Diversity and Dynamics of Coastal Dune Carabids
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Ann.ANN. Zool. ZOOL. Fennici FENNICI 33: 65–75 Vol. 33 • ISSN 0003-455X65 Helsinki 14 June 1996 © Finnish Zoological and Botanical Publishing Board 1996 Diversity and dynamics of coastal dune carabids Konjev R. C. Desender Department of Entomology, Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1000 Brussel, Belgium Received 29 September 1995, accepted 5 January 1996 Since 1990, populations of carabid beetles are continuously monitored in five coastal dune habitats in Belgium, situated along a transect from seaside marram dunes to inland moss dunes. In five complete year cycles about 14 000 carabids belonging to 76 species have been obtained. At most 50% of the species are continuously present in reproduc- ing populations. The remaining species are only rarely caught, mostly outside of their normal reproductive season, and thus are presumably non-residents or vagrants. Fur- thermore, observations on wing and flight muscle development and on possible source areas support the hypothesis of accidental immigration rather than of local populations passing through bottlenecks. Many well-established and typical dune indicator species show strong fluctuations in their population dynamics, significantly concordant be- tween different sampling stations and largely explained by climatological variables, either directly or indirectly. This study shows that the use of terrestrial invertebrates as bio-indicators in conservation ecology requires a population level approach. 1. Introduction by means of pitfall trapping. Nevertheless, much dis- cussion remains on the necessary methodologies in Carabid beetles are increasingly used as taxonomic sampling (details of techniques, intensity and dura- study group in biodiversity and as bio-indicators in tion of trapping) as well as in data analyses (multi- monitoring or site assessment studies for nature variate analysis techniques for community and indi- conservation purposes (e.g. Luff et al. 1989, 1992, Eyre cator analyses, see e.g. Dufrêne & Legendre 1996) & Luff 1990, 1994, Desender et al. 1991, 1992, Erwin or in diversity assessment (Southwood 1978). 1991, Loreau 1994, Heijerman & Turin 1995, Dufrêne One problem related to the study of carabid di- & Legendre 1996). The very high number of species, versity is to assess which part of the species caught estimated some ten years ago at about 40 000 de- at a certain site actually belongs to the local fauna scribed species (Noonan 1985), as well as the well- and has reproducing populations. Related to this studied pronounced habitat or even microhabitat problem is the question of observed turnover in spe- preference of many of these (Thiele 1977) are im- cies richness from year to year on a given site. A portant reasons for the increasing interest they get. short review of the literature shows that most au- Furthermore, the majority of carabid beetles (at least thors either deny the problem (i.e. assume that all in temperate or subarctic climates) are relatively species caught on a site belong to the local fauna easily collected in a more or less standardized way and/or that species caught in low numbers have a 66 Desender • ANN. ZOOL. FENNICI Vol. 33 Fig. 1. Map of the study area with the sampling sites (long term monitoring transect). small local population) or use a more or less arbi- 1993, Van Dijk 1994). Also, a recent paper by Den trary limit between so-called local species and acci- Boer and van Dijk (1994) shows that many of their dentally caught species. Surprisingly, there have only long term series on carabid dynamics seem to have been few attempts to discriminate between the two been influenced to a high degree by recent direc- by means of long term population studies or by in- tional changes in environmental conditions (e.g. air vestigating additional aspects of the biology (dis- pollution, changed drainage and vegetation cover) persal power and reproductive characteristics) and which could mean they have to be classified more as ecology (occurrence in surrounding or nearby other ‘succession’ studies. habitats). A comparable problem is also encountered In the above context, a long term monitoring on a larger geographical scale, where one recently project was started in coastal dunes in Belgium in has started to distinguish between core and satellite 1990 and has been continued ever since. This study species (e.g. Niemelä & Spence 1994). aims at understanding yearly dynamics at the com- A second problem is the lack of knowledge on munity as well as the population level in order to year-to-year variation in numbers of many carabid define more precisely the possibilities and especially species, in other words data on the magnitude of popu- the constraints of such data