Fundamentos de la sentencia

5.1 Introduction

5.1.1 Abiotic and biotic factors examined

Nest-site selection by H . rubicundus can be split into more than one level. A t the very broadest scale certain basic requirements must be satisfied. These include a need for: a substrate that can be burrowed into; an absence of dense vegetation; exposure to several hours of direct sunlight each day; and an area free of water­ logging. A complete list of prerequisites would be enormous, and examination of such broad characteristics is of little use when investigating nest-site selection in hypogeous Hymenoptera, since in a heterogeneous environment relatively little area w ill satisfy these basic requirements.

What is more interesting is the variation in suitability within a given patch of ground that meets the above criteria. Individual females then show differential preferences between points in close proximity to one another, and so a choice is exhibited. This is reflected in the relative densities within an aggregation, and analysis of this is the main aim of this study. In addition, at the individual nest level, the spatial patterning within an area also reveals other small scale factors that influence female preference.

There is a vast array of possible factors that could influence nest-site selection in ground nesting Hymenoptera. Most studies have examined only a very small number of these variables, and then attempted to use any findings to explain nest- site preferences (see chapter 1). There is no doubt that many of these factors are important in nest-site selection activity, but it is also very probable that other significant factors have been overlooked.

One study has, however, been much broader in its approach. Brockmann (1979) looked at a range of the biotic and abiotic factors which influenced nest-site preference in the wasp Sphex ichneumoneus. Several nesting requirements were identified, and significant correlations between nest density and some substrate properties were found. In addition nest-site fidelity was observed; thus high density aggregations were explained by both the utilisation of limited substrate and by philopatric behaviour. No other study has looked at a wide range of variables likely to influence nest-site selection in a fossorial bee species.

Cane (1991) investigated a few abiotic factors measured at the nest-sites of 32 bee species, but did not look at any relationship between these and the density of nesting. Certain substrate preferences were found, but these varied as much within a bee taxon than they did between taxa.

From the published work (reviewed in chapter 1), it has been possible to put together a comprehensive list of factors that have been found to alter the suitability of a nesting area for a wide range of bee and wasp species. The influences these have upon nesting density are given in Table 5.1, and the predictions made are explained in section 1.2.

These factors have then been examined, in detail, for the single bee species H.

ruhicundus in order to explain the high levels of density observed in the aggregations of this species; both within (Table 5.2) and across (Table 5.5) sites. As the number of ecological factors that could explain variation in nest density is very large and so the type of analysis that is undertaken is important. A series of linear regressions, with density as the response variable, would be highly susceptible to type I errors, with this number of predictors being tested. The

Bonferroni technique could be applied to a table of multiple correlations, but it is often considered too stringent a modification to make and seldom used (Rice 1989). Instead multiple regression has been used wherever possible, as this controls for type I errors.

Many of the factors tested are likely to be closely related, especially when examining edaphic and microclimatic variables. A female bee may test various potential points to initiate a nest and so use cues from the environment to make the decision whether to dig or not. It is easy to measure a large number of properties of the environment, but it may be difficult to determine which are the ultimate and which are the proximate factors determining nest-site suitability. For instance the maintenance of a warm nest is selectively advantageous for both adult and offspring (ultimate factor); and a moderately angled banking, free of vegetation, with a southern aspect are all proximal factors which help ensure that a nests temperature is kept elevated. It may then not be necessary for a searching female to perceive the ultimate factor determining nest-site suitability, but instead use one or more proximate factors that are closely correlated to it.

The various factors were tested by using areas of varying nest density within a site (Invergowrie), and areas of maximum nest density across sites in the UK. It is assumed that an area of maximum density is the most locally favoured nesting site within a heterogeneous environment. Maximum and mean nest density for a particular site are highly correlated (see section 5.3.5), and so both maximum and mean densities w ill vary similarly with respect to other variables.

5.1.2 Female searching and digging behaviour

When females emerge, after overwintering in their hibernacula, they begin to search for a site to initiate a new nest. Initially they fly just above the banking