David Pérez Padilla

Integration of invasive Opuntia spp. by native and alien seed dispersers in the Mediterranean area and the Canary Islands

The success of many alien plant species depends on mutualistic relationships with other species. We describe the assemblage of seed dispersers on three species of alien Opuntia invading Mediterranean and Macaronesian habitats, and examine the quality of such plant-animal interactions. We identified vertebrates consuming O. maxima, O. dillenii and O. stricta fruits by direct observation and collecting droppings and pellets. Phenology of the alien species, as well as that of coexisting native species, was monitored for an entire year. Germination tests of ingested and non-ingested seeds were performed both in the greenhouse and in the field. Seed coat thickness and viability were also measured for all treatments. A great variety of taxa, including reptiles, birds and mammals actively participate in the seed dispersal of Opuntia. Phenology of Opuntia fruits in Menorca and Tenerife overlaps with only a few native fleshy-fruited plants present in the study areas, which suggests an advantage for the invader. Most seeds germinated during the second year of the experiment, independently of the effect produced by the dispersers’ guts. We found great variation in the germination percentage of Opuntia after gut passage and in the effects of ingestion on seed coat thickness. Seed viability was somewhat reduced after gut passage compared to manually depulped seeds. Our results show how different Opuntia species are integrated into native communities by means of mutualistic interactions, with both native and alien dispersers. Although with heterogeneous effects, either type of disperser potentially contributes to the spread of these alien cacti in the recipient areas.

Differential seed dispersal systems of endemic junipers in two oceanic Macaronesian archipelagos: the influence of biogeographic and biological characteristics

This article evaluates the seed dispersal systems of two congeneric and endemic fleshy-fruited plants in the context of two relatively close oceanic archipelagos. For this purpose, representative populations of the endangered junipers Juniperus cedrus in the Canary Islands and Madeira, and Juniperus brevifolia in the Azores were studied. Despite both species sharing the same biogeographic region, we set out to test whether different conditions of the islands and biological characteristics of each juniper species determine the distinctive guilds of seed dispersers involved. We assessed the quantitative and qualitative role of the potential frugivores, showing that the wintering Turdus torquatus and the native Turdus merula were the main seed dispersers for J. cedrus and J. brevifolia, respectively (Frequency of occurrence: 74.9%, 80.2%; germination increase with respect to controls: 11.6%, 15.5%; for J. cedrus and J. brevifolia, respectively). The endemic lizard Gallotia galloti was quantitatively outstanding as seed disperser of J. cedrus, although its qualitative effect does not appear to be beneficial. The introduced rabbit Oryctolagus cuniculus acts as a disruptor in both natural seed dispersal systems, as inferred from the high percentage of damaged seeds found in their droppings. Our results indicate that J. cedrus and J. brevifolia are primarily adapted to ornithochory processes, T. torquatus and T. merula being their respective legitimate long-distance dispersers. Although these birds should be playing a key role in the connectivity of fragmented populations, the dependence of J. cedrus on a migrant bird involves a notable fragility of the system.

Contrasting phenology and female cone characteristics of the two Macaronesian island endemic cedars (Juniperus cedrus and J. brevifolia)

Phenology and female cone characteristics of the two endemic cedars (Juniperus cedrus and J. brevifolia) from the Macaronesian islands were studied. Despite their closely taxonomic affinity and their evolution under insular conditions, different trends were recorded. Mature J. cedrus female cones were present throughout the year, while those from J. brevifolia were only present in summer and autumn. J. cedrus female cone size was significantly larger than that of J. brevifolia, a trend consistent with the presence of larger vertebrates (lizards and birds) in the Canary Islands. However, water content was four times higher in J. brevifolia female cones, which can be related with the higher rainfall existing in the Azores. J. cedrus has two or three seeds per cone, whereas J. brevifolia frequently had three. Seeds from J. cedrus were clearly larger and heavier, coinciding with the female cone size trend. However, tetrazolium tests revealed higher viability values in J. brevifolia. The relatively low percentage of filled seeds in J. cedrus could be a consequence of the climatic stress and limits to pollination due to fragmented populations as described for other Juniperus species. In summary, our results reveal that some environmental factors such as the harsh conditions, high population fragmentation and the dependence on large dispersers have compromised the fitness of J. cedrus in the Canary Islands.

