Complex environmental gradients predict distributions of river-dependent plants in eastern Australia

Abstract

We used principal components analysis and multiple logistic regression to investigate the relationships between environmental variables and the distributions of 71 species of river-dependent vascular plants in northeastern New South Wales, Australia. Our analysis defined seven main environmental factors, summarised (in order of decreasing frequency of statistically significant association with species distributions) as exposure, salinity, stream size, stone scarcity, nutrient enrichment, grazing pressure and rockiness. The main environmental correlates of the presence or absence of macrophyte species in our study were broadly similar to those reported elsewhere, but the relatively low apparent importance of nutrients and grazing was unexpected. We were not able to fully separate the effects of climate-related and non-climatic environmental variables because variables of both types loaded strongly on some principal components, but we suggest that both types of variables should be included in models that aim to forecast potential shifts in plant distributions under projected climatic change. Vascular plants have been neglected in monitoring programs for Australian rivers and their conservation requires a better understanding of patterns and trends in distribution and abundance.

Native submerged macrophyte distribution in seasonally-flowing, south-western Australian streams in relation to stream condition

“…This is consistent with other Australian studies in which low riparian canopy cover has been associated with P. ochreatus, the ecologically similar Potamogeton perfoliatus and the floating-leaved Potamogeton tricarinatus (Mackay et al 2003;Chessman and Royal 2010). In lentic systems, where light is limited primarily by water transparency, these species tolerate turbid conditions by canopy formation near the surface (P. ochreatus) and development of floating leaves (P. drummondii and O. ovalifolia) .…”

“…Livestock access was noted as present or absent. Chessman and Royal (2010). Further classification of physical characteristic based on these scores for frequency analysis is described below.…”

“…Australian studies have also not found strong correlations between nutrient status and macrophyte distribution (Chessman and Royal 2010;Mackay et al 2010), and increased water column nutrients may have little positive effect on macrophytes in rivers and streams (Madsen andCedergreen 2002, Hilton et al 2006). …”

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

“…Long‐term diversity pattern at the three hydrologic site types is reminiscent of the ‘Goldilocks effect’ (Berlow, Brose & Martinez, 2008), with conditions at perennial sites being too consistently wet, those at the ephemeral sites being too consistently dry, and those at the intermittent sites being ‘just right’ for accumulating species‐rich plant assemblages over time. Although the distribution of riparian plants reflects many environmental gradients (Chessman & Royal, 2010), reach hydrology is a primary driving variable in the desert rivers studied here.…”

“…Although the distribution of riparian plants reflects many environmental gradients (Chessman & Royal, 2010), reach hydrology is a primary driving variable in the desert rivers studied here.…”