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Communities worldwide are losing multiple species at an unprecedented rate, but how communities reassemble after these losses is often an open question. It is well established that the order and timing of species arrival during community assembly shapes forthcoming community composition and function. Yet, whether the order and timing of species losses can lead to divergent community trajectories remains largely unexplored. Here, we propose a novel framework that sets testable hypotheses on the effects of the order and timing of species losses-inverse priority effects-and suggests its integration into the study of community assembly. We propose that the order and timing of species losses within a community can generate alternative reassembly trajectories, and suggest mechanisms that may underlie these inverse priority effects. To formalize these concepts quantitatively, we used a three-species Lotka-Volterra competition model, enabling to investigate conditions in which the order of species losses can lead to divergent reassembly trajectories. The inverse priority effects framework proposed here promotes the systematic study of the dynamics of species losses from ecological communities, ultimately aimed to better understand community reassembly and guide management decisions in light of rapid global change.

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Understanding the impacts of global change on ecological communities is a major challenge in modern ecology. The gain or loss of particular species and the disruption of key interactions are both consequences and drivers of global change that can lead to the disassembly of ecological networks. We examined whether the disruption of a hummingbird-mistletoe-marsupial mutualism by the invasion of non-native species can have cascading effects on both pollination and seed dispersal networks in the temperate forest of Patagonia, Argentina. We focused on network motifs, subnetworks composed of a small number of species exhibiting particular patterns of interaction, to examine the structure and diversity of mutualistic networks. We found that the hummingbird-mistletoe-marsupial mutualism plays a critical role in the community by increasing the complexity of pollination and seed dispersal networks through supporting a high diversity of interactions. Moreover, we found that the disruption of this tripartite mutualism by non-native ungulates resulted in diverse indirect effects that led to less complex pollination and seed dispersal networks. Our results demonstrate that the gains and losses of particular species and the alteration of key interactions can lead to cascading effects in the community through the disassembly of mutualistic networks.

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It is not uncommon for one or a few species, and their interactions, to have disproportionate effects on other species in ecological communities. Such keystone interactions might affect how communities respond to the invasion of non-native species by preventing or inhibiting the establishment, spread or impact of non-native species. We explore whether a keystone mutualism among a hummingbird-mistletoe-marsupial promotes ecological resistance to an invasive pollinator, the bumblebee Bombus terrestris, by comparing data collected at sites prior to bumblebee invasion to data collected 11 years after the invasion in sites with and without the keystone mutualism. We built pollination networks and focused on network motifs, regarded as building blocks of networks, to identify the central pollinators and estimate the change in their interactions after invasion of B. terrestris. We also estimated the interaction rewiring across the season in post-invasion networks and tested it as a possible mechanism explaining how the keystone mutualism increased ecological resistance to invasion. We found two times more species in post-invasion sites with the keystone mutualism than in post-invasion sites without the keystone mutualism. Moreover, we found that invasive bumblebee reduced the strength and interaction niche of the five central pollinator species while increasing its own strength and interaction niche, suggesting a replacement of interactions. Also, we found that the keystone mutualism promoted resistance to B. terrestris invasion by reducing its negative impacts on central species. In the presence of the keystone mutualism, central species had three times more direct interactions than in sites without this keystone mutualism. The higher interaction rewiring, after invasion of B. terrestris, in sites with the keystone mutualism indicates greater chances of central pollinators to form new interactions and reduces their competence for resources with the non-native bumblebee. Our results demonstrate that a keystone mutualism can enhance community resistance against the impacts of a non-native invasive pollinator by increasing species diversity and promoting interaction rewiring in the community. This study suggests that the conservation of mutualisms, especially those considered keystone, could be essential for long-term preservation of natural communities under current and future impacts of global change.

