dispersal. However, closer investigation of animal seed handling behaviour reveals that most interactions with seed dispersers are replaced by ecologically different interactions with seed predators. Therefore, restoration of seed dispersal functionality in this novel plant–frugivore community is unlikely.

The newly formed Mite Specialist Group of the IUCN’s Species Survival Commission and the conservation of global mite diversity

The most serious environmental challenge facing humanity is the massive, widespread and continuing loss of biodiversity due to human activities. The commonly reported root causes of the decline and extinction of species are the degradation, destruction and fragmentation of habitat; pollution; pesticide use; invasive species; climate change; and over-exploitation; with co-extinction cascades accelerating the losses. The current alarming rate of loss of species across the biodiversity spectrum has ecological, economic, social, aesthetic, cultural and spiritual impacts that directly undermine the welfare of all humanity. This unprecedented crisis demands an urgent, science-based, comprehensive, coordinated, global response. Among the organizations responding to the multifaceted challenge of biodiversity loss is the International Union for Conservation of Nature (IUCN). Its enormous pool of integrated expertise, technical capacity and policy experience makes the IUCN the global authority on the status of nature and the suite of measures needed to protect it. The largest of the IUCN’s six commissions is the Species Survival Commission, a science-based network of over 160 Specialist Groups, including 17 invertebrate groups; Red List Authorities; and Task Forces. Despite there being an estimated 500,000 – 1,500,000 mite species, and their ubiquity in global ecosystems and fundamental role in many ecological processes, mites have received minimal attention on the global conservation agenda. The role of the newly formed Mite Specialist Group, which gained official status in April 2021, is to redress that situation. The mission of the group, which currently includes 65 mite specialists, ecologists, botanists, environmentalists and conservation practitioners from 36 countries on five continents, is to contribute to a collaborative global effort to conserve mite diversity through research, education, advocacy, community engagement and specific conservation initiatives.

Interaction generalisation and demographic feedbacks drive the resilience of plant-insect networks to extinctions.

Understanding the processes driving ecological resilience, that is the extent to which systems retain their structure while absorbing perturbations, is a central challenge for theoretical and applied ecologists. Plant-insect assemblages are well-suited for the study of ecological resilience as they are species-rich and encompass a variety of ecological interactions that correspond to essential ecosystem functions. Mechanisms affecting community response to perturbations depend on both the natural history and structure of ecological interactions. Natural history attributes of the interspecific interactions, for example whether they are mutualistic or antagonistic, may affect the ecological resilience by controlling the demographic feedbacks driving ecological dynamics at the community level. Interaction generalisation may also affect resilience, by defining opportunities for interaction rewiring, the extent to which species are able to switch interactions in fluctuating environments. These natural history attributes may also interact with network structure to affect ecological resilience. Using adaptive network models, we investigated the resilience of plant-pollinator and plant-herbivore networks to species loss. We specifically investigated how fundamental natural history differences between these systems, namely the demographic consequences of the interaction and their level of generalisation-mediating rewiring opportunities-affect the resilience of dynamic ecological networks to extinctions. We also create a general benchmark for the effect of network structure on resilience simulating extinctions on theoretical networks with controlled structures. When network structure was static, pollination networks were less resilient than herbivory networks; this is related to their high levels of nestedness and the reciprocally positive feedbacks that define mutualisms, which made co-extinction cascades more likely and longer in plant-pollinator assemblages. When considering interaction rewiring, the high generalisation and the structure of pollination networks boosted their resilience to extinctions, which approached those of herbivory networks. Simulation results using theoretical networks suggested that the empirical structure of herbivory networks may protect them from collapse. Elucidating the ecological and evolutionary processes driving interaction rewiring is key to understanding the resilience of plant-insect assemblages. Accounting for rewiring requires ecologists to combine natural history with network models that incorporate feedbacks between species abundances, traits and interactions. This combination will elucidate how perturbations propagate at community level, reshaping biodiversity structure and ecosystem functions.

Pyrodiversity promotes interaction complementarity and population resistance.

Theory predicts that network characteristics may help anticipate how populations and communities respond to extreme climatic events, but local environmental context may also influence responses to extreme events. For example, altered fire regimes in many ecosystems may significantly affect the context for how species and communities respond to changing climate. In this study, I tested whether the responses of a pollinator community to extreme drought were influenced by the surrounding diversity of fire histories (pyrodiversity) which can influence their interaction networks via changing partner availability. I found that at the community level, pyrodiverse landscapes promote functional complementarity and generalization, but did not consistently enhance functional redundancy or resistance to simulated co‐extinction cascades. Pyrodiversity instead supported flexible behaviors that enable populations to resist perturbations. Specifically, pollinators that can shift partners and network niches are better able to take advantage of the heterogeneity generated by pyrodiversity, thereby buffering pollinator populations against changes in plant abundances. These findings suggest that pyrodiversity is unlikely to improve community‐level resistance to droughts, but instead promotes population resistance and community functionality. This study provides unique evidence that resistance to extreme climatic events depends on both network properties and historical environmental context.

