Daniel García

Age structure of Juniperus communis L. in the Iberian peninsula : Conservation of remnant populations in Mediterranean mountains

Abstract We examined the demographic structure of Juniperus communis L. populations in the Mediterranean high mountains of southern Spain in order to analyse its population viability. For this, we compared the age structure of these Mediterranean populations with those of northern Spain (Atlantic), and, on a local scale, the populations of different habitats within the Sierra Nevada. The populations from northern Spain showed age structures with high proportions at young stages. In contrast, the Mediterranean populations proved to be dominated by adult and senescent individuals, except for the few habitats with higher water availability during the summer, which have larger proportions of seedlings and juveniles. Dry habitats showed a lower seedling survival rate than did wet ones, mainly due to summer drought. Both the difference between Atlantic and Mediterranean categories, and the difference between habitats in Sierra Nevada reflect a serious limitation on recruitment of J. communis in Mediterranean mountains due to climatic stress imposed by summer drought. The Mediterranean populations provide a clear example of remnant dynamics, surviving for long periods due to great individual longevity which partially offsets losses to unfavourable environmental conditions. The low ability to recover after anthropic disturbances emphasizes the need for conservation of J. communis populations in south-eastern Spain.

Yew (Taxus baccata L.) regeneration is facilitated by fleshy-fruited shrubs in Mediterranean environments

Yew Taxus baccata is catalogued as a species endangered and prone to extinction in the Mediterranean mountains of southern Spain, due to the small size and senescent status of most populations. In this paper, we study the effects of herbivory and the protective role of woody shrubs in the regeneration ability of the yew in the Sierra Nevada. The estimated density of the yew in the study plot was 287.9 individuals/ha, more than 90% being juveniles (seedlings and saplings), which were mostly located under fleshy-fruited shrubs. Saplings suffered serious herbivore damage when unprotected by shrubs. Thus, fleshy-fruited shrubs proved to be the best habitat for seedling establishment and sapling survival and growth. The abundance of fleshy-fruited shrubs in our study site provides a yew population characterized by an active regeneration under natural conditions. We suggest that the maintenance of healthy populations of yew in Mediterranean mountains is strongly dependent on the conservation of well-developed fleshy-fruit understories and their community of avian dispersers.

Effects of seed dispersal on Juniperus communis recruitment on a Mediterranean mountain

. The recruitment of the relict shrub Juniperus communis on a mountain in SE Spain was studied during the period 1994–1998. The main objective was to determine both the quantitative and qualitative effects of bird dispersal on seedling establishment. Seed removal by birds, seed rain, post-dispersal seed predation, germination, and seedling emergence and survival were analysed in different microhabitats. Birds removed 53 - 89% of the seeds produced by plants. Seed rain was spatially irregular as most seeds accumulated near stones used by birds as perches and below mother plants while a few seeds were dropped in wet meadows and open ground areas. Post-dispersal seed predation by rodents affected < 10% of dispersed seeds but varied significantly among microhabitats. Only 3.6 - 5.5% of dispersed seeds appeared viable, as many seeds had aborted or showed wasp damage. Seeds germinated in the second and third springs after sowing, reaching a germination percentage of 36%. Seedling emergence was concentrated in wet meadows. Seedling mortality was high (75–80%), but significantly lower in wet meadows, the only microhabitat where seedlings could escape from summer drought, the main mortality cause. Seed abortion, germination and seedling mortality proved to be the main regeneration constraints of J. communis on Mediterranean mountains. Birds exerted a strong demographic effect, although their qualitative effect was limited by abiotic factors which caused the pattern of seed rain to differ from the final pattern of recruitment between microhabitats.

