15. Habitat characteristics

How do we observe a site where a particular species lives? Is it possible to recognize the spatial heterogeneity in the landscape that is essential for the survival of a population of that species? The fundamental question is how can a situation be described from a human point of view that relates to the requirements of an insect?

A start can be made at a site that has some special ecological qualities because of the presence of an interesting species. This site can be looked up on a topographical map scale 1:25.000 or 1:50.000; alternatively the coordinates of the site can be read from a GPS-receiver. In Google-Chrome the site can be found on the satellite picture using the coordinates in hexagonal or decimal notation. The picture on the screen can be watched step by step from the detailed scale to the scale 1:25.000 which corresponds to the topographic map. The satellite picture can be zoomed from micro, meso to macro-scale. But how does the landscape looks through the eyes of a butterfly? What are the butterflies looking for? They will require a site to oviposit where the larvae will survive so that the butterfly will reach maturity.
However, the first question to be answered is: in what kind of landscape is the site situated?

Landscapes
There are many systems of classifying landscapes. The most familiar system is the one that can be read from a topographic map scale 1:25.000 on which the different types of landscape have symbols, which are described in the legend of the map. A map on this scale has as grid cell unit of 1km² and its size is 4 x 4 cm on the 1:25.000 scale. This scale is about the same as at step 6 of a Google-Chrome satellite picture (see table 9-3in chapter 9 ‘Behaviour in space’). Arbitrarily this scale is chosen here as landscape macro-scale. Landscape types are listed in table 15-1.

Table 15-1. Types of landscapes at macro-scale

However, the relation between landscape characteristics and requirements of a particular species cannot be traced using these rough descriptions. In botanical science classifications have been developed which are useful in describing a landscape in detail. The vegetation types are described using the species composition at selected spots (the biotope) and analysed using the edaphic (soil characteristics) growing conditions and occurrence (the ecotope). These data are the basis of classifications and can be used in describing the heterogeneity of a particular landscape. The vegetation classification has a practical purpose: mainly to read the growing conditions from the composition of the local occurring flora. The history of this landscape classification goes back many years and several different systems have been developed. For the Netherlands a handy field guide of plant communities was published by Schaminée et al. in 2010 and this guide is taken as a basis from which more detailed landscape descriptions are introduced. A more detailed picture of vegetation variety assists in the recognition of heterogeneity in the landscape. A more detailed picture, expressing the variety of the vegetation is useful in the characterization of the habitat of a particular species. A numerical classification code enables a compact description. Appendix 8 ‘Codes of selected vegetation units’ presents a list of vegetation types that can be used for describing the landscape heterogeneity. Table 15-2 presents an example how main landscape types can be specified in detail.

Table 15-2. Types of landscapes at macro-scale

Parts of landscape
However, to know what vegetation types are present, one has to make a field investigation but to qualify the heterogeneity of the landscape the detailed satellite picture will show the configuration of the landscape, the area of a particular vegetation type and the boundaries with others. Management advice for a site that focuses on safeguarding a particular species should be presented in a way understandable for people responsible for the management. This advice will mainly present an advice for landscape development and management. The presentation of a detailed satellite picture and an explanation how the species behaves over time and in space and what bottlenecks it may meet, will be the most useful ingredients in conservation planning. In a detailed satellite picture or an aerial photograph at scale 1:4000 or 1:12.500, the boundaries and spatial patterns of different vegetation types are visible. The botanical approach is focussed on community composition and process, whereas the entomological approach is the arrangement of the differences within the landscape and the spatial scale.
Most important is the ecological function of these parts for the particular species. Is a specified given part a place for reproduction, feeding or hibernating? This sort of questions will pass when the characterization of a habitat is the target.

Habitat
The word habitat has several meanings, here it is restricted to the set of requirements of a particular species. A site that has all the qualities for a thriving population can be considered as a habitat. But this is only applicable for species having spatial ‘stay-at-home’ behaviour as described in chapter 9. In the case of nomads and migrants the site may be suitable for a temporary stay to fulfil an episode of the lifecycle and it can be considered as an essential part of the habitat of a particular species. In this case a habitat cannot be characterized by observation at a site but only by summarising all the different habitat parts involved in the landscape. In the cases of not site bound occurrence (‘nomads’ and ‘migrants’) the spots selected by the egg laying butterflies or where the larva can be found are the places to investigate. Recording the places where the butterflies are observed contributes little to the knowledge of the habitat characteristics; it is the assembly of spots which are essential in the reproduction and development that are most informative.
So a successful habitat characterization of a species should take the spatial behaviour into account. This leads to the conclusion that only a small number of species will be suitable for habitat characterization by the traditional method.
Observed habitat characteristics differ with climate. In a cool climate the butterflies will select the warmest places, in a warm climate the coolest ones; obviously this can lead to completely different opinions about the specific habitat of a species.

Space and species
There are species of which their populations inhabit very small sites of less than one hectare, e.g. small blue (Cupido minimus) but also butterflies that use half of the European continent in the course of the seasons , e.g. the migrant red admiral (Vanessa atalanta).
The former species selects a site very precisely with young flowers of kidney vetch in an open vegetation, whereas the latter uses the ubiquitous stinging nettle for oviposition. However, a female red admiral shows that it is also very precise in selecting both the growing condition and position of the plant.
Each butterfly species has a particular relationship with space and environmental heterogeneity;
in mapping programs such relationships are not apparent. Distribution maps give only a vague idea about the space a species requires. In the Netherlands, in surveys the recording of observations are presented either at a 1km² or at a 5km² grid (‘uurhok’), in the United Kingdom at a 10km² grid and for European surveys a 50km² grid. Table 15-3 gives an idea of what grid sizes are appropriate to the different types of spatial behaviours of species. These kind of data are possibly applicable for characterization of the habitat size. Asher et al. (2001) discuss extensively the practicability of distribution maps.

Table 15-3. Spatial behaviour type connected to size of grid cells on topographical maps

Appendix 6. Codes of selected vegetation units

Contact: Sylvain Cuvelier