Identifying the demographic determinants of population growth rate: a case study of red‐billed choughs Pyrrhocorax pyrrhocorax

Abstract

Summary 1.Identifying which age-specific demographic rates underlie variation in a population's growth rate ( λ ) is an important step towards understanding the population's dynamics. Using data from a 20-year study of marked individuals, we describe patterns of demographic variation and covariation in the Scottish red-billed chough population ( Pyrrhocorax pyrrhocorax ), and investigate which demographic rates have the greatest projected and realized influence on λ . 2. Survival, the probability of breeding and breeding success varied with age in this population. Data were sufficient to estimate year-specific probabilities of first-year, second-year and adult (all ages over 2) survival and mean breeding success. A population trajectory modelled using these parameter estimates closely matched census data, suggesting that estimates and simplifying assumptions were sufficient to accurately describe important demographic processes. 3. Elasticity analyses based on stage-classes for which year-specific survival was estimable suggested that λ was more elastic to variation in adult survival than first-or second-year survival or breeding success. These ranks were consistent across all 15 years for which λ could be estimated directly, although the elasticity of adult survival declined with population growth. 4. Survival and breeding success were positively correlated across years. λ remained most sensitive to adult survival when this demographic covariation was incorporated into elasticity analyses. 5. However, elasticities calculated from a fully age-structured model suggested that λ was more elastic to variation in first-and second-year survival than to survival at any individual older age class. These ranks were robust to realistic demographic variation, but sensitive to postulated patterns of demographic covariation. We emphasize that covariation should be measured and incorporated into elasticity analyses, and that estimated elasticities must be interpreted in the context of the way in which stage-classes are defined. 6. Of the demographic rates in which we quantified between-year variation, first-year survival varied most, followed by second-year survival, breeding success and adult survival. These rates consequently contributed more equally to variation in λ than elasticities predicted. Overall, variation in λ was caused primarily by variation in survival rather than breeding success, and variation in prebreeding survival accounted for 56% of the total variation in λ .

The Demography and Dynamics of an Expanding, Managed African Wild Dog Metapopulation

“…The deterministic population growth rate (l) was calculated from the dominant eigenvalue of the Leslie matrix. We validated our parameters by modelling the deterministic population trajectory correcting for newly reintroduced animals and comparing this to census data (Reid, Bignal, Bignal, McCracken & Monaghan, 2004). We used elasticities to determine which vital rates had the greatest impact on population growth (Caswell, 2001), and compared these to results from the larger, unmanaged wild dog populations examined by Creel et al (2004), which included the Selous Game Reserve in Tanzania (1991Tanzania ( -1997, Kruger National Park in South Africa (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) and Moremi Game Reserve and adjacent Wildlife Management Areas in Botswana (1989Botswana ( -2002.…”

Breeding status of ChoughsPyrrhocorax pyrrhocoraxin the UK and Isle of Man in 2002

“…There is evidence that breeding success and subsequent survival of Choughs are correlated with environmental conditions before breeding on Islay (Reid et al 2003), and that survival of pre-breeders most influences the population trend (Reid et al 2004). It is possible that beneficial environmental conditions have been more frequent in recent decades, and this has resulted in population growth and colonization of new sites at the edge of their range.…”

“…Choughs do not breed until two or more years old (Bullock et al 1983, Reid et al 2004, usually associating with other immature birds in flocks until then (Bignal et al 1997).…”

“…Therefore, a secondary aim was to estimate the size of non-breeding Chough populations, although it was anticipated that inconsistencies in methods between censuses would limit their interpretation. As part of several ongoing studies, additional visits are made annually to many Chough sites to measure breeding success (Reid et al 2004). Furthermore, colour-ring resightings enabled many individuals to be identified.…”

Food supplementation does not increase demographic rates in a passerine species of conservation concern

“…For instance, numerous studies have shown that avian λ is more heavily influenced by first year and adult survival than by reproductive success (e.g. Saether and Bakke 2000;Reid et al 2004;Schaub et al 2006). Thus, there would be little purpose in providing supplementary food to increase reproductive success, if this demographic rate was predicted to have little impact upon λ in the target species.…”