When an invasive species is introduced in a new environment, it inevitably interacts with other species in the recipient community. The most studied interactions are the negative ones, such as the competence, depredation, hybridization and disease transmission, since they have impacts on native species. However, a new study led by the Doñana Biological Station (CSIC) has demonstrated that there are positive interactions in some cases: native species can take profit from some invasive species, such as the monk parakeet.
The monk parakeet (Myiopsitta monachus) is a successful and globally widespread avian invader. Native to South American, it is internationally traded as a pet. Due to accidental or intentional escapes, this species has spread in many countries out of their native rang. Its special nesting strategy is one of the key of its success as an invader species: unlike the other parrot species, monk parakeets build their own nests, large structures of sticks that include several chambers, each occupied during the whole year, forming large colonies.
"We have observed that some species can occupy the nests of monk parakeets, although the ecological effects of this interactions are barely known, especially in the invaded areas", explains Dailos Hernández Brito, researcher at the Doñana Biological Station – CSIC and first author of the study. "That's why we have studied the role of monk parakeets as ecosystem engineers, who can modify habitats and provide novel resources (as nest-sites) that may be exploited by other species".
The research team conducted different monitoring campaigns that covered both rural and urban areas in seven countries from 2013 to 2020, in which they monitored native and invasive populations of monk parakeet and recorded the tenants occupying their nests. Thus, the researchers could understand the factors involved in the presence, abundance and richness of tenants in these colonies, as well as the interactions between monk parakeets and different tenant species.
The researchers monitored 2595 monk parakeet nests and recorded 2690 nests of 42 tenant species, mostly cavity-nesting birds, that were present in 26% of the monk parakeet nests. The proportion of tenant nests was lower in the areas invaded by monk parakeets than in their native areas. However, models have demonstrated than the presence, abundance and richness of tenants were higher in invaded than native areas, in nests with more chambers and especially, those located in rural areas.
These parameters, along with the different nesting strategies by tenants, resulted in mixed colonies of several species. "We could count up to 35 nests of tenant species, mostly sparrows, that formed a large interspecific community in a single tree together with the monk parakeet host", explains Hernández Brito. Besides, they could demonstrate that the monk parakeet nests were a valuable resource for the tenants, since these ones persisted longer in them when they were located in rural areas, have more chambers and were occupied by a low number of monk parakeets.
Host and tenants: a complex network of interactions
Regarding the interactions between host and tenants, only 21% of these encounters resulted in aggressions and were mainly initiated by monk parakeets, although less than half of these aggressions resulted in the expulsion of tenants from the colonies. Despite these aggressions and nest usurpations by some tenants, researchers also observed events of cooperative defense of colonies between tenant species and monk parakeets against predators, such as birds of prey and rats. The models showed that this cooperation facilitated the success of expelling a predator. "This type of interactions is common in social insects such as termites, but the complexity of relationships studied here were never described before in birds", explains co-author Martina Carrete, researcher at the Pablo de Olavide University.
This study shows the complexity of interactions between species in the context of biological invasions, in which both positive and negative relationships for native species can take place. As an ecosystem engineer, the monk parakeet provides nest sites for species that need cavities for breeding, but are unable to make them by their own. Besides nesting opportunities, there are other benefits derived from this association between monk parakeets and their tenants, such as the thermoregulation of the chambers and the interspecific cooperative nest defense, which directly facilitate the breeding success of tenants. This can be important for the conservation of threatened or rare native species. "For example, the recent expansion of western jackdaws and stock doves in urbanized areas of Madrid coincides with monk parakeet population growth and expansion", adds Guillermo Blanco, researcher at the National Museum of Natural Science in Spain.
However, the continued use of these nets promotes the parasite load that may negatively affect the breeding success of birds. Moreover, researchers have observed that monk parakeets introduce novel parasites into the recipient community that they coexist with. Besides, monk parakeets also provide nests for other non-native species both in the native and invaded areas, facilitating the invasion process, not only for non-native tenants but also for themselves, since they increase the efficiency of the defense against predators thanks to the interspecific cooperation. "These interactions are even more complex and we are involved in new studies about their ecological implications", comments Dailos Hernández Brito.
Mitigating the adverse effects on native species
Despite this duality of effects on native species, management and action plans on the invasive monk parakeet populations cannot be delayed any longer. These populations maintain a constant growth, especially in Mediterranean countries, which increases their damage not only to the ecosystem, but also to infrastructure and crops. "The implementation of control actions on these populations are urgent, but they must consider and mitigate potential negative effects on native tenant species during nest removals," adds José Luis Tella, researcher at the Doñana Biological Station and supervisor of the study. Therefore, the study of biological interactions in the scenario of biological invasions not only sheds light on the understanding of this global problem but also on the development and implementation of control plans.
Hernández-Brito, D.; Carrete, M.; Blanco, G.; Romero-Vidal, P.; Senar, J.C.; Mori, E.; White, T.H., Jr.; Luna, Á.; Tella, J.L. The Role of Monk Parakeets as Nest-Site Facilitators in Their Native and Invaded Areas. Biology 2021, 10, 683. https://doi.org/10.3390/biology10070683https://doi.org/10.3390/biology10070683