A discrete reaction-diffusion model was used to estimate long-term equilibrium populations of a hypothetical species inhabiting patchy landscapes to examine the relative importance of habitat amount and arrangement in explaining population size. When examined over a broad range of habitat amounts and arrangements, population size was largely determined by a pure amount effect (proportion of habitat in the landscape accounted for >96% of the total variation compared to <1% for the arrangement main effect). However, population response deviated from a pure amount effect as coverage was reduced below 30%-50%. That deviation coincided with a persistence threshold as indicated by a rapid decline in the probability of landscapes supporting viable populations. When we partitioned experimental landscapes into sets of "above" and "below" persistence threshold, habitat arrangement became an important factor in explaining population size below threshold conditions. Regression analysis on below-threshold landscapes using explicit measures of landscape structure (after removing the covariation with habitat amount) indicated that arrangement variables accounted for 33%-39% of the variation in population size, compared to 27%-49% for habitat amount. Thus, habitat arrangement effects became important when species persistence became uncertain due to dispersal mortality.

The aim of this study was to investigate whether mare follicular fluid (FF) induces the acrosome reaction (AR) in stallion spermatozoa and, if so, to identify the component in FF responsible for it. Furthermore, the effect of this component on sperm-zona binding and the subsequent AR was studied. Pooled FF, aspirated from the preovulatory follicles of mares in oestrous, was used and aliquots of the fluid were treated with charcoal to remove steroids (CFF). Charcoal treatment reduced the progesterone concentration in FF from 153 to < 2 ng/mL. Spermatozoa from fertile stallions collected by a swim-up procedure were preincubated in modified Tyrode's medium for 5 h and then incubated for 30 min at 37 degrees C with either (1) 50% FF + 50% CFF, (2) 50% FF + 50% CFF + 150 ng/mL progesterone, (3) 50% CFF + 150 ng/mL progesterone, (4)150 ng/mL progesterone or (5) modified Tyrode's medium alone. The sperm-hemizona assay was applied: (a) to compare the number of spermatozoa bound to a hemizona in the presence and absence of 1.5, 15 or 150 ng/mL progesterone after 1 h co-incubation of spermatozoa and hemizonae, (b) to compare the incidence of the AR in sperm-hemizona complexes incubated for 1 h in the presence and absence of 1 microgram/mL progesterone. Both spermatozoa in suspension and bound to a hemizona were treated with the supravital dye Ethidium homodimer and fixed. Their plasma membranes were permeabilized, and the outer acrosomal membranes were labelled with FITC-PNA. Viable spermatozoa without the outer acrosomal membrane were considered as physiologically acrosome-reacted. Results showed that (1) FF induced a higher percentage of AR than did CFF or modified Tyrode's medium, (2) addition of 150 ng/mL progesterone to CFF restored 77% of the AR-inducing activity and (3) CFF and modified Tyrode's medium both induced the AR to a similar extent when supplemented with 150 ng/mL progesterone. Neither FF nor progesterone treatment affected sperm viability severely. The number of spermatozoa bound to a hemizona in the presence of 15 and 150 ng/mL progesterone was significantly higher (p < 0.05) than the number of spermatozoa bound in the absence of progesterone. A higher incidence of the AR was found in sperm-hemizona complexes incubated in the presence of progesterone (55.6 +/- 3.4% vs. 27.1 +/- 4.3%, in the presence and absence of progesterone, respectively) (n = 15, p < 0.05). It is concluded that mare FF can induce the AR in stallion spermatozoa. Progesterone is the physiological component responsible for this AR-inducing capacity. Progesterone enhances sperm-zona binding activity and exerts an additive effect on the zona-induced AR.

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