Mario Díaz

Regeneration of a Mediterranean oak: A whole-cycle approach

ABSTRACT Using the holm oak (Quercus ilex) as a model system, we quantified the losses in the potential number of recruits from the flower to the sapling stage caused by abiotic factors, pre- and post-dispersal seed predators, and invertebrate and ungulate herbivores in one well-preserved forest plot (F) and in one dehesa (savanna-like) plot (D). Tree fecundity was an order of magnitude higher in D than in F. Predispersal (flower plus fruit) losses due to abiotic factors were much larger (71% in F and 90% in D) than those originated by biotic factors (29% in F and 10% in D). Post-dispersal predation of acorns under trees led to crop depletion in F, while predators did not deplete the acorn crop despite a much higher predation level in D. Surface acorns were invariably eaten, while most buried acorns survived to germination in both plots. Ninety-five percent of seedlings emerged under oaks in D and only 20% in F, the density of newly emerged seedlings being ten times greater in F. Emergence from experimentally sown acorns was very high on average (73%), while seedling survival to the 2-y sapling stage was primarily affected by water stress in the first summer, especially in the dehesa stand. A 75-fold difference in recruitment rates between populations was found (0.00150 in F and 0.00002 in D). This whole-cycle disparity was the result of differences in the conversion rate from viable fruits to newly emerged seedlings (two orders of magnitude larger in F). The inability to direct acorns to safe (shaded) sites by means of efficient dispersers appears to limit recruitment in open dehesa stands. This finding could explain the general lack of natural regeneration of dehesas as compared to the forests from which they developed.

Effects of landscape complexity on the ecological effectiveness of agri-environment schemes

Agricultural intensification is a major cause for biodiversity loss. It occurs at field scales through increased inputs and outputs, and at landscape scales through landscape simplification. Agri-environment schemes (AES) of the European Common Agricultural Policy (CAP) aim at reducing biodiversity loss by promoting extensification of agricultural practises mostly at field scales. We present a conceptual model for the relationship between landscape complexity and ecological effectiveness of AES based on (a) non-linear relationships between landscape complexity and abundance and diversity at field scales and (b) four possible interactive scenarios between landscape- and field scale effects on abundance and diversity. We then evaluated whether and how effectiveness of AES interacted with landscape-scale effects of intensification along a landscape complexity gradient established in central Spain. Pairs of cereal fields with and without AES but with the same landscape context were selected in three regions differing in landscape complexity. Effectiveness of AES was measured as differences between paired fields in species richness and abundance of five target groups (birds, grasshoppers and crickets, spiders, bees and plants). Landscape metrics were measured in 500–m radius circular plots around field centres. Positive, negative and no effects of landscape complexity on effectiveness of AES were found, suggesting that effects of complexity on effectiveness of AES changes from positive to negative along gradients of landscape complexity. Effectiveness of AES for improving biodiversity was then constrained by landscape. Compulsory measures aimed at enhancing or maintaining landscape complexity would enhance the effectiveness of AES for preserving biodiversity in farmed landscapes.

Satiation of predispersal seed predators: the importance of considering both plant and seed levels

Plants can reduce the fitness costs of granivory by satiating seed predators. The most common satiation mechanism is the production of large crops, which ensures that a proportion of the seeds survive predation. Nevertheless, satiation of small granivores at the seed level may also exist. Larger seeds would satiate more efficiently, enhancing the probability of seed survival after having been attacked. However, a larger seed size could compromise the efficiency of satiation by means of large crops if there were a negative relationship between seed size and the number of seeds produced by an individual plant. We analyze both types of satiation in the interaction between the holm oak Quercus ilex and the chestnut weevil Curculio elephas. Both crop size and acorn size differed strongly in a sample of 32 trees. Larger crop sizes satiated weevils, and higher proportions of the seeds were not attacked as crop size increased. Larger seeds also satiated weevil larvae, as a larger acorn size increased the likelihood of embryo survival. Seedling size was strongly related to acorn size and was reduced by weevil attack, but seedlings coming from large weeviled acorns were still larger. The number and the size of the acorns produced by individual trees were negatively related. Larger proportions of the crop were infested in oaks producing less numerous crops of larger acorns. However, contrary to expectations, these trees did not satiate more effectively at the seed level either. Effective satiation by larger acorns was precluded by larger multi-infestation rates associated to smaller seed crops, in such a way that the proportion of attacked seeds that survived did not vary among trees with different acorn sizes. These results highlight the need of considering satiation by means of large crops and large seeds in studies of predispersal seed predation. Long-term monitoring on individual oaks will help to assess whether there is a trade-off between the number and the size of the acorns and, if it existed, how it could condition the fitness consequences of both types of satiation.

