Remoción de semillas por Dorymyrmex insanus (Hymenoptera: Formicidae) en una reserva ecológica en la Ciudad de México, México / Seed removal by Dorymyrmex insanus (Hymenoptera: Formicidae) in an ecological reserve in Mexico City, Mexico

Abstract Introduction: Seed removal by ants is an interaction that may greatly affect the dynamic and structure of the vegetation. This aspect is well known for granivorous ants; however, there is little information on the effect of omnivorous ants. Objective: To assess the potential impact of the omnivorous ant Dorymyrmex insanus on vegetation. Methods: In the Pedregal Reserve, Mexico City, we identified the items in the refuse piles of ten ant colonies, for one year, covering the rainy and dry seasons. For each season we calculated seed diversity and analyzed the possible relationship between seed size and their abundance in the refuse piles, with regression models. We also did germination tests with seeds of Tagetes micrantha, comparing seeds from piles and from plants. Results: D. insanus removed seeds of 19 plant species as well as plant remains (such as leaves, twigs, roots), and remains of insects. Seed diversity was higher in the rainy season but the greatest abundance was in the dry season. When analyzing the relationship between seed length and abundance in the refuse piles, we found that the ants preferred seeds of around 10 mm. We also found that more seeds of T. micrantha germinated when they were previously handled by ants. Conclusions: The ant D. insanus actively participates in the removal of seeds from several species, favoring germination, and seasonality affects the selectivity of resources.

Resumen Introducción: La remoción de semillas por parte de las hormigas es una interacción que puede afectar en gran medida la dinámica y estructura de la vegetación. Este aspecto es bien conocido para las hormigas granívoras; sin embargo, hay poca información sobre el efecto de las omnívoras. Objetivo: Evaluar el impacto potencial de la hormiga omnívora Dorymyrmex insanus sobre la vegetación. Métodos: En la Reserva Pedregal, Ciudad de México, se identificaron los ítems en el área del basurero para diez colonias de hormigas, durante un año, cubriendo la época de lluvia y sequía. Para cada temporada calculamos la diversidad de semillas y analizamos la posible relación entre el tamaño de las semillas y su abundancia en los basureros, con modelos de regresión. También hicimos pruebas de germinación con semillas de Tagetes micrantha, comparando las encontradas en los basureros con las provenientes de las plantas. Resultados: D. insanus eliminó semillas de 19 especies de plantas, así como restos de plantas (hojas, ramitas, raíces) y restos de insectos. La diversidad de semillas fue mayor en la estación lluviosa pero la mayor abundancia lo fue en la estación seca. Al analizar la relación entre la longitud de las semillas y la abundancia en el área del basurero, encontramos que las hormigas preferían semillas de alrededor de 10 mm. También encontramos que germinaron más semillas de T. micrantha cuando fueron manipuladas previamente por hormigas. Conclusiones: La hormiga D. insanus participa activamente en la remoción de semillas de varias especies, favoreciendo la germinación, y la estacionalidad afecta la selectividad de recursos.

Intra annual seed production and availability of two morphotypes of Brosimum rubescens taubert in forests of the Colombian Amazon / Producción y disponibilidad intra anual de semillas de dos morfotipos de Brosimum rubescens Taubert en bosques de la amazonía colombiana

We assessed the reproductive phenology, production, and availability of seeds for two morphotypes of Brosimum rubescens Taub. (Moraceae), locally known as black palosangre (BP) and white palosangre (WP) during an annual cycle in a 20-ha mega plot located in a primary forest at the El Zafire Biological Station, in the Colombian Amazon. We found that 87% and 41% of potentially reproductive individuals of BP and WP respectively, was fertile and the production of reproductive structures was variable within and between morphotypes. The phenological pattern was seasonal in both morphotypes, characterized by flowering at the end of the dry season and fruiting at the start of the rainy season. BP produced and aborted large amounts of infructescences (approximately 21743), possibly as a response to satiate predators. Relative predation in terms of pre-dispersion of seeds was similar in both morphotypes, with a greater damage generated by a Scolytidae Curculionidae (Coleoptera) and to a lesser degree by frugivorous vertebrates. The number of mature fruits, total seeds, and removed seeds was similar for both morphotypes. Differences in the amount of reproductive structures and in the timing of phenophases between morphotypes decreased the competition and contributed to their coexistence. It seems that in undisturbed forests seed limitation could be more conditioning for WP, while other limitations after fruit and seed production could occur in BP.