Prey size selection of insular lizards by two sympatric predatory bird species

Abstract. n Size-related selection of insular lizards by two sympatric predatory bird species (Southern Grey Shrike and Eurasian Kestrel) was studied in an arid insular environment. The endemic lizard genus Gallotia was a key resource in the diet of both predators, constituting more than 50% of the total biomass. Shrikes captured smaller lizards than kestrels during all seasons (mean snout - vent length (SVL): 7.4 ±1.9 vs. 9.4 ± 2.1 cm respectively), presenting a sequential use of lizard sizes and avoiding potential competition. On the other hand, shrikes and kestrels showed the same seasonal variation pattern, capturing the largest lizards during their breeding periods (spring). Considering lizard availability, shrikes displayed less selective predation than kestrels in all seasons. Shrikes positively selected the medium lizard size (SVL: 5–10 cm) during the nestling period, but negatively selected the small lizard size (SVL: < 5 cm) in autumn and winter, probably due to an explosion of juvenile lizards. Lastly, kestrels appeared to be more selective, negatively choosing the small lizard size but positively selecting the largest ones all the year round (SVL: > 10 cm).

Comparison of three chemical tests to assess seed viability: the seed dispersal system of the Macaronesian endemic plant Rubia fruticosa (Rubiaceae) as an example

Summary.Three chemical viability tests were evaluated in the seed dispersal system of Rubia fruticosa, in which three main groups of dispersers participate: reptiles, birds and mammals. Tetrazolium chloride (TTC) and indigo carmine (IC) indicated a lower viability of seeds from droppings of introduced rabbits (Oryctolagus cuniculus) than of those from control plants and the native dispersers, lizards and gulls. In the rabbit seed treatment, significant differences were observed between results obtained with TTC and IC tests. Interpretation of these data, due to the presence of doubtful embryo staining, was more difficult using the IC test. Furthermore, some seeds that were clearly dead had been underestimated. In contrast with results obtained from the two staining methods, the EC test did not confirm that viability of control seeds and those seeds consumed by native dispersers were clearly higher than in seeds ingested by O. cuniculus. Further, compared to the other two tests, the EC method requires more careful handling of the embryo during the extraction process to avoid errors in viability estimation, since this method measures concentration of electrolytes that are released through cellular membranes. Thus, TTC was the most reliable test to assess seed viability in the seed dispersal system of R. fruticosa, and these results agree with those obtained in previous germination experiments made on the same set of seeds given the same treatments.

Trophic ecology of the Southern Grey Shrike (Lanius meridionalis) in insular environments: the influence of altitude and seasonality

The seasonal diet and prey selection of the Southern Grey Shrike (Lanius meridionalis) was studied in two different insular habitats: shrub environments of the Canary Islands in coastal and high mountain zones. We measured, in each season, food availability and prey size in order to determine prey size selection of shrikes along an altitudinal gradient. Moreover, we compared the diet patterns observed with those documented on the continent, to determine if Southern Grey Shrikes in the islands’ high mountain zone (which has a continental climate) showed seasonal diet variation similar to those in northern continental areas. We analysed a total of 1,139 shrike pellets collected in 1 year and identified 10,179 prey items. Numerically arthropods (91%), and in terms of biomass lizards (70%) were the main prey consumed by the shrikes. The proportions of the main prey items differed significantly between seasons and habitats. Diet in the coastal areas was less variable than in the high mountain zone. The greater seasonal climatic variation in the high mountain zone was associated with diet patterns similar to those found in some northern continental areas, such as the Iberian Peninsula and southern France. Finally, shrikes selected the largest prey in the high mountain habitat. This suggests that foraging behaviour in this species is related to climatic conditions, as the biggest and most profitable prey were consumed in the most harsh habitats.

The Key Role of a Ring Ouzel Tardus torquatus Wintering Population in Seed Dispersal of the Endangered Endemic Juniperus cedrus in an Insular Environment

Abstract. Although the Ring Ouzel has been considered accidental in the Canary Islands, it has been observed for the last five years in the high mountain zone of Tenerife (Canary Islands), showing it to be a regular winter visitor. Its interaction with female Juniperus cedrus cones and seeds was studied by analysing its wintering diet during January–April 2008. This juniper is endemic to the Canaries and Madeira and is classified as endangered by the IUCN. Previously, its sole long-distance disperser was known to be the Raven Corvus corax, which is unfortunately now extinct in the distribution range of J. cedrus. The results show that the diet of T. torquatus in Tenerife consists mostly of female J. cedrus cones (95% of the biomass consumed). Due to the large amount of seeds dispersed by Ring Ouzels and the effectiveness in seed germination, this winter visitor constitutes an important vector in the dispersal of J. cedrus that appears to have gone undetected in the past. This passerine plays a key role in its seed dispersal system, being able to move seeds over long distances, thus connecting fragmented populations of this plant. The present work is one of the few cases described in which a plant endemic to an oceanic island is so dependent on a winter visitor, and highlights the importance of insular environments in the wintering range of the Ring Ouzel. Owing to the absence of native long-distance seed dispersers, the future fate of this conifer is now probably dependent on the wintering population of this thrush.

The study of habitat use by censuses and molecular methods in birds: the case of two sympatric pigeons

Capsule Faecal DNA gives accurate results of habitat use.