Including Density Dependence Research Articles

Density dependence in an island population of silvereyes

The population of silvereyes Zosterops lateralis chlorocephalus, on Heron Island, Great Barrier Reef has been monitored accurately since 1965. Between 1979 and 1993, the breeding success of all birds was determined by monitoring nests. The population fluctuated between 225 and 483 individuals. Four cyclones led to substantial mortality. As this data set is long‐term, has little observation error, and is from an effectively closed population, it provides an unusual opportunity to examine density dependence in reproduction or mortality. Using a stochastic logistic model, we found clear evidence of density dependence in adult population size. Logistic regression suggested that fledgling survival decreased with the numbers of birds attempting to breed. There was also some suggestion that adult survival might be density dependent. The fitted stochastic logistic model predicts negligible risks of extinction for this population, in contrast to the predictions of a published population viability analysis. Whilst our statistical model including density dependence may provide better predictions of the “usual” behaviour of a population than a population viability analysis, we suggest that caution should be exercised when statistically fitted models are used to predict the behaviour of the population at extremes, such as near extinction.

EVALUATING THE EFFECTS OF HABITAT QUALITY, CONNECTIVITY, AND CATASTROPHES ON A THREATENED SPECIES

Conserving threatened and endangered species requires an understanding of the effects that variability in habitat, environment, demographics, and genetics have on the long-term viability of the species. The Florida Scrub Jay (Aphelocoma coerulescens), which has been the focus of intensive, long-term study and recent conservation efforts, makes an excellent candidate to illustrate how population modeling can be used to examine these effects on a threatened species. A stage-based population model, including density dependence and stochasticity, was used to explore the effects of habitat quality, connectivity, and catastrophes on long-term survival for four populations of Scrub Jays in Brevard County, Florida, United States. Restoring high-quality habitat was critical for long-term viability of these Scrub Jay populations. The quality of the scrub habitat, based on field surveys, is too overgrown to support the populations for the next 60 yr, and extinction is likely in <30 yr. These populations are unlikely to survive an epidemic or catastrophe without dispersal among the habitat patches and populations. The present research effectively illustrates how population modeling can be used to explore links between demographic processes and the environment, and to evaluate management strategies for habitat specialist species.

Factors Affecting Insect Population Dynamics: Differences Between Outbreak and Non-Outbreak Species

Even though outbreaks of insects are temporally rare events, they have aroused the interest of theoretical and applied entomologists alike. The pop­ ulation dynamics of spatially rare species have been the subject of long-term studies concerned with the extirpation of these species, the unusual habitats they occupy, or the unique adaptations they possess (5, 107, 112) . A compari­ son of the features of insects possessing different population processes may lead to a more thorough understanding of insect population dynamics. Past Annual Review of Entomology articles have addressed a number of population dynamics topics including density dependence (100), life tables (54), dynam­ ics (160), stability and diversity (153), and pest management (159). However, no review has specifically addressed insect pest outbreaks or attempted to compare population dynamics of outbreak and nonoutbreak species. Richards (118) pointed out that this comparison is essentially an issue of popUlation balance versus population fluctuation. Populations of organisms are never truly stable, but rise from some low density and then fall to approximately their original size. They may exhibit stable equilibrium points, stable cyclic oscillations between two popUlation points, stable cycles, or a regime of aperiodicity (87). Insect pests usually