Se evaluó la fenología reproductiva, producción y disponibilidad de semillas de dos morfotipos de Brosimum rubescens Taub. (Moraceae), conocidos localmente como palosangre negro (PN) y palosangre blanco (PB), durante un ciclo anual en una megaparcela de 20 ha localizada en bosques primarios de la Estación Biológica El Zafire, Amazonía colombiana. Se encontró que estuvieron fértiles el 87% y 41% de los individuos potencialmente reproductivos de PN y PB, respectivamente. La producción de estructuras reproductivas fue variable dentro y entre morfotipos. El patrón fenológico fue estacional en ambos: florecieron al final de la época seca y fructificaron al inició de la lluviosa. Aunque en PB aparentemente ocurre limitación en la polinización, la maduración de los óvulos fecundados es más eficiente que en PN. Este último produjo y abortó abundante cantidad de infrutescencias, posiblemente para saciar depredadores. En ambos morfotipos la depredación relativa pre-dispersión de las semillas fue similar, siendo mayor el daão generado por un Curculionidae Scolytidae (Coleóptera) y menor por frugívoros vertebrados. El número de frutos maduros, semillas totales y semillas removidas también fue similar entre morfotipos. Diferencias en la cantidad de estructuras reproductivas y en el tiempo de manifestación de las fenofases entre morfotipos, disminuyen la competencia y contribuyen a su coexistencia. Aparentemente, en bosques no disturbados la limitación de semillas podría ser más condicionante para PB, pero otras limitaciones posteriores a la producción de frutos y semillas podrían presentarse en PN.

Influências de Atta spp. (Hymenoptera: Formicidae) na recuperação da vegetação pós-fogo em floresta de transição amazônica / Influences of leafcutter ant Atta spp. (Hymenoptera: Formicidae) on vegetation recovery after fire in Amazonian transitional forest

Este estudo investigou o papel das saúvas na recuperação da vegetação pós-fogo. Foi hipotetizado que a perturbação do fogo aumenta a abundância de ninhos de saúva (1) e as atividades de remoção de sementes (2) e desfolha (3), especialmente no ambiente de borda. Os ninhos de saúva foram inventariados e mapeados e após 17 meses, checados para o registro de sua atividade e de novas colônias emergindo na área. Realizou-se experimentos comparativos de remoção de sementes e desfolha de plântulas em duas parcelas de 50 ha, uma submetida a fogo anual (tratamento) e outra sem fogo (controle). A abundância de ninhos de saúva foi maior na parcela tratamento que na parcela controle, especialmente na borda da floresta. As espécies encontradas foram: Atta cephalotes, A. laevigata e A. sexdens , sendo esta última a espécie mais abundante e que mostrou aumento em número de colônias ativas após 17 meses. O aumento na abundância de ninhos na parcela tratamento foi acompanhado por um aumento na atividade dessas formigas. Enquanto mais de duas folhas foram arrancadas por plântula nessa parcela, menos de uma foi registrada na parcela controle. Também a abundância média de galhos desfolhados e de sementes removidas por saúvas foi maior na parcela tratamento que na parcela controle. As saúvas tanto podem diminuir ou retardar o processo de regeneração florestal pós-fogo, quanto acelerá-lo, devido à elevada predação seletiva que favorece as espécies menos palatáveis. Desta forma, suas atividades podem modificador apenas a composição da vegetação recuperando-se do fogo.

