H. Siepel

Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats

One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984–2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.

Life-history tactics of soil microarthropods

A sound basis for defining life-history tactics was obtained by reviewing life-history traits. This involved an explanation of the ecological implications of thelytoky (automictic or apomictic), arrhenotoky, amphitoky, sexual reproduction, semelparity, iteroparity, seasonal iteroparity, and the relationship between semelparity and juvenile development. Several forms of synchronization of life cycles with environmental conditions were classified, ranging from the ability to overcome harsh seasonal conditions to obligate diapause dormancy. Ecologically, this involves adaptations to environmental conditions ranging from irregularly occurring and unpredictable events to regularly occurring and predictable conditions. Dispersal traits were grouped as directional migration (phoresy) and undirectional migration (anemochory). Distinctions were made between facultative and obligate phoresy and between carrier-specific and carrier-unspecific phoresy. A multi-dimensional system of tactics was outlined, based on well-defined underived traits. This system was compared with one- and two-dimensional schemes described by MacArthur and Wilson (1967), Grime (1977), and Southwood (1977) and with the multi-dimensional system described by Stearns (1976). For each tactic, relationships with the main biotopes in which it occurs were defined. Examples of species showing certain tactics were selected from the taxonomic orders of microarthropods. The generality of the traits for various groups of organisms and of the classification of life-history tactics developed was explored.

More than 75 percent decline over 27 years in total flying insect biomass in protected areas

Global declines in insects have sparked wide interest among scientists, politicians, and the general public. Loss of insect diversity and abundance is expected to provoke cascading effects on food webs and to jeopardize ecosystem services. Our understanding of the extent and underlying causes of this decline is based on the abundance of single species or taxonomic groups only, rather than changes in insect biomass which is more relevant for ecological functioning. Here, we used a standardized protocol to measure total insect biomass using Malaise traps, deployed over 27 years in 63 nature protection areas in Germany (96 unique location-year combinations) to infer on the status and trend of local entomofauna. Our analysis estimates a seasonal decline of 76%, and mid-summer decline of 82% in flying insect biomass over the 27 years of study. We show that this decline is apparent regardless of habitat type, while changes in weather, land use, and habitat characteristics cannot explain this overall decline. This yet unrecognized loss of insect biomass must be taken into account in evaluating declines in abundance of species depending on insects as a food source, and ecosystem functioning in the European landscape.

Museum specimens reveal loss of pollen host plants as key factor driving wild bee decline in The Netherlands

Significance Growing concern about bee declines and associated loss of pollination services has increased the urgency to identify the underlying causes. So far, the identification of the key drivers of decline of bee populations has largely been based on speculation. We assessed the relative importance of a range of proposed factors responsible for wild bee decline and show that loss of preferred host plant species is one of the main factors associated with the decline of bee populations in The Netherlands. Interestingly, species foraging on crop plant families have stable or increasing populations. These results indicate that mitigation strategies for loss of wild bees will only be effective if they target the specific host plants of declining bee species. Evidence for declining populations of both wild and managed bees has raised concern about a potential global pollination crisis. Strategies to mitigate bee loss generally aim to enhance floral resources. However, we do not really know whether loss of preferred floral resources is the key driver of bee decline because accurate assessment of host plant preferences is difficult, particularly for species that have become rare. Here we examine whether population trends of wild bees in The Netherlands can be explained by trends in host plants, and how this relates to other factors such as climate change. We determined host plant preference of bee species using pollen loads on specimens in entomological collections that were collected before the onset of their decline, and used atlas data to quantify population trends of bee species and their host plants. We show that decline of preferred host plant species was one of two main factors associated with bee decline. Bee body size, the other main factor, was negatively related to population trend, which, because larger bee species have larger pollen requirements than smaller species, may also point toward food limitation as a key factor driving wild bee loss. Diet breadth and other potential factors such as length of flight period or climate change sensitivity were not important in explaining twentieth century bee population trends. These results highlight the species-specific nature of wild bee decline and indicate that mitigation strategies will only be effective if they target the specific host plants of declining species.

Mites of different feeding guilds affect decomposition of organic matter

Abstract The effect of representatives of five feeding guilds of oribatid mites on microbial respiration during decomposition of organic matter was studied. Herbivorous grazers had no effect on microbial respiration; herbofungivorous grazers and fungivorous grazers had a stimulating effect on microbial respiration, while fungivorous browsers and opportunistic herbofungivores had an inhibiting effect on it. The guilds with stimulating effects on mierobial respiration have in common the ability to digest chitin, a main component of fungal cell-walls. Chitin consists of nitrogen, the release of which may stimulate fungal growth and respiration. Addition of nitrogen in experiments stimulated microbial respiration, but the positive effect of a herbofungivorous grazer could not be demonstrated in experiments where extra nitrogen was added. It is concluded that stimulation of microbial respiration may be the result of the release of nitrogen from fungal cell-walls. Micro-arthropod grazing on fungi is discussed with respect to effects of micro-arthropod density, nutrient availability in the substrate and the nature of grazing. Next to these effects, temporal and spatial factors may have some influence on the eventual result of fungal activity on overall decomposition rates.

