A fresh approach reveals how dispersal shapes metacommunity structure in a human‐altered landscape

Abstract

1.To understand species losses from disturbed landscapes, it is important to distinguish the effects of degraded environmental conditions from those caused by barriers to dispersal between habitat patches. To assess the relative importance of these effects, we developed a new approach using permutation and association tests applied to rank abundance data, using the invertebrate fauna of two rivers in two seasons. 2. Our study streams were Hughes Creek and Seven Creeks, in south-eastern Australia, which have both been degraded by agriculture in downstream sections. We collected benthic invertebrates and also dispersing individuals (drift, terrestrial adults) during two seasons in [2007][2008]. Study sites spanned strong environmental gradients as well as the main dispersal route (up-and down-channel). Environmental data were analysed to set up permutation tests on rank abundances. Survey and disperser data were contrasted using contingency table analyses. 3. The results suggest dispersal plays a strong role in community structure. Environmental effects were evident and strongest upstream, but evidence of environmental effects was weak over much of the gradient. Many species had different distributions in different data sets or dispersers that were abundant at locations distant from centres of benthic distribution. 4. Our results differ from many studies, but few have been able to evaluate dispersal effects directly. Our method provides a practical approach for evaluating the role dispersal plays in driving species abundance patterns across landscapes, thus bridging a gap between theory and practice. 5. Synthesis and applications. Managers typically use indices of ecosystem health that assume environmental conditions largely determine species diversity and abundance. Dispersal between habitat patches is known to be important, but there are no reliable methods to assess the role dispersal may play. We provide an approach that allows both dispersal and environmental effects on species distributions to be evaluated from survey data. This may open the way for dispersal information to be incorporated into management actions. Additionally, the approach should allow improved siting of restoration projects that depend greatly on successful dispersal of individuals for successful outcomes.

Spatial and temporal homogenisation of freshwater macrofaunal communities in ditches

“…Other studies also found that freshwater communities may be only weakly dispersal limited, even up to a distance of 300 km (Cañedo-Argüelles & Rieradevall, 2011;Viana et al 2015, but see Heino et al, 2015;Downes, Lancaster, Glaister, & Bovill, 2017). Other studies also found that freshwater communities may be only weakly dispersal limited, even up to a distance of 300 km (Cañedo-Argüelles & Rieradevall, 2011;Viana et al 2015, but see Heino et al, 2015;Downes, Lancaster, Glaister, & Bovill, 2017).…”

Itinerant, nomad or invader? A field experiment sheds light on the characteristics of successful dispersers and colonists

“…Unsuccessful colonisation may be due to environmental constraints. For example, water temperatures at downstream locations exceed 29°C when the air temperature reaches ~34°C, which is a relatively common occurrence during summers (Downes et al., ). Water temperatures of 30°C are extreme for most insects (Lancaster & Downes, ) and can be associated with low oxygen concentrations in Hughes Creek as well (Lancaster, Downes, & Glaister, ).…”

“…Sandy locations thus have sparsely treed riparian zones, low densities of detritus, a bed of shifting sand and shallow, warm water. Temperature loggers showed that water temperatures at downstream locations were often > 29°C and reached up to 36°C on hot days, for example where air temperature exceeded 40°C (Downes et al., ). Downstream locations have approximately half the number of invertebrate species found in cooler, more intact upstream areas (Downes et al., ).…”

“…Temperature loggers showed that water temperatures at downstream locations were often > 29°C and reached up to 36°C on hot days, for example where air temperature exceeded 40°C (Downes et al., ). Downstream locations have approximately half the number of invertebrate species found in cooler, more intact upstream areas (Downes et al., ). Movement of animals in these sandy zones is predominantly through swimming and the drift, with few individuals walking across the sand (Lancaster & Downes, unpublished observations).…”

“…Detritus and benthic densities were sampled premanipulation, 1, 4, 9 and 12 months following the start of the experiment in January 2013. Our sampling methods have been described in detail in previous studies (Downes & Lancaster, ; Downes et al., ). In brief, on each sampling occasion, we collected 10–15, randomly placed benthic samples per site using a Surber sampler (0.09 m 2 , 250 μm mesh).…”