In this study we investigated the role of leaf-cutting ants in the post-fire vegetation recovery. We hypothesized that a forest plot submitted to annual fire presents: (1) higher abundance of leaf-cutting ant nests; (2) higher removal of seeds; and (3) higher herbivory rates of leaf-cutting ants, when compared to the forest plots without fire (control). The leaf-cutting ant nests were sampled, mapped, and checked up (after 17 months) to register their activity and new colonies emerging from the plot. We made comparative experiments of seeds removal and seedlings herbivory in two 50 ha plots, one submitted to annual fire and another without fire. The abundance of leaf-cutting nests was higher in the plot submitted to fire than in the control plot. The species found were: Atta cephalotes, A. laevigata, and A. sexdens, being the latter the most abundant and the one that showed an increase of active colonies after 17 months. The plot submitted to fire showed a higher abundance of seeds removed by leaf-cutting ants than the control. While more than two leaves were ripped per seedling in the plot under fire, less than one was registered in the control plot. It was also observed that the average abundance of seedlings attacked by leaf-cutting ant in the fire plot was higher than in the plot without fire. This study shows that the leaf-cutting ants may reduce seed germination and seedling recruitment, affecting the reproductive success and the composition of vegetation recovering from fire. Alternatively, the leaf-cutting ants may also speed up the regeneration process of unpalatable plant species. In this sense, we may conclude that the presence of leaf-cutting may interfere in the species composition of post fire forest.

Seed predation by mammals in forest fragments in Monteverde, Costa Rica

Few studies have evaluated seed predation in fragmented landscapes, in which lower species diversity is expected to modifying ecological interactions. The rates of seed removal by mammals were investigated in a continuous forest and two fragmented patches of Premontane Tropical Moist Forest, in Monteverde, Costa Rica. The composition of mammalian seed-predators in each site was recorded during 16 months. The removal of four native tree species of experimental seeds: Ocotea valeriana and Ocotea whitei (Lauraceae), Panopsis costaricensis (Proteaceae) and Billia colombiana (Hippocastanaceae) in forest understories was followed during two annual fruiting seasons for each species. Results indicated similar species composition of seed-predators between continuous forest, the largest fragment (350 ha). However the smaller fragment (20 ha), had fewer seed predators. In this fragment, the specialized seed predator Heteromys desmarestianus (Rodentia) was more abundant. Unexpectedly, seed-predation in the two forest fragments and the continuous forest did not differ statistically for any of the seed species. Apparently, the higher abundance of small seed-predators in the fragments was compensated by the absence of medium and large seed-predators, like Agouti paca, Dasyprocta punctata (both Rodentia) and Pecari tajacu (Artiodactyla) recorded in continuous forest. Removal of experimentally-placed seeds was higher when the number of naturally occurring seeds in the sites was lower. This result could best be attributed to differential satiation of seed predators rather than differences in richness or abundance of seed predators. Rev. Biol. Trop. 57 (3): 865-877. Epub 2009 September 30.

Pocos estudios han evaluado la depredación de semillas en ambientes fragmentados, en éstos la menor diversidad de especies debe estar modificando las interacciones ecológicas. Se investigó la remoción de semillas por mamíferos en un bosque continuo y dos fragmentos en Monteverde, Costa Rica. Se registró la composición de mamíferos en cada sitio durante 16 meses y se observó la remoción de semillas experimentales del suelo del bosque. Se utilizó semillas de cuatro especies de árboles: Ocotea valeriana, Ocotea whitei, Panopsis costaricensis y Billia colombiana, durante dos periodos anuales de fructificación cada una. Los resultados indican una composición de depredadores de semillas similar entre el bosque continuo y el fragmento de 350 ha, más diversos que el fragmento de 20 ha, en el cual el especialista en semillas Heteromys desmarestianus fue más abundante. La depredación de semillas entre el bosque continuo y los fragmentos no fue estadísticamente diferente al considerar todas las especies de semillas, debido a la mayor abundancia de depredadores pequeños, que pudo compensar la ausencia de depredadores de talla mediana y grande que sí están presentes en el bosque continuo. La depredación sobre semillas fue mayor cuando la oferta de semillas no experimentales fue menor, indicando que la saciedad de los depredadores puede estar determinando la remoción de semillas en fragmentos, más que la riqueza o abundancia de especies depredadoras.