08 December 2008

Salt requirement may not be limiting the ranges of coastal animals

A. M. M. Richardson, R. Swain, C. J. McCoull (2001). Salt spray limits the inland penetration of a coastally restricted invertebrate: a field experiment using landhoppers (Crustacea: Amphipoda: Talitridae) Functional Ecology, 15:435-442.
DOI: 10.1046/j.0269-8463.2001.00545.x

There are many animal species whose habitats are restricted to coastal areas. Some of the snails in that group have been the subjects of several posts on this blog (for example, here, here and here). Why can’t such species survive away from the sea, their ancestral home?

Some species of terrestrial talitrid amphipods (Talitridae), also known as landhoppers, likewise live only in the coastal zone. In this paper, Richardson et al., (pdf) tried to shed some light on the factors that may be limiting the landward migrations of certain Tasmanian landhoppers. They note that there are 2 main hypotheses to explain the restricted ranges of coastal landhoppers, which, obviously, apply to all other strictly coastal animals:

1. Coastal species are limited by a physiological requirement for salt, which is present, for obvious reasons, in high enough concentrations only in coastal soils.

2. Coastal species may not need high salt levels in their habitats and have a superior tolerance of elevated salt concentrations nevertheless, but are excluded from inland habitats by competition or some other biological interaction.

To test these hypotheses, the authors did a relatively simple experiment. They collected batches of litter containing the landhopper Austrotroides maritimus, whose distribution, as its name implies, is confined to a zone less than 100 m from the high-water mark. These they transplanted into enclosures 30 m inland from the most inland record of the species. For a period of 5 months, they applied at monthly intervals to each enclosure either dry sea salt, salty water, fresh water or nothing. Initially, each enclosure contained approximately the same number of landhoppers. At monthly intervals, they also estimated the number of landhoppers in each enclosure and compared these with the numbers obtained from untreated enclosures within the natural coastal range of A. maritimus.

Here are the main results of the study.

The top curve shows the population increase within the natural coastal range of the landhoppers. This was probably an expected seasonal phenomenon, since the experiment was done during the summer months. Among the experimental enclosures away from the coast, only in those that received dry salt was there an initial increase in landhopper numbers prior to a sharp decline; in the rest, the populations remained very low.

The authors claim that according to these results, A. maritimus is restricted to the coasts because it depends on the salt spray originating from the sea.

I disagree. Their results don't demonstrate anything definite. If salt were the main requirement, then why did the landhopper populations decline in the enclosures that received salty water? They don't offer a good answer to that. Also, why did the landhopper populations decline after January in the enclosures that received dry salt? That too remains unanswered.

The authors acknowledge 2 problems with their study. First, soil conditions at the coast where A. maritimus is naturally present differed from those at the experimental site. So, the presence or absence of salt wasn't the only factor that may have affected the results. Second, A. maritimus was transplanted to an area where there were already terrestrial landhoppers. The design of this study totally ignores the potential interactions of the pre-existing and transplanted animals (see hypothesis #2).

Another problem, not discussed in the paper is that the 5-month period seems too short for a study like this. If the landhopper population in the experimental enclosures that received salt had increased to and stayed at levels comparable to those of the coastal populations, could they state conclusively that salt was indeed the crucial factor? They probably would, but that would be erronous. How could they know that they had established stable colonies using data from 5 months, which probably represented just one season of reproduction. An experiment like this should run for several seasons to demonstrate that stable, self-sustaining colonies can or cannot be established.

We still don't know why the coastal animals are restricted to the coasts.


Anonymous said...

I agree. I think it's a pretty lame experiment all round. The design of it is full of holes. The plant hoppers are presumably quite active. In their coastal site there is probably constant mixing as the hoppers move around. Maybe a patch of hoppers need to be surrounded by a good density of others of their kind in order to flourish. I mean there are so many more questions raised by this study than there are answers. I think the only thing that the results show for sure is that this was a poor experiment.


Anonymous said...

What do they eat? If that component of the litter dropped off quickly, so would the hoppers, regardless of their own need for salt


That's good point. Their food is not addressed in the paper.