Soil and aquatic invertebrate populations in the inter-tidal zone.
One of the consequences of the destruction of salt marsh swards may be a decline in both species composition and numbers of invertebrates in this habitat. Given that they are a significant food source for shorebirds, small landbirds and ducks, and that some are even eaten by geese, a decline in invertebrates as a consequence of the trophic cascade may have serious consequences for these bird populations.
Eighty ponds were selected along a broad transect which traversed areas where the vegetation was intact and where the ground vegetation had been removed by the geese and the willows had subsequently died. The ponds were sampled for aquatic invertebrates on several occasions. At some ponds, three samples of the water column and sediments (up to 5 cm depth) were taken on each occasion in order to determine intra-pond variability in populations of invertebrates. In addition, pH, redox level, total C and N in the sediments, and water temperature were measured in the eighty ponds.
By early August all ponds were dry. The range of salinities in the water of ponds just prior to their drying out was from 0.5 0/00 S (0.5 g of dissolved solids per liter) to 33 0/00 S (equivalent to that of oceanic sea water). The water column above sediments with a high organic content had a low salinity. In ponds that were intensively sampled during the snow-free season, there are indications of higher numbers of individuals and a higher species diversity in ponds which are surrounded by willow shrubs than in ponds in degraded sites. These pools maintain transient populations of aquatic invertebrates and the pools dry up each summer. However, the highest salinities, lowest redox values (Eh) and lowest species diversity all occurred in ponds in degraded sites where the vegetation had been removed.
Similarly, pit-fall traps were used to collect samples of terrestrial invertebrates from intact grassland swards, swards beneath live willow bushes and from sites where the vegetation was destroyed. The biomass and the diversity of invertebrates was considerably higher in swards beneath live willow bushes and lowest in pit-fall traps set-up in degraded areas. This project is primarily the work of Brian Milakovic. milakovic@botany.utoronto.ca.
- revised 10/11/97 -