Population dynamics across a parapatric range boundary: <i>Ulex gallii</i> and <i>Ulex minor</i>

Abstract

1 Ulex gallii and U. minor show a parapatric range distribution across southern England. Their population dynamics were explored along an environmental gradient that encompasses the range limits of the two species. Matrix population models were constructed to provide estimates of population growth rate (λ) at various sites. 2 Contrary to expectation no decline in λ was observed for populations of either species at their distribution limit. Density-independent models gave values considerably in excess of unity at the range margin of both species, indicating potential for expansion into each other's range. 3 Inclusion of density-dependent seedling emergence and flowering within the matrix models alters the relative importance of the elasticity values, emphasizing the role played by the mortality of adults in determining population size and structure, through modulating the flux of individuals from the seedling stage through the intermediate size classes. 4 Estimates of λ obtained from the generalized density-dependent model run at low density were approximately 22% and 63% higher for U. gallii and U. minor , respectively, than the minimum λ -values obtained from the site-specific density-independent models. 5 The predicted stable stage distribution differs between the density-independent and equilibrium models (approximately log-linear vs. U-shaped). Size structures seen in the field were intermediate between the two extremes, suggesting that the observed populations are subject to density-dependence but are not at equilibrium.