André P. Schaffers

Arthropod assemblages are best predicted by plant species composition

Insects and spiders comprise more than two-thirds of the Earths total species diversity. There is wide concern, however, that the global diversity of arthropods may be declining even more rapidly than the diversity of vertebrates and plants. For adequate conservation planning, ecologists need to understand the driving factors for arthropod communities and devise methods, that provide reliable predictions when resources do not permit exhaustive ground surveys. Which factor most successfully predicts arthropod community structure is still a matter of debate, however. The purpose of this study was to identify the factor best predicting arthropod assemblage composition. We investigated the species composition of seven functionally different arthropod groups (epigeic spiders, grasshoppers, ground beetles, weevils, hoppers, hoverflies, and bees) at 47 sites in The Netherlands comprising a range of seminatural grassland types and one heathland type. We then compared the actual arthropod composition with predictions based on plant species composition, vegetation structure, environmental data, flower richness, and landscape composition. For this we used the recently published method of predictive co-correspondence analysis, and a predictive variant of canonical correspondence analysis, depending on the type of predictor data. Our results demonstrate that local plant species composition is the most effective predictor of arthropod assemblage composition, for all investigated groups. In predicting arthropod assemblages, plant community composition consistently outperforms both vegetation structure and environmental conditions (even when the two are combined), and also performs better than the surrounding landscape. These results run against a common expectation of vegetation structure as the decisive factor. Such expectations, however, have always been biased by the fact that until recently no methods existed that could use an entire (plant) species composition in the explanatory role. Although more recent experimental diversity work has reawakened interest in the role of plant species, these studies still have not used (or have not been able to use) entire species compositions. They only consider diversity measures, both for plant and insect assemblages, which may obscure relationships. The present study demonstrates that the species compositions of insect and plant communities are clearly linked.

CO‐CORRESPONDENCE ANALYSIS: A NEW ORDINATION METHOD TO RELATE TWO COMMUNITY COMPOSITIONS

A new ordination method, called co-correspondence analysis, is developed to relate two types of communities (e.g., a plant community and an animal community) sampled at a common set of sites in a direct way. The method improves the simple, indirect approach of applying correspondence analysis (reciprocal averaging) to the separate species data sets and correlating the resulting ordination axes. Co-correspondence analysis maximizes the weighted covariance between weighted averaged species scores of one community and weighted averaged species scores of the other community. It thus attempts to identify the patterns that are common to both communities. Both a symmetric descriptive and an asymmetric predictive form are developed. The symmetric form relates to co-inertia analysis and the asymmetric, predictive form to partial least-squares regression. In two examples the predictive power of co-correspondence analysis is compared with that of canonical correspondence analyses on syntaxonomic and environmental data. In the first example, carabid beetles in roadside verges are shown to be more closely related to plant species composition than to vegetation structure (biomass, height, roughness, among others), and, in the second example, bryophytes in spring meadows are shown to be more closely related to the species composition of the vascular plants than to the measured water chemistry.

Soil, biomass, and management of semi-natural vegetation Part I. Interrelationships

The interrelationships between biomass characteristics and soil properties (including in situ annual nitrogen mineralization) were statistically investigated in a descriptive study using a broad range of plant communities in unfertilized road verges. Not only the dependence of biomass characteristics on soil properties, but also the possibility of inferring soil nutrient availability from biomass characteristics was investigated. Possible effects of vegetation management (mowing) and overstory trees (shading) were accounted for. Annual aboveground biomass production was mainly explained by annual N mineralization, average soil moisture content, shading intensity, and soil pH (optimum at pH-CaCl2 5.7). Average tissue nutrient concentrations were primarily explained by mowing frequency, shading intensity, the availability of the corresponding soil nutrient, and pH (optima between 5.5 and 6.0). Results also implied that making hay twice per year removes more nutrients than a single cut at the end of the season. N mineralization may be inferred from the aboveground biomass production, but only across sites with equal moisture and shading conditions (partial r=0.74). In general, it is concluded that nutrient availability can only be deduced from biomass characteristics if sites with similar moisture content are compared. The only exception to this general rule is K availability. The latter was mainly indicated by the K concentration in vegetation biomass (partial r=0.80), and the confounding effect of other factors was small (bivariate r was still 0.71). Soil available P could not be satisfactorily indicated, even across sites with equal moisture and shading conditions. Also, different nutrients appeared to interact, and should not be considered in isolation. K was the only element with a strong relationship between its soil and tissue concentration. For the other nutrients, tissue concentrations did not depend predominantly on the soil availability. It seems most likely that the species occurring in semi-natural vegetation are adapted to the local fertility by means of their physiology or growth rate. It is concluded that, with the possible exception of K, simple relationships between soil properties and biomass characteristics cannot be expected over wide environmental gradients.