Applications of microarthropod life-history tactics in nature management and ecotoxicology

Examples of the practical use of microarthropod life-history tactics in nature management and ecotoxicology were selected. A key to the tactics was developed to facilitate the use of the life-history tactics. Examples of the microarthropod distribution over these tacties were selected from a forest, a grassland, and a salt marsh. The process of decomposition of grass leaves in litterbags was described using life-history tactic diagrams. The effects of various disturbances were described in terms of shifts in the distribution of life-history tactics of species. Irregular unpredictable disturbances lead to an increased representation of phoresy tactics. Regular disturbances lead to an increased representation of synchronization tactics. Permanent and persistent pollution, finally, leads to an increased representation of tactics with thelytokous reproduction. The consequences of the latter for ecotoxicology were examined. It was concluded that microarthropod life-history tactics meet the criteria described, permitting comparisons between effects of management measures and pollution in different biotopes and countries.

Are some mites more ecologically exposed to pollution with lead than others

Siepel, H. 1995, Are some mites more ecologically exposed to pollution with lead than others? Exp. Appl. Acerol. 19: 000-000. The hypothesis that soil mites capable of digesting fungal cell-walls by chitinase activity in their guts are more exposed to heavy-metal pollution, for instance lead, than other fungivorous soil mites was tested. Heavy metals accumulated in fungi are located in particular in the cell wall. Mites piercing the fungi and digesting the contents will not raise their body burden of metals significantly. Mites ingesting fungal material but unable to digest fungal cell walls (fungivorous browsers), may have a somewhat higher exposure to heavy-metal pollution. However, mites ingesting fungal hyphae and digesting all the cell walls (fungivorous grazers) will encounter the most pronounced effects. A test of this hypothesis in an industrial site predominantly contaminated with lead showed an almost complete absence of the most vulnerable group: the fungivorous grazers. A second test in a clay pigeon shooting range showed an inverse relationship between the density of fungivorous grazers and the amount of lead in the soil. Fungivorous grazers at that site also appeared to have a much higher body burden of lead than other mites. Finally in laboratory experiments, it is shown that fungivorous grazers accumulate lead faster than fungivorous browsers.

A tool to assess the influence of management practices on grassland surface macrofaunas

We present a method for involving the invertebrate fauna into nature management by the selection of species indicating effects of fertilization, mowing and grazing on the surface macrofauna of grasslands. Low fertilization levels result in the highest numbers of individuals. Grazing results in lowered diversity and a decrease in number of species as compared with no management and with mowing. Mowing results in higher numbers of individuals compared with no management and with grazing. Indicator species are given for assessing the main difference between grasslands which turns out to be fertilization/yield, and grazing. Management effects on the surface macrofauna, the usefulness of the method, and the indicator species concerned are discussed.

Habitat selection of brood-rearing northern wheatears oenanthe oenanthe and their invertebrate prey

Birds consider both variation in prey abundance and accessibility in their decision of where to forage. Acidification and nitrogen deposition affect both prey abundance and accessibility by stimulating growth of nitrophilic grasses at the expense of forbs. Management practises such as mowing or grazing primarily affect vegetation structure which also influences the abundance and accessibility of invertebrates. Hence, for effective management and conservation purposes It is paramount to understand the relationships between vegetation structure, densities of preferred prey and habitat-use of birds. In this study we explore such relationships for the nationally endangered Northern Wheatear Oenanthe oenanthe in dune grasslands along the Dutch coast. Our findings support the hypothesis that forager mobility and food accessibility are of greater importance during patch selection than food abundance per se in ground foraging birds. The abundance of all potential prey and three of the four most important actual prey groups was highest in tall grass, but Northern Wheatears foraged preferentially in short grass. Clearly, encroachment by tall grass will diminish habitat suitability for Northern Wheatears due to lowered prey accessibility. We propose that a mixture of short and tall vegetation and landscape management allows for dispersal of arthropods between different (micro)habitats. We provide densities of the important prey in a coastal area where Northern Wheatears still successfully breed. This enables site-managers to efficiently investigate presence and abundance of important prey In seemingly suitable areas but where Northern Wheatears do not breed. Eventually we may be able to discern whether food shortage poses a bottleneck for Northern Wheatears in these uninhabited areas.

Economy and ecology of heathlands

Heathlands in Europe reflect a long history of human activity and use. Through the ages heathlands have been of importance as (semi)agricultural areas, hunting territories and various other uses of mankind. This diversity of use has created a cultural diversity over the whole Atlantic area of Europe, from Portugal and Spain in the south to Norway in the north and from Germany in the east to Ireland in the west. Economy and ecology of heathlands does show us both the diversity in use all over Europe combining this with the newest insights in ecology and the Natura 2000 status most of these heathlands have got nowadays. Central theme is how to cover the costs of maintenance of these heathlands. Is their future in new types of commons, or do other types of land ownership using the revenues of heathland ecosystem services give better opportunities. The editors combine a broad variance in knowledge of heathlands varying from all aspects of ecology, land use, as well as economy.