patterns of variation in seed traits
Ant-dispersed plants largely rely on elaiosomes for seed dispersal (Van der Pijl 1982; Beattie 1985). Interspecific differences in a variety of diaspore traits have been related to attractiveness (Hughes & Westoby 1992b; Gorb & Gorb 1995) but data on intraspecific variation are scarce (but see Mark & Olesen 1996). Absolute reward for ants (elaiosome mass), relative reward (elaiosome to seed mass ratio), or total amount of bulk to be transported (seed mass), which vary within or among populations, as found in this study, must account for intraspecific patterns.
Seed mass is also frequently recognized as a fundamental trait determining the future survival of seedlings (Jurado & Westoby 1992; Saverimuttu & Westoby 1996; Eriksson & Eriksson 1998; Walters & Reich 2000). Diaspore traits may therefore be classified as related to ant reward (dispersability) and related to size (survival). The partitioning of variance of H. foetidus diaspore traits clearly differed between ant reward-related traits (variability in elaiosome mass and elaiosome to seed mass ratio occurred mostly within plants) and size-related traits, where among-locality differences may reflect possible divergence in seed size. Elaiosome mass did not differ among localities, and seed mass is therefore the main determinant of both relative reward and total ant load. The elaiosome-bearing seeds of H. foetidus thus differ somewhat from other types of seeds, where variation in seed size is often less pronounced among plants within the same population and within individual plants (e.g. Thompson 1984; Thompson & Pellmyr 1989; Obeso 1993; Méndez 1997; Vaughton & Ramsey 1997, 1998).
Principal component analysis (Fig. 3) clearly segregates localities according to seed size and ant reward factors. Interestingly, the upper left quadrant in the PCA (corresponding to the most suitable diaspores for the ants) is often empty for several localities, suggesting a trade-off between seed mass and elaiosome mass i.e. survival (providing resources for future growth) vs. dispersability (Ganeshaiah & Uma Saanker 1991; Greene & Johnson 1993; Morse & Schmitt 1995). In some localities H. foetidus diaspores may be responding mainly to ant dispersal selective pressure by reducing seed mass, whereas in others different factors control the allocation of resources into the diaspore.
Pairwise comparisons of localities (Fig. 3 and analyses of Mseed) suggest the existence of a large scale mosaic for diaspore traits, with similarity unrelated to distance.
patterns of variation in ant community composition and function
Ant community composition varied considerably across the broad range of ecological conditions studied. There was a broad-scale distance-dependent pattern so that, for instance, typical Central European ants like Myrmica scabrinodis and Formica lugubris appeared only in some of the northern populations, but differences in habitat and/or vegetation type also contribute to create variation at smaller spatial scales (Wolff & Debussche 1999; Retana & Cerdá 2000), as between adjacent sites such as Caurel Forest and Caurel Scrubland. Combination of these two patterns creates a complex large-scale mosaic.
Almost all ant species responded positively to seed offerings and preferred seeds with elaiosomes (see also Wolff & Debussche 1999), and could therefore affect the ant–seed interaction. This preference was more marked in larger species (Gorb & Gorb 1995), although many of the smaller species carried more than 50% of the diaspores offered, and could therefore be effective dispersers. Given the positive influence of ant size on the response to diaspore, ant size at the community level could influence H. foetidus dispersal. Although average species size in the ant assemblage tends to increase with increasing latitude (Cushman et al. 1993) over much of Europe, only one of our localities had a significantly different average size and, furthermore, this parameter was not distance-dependent.
Do any differences in ant community composition translate into differential dispersal probabilities for H. foetidus? Local dispersal success, as estimated here (Fig. 3), suggests that dispersal probabilities will vary greatly, but this was due to differences in the absolute abundance of ants rather than in species composition. Thus community-level AR did not vary, showing that, once found, the probability of a seed being dispersed was independent of the substantial variation in ant community composition. Differences in community composition do not therefore necessarily result in functional differences in the quality of the disperser guild of H. foetidus diaspores.
relationships between ant communities and diaspore traits: evolutionary remarks
Local matching between animal and plant traits has generally been considered to be evidence of evolutionary adjustment between the interactors. However, recent formulations of the coevolutionary theory (Thompson 1994, 1997, 1999) have emphasized the role of the geographical structure of the interactions, Following this recent view, we have tested for correlation between the geographical variation in diaspore traits and dispersal-related characteristics of the local ant communities.
The absence of such a relationship found in this study was partially expected from the lack of distance-dependent variation in diaspore traits, although distance-independent matching may still occur. Seed size was the most variable, and therefore most selectable, trait in a broad geographical context but, as expected from a functional equivalence in disperser assemblages (Zamora 2000), mean local seed mass was not significantly related either to ant community structure, ant size at the community level, or ant response to diaspore at the community level (Fig. 5).
The general absence of relationships in this study shows that the spatial mosaic in seed traits of H. foetidus does not match with that of ant communities, but recent theory (Thompson 1994, 1997, 1999) does not require, or even predict, that any specific interaction should generally show accurate matching. Instead a geographical mosaic of mismatched and well-matched situations will result from local selective forces and also, to some extent, from gene flow and genetic drift. Then, could the large-scale lack of adjustment reflected in this study be a consequence of such a mosaic? Our results suggest that this could be the case here: only at some sites is community-level AS correlated with their relative position in the PCA (Fig. 3). Thus Mágina Scrubland and Sierra de Baza, with relatively small ant sizes and small seeds, and Roblehondo and Caurel Forest, with the largest community-level AS and dominated by large-seeded plants, are well matched, unlike any of the other sites. In particular, Caurel Scrubland, one of the northern populations, and Sierra Nevada tend to have large seeds and medium to small ants. The mosaic pattern thus probably obscures any large-scale adjustment.
Well-matched situations may be interpreted as resulting from the selective pressures by the ant assemblage or, in contrast, from random variation in space of diaspore traits and ant assemblages. Most of the good matches (Caurel Forest, Sierra de Baza, and Roblehondo) also have high dispersal success, compared to lower values in mismatches (e.g. Sierra Nevada, Correhuelas) (Fig. 3) and may indicate the operation of selective pressure rather than independent random variation in seed traits and ant communities.
This study represents a first step in establishing the coevolutionary potential of the interaction between H. foetidus and its ant dispersers by studying its geographical structure. The study system potentially reflect a selection mosaic, where distance-dependent patterns are rare and where matching patterns are not established on a species by species basis (as suggested by Thompson 1994). Instead, habitat type, which strongly influences ant community composition and abundance, and the functional equivalence of the disperser assemblages (Zamora 2000) play more important roles.
Results of this study also suggest that close adjustments between plant and animal communities may not be apparent across multiple sites, because interactions may take different directions in different localities or even because at many localities such an adjustment may not occur. Thus, findings based on one or few localities can lead to erroneous conclusions about the evolution of the interactions (Travis 1996; Thompson 1999).