Facilitation of Quercus ilex recruitment by shrubs in Mediterranean open woodlands

Abstract Question: Insufficient tree regeneration threatens the long-term persistence of biodiverse Mediterranean open oak woodlands. Could shrubs, scarce due to decades of management (clearing and ploughing), facilitate holm oak recruitment at both acorn and seedling stages? Location: Open oak woodlands in Central Spain. Methods: Plots with four acorns were planted: (1) under the canopy of the spiny shrub Genista hirsuta, (2) in a small cage, protecting against ungulates, (3) in a shaded cage, protecting against ungulates and sun, and (4) in open grassland. Sets of these four treatments were spatially grouped according to a randomised block design, with 16 blocks near (< 10 m) and 16 away from (> 20 m) parent trees to test for distance-related survival. Plots were regularly checked for seed removal. After emergence one seedling per plot (97 in total) was selected and its survival monitored. Results: Three months after sowing, 199 of 512 acorns were removed, predominantly by rodents. Acorn removal occurred at each treatment but was highest under shrubs. Eight months after sowing, seedling survival was highest under shrubs (50%), followed by shaded cages (16%), open grassland (4%) and cages (0%). Main mortality cause was drought (90%), killing most seedlings between June and July. No seedlings died from ungulate browsing. Conclusion: Shrubs demonstrated clear net facilitative effects for Quercus ilex recruitment, despite higher seed removal. Shading appears the crucial factor facilitating seedling survival. We therefore propose that lack of shrubs contributes largely to tree recruitment failure in Mediterranean open woodlands; management should aim at conserving shrubs.

Food choice by seed-eating birds in relation to seed chemistry☆

Abstract The role of seed chemical traits (nutrient composition, energy contents and presence of secondary compounds) for the selection of foods by granivorous birds and for the evolution of bird granivory is analyzed and compared with the roles of other seed traits such as seed distribution and seed morphology. A literature review was made, focusing on: 1) seasonal dietary shifts from seeds to other food items; 2) dietary effects of between-seeds differences in energy and nutrient contents; and 3) dietary effects of seed secondary chemistry. Results suggest that the effects of seed chemistry on seed-eating bird diets are much less important than the effects of morphologic traits related to seed processing speed. Birds generally tend to select the food items they can process faster, their chemical composition being of secondary importance. Several experiments aimed to test this hypothesis are proposed. This pattern of food selection suggests that the evolution of bird granivory has been mainly focused on the development of morphologic adaptations for the mechanical digestion of seeds, whereas physiologic adaptations for their chemical digestion appear to be secondary. Additional data on the physiology of seed-eating birds in the wild would be necessary to corroborate this idea.

Spatial and temporal patterns of granivorous ant seed predation in patchy cereal crop areas of central Spain

SummaryPatterns of granivorous ant seed predation in extensive cereal croplands of central Spain were investigated by measuring seed removal rates on artificial seed patches. Sampling was designed to cope with the seasonal and daily foraging cycle of ant colonies. Simultaneously with removal rates, I measured seed availability, habitat physiognomy at two spatial scales (landscape and microhabitat), weather variables (temperature and rainfall), and distance to the nearest ant nest. Ant seed predation was concentrated on shrublands, and associated with places with high covers of shrubs, chamaephytes and stones. These results were in close agreement with those obtained by analyzing the spatial distribution of granivorous ant nests (Díaz 1991). Moreover, there was a close relationship between seed removal rates and distance to the nearest ant nest, that fitted the predictions of the optimal foraging model developed by Reyes-López (1987). Seasonal and daily patterns of ant foraging activity seemed to depend more on endogenous factors than on environmental variation. I conclude that ants were not able to track the spatial and temporal variation of their food resources in these man-modified habitats, so that their potential to interact with other members of the granivore system is greatly reduced by human activities.

Habitat selection patterns of common cranes Grus grus wintering in holm oak Quercus ilex dehesas of central Spain: Effects of human management

Abstract Most of the western European population of common cranes Grus grus spends the winter in Iberian wooded dehesas, a kind of wood-pasture composed of grasslands, cereal croplands and Mediterranean scrub, densely interspersed with holm oak trees Quercus ilex in a savanna-like landscape. Three main types of wooded dehesas can be distinguished according to management: grazed dehesas, shrubby dehesas with occasional grazing, and cultivated dehesas without livestock. Cranes depend largely on acorns during winter and mainly select dehesas cultivated with cereals where acorn abundance is not reduced by livestock. Apparent positive effects of livestock on earth-worm abundance, the main alternative food source for cranes, does not compensate for acorn depletion. Thus, any increase in livestock grazing pressure would have a strong impact on European crane populations during their wintering in Spain.

