The dusky gopher frog is a somewhat defenseless creature, confined these days to a handful of pampered ponds in a Mississippi forest and at “serious risk of extinction,” according to a government lawyer.

Yet it managed Monday to divide the understaffed Supreme Court into familiar camps and raised the possibility that the first case of the 2018 term might end in a tie. Or that the outcome might have to wait until the Senate confirms a ninth justice — Brett M. Kavanaugh or someone else.

There was talk of “draining the swamp,” but in this case, a literal one.

The term began with an empty space at the right end of the bench; the court was rearranged in anticipation of a new member.

Seats are assigned by seniority, and Anthony M. Kennedy’s retirement meant that Justice Clarence Thomas is now the court’s longest-serving member. By tradition, he sits at the right hand of Chief Justice John G. Roberts Jr. Justice Ruth Bader Ginsburg now sits to his left.

Roberts noted before argument began that the day marked the 25th anniversary of Ginsburg’s investiture. “We all look forward to sharing many more years with you in our common calling,” Roberts said. Ginsburg, 85, smiled, but did not respond.

But her silence did not last long. Ginsburg and the court’s other liberals seemed inclined to believe that the U.S. Fish and Wildlife Service had not exceeded its authority in designating more than 1,500 acres in Louisiana — about 50 miles from the places the frogs are known to live now — as “critical habitat” for the species’ future survival.

The frogs once lived there, although they haven’t been seen since the 1960s.

The court’s conservatives were concerned about the rights of the property owners, in this case, a family that leases its land to Weyerhaeuser, the timber giant that harvests trees there, but also has development plans for the land.

The Louisiana property has something essential for the dusky gopher frog’s reproduction, the poetically named “ephemeral ponds.” That refers to low-lying areas that fill with water at certain times of the year — when the frog lays eggs — but then dry out. Because of that, the ponds can’t support fish, which would eat the eggs.

But everyone agrees that, although the land has the ponds, it does not have other physical characteristics the frogs need.

Chicago lawyer Timothy S. Bishop, representing Weyerhaeuser, said the law only allows the government to designate as critical habitat lands where the species could now live. (The land is something of an insurance policy, should things go bad in the frogs’ current home.)

Justice Elena Kagan disputed that. What if the land could be made habitable with reasonable alterations, she wondered.

“To my mind, it is a counterintuitive result that the statute would prefer extinction of the species to the designation of an area which requires only certain reasonable improvements in order to support the species,” she said.

And Ginsburg questioned whether the company and the family that owns most of the land had yet suffered any loss that would make the case ripe for a Supreme Court decision.

Bishop said the government had estimated the land value could decline by $33 million if the government restrictions stand.

“The immediate effect of this overlay of a critical habitat on this 1,500 acres is a diminution in value of tens of millions of dollars,” the lawyer said, adding: “Any buyer coming in will recognize that, down the road, they have to deal with the critical habitat designation.”

Justice Samuel A. Alito Jr. agreed with Bishop.

The question is not whether the frogs will become extinct, Alito said. “The question is, who is going to have to pay and who should pay for the preservation of this public good?”

The government hasn’t offered to buy the land or pay for the improvements needed to make it work for the frog.

That worried Roberts. “If you permit the designation of something as critical habitat that cannot be occupied by the animal, because you think they can do something down the road that will cure the problem . . . you ought to be able to articulate what the limit is on what you require down the road,” Roberts said.

Deputy Solicitor General Edwin S. Kneedler said the improvements would have to be reasonable, but he was unable to come up with a rule that seemed to satisfy Roberts.

When they are evenly divided, justices often try to find a narrow way to decide the case. If they remained split, the lower court decision supporting the government would stand, but it would not set a national precedent.

The court can also wait and rehear the case so that the new justice can help decide it.