Including rewiring in the estimation of the robustness of mutualistic networks

Abstract Species are entangled within communities by their interactions in such a manner that their local extinction may unchain coextinction cascades and impact community dynamics and stability. Despite increasing attention, simulation models to estimate the robustness of interaction networks largely neglect the important role of interaction rewiring, that is, the ability of species to switch partners. Here we propose a new method to incorporate the potential of species to replace lost partners into a widely used coextinction model to estimate network robustness. In this model, species are allowed to rewire their interactions after initial loss of partners according to probabilities derived from well‐known mechanisms that determine mutualistic interactions, for example, trait matching, phenological overlap and abundances. To illustrate the use of this method, we analyzed a well‐sampled dataset of mutualistic plant–hummingbird interactions. We found that (a) in general, rewiring increases the estimated robustness, (b) networks are similarly robust to the loss of pollinators or plants, (c) morphological matching and phenological overlap leads to higher robustness, (d) when multiple rewiring mechanisms are combined, however, robustness increases little, and (e) rewiring tends to increase robustness more in scenarios when the loss of generalist species occurs first. Our results suggest that the same mechanisms known to drive plant–hummingbird network structure are relevant in buffering the effects of species loss via rewiring. The method proposed here can be applied to a wide range of interaction networks and is flexible to the inclusion of other variables important in determining interactions for specific systems. It also allows changes on the assumptions regarding the importance of distinct mechanisms, for instance including a hierarchical relationship, which facilitates insights into the relative importance of multiple variables influencing network disassembling. The analytical framework we offer here represents a step towards a more realistic estimation on how species loss may affect the integrity of interaction networks.

Network structure embracing mutualism–antagonism continuums increases community robustness

Theory predicts that contrasting properties of mutualistic and antagonistic networks differentially promote community resilience to species loss. However, the outcome of most ecological interactions falls within a continuum between mutualism and antagonism, and we ignore the extent to which this interactions' continuum might influence community stability. Using a large dataset of interactions, we compared co-extinction cascades that either consider or ignore the mix of beneficial and detrimental actions that parrots exert on plants. When the antagonism-mutualism continuum was considered, a combination of the properties that separately enhance community stability in ecological networks emerged. This combination of properties led to an overall increase of the parrot community robustness to face plant species loss. Our results highlight that the conditional outcomes of interactions can influence the structure of ecological networks, thus affecting our predictions of community stability against eventual changes.

Plant Facilitation and Phylogenetics

The relationship between facilitation and evolutionary ecology is poorly understood. We review five issues elucidating how the phylogenetic relatedness of species provides insight into the role of facilitation in community assembly: (a) Are the facilitative interactions more common between species that differ in a regeneration niche? (b) Are facilitative interactions more common between distantly related species? (c) Do communities governed by facilitation (rather than competition) have higher phylogenetic diversity? (d) As facilitated juvenile plants mature, do they compete with their nurses more often if they are closely related to them? (e) How does the phylogenetic signature in a community reveal ecological processes, such as succession, regeneration dynamics, indirect interactions, and coextinction cascades? The evolutionary history of lineages explains the regeneration niche of species, which ultimately determines the facilitation-competition balance and therefore community assembly and dynamics. We apply this framework to the conservation of biodiversity and propose future research avenues.

Effects of Dam‐Induced Landscape Fragmentation on Amazonian Ant–Plant Mutualistic Networks

Mutualistic networks are critical to biological diversity maintenance; however, their structures and functionality may be threatened by a swiftly changing world. In the Amazon, the increasing number of dams poses a large threat to biological diversity because they greatly alter and fragment the surrounding landscape. Tight coevolutionary interactions typical of tropical forests, such as the ant-myrmecophyte mutualism, where the myrmecophyte plants provide domatia nesting space to their symbiotic ants, may be jeopardized by the landscape changes caused by dams. We analyzed 31 ant-myrmecophyte mutualistic networks in undisturbed and disturbed sites surrounding Balbina, the largest Central Amazonian dam. We tested how ant-myrmecophyte networks differ among dam-induced islands, lake edges, and undisturbed forests in terms of species richness, composition, structure, and robustness (number of species remaining in the network after partner extinctions). We also tested how landscape configuration in terms of area, isolation, shape, and neighborhood alters the structure of the ant-myrmecophyte networks on islands. Ant-myrmecophytic networks were highly compartmentalized in undisturbed forests, and the compartments had few strongly connected mutualistic partners. In contrast, networks at lake edges and on islands were not compartmentalized and were negatively affected by island area and isolation in terms of species richness, density, and composition. Habitat loss and fragmentation led to coextinction cascades that contributed to the elimination of entire ant-plant compartments. Furthermore, many myrmecophytic plants in disturbed sites lost their mutualistic ant partners or were colonized by opportunistic, nonspecialized ants. Robustness of ant-myrmecophyte networks on islands was lower than robustness near lake edges and in undisturbed forest and was particularly susceptible to the extinction of plants. Beyond the immediate habitat loss caused by the building of large dams in Amazonia, persistent edge effects and habitat fragmentation associated with dams had large negative effects on animal-plant mutualistic networks.

