José Ramón Obeso

Beyond species loss: the extinction of ecological interactions in a changing world

Summary 1. The effects of the present biodiversity crisis have been largely focused on the loss of species. However, a missed component of biodiversity loss that often accompanies or even precedes species disappearance is the extinction of ecological interactions. 2. Here, we propose a novel model that (i) relates the diversity of both species and interactions along a gradient of environmental deterioration and (ii) explores how the rate of loss of ecological functions, and consequently of ecosystem services, can be accelerated or restrained depending on how the rate of species loss covaries with the rate of interactions loss. 3. We find that the loss of species and interactions are decoupled, such that ecological interactions are often lost at a higher rate. This implies that the loss of ecological interactions may occur well before species disappearance, affecting species functionality and ecosystems services at a faster rate than species extinctions. We provide a number of empirical case studies illustrating these points. 4. Our approach emphasizes the importance of focusing on species interactions as the major biodiversity component from which the ‘health’ of ecosystems depends.

Sex ratios, size distributions, and sexual dimorphism in the dioecious tree Ilex aquifolium (Aquifoliaceae).

Sex ratio and sexual dimorphism in physiology and growth were studied in the dioecious tree Ilex aquifolium at two localities in northern Spain. Genet sex ratio was significantly male biased in one locality but not in the other. However, ramet and flowering ramet sex ratios were male biased at both study sites. Males had significantly thicker main trunks than females in one locality and produced more ramets in the other. Growth rate, estimated from mean width of annual rings, did not differ between localities, but males produced wider rings than females at both sites. Mean annual growth rates over the last 10, 20, and 30 yr were significantly higher for males. Measurements of chlorophyll fluorescence indicated that the efficiency of photosynthesis of leaves on nonfruiting branches of females was higher than for leaves on branches of male plants under low-light conditions, though not under saturating-light conditions. Efficiency of photosynthesis was significantly lower on fruiting branches of female plants than on nonfruiting branches. We discuss whether the observed between-sex differences are attributable to the higher cost of reproduction in females and/or to pollen competition.

Seed mass variation in the perennial herb Asphodelus albus: sources of variation and position effect

Patterns of seed mass variation in the perennial herb Asphodelus albus (Liliaceae) were studied in one population over 3 years (1988–1990) and in three populations during 1989. Plant size, phenology and several components of plant fecundity showed no effect on mean seed mass per plant. Mean seed mass varied among populations and among plants within populations. Significant variation was also found among years and among plants within year, but most of the variation was accounted for by the within-plant component. Withinfruit variation may be as important as between fruits within plant. Fruit position within the plant influenced seed mass, being heavier the seeds at the bottom of the stalk. However, the plants markedly differed in the proportion of the variation accounted for by the position effect. The correlation between seed number per fruit and seed mass showed an interaction with fruit position. Seeds from small broods were heavier than those from large ones, but only in the lower part of the stalk. Decline in seed mass towards the top of the stalk may be attributed to seasonal reduction in resource availability. In addition, the change in the relationship between seed number and mass might be due to changes in the “resolution” of the parent-offspring conflict, also related to nutrient availability.

Allometric allocation in fruit and seed packaging conditions the conflict among selective pressures on seed size

Selective pressures on seed size could vary among the different stages of plant life cycles, so no simple relation could explain a priori its evolution. Here, we determined the relationships between seed size and two fitness components—seed dispersal and survival from predation—in a bird-dispersed tree, Crataegus monogyna. We interpret these relationships in relation to the patterns of mass allocation to fruit and seed components. Selection patterns were assessed at two levels (1) selection pressures on the parent tree; comparing seed dispersal efficiency among individual plants and (2) selection pressures at the individual seed level; comparing seed size variation (i) before and after dispersal, and (ii) before and after postdispersal seed predation. Dispersal efficiency (percentage of seed crop dispersed) was positively correlated with fruit mass and fruit width. Differences in crop size did not offset this effect, and larger seeds were overrepresented in the seed rain relative to the seed pool before dispersal. However, the advantage of larger seeds during the dispersal stage was cancelled later by an opposite selection pressure exerted by seed predators. As a result, smaller seeds had a higher probability of surviving postdispersal seed predation, establishing an evolutionary conflict imposed by the need for dispersal and the danger of being predated. Birds and rodents preferentially selected highly profitable fruits and seeds in terms of the relative proportion of their components. Larger fruits had a higher pulp to seed proportion than smaller ones, and all seeds had the same proportion of coat relative to the embryo-plus-endosperm fraction. Hence, although predator pressures were stronger than disperser ones, larger seeds invested proportionally less in structural defense than in dispersal.

Selective fruit and seed maturation in Asphodelus albus Miller (Liliaceae)

The fruiting patterns of the rhizomatous perennial Asphodelus albus Miller (Liliaceae) were studied in five populations during 1989 and in one population over 3 years (1988–1990). Fruit/flower (Fr/Fl) ratio and seed/ovule (S/O) ratio varied markedly between populations. Although there were differences between years within the population studied over 3 years, these variations, although statistically signifcant, were less important than those between populations. Neither flowering phenology nor plant size influenced Fr/Fl or S/O ratios. Field experiments tested whether fruit and seed set were pollenor resource-limited. Hand-pollination had no effect on Fr/Fl or S/O ratios, but the reduction of resources by defoliation at the time of flowering decreased both relative components of fecundity. Manipulation of resources by hand-thinning flowers and tiny fruits had no demonstrable effect on these ratios, although brood size of individual fruits was significantly affected. It may be concluded that fruit maturation is resource-limited rather than pollen-limited. Most of the fruits aborted early in the fruiting season, and fruits with higher numbers of developing seeds had a lower probability of abortion than fruits with fewer seeds. Analyses of position effects revealed that the fruits in lower positions in the inflorescence matured preferentially. Furthermore, the two ovules in the same carpel tended either both to fail or both to develop into seeds. The plants ability to selectively mature only high quality embryos may be limited.