Es común que una o unas pocas especies y sus interacciones tengan efectos desproporcionado sobre otras especies en las comunidades. Estas especies y sus interacciones claves podrían afectar el modo en que las comunidades responden a la invasión de especies no nativas al prevenir o disminuir su establecimiento, su propagación o el impacto de las mismas. En este estudio evaluamos si un mutualismo clave entre un colibrí, un muérdago y un marsupial promueve la resistencia de la comunidad frente a un polinizador invasor, el abejorro Bombus terrestris, mediante la comparación de datos colectados en sitios previos a la invasión del abejorro y datos colectados 11 años después de su invasión, en sitios con y sin el mutualismo clave. Construimos redes ecológicas planta-polinizador y nos centramos en los modos de interacción ("interaction motifs"), los cuales son usados como bloques en la construcción de las redes, para identificar los polinizadores centrales y estimar el cambio en sus interacciones después de la invasión de B. terrestris. Además, en las redes posteriores a la invasión estimamos la reconexión de interacciones a lo largo de la temporada y la evaluamos como un posible mecanismo mediante la cual el mutualismo clave aumentó la resistencia a la invasión. En sitios posteriores a la invasión con el mutualismo clave encontramos dos veces más especies que en sitios posteriores a la invasión ausentes de éste. Además, en los sitios ausentes del mutualismo clave, encontramos que el abejorro invasor redujo la fuerza y el nicho de interacción de los cinco polinizadores centrales mientras incrementó su propia fuerza y nicho de interacciones, sugiriendo un reemplazo de interacciones. Asimismo, encontramos que el mutualismo clave promovió la resistencia de la comunidad a la invasión de B. terrestris al reducir sus impactos negativos sobre las especies centrales. En presencia del mutualismo clave, las especies centrales presentaron tres veces más interacciones directas que en sitios ausentes de esta interacción. La gran reconexión de interacciones encontrada en sitios posteriores a la invasión con el mutualismo clave indica mayores probabilidades de que los polinizadores centrales formen nuevas interacciones y reduzcan la competencia por recursos con el abejorro no nativo. Nuestros resultados demuestran que un mutualismo clave puede mejorar la resistencia de la comunidad frente a los impactos de especies invasoras al incrementar la diversidad de especies y promover la reconexión de interacciones en la comunidad. Este estudio sugiere que la conservación de las interacciones mutualistas, principalmente aquellas consideradas claves, podría ser esencial para preservar las comunidades naturales frente a los impactos del cambio global.

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The ideal-weed hypothesis predicts that invasive plants should be less dependent on mutualisms. However, evidence in favor of or against this hypothesis comes mainly from observational studies. Here, we experimentally tested this hypothesis using a two-factor greenhouse experiment, comparing the seedling growth response of different Pinus species (varying in invasiveness) to ectomycorrhizal fungal inoculation. Most species showed no response until they were 6 mo old, at which point inoculation increased growth between 10 and 260% among the different species. This growth response was higher for species with lower seed mass, higher dispersal ability, higher Z score (a proxy for invasiveness) and higher number of naturalized regions, all of which correspond to higher invasiveness. Our results show that timing is a crucial factor when comparing mycorrhizal dependency of different species. Dependence on mutualistic microorganisms could be part of a strategy in which invasive species produce smaller seeds, in greater number, which can disperse further, but where seedlings are more reliant on mycorrhizas to improve access to water, nutrients, and protection from pathogens. Our results suggest that reliance on mutualisms may enhance, rather than limit, nonnative species in their ability to spread, establish, and colonize.

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Why some species become invasive while others do not remains an elusive question. It has been proposed that invasive species should depend less on mutualisms, because their spread would then be less constrained by the availability of mutualistic partners. We tested this idea with the genus Pinus, whose degree of invasiveness is known at the species level (being highly and negatively correlated with seed size), and which forms obligate mutualistic associations with ectomycorrhizal fungi (EMF). Mycorrhizal dependence is defined as the degree to which a plant needs the mycorrhizal fungi to show the maximum growth. In this regard, we use plant growth response to mycorrhizal fungi as a proxy for mycorrhizal dependence. We assessed the responsiveness of Pinus species to EMF using 1,206 contrasts published on 34 species, and matched these data with data on Pinus species invasiveness. Surprisingly, we found that species that are more invasive depend more on mutualisms (EMF). Seedling growth of species with smaller seeds benefited more from mutualisms, indicating a higher dependence. A higher reliance on EMF could be part of a strategy in which small-seeded species produce more seeds that can disperse further, and these species are likely to establish only if facilitated by mycorrhizal fungi. On the contrary, big-seeded species showed a lower dependence on EMF, which may be explained by their tolerance to stressful conditions during establishment. However, the limited dispersal of larger seeds may limit the spread of these species. We present strong evidence against a venerable belief in ecology that species that rely more on mutualisms are less prone to invade, and suggest that in certain circumstances greater reliance on mutualists can increase spread capacity.

