NEW ORLEANS (AP) -- Nearly two years after oil from BP's busted well in the Gulf of Mexico fouled coastal marshes, scientists are studying the impact on some of the area's tiniest residents: bugs.
It's an important effort because insects and spiders are a food source for larger animals, and also because changes in their populations can affect the ecosystem's health.
Findings so far are mixed.
A University of Houston ecologist says insects and spiders that live on marsh grass have rebounded. But a Louisiana State University entomologist says her more detailed study shows falling numbers for many kinds of bugs.
They agree that it's too soon to know the lasting effects of the oil on marsh ecosystems. The well blew wild in April 2010, leading to the nation's worst offshore oil disaster.
More than 200 million gallons of crude oil flowed from the well, soiling the northern Gulf coast.
Many of the areas where it came ashore were salt marshes where grass roots knit the loose soil and keep it from washing away. Both the Houston and LSU researchers looked at insects and spiders that live in and around the most common of these grasses.
In August 2010, the month after the well was capped, University of Houston ecologist Steven Pennings and graduate student Brittany McCall used a vacuum to suck up bugs from marsh grass in six oil-free areas and on plants growing in areas where oil had washed up.
They found half as many insects and spiders in the oiled areas. A year later, they checked the four oiled sites and two more in Terrebonne Bay, and found nearly equal numbers of bugs in both oiled and never-oiled areas.
Linda Hooper-Bui of the LSU Agricultural Center collected insects about 20 or 25 times last year at 45 sites from Cocodrie to Breton Sound, using both vacuums and nets. She has been studying those sites since 2009 and is making monthly checks this year. In those hit by the spill, some kinds of insects and spiders are still far less numerous than before, she said.
Hooper-Bui's pre-spill surveys mean before-and-after data for some oiled plots.
'That's wonderful. That's going to be really important,' said Gina Wimp, who studies arthropods in Atlantic salt marshes as an assistant biology professor at Georgetown University.
Which species make the biggest comebacks can affect the health of the marsh grasses they live in, Wimp said.
She wouldn't compare the University of Houston and LSU studies, but said both are good science.
'As we start essentially assimilating the results of multiple studies, you get a much clearer picture of what's happening,' she said.
The oiled sites that Pennings checked both years were in Grand Isle, Bay St. Louis and Barataria Bay. In 2011, he added two oiled sites in Terrebonne Bay. In all oiled sites, he and McCall collected bugs from healthy plants growing just beyond where oil had killed those closer to the water.
He said insects and spiders were 'hammered' right after the spill. 'But by a year later, the insects and spiders had all bounced back. That's encouraging,' he said.
The marsh grass ecosystem is 'prone to disturbance' from hurricanes, drought and other factors, Wimp said. 'So a lot of these insects are really good at recolonizing once that disturbance has passed.'
Hooper-Bui said she found a drop in numbers immediately after the oil spill comparable to that found by Pennings, but is seeing growth only in some species while others are still low in numbers or even collapsing.
'Every single time we go out there, the Pollyanna part of me thinks, `Now we're going to measure recovery,'' she said. 'Then I get out there and say, `Whaaat?''
She said she'd expected that one group of arthropods might be hit hard while others recovered, but her work, still incomplete, shows a large downturn among many kinds. 'We never thought it would be this big, this widespread,' she said.
Pennings said he wasn't shocked by Hooper-Bui's findings, because he had expected a longer-lasting effect than he found.
Aside from the fact that Hooper-Bui is still going out on collecting trips, one reason her study isn't complete is that she has so much to count. Another is that she's doing before-and-after analyses of most individual species that live in and around the marsh grass called Spartina alterniflora, and has yet to identify a number of them. Pennings' analysis groups the 100 or so species by what they eat: sap-suckers, stem-boring herbivores, those that eat dead matter, predators and parasites.
One of the most dramatic changes is in the numbers of ants that live in the hollow stems of the marsh grass and scurry around outside it to forage. In oiled areas, their numbers have dropped steadily since the spill, Hooper-Bui said.
'This year we're hard-pressed to find ants in previously oiled areas,' she said. It's usually easy to find them on chilly winter days because they stay inside. In January and February, she said, an hour's search by breaking stems didn't turn up a single colony in areas where oil had landed, compared to six to eight colonies in areas not exposed to the oil.
In warm weather, sweeping a net through the stems picked up an average of 32 ants before the spill. That was down to about 10 in September 2010 in oiled areas. Last September, that average was down to 0.5 ants per net in oiled areas, compared to 17 in non-oiled areas.
Hooper-Bui said large-bodied spiders appear to have been hit harder than small spiders.
In the Atlantic salt marshes, Wimp said, some of the largest spiders are at the top of the Spartina ecosystem food chain. 'It's pretty common for things at the top of the food chain to be most heavily impacted when you have some sort of disturbance,' she said.
The Houston study used two 'snapshots,' which can be misleading, Hooper-Bui said. For instance she said, if the other researchers happened to show up after water had been covering the marsh for a long period, they would get larger numbers of flying insects.
Pennings' analysis didn't include dragonflies, mosquitoes and other 'transient species.' Those and species rarely found in stands of Spartina made up about 30 percent of what he collected, he said.
He said he also didn't look at ants, partly because he believes they tend to be 'patchy' in Spartina and partly because the vacuum isn't suited to collecting them.
Both studies also looked at crabs; Hooper-Bui took soil samples to check whether they contained oil from the BP well.
Oil can remain in wet sediments for 'a really long time,' Pennings said.
'I wouldn't say we can study the system for a couple of years and know the answer,' he said. 'We'll be studying these systems probably for a couple of decades.'