for studies on site as- lation dynamics in more or less natural situations. sessment, biodiversity and monitoring. Such studies of course require a continuous long term In this paper I will first evaluate the observed sampling effort, which is probably the most obvious turnover in carabid species from year to year, I then reason for their scarcity. If one does not take succes- will have a look at the dispersal power and reproduc- sion studies into account (which address different tive state from discontinously present species in or- questions (e.g. Meijer 1980, Gruttke & Willecke der to find evidence in support of one of the two alter- 1993, Verschoor & Krebs 1995ab) but not always native hypotheses for their occurrence: are they acci- are able to discriminate natural dynamics from those dental immigrators or do they belong to local linked to directional changes), there are indeed only populations passing through bottlenecks (years with a few studies where sampling has continued for over very low abundance). Finally, for the well-established 5 years. As a result, until now relatively few authors species, I will briefly discuss the extent of their popu- have tried to document and explain these dynamics lation dynamics, whether or not their numbers fluctu- in carabids, and if and how these might be regulated ate in synchrony in different populations and whether (Weber & Klenner 1987, Den Boer 1990, 1991, Luff climatological data can at least partly explain the ob- 1990, Van Dijk & den Boer 1992, Den Boer et al. served dynamics. Of course, analyses on the regula- ANN. ZOOL. FENNICI Vol. 33 • Diversity and dynamics of coastal dune carabids 67 tion of population dynamics have to be viewed as pre- sected in order to distinguish those in reproduction (at least liminary because of the still relatively short nature of carrying ripening or ripe eggs) from those out of their repro- the available sampling series database. ductive period (teneral, immature or spent). The extent of population dynamics in the most abundant species was evaluated by calculating mean R values based on data for each sampling station (‘coefficient of net reproduc- 2. Material and methods tion = total number in year cycle i/number in year cycle i–1’, after Den Boer 1990, Den Boer et al. 1993). Since 1990, populations of carabid beetles and spiders are Analyses of concordance between yearly totals caught in continuously monitored in five coastal dune habitats, situated different sampling stations were performed by means of along a transect from seaside marram (Ammophila arenaria) Kendall’s Test of Concordance (species occurring in suffi- dunes to inland moss dunes and dune grasslands. cient numbers in at least three sampling stations) or Spearman The investigated dune habitats are located near the river Rank Correlation (species from two sampling stations only) IJzer estuary (Fig. 1), close to a field station of the R.B.I.N.Sc. (Siegel, 1956). These dunes are relatively undisturbed by tourism, which is The yearly totals of the most numerous species were ana- extremely rare for Belgian standards. The transect includes lysed in an ordination (Detrended Canonical Correspondence seaward marram dunes (station 1), dune top (station 2) and Analysis) in order to try to simplify the patterns of observed landward side of the same marram dunes (station 3). These dynamics. At this stage, climatological variables were included three habitats are all dominated by marram grass (A. arenaria). in the analysis (temperature, precipitation, relative humidity The remaining two sampling stations are situated more in- and wind speed, all z-standardized, regrouped per season for land, one station close to the marram dunes (station 4), one each year or previous year, in other words including a time- station further inland (station 5). These sites are dominated lag, and after removal of redundant variables). Ordinations by mosses, low herbs and lichens, which especially dominate were performed by means of CANOCO (Ter Braak 1988). in the fifth station. During the autumn of the fourth year cycle Detailed time series are presented after interpolating the monitoring work was disturbed due to the temporary loss the actual data to monthly totals, corrected for (small) dif- of the seaward marram dune station, which vanished during ferences in the number of days in each month. several heavy northwestern storms. The authorities then de- In an earlier paper (Desender & Baert 1995) the sam- cided to protect this area with a concrete wall, which was pling stations of this long term monitoring transect are pre- more or less covered with sand, and is now monitored again sented in more detail along with analyses of the first four for beetles and spiders. complete year cycles, with emphasis on the observed com- Other available habitats