Fruit removal, pyrene dispersal, post-dispersal predation and seedling establishment of a bird-dispersed tree

The recruitment of a dioecious bird-dispersed tree, the hollyIlex aquifolium (Aquifoliaceae), was studied consideringthe stages of fruit removal by birds, seed rain, post-dispersal seed predation,seed germination and seedling survival. The main objective was to test theeffect of different microhabitats within a beech forest on recruitment stages.Migrant thrushes were the main dispersers of this tree whose fruit crops wereentirely removed during two study years. Seed rain was greatest beneath hollytrees regardless of their sex and lowest in the open sites. Post-dispersal seedpredation was examined by two experiments and did not differ betweenmicrohabitats despite its quantitative importance (about 70%). Seedlingemergence, which probably corresponded to seeds from several cohorts, wasgreater beneath trees than in open sites and the density of second-yr to 5cm seedlings depended on the presence-absence of ungulateherbivores and litter. While the former had a detrimental effect, the latterhada beneficial effect on seedling abundance. Seedling survival showed nosignificant variations between microhabitats but depended on seedling densityinsome microhabitats (holly, beech). Finally, the initial seed arrival seemed todetermine microhabitat suitability for holly seedling establishment. However,under heavy browsing the density of seedlings may be strongly reduced leadingtomicrohabitat homogeneity for holly seedling establishment.

Geographic distribution and community structure of bumblebees in the northern Iberian peninsula

SummaryRelationships between species distribution and abundance, the influence of proboscis length on species-packing, and species associations within the local assemblages were studied in local communities of bumblebees in northern Spain along an altitudinal gradient. Local species abundance and altitudinal range occupied accounted for much of the variation in species distribution. Altitudinal range occupied by species was related to species distribution, but the most important variable accounting for species distribution was the local percentage abundance. Despite this, there was no evidence for age abundance. Despite this, there was no evidence for bimodality in the distributions of species incidence. A general trends for mean proboscis length in each locality to be greater in lowland localities exists, but this variable was not related to species distribution or abundance. Proboscis length spacings were studied among species in local assemblages and in most of the cases observed spacing did not differ from random expectations. The same patterns were demonstrated calculating spacings for ‘core’ species in each local assemblage. Furthermore, species showed little tendency towards associations, so it may be concluded that bumblebee assemblages were irregularly structured and no clear patterns emerged from the present study.

Habitat selection by Cantabrian capercaillie Tetrao urogallus cantabricus at the edge of the species' distribution

Abstract We studied the patterns of habitat availability and use by the Cantabrian capercaillie Tetrao urogallus cantabricus, an isolated and endangered population at the southwestern edge of the species distribution. We combined field surveys and GIS analyses to show that this population differs from its conifer-specialist conspecifics in that it inhabits beech Fagus sylvatica and oak Quercus petraea forests, without specialisation as regards the tree species as long as enough forest cover remains. The habitat of Cantabrian capercaillie is highly fragmented, and smaller forest patches have been abandoned during the last few decades; the display areas that remain occupied are now located farther from forest edges. Lower tree density and more widespread distribution of bilberry Vaccinium myrtillus characterised the occupied display grounds. However, even abandoned areas showed tree densities well within the optimum range for capercaillie and bilberry in other populations, a result that should be taken into account before any habitat management action is considered. We found that some non-forested habitats were also used by capercaillie, especially during the autumn and winter. We suggest that the protection of Cantabrian capercaillie should be tightly coupled with an effective, strict protection of the few remaining large forest fragments in the range, and that information about nesting and brood rearing habitat should be obtained. Conditions for understory development should be favoured, preventing overgrazing by ungulates. The natural, non-forested matrix should also be considered in management plans, as a direct source of food and shelter and an important feature determining connectivity among patches.

Long-term trends in food habits of a relict brown bear population in northern Spain: the influence of climate and local factors

SUMMARY Relict endangered populations may be especially vulnerable to the effects of climate change. A data series for the period 1974‐2003 was used to examine shifts in brown bear Ursus arctos food habits in its south-westernmostEuropeanpopulation.Thisfocused on the hyperphagic season, when bears gain the fat thatisessentialforwinterdormancyandreproduction. General climatic indicators were predictors of diet trends. Other variables potentially able to modify brownbearfoodhabitsatthelocalscalewereaccounted for by considering two areas where local conditions changed in different ways during the study period. General climatic indicators such as temperature and the winter North Atlantic Oscillation index were good predictors of some diet trends, although local factors seemed to modulate the potential response of food habits to recent climate change. Boreal and temperate fooditemsdecreasinglycontributedtobrownbeardiet, replacedbyincreasingcontributionsofsouthernfoods, which suggested that warmer temperatures might determine the occurrence of some food items in the diet of Cantabrian brown bears through effects on plant distribution and phenology. At the local scale, high cattle density in one of the study areas limited foodsourcesavailableforbrownbears.Importantfood sources for brown bears, such as heath-like Vaccinium formationsandold-growthoakforestintheCantabrian Mountains, require increased levels of protection.

Patterns of variation in Ilex aquifolium fruit traits related to fruit consumption by birds and seed predation by rodents

The drupes of Ilex aquifolium contain up to four pyrenes, but brood size reduction by both pyrene number reduction and/or seed abortion producing blank pyrenes affected 2/3 of fruit production. The...