Tomás Santos

INFLUENCE OF FOREST FRAGMENTATION ON SEED CONSUMPTION AND DISPERSAL OF SPANISH JUNIPER Juniperus thurifera

Abstract We examined the effects of fragmentation on Spanish juniper Juniperus thurifera in central Spain by comparing eight small forest fragments (SF: 0·2–16 ha) with two large forests (LF:150 and 270 ha). Wood mice Apodemus sylvaticus , the only rodent seed eaters, were 8·9 times more dense in SF, whereas thrushes Turdus spp., the main avian seed dispersers, were 4·6 times more abundant in LF. Finches (seed eaters) were scarce in both forest groups. Mean fruit abundance was significantly higher in LF. Seed consumption was mainly by mice in SF and by finches in LF. Thrush pellets with intact seeds, seedling abundance and the proportion of trees with nearby seedlings, were all higher in LF. This evidence points to a decrease in the dispersal efficiency of Spanish juniper in SF. We suggest that the processes leading to reduced dispersal ability might be inherent to fragmentation and represent a threat for the survival of fruit-bearing plants in patchy environments.

Vertebrate predation on Holm Oak, Quercus ilex, acorns in a fragmented habitat: effects on seedling recruitment

Abstract Rodent and avian consumption of Holm Oak, Quercus ilex, acorns were examined in central Spain during two high-production seasons, 1990–1991 and 1993–1994, in 1 and 3 large stands and in 6 and 10 small stands (0.2–12 ha) respectively, to study the effects of vertebrate predation on seedling recruitment. Previous data indicate that wood mouse abundance is much higher in small stands. Tree size, frequency of acorn-bearing oaks and acorn abundance were similar in the two sizes of forest stands. Overall acorn consumption was 33.7 times higher in the small stands compared to that in the large stands in 1991, and 3.1 times larger in 1994. Consumption of acorns by mice in the small stands was 49 and 95 times greater than that by birds in the first and second year, respectively. Seedling recruitment was significantly higher in the large stands in both study seasons. Regeneration by ramets was roughly similar in small and large stands in both seasons. Thus, predation by mice was apparently responsible for the failure of sexual reproduction in the small stands. The results suggest that vegetative regeneration might be the prevalent reproductive method of Holm Oaks in small forest tracts.

Bird conservation in fragmented Mediterranean forests of Spain: effects of geographical location, habitat and landscape degradation

Abstract The effects of habitat fragmentation on forest bird assemblages were analysed in 214 holm oak ( Quercus ilex ) remnants spread across the northern and southern plateaux of central Spain. Bird richness was highly dependent on fragment area for all species regardless of isolation, and barely affected by habitat traits. Geographical location was associated with high differences in richness of bird assemblages, which included 17 species exclusive to northern remnants and one exclusive to southern remnants. This supports the hypothesis that habitat suitability deteriorates sharply from north to south for forest birds in Spain. The species-area relationships of bird assemblages sampled in fragmented forests along a broad continental gradient (from Norway to southern Spain) showed that true forest birds only nest in woodlands >100 ha in southern Spain, whereas the full complement of forest species occurs in much smaller fragments in central-western Europe. Loss of species that are particularly sensitive to habitat fragmentation accounts for these differences between dry Spanish and mesic European woodlands. These results are explained by the low habitat suitability of Spanish woodlands, associated with the restrictive conditions for plant regeneration in the Mediterranean climate and long-standing human usage. There is, therefore, a particular need to develop management strategies that conserve birds, and probably other forest organisms, in Mediterranean regions by preventing habitat deterioration and decreases in fragment size, and by conserving all woods >100 ha.

Effects of forest fragmentation on a guild of wintering passerines: the role of habitat selection

Abstract This study analyses the winter colonization of an archipelago of 31 forests (0·1–350 ha) in central Spain by the guild of pariforms ( Parus, Aegithalos, Regulus, Sitta and Certhia ). Two hypotheses are considered: (a) that birds with similar habitat preferences tend to disappear simultaneously with the reduction in forest size, leading to a ‘nested’ pattern of species distribution; or (b) that the species in the smallest forests are a random sample of those found in the larger ones. The results support hypothesis (a). The species that depend on relatively scarce resources, such as tree trunks and junipers Juniperus thurifera ( Sitta europaea, Certhia brachydactyla, Parus cristatus and P.ater ) only occupied the largest forests. On the other hand, species that exploit abundant, ubiquitous resources, such as holm oak Quercus ilex foliage ( Regulus ignicapillus and Parus caeruleus ), were distributed uniformly throughout all the fragments. These results emphasize the need for a better understanding of habitat selection by species when designing conservation strategies for fragmented populations.

A comparison on the response to forest fragmentation by medium-sized Iberian carnivores in central Spain

We studied the use of forest fragments by five medium-sized carnivorespecies in 280 forest fragments on the two Iberian plateaus. We looked forindirect evidence (faeces, tracks, dens) of fragment use (occurrence) by thespecies and analysed whether occurrence could be related to four groups ofvariables: local (vegetation structure and patch size), landscape (distance topossible colonisation sources), regional (fragment location on the northern orsouthern plateau) and the vegetation type of the fragments. We analysed thedifferential response of species according to their life-history and behaviouraltraits. The relationship between use by each species and the factors studied wasanalysed using stepwise logistic regressions. Results indicate that threefactors are crucial to explain fragment use: fragment size, geographic locationand vegetation type. Large fragments are used more than smaller ones, thenorthern plateau is more suitable than the southern plateau, and holm oak andbroad-leaved oak forests are more used than pine woods. The effects andmagnitude varied slightly among species, depending on habitat requirements orlife-history traits. Data indicate that conservation strategies in fragmentedenvironments must take into account elements functioning at different spatialscales, and that it is essential to consider each case within a characteristicregional context.

