A simple model relating habitat features to a diapause egg bank

As a way to escape from adverse conditions, many zooplankton populations produce diapausing eggs that accumulate in the sediments and hatch when suitable environmental conditions are restored. While buried in the sediment, diapausing eggs may be affected by several processes (i.e., production, hatching, deterioration, and loss). We present a simple mathematical model for the dynamics of diapausing eggs in the sediment. We were mainly inspired by the model organism Brachionus plicatilis, a cyclical parthenogen rotifer, but the model is applicable to other zooplankters. Three diapausing egg categories are used as variables in our model: (1) healthy-looking eggs, assumed to represent the fraction of viable eggs; (2) deteriorated eggs, considered unviable; and (3) hatched eggs, shells remaining in the egg bank from past emergences. The model is used to relate the abundances of these egg categories to production, hatching, deterioration, and loss rates. Then, we propose how relationships between these variables are related to habitat features for temporary populations. Size of the egg bank, here considered as the summation of the three egg categories, is indicative of the quality conditions in the water column (i.e., high production of diapausing eggs). The ratio among deteriorated and healthy-looking eggs is indicative of deterioration rates in the sediment, and high ratios are expected when sediment adversity is high. Our analysis also indicates that the ratio among hatched and healthy-looking eggs is indicative of the hatching rate, which we hypothesize is positively related to both sediment adversity and water-column predictability. Population zooplankton studies have been classically focused on the population dynamics in the water column. For a long time, limnologists have been aware of the importance of (1) the production of diapausing stages in the water column (Weismann 1876; Sars 1885; both cited by Hairston and Kearns 2002), which allow zooplankters to resist adverse conditions in which they cannot survive and reproduce, and (2) the colonization patterns of the water column from diapausing stages located in the pond and lake sediments, when favorable conditions resume. However, only in the last two decades have the processes affecting diapausing stages in the sediment received increasing attention (De Stasio 1989; Cáceres and Hairston 1998; Hairston and Kearns 2002). It is now recognized that diapausing eggs accumulate and form banks and that these banks have dynamic properties that are coupled to ecological and evolutionary processes occurring in the water column. Diapausing egg banks influence ecological and evolutionary processes in several ways. First, they are demographic and genetic reservoirs, providing inertia to populations, which in this way resist biotic and abiotic environmental changes. This inertia may have implications for population persistence, zooplankton community diversity, rates of adaptation, and population differentiation (Hairston 1998; Brendonck and De Meester 2003). Second, features exhibited by diapausing egg banks are expected to be at least partially shaped by evolutionary forces. Patterns of production and hatching of diapausing eggs are fitness components, and their evolution can be understood using life-history theory. Therefore, habitat features (e.g., temporality, adversity, predictability) are expected to be correlated with patterns of diapause (Cohen 1970; Cáceres and Tessier 2003, 2004). In this article, we aim to clarify how demographic processes are related to observable features of the diapausing egg banks, namely, abundances of different categories of diapausing eggs or their remains. For this purpose, we construct a dynamic model on diapausing egg categories. In our model, we omit a number of factors that are likely to affect sediment banks. This simplification makes our model practical and relates it to observed properties of natural sediments. It is thus assumed that the omitted factors exert minor effects if compared with the processes taken into account by our model. By formally modeling the dynamics of diapausing egg banks, we make much more explicit the assumptions needed to interpret field data than when verbal arguments are used. After modeling the dynamics of the egg bank, we use the standard theory of life-history evolution in order to elaborate qualitative expectations for the diapausing eggbank features in relation to habitat type. Our interest is to show that important insights into the egg-bank dynamics can be gained by studying the abundances of several egg categories and that these dynamics may be the adaptive response to the features of water-column and sediment conditions. A simple model for the dynamics of diapausing eggs in the sediment—We will consider that three categories of diapausing eggs can be distinguished in the sediment: (1) healthy-looking eggs, regarded as viable (He); (2) deteriorated, but still identifiable eggs, regarded as unviable (D); and (3) hatched eggs (Ha). In the rotifer species Brachionus, this distinction is easy to observe under the stereomicroscope. The dynamics of these three egg categories are controlled by the following processes: (1) egg production, which happens in the water column; (2) hatching; (3) degradation,