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Why some non-native plant species invade, and others fail remains an elusive question. Plant invasion success has been associated with specific species traits. Yet, we have limited knowledge of the mechanisms relating these traits to invasion potential. General patterns of biotic resistance by seed predation may provide a mechanism that helps separate invasive from non-invasive plants. Seed predation is an important barrier against plant establishment for many plant species. It may, therefore, create a selective filter against non-native plant establishment based on plant traits related to seed predation rate. In two cafeteria-style seed predation experiments in a steppe ecosystem in Patagonia (Argentina) we provided seeds of 16 non-native Pinaceae covering a 300-fold variation in seed mass, a 200-fold variation in seed volume and 75-fold variation in seed toughness. Seed removal decreased with seed mass and seed volume. Seed toughness was not a significant predictor while seed volume was the best predictor of predators' preference. However, for species of this family small seed size is the most important predictor of species invasiveness. Our results show that seed predation does not explain Pinaceae invasive success. In our system, species that have smaller seeds (i.e., more invasive) are preferentially consumed by seed predators. Seed mass was not the best predictor of granivory rates, despite being the seed trait on which most studies have been focused. Our ability to predict future invasion and understand invasion success could benefit from other studies that focus on the mechanisms behind invasive traits.

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Interaction webs summarize the diverse interactions among species in communities. The addition or loss of particular species and the alteration of key interactions can lead to the disassembly of the entire interaction web, although the nontrophic effects of species loss on interaction webs are poorly understood. We took advantage of ongoing invasions by a suite of exotic species to examine their impact in terms of the disassembly of an interaction web in Patagonia, Argentina. We found that the reduction of one species (a host of a keystone mistletoe species) resulted in diverse indirect effects that led to the disassembly of an interaction web through the loss of the mistletoe, two key seed-dispersers (a marsupial and a bird), and a pollinator (hummingbird). Our results demonstrate that the gains and losses of species are both consequences and drivers of global change that can lead to underappreciated cascading coextinctions through the disruption of mutualisms.

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Dromiciops gliroides is an arboreal marsupial found in the temperate forests of South America (36-43 °S). This species is the sole extant representative of the order Microbiotheria, and is a key seed disperser of many native plant species, including the keystone mistletoe Tristerix corymbosus. Here, we synthesized the current knowledge on the ecological aspects of this species, and compared the available information from Argentina and Chile. Population density (23 ± 2 (mean ± SE) individual/ha) and home range (1.6 ± 0.6 ha) appear to be relatively similar across a marked ecological gradient in the mainland, but lower densities (7 ± 2 individual/ha) and smaller home ranges (0.26 ± 0.04 ha) were detected at island sites. We detected regional variation in body condition in Chile, but there were no significant differences across a wider E-W gradient. Movement patterns fit a random walk model; such behavior might have important consequences in shaping plant's spatial patterns. Although our data suggest that D. gliroides is more tolerant to habitat disturbance than previously thought, its incapability to disperse across non-forested areas suggests that the rapid rate of habitat loss and fragmentation that characterizes southern temperate forests likely poses a serious threat to this species. These ecological similarities are surprising given that forests studied receive dramatically different rainfall and correspond to distinct forest types. The evidence synthetized here dispels some of the myths about this species but also stresses the need for more comprehensive ecological studies across its distribution range.

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Although it is increasingly clear that exotic invasive species affect seed-dispersal mutualisms, a synthetic examination of the effect of exotic invasive species on seed-dispersal mutualisms is lacking. Here, we review the impacts of the invasive Argentine ant (Linepithema humile) on seed dispersal. We found that sites with L. humile had 92 per cent fewer native ant seed dispersers than did sites where L. humile was absent. In addition, L. humile did not replace native seed dispersers, as rates of seed removal and seedling establishment were all lower in the presence of L. humile than in its absence. We conclude that potential shifts in plant diversity and concomitant changes in ecosystem function may be a consequence of Argentine ant invasions, as well as invasions by other ant species. Because very few studies have examined the effects of non-ant invasive species on seed-dispersal mutualisms, the prevalence of disruption of seed-dispersal mutualisms by invasive species is unclear.

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