Abundance and Food-Searching Intensity of Wood Mice (Apodemus sylvaticus) in Fragmented Forests

Abundance, food-searching intensity, physiological status, and population attributes of wood mice ( Apodemus sylvaticus ) were studied in forest islands of central Spain. Seventeen isolated woodlots ( Quercus rotundifolia ) ranging from 0.1 to 280 ha were studied. The results show high densities of mice and high rates of predation on acorns in the smaller forests.

Edge effects on nest predation in Mediterranean fragmented forests

Abstract Edge effects on predation of artificial avian nests were studied in a forest archipelago mixed with agricultural land in Central Spain. Predation rates were lower on farmland than in the forest habitat. There was a trend towards lower predation on the forest edge than in the interior. This edge-core predation gradient differed from the usual pattern of nest predation found in other temperate fragmented forests. Predation by rodents was almost completely restricted to the forest habitat. We suggest that in very small fragments, such as those studied here, a ‘packing effect’ of small specialist forest predators could be the cause of high predation rates throughout the forest.

Distributional patterns of insectivorous passerines in the Iberian Forests: does abundance decrease near the border?

In this paper we analyse whether the abundance of breeding insectivorous birds decreases towards the South along the Iberian forests (as predicted by the model of Brown (1984)), and try also to relate these changes of abundance to several environmental features. Abundances of six foliage insectivorous passerines that are not tree-hole nesters, Troglodytes troglodytes (L., 1758), Erithacus rubecula (L., 1758), Sylvia atricapilla (L., 1758), Phylloscopus collybita (Vieillot, 1817), Regulus ignicapillus (Temminck, 1820), and Aegithalos caudatus (L., 1758) were recorded in fifty-eight large woodlands along a 850 km belt crossing the Iberian Peninsula. By means of simple, partial and stepwise multiple regression analyses bird abundances were related with distances to the north of each woodland and with the mean scores of climatic (mean temperature and precipitation), physiognomic (tree trunk densities under 20 cm diameter and over 30 cm) and floristic (conifer v. broadleaved tree species) variables. Five of the six species showed significant, negative correlations between their abundances and the distances of forests to the north, thus corroborating Browns model. A. caudatus did not show, however, any clear pattern of abundance distribution. Simple correlation analyses showed also the importance of climatic and physiognomic variables in predicting bird abundance. When the problem of interaction between variables was solved, all the five species showed significant positive partial correlation with precipitation, although distance continued to be important for three species (T. troglodytes, R. ignicapillus and P. collybita), suggesting some additional role of this variable in the processes involved in the observed distribution. The importance of precipitation and distance to the north as predictors of abundance was also showed by the stepwise regression analysis. The mean variance explained by these models was around 30%, higher than or similar to that obtained in other studies performed at the local or intrahabitat level. This suggests the potential usefulness of multivariate approaches in studies of bird distribution undertaken on large scales.

Factors involved in the distribution of forest birds in the Iberian Peninsula

The relationships between a series of environmental variables and the abundance of individual species of forest birds were examined in 58 large woodlands along a band crossing the Iberian Peninsula from north to south. The variables were mean annual rainfall, mean annual temperature, density of trees 30 cm in diameter, floristic composition (conifer or broadleaved trees) and geographical situation (distance from northern extreme of study band). The variable most highly correlated with bird density was mean annual rainfall (40% of the species had significant partial correlations with this parameter). The northern species (Chiffchaff, Goldcrest, Robin, Song Thrush, Bullfinch, etc.) generally correlated positively with mean annual rainfall, while the Mediterranean species (Sardinian Warbler, Bonellis Warbler, Black-eared Wheatear, Rock Sparrow, Spotless Starling, Azurewinged Magpie, etc.) exhibited an inverse pattern. Mean annual temperature (36%) was also closely assoc...

Seasonal and interannual occupation of a forest archipelago by insectivorous passerines

The distribution of insectivorous passerines was studied in 31 fragments (0.1 to 350 ha) of a holm oak (Quercus ilex) Mediterranean forest archipelago in central Spain for two consecutive springs and winters (1988 to 1990). The study aimed to assess the role of forest size, abundance and other biological traits (nesting site and foraging substrata) of the species in their ability to occupy the fragments. The number of fragments occupied by the species(f i ) was correlated to their abundance (d i ) in spring and winter. Spring-to-winter changes of d i correlated positively to changes of f i due to the incidence of summer migrants but failed to show any significant pattern when restricted to species occurring in both seasons. The between-winter variations of d i correlated positively with changes in f i, while the between-spring changes of both parameters did not correlate, probably because of stronger effects of site fidelity on bird distribution in this period. Between-year persistence in the occupation of fragments was directly related to bird species abundance and, for a given abundance, persistence decreased in the smallest fragments. Hole-nesting species during the 1988 spring and the exploiters of scarce trees or foraging substrata during both winters showed an apparent restriction to colonize the smaller fragments due in part to changes in habitat structure related to fragmentation. These results support the hypothesis that abundance is a good predictor of the ability of species to occupy and persist in fragments. However, abundance alone does not fully explain the processes that determine these occupancy patterns, since other traits related to the spatial scale of analysis and the suitability of fragments for each bird species are also involved.