Density‐dependence at multiple scales in experimental and natural plant populations

Abstract

Summary 1.The survival and reproduction of individual plants may be related to the distribution of conspecific neighbours with which they interact. 2. We used 2 years' data from a field experiment to relate the reproduction and survival of focal Silene latifolia plants to the numbers of conspecific plants and flowers within a range of distances (0.16-150 m) from the focal plants. Thus we consider spatial scale as a continuously-varying component of population density. 3. Because the data come from a single continuous population, we fitted regression models in two stages in order to remove spatial autocorrelation induced by environmental factors before modelling density effects. As well as single-scale models, we fitted multiple-scale models to explore interaction kernels. We statistically standardized plant size in some analyses to isolate pollination effects. 4. Reproductive output per plant generally decreased with greater densities of plants and flowers; for example there was a negative effect of female flowers within a 70-m radius in the second year. However, at low female densities there was a positive effect of male flowers at a scale of 13 m in the first year. Seed-set per flower decreased with greater densities of flowers in the second year (at radii < 10 m). Seed predation was greater when more female flowers were present within 5-7 m. Plants' survival probability increased with density within 0.28 m for all but the smallest plants. 5.We also collected data on the pattern and reproductive success of S. latifolia plants in a natural population nearby and applied similar methodology. Here the negative effects of female flower densities occurred at finer scales and gave way to positive effects above 2 m, contrasting with some patterns from the experiment. 6. Synthesis. In a sessile population interacting with mobile biota, the effects of density on individual fitness may vary with the way in which density is measured. This, together with the methodological challenges for analysing spatial data from continuous populations, should be considered when designing and interpreting studies of spatial density-dependence. Taking these issues into account, we may begin to explain the spatial patterns of natural populations.

Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

“…The differences between sites appear to be robust if one considers that our lack of knowledge regarding several parameters (pollinator species composition, age, sex, experience of individuals) introduced potential variance in our data that would favour the null hypothesis that no differences exist. While the majority of studies have observed that the density of flowering species can be positively correlated with pollinator visitation rates and plant reproductive success (Gunton and Kunin 2009;Kunin 1993Kunin , 1997Sih and Baltus 1987;Sun et al 2010), so far no one has explicitly focussed on the phenotypic plasticity of pollinator foraging behaviour facing cheaters derived from a rewarding species in dense and sparse populations.…”

“…Plant populations/communities with higher densities generally attract a larger number of pollinators and have a higher reproductive output than lower densities (Bernhardt et al 2008;Gunton and Kunin 2009;Kunin 1997). Large floral aggregations supposedly emit more olfactory and visual cues to attract pollinators (Shao et al 2008).…”

Dinâmica de regeneração de Merostachys skvortzovii Send. em remanescente de Floresta com Araucaria

“…Por outro lado, a maior densidade de plântulas, devido ao microambiente favorável, parece determinar valores mais altos de mortalidade, resultada de controle populacional denso-dependente, como o observado para na clareira B (GUNTON & KUNIN, 2009). No entanto, apesar do crescimento populacional diferenciado, em ambas as clareiras estudadas, registrou-se efetiva capacidade de produção e manutenção de estoque de plântulas de M. skvortzovii, promovendo a sua permanência na área.…”

Back to the basics: using density series to test regulation versus limitation for invasive plants

“…A lack of these feedbacks in the responses of species to density is termed neutral density dependence and logically leads to the broader scope of ecological inquiry in plants and animals whether regulation or limitation is determining the relative abundance of a given species (White 2001). Regulation is best defined as the direct effects of other species (or intra-specific interactions) which reduce the numbers of a species below the capacity of the environment-sometimes called boundedness (Gunton and Kunin 2009;Hixon et al 2002;White 2001). Limitation is the capacity of environment to support a certain number of individuals (White 2001).…”

“…This 'mean field' or neighbourhood-level approach to species density (Gunton and Kunin 2009) can also be scaled up to community-level manipulations (Goldberg et al 2001;Lortie and Turkington 2002) whereby the total number of individuals for all species is increased. At both scales, population dynamics are inferred by comparing changing germination, recruitment, survival, etc.…”