Land‐use and isolation interact to affect wetland plant assemblages

Abstract

Different management regimes imposed on similar habitat types provide opportunities to investigate mechanisms driving community assembly and changes in species composition. We investigated the effect of pasture management on vegetation composition in wetlands with varying spatial isolation on a Florida cattle ranch. We hypothesized that increased pasture management intensity would dampen the expected negative effect of wetland isolation on native species richness due to a change from dispersal‐driven community assembly to niche‐driven assembly by accentuated environmental tolerance. We used native plant richness, exotic plant richness and mean coefficient of conservatism (CC) to assess wetland plant assemblage composition. Sixty wetlands were sampled, stratified by three levels of isolation across two pasture management intensities; semi‐native (less intensely managed; mostly native grasses, never fertilized) and agronomically improved (intensely managed, planted with exotic grasses, and fertilized). Improved pasture wetlands had lower native richness and CC scores, and greater total soil phosphorus and exotic species coverage compared to semi‐native pasture wetlands. Increased wetland isolation was significantly associated with decreases in native species richness in semi‐native pasture wetlands but not in improved pasture wetlands. Additionally, the species–area relationship was stronger in wetlands in improved pastures than semi‐native pastures. Our results indicate that a) native species switch from dispersal‐based community assembly in semi‐native pastures to a species‐sorting process in improved pastures, and b) recently‐introduced exotic species already sorted for more intensive management conditions are primarily undergoing dispersal‐based community assembly. That land‐use may alter the relative importance of assembly processes and that different processes drive native and exotic richness has implications for both ecosystem management and restoration planning.

Do geographically isolated wetlands influence landscape functions?

“…Variation in wetland connectivity affects biota and, thus, ecological condition (60, 61) within and across wetlands (110)(111)(112)(113). However, biological connectivity, unlike water and solutes, is not always constrained by flow direction.…”

“…Travel distances also vary in time, responding strongly and nonlinearly to climate forcing (114), fire and other natural disturbances, and human impacts. Wetlands contiguous to surface dispersal pathways (e.g., streams, flyways) or near refugia (e.g., lacustrine habitats) differ in community structure from wetlands where dispersal is restricted, or desiccation more frequent, either due to shallow basin form or exclusively subsurface hydrologic connectivity (113). Geographic isolation does not imply biological isolation (115) but can constrain aquatic plant and animal movement.…”

“…Spatial and temporal heterogeneity in the frequency, timing, and duration of connectivity affects water-mediated movement and thus community composition (116), with historical connectivity imprinted on contemporary diversity patterns (117). Geographic isolation selects plants with long-lived seeds (118) or long-distance dispersal strategies (e.g., via fauna or wind) (113), and animals such as amphibians that require competitor exclusion for all or part of their life cycles (119), or that rely on dynamic heterogeneity in aquatic resources (120,121). For example, increased nearest wetland distance reduces local species richness of both native and nonnative fauna (36) and flora (113) but enhances both landscape biodiversity (122,123), by creating taxonomically distinct communities, and metapopulation stability (124), by creating spatial heterogeneity in the drivers of subpopulation dynamics.…”

“…Wetland losses are strongly biased toward removing wetlands far from drainage features, changing network topology (55), and impacting services derived from longer residence times, including sediment storage (87), nutrient and floodwater retention (25,87), and controls on flood timing and magnitude (136). Network changes also impact aquatic ecosystem structure (137) and composition (24,113) by affecting dispersal (17,138,139). Land use intensification (i.e., cropping, urbanization) also leads to more regular wetland shapes (55), lowering perimeter-to-area ratios and impacting functions associated with wetland edges (140).…”

Intense ranchland management tips the balance of regional and local factors affecting wetland community structure

“…For example, agricultural practices alter soil structure, nutrient levels and vegetation, which affect local conditions in embedded aquatic ecosystems and also change factors acting at regional scales such as dispersal (Boughton et al, 2010;Dickson, 1986;Rabalais et al, 2002). These changes can reduce local species richness, but also permit invasion of non-native species (Limpens et al, 2003;Hobbs and Huenneke, 1992).…”