Effect of browsing by ungulates on sapling growth of Scots pine in a Mediterranean environment: consequences for forest regeneration

The impact of mammalian herbivory on Scots pine (Pinus sylvestris L.) sapling performance was quantified in three native forests located in two Mediterranean mountains, the Sierra Nevada and Sierra de Baza (SE Spain). More than 98% of the damage to terminal shoots was caused by goats and Spanish ibex in Sierra Nevada and sheep in Sierra de Baza. Some 72% of the tagged saplings (na 619) were browsed during at least 1 year of monitoring (1995‐1997). There were between-forest differences in herbivory pressure. Moreover, the herbivory pressure was significantly higher during a dry year (1995) than during wet ones (1996 and 1997). Overall, when browsing a sapling, ungulates consumed almost 30% of its apical shoots, and 85% of saplings were browsed more than once after establishment. As a consequence, ungulates severely affected the Scots pine sapling growth rate, and therefore browsed saplings grew slower than saplings unbrowsed by ungulates. Thus, according to exponential growth equations, the time necessary to attain a height threshold to escape from mammalian herbivores (150 cm height in our study forests) and start reproduction was retarded by the herbivory up to 12 years. Ungulates are a major factor hindering the natural regeneration and conservation of the last relict forests of Scots pines in SE Spain. # 2001 Elsevier Science B.V. All rights reserved.

Experimental study of pollination by ants in Mediterranean high mountain and arid habitats

In this paper, we report the results of an experimental study on ant pollination of three plant species inhabiting the Mediterranean high mountains (Alyssum purpureum, Arenaria tetraquetra and Sedum anglicum) and four species inhabiting the aridlands (Lepidium subulatum, Gypsophyla struthium, Frankenia thymifolia and Retama sphaerocarpa) of South-eastern Spain. We determined several plant and ant traits, as well as the composition and abundance of the pollinator assemblage. Insects belonging to 29 families and five orders visited the flowers of the plant species studied. In all but two, L. subulatum and G. struthium, the ants comprised 70–100% of the flower visitors. The results clearly show that five out of seven of these plant species were pollinated by ants. The role of the ants as pollinators seems to depend heavily on the relative abundance of the ants with respect to the other species of the pollinator assemblage, ant pollination becoming evident when ants outnumber other floral visitors. The ant-pollination systems analysed in this study may be the result of prevailing ecological conditions more than an evolutionary result of a specialized interaction.

Bird rejection of unhealthy fruits reinforces the mutualism between juniper and its avian dispersers

We investigated fruit damage by insects as well as fruit abortion in relation to the mutualism between Juniperus communis, a fleshy-fruited plant dominant in the high mountains of southeastern Spain, and its bird disperser assemblage. For two years. we performed field experiments to analyse fruit selection by birds, offering birds different types of anomalous fruits (unripe, aborted, pulp-sucker infested and seed-predator attacked) and comparing the removal rate to that of ripe, healthy, control fruits. In addition, we studied the proportion of fruits attacked by the seed predator in samples of fruits which, after manipulation and rejection by birds, we found lying underneath plants. We compared these data to values in samples of fruits which we took directly from plants. Finally, over four years, the abundance of predispersaldepredated seeds in the seed rain dispersed by birds was compared with the abundance in seeds taken directly from plants. Fruit-choice experiments showed that unripe, aborted and fruits attacked by pest insects (both pulp sucker and seed predator) were strongly counterselected by these frugivorous birds. The proportion of fruits attacked by seed-predator in the sample of fruits manipulated and rejected was significantly higher than in the fruits taken from plants. For all study years, the proportion of depredated seeds was significantly lower in the sample of seeds dispersed by birds than in the sample of seeds taken from plants. Bird response to pests was not categorically to accept or reject fruit, but rather was influenced by pest density. Birds showed two different levels of fruit selection, depending on the type of fruit: visual discrimination, against fruits that are unripe, aborted and infested by the pulp sucker: and within-beak discrimination, against fruits attacked by the seed predator. In the study, both pests either died or left the fruit when ripe, and therefore frugivorous birds did not interfere directly with frugivorous insects. On the contrary, insects did interfere indirectly with birds, promoting the rejection of pest-attacked fruits by birds. Bird dispersers overcame the predispersal interference of pest fruit damage and fruit abortion and increased the proportion of healthy seeds in the seed rain. This fact, together with the great quantity of seeds dispersed by birds, reinforces the importance of birds as plant mutualists.