Successful overwintering of arthropods in roadside verges

In densely populated areas, roadside verges often provide the last semi-natural habitats available. Their ecological value is often stressed by bio survey results. Yet insect summer surveys potentially misjudge the value of a biotope (roadside or otherwise) since the occurrences of species may only be seasonal, or even transient. To effectively ascertain the importance of a site for insects, species must be shown to complete their life cycle there or at least be shown to successfully accomplish critical life stages, such as reproduction or overwintering. To confirm overwintering of arthropods in roadsides, sods were cut in late winter in a verge where several years of summer survey data were available. The sods were placed in transparent semi-permeable cages and kept alive during subsequent spring, summer and autumn. All emerging arthropods were caught inside the cages using pitfall and funnel traps and identified to order or family level. Most of the terrestrial arthropod phyla and orders occurring in NW-Europe appeared to be represented. Several groups were further identified to the species level: Carabidae (Coleoptera), Curculionidae (Coleoptera), Araneae, Orthoptera, Apidae (Hymenoptera), Syrphidae (Diptera) and Dephacidae/Cercopidae (Hemiptera). Particularly for the Carabidae, Araneae and Curculionidae, many species recorded in summer were also found to overwinter; species overlap amounted to approximately 67%. Rarefaction of the summer sample or excluding possible summer vagrants, raised this overlap to as much as 88% for the Carabidae. Many of the species successfully overwintering in the roadside verge were generalists, but less common, more stenotopic species were present as well. Not only species hibernating as adults were involved, but also species overwintering in immature stages, indicating reproduction also takes place in the roadside verge. Apparently the roadside occurrence of many arthropod species, including stenotopic and declining ones, is not merely seasonal or incidental, and roadside verges do not necessarily act as a sink only. The ecological importance often attributed to roadside verges should clearly be taken seriously.

Food Availability for Meadow Bird Families in Grass Field Margins

Agricultural intensification in grasslands has led to the decline of meadow bird populations in The Netherlands in the last 60 years. Habitat for meadow bird chicks has declined in quality and quantity, thereby reducing food availability. Agri-environment schemes (AES) to halt the decline in meadow bird numbers have thus far been insufficient. These AES are on the level of entire fields, but recent research suggests that margins of fields may be more suitable chick habitat than centres of fields. Therefore, it could be productive to specifically target grass field margins as part of meadow bird AES. Our study examined the differences in food availability for meadow bird families in different portions of a grass field. Invertebrates were sampled in different locations on the field and results were compared to known dietary preferences of four species of meadow bird chicks. We show strong differences in food availability within fields, depending on meadow bird species. The preferred prey species of chicks of Black-tailed Godwit Limosa limosa and Redshank Tringa totanus predominantly occurred in field margins, whereas those of Oystercatcher Haematopus ostralegus chicks were found mostly in the main part of the field. The prey species of Northern Lapwing Vanellus vanellus chicks showed no clear pattern within fields. We conclude that food availability within a field differs spatially between meadow bird species. Particularly for Black-tailed Godwit and Redshank, grass field margins constitute an important part of the field. Therefore, specific management to further enhance food availability in these margins may constitute an important addition to the existing mosaic approach.