The phylogenetic structure of plant facilitation networks changes with competition

Summary 1. Ecological communities are assembled as complex networks of both positive (i.e. facilitation) and negative (i.e. competition) interactions. In networks established among plant species, many facilitative interactions occurring between a benefactor – a nurse – and a beneficiary – a facilitated seedling ‐ turn into competition over time as the facilitated seedling grows and outcompetes the nurse. The facilitative associations that disappear over time are mainly restricted to closely related taxa, because close relatives tend to share niche requirements and compete more strongly for the same resources. In consequence, the phylogenetic structure of a network might change as positive associations become negative. 2. This study is aimed to characterize how the overall (i.e. nestedness and connectance) and the phylogenetic structure of facilitation networks in semi‐arid communities change when facilitation turns into competition and some of the early species associations established by facilitation disappear. 3. We show that the initial facilitation networks retain the overall, but not the phylogenetic, structure. Phylogenetic analyses show that as seedlings, facilitated species tend to associate with the same subset of nurses while, on the contrary, nurses are indifferent to the identity of their facilitated seedlings. But when competition becomes important, closely related nurse species appear associated with only a subset of facilitated species in the community. 4. Synthesis. Temporal rearrangements in the phylogenetic structure of the facilitation networks provide evidence that plant–plant species interactions lead to highly species‐specific networks in which the phylogenetic history has a pervasive influence not only on recruitment but also on adult community composition. The use of phylogenetic methods combined with complex network approaches opens the possibility to understand the complexity of ecological interactions occurring in nature as for example those occurring when biological invasions take place or those producing coextinction cascades following species removal from ecosystems.

The assembly and disassembly of ecological networks

Global change has created a severe biodiversity crisis. Species are driven extinct at an increasing rate, and this has the potential to cause further coextinction cascades. The rate and shape of these coextinction cascades depend very much on the structure of the networks of interactions across species. Understanding network structure and how it relates to network disassembly, therefore, is a priority for system-level conservation biology. This process of network collapse may indeed be related to the process of network build-up, although very little is known about both processes and even less about their relationship. Here we review recent work that provides some preliminary answers to these questions. First, we focus on network assembly by emphasizing temporal processes at the species level, as well as the structural building blocks of complex ecological networks. Second, we focus on network disassembly as a consequence of species extinctions or habitat loss. We conclude by emphasizing some general rules of thumb that can help in building a comprehensive framework to understand the responses of ecological networks to global change.

Structure in mutualistic networks

Statistical analyses of the networks formed by plant–animal mutualisms can now take account of the relatedness of the players on either side. How helpful is this innovation for understanding network dynamics? Plants and their pollinators and seed dispersers form complex networks of interdependences. These networks have a well-defined architecture that strongly affects biodiversity maintenance. Using a phylogenetic approach, Rezende et al. show that past evolutionary history of plants and animals partly explains the network patterns. Closely related species tend to play similar roles in the network. As a result, coextinction cascades following a species extinction affect taxonomically related species, resulting in a non-random pruning of the evolutionary tree. From a conservation standpoint, this means that cascades of coextinction may spread across related species, further increasing the erosion of taxonomic diversity.

Non-random coextinctions in phylogenetically structured mutualistic networks

The interactions between plants and their animal pollinators and seed dispersers have moulded much of Earth's biodiversity. Recently, it has been shown that these mutually beneficial interactions form complex networks with a well-defined architecture that may contribute to biodiversity persistence. Little is known, however, about which ecological and evolutionary processes generate these network patterns. Here we use phylogenetic methods to show that the phylogenetic relationships of species predict the number of interactions they exhibit in more than one-third of the networks, and the identity of the species with which they interact in about half of the networks. As a consequence of the phylogenetic effects on interaction patterns, simulated extinction events tend to trigger coextinction cascades of related species. This results in a non-random pruning of the evolutionary tree and a more pronounced loss of taxonomic diversity than expected in the absence of a phylogenetic signal. Our results emphasize how the simultaneous consideration of phylogenetic information and network architecture can contribute to our understanding of the structure and fate of species-rich communities.