Effects of temperature and precipitation on a flood plain isopod community: a field study

Abstract

Effects of a changing climate at individual, population and community levels on biocoenoses will cause profound changes in ecosystem functioning. Considering detritivore communities, we expect severe impacts on the balance of nutrient input through annual leaf fall and nutrient cycling through leaf litter decomposition. Thus, fluctuations of detritivore populations under changing climatic conditions will affect the degradation of detritus. Monitoring populations of four isopod species, that represent the most abundant detritivores in a floodplain forest near Cologne, Germany, revealed three distinct patterns of breeding phenology. Porcellio scaber and Philoscia muscorum breed early in the season (late spring to summer), while Oniscus asellus breeds during summer and early fall. Hyloniscus riparius appears to have a prolonged breeding season, ranging from spring to fall. Over 5 years, I observed effects of annual variations in temperature and precipitation on isopod populations and the composition of the community of these detritivores. With respect to the onset of breeding and to overall population size, as estimated from the number of captured individuals, summer and winter temperature proved to be particularly important, while temperature during fall and spring had little influence on isopod number or phenology. Considering precipitation, mostly summer and spring were of importance, whereas fall and winter precipitation only played a role in determining phenology but did not influence population densities. Predominant effects of climatic factors were low population densities of P. scaber, P. muscorum and O. asellus after warm winters and moist springs and during dry summers, while moist springs increased numbers of H. riparius. After moist springs, in turn, P. scaber and O. asellus started breeding earlier than after dry springs. According to the present field data, isopods (and maybe other detritivores) will be affected by regional climate change in that numbers will be reduced in response to increasing winter temperature, more intense precipitation during spring and dry summers as predicted by current climate models. Thus, climate change will result in reduced faunal contributions to decomposition processes that, in turn, will lead to slowed-down microbial degradation of detritus and nutrient cycling.

Introduction

Besides earthworms and millipedes, woodlice (terrestrial isopods; Isopoda: Oniscidea) are important macro-detritivores in many temperate grass- and woodlands, contributing directly and indirectly to decomposition processes (summarized in Ref. [32]). Their population dynamics and density—and thus, the extent to which they contribute to decomposition—are affected by both the chemical and physical characteristics of leaf litter [13], [19], [38] and climatic factors [6], [16], [17], [18], [20], [27], [36], the latter, in turn, biasing the quality of leaf litter as a food source for detritivores (cf. [3], [8]). Thus, both climatic conditions and food supply vary seasonally and annually, only partially being predictable. Both neonates and juveniles show high mortality rates [4], [33], [38] and can be expected to be particularly sensitive to unfavourable climatic conditions (e.g., Ref. [29]) or limitations in food quality or quantity (e.g., Ref. [33]). Hence, the timing of breeding is essential for successful reproduction (from an individual point of view) and recruitment (from a population-ecological point of view).

Breeding of isopods in deciduous forests of moderate climates takes place from late spring to early fall. The individual onset of breeding exhibits phenotypic plasticity and is, thus, determined by environmental factors (climatic conditions: e.g., Ref. [36]) within a genetically fixed range [6], [15]. Further, growth conditions and individual developmental tactics [37] are important in this regard, because females have to reach a minimum reproductive size [19], [33]. In this context, it is unclear whether and how isopod species differ in terms of their phenological response to annual variation in climatic conditions. Yet, due to their contributions to decomposition processes, it is of interest to be able to predict their population responses to man-made changes in climate. According to recent predictions of climate change during the next couple of decades, both average temperature and annual precipitation will increase, but neither increases in temperature nor in precipitation will be distributed equally over the entire year, but seasons will differ in how strongly they will exhibit climate change (e.g., Refs. [2], [12]). With respect to temperature, mostly winters will become warmer, while summer temperature will change to a lesser extent. Similarly, we expect mostly winter and spring to exhibit more intense precipitation, while summers may even become drier.

Within the community of four isopod species that consistently co-occur in a floodplain forest near Cologne, Germany, three different breeding-phenological strategies have been described ([35]: Fig. 1). Due to slight annual differences in isopod breeding phenology, I was able to estimate the effects of annual variation in temperature and precipitation on the onset of breeding and on population densities of these isopod species. Based on these data, the aim of the present analysis was to predict effects of rising winter temperatures and increasing amounts of spring precipitation, as predicted by current climate models, on isopod communities in temperate deciduous forests. Thus, this analysis serves as a first approach to predicting the effects of a changing climate on decomposition processes in temperate deciduous forests.