Interaction between juniper Juniperus communis L. and its fruit pest insects: Pest abundance, fruit characteristics and seed viability

The relationships between the fruit features of Juniperus communis and the presence of fruit pests were studied in Sierra Nevada, SE Spain. The abundance of two insect species — a pulp-sucking scale and a seed-predator wasp — was surveyed with respect both to fruit characteristics and to viability of seeds contained therein. Seed-predator pressure was not significantly related to any fruit characteristics; however, pulp suckers tended to be more abundant in plants with low pulp: seed ratios and high fruit-water content. In addition, fruits with high levels of pulp-sucker attack tended to have higher water content. A multi-factor ANOVA, considering the identity of the plant and the attack of the different pests as factors, showed that plant identity accounts for most of the variation in fruit characteristics. The viability of seeds tended to be lower in plants strongly attacked by both pests. Fruits attacked by seed predators showed significantly lower proportions of viable and unviable seeds than did unattacked fruits. Seed viability was also lower in those fruits heavily attacked by pulp suckers, but this pattern is strongly mediated by plant identity. Pest activity proved to be clearly associated with a direct decrease in juniper reproductive capacity. This loss involved a reduction of the viable-seed number, mainly related to the seed predator, as well as a reduction of fruit attractiveness to frugivorous dispersers, related to the pulp sucker.

Do empty Juniperus communis seeds defend filled seeds against predation by Apodemus sylvaticus

Abstract The juniper Juniperus communis, a dominant plant in the high mountains of SE Spain, produces a high proportion of empty seeds within well-developed cones. We tested the hypothesis that the production of empty seeds by juniper reduces seed predation by the woodmouse Apodemus sylvaticus, thereby benefitting the plant. We performed laboratory and field experiments to determine i) woodmouse discrimination ability between filled and empty seeds, and ii) woodmouse response to changes in the proportion of empty versus filled seeds and in the seed density in seed clusters. In addition, we estimated, for six juniper populations over three years, whether plants or populations showing a higher proportion of empty seeds suffered reduced woodmouse predation. Experiments showed that woodmice can eventually discriminate externally between filled and empty seeds, but in most cases had to bite the seeds to identify and reject empty ones. The probability of predation for filled seeds was independent of changes in the proportion of empty versus filled seeds and in seed density per cluster. Seed predation suffered by plants in the field was unrelated to the proportion of empty seeds per plant for all populations and years. The presence of empty seeds did not benefit juniper against woodmouse predation, either in terms of individual reproductive output or in terms of offspring escape probability. Our study suggests that the effect of empty seeds on seed predation should be rare in plant-seed predator interactions where predators are polyphagous and eventually able to discriminate against empty seeds, and therefore suffer a low cost when coping with empty seeds.

Interactions between a high-mountain shrub, Genista versicolor (Fabaceae), and its seed predators

Abstract:For three years we studied the interaction between Genista versicolor (Fabaceae) and three seed-predator species, one moth (Coleophora brunneosignata; Coleophoridae) and two weevil species (Exapion compactum and Exapion nov. sp.; Apionidae), to assess the impact of the insects on seed production and the reciprocal effect of the plant reproductive strategies on the insect’s survival. Plants experienced three predispersal mortality factors of ovules: early death of ovules, seed abortion and seed predation. Each fruit bore an average of 5.4 ovules, of which 4.0 ovules died early in their development and 0.1 aborted. Each fruit sets an average of 1.3 mature seeds. Seed predators invariably infested more than 65% of the fruits every year, killing 87% of the seeds ripened by the plants. More than 90% of the seeds predators were weevils. However, including the other two predispersal mortality factors, we found that the main loss was to ovule death (74% of the initial number of ovules), whereas seed pred...