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Theory predicts that reproduction will change as individuals near the end of their lives by either increasing reproductive allocation (terminal allocation hypothesis) or decreasing allocation (senescence hypothesis) toward the end of life. Although senescence has received more support, few studies examine how both age and years to death influence late-life reproduction. We used a 37-yr study of Song Sparrows (Melospiza melodia) to ask how age and years to death influenced reproductive allocation late in life. We observed both senescence and terminal allocation, and that age and years to death interacted to influence individual variation in allocation tactics. In particular, we observed a decline in allocation in older individuals (senescence), but only when comparing individuals with the same number of years to death. Likewise, we observed terminal allocation, but only in young females. Reproductive tactics were most variable in young females, with many exerting high effort but living 1-2 yr, and others exerting low effort annually and living longer. Our results suggest that late-life reproductive tactics are influenced by both chronological age and years to death, and that short- and long-lived females with differing reproductive tactics exist within this population of Song Sparrows.

The dominance of nonnative plants coupled with declines of native plants suggests that competitive displacement drives extinctions, yet empirical examples are rare. Herbivores, however, can alter vegetation structure and reduce diversity when abundant. Herbivores may act on mature, reproductive life stages whereas some of the strongest competitive effects might occur at early life stages that are difficult to observe. For example, competition by perennial nonnative grasses can interfere with the establishment of native seeds. We contrasted the effects of ungulate herbivory and competition by neighboring plants on the performance of native plant species at early and established life stages in invaded oak meadows. We recorded growth, survival, and flowering in two native species transplanted as established plants, six native species grown from seed, and five extant lily species as part of two 2 x 2 factorial experiments that manipulated herbivory and competition. Herbivory reduced the performance of nearly all focal native species at early and established life stages, whereas competition had few measurable effects. Our results suggest that herbivory has a greater local influence on native plant species than competition and that reducing herbivore impacts will be required to successfully restore endangered oak meadows where ungulates are now abundant.

Theory predicts that animals breeding in heterogeneous landscapes preferentially occupy habitats likely to maximize individual fitness, but identifying those habitats has proved problematic. Many studies develop metrics of habitat quality linked to site-specific reproductive output measured in successive years, but few separate the independent effects of individual "intrinsic quality" from those due solely to the attributes of the habitats themselves. In many populations, processes such as competitive territory defense, longevity, site-fidelity, and variation in breeding density and territory size over time have the potential to limit the degree to which individual and habitat quality will be positively related in nature. However, the effects of these processes on estimates of habitat or site-specific reproductive output have not been thoroughly investigated. We show that, in an insular population of Song Sparrows (Melospiza melodia), females nested preferentially in breeding sites with high mean reproductive output assessed over 35 years, and that variation in site-specific reproductive output was positively related to female intrinsic quality, measured here as the lifetime reproductive success of individual females relative to others hatched the same year (rLRS). In contrast, vegetation traits (shrub cover, edge, and soil depth) predicted female preference for breeding sites but did not predict site-specific variation in annual reproductive output. Female quality also did not predict which females occupied more- or less-preferred breeding sites over the study period. However, mean annual reproductive output of breeding sites estimated over 35 years was strongly positively related to the quality of the females that nested in them. Overall, these results indicate that site-specific estimates of habitat quality that do not consider the quality of the individuals occupying those sites may include substantial bias due to variation in occupant quality, and thus may not reliably predict the intrinsic effects of habitat quality on individual or population fitness.

Birth seasons of ungulates in tropical regions show a complex pattern varying from asynchronous to highly synchronous and at different times of year. We examine the factors determining the phenology and synchrony of birth seasons of 13 species of ungulates in the Serengeti ecosystem, Tanzania. We propose that phenology of births (time of year) is determined by food supply, whereas birth synchrony (degree of coordination or spread) is an antipredator adaptation that functions in two different ways. High synchrony may occur through “predator satiation” in species with precocious newborn (“followers”), whereas asynchrony may occur through “predator avoidance” in species with nonprecocial young (“hiders”). We used green biomass of grass or tree shoots and percentage crude protein as measures of food supply. Births were determined from monthly sample counts covering the period 1967–1997. The frequency distribution of births was compared to that predicted by the abundance of green biomass and percentage protein...

The "predation-sensitive food" (PSF) hypothesis proposes that both food and predation necessarily limit populations, because as food becomes limiting animals take greater risks to obtain more food, and some of these are killed. Alternative hypotheses are "predator regulation" where predators hold the prey population well below starvation levels; and "surplus" predation where predators kill only those prey that are excluded from optimal habitat and are dying from starvation. The predictions from these hypotheses were tested by examining body condition of Serengeti wildebeest (Connochaetes taurinus) over 24 yr (1968-1991). Two phases of population growth were examined: 1968-1973 when the population was increasing with superabundant food; and 1977-1991 when the population was stationary and regulated by intraspecific competition for food. Three categories of data were compared: live animals, predation kills, and nonpredation deaths. Body condition was measured from bone marrow, the last reserves of fat in ungulates. The predator regulation hypothesis predicts that the marrow condition should be similar in the predation and live samples. The surplus hy- pothesis predicts the predation and nonpredation samples should be similar. The PSF hy- pothesis predicts that marrow condition of the predation sample should be (1) poorer than that of the live sample, (2) better than that of the nonpredation sample, and (3) better when food is limiting than when it is abundant. Analyses of the frequency distribution of marrow categories showed that both the pre- dation and nonpredation samples were significantly poorer than that of the live population. In both increase and stationary phases of population growth, the predation sample was in better condition than the nonpredation sample. The predation sample was not quite sig- nificantly better (P = 0.052) when food was limiting. These results are consistent with the PSF hypothesis and inconsistent with both of the alternative hypotheses. Female and male condition was similar in the predation sample, but females were killed at a younger age. Lions and hyenas killed animals in similar condition, but lions took animals at a younger age. The results suggest that (1) body condition affects the vulnerability of individual wildebeest to predation, and (2) predation jointly limits the population with intraspecific competition by removing animals from the population that are in better condition than those that are starving.

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, becaus...