Variability in seed size selection by granivorous passerines: effects of bird size, bird size variability, and ecological plasticity

The niche variation hypothesis predicts a direct relationship between intraspecific variability in feeding ecology and the variability of the morphological traits related to feeding behaviour. The following study tests this prediction by measuring in captivity the seed size preferences and the morphology of 9–11 individuals of seven specialized granivorous bird species. The average seed size preferences of these birds have previously been shown to be related to components of bill size. The ranges of seed sizes selected were related to the mean bill sizes of birds in a way that paralleled the patterns found when analysing average values. Bill and body size variability were not related, however, to the range of seed preferences after controlling for the significant mean-variance relationship showed by morphological traits. Thus, results do not support the niche variation hypothesis. the significant effect of average bill size on diet variability was consistent with the direct relationship between bird size and ecological plasticity expected on the basis of the shape of the family of functions relating seed size and seed profitability for different-sized birds. These findings suggest morphological mechanisms for ecological plasticity whose generality and evolutionary significance merit further research.

Complex edge effect fields as additive processes in patches of ecological systems

Abstract The edge effect is one of the main phenomena studied by landscape ecology, since it plays a decisive role in determining the structure and dynamics of ecological patches. Most conservation issues involving spatial considerations (design of reserves, land use planning, etc.) require—implicitly or explicitly—an analysis of the edge effects under operation and of their consequences. These are classically analyzed through empirical measurements of linear gradients referenced to the patch border, which yield models of edge effects that: (i) wrongly assume a simple spatial relationship to a single point of the border (the nearest one), (ii) provide flawed predictions on the effect fields generated in ecological patches. Malcolm (1994) has recently addressed these problems by considering an alternative approach that formally integrates the edge effect of every point along a patch border (‘point edge effect’), to generate edge effect fields. In this paper, we deepen the analysis of this concept of addition of multiple point edge effects to generate total effect fields, revealing some important restrictions of Malcolms method. These restrictions are: (i) linear point edge effects, and (ii) homogeneity of the point edge effect along the patch border. Here we carry out an extended application of this method to patch borders (homogeneous or heterogeneous in their point edge effects) of any geometry and to point edge effects of any kind, no matter how irregular or complex these might be. We provide a general numerical solution for the calculation of edge effect fields by means of a topological parameterization, which has been implemented in a program that can be easily run in MATHEMATICA software. We further show how patch shape and size can interfere with the point edge effect, giving rise to complex effect fields within the patches, which cannot be explained by edge effect penetration alone.

Effects of forest fragmentation on the winter body condition and population parameters of an habitat generalist, the wood mouse Apodemus sylvaticus: a test of hypotheses

Three main causal hypotheses have been proposed to explain the inverse relationships between habitat patch size and density of gen- eralist mouse species in fragmented habitats: 1) enhanced habitat conditions as habitat patch size decreases; 2) inhibited emigration of excess indi- viduals in small and isolated habitat patches; and 3). reduced territoriality in small patches because they are occupied temporarily by non- reproductive individuals. From the mechanism underlying each hypothesis, we derived predictions on the effects of fragment size on the body condition of individuals (measured both as absolute body size and as body mass relative to body size) and some demographic parameters of mouse populations redated to reproductive output (sex-ratio and proportions of sexually active and recently-born individuals), and we tested such predic- tions with data from wood mice Apodemus sylvaticus wintering in three Spanish forest archipelagos in which the inverse relationship between for- est patch size and mouse abundance had been previously proven. No differences in average body size or in average body mass relative to body size were detected among fragments. Mouse populations wintering in small fragments showed more male-biased sex-ratios, a larger proportion of sex- ually active adults and fewer juveniles as compared to mouse populations wintering in large fragments nearby. Results clearly rejected the third hypothesis and did not support the second one. It thus seemed that habitat conditions for mice improved as forest fragment size decreased, although the expected positive effects on individuals could have been prevented by relaxed territoriality and increased food resource depletion by denser mouse populations. Bearing in mind the negative effects of dense wood mice populations on the distribution, abundance and population dynamics of forest species, this apparent enhancement of habitat conditions for mice in small forest fragments could have far-reaching conse- quences for the long-term persistence of such fragments. 0 Elsevier, Paris